@article{GrossMuellerNothoferetal.2000, author = {Gross, M. and M{\"u}ller, David C. and Nothofer, Heinz-Georg and Scherf, Ullrich and Neher, Dieter and Br{\"a}uchler, C. and Meerholz, Klaus}, title = {Improving the performance of doped p-conjugated polymers for use in organic light-emitting diodes}, year = {2000}, language = {en} } @article{JoshiPingelGrigorianetal.2009, author = {Joshi, Siddharth and Pingel, Patrick and Grigorian, Souren and Panzner, Tobias and Pietsch, Ullrich and Neher, Dieter and Forster, Michael and Scherf, Ullrich}, title = {Bimodal temperature behavior of structure and mobility in high molecular weight p3ht thin films}, issn = {0024-9297}, doi = {10.1021/Ma900021w}, year = {2009}, abstract = {We report a temperature dependent crystalline structure of spin-coated thin films of high molecular weight regioregular poly(3-hexylthiophene) (P3HT) (M-n similar to 30000 g/mol) and its correlation with charge carrier mobility. These investigations show a reversible change of the crystalline structure, where the interlayer lattice spacing (100)along the alkyl side chains continuously increases up to a temperature of about 220 degrees C; in contrast, the in-plane pi-pi distance reduces with increasing temperature. These changes in structure are reversible and can be repeated several times. The temperature-induced structural properties differ for thick and thin films, pointing to a surface/interface role in stabilization of the layer morphology. In contrast to the structural changes, the carrier mobility is rather constant in the temperature range from room temperature up to 100-120 degrees C, followed by a continuous decrease. For thick layers this drop is significant and the transistor performance almost vanishes at high temperature, however, it completely recovers upon cooling back to roorn temperature. The drop of the charge carrier mobility at higher temperatures is in contrast with expectations front the structural studies, considering the increase of crystalline fraction of the polycrystalline layer. our electrical measurements Underscore that the reduction of the macroscopic mobility is mostly caused by it pronounced decrease of the intergrain transport. The thermally induced crystallization along(100) direction and the creation of numerous small crystallites at the film-substrate interface reduce the number of long polymer chain, bridging crystalline domains, which ultimately limits the macroscopic charge transport.}, language = {en} } @article{SchubertSteyrleuthnerBangeetal.2009, author = {Schubert, Marcel and Steyrleuthner, Robert and Bange, Sebastian and Sellinger, Alan and Neher, Dieter}, title = {Charge transport and recombination in bulk heterojunction solar cells containing a dicyanoimidazole-based molecular acceptor}, issn = {1862-6300}, doi = {10.1002/pssa.200925312}, year = {2009}, abstract = {Carrier transport and recombination have been studied in single component layers and blends of the soluble PPV- derivative poly[2,5-dimethoxy-1,4-phenylenevinylene-2-methoxy-5-(2-ethyl-hexyloxy)- 1,4-phenylenevinylene] (M3EH-PPV) and the small molecule acceptor 4,7-bis(2-(1-hexyl-4,5-dicyanoimidazole-2-yl)vinyl) benzo[c][1,2,5]-thiadiazole (HV-BT). Measurements on single carrier devices show significantly smaller electron mobility in the blend compared to the pure HV- BT layer, which is suggestive of the formation of isolated clusters of the acceptor in a continuous polymer matrix. The significant change in fill factor (FF) with increasing illumination intensity is consistently explained by a model taking into account bimolecular recombination and space charge effects. The decay of the carrier density after photoexcitation has been studied by performing photo-CELIV measurements on pure and blend layers. It is found that the decay at long delay times follows a power-law dependence, which is, however, not consistent with a Langevin-type bimolecular recombination of free charges. A good description of the data is obtained by assuming trimolecular recombination to govern the charge carrier dynamics in these systems.}, language = {en} } @article{SchubertYinCastellanietal.2009, author = {Schubert, Marcel and Yin, Chunhong and Castellani, Mauro and Bange, Sebastian and Tam, Teck Lip and Sellinger, Alan and Hoerhold, Hans-Heinrich and Kietzke, Thomas and Neher, Dieter}, title = {Heterojunction topology versus fill factor correlations in novel hybrid small-molecular/polymeric solar cells}, issn = {0021-9606}, doi = {10.1063/1.3077007}, year = {2009}, abstract = {The authors present organic photovoltaic (OPV) devices comprising a small molecule electron acceptor based on 2- vinyl-4,5-dicyanoimidazole (Vinazene (TM)) and a soluble poly(p-phenylenevinylene) derivative as the electron donor. A strong dependence of the fill factor (FF) and the external quantum efficiency [incident photons converted to electrons (IPCE)] on the heterojunction topology is observed. As-prepared blends provided relatively low FF and IPCE values of 26\% and 4.5\%, respectively, which are attributed to significant recombination of geminate pairs and free carriers in a highly intermixed blend morphology. Going to an all-solution processed bilayer device, the FF and IPCE dramatically increased to 43\% and 27\%, respectively. The FF increases further to 57\% in devices comprising thermally deposited Vinazene layers where there is virtually no interpenetration at the donor/acceptor interface. This very high FF is comparable to values reported for OPV using fullerenes as the electron acceptor. Furthermore, the rather low electron affinity of Vinazene compound near 3.5 eV enabled a technologically important open circuit voltage (V-oc) of 1.0 V.}, language = {en} } @article{InalCastellaniSellingeretal.2009, author = {Inal, Sahika and Castellani, Mauro and Sellinger, Alan and Neher, Dieter}, title = {Relationship of photophysical properties and the device performance of novel hybrid small-molecular/polymeric solar cells}, issn = {1022-1336}, doi = {10.1002/marc.200900221}, year = {2009}, abstract = {We investigate solar cells comprised of a vinazene derivative (HV-BT) as the electron acceptor and the well- known polymer poly(3-hexylthiophene) as the electron donor. In the as-prepared blend, most of the excited state species, including the excimers on HV-BT, are quenched at the heterojunction. Although the photophysical properties of the blends change upon annealing, the blend solar cells largely remain uninfluenced by such treatments. A significant improvement is, however, observed when inducing phase separation at a longer length scale, for example, in solution-processed bilayer devices. Hereby, both the fill factor (FF) and the open circuit voltage are considerably increased, pointing to the importance of the heterojunction topology and the layer composition at the charge extracting contacts. An optimized device exhibits a power conversion efficiency of close to 1\%.}, language = {en} } @article{SteyrleuthnerBangeNeher2009, author = {Steyrleuthner, Robert and Bange, Sebastian and Neher, Dieter}, title = {Reliable electron-only devices and electron transport in n-type polymers}, issn = {0021-8979}, doi = {10.1063/1.3086307}, year = {2009}, abstract = {Current-voltage analysis of single-carrier transport is a popular method for the determination of charge carrier mobilities in organic semiconductors. Although in widespread use for the analysis of hole transport, only a few reports can be found where the method was applied to electron transport. Here, we summarize the experimental difficulties related to the metal electrode leakage currents and nonlinear differential resistance (NDR) effects and explain their origin. We present a modified preparation technique for the metal electrodes and show that it significantly increases the reliability of such measurements. It allows to produce test devices with low leakage currents and without NDR even for thin organic layers. Metal oxides were often discussed as a possible cause of NDR. Our measurements on forcibly oxidized metal electrodes demonstrate that oxide layers are not exclusively responsible for NDR effects. We present electron transport data for two electron-conducting polymers often applied in all-polymer solar cells for a large variety of layer thicknesses and temperatures. The results can be explained by established exponential trapping models.}, language = {en} } @article{RengelAltmannNeheretal.1999, author = {Rengel, Heiko and Altmann, Markus and Neher, Dieter and Harrison, Craig B. and Myrick, Michael L. and Bunz, Uwe H. F.}, title = {Assignment of the optical transitions in 1,3- diethynylcyclobutadiene (cyclopentadienyl)cobalt oligomers}, year = {1999}, language = {en} } @article{MitevaMeiselNothoferetal.2000, author = {Miteva, T. and Meisel, A. and Nothofer, Heinz-Georg and Scherf, Ullrich and Knoll, W. and Neher, Dieter and Grell, M. and Lupo, D. and Yasuda, A.}, title = {Polarized electroluminescence from highly aligned liquid-crystalline polymers}, year = {2000}, language = {en} } @article{MitevaKloppenburgNeheretal.2000, author = {Miteva, T. and Kloppenburg, L. and Neher, Dieter and Bunz, Uwe H. F.}, title = {Interplay of Thermochromicity and Liquid Crystalline Behavior in Poly(p-phenyleneethynylen)s:p-p-Interactions or Planarization of the Conjugated Backbone?}, year = {2000}, language = {en} } @article{BauerUmbaschGiessenetal.2000, author = {Bauer, C. and Umbasch, G. and Giessen, H. and Meisel, A. and Nothofer, Heinz-Georg and Neher, Dieter and Scherf, Ullrich and Marth, R.}, title = {Polarized Photoluminescence and Spectral Narrowing in an oriented Polyfluorene Thin Film}, year = {2000}, language = {en} } @article{GrellKnollLupoetal.1999, author = {Grell, M. and Knoll, W. and Lupo, D. and Meisel, A. and Miteva, T. and Neher, Dieter and Nothofer, Heinz-Georg and Scherf, Ullrich and Yasuda, H.}, title = {Blue polarized electroluminescence from a liquid crystalline polyfluorene}, year = {1999}, language = {en} } @article{UrayamaKircherBoehmeretal.1999, author = {Urayama, Kenji and Kircher, Oliver and B{\"o}hmer, Roland and Neher, Dieter}, title = {Investigations of ferroelectric-to-paraelectric phase transition of vinylidenefluoride trifluoroethylene copolymer thin films by electromechanical interferometry}, year = {1999}, language = {en} } @article{BittnerDaeublerNeheretal.1999, author = {Bittner, Reinhard and D{\"a}ubler, Thomas Karl and Neher, Dieter and Meerholz, Klaus}, title = {Influence of the glass-transition and the chromophore content on the steady-state performance of PVK-based photorefractive polymers}, year = {1999}, language = {en} } @article{FormerWagnerRichertetal.1999, author = {Former, C. and Wagner, H. and Richert, R. and Neher, Dieter and M{\"u}llen, K.}, title = {Orientation and dynamics of chainlike dipole arrays: Donor-acceptor-substituted oligophenylenevinylenes in a polymer matrix}, year = {1999}, language = {en} } @article{HosseiniTokmoldinLeeetal.2020, author = {Hosseini, Seyed Mehrdad and Tokmoldin, Nurlan and Lee, Young Woong and Zou, Yingping and Woo, Han Young and Neher, Dieter and Shoaee, Safa}, title = {Putting order into PM6:Y6 solar cells to reduce the langevin recombination in 400 nm thick junction}, series = {Solar RRL}, volume = {4}, journal = {Solar RRL}, number = {11}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {2367-198X}, doi = {10.1002/solr.202000498}, pages = {7}, year = {2020}, abstract = {Increasing the active layer thickness without sacrificing the power conversion efficiency (PCE) is one of the great challenges faced by organic solar cells (OSCs) for commercialization. Recently, PM6:Y6 as an OSC based on a non-fullerene acceptor (NFA) has excited the community because of its PCE reaching as high as 15.9\%; however, by increasing the thickness, the PCE drops due to the reduction of the fill factor (FF). This drop is attributed to change in mobility ratio with increasing thickness. Furthermore, this work demonstrates that by regulating the packing and the crystallinity of the donor and the acceptor, through volumetric content of chloronaphthalene (CN) as a solvent additive, one can improve the FF of a thick PM6:Y6 device (approximate to 400 nm) from 58\% to 68\% (PCE enhances from 12.2\% to 14.4\%). The data indicate that the origin of this enhancement is the reduction of the structural and energetic disorders in the thick device with 1.5\% CN compared with 0.5\% CN. This correlates with improved electron and hole mobilities and a 50\% suppressed bimolecular recombination, such that the non-Langevin reduction factor is 180 times. This work reveals the role of disorder on the charge extraction and bimolecular recombination of NFA-based OSCs.}, language = {en} } @article{VandewalAlbrechtHokeetal.2014, author = {Vandewal, Koen and Albrecht, Steve and Hoke, Eric T. and Graham, Kenneth R. and Widmer, Johannes and Douglas, Jessica D. and Schubert, Marcel and Mateker, William R. and Bloking, Jason T. and Burkhard, George F. and Sellinger, Alan and Frechet, Jean M. J. and Amassian, Aram and Riede, Moritz K. and McGehee, Michael D. and Neher, Dieter and Salleo, Alberto}, title = {Efficient charge generation by relaxed charge-transfer states at organic interfaces}, series = {Nature materials}, volume = {13}, journal = {Nature materials}, number = {1}, publisher = {Nature Publ. Group}, address = {London}, issn = {1476-1122}, doi = {10.1038/NMAT3807}, pages = {63 -- 68}, year = {2014}, abstract = {carriers on illumination. Efficient organic solar cells require a high yield for this process, combined with a minimum of energy losses. Here, we investigate the role of the lowest energy emissive interfacial charge-transfer state (CT1) in the charge generation process. We measure the quantum yield and the electric field dependence of charge generation on excitation of the charge-transfer (CT) state manifold viaweakly allowed, low-energy optical transitions. For a wide range of photovoltaic devices based on polymer: fullerene, small-molecule:C-60 and polymer: polymer blends, our study reveals that the internal quantum efficiency (IQE) is essentially independent of whether or not D, A or CT states with an energy higher than that of CT1 are excited. The best materials systems show an IQE higher than 90\% without the need for excess electronic or vibrational energy.}, language = {en} } @article{SchubertDolfenFrischetal.2012, author = {Schubert, Marcel and Dolfen, Daniel and Frisch, Johannes and Roland, Steffen and Steyrleuthner, Robert and Stiller, Burkhard and Chen, Zhihua and Scherf, Ullrich and Koch, Norbert and Facchetti, Antonio and Neher, Dieter}, title = {Influence of aggregation on the performance of All-Polymer Solar Cells containing Low-Bandgap Naphthalenediimide Copolymers}, series = {dvanced energy materials}, volume = {2}, journal = {dvanced energy materials}, number = {3}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1614-6832}, doi = {10.1002/aenm.201100601}, pages = {369 -- 380}, year = {2012}, abstract = {The authors present efficient all-polymer solar cells comprising two different low-bandgap naphthalenediimide (NDI)-based copolymers as acceptors and regioregular P3HT as the donor. It is shown that these naphthalene copolymers have a strong tendency to preaggregate in specific organic solvents, and that preaggregation can be completely suppressed when using suitable solvents with large and highly polarizable aromatic cores. Organic solar cells prepared from such nonaggregated polymer solutions show dramatically increased power conversion efficiencies of up to 1.4\%, which is mainly due to a large increase of the short circuit current. In addition, optimized solar cells show remarkable high fill factors of up to 70\%. The analysis of the blend absorbance spectra reveals a surprising anticorrelation between the degree of polymer aggregation in the solid P3HT:NDI copolymer blends and their photovoltaic performance. Scanning near-field optical microscopy (SNOM) and atomic force microscopy (AFM) measurements reveal important information on the blend morphology. It is shown that films with high degree of aggregation and low photocurrents exhibit large-scale phase-separation into rather pure donor and acceptor domains. It is proposed that, by suppressing the aggregation of NDI copolymers at the early stage of film formation, the intermixing of the donor and acceptor component is improved, thereby allowing efficient harvesting of photogenerated excitons at the donoracceptor heterojunction.}, language = {en} } @article{FrischSchubertPreisetal.2012, author = {Frisch, Johannes and Schubert, Marcel and Preis, Eduard and Rabe, J{\"u}rgen P. and Neher, Dieter and Scherf, Ullrich and Koch, Norbert}, title = {Full electronic structure across a polymer heterojunction solar cell}, series = {Journal of materials chemistry}, volume = {22}, journal = {Journal of materials chemistry}, number = {10}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {0959-9428}, doi = {10.1039/c1jm14968g}, pages = {4418 -- 4424}, year = {2012}, abstract = {We correlate the morphology and energy level alignment of bilayer structures comprising the donor poly(3-hexylthiophene) (P3HT) and the acceptor polyfluorene copolymer poly(9,90dialklylfluorene-alt-4,7-bis(2,5-thiendiyl)-2,1,3-benzothiadiazole) (PFTBTT) with the performance of these bilayers in organic photovoltaic cells (OPVCs). The conducting polymer poly(ethylenedioxythiophene): poly (styrenesulfonate) (PEDT:PSS) was used as the bottom electrode and Ca as the top electrode. Ultraviolet photoelectron spectroscopy (UPS) revealed that notable interface dipoles occur at all interfaces across the OPVC structure, highlighting that vacuum level alignment cannot reliably be used to estimate the electronic properties for device design. Particularly the effective electrode work function values (after contact formation with the conjugated polymers) differ significantly from those of the pristine electrode materials. Chemical reactions between PEDT: PSS and P3HT on the one hand and Ca and PFTBTT on the other hand are identified as cause for the measured interface dipoles. The vacuum level shift between P3HT and PFTBTT is related to mutual energy level pinning at gap states. Annealing induced morphological changes at the P3HT/PFTBTT interface increased the efficiency of OPVCs, while the electronic structure was not affected by thermal treatment.}, language = {en} } @article{AlbrechtSchaeferLangeetal.2012, author = {Albrecht, Steve and Schaefer, Sebastian and Lange, Ilja and Yilmaz, Seyfullah and Dumsch, Ines and Allard, Sybille and Scherf, Ullrich and Hertwig, Andreas and Neher, Dieter}, title = {Light management in PCPDTBT:PC70BM solar cells: A comparison of standard and inverted device structures}, series = {Organic electronics : physics, materials and applications}, volume = {13}, journal = {Organic electronics : physics, materials and applications}, number = {4}, publisher = {Elsevier}, address = {Amsterdam}, issn = {1566-1199}, doi = {10.1016/j.orgel.2011.12.019}, pages = {615 -- 622}, year = {2012}, abstract = {We compare standard and inverted bulk heterojunction solar cells composed of PCPDTBT:PC70BM blends. Inverted devices comprising 100 nm thick active layers exhibited short circuit currents of 15 mA/cm(2), 10\% larger than in corresponding standard devices. Modeling of the optical field distribution in the different device stacks proved that this enhancement originates from an increased absorption of incident light in the active layer. Internal quantum efficiencies (IQEs) were obtained from the direct comparison of experimentally derived and modeled currents for different layer thicknesses, yielding IQEs of similar to 70\% for a layer thickness of 100 nm. Simulations predict a significant increase of the light harvesting efficiency upon increasing the layer thickness to 270 nm. However, a continuous deterioration of the photovoltaic properties with layer thickness was measured for both device architectures, attributed to incomplete charge extraction. On the other hand, our optical modeling suggests that inverted devices based on PCPDTBT should be able to deliver high power conversion efficiencies (PCEs) of more than 7\% provided that recombination losses can be reduced.}, language = {en} } @article{AlbrechtSchindlerKurpiersetal.2012, author = {Albrecht, Steve and Schindler, Wolfram and Kurpiers, Jona and Kniepert, Juliane and Blakesley, James C. and Dumsch, Ines and Allard, Sybille and Fostiropoulos, Konstantinos and Scherf, Ullrich and Neher, Dieter}, title = {On the field dependence of free charge carrier generation and recombination in blends of PCPDTBT/PC70BM influence of solvent additives}, series = {The journal of physical chemistry letters}, volume = {3}, journal = {The journal of physical chemistry letters}, number = {5}, publisher = {American Chemical Society}, address = {Washington}, issn = {1948-7185}, doi = {10.1021/jz3000849}, pages = {640 -- 645}, year = {2012}, abstract = {We have applied time-delayed collection field (TDCF) and charge extraction by linearly increasing voltage (CELIV) to investigate the photogeneration, transport, and recombination of charge carriers in blends composed of PCPDTBT/PC70BM processed with and without the solvent additive diiodooctane. The results suggest that the solvent additive has severe impacts on the elementary processes involved in the photon to collected electron conversion in these blends. First, a pronounced field dependence of the free carrier generation is found for both blends, where the field dependence is stronger without the additive. Second, the fate of charge carriers in both blends can be described with a rather high bimolecular recombination coefficients, which increase with decreasing internal field. Third, the mobility is three to four times higher with the additive. Both blends show a negative field dependence of mobility, which we suggest to cause bias-dependent recombination coefficients.}, language = {en} } @article{IlnytskyiSaphiannikovaNeheretal.2012, author = {Ilnytskyi, Jaroslav M. and Saphiannikova, Marina and Neher, Dieter and Allen, Michael P.}, title = {Modelling elasticity and memory effects in liquid crystalline elastomers by molecular dynamics simulations}, series = {Soft matter}, volume = {8}, journal = {Soft matter}, number = {43}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {1744-683X}, doi = {10.1039/c2sm26499d}, pages = {11123 -- 11134}, year = {2012}, abstract = {We performed molecular dynamics simulations of a liquid crystal elastomer of side-chain architecture. The network is formed from a melt of 28 molecules each having a backbone of 100 hydrocarbon monomers, to which 50 side chains are attached in a syndiotactic way. Crosslinking is performed in the smectic A phase. We observe an increase of the smectic-isotropic phase transition temperature of about 5 degrees as compared to the uncrosslinked melt. Memory effects in liquid crystalline order and in sample shape are well reproduced when the elastomer is driven through the smectic-isotropic transition. Above this transition, in the isotropic phase, the polydomain smectic phase is induced by a uniaxial load. Below the transition, in a monodomain smectic A phase, both experimentally observed effects of homogeneous director reorientation and stripe formation are reproduced when the sample is stretched along the director. When the load is applied perpendicularly to the director, the sample demonstrates reversible deformation with no change of liquid crystalline order, indicating elasticity of the two-dimensional network of polymer layers.}, language = {en} } @article{PingelSchwarzlNeher2012, author = {Pingel, P. and Schwarzl, R. and Neher, Dieter}, title = {Effect of molecular p-doping on hole density and mobility in poly(3-hexylthiophene)}, series = {Applied physics letters}, volume = {100}, journal = {Applied physics letters}, number = {14}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0003-6951}, doi = {10.1063/1.3701729}, pages = {3}, year = {2012}, abstract = {Employing impedance spectroscopy, we have studied the hole density, conductivity, and mobility of poly(3-hexylthiophene), P3HT, doped with the strong molecular acceptor tetrafluorotetracyanoquinodimethane, F(4)TCNQ. We find that the hole density increases linearly with the F(4)TCNQ concentration. Furthermore, the hole mobility is decreased upon doping at low-to-medium doping level, which is rationalized by an analytic model of carrier mobility in doped organic semiconductors [V. I. Arkhipov, E. V. Emelianova, P. Heremans, and H. Bassler, Phys. Rev. B 72, 235202 (2005)]. We infer that the presence of ionized F(4)TCNQ molecules in the P3HT layer increases energetic disorder, which diminishes the carrier mobility.}, language = {en} } @article{NikolisMischokSiegmundetal.2019, author = {Nikolis, Vasileios C. and Mischok, Andreas and Siegmund, Bernhard and Kublitski, Jonas and Jia, Xiangkun and Benduhn, Johannes and H{\"o}rmann, Ulrich and Neher, Dieter and Gather, Malte C. and Spoltore, Donato and Vandewal, Koen}, title = {Strong light-matter coupling for reduced photon energy losses in organic photovoltaics}, series = {Nature Communications}, volume = {10}, journal = {Nature Communications}, publisher = {Nature Publ. Group}, address = {London}, issn = {2041-1723}, doi = {10.1038/s41467-019-11717-5}, pages = {8}, year = {2019}, abstract = {Strong light-matter coupling can re-arrange the exciton energies in organic semiconductors. Here, we exploit strong coupling by embedding a fullerene-free organic solar cell (OSC) photo-active layer into an optical microcavity, leading to the formation of polariton peaks and a red-shift of the optical gap. At the same time, the open-circuit voltage of the device remains unaffected. This leads to reduced photon energy losses for the low-energy polaritons and a steepening of the absorption edge. While strong coupling reduces the optical gap, the energy of the charge-transfer state is not affected for large driving force donor-acceptor systems. Interestingly, this implies that strong coupling can be exploited in OSCs to reduce the driving force for electron transfer, without chemical or microstructural modifications of the photoactive layer. Our work demonstrates that the processes determining voltage losses in OSCs can now be tuned, and reduced to unprecedented values, simply by manipulating the device architecture.}, language = {en} } @article{GarciaBenitoQuartiQuelozetal.2020, author = {Garc{\´i}a-Benito, In{\´e}s and Quarti, Claudio and Queloz, Valentin I. E. and Hofstetter, Yvonne J. and Becker-Koch, David and Caprioglio, Pietro and Neher, Dieter and Orlandi, Simonetta and Cavazzini, Marco and Pozzi, Gianluca and Even, Jacky and Nazeeruddin, Mohammad Khaja and Vaynzof, Yana and Grancini, Giulia}, title = {Fluorination of organic spacer impacts on the structural and optical response of 2D perovskites}, series = {Frontiers in Chemistry}, volume = {7}, journal = {Frontiers in Chemistry}, publisher = {Frontiers Media}, address = {Lausanne}, issn = {2296-2646}, doi = {10.3389/fchem.2019.00946}, pages = {1 -- 11}, year = {2020}, abstract = {Low-dimensional hybrid perovskites have triggered significant research interest due to their intrinsically tunable optoelectronic properties and technologically relevant material stability. In particular, the role of the organic spacer on the inherent structural and optical features in two-dimensional (2D) perovskites is paramount for material optimization. To obtain a deeper understanding of the relationship between spacers and the corresponding 2D perovskite film properties, we explore the influence of the partial substitution of hydrogen atoms by fluorine in an alkylammonium organic cation, resulting in (Lc)(2)PbI4 and (Lf)(2)PbI4 2D perovskites, respectively. Consequently, optical analysis reveals a clear 0.2 eV blue-shift in the excitonic position at room temperature. This result can be mainly attributed to a band gap opening, with negligible effects on the exciton binding energy. According to Density Functional Theory (DFT) calculations, the band gap increases due to a larger distortion of the structure that decreases the atomic overlap of the wavefunctions and correspondingly bandwidth of the valence and conduction bands. In addition, fluorination impacts the structural rigidity of the 2D perovskite, resulting in a stable structure at room temperature and the absence of phase transitions at a low temperature, in contrast to the widely reported polymorphism in some non-fluorinated materials that exhibit such a phase transition. This indicates that a small perturbation in the material structure can strongly influence the overall structural stability and related phase transition of 2D perovskites, making them more robust to any phase change. This work provides key information on how the fluorine content in organic spacer influence the structural distortion of 2D perovskites and their optical properties which possess remarkable importance for future optoelectronic applications, for instance in the field of light-emitting devices or sensors.}, language = {en} } @article{KniepertLangevanderKaapetal.2014, author = {Kniepert, Juliane and Lange, Ilja and van der Kaap, Niels J. and Koster, L. Jan Anton and Neher, Dieter}, title = {A conclusive view on charge generation, recombination, and extraction in As-prepared and annealed P3HT:PCBM blends: combined experimental and simulation work}, series = {dvanced energy materials}, volume = {4}, journal = {dvanced energy materials}, number = {7}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1614-6832}, doi = {10.1002/aenm.201301401}, pages = {11}, year = {2014}, abstract = {Time-delayed collection field (TDCF) and bias-amplified charge extraction (BACE) are applied to as-prepared and annealed poly(3-hexylthiophene):[6,6]-phenyl C-71 butyric acid methyl ester (P3HT:PCBM) blends coated from chloroform. Despite large differences in fill factor, short-circuit current, and power conversion efficiency, both blends exhibit a negligible dependence of photogeneration on the electric field and strictly bimolecular recombination (BMR) with a weak dependence of the BMR coefficient on charge density. Drift-diffusion simulations are performed using the measured coefficients and mobilities, taking into account bimolecular recombination and the possible effects of surface recombination. The excellent agreement between the simulation and the experimental data for an intensity range covering two orders of magnitude indicates that a field-independent generation rate and a density-independent recombination coefficient describe the current-voltage characteristics of the annealed P3HT: PCBM devices, while the performance of the as-prepared blend is shown to be limited by space charge effects due to a low hole mobility. Finally, even though the bimolecular recombination coefficient is small, surface recombination is found to be a negligible loss mechanism in these solar cells.}, language = {en} } @article{PoelkingBenduhnSpoltoreetal.2022, author = {Poelking, Carl and Benduhn, Johannes and Spoltore, Donato and Schwarze, Martin and Roland, Steffen and Piersimoni, Fortunato and Neher, Dieter and Leo, Karl and Vandewal, Koen and Andrienko, Denis}, title = {Open-circuit voltage of organic solar cells}, series = {Communications physics}, volume = {5}, journal = {Communications physics}, number = {1}, publisher = {Nature portfolio}, address = {Berlin}, issn = {2399-3650}, doi = {10.1038/s42005-022-01084-x}, pages = {7}, year = {2022}, abstract = {Organic photovoltaics (PV) is an energy-harvesting technology that offers many advantages, such as flexibility, low weight and cost, as well as environmentally benign materials and manufacturing techniques. Despite growth of power conversion efficiencies to around 19 \% in the last years, organic PVs still lag behind inorganic PV technologies, mainly due to high losses in open-circuit voltage. Understanding and improving open circuit voltage in organic solar cells is challenging, as it is controlled by the properties of a donor-acceptor interface where the optical excitations are separated into charge carriers. Here, we provide an electrostatic model of a rough donor-acceptor interface and test it experimentally on small molecule PV materials systems. The model provides concise relationships between the open-circuit voltage, photovoltaic gap, charge-transfer state energy, and interfacial morphology. In particular, we show that the electrostatic bias generated across the interface reduces the photovoltaic gap. This negative influence on open-circuit voltage can, however, be circumvented by adjusting the morphology of the donor-acceptor interface. Organic solar cells, despite their high power conversion efficiencies, suffer from open circuit voltage losses making them less appealing in terms of applications. Here, the authors, supported with experimental data on small molecule photovoltaic cells, relate open circuit voltage to photovoltaic gap, charge-transfer state energy, and donor-acceptor interfacial morphology.}, language = {en} } @article{VollbrechtTokmoldinSunetal.2022, author = {Vollbrecht, Joachim and Tokmoldin, Nurlan and Sun, Bowen and Brus, Viktor V. and Shoaee, Safa and Neher, Dieter}, title = {Determination of the charge carrier density in organic solar cells}, series = {Journal of applied physics}, volume = {131}, journal = {Journal of applied physics}, number = {22}, publisher = {American Institute of Physics}, address = {Melville, NY}, issn = {0021-8979}, doi = {10.1063/5.0094955}, pages = {18}, year = {2022}, abstract = {The increase in the performance of organic solar cells observed over the past few years has reinvigorated the search for a deeper understanding of the loss and extraction processes in this class of device. A detailed knowledge of the density of free charge carriers under different operating conditions and illumination intensities is a prerequisite to quantify the recombination and extraction dynamics. Differential charging techniques are a promising approach to experimentally obtain the charge carrier density under the aforementioned conditions. In particular, the combination of transient photovoltage and photocurrent as well as impedance and capacitance spectroscopy have been successfully used in past studies to determine the charge carrier density of organic solar cells. In this Tutorial, these experimental techniques will be discussed in detail, highlighting fundamental principles, practical considerations, necessary corrections, advantages, drawbacks, and ultimately their limitations. Relevant references introducing more advanced concepts will be provided as well. Therefore, the present Tutorial might act as an introduction and guideline aimed at new prospective users of these techniques as well as a point of reference for more experienced researchers. Published under an exclusive license by AIP Publishing.}, language = {en} } @article{SunSandbergNeheretal.2022, author = {Sun, Bowen and Sandberg, Oskar and Neher, Dieter and Armin, Ardalan and Shoaee, Safa}, title = {Wave optics of differential absorption spectroscopy in thick-junction organic solar cells}, series = {Physical review applied / The American Physical Society}, volume = {17}, journal = {Physical review applied / The American Physical Society}, number = {5}, publisher = {American Physical Society}, address = {College Park}, issn = {2331-7019}, doi = {10.1103/PhysRevApplied.17.054016}, pages = {12}, year = {2022}, abstract = {Differential absorption spectroscopy techniques serve as powerful techniques to study the excited species in organic solar cells. However, it has always been challenging to employ these techniques for characterizing thick-junction organic solar cells, especially when a reflective top contact is involved. In this work, we present a detailed and systematic study on how a combination of the presence of the interference effect and a nonuniform charge-distribution profile, severely manipulates experimental spectra and the decay dynamics. Furthermore, we provide a practical methodology to correct these optical artifacts in differential absorption spectroscopies. The results and the proposed correction method generally apply to all kinds of differential absorption spectroscopy techniques and various thin-film systems, such as organics, perovskites, kesterites, and two-dimensional materials. Notably, it is found that the shape of differential absorption spectra can be strongly distorted, starting from 150-nm active-layer thickness; this matches the thickness range of thick-junction organic solar cells and most perovskite solar cells and needs to be carefully considered in experiments. In addition, the decay dynamics of differential absorption spectra is found to be disturbed by optical artifacts under certain conditions. With the help of the proposed correction formalism, differential spectra and the decay dynamics can be characterized on the full device of thin-film solar cells in transmission mode and yield accurate and reliable results to provide design rules for further progress.}, language = {en} } @article{FritschKurpiersRolandetal.2022, author = {Fritsch, Tobias and Kurpiers, Jona and Roland, Steffen and Tokmoldin, Nurlan and Shoaee, Safa and Ferron, Thomas and Collins, Brian A. and Janietz, Silvia and Vandewal, Koen and Neher, Dieter}, title = {On the interplay between CT and singlet exciton emission in organic solar cells with small driving force and its impact on voltage loss}, series = {Advanced energy materials}, volume = {12}, journal = {Advanced energy materials}, number = {31}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1614-6832}, doi = {10.1002/aenm.202200641}, pages = {11}, year = {2022}, abstract = {The interplay between free charge carriers, charge transfer (CT) states and singlet excitons (S-1) determines the recombination pathway and the resulting open circuit voltage (V-OC) of organic solar cells. By combining a well-aggregated low bandgap polymer with different blend ratios of the fullerenes PCBM and ICBA, the energy of the CT state (E-CT) is varied by 130 meV while leaving the S-1 energy of the polymer (ES1\[{E_{{{\rm{S}}_1}}}\]) unaffected. It is found that the polymer exciton dominates the radiative properties of the blend when ECT\[{E_{{\rm{CT}}}}\] approaches ES1\[{E_{{{\rm{S}}_1}}}\], while the V-OC remains limited by the non-radiative decay of the CT state. It is concluded that an increasing strength of the exciton in the optical spectra of organic solar cells will generally decrease the non-radiative voltage loss because it lowers the radiative V-OC limit (V-OC,V-rad), but not because it is more emissive. The analysis further suggests that electronic coupling between the CT state and the S-1 will not improve the V-OC, but rather reduce the V-OC,V-rad. It is anticipated that only at very low CT state absorption combined with a fairly high CT radiative efficiency the solar cell benefit from the radiative properties of the singlet excitons.}, language = {en} } @article{GerhardWegenerKuenstleretal.2000, author = {Gerhard, Reimund and Wegener, Michael and K{\"u}nstler, Wolfgang and Wirges, Werner and G{\"o}rne, Thomas and Urayama, D. and Neher, Dieter}, title = {Inverse piezoelectricity of porous PTFE films with bipolar space charge}, year = {2000}, language = {en} } @article{SainovaMitevaNothoferetal.2000, author = {Sainova, Dessislava and Miteva, T. and Nothofer, Heinz-Georg and Scherf, Ullrich and Fujikawa, H. and Glowacki, Ireneusz and Ulanski, J. and Neher, Dieter}, title = {Control of color and efficiency of light-emitting diodes based on polyfluorenes blended with hole-transporting molecules}, year = {2000}, language = {en} } @article{SchubertFrischAllardetal.2017, author = {Schubert, Marcel and Frisch, Johannes and Allard, Sybille and Preis, Eduard and Scherf, Ullrich and Koch, Norbert and Neher, Dieter}, title = {Tuning side chain and main chain order in a prototypical donor-acceptor copolymer}, series = {Elementary Processes in Organic Photovoltaics}, volume = {272}, journal = {Elementary Processes in Organic Photovoltaics}, publisher = {Springer}, address = {Berlin}, isbn = {978-3-319-28338-8}, issn = {0065-3195}, doi = {10.1007/978-3-319-28338-8_10}, pages = {243 -- 265}, year = {2017}, abstract = {The recent development of donor-acceptor copolymers has led to an enormous improvement in the performance of organic solar cells and organic field-effect transistors. Here we describe the synthesis, detailed characterisation, and application of a series of structurally modified copolymers to investigate fundamental structure-property relationships in this class of conjugated polymers. The interplay between chemical structure and optoelectronic properties is investigated. These are further correlated to the charge transport and solar cell performance, which allows us to link their chemical structure to the observed physical properties.}, language = {en} } @article{ScharsichLohwasserSommeretal.2012, author = {Scharsich, Christina and Lohwasser, Ruth H. and Sommer, Michael and Asawapirom, Udom and Scherf, Ullrich and Thelakkat, Mukundan and Neher, Dieter and Koehler, Anna}, title = {Control of aggregate formation in poly(3-hexylthiophene) by solvent, molecular weight, and synthetic method}, series = {Journal of polymer science : B, Polymer physics}, volume = {50}, journal = {Journal of polymer science : B, Polymer physics}, number = {6}, publisher = {Wiley-Blackwell}, address = {Hoboken}, issn = {0887-6266}, doi = {10.1002/polb.23022}, pages = {442 -- 453}, year = {2012}, abstract = {Aggregate formation in poly(3-hexylthiophene) depends on molecular weight, solvent, and synthetic method. The interplay of these parameters thus largely controls device performance. In order to obtain a quantitative understanding on how these factors control the resulting electronic properties of P3HT, we measured absorption in solution and in thin films as well as the resulting field effect mobility in transistors. By a detailed analysis of the absorption spectra, we deduce the fraction of aggregates formed, the excitonic coupling within the aggregates, and the conjugation length within the aggregates, all as a function of solvent quality for molecular weights from 5 to 19 kDa. From this, we infer in which structure the aggregated chains pack. Although the 5 kDa samples form straight chains, the 11 and 19 kDa chains are kinked or folded, with conjugation lengths that increase as the solvent quality reduces. There is a maximum fraction of aggregated chains (about 55 +/- 5\%) that can be obtained, even for poor solvent quality. We show that inducing aggregation in solution leads to control of aggregate properties in thin films. As expected, the field-effect mobility correlates with the propensity to aggregation. Correspondingly, we find that a well-defined synthetic approach, tailored to give a narrow molecular weight distribution, is needed to obtain high field effect mobilities of up to 0.01 cm2/Vs for low molecular weight samples (=11 kDa), while the influence of synthetic method is negligible for samples of higher molecular weight, if low molecular weight fractions are removed by extraction.}, language = {en} } @article{LangeReiterPaetzeletal.2014, author = {Lange, Ilja and Reiter, Sina and Paetzel, Michael and Zykov, Anton and Nefedov, Alexei and Hildebrandt, Jana and Hecht, Stefan and Kowarik, Stefan and Woell, Christof and Heimel, Georg and Neher, Dieter}, title = {Tuning the work function of polar zinc oxide surfaces using modified phosphonic acid self-assembled monolayers}, series = {Advanced functional materials}, volume = {24}, journal = {Advanced functional materials}, number = {44}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1616-301X}, doi = {10.1002/adfm.201401493}, pages = {7014 -- 7024}, year = {2014}, abstract = {Zinc oxide (ZnO) is regarded as a promising alternative material for transparent conductive electrodes in optoelectronic devices. However, ZnO suffers from poor chemical stability. ZnO also has a moderate work function (WF), which results in substantial charge injection barriers into common (organic) semiconductors that constitute the active layer in a device. Controlling and tuning the ZnO WF is therefore necessary but challenging. Here, a variety of phosphonic acid based self-assembled monolayers (SAMs) deposited on ZnO surfaces are investigated. It is demonstrated that they allow the tuning the WF over a wide range of more than 1.5 eV, thus enabling the use of ZnO as both the hole-injecting and electron-injecting contact. The modified ZnO surfaces are characterized using a number of complementary techniques, demonstrating that the preparation protocol yields dense, well-defined molecular monolayers.}, language = {en} } @article{InalKoelschChiappisietal.2013, author = {Inal, Sahika and Koelsch, Jonas D. and Chiappisi, Leonardo and Janietz, Dietmar and Gradzielski, Michael and Laschewsky, Andr{\´e} and Neher, Dieter}, title = {Structure-related differences in the temperature-regulated fluorescence response of LCST type polymers}, series = {Journal of materials chemistry : C, Materials for optical and electronic devices}, volume = {1}, journal = {Journal of materials chemistry : C, Materials for optical and electronic devices}, number = {40}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {2050-7526}, doi = {10.1039/c3tc31304b}, pages = {6603 -- 6612}, year = {2013}, abstract = {We demonstrate new fluorophore-labelled materials based on acrylamide and on oligo(ethylene glycol) (OEG) bearing thermoresponsive polymers for sensing purposes and investigate their thermally induced solubility transitions. It is found that the emission properties of the polarity-sensitive (solvatochromic) naphthalimide derivative attached to three different thermoresponsive polymers are highly specific to the exact chemical structure of the macromolecule. While the dye emits very weakly below the LCST when incorporated into poly(N-isopropylacrylamide) (pNIPAm) or into a polyacrylate backbone bearing only short OEG side chains, it is strongly emissive in polymethacrylates with longer OEG side chains. Heating of the aqueous solutions above their cloud point provokes an abrupt increase of the fluorescence intensity of the labelled pNIPAm, whereas the emission properties of the dye are rather unaffected as OEG-based polyacrylates and methacrylates undergo phase transition. Correlated with laser light scattering studies, these findings are ascribed to the different degrees of pre-aggregation of the chains at low temperatures and to the extent of dehydration that the phase transition evokes. It is concluded that although the temperature-triggered changes in the macroscopic absorption characteristics, related to large-scale alterations of the polymer chain conformation and aggregation, are well detectable and similar for these LCST-type polymers, the micro-environment provided to the dye within each polymer network differs substantially. Considering sensing applications, this finding is of great importance since the temperature-regulated fluorescence response of the polymer depends more on the macromolecular architecture than the type of reporter fluorophore.}, language = {en} } @article{AlbrechtJanietzSchindleretal.2012, author = {Albrecht, Steve and Janietz, Silvia and Schindler, Wolfram and Frisch, Johannes and Kurpiers, Jona and Kniepert, Juliane and Inal, Sahika and Pingel, Patrick and Fostiropoulos, Konstantinos and Koch, Norbert and Neher, Dieter}, title = {Fluorinated Copolymer PCPDTBT with enhanced open-circuit voltage and reduced recombination for highly efficient polymer solar cells}, series = {Journal of the American Chemical Society}, volume = {134}, journal = {Journal of the American Chemical Society}, number = {36}, publisher = {American Chemical Society}, address = {Washington}, issn = {0002-7863}, doi = {10.1021/ja305039j}, pages = {14932 -- 14944}, year = {2012}, abstract = {A novel fluorinated copolymer (F-PCPDTBT) is introduced and shown to exhibit significantly higher power conversion efficiency in bulk heterojunction solar cells with PC70BM compared to the well-known low-band-gap polymer PCPDTBT. Fluorination lowers the polymer HOMO level, resulting in high open-circuit voltages well exceeding 0.7 V. Optical spectroscopy and morphological studies with energy-resolved transmission electron microscopy reveal that the fluorinated polymer aggregates more strongly in pristine and blended layers, with a smaller amount of additives needed to achieve optimum device performance. Time-delayed collection field and charge extraction by linearly increasing voltage are used to gain insight into the effect of fluorination on the field dependence of free charge-carrier generation and recombination. F-PCPDTBT is shown to exhibit a significantly weaker field dependence of free charge-carrier generation combined with an overall larger amount of free charges, meaning that geminate recombination is greatly reduced. Additionally, a 3-fold reduction in non-geminate recombination is measured compared to optimized PCPDTBT blends. As a consequence of reduced non-geminate recombination, the performance of optimized blends of fluorinated PCPDTBT with PC70BM is largely determined by the field dependence of free-carrier generation, and this field dependence is considerably weaker compared to that of blends comprising the non-fluorinated polymer. For these optimized blends, a short-circuit current of 14 mA/cm(2), an open-circuit voltage of 0.74 V, and a fill factor of 58\% are achieved, giving a highest energy conversion efficiency of 6.16\%. The superior device performance and the low band-gap render this new polymer highly promising for the construction of efficient polymer-based tandem solar cells.}, language = {en} } @article{SchattauerReinholdAlbrechtetal.2012, author = {Schattauer, Sylvia and Reinhold, Beate and Albrecht, Steve and Fahrenson, Christoph and Schubert, Marcel and Janietz, Silvia and Neher, Dieter}, title = {Influence of sintering on the structural and electronic properties of TiO2 nanoporous layers prepared via a non-sol-gel approach}, series = {Colloid and polymer science : official journal of the Kolloid-Gesellschaft}, volume = {290}, journal = {Colloid and polymer science : official journal of the Kolloid-Gesellschaft}, number = {18}, publisher = {Springer}, address = {New York}, issn = {0303-402X}, doi = {10.1007/s00396-012-2708-9}, pages = {1843 -- 1854}, year = {2012}, abstract = {In this work, a nonaqueous method is used to fabricate thin TiO2 layers. In contrast to the common aqueous sol-gel approach, our method yields layers of anatase nanocrystallites already at low temperature. Raman spectroscopy, electron microscopy and charge extraction by linearly increasing voltage are employed to study the effect of sintering temperature on the structural and electronic properties of the nanocrystalline TiO2 layer. Raising the sintering temperature from 120 to 600 A degrees C is found to alter the chemical composition, the layer's porosity and its surface but not the crystal phase. The room temperature mobility increases from 2 x 10(-6) to 3 x 10(-5) cm(2)/Vs when the sinter temperature is increased from 400 to 600 A degrees C, which is explained by a better interparticle connectivity. Solar cells comprising such nanoporous TiO2 layers and a soluble derivative of cyclohexylamino-poly(p-phenylene vinylene) were fabricated and studied with regard to their structural and photovoltaic properties. We found only weak polymer infiltration into the oxide layer for sintering temperatures up to 550 A degrees C, while the polymer penetrated deeply into titania layers that were sintered at 600 A degrees C. Best photovoltaic performance was reached with a nanoporous TiO2 film sintered at 550 A degrees C, which yielded a power conversion efficiency of 0.5 \%. Noticeably, samples with the TiO2 layer dried at 120 A degrees C displayed short-circuit currents and open circuit voltages only about 15-20 \% lower than for the most efficient devices, meaning that our nonaqueous route yields titania layers with reasonable transport properties even at low sintering temperatures.}, language = {en} } @article{SchubertPreisBlakesleyetal.2013, author = {Schubert, Marcel and Preis, Eduard and Blakesley, James C. and Pingel, Patrick and Scherf, Ullrich and Neher, Dieter}, title = {Mobility relaxation and electron trapping in a donor/acceptor copolymer}, series = {Physical review : B, Condensed matter and materials physics}, volume = {87}, journal = {Physical review : B, Condensed matter and materials physics}, number = {2}, publisher = {American Physical Society}, address = {College Park}, issn = {1098-0121}, doi = {10.1103/PhysRevB.87.024203}, pages = {12}, year = {2013}, abstract = {To address the nature of charge transport and the origin of severe (intrinsic) trapping in electron-transporting polymers, transient and steady-state charge transport measurements have been conducted on the prototype donor/acceptor copolymer poly[2,7-(9,9-dialkyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PFTBTT). A charge-generation layer technique is used to selectively address transport of the desired charge carrier type, to perform time-of-flight measurements on samples with < 200 nm thickness, and to combine the time-of-flight and the photocharge extraction by linearly increasing voltage (photo-CELIV) techniques to investigate charge carrier dynamics over a wide time range. Significant trapping of free electrons is observed in the bulk of dioctyl-substituted PFTBTT (alt-PF8TBTT), introducing a strong relaxation of the charge carrier mobility with time. We used Monte-Carlo simulation to simulate the measured transient data and found that all measurements can be modeled with a single parameter set, with the charge transport behavior determined by multiple trapping and detrapping of electrons in an exponential trap distribution. The influence of the concomitant mobility relaxation on the transient photocurrent characteristics in photo-CELIV experiments is discussed and shown to explain subtle features that were seen in former publications but were not yet assigned to electron trapping. Comparable studies on PFTBTT copolymers with chemical modifications of the side chains and backbone suggest that the observed electron trapping is not caused by a distinct chemical species but rather is related to interchain interactions.}, language = {en} } @article{LuBlakesleyHimmelbergeretal.2013, author = {Lu, Guanghao and Blakesley, James C. and Himmelberger, Scott and Pingel, Patrick and Frisch, Johannes and Lieberwirth, Ingo and Salzmann, Ingo and Oehzelt, Martin and Di Pietro, Riccardo and Salleo, Alberto and Koch, Norbert and Neher, Dieter}, title = {Moderate doping leads to high performance of semiconductor/insulator polymer blend transistors}, series = {Nature Communications}, volume = {4}, journal = {Nature Communications}, number = {1-2}, publisher = {Nature Publ. Group}, address = {London}, issn = {2041-1723}, doi = {10.1038/ncomms2587}, pages = {8}, year = {2013}, abstract = {Polymer transistors are being intensively developed for next-generation flexible electronics. Blends comprising a small amount of semiconducting polymer mixed into an insulating polymer matrix have simultaneously shown superior performance and environmental stability in organic field-effect transistors compared with the neat semiconductor. Here we show that such blends actually perform very poorly in the undoped state, and that mobility and on/off ratio are improved dramatically upon moderate doping. Structural investigations show that these blend layers feature nanometre-scale semiconductor domains and a vertical composition gradient. This particular morphology enables a quasi three-dimensional spatial distribution of semiconductor pathways within the insulating matrix, in which charge accumulation and depletion via a gate bias is substantially different from neat semiconductor, and where high on-current and low off-current are simultaneously realized in the stable doped state. Adding only 5 wt\% of a semiconducting polymer to a polystyrene matrix, we realized an environmentally stable inverter with gain up to 60.}, language = {en} } @article{SteyrleuthnerSchubertHowardetal.2012, author = {Steyrleuthner, Robert and Schubert, Marcel and Howard, Ian and Klaum{\"u}nzer, Bastian and Schilling, Kristian and Chen, Zhihua and Saalfrank, Peter and Laquai, Frederic and Facchetti, Antonio and Neher, Dieter}, title = {Aggregation in a high-mobility n-type low-bandgap copolymer with implications on semicrystalline morphology}, series = {Journal of the American Chemical Society}, volume = {134}, journal = {Journal of the American Chemical Society}, number = {44}, publisher = {American Chemical Society}, address = {Washington}, issn = {0002-7863}, doi = {10.1021/ja306844f}, pages = {18303 -- 18317}, year = {2012}, abstract = {We explore the photophysics of P(NDI2OD-T2), a high-mobility and air-stable n-type donor/acceptor polymer. Detailed steady-state UV-vis and photoluminescence (PL) measurements on solutions of P(NDI2OD-T2) reveal distinct signatures of aggregation. By performing quantum chemical calculations, we can assign these spectral features to unaggregated and stacked polymer chains. NMR measurements independently confirm the aggregation phenomena of P(NDI2OD-T2) in solution. The detailed analysis of the optical spectra shows that aggregation is a two-step process with different types of aggregates, which we confirm by time-dependent PL measurements. Analytical ultracentrifugation measurements suggest that aggregation takes place within the single polymer chain upon coiling. By transferring these results to thin P(NDI2OD-T2) films, we can conclude that film formation is mainly governed by the chain collapse, leading in general to a high aggregate content of similar to 45\%. This process also inhibits the formation of amorphous and disordered P(NDI2OD-T2) films.}, language = {en} } @article{PingelNeher2013, author = {Pingel, P. and Neher, Dieter}, title = {Comprehensive picture of p-type doping of P3HT with the molecular acceptor F(4)TCNQ}, series = {Physical review : B, Condensed matter and materials physics}, volume = {87}, journal = {Physical review : B, Condensed matter and materials physics}, number = {11}, publisher = {American Physical Society}, address = {College Park}, issn = {1098-0121}, doi = {10.1103/PhysRevB.87.115209}, pages = {9}, year = {2013}, abstract = {By means of optical spectroscopy, Kelvin probe, and conductivity measurements, we study the p-type doping of the donor polymer poly(3-hexylthiophene), P3HT, with the molecular acceptor tetrafluorotetracyanoquin-odimethane, F(4)TCNQ, covering a broad range of molar doping ratios from the ppm to the percent regime. Thorough quantitative analysis of the specific near-infrared absorption bands of ionized F(4)TCNQ reveals that almost every F(4)TCNQ dopant undergoes integer charge transfer with a P3HT site. However, only about 5\% of these charge carrier pairs are found to dissociate and contribute a free hole for electrical conduction. The nonlinear behavior of the conductivity on doping ratio is rationalized by a numerical mobility model that accounts for the broadening of the energetic distribution of transport sites by the Coulomb potentials of ionized F(4)TCNQ dopants. DOI: 10.1103/PhysRevB.87.115209}, language = {en} } @article{HofmannZuefleShimizuetal.2019, author = {Hofmann, Alexander J. L. and Z{\"u}fle, Simon and Shimizu, Kohei and Schmid, Markus and Wessels, Vivien and J{\"a}ger, Lars and Altazin, Stephane and Ikegami, Keitaro and Khan, Motiur Rahman and Neher, Dieter and Ishii, Hisao and Ruhstaller, Beat and Br{\"u}tting, Wolfgang}, title = {Dipolar Doping of Organic Semiconductors to Enhance Carrier Injection}, series = {Physical review applied}, volume = {12}, journal = {Physical review applied}, number = {6}, publisher = {American Physical Society}, address = {College Park}, issn = {2331-7019}, doi = {10.1103/PhysRevApplied.12.064052}, pages = {11}, year = {2019}, abstract = {If not oriented perfectly isotropically, the strong dipole moment of polar organic semiconductor materials such as tris-(8-hydroxyquinolate)aluminum (Alq3) will lead to the buildup of a giant surface potential (GSP) and thus to a macroscopic dielectric polarization of the organic film. Despite this having been a known fact for years, the implications of such high potentials within an organic layer stack have only been studied recently. In this work, the influence of the GSP on hole injection into organic layers is investigated. Therefore, we apply a concept called dipolar doping to devices consisting of the prototypical organic materials N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) as nonpolar host and Alq3 as dipolar dopant with different mixing ratios to tune the GSP. The mixtures are investigated in single-layer monopolar devices as well as bilayer metal/insulator/semiconductor structures. Characterization is done electrically using current-voltage (I-V) characteristics, impedance spectroscopy, and charge extraction by linearly increasing voltage and time of flight, as well as with ultraviolet photoelectron spectroscopy. We find a maximum in device performance for moderate to low doping concentrations of the polar species in the host. The observed behavior can be described on the basis of the Schottky effect for image-force barrier lowering, if the changes in the interface dipole, the carrier mobility, and the GSP induced by dipolar doping are taken into account.}, language = {en} } @misc{LaquaiAndrienkoDeibeletal.2017, author = {Laquai, Frederic and Andrienko, Denis and Deibel, Carsten and Neher, Dieter}, title = {Charge carrier generation, recombination, and extraction in polymer-fullerene bulk heterojunction organic solar cells}, series = {Elementary processes in organic photovoltaics}, volume = {272}, journal = {Elementary processes in organic photovoltaics}, publisher = {Springer}, address = {Berlin}, isbn = {978-3-319-28338-8}, issn = {0065-3195}, doi = {10.1007/978-3-319-28338-8_11}, pages = {267 -- 291}, year = {2017}, abstract = {In this chapter we review the basic principles of photocurrent generation in bulk heterojunction organic solar cells, discuss the loss channels limiting their efficiency, and present case studies of several polymer-fullerene blends. Using steady-state and transient, optical, and electrooptical techniques, we create a precise picture of the fundamental processes that ultimately govern solar cell efficiency.}, language = {en} } @article{HahnTscheuschnerSalleretal.2016, author = {Hahn, Tobias and Tscheuschner, Steffen and Saller, Christina and Strohriegl, Peter and Boregowda, Puttaraju and Mukhopadhyay, Tushita and Patil, Satish and Neher, Dieter and B{\"a}ssler, Heinz and K{\"o}hler, Anna}, title = {Role of Intrinsic Photogeneration in Single Layer and Bilayer Solar Cells with C-60 and PCBM}, series = {The journal of physical chemistry : C, Nanomaterials and interfaces}, volume = {120}, journal = {The journal of physical chemistry : C, Nanomaterials and interfaces}, publisher = {American Chemical Society}, address = {Washington}, issn = {1932-7447}, doi = {10.1021/acs.jpcc.6b08471}, pages = {25083 -- 25091}, year = {2016}, language = {en} } @article{YuanZhangQiuetal.2022, author = {Yuan, Jun and Zhang, Chujun and Qiu, Beibei and Liu, Wei and So, Shu Kong and Mainville, Mathieu and Leclerc, Mario and Shoaee, Safa and Neher, Dieter and Zou, Yingping}, title = {Effects of energetic disorder in bulk heterojunction organic solar cells}, series = {Energy \& environmental science}, volume = {15}, journal = {Energy \& environmental science}, number = {7}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {1754-5692}, doi = {10.1039/d2ee00271j}, pages = {2806 -- 2818}, year = {2022}, abstract = {Organic solar cells (OSCs) have progressed rapidly in recent years through the development of novel organic photoactive materials, especially non-fullerene acceptors (NFAs). Consequently, OSCs based on state-of-the-art NFAs have reached significant milestones, such as similar to 19\% power conversion efficiencies (PCEs) and small energy losses (less than 0.5 eV). Despite these significant advances, understanding of the interplay between molecular structure and optoelectronic properties lags significantly behind. For example, despite the theoretical framework for describing the energetic disorder being well developed for the case of inorganic semiconductors, the question of the applicability of classical semiconductor theories in analyzing organic semiconductors is still under debate. A general observation in the inorganic field is that inorganic photovoltaic materials possessing a polycrystalline microstructure exhibit suppressed disorder properties and better charge carrier transport compared to their amorphous analogs. Accordingly, this principle extends to the organic semiconductor field as many organic photovoltaic materials are synthesized to pursue polycrystalline-like features. Yet, there appears to be sporadic examples that exhibit an opposite trend. However, full studies decoupling energetic disorder from aggregation effects have largely been left out. Hence, the potential role of the energetic disorder in OSCs has received little attention. Interestingly, recently reported state-of-the-art NFA-based devices could achieve a small energetic disorder and high PCE at the same time; and interest in this investigation related to the disorder properties in OSCs was revived. In this contribution, progress in terms of the correlation between molecular design and energetic disorder is reviewed together with their effects on the optoelectronic mechanism and photovoltaic performance. Finally, the specific challenges and possible solutions in reducing the energetic disorder of OSCs from the viewpoint of materials and devices are proposed.}, language = {en} } @article{MuellerNanovaGlaseretal.2016, author = {Mueller, Lars and Nanova, Diana and Glaser, Tobias and Beck, Sebastian and Pucci, Annemarie and Kast, Anne K. and Schroeder, Rasmus R. and Mankel, Eric and Pingel, Patrick and Neher, Dieter and Kowalsky, Wolfgang and Lovrincic, Robert}, title = {Charge-Transfer-Solvent Interaction Predefines Doping Efficiency in p-Doped P3HT Films}, series = {Chemistry of materials : a publication of the American Chemical Society}, volume = {28}, journal = {Chemistry of materials : a publication of the American Chemical Society}, publisher = {American Chemical Society}, address = {Washington}, issn = {0897-4756}, doi = {10.1021/acs.chemmater.6b01629}, pages = {4432 -- 4439}, year = {2016}, abstract = {Efficient electrical doping of organic semiconductors is a necessary prerequisite for the fabrication of high performance organic electronic devices. In this work, we study p-type doping of poly(3-hexylthiophene) (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F(4)TCNQ) spin-cast from two different solvents. Using electron diffraction, we find strong dopant-induced pi-pi-stacking for films from the solvent chloroform, but not from chlorobenzene. This image is confirmed and expanded by the analysis of vibrational features of P3HT and polaron absorptions using optical spectroscopy. Here, a red-shifted polaron absorption is found in doped films from chloroform, caused by a higher conjugation length of the polymer backbone. These differences result in a higher conductivity of films from chloroform. We use optical spectroscopy on the corresponding blend solutions to shed light on the origin of this effect and propose a model to explain why solutions of doped P3HT reveal more aggregation of charged molecules in chlorobenzene, whereas more order is finally observed in dried films from chloroform. Our study emphasizes the importance of solvent parameters exceeding the bare solubility of pure dopant and host material for the preparation of highly conductive doped films.}, language = {en} } @article{BenduhnPiersimoniLondietal.2018, author = {Benduhn, Johannes and Piersimoni, Fortunato and Londi, Giacomo and Kirch, Anton and Widmer, Johannes and Koerner, Christian and Beljonne, David and Neher, Dieter and Spoltore, Donato and Vandewal, Koen}, title = {Impact of triplet excited states on the open-circuit voltage of organic solar cells}, series = {dvanced energy materials}, volume = {8}, journal = {dvanced energy materials}, number = {21}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1614-6832}, doi = {10.1002/aenm.201800451}, pages = {7}, year = {2018}, abstract = {The best organic solar cells (OSCs) achieve comparable peak external quantum efficiencies and fill factors as conventional photovoltaic devices. However, their voltage losses are much higher, in particular those due to nonradiative recombination. To investigate the possible role of triplet states on the donor or acceptor materials in this process, model systems comprising Zn- and Cu-phthalocyanine (Pc), as well as fluorinated versions of these donors, combined with C-60 as acceptor are studied. Fluorination allows tuning the energy level alignment between the lowest energy triplet state (T-1) and the charge-transfer (CT) state, while the replacement of Zn by Cu as the central metal in the Pcs leads to a largely enhanced spin-orbit coupling. Only in the latter case, a substantial influence of the triplet state on the nonradiative voltage losses is observed. In contrast, it is found that for a large series of typical OSC materials, the relative energy level alignment between T-1 and the CT state does not substantially affect nonradiative voltage losses.}, language = {en} } @article{HoermannZeiskePiersimonietal.2018, author = {H{\"o}rmann, Ulrich and Zeiske, Stefan and Piersimoni, Fortunato and Hoffmann, Lukas and Schlesinger, Raphael and Koch, Norbert and Riedl, Thomas and Andrienko, Denis and Neher, Dieter}, title = {Stark effect of hybrid charge transfer states at planar ZnO/organic interfaces}, series = {Physical review : B, Condensed matter and materials physics}, volume = {98}, journal = {Physical review : B, Condensed matter and materials physics}, number = {15}, publisher = {American Physical Society}, address = {College Park}, issn = {2469-9950}, doi = {10.1103/PhysRevB.98.155312}, pages = {9}, year = {2018}, abstract = {We investigate the bias dependence of the hybrid charge transfer state emission at planar heterojunctions between the metal oxide acceptor ZnO and three donor molecules. The electroluminescence peak energy linearly increases with the applied bias, saturating at high fields. Variation of the organic layer thickness and deliberate change of the ZnO conductivity through controlled photodoping allow us to confirm that this bias-induced spectral shift relates to the internal electric field in the organic layer rather than the filling of states at the hybrid interface. We show that existing continuum models overestimate the hole delocalization and propose a simple electrostatic model in which the linear and quadratic Stark effects are explained by the electrostatic interaction of a strongly polarizable molecular cation with its mirror image.}, language = {en} } @article{MansourLungwitzSchultzetal.2020, author = {Mansour, Ahmed E. and Lungwitz, Dominique and Schultz, Thorsten and Arvind, Malavika and Valencia, Ana M. and Cocchi, Caterina and Opitz, Andreas and Neher, Dieter and Koch, Norbert}, title = {The optical signatures of molecular-doping induced polarons in poly(3-hexylthiophene-2,5-diyl)}, series = {Journal of materials chemistry : C, Materials for optical and electronic devices}, volume = {8}, journal = {Journal of materials chemistry : C, Materials for optical and electronic devices}, number = {8}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {2050-7526}, doi = {10.1039/c9tc06509a}, pages = {2870 -- 2879}, year = {2020}, abstract = {Optical absorption spectroscopy is a key method to investigate doped conjugated polymers and to characterize the doping-induced charge carriers, i.e., polarons. For prototypical poly(3-hexylthiophene-2,5-diyl) (P3HT), the absorption intensity of molecular dopant induced polarons is widely used to estimate the carrier density and the doping efficiency, i.e., the number of polarons formed per dopant molecule. However, the dependence of the polaron-related absorption features on the structure of doped P3HT, being either aggregates or separated individual chains, is not comprehensively understood in contrast to the optical absorption features of neutral P3HT. In this work, we unambiguously differentiate the optical signatures of polarons on individual P3HT chains and aggregates in solution, notably the latter exhibiting the same shape as aggregates in solid thin films. This is enabled by employing tris(pentafluorophenyl)borane (BCF) as dopant, as this dopant forms only ion pairs with P3HT and no charge transfer complexes, and BCF and its anion have no absorption in the spectral region of P3HT polarons. Polarons on individual chains exhibit absorption peaks at 1.5 eV and 0.6 eV, whereas in aggregates the high-energy peak is split into a doublet 1.3 eV and 1.65 eV, and the low-energy peak is shifted below 0.5 eV. The dependence of the fraction of solvated individual chains versus aggregates on absolute solution concentration, dopant concentration, and temperature is elucidated, and we find that aggregates predominate in solution under commonly used processing conditions. Aggregates in BCF-doped P3HT solution can be effectively removed upon simple filtering. From varying the filter pore size (down to 200 nm) and thin film morphology characterization with scanning force microscopy we reveal the aggregates' size dependence on solution absolute concentration and dopant concentration. Furthermore, X-ray photoelectron spectroscopy shows that the dopant loading in aggregates is higher than for individual P3HT chains. The results of this study help understanding the impact of solution pre-aggregation on thin film properties of molecularly doped P3HT, and highlight the importance of considering such aggregation for other doped conjugated polymers in general.}, language = {en} } @misc{KurpiersNeher2016, author = {Kurpiers, Jona and Neher, Dieter}, title = {Dispersive Non-Geminate Recombination in an Amorphous Polymer:Fullerene Blend}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-91541}, pages = {10}, year = {2016}, abstract = {Recombination of free charge is a key process limiting the performance of solar cells. For low mobility materials, such as organic semiconductors, the kinetics of non-geminate recombination (NGR) is strongly linked to the motion of charges. As these materials possess significant disorder, thermalization of photogenerated carriers in the inhomogeneously broadened density of state distribution is an unavoidable process. Despite its general importance, knowledge about the kinetics of NGR in complete organic solar cells is rather limited. We employ time delayed collection field (TDCF) experiments to study the recombination of photogenerated charge in the high-performance polymer:fullerene blend PCDTBT:PCBM. NGR in the bulk of this amorphous blend is shown to be highly dispersive, with a continuous reduction of the recombination coefficient throughout the entire time scale, until all charge carriers have either been extracted or recombined. Rapid, contact-mediated recombination is identified as an additional loss channel, which, if not properly taken into account, would erroneously suggest a pronounced field dependence of charge generation. These findings are in stark contrast to the results of TDCF experiments on photovoltaic devices made from ordered blends, such as P3HT:PCBM, where non-dispersive recombination was proven to dominate the charge carrier dynamics under application relevant conditions.}, language = {en} } @misc{LiuTkachovKomberetal.2014, author = {Liu, W. and Tkachov, R. and Komber, H. and Senkovskyy, V. and Schubert, M. and Wei, Z. and Facchetti, A. and Neher, Dieter and Kiriy, A.}, title = {Chain-growth polycondensation of perylene diimide-based copolymers}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-98724}, pages = {8}, year = {2014}, abstract = {Herein, we report the chain-growth tin-free room temperature polymerization method to synthesize n-type perylene diimide-dithiophene-based conjugated polymers (PPDIT2s) suitable for solar cell and transistor applications. The palladium/electron-rich tri-tert-butylphosphine catalyst is effective to enable the chain-growth polymerization of anion-radical monomer Br-TPDIT-Br/Zn to PPDIT2 with a molecular weight up to Mw ≈ 50 kg mol-1 and moderate polydispersity. This is the second example of the polymerization of unusual anion-radical aromatic complexes formed in a reaction of active Zn and electron-deficient diimide-based aryl halides. As such, the discovered polymerization method is not a specific reactivity feature of the naphthalene-diimide derivatives but is rather a general polymerization tool. This is an important finding, given the significantly higher maximum external quantum efficiency that can be reached with PDI-based copolymers (32-45\%) in all-polymer solar cells compared to NDI-based materials (15-30\%). Our studies revealed that PPDIT2 synthesized by the new method and the previously published polymer prepared by step-growth Stille polycondensation show similar electron mobility and all-polymer solar cell performance. At the same time, the polymerization reported herein has several technological advantages as it proceeds relatively fast at room temperature and does not involve toxic tin-based compounds. Because several chain-growth polymerization reactions are well-suited for the preparation of well-defined multi-functional polymer architectures, the next target is to explore the utility of the discovered polymerization in the synthesis of end-functionalized polymers and block copolymers. Such materials would be helpful to improve the nanoscale morphology of polymer blends in all-polymer solar cells.}, language = {en} } @misc{NeherKniepertElimelechetal.2016, author = {Neher, Dieter and Kniepert, Juliane and Elimelech, Arik and Koster, L. Jan Anton}, title = {A New Figure of Merit for Organic Solar Cells with Transport-limited Photocurrents}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-91414}, pages = {9}, year = {2016}, abstract = {Compared to their inorganic counterparts, organic semiconductors suffer from relatively low charge carrier mobilities. Therefore, expressions derived for inorganic solar cells to correlate characteristic performance parameters to material properties are prone to fail when applied to organic devices. This is especially true for the classical Shockley-equation commonly used to describe current-voltage (JV)-curves, as it assumes a high electrical conductivity of the charge transporting material. Here, an analytical expression for the JV-curves of organic solar cells is derived based on a previously published analytical model. This expression, bearing a similar functional dependence as the Shockley-equation, delivers a new figure of merit α to express the balance between free charge recombination and extraction in low mobility photoactive materials. This figure of merit is shown to determine critical device parameters such as the apparent series resistance and the fill factor.}, language = {en} } @article{KurpiersNeher2016, author = {Kurpiers, Jona and Neher, Dieter}, title = {Dispersive Non-Geminate Recombination in an Amorphous Polymer:Fullerene Blend}, series = {Scientific reports}, volume = {6}, journal = {Scientific reports}, publisher = {Nature Publishing Group}, address = {London}, issn = {2045-2322}, doi = {10.1038/srep26832}, pages = {10}, year = {2016}, abstract = {Recombination of free charge is a key process limiting the performance of solar cells. For low mobility materials, such as organic semiconductors, the kinetics of non-geminate recombination (NGR) is strongly linked to the motion of charges. As these materials possess significant disorder, thermalization of photogenerated carriers in the inhomogeneously broadened density of state distribution is an unavoidable process. Despite its general importance, knowledge about the kinetics of NGR in complete organic solar cells is rather limited. We employ time delayed collection field (TDCF) experiments to study the recombination of photogenerated charge in the high-performance polymer:fullerene blend PCDTBT:PCBM. NGR in the bulk of this amorphous blend is shown to be highly dispersive, with a continuous reduction of the recombination coefficient throughout the entire time scale, until all charge carriers have either been extracted or recombined. Rapid, contact-mediated recombination is identified as an additional loss channel, which, if not properly taken into account, would erroneously suggest a pronounced field dependence of charge generation. These findings are in stark contrast to the results of TDCF experiments on photovoltaic devices made from ordered blends, such as P3HT:PCBM, where non-dispersive recombination was proven to dominate the charge carrier dynamics under application relevant conditions.}, language = {en} } @article{NeherKniepertElimelechetal.2016, author = {Neher, Dieter and Kniepert, Juliane and Elimelech, Arik and Koster, L. Jan Anton}, title = {A New Figure of Merit for Organic Solar Cells with Transport-limited Photocurrents}, series = {Scientific reports}, volume = {6}, journal = {Scientific reports}, publisher = {Nature Publishing Group}, address = {London}, issn = {2045-2322}, doi = {10.1038/srep24861}, pages = {9}, year = {2016}, abstract = {Compared to their inorganic counterparts, organic semiconductors suffer from relatively low charge carrier mobilities. Therefore, expressions derived for inorganic solar cells to correlate characteristic performance parameters to material properties are prone to fail when applied to organic devices. This is especially true for the classical Shockley-equation commonly used to describe current-voltage (JV)-curves, as it assumes a high electrical conductivity of the charge transporting material. Here, an analytical expression for the JV-curves of organic solar cells is derived based on a previously published analytical model. This expression, bearing a similar functional dependence as the Shockley-equation, delivers a new figure of merit α to express the balance between free charge recombination and extraction in low mobility photoactive materials. This figure of merit is shown to determine critical device parameters such as the apparent series resistance and the fill factor.}, language = {en} } @article{IlnytskyiSaphiannikovaNeher2006, author = {Ilnytskyi, Jaroslav and Saphiannikova, Marina and Neher, Dieter}, title = {Photo-induced deformations in azobenzene-containing side-chain polymers : molecular dynamics study}, issn = {1607-324X}, year = {2006}, abstract = {We perform molecular dynamics simulations of azobenzene containing side-chain liquid crystalline polymer subject to an external model field that mimicks the reorientations of the azobenzenes upon irradiation with polarized light. The smectic phase of the polymer is studied with the field applied parallel to the nematic director, forcing the trans isomers to reorient perpendicularly to the field (the direction of which can be assosiated with the light polarization). The coupling between the reorientation of azobenzenes and mechanical deformation of the sample is found to depend on the field strength. In a weak field the original smectic order is melted gradually with no apparent change in the simulation box shape, whereas in a strong field two regimes are observed. During the first one a rapid melting of the liquid crystalline order is accompanied by the contraction of the polymer along the field direction (the effect similar to the one observed experimentally in azopenzene containing elastomers). During the slower second regime, the smectic layers are rebuilt to accomodate the preferential direction of chromophores perperdicular to the field.}, language = {en} } @article{KietzkeHorholdNeher2005, author = {Kietzke, Thomas and Horhold, H. H. and Neher, Dieter}, title = {Efficient polymer solar cells based on M3EH-PPV}, year = {2005}, abstract = {We report on polymer blend solar cells with an external quantum efficiency of more than 30\% and a hi-h overall energy conversion efficiency (ECE) under white light illumination (100 mW/cm(2)) Of Lip to 1.7\% using a blend of M3EH- PPV (poly [2,5-dimethoxy-1,4-phenylene-1,2-ethenylene-2-methoxy-5(2-ethylhexyloxy)-(1,4-pheiiylene-1,2-ethenylene)]) and CN-ether-PPV (poly[oxa-1,4-phenylene-1,2(1-cyano)ethenylene-2,5-dioctyloxy-1,4-phenylene-1,2-(2-cyano)ethellyiene-1,4- phenylene]). We attribute these high efficiencies to the formation of a vertically composition graded structure during spin coating Photoluminescence measurements performed on the blend layers indicated the formation of exciplexes between both types of polymers, which we propose to be one factor preventing even higher efficiencies}, language = {en} } @article{YangMuellerNeheretal.2006, author = {Yang, Xiaohui and M{\"u}ller, David C. and Neher, Dieter and Meerholz, Klaus}, title = {Highly efficient polymeric electrophosphorescent diodes}, year = {2006}, abstract = {Polymeric electrophosphorescent LEDs with internal quantum efficiencies approaching unity have been fabricated. Such performance levels are previously unknown for OLEDs. The key to this success is redox chemically doped oxetane- crosslinkable hole-transporting layers with multilayer capability (see figure). They improve hole injection and act as electron-blocking layers, without the need to include exciton-or hole-blocking layers}, language = {en} } @article{EgbeCarbonnierPauletal.2005, author = {Egbe, D. A. M. and Carbonnier, B. and Paul, E. L. and Muhlbacher, D. and Kietzke, Thomas and Birckner, Eckhard and Neher, Dieter and Grummt, U. W. and Pakula, T.}, title = {Diyne-containing PPVs : Solid-state properties and comparison of their photophysical and electrochemical properties with those of their Yne-containing counterparts}, issn = {0024-9297}, year = {2005}, abstract = {Diyne-containing poly(p-phenylene-vinylene)s, 4a-d, of general chemical structure-(Ph-C\&3bond; C-C\&3bond; C-Ph- CH\&3bond; CH-Ph-CH\&3bond; CH-)(n), obtained through polycondensation reactions of 1,4-bis(4-formyl-2,5-dioctyloxyphenyl)- buta-1,3-diyne (2) with various 2,5-dialkoxy-p-xylylenebis(diethylphosphonates), 3a-d, are the subject of this report. The polymers exhibit great disparity in their degree of polymerization, n, which might be ascribed to side-chain-related differences in reactivity of the reactive species during the polycondensation process and which led to n-dependent absorption (solution and solid state) and emission (solution) behaviors of the polymers. Polarizing optical microscopy and differential scanning calorimetry are employed to probe their thermal behavior. The structure is investigated by means of wide-angle X-ray diffraction for both isotropic and macroscopically oriented samples. Comparison of photophysical (experimental and theoretical) and electrochemical properties of the polymers with those of their yne- containing counterparts 6a-d [-(Ph-C\&3bond; C-Ph-CH\&3bond; CH-Ph-CH\&3bond; CH-)(n)] has been carried out. Similar photophysical behavior was observed for both types of polymers despite the difference in backbone conjugation pattern. The introduction of a second yne unit in 4 lowers the HOMO and LUMO levels, thereby enhancing the electron affinity of polymers 4 compared to polymers 6. The "wider opening" introduced by the second yne unit facilitates moreover the movement of charges during the electrochemical processes leading to minimal discrepancy, Delta E-g between the optical and electrochemical band gap energies. Polymers 6, in contrast, show significant side-chain-dependent Delta E-g values. Low turn-on voltages between 2 and 3 V and maximal luminous efficiencies between 0.32 and 1.25 cd/A were obtained from LED devices of configuration ITO/PEDOT:PSS/polymer 4/Ca/Al}, language = {en} } @article{EgbeUlbrichtOrgisetal.2005, author = {Egbe, D. A. M. and Ulbricht, C. and Orgis, Thomas and Carbonnier, B. and Kietzke, Thomas and Peip, M. and Metzner, M. and Gericke, M. and Birckner, Eckhard and Pakula, T. and Neher, Dieter and Grumm, U. W.}, title = {Odd-even effects and the influence of length and specific positioning of alkoxy side chains on the optical properties of PPE-PPV polymers}, issn = {0897-4756}, year = {2005}, abstract = {This contribution reports the combined influences of odd-even effects and the specific positioning of alkoxy side chains OR1 = (OCn+H-10(2(n+10)+1)) and OR2 = (OCnH2n+1) (with n = 6, 7, 8, 9) on the phenylene-ethynylene and phenylene- vinylene segments, respectively, on the optical properties of hybrid polymers P(n+10)/n of general repeating unit: -Ph-C equivalent to C-Ph-C equivalent to C-Ph-CH=CH-Ph-CH=CH-. For the polymeric materials, visual color impression varies alternatively between orange red (P16/6 and P18/8) and yellow (P17/7 and P19/9) according to the odd and even features of the alkoxy side chains, where odd or even relates to the total number of sp(3)-hybridized atoms within the side chains. This side chain related effect is ascribed to both absorptive and emissive behaviors of the polymers on the basis of photophysical investigations in the bulk. Almost identical thin film absorption spectra were obtained for all four materials; however, the photoluminescence of the odd polymers, P16/6 (lambda(f) = 556 nm) and P18/ 8 (lambda(f) = 614 nm), was red-shifted relative to that of their even counterparts (lambda(f) = 535 nm). Further, the P18/8 maximum at 614 nm can be readily assigned to excimer emission, as evidenced by the largest Stokes shift (5600 cm(- 1)), largest fwhmf-value (3700 cm(-1))(,) and the lowest Phi(f)-value of 24\%. The strong pi-pi interchain interaction in P18/8, due to loose alkoxy side chains packing, does not only favor fluorescence quenching but also enable an effective inter- as well as intra-molecular recombination of the generated positive and negative polarons in electrolurninescence, which explains the good EL properties of this polymer irrespective of the solvent used. A voltage-dependent blue shift of the EL spectra of up to 100 nm was observed for P18/8 devices prepared from aromatic solvents. This red to green EL shift as observed with increasing voltage is assigned to conformational changes of the polymer chains with increasing temperature}, language = {en} } @article{KurpiersNeher2016, author = {Kurpiers, Jona and Neher, Dieter}, title = {Dispersive Non-Geminate Recombination in an Amorphous Polymer: Fullerene Blend}, series = {Scientific reports}, volume = {6}, journal = {Scientific reports}, publisher = {Nature Publ. Group}, address = {London}, issn = {2045-2322}, doi = {10.1038/srep26832}, pages = {10}, year = {2016}, abstract = {Recombination of free charge is a key process limiting the performance of solar cells. For low mobility materials, such as organic semiconductors, the kinetics of non-geminate recombination (NGR) is strongly linked to the motion of charges. As these materials possess significant disorder, thermalization of photogenerated carriers in the inhomogeneously broadened density of state distribution is an unavoidable process. Despite its general importance, knowledge about the kinetics of NGR in complete organic solar cells is rather limited. We employ time delayed collection field (TDCF) experiments to study the recombination of photogenerated charge in the high-performance polymer: fullerene blend PCDTBT:PCBM. NGR in the bulk of this amorphous blend is shown to be highly dispersive, with a continuous reduction of the recombination coefficient throughout the entire time scale, until all charge carriers have either been extracted or recombined. Rapid, contact-mediated recombination is identified as an additional loss channel, which, if not properly taken into account, would erroneously suggest a pronounced field dependence of charge generation. These findings are in stark contrast to the results of TDCF experiments on photovoltaic devices made from ordered blends, such as P3HT:PCBM, where non-dispersive recombination was proven to dominate the charge carrier dynamics under application relevant conditions.}, language = {en} } @article{KietzkeStillerLandfesteretal.2005, author = {Kietzke, Thomas and Stiller, Burkhard and Landfester, Katharina and Montenegro, Rivelino V. D. and Neher, Dieter}, title = {Probing the local optical properties of layers prepared from polymer nanoparticles}, issn = {0379-6779}, year = {2005}, abstract = {It is well known that the performance of solar cells based on a blend of hole-accepting and electron-accepting conjugated polymers as the active material depend crucially on the length scale of the resulting phase separated morphology. However, a direct control of this morphology is difficult if the layer is prepared from an organic solvent. To circumvent this difficulty, recently a universal method to fabricate defined nano-structured blend layer using nanoparticles dispersed in water was demonstrated. These nanoparticles were prepared with the miniemulsion method, which allows for the preparation of semiconducting polymer nanospheres (SPNs) with diameters in the range of 30 to 300 nanometres. Since the process starts from the active material dissolved in a common solvent, it can be applied to the fabrication of nanoparticles of blends of polymers with oligomers or even with inorganic materials. We present here for the first time scanning near field optical microscopy (SNOM) investigations on these novel nanostructured polymer layers. We show that by spin-coating a mixture of two different dispersions a nanoparticle monolayer with a statistically distribution of the nanoparticles can be obtained. Mixing conjugated polymer nanoparticles with some inert particles like polystyrene beads may allow for the preparation of nano-sized light emitters}, language = {en} } @article{KarageorgievNeherSchulzetal.2005, author = {Karageorgiev, Peter and Neher, Dieter and Schulz, Burkhard and Stiller, Burkhard and Pietsch, Ullrich and Giersig, Michael and Brehmer, Ludwig}, title = {From anisotropic photo-fluidity towards nanomanipulation in the optical near-field}, issn = {1476-1122}, year = {2005}, abstract = {An increase in random molecular vibrations of a solid owing to heating above the melting point leads to a decrease in its long-range order and a loss of structural symmetry. Therefore conventional liquids are isotropic media. Here we report on a light-induced isothermal transition of a polymer film from an isotropic solid to an anisotropic liquid state in which the degree of mechanical anisotropy can be controlled by light. Whereas during irradiation by circular polarized light the film behaves as an isotropic viscoelastic fluid, it shows considerable fluidity only in the direction parallel to the light field vector under linear polarized light. The fluidization phenomenon is related to photoinduced motion of azobenzene-functionalized molecular units, which can be effectively activated only when their transition dipole moments are oriented close to the direction of the light polarization. We also describe here how the photofluidization allows nanoscopic elements of matter to be precisely manipulated}, language = {en} } @article{MechauSaphiannikovaNeher2005, author = {Mechau, Norman and Saphiannikova, Marina and Neher, Dieter}, title = {Dielectric and mechanical properties of azobenzene polymer layers under visible and ultraviolet irradiation}, issn = {0024-9297}, year = {2005}, abstract = {Photoinduced changes in the mechanical and dielectric properties of azobenzene polymer films were measured utilizing the method of electromechanical spectroscopy. The measurements revealed a strong correlation between the time- dependent behavior of the plate compliance and the dielectric constant under irradiation. Actinic light causes a light softening of the film that also manifests itself in the increase of the dielectric constant, whereas ultraviolet irradiation results in an initial plasticization of the film followed by its hardening. The latter is accompanied by decrease of the dielectric constant. A semiquantitative model based on the kinetics of the photoisomerization process in azobenzene polymers is proposed. We assume that both visible and ultraviolet irradiation increase the free volume in the layer due to photoisomerization. Additionally, ultraviolet light increases the modulus of the polymer matrix due to the presence of a high density of azobenzene moieties in the cis state. These assumptions allowed us to reproduce the time- dependent behavior of the bulk compliance as well as the dielectric constant at different irradiation intensities, for both visible and ultraviolet light, with only two adjustable parameters}, language = {en} } @article{EgbeKietzkeCarbonnieretal.2004, author = {Egbe, D. A. M. and Kietzke, Thomas and Carbonnier, B. and Muhlbacher, D. and Horhold, H. H. and Neher, Dieter and Pakula, T.}, title = {Synthesis, characterization, and photophysical, electrochemical, electroluminescent, and photovoltaic properties of yne-containing CN-PPVs}, year = {2004}, abstract = {Alkoxy-substituted CN-containing phenylene-vinylene-alt-phenylene-ethynylene hybrid polymers (CN-PPV-PPE), 3a, 3b, and 7a, were obtained from luminophoric dialdehydes 1 by step growth polymerization via Knoevenagel reaction as high molecular-weight materials. Corresponding CN-free polymers 3c and 7b and an ethynylene-free polymer 5 with similar side chains were synthesized for the purpose of comparison. The chemical structures of the polymers were confirmed by IR, H-1 and C-13 NMR, and elemental analysis. Thermal characterization was conducted by means of thermogravimetric analysis and differential scanning calorimetry. Morphology was investigated by means of optical microscopy and small-angle light scattering. The final morphologies are determined by the molecular characteristics (side chains volume fraction, backbone stiffness) of the studied polymers. All the CN-containing polymers 3b, 5, and 7a exhibit higher fluorescence quantum yield in solid state (50 to 60\%), but lower quantum yields (12-40\%) in dilute chloroform solution, in total contrast to CN-free polymers 3c, 3d, and 7b. Identical optical, E-g(opt), and electrochemical band gap energies, E- g(ec), were obtained for 3b, 3c and 3d with intrinsic self-assembly ability, whereas a discrepancy, DeltaE(g), was observed in the cases of the fully substituted polymers 5, 7a, and 7b, whose values are dependent on the level of backbone stiffness and length of the side groups combined with the presence or absence of CN units. The incorporation of CN units in 3b and 7a lowers their respective LUMO level by 220 and 350 meV compared to their corresponding CN-free counterparts 3c and 7b, suggesting an improvement of the electron-accepting strength. Polymers 3b and 7a are efficient electron acceptors suitable for photovoltaic application. The experiments indicate that 3b is a better electron acceptor when used together with M3EH-PPV, but transport properties seem to be better for 7a. With 3b, high external quantum efficiencies of up to 23\%, an open circuit voltage of up to 1.52 V, and a white light energy efficiency of 0.65\% could be realized in bilayer solar cell devices. LED-devices of configuration ITO/PEDOT:PSS/polymer/Ca/Al from 3b, 3c, 7a, and 7b showed low turn-on voltages between 2 and 2.5 V. The CN-free polymers 3c and 7b exhibit far better EL parameters than their corresponding CN containing counterparts 3b and 7a}, language = {en} } @article{ZenSaphiannikovaNeheretal.2005, author = {Zen, Achmad and Saphiannikova, Marina and Neher, Dieter and Asawapirom, Udom and Scherf, Ullrich}, title = {Comparative study of the field-effect mobility of a copolymer and a binary blend based on poly(3- alkylthiophene)s}, issn = {0897-4756}, year = {2005}, abstract = {The performance of highly soluble regioregular poly[ (3-hexylthiophene)-co-(3-octylthiophetie)] (P3HTOT) as a semiconducting material in organic field-effect transistors (OFETs) is presented in comparison to that of the corresponding homopolymers. Transistors made from as-prepared layers of P3HTOT exhibit a mobility of ca. 7 x 10(-3) cm(2) V-1 s(-1), which is comparable to the performance of transistors made from as-prepared poly(3-hexylthiophene) (P3HT) and almost 6 times larger than the mobility of transistors prepared with poly(3-octylthiophene) (P3OT). On the other hand, the solubility parameter delta(p) of P3HTOT is close to that of the highly soluble P3OT. Moreover, compared to a physical blend of poly(3-hexylthiophene) and poly(3-octylthiophene), the mobility of P3HTOT devices is almost twice as large and the performance does not degrade upon annealing at elevated temperatures. Therefore, the copolymer approach outlined here may be one promising step toward an optimum balance between a Sufficient processability of the polymers from common organic solvents, a high solid state order, and applicable OFET performances}, language = {en} } @article{KietzkeNeherKumkeetal.2004, author = {Kietzke, Thomas and Neher, Dieter and Kumke, Michael Uwe and Montenegro, Rivelino V. D. and Landfester, Katharina and Scherf, Ullrich}, title = {A nanoparticle approach to control the phase separation in polyfluorene photovoltaic devices}, year = {2004}, abstract = {Polymer solar cell devices with nanostructured blend layers have been fabricated using single- and dual- component polymer nanospheres. Starting from an electron-donating and an electron-accepting polyfluorene derivative, PFB and F8BT, dissolved in suitable organic solvents, dispersions of solid particles with mean diameters of ca. 50 nm, containing either the pure polymer components or a mixture of PFB and F8BT in each particle, were prepared with the miniemulsion process. Photovoltaic devices based on these particles have been studied with respect to the correlation between external quantum efficiency and layer composition. It is shown that the properties of devices containing a blend of single-component PFB and F8BT particles differ significantly from those of solar cells based on blend particles, even for the same layer composition. Various factors determining the quantum efficiency in both kinds of devices are identified and discussed, taking into account the spectroscopic properties of the particles. An external quantum efficiency of ca. 4\% is measured for a device made from polymer blend nanoparticles containing PFB:F8BT at a weight ratio of 1:2 in each individual nanosphere. This is among the highest values reported so far for photovoltaic cells using this material combination}, language = {en} } @article{BagnichBasslerNeher2004, author = {Bagnich, Sergey A. and Bassler, H. and Neher, Dieter}, title = {Sensitized phosphorescence of benzil-doped ladder-type methyl-poly(para-phenylene)}, issn = {0021-9606}, year = {2004}, abstract = {The delayed luminescence and phosphorescence of ladder-type methyl-poly(para-phenylene) (MeLPPP) doped with benzil at a concentration of 20\% by weight has been measured. The introduction of benzil leads to a dramatic reduction of the polymer singlet emission. At the same time, a new band with maximum at 611 nm appears, corresponding to the phosphorescence of MeLPPP. The phosphorescence decay on the short time scale is close to an exponential law with a time decay of 15 ms. This indicates that benzil can efficiently sensitize the phosphorescence of the polymer. In addition, a broad and featureless emission is observed in the delayed luminescence spectra of benzil-doped MeLPPP, which is attributed to an exciplex formed between the polymer host and the dopant. We further observe that the delayed fluorescence is enhanced by the addition of benzil. It is concluded that the delayed fluorescence of benzil-doped MeLPPP is mainly due to the annihilation of triplet excitons on the polymer. Finally, efficient triplet-triplet energy transfer from the benzil-doped polymer to the red-emitting phosphorescent dye Pt(II)octaethylporphyrin is established. (C) 2004 American Institute of Physics}, language = {en} } @article{SaphiannikovaNeher2005, author = {Saphiannikova, Marina and Neher, Dieter}, title = {Thermodynamic theory of light-induced material transport in amorphous azobenzene polymer films}, issn = {1520-6106}, year = {2005}, abstract = {It was discovered 10 years ago that the exposure of an initially flat layer of an azobenzene-containing polymer to an inhomogeneous light pattern leads to the formation of surface relief structures, accompanied by a mass transport over several micrometers. However, the driving force of this process is still unclear. We propose a new thermodynamic approach that explains a number of experimental findings including the light-induced deformation of free-standing films and the formation of surface relief gratings for main inscription geometries. Our basic assumption is that under homogeneous illumination, an initially isotropic sample should stretch itself along the polarization direction to compensate the entropy decrease produced by the photoinduced reorientation of azobenzene chromophores. The magnitude of the elastic stress, estimated by taking the derivative of the free energy over the sample deformation, is shown to be sufficient to induce plastic deformation of the polymer film. Orientational distributions of chromophores predicted by our model are compared with those deduced from Raman intensity measurements}, language = {en} } @article{KulikovskyNeherMecheretal.2004, author = {Kulikovsky, Lazar and Neher, Dieter and Mecher, E. and Meerholz, Klaus and Horhold, H. H. and Ostroverkhova, O.}, title = {Photocurrent dynamics in a poly(phenylene vinylene)-based photorefractive composite}, issn = {1098-0121}, year = {2004}, abstract = {All parameters describing the charge carrier dynamics in a poly(phenylene vinylene)-based photorefractive (PR) composite relevant to PR grating dynamics were determined using photoconductivity studies under various illumination conditions. In particular, the values of the coefficients for trap filling and recombination of charges with ionized sensitizer molecules could be extracted independently. It is concluded that the PR growth time without preillumination is mostly determined by the competition between deep trap filling and recombination with ionized sensitizer molecules. Further, the pronounced increase in PR speed upon homogeneous preillumination (gating) as reported recently is quantitatively explained by deep trap filling}, language = {en} } @article{BagnichBasslerNeher2005, author = {Bagnich, Sergey A. and Bassler, H. and Neher, Dieter}, title = {Exciton dynamics in ladder-type methyl-poly(para-phenylene) doped with phosphorescent dyes}, issn = {0022-2313}, year = {2005}, abstract = {The luminescence of a ladder-type methyl-poly(para-phenylene) (MeLPPP) doped with platinum-porphyrin dye PtOEP covering the concentration 10(-3)-5\% by weight has been measured employing cw and transient techniques. Upon excitation into the range of absorption of the host, strong phosphorescence of the dopant is observed. Possible ways of populating the dopant triplet state are considered. (c) 2004 Elsevier B.V. All rights reserved}, language = {en} } @article{GalbrechtYangNehlsetal.2005, author = {Galbrecht, Frank and Yang, X. H. and Nehls, B. S. and Neher, Dieter and Farrell, Tony and Scherf, Ullrich}, title = {Semiconducting polyfluorenes with electrophosphorescent on-chain platinum-salen chromophores}, issn = {1359-7345}, year = {2005}, abstract = {The synthesis of statistical fluorene-type copolymers with on-chain Pt-salen phosphorescent units and their use in electrophosphorescent OLEDs is reported}, language = {en} } @article{StillerKarageorgievGeueetal.2004, author = {Stiller, Burkhard and Karageorgiev, Peter and Geue, Thomas and Morawetz, Knut and Saphiannikova, Marina and Mechau, Norman and Neher, Dieter}, title = {Optically induced mass transport studied by scanning near-field optical- and atomic force microscopy}, issn = {0204-3467}, year = {2004}, abstract = {Some functionalised thin organic films show a very unusual property, namely the light induced material transport. This effect enables to generate three-dimensional structures on surfaces of azobenzene containing films only caused by special optical excitation. The physical mechanisms underlying this phenomenon have not yet been fully understood, and in addition, the dimensions of structures created in that way are macroscopic because of the optical techniques and the wavelength of the used light. In order to gain deeper insight into the physical fundamentals of this phenomenon and to open possibilities for applications it is necessary to create and study structures not only in a macroscopic but also in nanometer range. We first report about experiments to generate optically induced nano structures even down to 100 nm size. The optical stimulation was therefore made by a Scanning Near-field Optical Microscope (SNOM). Secondly, physical conditions inside optically generated surface relief gratings were studied by measuring mechanical properties with high lateral resolution via pulse force mode and force distance curves of an AFM}, language = {en} } @article{YangNeher2004, author = {Yang, X. H. and Neher, Dieter}, title = {Polymer electrophosphorescence devices with high power conversion efficiencies}, issn = {0003-6951}, year = {2004}, abstract = {We demonstrate efficient single-layer polymer phosphorescent light-emitting devices based on a green-emitting iridium complex and a polymer host co-doped with electron-transporting and hole-transporting molecules. These devices can be operated at relatively low voltages, resulting in a power conversion efficiency of up to 24 lm/W at luminous efficiencies exceeding 30 cd/A. The overall performances of these devices suggest that efficient electrophosphorescent devices with acceptable operating voltages can be achieved in very simple device structures fabricated by spin coating. (C) 2004 American Institute of Physics}, language = {en} } @article{YangNeherHerteletal.2004, author = {Yang, X. H. and Neher, Dieter and Hertel, D. and Daubler, T. K.}, title = {Highly efficient single-layer polymer electrophosphorescent devices}, issn = {0935-9648}, year = {2004}, abstract = {A commercially available Ir complex has been employed for the preparation of highly efficient (see Figure) single-layer phosphorescent polymer light,emitting diodes by use of appropriate thermal treatment and proper adjustment of the layer composition. These devices exhibit essentially no dependence of the driving field on the concentration of the Ir complex, suggesting that the build-up of space-charge in the layer is insignificant}, language = {en} } @article{SrikhirinCimrovaSchieweetal.2002, author = {Srikhirin, T. and Cimrova, V. and Schiewe, B. and Tzolov, M. and Hagen, R. and Kostromine, S. and Bieringer, Thomas and Neher, Dieter}, title = {An Investigation of the photoinduced changes of absoprtion of high-performance photoaddressable Polymers}, year = {2002}, language = {en} } @article{SianovaZenNothoferetal.2002, author = {Sianova, D. and Zen, Achmad and Nothofer, Heinz-Georg and Asawapirom, Udom and Scherf, Ullrich and Hagen, R. and Bieringer, Thomas and Kostromine, S. and Neher, Dieter}, title = {Photoaddressable alignment layers for fluorescent polymers in polarized electroluminescence devices}, year = {2002}, language = {en} } @article{WilsonSteffenMcKenzieetal.2002, author = {Wilson, J. N. and Steffen, W. and McKenzie, T. G. and Lieser, G. and Oda, Masao and Neher, Dieter and Bunz, Uwe H. F.}, title = {Chiroptcial properties of poly(p-phenyleneethynylene) copolymers in thin films : large g-values}, year = {2002}, language = {en} } @article{ZenNeherBaueretal.2002, author = {Zen, Achmad and Neher, Dieter and Bauer, C. and Asawapirom, Udom and Scherf, Ullrich and Hagen, R. and Kostromine, S. and Mahrt, R. F.}, title = {Polarization-sensitive photoconductivity in aligned polyfluorene layers}, year = {2002}, language = {en} } @article{SrikhirinLaschitschNeheretal.2000, author = {Srikhirin, Toemsak and Laschitsch, Alexander and Neher, Dieter and Johannsmann, Diethelm}, title = {Light-induced softening of azobenzene dye-doped polymer films probed with quartz crystal resonators}, year = {2000}, language = {en} } @article{NothoferMeiselMitevaetal.2000, author = {Nothofer, Heinz-Georg and Meisel, A. and Miteva, T. and Neher, Dieter and Forster, M. and Oda, Masao and Lieser, G. and Sainova, Dessislava and Yasuda, A. and Lupo, D. and Knoll, W. and Scherf, Ullrich}, title = {Liquid crystalline polyfluorenes for blue polarized electroluminescence}, year = {2000}, language = {en} } @article{LieserOdaMitevaetal.2000, author = {Lieser, G. and Oda, Masao and Miteva, T. and Nothofer, Heinz-Georg and Scherf, Ullrich and Neher, Dieter}, title = {Ordering, graphoepitaxial orientation, and conformation of a polyfluorene derivative of the "hairy-rod" type on an oriented substrate of polyimide}, year = {2000}, language = {en} } @article{OdaNothoferLieseretal.2000, author = {Oda, Masao and Nothofer, Heinz-Georg and Lieser, G. and Scherf, Ullrich and Meskers, S. C. J. and Neher, Dieter}, title = {Circularly-polarized electroluminescence from liquid-crystalline chiral polyfluorenes}, year = {2000}, language = {en} } @article{UrayamaTsujiNeher2000, author = {Urayama, Kenji and Tsuji, M. and Neher, Dieter}, title = {Layer-thinning effects on ferroelectricity and the ferroelectric-to-paraelectric phase transition of vinylidene fluoride-trifluoroethylene copolymer layers}, year = {2000}, language = {en} } @article{BauerBoehmerMorenoFloresetal.2000, author = {Bauer, C. and B{\"o}hmer, Roland and Moreno-Flores, S. and Richert, R. and Sillescu, H. and Neher, Dieter}, title = {Capacitive scanning dilatometry and frequency dependent thermal expansion of polymer films}, year = {2000}, language = {en} } @article{PralleUrayamaTewetal.2000, author = {Pralle, Martin U. and Urayama, Kenji and Tew, Gregory N. and Neher, Dieter and Wegner, Gerhard and Stupp, Samuel I.}, title = {Piezoelectricity in polar supramolecular materials}, year = {2000}, language = {en} } @article{OdaMeskersNothoferetal.2000, author = {Oda, Masao and Meskers, S. C. J. and Nothofer, Heinz-Georg and Scherf, Ullrich and Neher, Dieter}, title = {Chiroptical properties of chiral-substituted polyfluorenes}, year = {2000}, language = {en} } @article{PranavBenduhnNymanetal.2021, author = {Pranav, Manasi and Benduhn, Johannes and Nyman, Mathias and Hosseini, Seyed Mehrdad and Kublitski, Jonas and Shoaee, Safa and Neher, Dieter and Leo, Karl and Spoltore, Donato}, title = {Enhanced charge selectivity via anodic-C60 layer reduces nonradiative losses in organic solar cells}, series = {ACS applied materials \& interfaces}, volume = {13}, journal = {ACS applied materials \& interfaces}, number = {10}, publisher = {American Chemical Society}, address = {Washington}, issn = {1944-8244}, doi = {10.1021/acsami.1c00049}, pages = {12603 -- 12609}, year = {2021}, abstract = {Interfacial layers in conjunction with suitable charge-transport layers can significantly improve the performance of optoelectronic devices by facilitating efficient charge carrier injection and extraction. This work uses a neat C-60 interlayer on the anode to experimentally reveal that surface recombination is a significant contributor to nonradiative recombination losses in organic solar cells. These losses are shown to proportionally increase with the extent of contact between donor molecules in the photoactive layer and a molybdenum oxide (MoO3) hole extraction layer, proven by calculating voltage losses in low- and high-donor-content bulk heterojunction device architectures. Using a novel in-device determination of the built-in voltage, the suppression of surface recombination, due to the insertion of a thin anodic-C-60 interlayer on MoO3, is attributed to an enhanced built-in potential. The increased built-in voltage reduces the presence of minority charge carriers at the electrodes-a new perspective on the principle of selective charge extraction layers. The benefit to device efficiency is limited by a critical interlayer thickness, which depends on the donor material in bilayer devices. Given the high popularity of MoO3 as an efficient hole extraction and injection layer and the increasingly popular discussion on interfacial phenomena in organic optoelectronic devices, these findings are relevant to and address different branches of organic electronics, providing insights for future device design.}, language = {en} } @article{LiBenduhnQiaoetal.2019, author = {Li, Tian-yi and Benduhn, Johannes and Qiao, Zhi and Liu, Yuan and Li, Yue and Shivhare, Rishi and Jaiser, Frank and Wang, Pei and Ma, Jie and Zeika, Olaf and Neher, Dieter and Mannsfeld, Stefan C. B. and Ma, Zaifei and Vandewal, Koen and Leo, Karl}, title = {Effect of H- and J-Aggregation on the Photophysical and Voltage Loss of Boron Dipyrromethene Small Molecules in Vacuum-Deposited Organic Solar Cells}, series = {The journal of physical chemistry letters}, volume = {10}, journal = {The journal of physical chemistry letters}, number = {11}, publisher = {American Chemical Society}, address = {Washington}, issn = {1948-7185}, doi = {10.1021/acs.jpclett.9b01222}, pages = {2684 -- 2691}, year = {2019}, abstract = {An understanding of the factors limiting the open-circuit voltage (V-oc) and related photon energy loss mechanisms is critical to increase the power conversion efficiency (PCE) of small-molecule organic solar cells (OSCs), especially those with near-infrared (NIR) absorbers. In this work, two NIR boron dipyrromethene (BODIPY) molecules are characterized for application in planar (PHJ) and bulk (BHJ) heterojunction OSCs. When two H atoms are substituted by F atoms on the peripheral phenyl rings of the molecules, the molecular aggregation type in the thin film changes from the H-type to J-type. For PHJ devices, the nonradiative voltage loss of 0.35 V in the J-aggregated BODIPY is lower than that of 0.49 V in the H-aggregated device. In BHJ devices with a nonradiative voltage loss of 0.35 V, a PCE of 5.5\% is achieved with an external quantum efficiency (EQE) maximum of 68\% at 700 nm.}, language = {en} } @article{YangJaiserStilleretal.2006, author = {Yang, Xiao Hui and Jaiser, Frank and Stiller, Burkhard and Neher, Dieter and Galbrecht, Frank and Scherf, Ullrich}, title = {Efficient polymer electrophosphoreseent devices with interfacial layers}, series = {Advanced functional materials}, volume = {16}, journal = {Advanced functional materials}, number = {16}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1616-301X}, doi = {10.1002/adfm.200500834}, pages = {2156 -- 2162}, year = {2006}, abstract = {It is shown that several polymers can form insoluble interfacial layers on a poly (ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) layer after annealing of the double-layer structure. The thickness of the interlayer is dependent on the characteristics of the underlying PEDOT.PSS and the molecular weight of the polymers. It is further shown that the electronic structures of the interlayer polymers have a significant effect on the properties of red-light-emitting polymer-based electrophosphorescent devices. Upon increasing the highest occupied molecular orbital and lowest unoccupied molecular orbital positions, a significant increase in current density and device efficiency is observed. This is attributed to efficient blocking of electrons in combination with direct injection of holes from the interlayer to the phosphorescent dye. Upon proper choice of the interlayer polymer, efficient red, polymer-based electrophosphorescent devices with a peak luminance efficiency of 5.5 cd A(-1) (external quantum efficiency = 6 \%) and a maximum power-conversion efficiency of 5 Im W-1 can be realized.}, language = {en} } @article{KurpiersFerronRolandetal.2018, author = {Kurpiers, Jona and Ferron, Thomas and Roland, Steffen and Jakoby, Marius and Thiede, Tobias and Jaiser, Frank and Albrecht, Steve and Janietz, Silvia and Collins, Brian A. and Howard, Ian A. and Neher, Dieter}, title = {Probing the pathways of free charge generation in organic bulk heterojunction solar cells}, series = {Nature Communications}, volume = {9}, journal = {Nature Communications}, publisher = {Nature Publ. Group}, address = {London}, issn = {2041-1723}, doi = {10.1038/s41467-018-04386-3}, pages = {11}, year = {2018}, abstract = {The fact that organic solar cells perform efficiently despite the low dielectric constant of most photoactive blends initiated a long-standing debate regarding the dominant pathways of free charge formation. Here, we address this issue through the accurate measurement of the activation energy for free charge photogeneration over a wide range of photon energy, using the method of time-delayed collection field. For our prototypical low bandgap polymer:fullerene blends, we find that neither the temperature nor the field dependence of free charge generation depend on the excitation energy, ruling out an appreciable contribution to free charge generation though hot carrier pathways. On the other hand, activation energies are on the order of the room temperature thermal energy for all studied blends. We conclude that charge generation in such devices proceeds through thermalized charge transfer states, and that thermal energy is sufficient to separate most of these states into free charges.}, language = {en} } @article{LuszczynskaDobruchowskaGlowackietal.2006, author = {Luszczynska, Beata and Dobruchowska, Ewa and Glowacki, Ireneusz and Ulanski, Jacek and Jaiser, Frank and Yang, Xiaohui and Neher, Dieter and Danel, Andrzej}, title = {Poly(N-vinylcarbazole) doped with a pyrazoloquinoline dye : a deep blue light-emitting composite for light- emitting diode applications}, issn = {0021-8979}, doi = {10.1063/1.2162268}, year = {2006}, abstract = {We investigated the spectral properties of light-emitting diodes based on a deep blue-emitting pyrazoloquinoline dye doped into a poly(N-vinylcarbazole)-based matrix. Even though the electroluminescence (EL) of the host is redshifted and broadened with respect to the emission of the dye, the EL spectrum becomes fully dominated by the dye emission at concentrations of ca. 2 wt \%. This is attributed to a competition of exciplex formation on the matrix and exciton formation on the dye.}, language = {en} } @article{QuZhangGrimsdaleetal.2004, author = {Qu, J. Q. and Zhang, J. Y. and Grimsdale, A. C. and Mullen, K. and Jaiser, Frank and Yang, X. H. and Neher, Dieter}, title = {Dendronized perylene diimide emitters : Synthesis, luminescence, and electron and energy transfer studies}, issn = {0024-9297}, year = {2004}, abstract = {Aggregation of chromophores in the solid state commonly causes undesirable red shifts in the emission spectra and/or emission quenching. To overcome this problem, we have prepared soluble perylenetetracarboxidiimide dyes in which the chromophores are effectively shielded by polyphenylene dendrimers attached in the bay positions. Models show that attachment of the shielding units in the bay position should provide more efficient shielding than attaching them via the imide moieties. The dendrimers possess excellent film-forming properties due to alkyl substituents on their peripheries. The lack of a red shift in emission upon going from solution to the solid state indicates the dendrons suppress interaction of the emissive cores, leading to pure red-orange emission. Single-layer LEDs produce red-orange emission with relatively low efficiency especially for the higher generation dendrons, which is attributed to poor charge conduction. LEDs using blends of the dendrimers and the undendronized dye as a model compound in PVK have been investigated, and a model to extract relative charge injection rates through the dendritic scaffold from the spectral contributions in the EL spectra is developed}, language = {en} } @article{YangJaiserKlingeretal.2006, author = {Yang, X. H. and Jaiser, Frank and Klinger, S and Neher, Dieter}, title = {Blue polymer electrophosphorescent devices with different electron-transporting oxadiazoles}, doi = {10.1063/1.2162693}, year = {2006}, abstract = {We report that the performances of blue polymer electrophosphorescent devices are crucially depending on the choice of the electron transporting material incorporated into the emissive layer. Devices with 1,3-bis[(4-tert- butylphenyl)-1,3,4-oxidiazolyl]phenylene (OXD-7) doped at similar to 40 wt\% into a poly(vinylcarbazole) matrix exhibited significantly higher efficiencies than those with 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), yielding maximum luminous and power efficiency values of 18.2 Cd/A and 8.8 lm/W, respectively. Time resolved photoluminescence measurements revealed a long lifetime phosphorescence component in layers with PBD, which we assign to significant triplet harvesting by this electron-transporting component. (c) 2006 American Institute of Physics}, language = {en} } @article{SteyrleuthnerSchubertJaiseretal.2010, author = {Steyrleuthner, Robert and Schubert, Marcel and Jaiser, Frank and Blakesley, James C. and Chen, Zhihua and Facchetti, Antonio and Neher, Dieter}, title = {Bulk electron transport and charge injection in a high mobility n-type semiconducting polymer}, issn = {0935-9648}, doi = {10.1002/adma.201000232}, year = {2010}, abstract = {Bulk electron transport in a high mobility n-type polymer is studied by time-of-flight photocurrent measurements and electron-only devices. Bulk electron mobilities of similar to 5 x 10(-3) cm(2)/Vs are obtained. The analysis of the electron currents suggests the presence of an injection barrier for all conventionally used low workfunction cathodes.}, language = {en} } @article{SiniSchubertRiskoetal.2018, author = {Sini, Gjergji and Schubert, Marcel and Risko, Chad and Roland, Steffen and Lee, Olivia P. and Chen, Zhihua and Richter, Thomas V. and Dolfen, Daniel and Coropceanu, Veaceslav and Ludwigs, Sabine and Scherf, Ullrich and Facchetti, Antonio and Frechet, Jean M. J. and Neher, Dieter}, title = {On the Molecular Origin of Charge Separation at the Donor-Acceptor Interface}, series = {Advanced energy materials}, volume = {8}, journal = {Advanced energy materials}, number = {12}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1614-6832}, doi = {10.1002/aenm.201702232}, pages = {15}, year = {2018}, abstract = {Fullerene-based acceptors have dominated organic solar cells for almost two decades. It is only within the last few years that alternative acceptors rival their dominance, introducing much more flexibility in the optoelectronic properties of these material blends. However, a fundamental physical understanding of the processes that drive charge separation at organic heterojunctions is still missing, but urgently needed to direct further material improvements. Here a combined experimental and theoretical approach is used to understand the intimate mechanisms by which molecular structure contributes to exciton dissociation, charge separation, and charge recombination at the donor-acceptor (D-A) interface. Model systems comprised of polythiophene-based donor and rylene diimide-based acceptor polymers are used and a detailed density functional theory (DFT) investigation is performed. The results point to the roles that geometric deformations and direct-contact intermolecular polarization play in establishing a driving force ( energy gradient) for the optoelectronic processes taking place at the interface. A substantial impact for this driving force is found to stem from polymer deformations at the interface, a finding that can clearly lead to new design approaches in the development of the next generation of conjugated polymers and small molecules.}, language = {en} } @article{FangHolzmuellerMatulaitisetal.2016, author = {Fang, Lijia and Holzmueller, Felix and Matulaitis, Tomas and Baasner, Anne and Hauenstein, Christoph and Benduhn, Johannes and Schwarze, Martin and Petrich, Annett and Piersimoni, Fortunato and Scholz, Reinhard and Zeika, Olaf and Koerner, Christian and Neher, Dieter and Vandewal, Koen and Leo, Karl}, title = {Fluorine-containing low-energy-gap organic dyes with low voltage losses for organic solar cells}, series = {Synthetic metals : the journal of electronic polymers and electronic molecular materials}, volume = {222}, journal = {Synthetic metals : the journal of electronic polymers and electronic molecular materials}, publisher = {Elsevier}, address = {Lausanne}, issn = {0379-6779}, doi = {10.1016/j.synthmet.2016.10.025}, pages = {232 -- 239}, year = {2016}, abstract = {Fluorine-containing donor molecules TFTF, CNTF and PRTF are designed and isomer selectively synthesized for application in vacuum-deposited organic solar cells. These molecules comprise a donor acceptor molecular architecture incorporating thiophene and benzothiadiazole derivatives as the electron-donating and electron-withdrawing moieties, respectively. As opposed to previously reported materials from this class, PRTF can be purified by vacuum sublimation at moderate to high yields because of its higher volatility and better stabilization due to a stronger intramolecular hydrogen bond, as compared to TFTF and CNTF. The UV-vis absorption spectra of the three donors show an intense broadband absorption between 500 nm and 800 nm with, similar positions of their frontier energy levels. The photophysical properties of the three donor molecules are thoroughly tested and optimized in bulk heterojunction solar cells with C-60 as acceptor. PRTF shows the best performance, yielding power conversion efficiencies of up to 3.8\%. Moreover, the voltage loss for the PRTF device due to the non radiative recombination of free charge carriers is exceptionally low (0.26 V) as compared to typical values for organic solar cells (>0.34V). (C) 2016 Published by Elsevier B.V.}, language = {en} } @article{LuDiPietroKoellnetal.2016, author = {Lu, Guanghao and Di Pietro, Riccardo and K{\"o}lln, Lisa Sophie and Nasrallah, Iyad and Zhou, Ling and Mollinger, Sonya and Himmelberger, Scott and Koch, Norbert and Salleo, Alberto and Neher, Dieter}, title = {Dual-Characteristic Transistors Based on Semiconducting Polymer Blends}, series = {Advanced electronic materials}, volume = {2}, journal = {Advanced electronic materials}, publisher = {Wiley-Blackwell}, address = {Hoboken}, issn = {2199-160X}, doi = {10.1002/aelm.201600267}, pages = {2344 -- 2351}, year = {2016}, abstract = {A dual-characteristic polymer field-effect transistor has markedly different characteristics in low and high voltage operations. In the low-voltage range (<5 V) it shows sharp subthreshold slopes (0.3-0.4 V dec\&\#8722;1), using which a low-voltage inverter with gain 8 is realized, while high-voltage (>5 V) induces symmetric current with regard to drain and gate voltages, leading to discrete differential (trans) conductances.}, language = {en} } @article{UllbrichBenduhnJiaetal.2019, author = {Ullbrich, Sascha and Benduhn, Johannes and Jia, Xiangkun and Nikolis, Vasileios C. and Tvingstedt, Kristofer and Piersimoni, Fortunato and Roland, Steffen and Liu, Yuan and Wu, Jinhan and Fischer, Axel and Neher, Dieter and Reineke, Sebastian and Spoltore, Donato and Vandewal, Koen}, title = {Emissive and charge-generating donor-acceptor interfaces for organic optoelectronics with low voltage losses}, series = {Nature materials}, volume = {18}, journal = {Nature materials}, number = {5}, publisher = {Nature Publ. Group}, address = {London}, issn = {1476-1122}, doi = {10.1038/s41563-019-0324-5}, pages = {459 -- 464}, year = {2019}, abstract = {Intermolecular charge-transfer states at the interface between electron donating (D) and accepting (A) materials are crucial for the operation of organic solar cells but can also be exploited for organic light-emitting diodes(1,2). Non-radiative charge-transfer state decay is dominant in state-of-the-art D-A-based organic solar cells and is responsible for large voltage losses and relatively low power-conversion efficiencies as well as electroluminescence external quantum yields in the 0.01-0.0001\% range(3,4). In contrast, the electroluminescence external quantum yield reaches up to 16\% in D-A-based organic light-emitting diodes(5-7). Here, we show that proper control of charge-transfer state properties allows simultaneous occurrence of a high photovoltaic and emission quantum yield within a single, visible-light-emitting D-A system. This leads to ultralow-emission turn-on voltages as well as significantly reduced voltage losses upon solar illumination. These results unify the description of the electro-optical properties of charge-transfer states in organic optoelectronic devices and foster the use of organic D-A blends in energy conversion applications involving visible and ultraviolet photons(8-11).}, language = {en} } @article{LysyakovaLomadzeNeheretal.2015, author = {Lysyakova, Liudmila and Lomadze, Nino and Neher, Dieter and Maximova, Ksenia and Kabashin, Andrei V. and Santer, Svetlana}, title = {Light-Tunable Plasmonic Nanoarchitectures Using Gold Nanoparticle-Azobenzene-Containing Cationic Surfactant Complexes}, series = {The journal of physical chemistry : C, Nanomaterials and interfaces}, volume = {119}, journal = {The journal of physical chemistry : C, Nanomaterials and interfaces}, number = {7}, publisher = {American Chemical Society}, address = {Washington}, issn = {1932-7447}, doi = {10.1021/jp511232g}, pages = {3762 -- 3770}, year = {2015}, abstract = {When arranged in a proper nanoaggregate architecture, gold nanoparticles can offer controllable plasmon-related absorption/scattering, yielding distinct color effects that depend critically on the relative orientation and distance between nanoparticle constituents. Herein, we report on the implementation of novel plasmonic nanoarchitectures based on complexes between gold nanoparticles and an azobenzene-modified cationic surfactant that can exhibit a light-tunable plasmonic response. The formation of such complexes becomes possible through the use of strongly negatively charged bare gold nanoparticles (similar to 10-nm diameter) prepared by the method of laser ablation in deionized water. Driven by electrostatic interactions, the cationic surfactant molecules attach and form a shell around the negatively charged nanoparticles, resulting in neutralization of the particle charge or even overcompensation beyond which the nanoparticles become positively charged. At low and high surfactant concentrations, Au nanoparticles are negatively and positively charged, respectively, and are represented by single species due to electric repulsion effects having absorption peaks around 523-527 nm, whereas at intermediate concentrations, the Au nanoparticles become neutral, forming nanoscale 100-nm clusterlike aggregates and exhibiting an additional absorption peak at gimel > 600 nm and a visible change in the color of the solution from red to blue. Because of the presence of the photosensitive azobenzene unit in the surfactant tail that undergoes trans-to-cis isomerization under irradiation with UV light, we then demonstrate a light-controlled nanoclustering of nanoparticles, yielding a switch in the plasmonic absorption band and a related change in the solution color. The formed hybrid architectures with a light-controlled plasmonic response could be important for a variety of tasks, including biomedical, surface-enhanced Raman spectroscopy (SERS), data transmission, and storage applications.}, language = {en} } @article{PaulkeStranksKniepertetal.2016, author = {Paulke, Andreas and Stranks, Samuel D. and Kniepert, Juliane and Kurpiers, Jona and Wolff, Christian Michael and Sch{\"o}n, Natalie and Snaith, Henry J. and Brenner, Thomas J. K. and Neher, Dieter}, title = {Charge carrier recombination dynamics in perovskite and polymer solar cells}, series = {Applied physics letters}, volume = {108}, journal = {Applied physics letters}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0003-6951}, doi = {10.1063/1.4944044}, pages = {252 -- 262}, year = {2016}, abstract = {Time-delayed collection field experiments are applied to planar organometal halide perovskite (CH3NH3PbI3) based solar cells to investigate charge carrier recombination in a fully working solar cell at the nanosecond to microsecond time scale. Recombination of mobile (extractable) charges is shown to follow second-order recombination dynamics for all fluences and time scales tested. Most importantly, the bimolecular recombination coefficient is found to be time-dependent, with an initial value of ca. 10(-9) cm(3)/s and a progressive reduction within the first tens of nanoseconds. Comparison to the prototypical organic bulk heterojunction device PTB7:PC71BM yields important differences with regard to the mechanism and time scale of free carrier recombination. (C) 2016 AIP Publishing LLC.}, language = {en} }