TY  - JOUR
A1  - Däubler, Thomas Karl
A1  - Cimrova, V.
A1  - Pfeiffer, S.
A1  - Hörhold, Hans-Heinrich
A1  - Neher, Dieter
T1  - Field and wavelength dependence of charge carrier photogeneration in soluble PPV derivatives
Y1  - 1999
ER  - 
TY  - JOUR
A1  - Rengel, Heiko
A1  - Gattinger, P.
A1  - Silverova, R.
A1  - Neher, Dieter
T1  - Conductivity measurements of electrochemically oxidized Langmuir-Blodgett films of phthalocyaninato- polysiloxanes
Y1  - 1999
ER  - 
TY  - JOUR
A1  - Däubler, Thomas Karl
A1  - Neher, Dieter
A1  - Rost, H.
A1  - Hörhold, Hans-Heinrich
T1  - Efficient bulk photogeneration of charge carriers in arylamino-PPV polymer sandwich cells
Y1  - 1999
ER  - 
TY  - JOUR
A1  - Däubler, Thomas Karl
A1  - Pfeiffer, S.
A1  - Hörhold, Hans-Heinrich
A1  - Neher, Dieter
T1  - Photogeneration of charge carriers in segmented arylamino-PPV derivatives
Y1  - 1999
ER  - 
TY  - JOUR
A1  - Sainova, Dessislava
A1  - Fujikawa, H.
A1  - Scherf, Ullrich
A1  - Neher, Dieter
T1  - The effect of hole traps on the performance of single layer polymer light emitting diodes
Y1  - 1999
ER  - 
TY  - JOUR
A1  - Fiesel, R.
A1  - Neher, Dieter
A1  - Scherf, Ullrich
T1  - On the solid state aggregation of chiral substituted poly(para-phenylene)s (PPPs)
Y1  - 1999
ER  - 
TY  - JOUR
A1  - Cimrová, V.
A1  - Neher, Dieter
A1  - Kostromine, S.
A1  - Bieringer, Thomas
T1  - Optical anisotropy in films of photoaddressable polymers
Y1  - 1999
ER  - 
TY  - JOUR
A1  - Roland, Steffen
A1  - Kniepert, Juliane
A1  - Love, John A.
A1  - Negi, Vikas
A1  - Liu, Feilong
A1  - Bobbert, Peter
A1  - Melianas, Armantas
A1  - Kemerink, Martijn
A1  - Hofacker, Andreas
A1  - Neher, Dieter
T1  - Equilibrated Charge Carrier Populations Govern Steady-State Nongeminate Recombination in Disordered Organic Solar Cells
JF  - The journal of physical chemistry letters
N2  - We employed bias-assisted charge extraction techniques to investigate the transient and steady-state recombination of photogenerated charge carriers in complete devices of a disordered polymer-fullerene blend. Charge recombination is shown to be dispersive, with a significant slowdown of the recombination rate over time, consistent with the results from kinetic Monte Carlo simulations. Surprisingly, our experiments reveal little to no contributions from early time recombination of nonequilibrated charge carriers to the steady-state recombination properties. We conclude that energetic relaxation of photogenerated carriers outpaces any significant nongeminate recombination under application-relevant illumination conditions. With equilibrated charges dominating the steady-state recombination, quasi-equilibrium concepts appear suited for describing the open-circuit voltage of organic solar cells despite pronounced energetic disorder.
Y1  - 2019
U6  - https://doi.org/10.1021/acs.jpclett.9b00516
SN  - 1948-7185
VL  - 10
IS  - 6
SP  - 1374
EP  - 1381
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Inal, Sahika
A1  - Chiappisi, Leonardo
A1  - Kölsch, Jonas D.
A1  - Kraft, Mario
A1  - Appavou, Marie-Sousai
A1  - Scherf, Ullrich
A1  - Wagner, Manfred
A1  - Hansen, Michael Ryan
A1  - Gradzielski, Michael
A1  - Laschewsky, André
A1  - Neher, Dieter
T1  - Temperature-regulated fluorescence and association of an Oligo(ethyleneglycol)methacrylate-based copolymer with a conjugated Polyelectrolyte-the effect of solution ionic strength
JF  - The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces & biophysical chemistry
N2  - Aqueous mixtures of a dye-labeled non-ionic thermoresponsive copolymer and a conjugated cationic polyelectrolyte are shown to exhibit characteristic changes in fluorescence properties in response to temperature and to the presence of salts, enabling a double-stimuli responsiveness. In such mixtures at room temperature, i.e., well below the lower critical solution temperature (LCST), the emission of the dye is strongly quenched due to energy transfer to the polycation, pointing to supramolecular interactions between the two macromolecules. Increasing the concentration of salts weakens the interpolymer interactions, the extent of which is simultaneously monitored from the change in the relative emission intensity of the components. When the mixture is heated above its LCST, the transfer efficiency is significantly reduced, signaling a structural reorganization process, however, surprisingly only if the mixture contains salt ions. To elucidate the reasons behind such thermo- and ion-sensitive fluorescence characteristics, we investigate the effect of salts of alkali chlorides, in particular of NaCl, on the association behavior of these macromolecules before and after the polymer phase transition by a combination of UV-vis, fluorescence, and H-1 NMR spectroscopy with light scattering and small-angle neutron scattering measurements.
Y1  - 2013
U6  - https://doi.org/10.1021/jp408864s
SN  - 1520-6106
VL  - 117
IS  - 46
SP  - 14576
EP  - 14587
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Lange, Ilja
A1  - Kniepert, Juliane
A1  - Pingel, Patrick
A1  - Dumsch, Ines
A1  - Allard, Sybille
A1  - Janietz, Silvia
A1  - Scherf, Ullrich
A1  - Neher, Dieter
T1  - Correlation between the open circuit voltage and the energetics of organic bulk heterojunction solar cells
JF  - The journal of physical chemistry letters
N2  - A detailed investigation of the open circuit voltage (V-OC) of organic bulk heterojunction solar cells comprising three different donor polymers and two different fullerene-based acceptors is presented. Bias amplified charge extraction (BACE) is combined with Kelvin Probe measurements to derive information on the relevant energetics in the blend. On the example of P3HT:PC70BM the influence of composition and preparation conditions on the relevant transport levels will be shown. Moderate upward shifts of the P3HT HOMO depending on crystallinity are observed, but contrarily to common believe, the dependence of V-OC on blend composition and thermal history is found to be largely determined by the change in the PCBM LUMO energy. Following this approach, we quantified the energetic contribution to the V-OC in blends with fluorinated polymers or higher adduct fullerenes.
Y1  - 2013
U6  - https://doi.org/10.1021/jz401971e
SN  - 1948-7185
VL  - 4
IS  - 22
SP  - 3865
EP  - 3871
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Gross, M.
A1  - Müller, David C.
A1  - Nothofer, Heinz-Georg
A1  - Scherf, Ullrich
A1  - Neher, Dieter
A1  - Bräuchler, C.
A1  - Meerholz, Klaus
T1  - Improving the performance of doped p-conjugated polymers for use in organic light-emitting diodes
Y1  - 2000
ER  - 
TY  - JOUR
A1  - Joshi, Siddharth
A1  - Pingel, Patrick
A1  - Grigorian, Souren
A1  - Panzner, Tobias
A1  - Pietsch, Ullrich
A1  - Neher, Dieter
A1  - Forster, Michael
A1  - Scherf, Ullrich
T1  - Bimodal temperature behavior of structure and mobility in high molecular weight p3ht thin films
N2  - 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.
Y1  - 2009
UR  - http://pubs.acs.org/journal/mamobx
U6  - https://doi.org/10.1021/Ma900021w
SN  - 0024-9297
ER  - 
TY  - JOUR
A1  - Schubert, Marcel
A1  - Steyrleuthner, Robert
A1  - Bange, Sebastian
A1  - Sellinger, Alan
A1  - Neher, Dieter
T1  - Charge transport and recombination in bulk heterojunction solar cells containing a dicyanoimidazole-based molecular acceptor
N2  - 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.
Y1  - 2009
UR  - http://www3.interscience.wiley.com/cgi-bin/jhome/40000761
U6  - https://doi.org/10.1002/pssa.200925312
SN  - 1862-6300
ER  - 
TY  - JOUR
A1  - Schubert, Marcel
A1  - Yin, Chunhong
A1  - Castellani, Mauro
A1  - Bange, Sebastian
A1  - Tam, Teck Lip
A1  - Sellinger, Alan
A1  - Hoerhold, Hans-Heinrich
A1  - Kietzke, Thomas
A1  - Neher, Dieter
T1  - Heterojunction topology versus fill factor correlations in novel hybrid small-molecular/polymeric solar cells
N2  - 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.
Y1  - 2009
UR  - http://jcp.aip.org/
U6  - https://doi.org/10.1063/1.3077007
SN  - 0021-9606
ER  - 
TY  - JOUR
A1  - Inal, Sahika
A1  - Castellani, Mauro
A1  - Sellinger, Alan
A1  - Neher, Dieter
T1  - Relationship of photophysical properties and the device performance of novel hybrid small-molecular/polymeric solar cells
N2  - 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%.
Y1  - 2009
UR  - http://www3.interscience.wiley.com/cgi-bin/jhome/10003270
U6  - https://doi.org/10.1002/marc.200900221
SN  - 1022-1336
ER  - 
TY  - JOUR
A1  - Steyrleuthner, Robert
A1  - Bange, Sebastian
A1  - Neher, Dieter
T1  - Reliable electron-only devices and electron transport in n-type polymers
N2  - 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.
Y1  - 2009
UR  - http://jap.aip.org/
U6  - https://doi.org/10.1063/1.3086307
SN  - 0021-8979
ER  - 
TY  - JOUR
A1  - Rengel, Heiko
A1  - Altmann, Markus
A1  - Neher, Dieter
A1  - Harrison, Craig B.
A1  - Myrick, Michael L.
A1  - Bunz, Uwe H. F.
T1  - Assignment of the optical transitions in 1,3- diethynylcyclobutadiene (cyclopentadienyl)cobalt oligomers
Y1  - 1999
ER  - 
TY  - JOUR
A1  - Miteva, T.
A1  - Meisel, A.
A1  - Nothofer, Heinz-Georg
A1  - Scherf, Ullrich
A1  - Knoll, W.
A1  - Neher, Dieter
A1  - Grell, M.
A1  - Lupo, D.
A1  - Yasuda, A.
T1  - Polarized electroluminescence from highly aligned liquid-crystalline polymers
Y1  - 2000
ER  - 
TY  - JOUR
A1  - Miteva, T.
A1  - Kloppenburg, L.
A1  - Neher, Dieter
A1  - Bunz, Uwe H. F.
T1  - Interplay of Thermochromicity and Liquid Crystalline Behavior in Poly(p-phenyleneethynylen)s:p-p-Interactions or Planarization of the Conjugated Backbone?
Y1  - 2000
ER  - 
TY  - JOUR
A1  - Bauer, C.
A1  - Umbasch, G.
A1  - Giessen, H.
A1  - Meisel, A.
A1  - Nothofer, Heinz-Georg
A1  - Neher, Dieter
A1  - Scherf, Ullrich
A1  - Marth, R.
T1  - Polarized Photoluminescence and Spectral Narrowing in an oriented Polyfluorene Thin Film
Y1  - 2000
ER  - 
TY  - JOUR
A1  - Grell, M.
A1  - Knoll, W.
A1  - Lupo, D.
A1  - Meisel, A.
A1  - Miteva, T.
A1  - Neher, Dieter
A1  - Nothofer, Heinz-Georg
A1  - Scherf, Ullrich
A1  - Yasuda, H.
T1  - Blue polarized electroluminescence from a liquid crystalline polyfluorene
Y1  - 1999
ER  - 
TY  - JOUR
A1  - Urayama, Kenji
A1  - Kircher, Oliver
A1  - Böhmer, Roland
A1  - Neher, Dieter
T1  - Investigations of ferroelectric-to-paraelectric phase transition of vinylidenefluoride trifluoroethylene copolymer thin films by electromechanical interferometry
Y1  - 1999
ER  - 
TY  - JOUR
A1  - Bittner, Reinhard
A1  - Däubler, Thomas Karl
A1  - Neher, Dieter
A1  - Meerholz, Klaus
T1  - Influence of the glass-transition and the chromophore content on the steady-state performance of PVK-based photorefractive polymers
Y1  - 1999
ER  - 
TY  - JOUR
A1  - Former, C.
A1  - Wagner, H.
A1  - Richert, R.
A1  - Neher, Dieter
A1  - Müllen, K.
T1  - Orientation and dynamics of chainlike dipole arrays: Donor-acceptor-substituted oligophenylenevinylenes in a polymer matrix
Y1  - 1999
ER  - 
TY  - JOUR
A1  - Hosseini, Seyed Mehrdad
A1  - Tokmoldin, Nurlan
A1  - Lee, Young Woong
A1  - Zou, Yingping
A1  - Woo, Han Young
A1  - Neher, Dieter
A1  - Shoaee, Safa
T1  - Putting order into PM6:Y6 solar cells to reduce the langevin recombination in 400 nm thick junction
JF  - Solar RRL
N2  - 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.
KW  - charge carrier extraction
KW  - energetic disorders
KW  - non-fullerene acceptors
KW  - non-Langevin reduction factors
KW  - thick junctions
Y1  - 2020
U6  - https://doi.org/10.1002/solr.202000498
SN  - 2367-198X
VL  - 4
IS  - 11
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Vandewal, Koen
A1  - Albrecht, Steve
A1  - Hoke, Eric T.
A1  - Graham, Kenneth R.
A1  - Widmer, Johannes
A1  - Douglas, Jessica D.
A1  - Schubert, Marcel
A1  - Mateker, William R.
A1  - Bloking, Jason T.
A1  - Burkhard, George F.
A1  - Sellinger, Alan
A1  - Frechet, Jean M. J.
A1  - Amassian, Aram
A1  - Riede, Moritz K.
A1  - McGehee, Michael D.
A1  - Neher, Dieter
A1  - Salleo, Alberto
T1  - Efficient charge generation by relaxed charge-transfer states at organic interfaces
JF  - Nature materials
N2  - 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.
Y1  - 2014
U6  - https://doi.org/10.1038/NMAT3807
SN  - 1476-1122
SN  - 1476-4660
VL  - 13
IS  - 1
SP  - 63
EP  - 68
PB  - Nature Publ. Group
CY  - London
ER  - 
TY  - JOUR
A1  - Schubert, Marcel
A1  - Dolfen, Daniel
A1  - Frisch, Johannes
A1  - Roland, Steffen
A1  - Steyrleuthner, Robert
A1  - Stiller, Burkhard
A1  - Chen, Zhihua
A1  - Scherf, Ullrich
A1  - Koch, Norbert
A1  - Facchetti, Antonio
A1  - Neher, Dieter
T1  - Influence of aggregation on the performance of All-Polymer Solar Cells containing Low-Bandgap Naphthalenediimide Copolymers
JF  - dvanced energy materials
N2  - 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.
KW  - aggregation
KW  - morphology
KW  - naphthalenediimide
KW  - organic semiconductors
KW  - organic photovoltaics
Y1  - 2012
U6  - https://doi.org/10.1002/aenm.201100601
SN  - 1614-6832
VL  - 2
IS  - 3
SP  - 369
EP  - 380
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Frisch, Johannes
A1  - Schubert, Marcel
A1  - Preis, Eduard
A1  - Rabe, Jürgen P.
A1  - Neher, Dieter
A1  - Scherf, Ullrich
A1  - Koch, Norbert
T1  - Full electronic structure across a polymer heterojunction solar cell
JF  - Journal of materials chemistry
N2  - 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.
Y1  - 2012
U6  - https://doi.org/10.1039/c1jm14968g
SN  - 0959-9428
VL  - 22
IS  - 10
SP  - 4418
EP  - 4424
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Albrecht, Steve
A1  - Schaefer, Sebastian
A1  - Lange, Ilja
A1  - Yilmaz, Seyfullah
A1  - Dumsch, Ines
A1  - Allard, Sybille
A1  - Scherf, Ullrich
A1  - Hertwig, Andreas
A1  - Neher, Dieter
T1  - Light management in PCPDTBT:PC70BM solar cells: A comparison of standard and inverted device structures
JF  - Organic electronics : physics, materials and applications
N2  - 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.
KW  - Organic solar cells
KW  - Inverted solar cells
KW  - PCPDTBT
KW  - Low band-gap
KW  - Optical modeling
Y1  - 2012
U6  - https://doi.org/10.1016/j.orgel.2011.12.019
SN  - 1566-1199
VL  - 13
IS  - 4
SP  - 615
EP  - 622
PB  - Elsevier
CY  - Amsterdam
ER  - 
TY  - JOUR
A1  - Albrecht, Steve
A1  - Schindler, Wolfram
A1  - Kurpiers, Jona
A1  - Kniepert, Juliane
A1  - Blakesley, James C.
A1  - Dumsch, Ines
A1  - Allard, Sybille
A1  - Fostiropoulos, Konstantinos
A1  - Scherf, Ullrich
A1  - Neher, Dieter
T1  - On the field dependence of free charge carrier generation and recombination in blends of PCPDTBT/PC70BM influence of solvent additives
JF  - The journal of physical chemistry letters
N2  - 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.
Y1  - 2012
U6  - https://doi.org/10.1021/jz3000849
SN  - 1948-7185
VL  - 3
IS  - 5
SP  - 640
EP  - 645
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Ilnytskyi, Jaroslav M.
A1  - Saphiannikova, Marina
A1  - Neher, Dieter
A1  - Allen, Michael P.
T1  - Modelling elasticity and memory effects in liquid crystalline elastomers by molecular dynamics simulations
JF  - Soft matter
N2  - 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.
Y1  - 2012
U6  - https://doi.org/10.1039/c2sm26499d
SN  - 1744-683X
VL  - 8
IS  - 43
SP  - 11123
EP  - 11134
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Pingel, P.
A1  - Schwarzl, R.
A1  - Neher, Dieter
T1  - Effect of molecular p-doping on hole density and mobility in poly(3-hexylthiophene)
JF  - Applied physics letters
N2  - 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.
Y1  - 2012
U6  - https://doi.org/10.1063/1.3701729
SN  - 0003-6951
VL  - 100
IS  - 14
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - JOUR
A1  - Nikolis, Vasileios C.
A1  - Mischok, Andreas
A1  - Siegmund, Bernhard
A1  - Kublitski, Jonas
A1  - Jia, Xiangkun
A1  - Benduhn, Johannes
A1  - Hörmann, Ulrich
A1  - Neher, Dieter
A1  - Gather, Malte C.
A1  - Spoltore, Donato
A1  - Vandewal, Koen
T1  - Strong light-matter coupling for reduced photon energy losses in organic photovoltaics
JF  - Nature Communications
N2  - 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.
Y1  - 2019
U6  - https://doi.org/10.1038/s41467-019-11717-5
SN  - 2041-1723
VL  - 10
PB  - Nature Publ. Group
CY  - London
ER  - 
TY  - JOUR
A1  - García-Benito, Inés
A1  - Quarti, Claudio
A1  - Queloz, Valentin I. E.
A1  - Hofstetter, Yvonne J.
A1  - Becker-Koch, David
A1  - Caprioglio, Pietro
A1  - Neher, Dieter
A1  - Orlandi, Simonetta
A1  - Cavazzini, Marco
A1  - Pozzi, Gianluca
A1  - Even, Jacky
A1  - Nazeeruddin, Mohammad Khaja
A1  - Vaynzof, Yana
A1  - Grancini, Giulia
T1  - Fluorination of organic spacer impacts on the structural and optical response of 2D perovskites
JF  - Frontiers in Chemistry
N2  - 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.
KW  - fluorinated organic spacer
KW  - 2D perovskites
KW  - phase transition
KW  - temperature dependence
KW  - excitonic materials
Y1  - 2020
U6  - https://doi.org/10.3389/fchem.2019.00946
SN  - 2296-2646
VL  - 7
SP  - 1
EP  - 11
PB  - Frontiers Media
CY  - Lausanne
ER  - 
TY  - JOUR
A1  - Kniepert, Juliane
A1  - Lange, Ilja
A1  - van der Kaap, Niels J.
A1  - Koster, L. Jan Anton
A1  - Neher, Dieter
T1  - A conclusive view on charge generation, recombination, and extraction in As-prepared and annealed P3HT:PCBM blends: combined experimental and simulation work
JF  - dvanced energy materials
N2  - 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.
Y1  - 2014
U6  - https://doi.org/10.1002/aenm.201301401
SN  - 1614-6832
SN  - 1614-6840
VL  - 4
IS  - 7
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Poelking, Carl
A1  - Benduhn, Johannes
A1  - Spoltore, Donato
A1  - Schwarze, Martin
A1  - Roland, Steffen
A1  - Piersimoni, Fortunato
A1  - Neher, Dieter
A1  - Leo, Karl
A1  - Vandewal, Koen
A1  - Andrienko, Denis
T1  - Open-circuit voltage of organic solar cells
BT  - interfacial roughness makes the difference
JF  - Communications physics
N2  - 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.
Y1  - 2022
U6  - https://doi.org/10.1038/s42005-022-01084-x
SN  - 2399-3650
VL  - 5
IS  - 1
PB  - Nature portfolio
CY  - Berlin
ER  - 
TY  - JOUR
A1  - Vollbrecht, Joachim
A1  - Tokmoldin, Nurlan
A1  - Sun, Bowen
A1  - Brus, Viktor V.
A1  - Shoaee, Safa
A1  - Neher, Dieter
T1  - Determination of the charge carrier density in organic solar cells
BT  - a tutorial
JF  - Journal of applied physics
N2  - 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.
KW  - Electrical properties and parameters
KW  - Organic semiconductors
KW  - Solar cells
KW  - Photoconductivity
KW  - Capacitance spectroscopy
Y1  - 2022
U6  - https://doi.org/10.1063/5.0094955
SN  - 0021-8979
SN  - 1089-7550
SN  - 1520-8850
VL  - 131
IS  - 22
PB  - American Institute of Physics
CY  - Melville, NY
ER  - 
TY  - JOUR
A1  - Sun, Bowen
A1  - Sandberg, Oskar
A1  - Neher, Dieter
A1  - Armin, Ardalan
A1  - Shoaee, Safa
T1  - Wave optics of differential absorption spectroscopy in thick-junction organic solar cells
BT  - optical artifacts and correction strategies
JF  - Physical review applied / The American Physical Society
N2  - 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.
Y1  - 2022
U6  - https://doi.org/10.1103/PhysRevApplied.17.054016
SN  - 2331-7019
VL  - 17
IS  - 5
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - JOUR
A1  - Fritsch, Tobias
A1  - Kurpiers, Jona
A1  - Roland, Steffen
A1  - Tokmoldin, Nurlan
A1  - Shoaee, Safa
A1  - Ferron, Thomas
A1  - Collins, Brian A.
A1  - Janietz, Silvia
A1  - Vandewal, Koen
A1  - Neher, Dieter
T1  - On the interplay between CT and singlet exciton emission in organic solar cells with small driving force and its impact on voltage loss
JF  - Advanced energy materials
N2  - 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.
KW  - external quantum efficiency
KW  - organic photovoltaics
KW  - ternary blends
KW  - voltage losses
Y1  - 2022
U6  - https://doi.org/10.1002/aenm.202200641
SN  - 1614-6832
SN  - 1614-6840
VL  - 12
IS  - 31
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Gerhard, Reimund
A1  - Wegener, Michael
A1  - Künstler, Wolfgang
A1  - Wirges, Werner
A1  - Görne, Thomas
A1  - Urayama, D.
A1  - Neher, Dieter
T1  - Inverse piezoelectricity of porous PTFE films with bipolar space charge
Y1  - 2000
ER  - 
TY  - JOUR
A1  - Sainova, Dessislava
A1  - Miteva, T.
A1  - Nothofer, Heinz-Georg
A1  - Scherf, Ullrich
A1  - Fujikawa, H.
A1  - Glowacki, Ireneusz
A1  - Ulanski, J.
A1  - Neher, Dieter
T1  - Control of color and efficiency of light-emitting diodes based on polyfluorenes blended with hole-transporting molecules
Y1  - 2000
ER  - 
TY  - JOUR
A1  - Schubert, Marcel
A1  - Frisch, Johannes
A1  - Allard, Sybille
A1  - Preis, Eduard
A1  - Scherf, Ullrich
A1  - Koch, Norbert
A1  - Neher, Dieter
T1  - Tuning side chain and main chain order in a prototypical donor-acceptor copolymer
BT  - implications for optical, electronic, and photovoltaic characteristics
JF  - Elementary Processes in Organic Photovoltaics
N2  - 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.
KW  - Aggregate states
KW  - All-polymer heterojunctions
KW  - Alternating copolymers
KW  - Ambipolar charge transport
KW  - Ambipolar materials
KW  - Backbone modifications
KW  - Bilayer solar cells
KW  - Charge separation
KW  - Conformational disorder
KW  - Crystalline phases
KW  - Donor-acceptor copolymers
KW  - Electron traps
KW  - Energetic disorder
KW  - Energy-level alignment
KW  - Fermi-level alignment
KW  - Fermi-level pinning
KW  - Interface dipole
KW  - Interlayer
KW  - Intrachain order
KW  - Intragap states
KW  - Microscopic morphology
KW  - Mobility imbalance
KW  - Mobility relaxation
KW  - Monte Carlo simulation
KW  - Multiple trapping model
KW  - Nonradiative recombination
KW  - OFET
KW  - Open-circuit voltage
KW  - Optoelectronic properties
KW  - Partially alternating copolymers
KW  - Photo-CELIV
KW  - Photocurrent
KW  - Photovoltaic gap
KW  - Polymer intermixing
KW  - Recombination losses
KW  - Spectral diffusion
KW  - Statistical copolymers
KW  - Stille-type cross-coupling
KW  - Structure-property relationships
KW  - Time-dependent mobility
KW  - Time-of-flight (TOF)
KW  - Transient photocurrent
KW  - Ultraviolet photoelectron spectroscopy
KW  - Vacuum-level alignment
KW  - X-ray photoelectron spectroscopy
Y1  - 2016
SN  - 978-3-319-28338-8
SN  - 978-3-319-28336-4
U6  - https://doi.org/10.1007/978-3-319-28338-8_10
SN  - 0065-3195
VL  - 272
SP  - 243
EP  - 265
PB  - Springer
CY  - Berlin
ER  - 
TY  - JOUR
A1  - Scharsich, Christina
A1  - Lohwasser, Ruth H.
A1  - Sommer, Michael
A1  - Asawapirom, Udom
A1  - Scherf, Ullrich
A1  - Thelakkat, Mukundan
A1  - Neher, Dieter
A1  - Koehler, Anna
T1  - Control of aggregate formation in poly(3-hexylthiophene) by solvent, molecular weight, and synthetic method
JF  - Journal of polymer science : B, Polymer physics
N2  - 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.
KW  - conformational analysis
KW  - conjugated polymers
KW  - crystallization
KW  - films
KW  - interaction parameter
KW  - molecular weight distribution
KW  - molar mass distribution
KW  - nucleation
KW  - photophysics
KW  - structure
KW  - UV-vis spectroscopy
Y1  - 2012
U6  - https://doi.org/10.1002/polb.23022
SN  - 0887-6266
VL  - 50
IS  - 6
SP  - 442
EP  - 453
PB  - Wiley-Blackwell
CY  - Hoboken
ER  - 
TY  - JOUR
A1  - Lange, Ilja
A1  - Reiter, Sina
A1  - Paetzel, Michael
A1  - Zykov, Anton
A1  - Nefedov, Alexei
A1  - Hildebrandt, Jana
A1  - Hecht, Stefan
A1  - Kowarik, Stefan
A1  - Woell, Christof
A1  - Heimel, Georg
A1  - Neher, Dieter
T1  - Tuning the work function of polar zinc oxide surfaces using modified phosphonic acid self-assembled monolayers
JF  - Advanced functional materials
N2  - 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.
KW  - ZnO
KW  - self-assembled monolayers
KW  - phosphonic acid
KW  - surface modification
KW  - electrodes
Y1  - 2014
U6  - https://doi.org/10.1002/adfm.201401493
SN  - 1616-301X
SN  - 1616-3028
VL  - 24
IS  - 44
SP  - 7014
EP  - 7024
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Inal, Sahika
A1  - Koelsch, Jonas D.
A1  - Chiappisi, Leonardo
A1  - Janietz, Dietmar
A1  - Gradzielski, Michael
A1  - Laschewsky, André
A1  - Neher, Dieter
T1  - Structure-related differences in the temperature-regulated fluorescence response of LCST type polymers
JF  - Journal of materials chemistry : C, Materials for optical and electronic devices
N2  - 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.
Y1  - 2013
U6  - https://doi.org/10.1039/c3tc31304b
SN  - 2050-7526
VL  - 1
IS  - 40
SP  - 6603
EP  - 6612
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Albrecht, Steve
A1  - Janietz, Silvia
A1  - Schindler, Wolfram
A1  - Frisch, Johannes
A1  - Kurpiers, Jona
A1  - Kniepert, Juliane
A1  - Inal, Sahika
A1  - Pingel, Patrick
A1  - Fostiropoulos, Konstantinos
A1  - Koch, Norbert
A1  - Neher, Dieter
T1  - Fluorinated Copolymer PCPDTBT with enhanced open-circuit voltage and reduced recombination for highly efficient polymer solar cells
JF  - Journal of the American Chemical Society
N2  - 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.
Y1  - 2012
U6  - https://doi.org/10.1021/ja305039j
SN  - 0002-7863
VL  - 134
IS  - 36
SP  - 14932
EP  - 14944
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Schattauer, Sylvia
A1  - Reinhold, Beate
A1  - Albrecht, Steve
A1  - Fahrenson, Christoph
A1  - Schubert, Marcel
A1  - Janietz, Silvia
A1  - Neher, Dieter
T1  - Influence of sintering on the structural and electronic properties of TiO2 nanoporous layers prepared via a non-sol-gel approach
JF  - Colloid and polymer science : official journal of the Kolloid-Gesellschaft
N2  - 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.
KW  - Nonaqueous sol-gel
KW  - Thin nanocrystalline TiO2 layer
KW  - Solar cells
KW  - Effect of sintering
KW  - Linearly increasing voltage (CELIV)
KW  - Polymer infiltration
KW  - Transport properties titania
KW  - Transient fluorescence
Y1  - 2012
U6  - https://doi.org/10.1007/s00396-012-2708-9
SN  - 0303-402X
VL  - 290
IS  - 18
SP  - 1843
EP  - 1854
PB  - Springer
CY  - New York
ER  - 
TY  - JOUR
A1  - Schubert, Marcel
A1  - Preis, Eduard
A1  - Blakesley, James C.
A1  - Pingel, Patrick
A1  - Scherf, Ullrich
A1  - Neher, Dieter
T1  - Mobility relaxation and electron trapping in a donor/acceptor copolymer
JF  - Physical review : B, Condensed matter and materials physics
N2  - 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.
Y1  - 2013
U6  - https://doi.org/10.1103/PhysRevB.87.024203
SN  - 1098-0121
VL  - 87
IS  - 2
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - JOUR
A1  - Lu, Guanghao
A1  - Blakesley, James C.
A1  - Himmelberger, Scott
A1  - Pingel, Patrick
A1  - Frisch, Johannes
A1  - Lieberwirth, Ingo
A1  - Salzmann, Ingo
A1  - Oehzelt, Martin
A1  - Di Pietro, Riccardo
A1  - Salleo, Alberto
A1  - Koch, Norbert
A1  - Neher, Dieter
T1  - Moderate doping leads to high performance of semiconductor/insulator polymer blend transistors
JF  - Nature Communications
N2  - 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.
Y1  - 2013
U6  - https://doi.org/10.1038/ncomms2587
SN  - 2041-1723
VL  - 4
IS  - 1-2
PB  - Nature Publ. Group
CY  - London
ER  - 
TY  - JOUR
A1  - Steyrleuthner, Robert
A1  - Schubert, Marcel
A1  - Howard, Ian
A1  - Klaumünzer, Bastian
A1  - Schilling, Kristian
A1  - Chen, Zhihua
A1  - Saalfrank, Peter
A1  - Laquai, Frederic
A1  - Facchetti, Antonio
A1  - Neher, Dieter
T1  - Aggregation in a high-mobility n-type low-bandgap copolymer with implications on semicrystalline morphology
JF  - Journal of the American Chemical Society
N2  - 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.
Y1  - 2012
U6  - https://doi.org/10.1021/ja306844f
SN  - 0002-7863
VL  - 134
IS  - 44
SP  - 18303
EP  - 18317
PB  - American Chemical Society
CY  - Washington
ER  -