@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{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{LangeBlakesleyFrischetal.2011, author = {Lange, Ilja and Blakesley, James C. and Frisch, Johannes and Vollmer, Antje and Koch, Norbert and Neher, Dieter}, title = {Band bending in conjugated polymer layers}, series = {Physical review letters}, volume = {106}, journal = {Physical review letters}, number = {21}, publisher = {American Physical Society}, address = {College Park}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.106.216402}, pages = {4}, year = {2011}, abstract = {We use the Kelvin probe method to study the energy-level alignment of four conjugated polymers deposited on various electrodes. Band bending is observed in all polymers when the substrate work function exceeds critical values. Through modeling, we show that the band bending is explained by charge transfer from the electrodes into a small density of states that extends several hundred meV into the band gap. The energetic spread of these states is correlated with charge-carrier mobilities, suggesting that the same states also govern charge transport in the bulk of these polymers.}, 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{GhaniOpitzPingeletal.2015, author = {Ghani, Fatemeh and Opitz, Andreas and Pingel, Patrick and Heimel, Georg and Salzmann, Ingo and Frisch, Johannes and Neher, Dieter and Tsami, Argiri and Scherf, Ullrich and Koch, Norbert}, title = {Charge Transfer in and Conductivity of Molecularly Doped Thiophene-Based Copolymers}, series = {Journal of polymer science : B, Polymer physics}, volume = {53}, journal = {Journal of polymer science : B, Polymer physics}, number = {1}, publisher = {Wiley-Blackwell}, address = {Hoboken}, issn = {0887-6266}, doi = {10.1002/polb.23631}, pages = {58 -- 63}, year = {2015}, abstract = {The electrical conductivity of organic semiconductors can be enhanced by orders of magnitude via doping with strong molecular electron acceptors or donors. Ground-state integer charge transfer and charge-transfer complex formation between organic semiconductors and molecular dopants have been suggested as the microscopic mechanisms causing these profound changes in electrical materials properties. Here, we study charge-transfer interactions between the common molecular p-dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane and a systematic series of thiophene-based copolymers by a combination of spectroscopic techniques and electrical measurements. Subtle variations in chemical structure are seen to significantly impact the nature of the charge-transfer species and the efficiency of the doping process, underlining the need for a more detailed understanding of the microscopic doping mechanism in organic semiconductors to reliably guide targeted chemical design.}, 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} }