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 - Zen, Achmad A1 - Saphiannikova, Marina A1 - Neher, Dieter A1 - Grenzer, Jörg A1 - Grigorian, Souren A. A1 - Pietsch, Ullrich A1 - Asawapirom, Udom A1 - Janietz, Silvia A1 - Scherf, Ullrich A1 - Lieberwirth, Ingo A1 - Wegner, Gerhard T1 - Effect of molecular weight on the structure and crystallinity of poly(3-hexylthiophene) N2 - Recently, two different groups have reported independently that the mobility of field-effect transistors made from regioregular poly(3-hexylthiophene) (P3HT) increases strongly with molecular weight. Two different models were presented: one proposing carrier trapping at grain boundaries and the second putting emphasis on the conformation and packing of the polymer chains in the thin layers for different molecular weights. Here, we present the results of detailed investigations of powders and thin films of deuterated P3HT fractions with different molecular weight. For powder samples, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) were used to investigate the structure and crystallization behavior of the polymers. The GPC investigations show that all weight fractions possess a rather broad molecular weight distribution. DSC measurements reveal a strong decrease of the crystallization temperature and, most important, a significant decrease of the degree of crystallinity with decreasing molecular weight. To study the structure of thin layers in lateral and vertical directions, both transmission electron microscopy (TEM) and X-ray grazing incidence diffraction (GID) were utilized. These methods show that thin layers of the low molecular weight fraction consist of well-defined crystalline domains embedded in a disordered matrix. We propose that the transport properties of layers prepared from fractions of poly(3-hexylthiophene) with different molecular weight are largely determined by the crystallinity of the samples and not by the perfection of the packing of the chains in the individual crystallites Y1 - 2006 UR - http://pubs.acs.org/doi/full/10.1021/ma0521349 U6 - https://doi.org/10.1021/Ma0521349 ER - TY - JOUR A1 - Janietz, Silvia A1 - Schulz, Burkhard T1 - Electrochemical investigation on poly(arylene-1,3,4-oxadiazole)s Y1 - 1996 ER - TY - JOUR A1 - Schulz, Burkhard A1 - Janietz, Silvia T1 - Electrochemical studies on thin films of poly(arylene 1,3,4-oxadiazole)s Y1 - 1994 ER - TY - JOUR A1 - Janietz, Silvia A1 - Schulz, Burkhard T1 - Electrochemical studies on thin films of poly(arylene-1,3,4-oxadiazole)s Y1 - 1994 ER - TY - JOUR A1 - Janietz, Silvia A1 - Schulz, Burkhard T1 - Elektrochemische Untersuchungen an aromatischen Poly(1,3,4-oxadiazol)en Y1 - 1995 ER - TY - JOUR A1 - Janietz, Silvia A1 - Freydank, A. T1 - Elektrochemische Untersuchungen aromatischer 1,3,4-Oxadiazole in Lösungen und als dünne Schichten Y1 - 1995 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 - 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 -