TY - JOUR A1 - Gorenflot, Julien A1 - Paulke, Andreas A1 - Piersimoni, Fortunato A1 - Wolf, Jannic A1 - Kan, Zhipeng A1 - Cruciani, Federico A1 - El Labban, Abdulrahman A1 - Neher, Dieter A1 - Beaujuge, Pierre M. A1 - Laquai, Frederic T1 - From recombination dynamics to device performance BT - quantifying the efficiency of exciton dissociation, charge separation, and extraction in bulk heterojunction solar cells with Fluorine-Substituted polymer donors JF - dvanced energy materials N2 - An original set of experimental and modeling tools is used to quantify the yield of each of the physical processes leading to photocurrent generation in organic bulk heterojunction solar cells, enabling evaluation of materials and processing condition beyond the trivial comparison of device performances. Transient absorption spectroscopy, “the” technique to monitor all intermediate states over the entire relevant timescale, is combined with time-delayed collection field experiments, transfer matrix simulations, spectral deconvolution, and parametrization of the charge carrier recombination by a two-pool model, allowing quantification of densities of excitons and charges and extrapolation of their kinetics to device-relevant conditions. Photon absorption, charge transfer, charge separation, and charge extraction are all quantified for two recently developed wide-bandgap donor polymers: poly(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-3,4-difluorothiophene) (PBDT[2F]T) and its nonfluorinated counterpart poly(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-3,4-thiophene) (PBDT[2H]T) combined with PC71BM in bulk heterojunctions. The product of these yields is shown to agree well with the devices' external quantum efficiency. This methodology elucidates in the specific case studied here the origin of improved photocurrents obtained when using PBDT[2F]T instead of PBDT[2H]T as well as upon using solvent additives. Furthermore, a higher charge transfer (CT)-state energy is shown to lead to significantly lower energy losses (resulting in higher VOC) during charge generation compared to P3HT:PCBM. KW - bulk heterojunction KW - charge generation yield KW - charge recombination yield KW - polymer solar cells KW - transient absorption spectroscopy Y1 - 2018 U6 - https://doi.org/10.1002/aenm.201701678 SN - 1614-6832 SN - 1614-6840 VL - 8 IS - 4 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Sini, Gjergji A1 - Schubert, Marcel A1 - Risko, Chad A1 - Roland, Steffen A1 - Lee, Olivia P. A1 - Chen, Zhihua A1 - Richter, Thomas V. A1 - Dolfen, Daniel A1 - Coropceanu, Veaceslav A1 - Ludwigs, Sabine A1 - Scherf, Ullrich A1 - Facchetti, Antonio A1 - Frechet, Jean M. J. A1 - Neher, Dieter T1 - On the Molecular Origin of Charge Separation at the Donor-Acceptor Interface JF - Advanced energy materials N2 - 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. KW - donor-acceptor interfaces KW - energy gradients KW - geometrical deformations KW - nonfullerene acceptors KW - organic photovoltaics KW - photocurrent generation KW - polymer solar cells Y1 - 2018 U6 - https://doi.org/10.1002/aenm.201702232 SN - 1614-6832 SN - 1614-6840 VL - 8 IS - 12 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Samson, Stephanie A1 - Rech, Jeromy A1 - Perdigon-Toro, Lorena A1 - Peng, Zhengxing A1 - Shoaee, Safa A1 - Ade, Harald A1 - Neher, Dieter A1 - Stolterfoht, Martin A1 - You, Wei T1 - Organic solar cells with large insensitivity to donor polymer molar mass across all acceptor classes JF - ACS applied polymer materials N2 - Donor polymer number-average molar mass (M-n) has long been known to influence organic photovoltaic (OPV) performance via changes in both the polymer properties and the resulting bulk heterojunction morphology. The exact nature of these M-n effects varies from system to system, although there is generally some intermediate M-n that results in optimal performance. Interestingly, our earlier work with the difluorobenzotriazole (FTAZ)-based donor polymer, paired with either N2200 (polymer acceptor) or PC61BM (fullerene acceptor), PcBm demonstrated <10% variation in power conversion efficiency and a consistent morphology over a large span of M-n (30 kg/mol to over 100 kg/mol). Would such insensitivity to polymer M-n still hold true when prevailing small molecular acceptors were used with FTAZ? To answer this question, we explored the impact of FTAZ on OPVs with ITIC, a high-performance small-molecule fused-ring electron acceptor (FREA). By probing the photovoltaic characteristics of the resulting OPVs, we show that a similar FTAZ mn insensitivity is also found in the FTAZ:ITIC system. This study highlights a single-donor polymer which, when paired with an archetypal fullerene, polymer, and FREA, results in systems that are largely insensitive to donor M. Our results may have implications in polymer batch-to-batch reproducibility, in particular, relaxing the need for tight M-n control during synthesis. KW - polymer solar cells KW - conjugated polymers KW - fullerenes KW - fluorination KW - molecular weight KW - non-fullerene acceptors KW - power conversion efficiency Y1 - 2020 U6 - https://doi.org/10.1021/acsapm.0c01041 SN - 2637-6105 VL - 2 IS - 11 SP - 5300 EP - 5308 PB - American Chemical Society CY - Washington ER -