TY - JOUR A1 - Alqahtani, Obaid A1 - Babics, Maxime A1 - Gorenflot, Julien A1 - Savikhin, Victoria A1 - Ferron, Thomas A1 - Balawi, Ahmed H. A1 - Paulke, Andreas A1 - Kan, Zhipeng A1 - Pope, Michael A1 - Clulow, Andrew J. A1 - Wolf, Jannic A1 - Burn, Paul L. A1 - Gentle, Ian R. A1 - Neher, Dieter A1 - Toney, Michael F. A1 - Laquai, Frederic A1 - Beaujuge, Pierre M. A1 - Collins, Brian A. T1 - Mixed Domains Enhance Charge Generation and Extraction in Bulk-Heterojunction Solar Cells with Small-Molecule Donors JF - Advanced energy materials N2 - The interplay between nanomorphology and efficiency of polymer-fullerene bulk-heterojunction (BHJ) solar cells has been the subject of intense research, but the generality of these concepts for small-molecule (SM) BHJs remains unclear. Here, the relation between performance; charge generation, recombination, and extraction dynamics; and nanomorphology achievable with two SM donors benzo[1,2-b:4,5-b]dithiophene-pyrido[3,4-b]-pyrazine BDT(PPTh2)(2), namely SM1 and SM2, differing by their side-chains, are examined as a function of solution additive composition. The results show that the additive 1,8-diiodooctane acts as a plasticizer in the blends, increases domain size, and promotes ordering/crystallinity. Surprisingly, the system with high domain purity (SM1) exhibits both poor exciton harvesting and severe charge trapping, alleviated only slightly with increased crystallinity. In contrast, the system consisting of mixed domains and lower crystallinity (SM2) shows both excellent exciton harvesting and low charge recombination losses. Importantly, the onset of large, pure crystallites in the latter (SM2) system reduces efficiency, pointing to possible differences in the ideal morphologies for SM-based BHJ solar cells compared with polymer-fullerene devices. In polymer-based systems, tie chains between pure polymer crystals establish a continuous charge transport network, whereas SM-based active layers may in some cases require mixed domains that enable both aggregation and charge percolation to the electrodes. KW - charge transport KW - domain purity KW - microscopy KW - mixed domains KW - organic solar cells KW - photovoltaic devices KW - resonant X-ray scattering KW - small molecules KW - transient spectroscopy Y1 - 2018 U6 - https://doi.org/10.1002/aenm.201702941 SN - 1614-6832 SN - 1614-6840 VL - 8 IS - 19 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Roland, Steffen A1 - Schubert, Marcel A1 - Collins, Brian A. A1 - Kurpiers, Jona A1 - Chen, Zhihua A1 - Facchetti, Antonio A1 - Ade, Harald W. A1 - Neher, Dieter T1 - Fullerene-free polymer solar cells with highly reduced bimolecular recombination and field-independent charge carrier generation JF - The journal of physical chemistry letters N2 - Photogeneration, recombination, and transport of free charge carriers in all-polymer bulk heterojunction solar cells incorporating poly(3-hexylthiophene) (P3HT) as donor and poly([N,N'-bis(2-octyldodecyl)-naphthelene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)) (P(NDI2OD-T2)) as acceptor polymer have been investigated by the use of time delayed collection field (TDCF) and time-of-flight (TOF) measurements. Depending on the preparation procedure used to dry the active layers, these solar cells comprise high fill factors (FFs) of up to 67%. A strongly reduced bimolecular recombination (BMR), as well as a field-independent free charge carrier generation are observed, features that are common to high performance fullerene-based solar cells. Resonant soft X-ray measurements (R-SoXS) and photoluminescence quenching experiments (PQE) reveal that the BMR is related to domain purity. Our results elucidate the similarities of this polymeric acceptor with the superior recombination properties of fullerene acceptors. Y1 - 2014 U6 - https://doi.org/10.1021/jz501506z SN - 1948-7185 VL - 5 IS - 16 SP - 2815 EP - 2822 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Steyrleuthner, Robert A1 - Di Pietro, Riccardo A1 - Collins, Brian A. A1 - Polzer, Frank A1 - Himmelberger, Scott A1 - Schubert, Marcel A1 - Chen, Zhihua A1 - Zhang, Shiming A1 - Salleo, Alberto A1 - Ade, Harald W. A1 - Facchetti, Antonio A1 - Neher, Dieter T1 - The Role of Regioregularity, Crystallinity, and Chain Orientation on Electron Transport in a High-Mobility n-Type Copolymer JF - Journal of the American Chemical Society Y1 - 2014 U6 - https://doi.org/10.1021/ja4118736 SN - 0002-7863 VL - 136 IS - 11 SP - 4245 EP - 4256 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Schubert, Marcel A1 - Collins, Brian A. A1 - Mangold, Hannah A1 - Howard, Ian A. A1 - Schindler, Wolfram A1 - Vandewal, Koen A1 - Roland, Steffen A1 - Behrends, Jan A1 - Kraffert, Felix A1 - Steyrleuthner, Robert A1 - Chen, Zhihua A1 - Fostiropoulos, Konstantinos A1 - Bittl, Robert A1 - Salleo, Alberto A1 - Facchetti, Antonio A1 - Laquai, Frederic A1 - Ade, Harald W. A1 - Neher, Dieter T1 - Correlated donor/acceptor crystal orientation controls photocurrent generation in all-polymer solar cells JF - Advanced functional materials N2 - New polymers with high electron mobilities have spurred research in organic solar cells using polymeric rather than fullerene acceptors due to their potential of increased diversity, stability, and scalability. However, all-polymer solar cells have struggled to keep up with the steadily increasing power conversion efficiency of polymer: fullerene cells. The lack of knowledge about the dominant recombination process as well as the missing concluding picture on the role of the semi-crystalline microstructure of conjugated polymers in the free charge carrier generation process impede a systematic optimization of all-polymer solar cells. These issues are examined by combining structural and photo-physical characterization on a series of poly(3-hexylthiophene) (donor) and P(NDI2OD-T2) (acceptor) blend devices. These experiments reveal that geminate recombination is the major loss channel for photo-excited charge carriers. Advanced X-ray and electron-based studies reveal the effect of chloronaphthalene co-solvent in reducing domain size, altering domain purity, and reorienting the acceptor polymer crystals to be coincident with those of the donor. This reorientation correlates well with the increased photocurrent from these devices. Thus, effi cient split-up of geminate pairs at polymer/polymer interfaces may necessitate correlated donor/acceptor crystal orientation, which represents an additional requirement compared to the isotropic fullerene acceptors. Y1 - 2014 U6 - https://doi.org/10.1002/adfm.201304216 SN - 1616-301X SN - 1616-3028 VL - 24 IS - 26 SP - 4068 EP - 4081 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Kurpiers, Jona A1 - Ferron, Thomas A1 - Roland, Steffen A1 - Jakoby, Marius A1 - Thiede, Tobias A1 - Jaiser, Frank A1 - Albrecht, Steve A1 - Janietz, Silvia A1 - Collins, Brian A. A1 - Howard, Ian A. A1 - Neher, Dieter T1 - Probing the pathways of free charge generation in organic bulk heterojunction solar cells JF - Nature Communications N2 - The fact that organic solar cells perform efficiently despite the low dielectric constant of most photoactive blends initiated a long-standing debate regarding the dominant pathways of free charge formation. Here, we address this issue through the accurate measurement of the activation energy for free charge photogeneration over a wide range of photon energy, using the method of time-delayed collection field. For our prototypical low bandgap polymer:fullerene blends, we find that neither the temperature nor the field dependence of free charge generation depend on the excitation energy, ruling out an appreciable contribution to free charge generation though hot carrier pathways. On the other hand, activation energies are on the order of the room temperature thermal energy for all studied blends. We conclude that charge generation in such devices proceeds through thermalized charge transfer states, and that thermal energy is sufficient to separate most of these states into free charges. Y1 - 2018 U6 - https://doi.org/10.1038/s41467-018-04386-3 SN - 2041-1723 VL - 9 PB - Nature Publ. Group CY - London ER -