TY - JOUR A1 - Gehrig, Dominik W. A1 - Roland, Steffen A1 - Howard, Ian A. A1 - Kamm, Valentin A1 - Mangold, Hannah A1 - Neher, Dieter A1 - Laquai, Frederic T1 - Efficiency-limiting processes in low-bandgap polymer:Perylene diimide photovoltaic blends JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - The charge generation and recombination processes following photo-excitation of a low-bandgap polymer:perylene diimide photovoltaic blend are investigated by transient absorption pump-probe spectroscopy covering a dynamic range from femto-to microseconds to get insight into the efficiency-limiting photophysical processes. The several tens of picoseconds, and its efficiency is only half of that in a polymer:fullerene photoinduced electron transfer from the polymer to the perylene acceptor takes up to blend. This reduces the short-circuit current. Time-delayed collection field experiments reveal that the subsequent charge separation is strongly field-dependent, limiting the fill factor and lowering the short-circuit current in polymer:PDI devices. Upon excitation of the acceptor in the low-bandgap polymer blend, the PDI exciton undergoes charge transfer on a time scale of several tens of picoseconds. However, a significant fraction of the charges generated at the interface are quickly lost because of fast geminate recombination. This reduces the short-circuit current even further, leading to a scenario in which only around 2596 of the initial photoexcitations generate free charges that can potentially contribute to the photocurrent. In summary, the key photophysical limitations of perylene diimide as an acceptor in low-bandgap polymer blends appear at the interface between the materials, with the kinetics of both charge generation and separation inhibited as compared to that of fullerenes. Y1 - 2014 U6 - https://doi.org/10.1021/jp503366m SN - 1932-7447 VL - 118 IS - 35 SP - 20077 EP - 20085 PB - American Chemical Society CY - Washington 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 - 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 - 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 -