Proceedings of nanoGe September Meeting 2015 (NFM15)
Publication date: 8th June 2015
A complex interplay between structure, morphology, and photophysics determines the efficiency of bulk heterojunction organic photovoltaic devices. As an example we show in the first part, that PBDTTPD polymers substituted with linear side-chains exhibit efficiencies of only 4%, while the same polymer backbone substituted with a combination of branched and linear side chains yields 8% PCE. We demonstrate by transient absorption spectroscopy that while the exciton dissociation and ultrafast charge generation steps are not strongly affected by the side chain modifications, the polymer with branched side chains exhibits a decreased rate of non-geminate recombination and also a lower fraction of sub-ns geminate recombination, which can be explained by the difference in aggregation in the bulk polymer domains and the specific interaction of the polymer backbone with the fullerene molecules both tuned by the side chain pattern.[1] In the second part of the talk, the observation of triplet state formation in OPV blends is discussed, which has recently been identified as a potential loss channel in low-bandgap:fullerene solar cells. We show spectroscopic experiments performed on the prototypic low-bandgap donor-acceptor copolymers PCPDTBT and its silicon-substituted analogue PSBTBT that demonstrate less triplet state formation by non-geminate recombination in the latter, indicating that the solid-state morphology and interfacial structure of PSBTBT:PC70BM blends reduce triplet state formation.[2] These studies have recently been extended to DPP-based donor polymers [3] and PBDTTT-C:fullerene blends, two systems for which we observe almost quantitative triplet state formation following exciton dissociation and charge generation.[4]
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