Identify The Physical Processes occurring in Poly-Small-Molecule Acceptor-Based All-Polymer Bulk Heterojunction Solar Cells

<i>For some fundamental reasons, the performance of all-polymer solar cells is behind the state-of-the-art in small molecule non-fullerene acceptor bulk heterojunction organic solar cells. This work investigates the efficiency-limiting processes in all-polymer solar cells using blends of the common donor polymer PBDB-T and PM6 with two acceptor polymers, namely PYN-BDT and PYN-BDTF. These compounds function as macromolecular absorbers with large optical cross-sections extending to around 900 nm due to their </i><i>π</i><i>-extended </i><i>naphthalene rings. Combining data from steady-state optical spectroscopy and time-resolved photoluminescence, transient absorption, electron spin resonance, and time-delayed collection field experiments provides not only a concise but also quantitative assessment of the losses due to limited photon absorption, geminate and non-geminate charge carrier recombination, field-dependent charge generation, and inefficient carrier extraction. Kinetic parameters obtained by pulsed laser spectroscopy are used to reproduce the experimentally-measured device IV characteristics and indicate that low fill factors originate either from non-geminate recombination competing with charge extraction or from a pronounced field dependence of charge generation, depending on the donor polymer. The methodology presented here is generic and can be used to quantify the loss processes in BHJ OSCs, including both all-polymer and small-molecule NFA systems.</i>