Charge Generation and Recombination Pathways in All‐Polymer Bulk Heterojunction Solar Cells

ABSTRACT All‐polymer solar cells lag behind state‐of‐the‐art small‐molecule non‐fullerene acceptor (NFA)‐based bulk heterojunction (BHJ) organic solar cells (OSCs) in terms of power conversion efficiency. Here, the efficiency‐limiting processes in all‐polymer solar cells using blends of the donor polymer PBDB‐T or PBDB‐T‐2F (PM6) and either of two recently developed acceptor polymers, coded PYN‐BDT and PYN‐BDTF, are investigated. The acceptor polymers exhibit strong light absorption up to 900 nm due to π‐extended naphthalene moieties. Combining results from steady‐state optical spectroscopies, as well as time‐resolved photoluminescence, transient absorption, photoluminescence detected magnetic resonance, and time‐delayed collection field experiments, provides a concise and quantitative evaluation of loss channels. Kinetic parameters and process yields obtained by pulsed laser spectroscopy are shown to reproduce the experimentally measured device current‐voltage characteristics, indicating that the moderate fill factors are caused by non‐geminate recombination competing with charge extraction, as well as field‐dependent charge generation, the impact of either changing with the acceptor polymer. The methodology described here is generally applicable to quantify loss processes in BHJ OSCs, not only in all‐polymer, but also in small‐molecule NFA systems.