Linking morphology and performance of organic solar cells based on decacyclene triimide acceptors

Bulk heterojunction photovoltaic devices consisting of a novel nonfullerene acceptor based on a decacyclene triimide core and the common polymer donor poly(3-hexylthiophene) exhibit good power conversion efficiency (similar to 1.6%) as-cast. However, thermal annealing results in a drastic decrease in both the efficiency and the electron current. Polarized soft Xray spectroscopy and grazing incidence Xray scattering reveal that thermal annealing results in a reorientation of the acceptor molecules to an edge-on orientation with their pp stacking direction predominantly inplane with the substrate and overall hexagonal packing among planar columnar structures. This packing motif greatly hinders vertical electron transport in bulk heterojunction films. Furthermore, in situ Xray scattering studies show that this critical reorientation occurs even at relatively low temperatures (similar to 60 degrees C). These results are some of the first to highlight differences between the morphology of nonfullerene and fullerene-based bulk heterojunctions and critical parameters that must be controlled when designing future high-performance nonfullerene acceptor molecules.