Dithienogermole-Based Nonfullerene Acceptors: Roles of the Side-Chains' Direction and Development of Green-Tinted Efficient Semitransparent Organic Solar Cells

Inspired by the encouraging properties of Ge-fused heterocyclic dithienogermole (DTG) in optoelectronic applications, we here report two narrow-bandgap acceptor-donor'-donor-donor'-acceptor (A-D'-D-D'-A)-type isomeric nonfullerene acceptors based on DTG (DTG-IW with inward-facing side chains vs DTG-OW with outward-facing side chains) for use in organic solar cells (OSCs). The introduction of the inward-facing side chains into the backbone results in extremely confined face-on crystallites in the solid state, as verified by grazing-incidence wide-angle X-ray scattering measurements. This result is attributed mainly to a better power conversion efficiency (PCE) of 9.16% in the OSC based on a blend of DTG-IW with a narrow-bandgap PTB7-Th donor polymer, as compared with the corresponding DTG-OW-based one. Furthermore, the appealing feature of the blend mixing of the narrow-bandgap donor and acceptor pair is that it enables the construction of a green-tinted efficient semitransparent OSC with a PCE of 6.19% and transmittance of 50.4% in the green wavelength region through incorporation of the recently formulated semitransparent Ag/Sb2O3/Ag electrode. Overall, in addition to providing useful perspectives into the side-chain engineering of nonfullerene acceptors, this work highlights that OSC based on the A-D'-D-D'-A-type DTG is a promising narrow-bandgap acceptor for further improvement of the performance of semitransparent OSCs.