Increasing donor-acceptor spacing for reduced voltage loss in organic solar cells

The high voltage losses (V-loss), originating from inevitable electron-phonon coupling in organic materials, limit the power conversion efficiency of organic solar cells to lower values than that of inorganic or perovskite solar cells. In this work, we demonstrate that this V-loss can in fact be suppressed by controlling the spacing between the donor (D) and the acceptor (A) materials (DA spacing). We show that in typical organic solar cells, the DA spacing is generally too small, being the origin of the too-fast non-radiative decay of charge carriers (k(nr)), and it can be increased by engineering the non-conjugated groups, i.e., alkyl chain spacers in single component DA systems and side chains in high-efficiency bulk-hetero-junction systems. Increasing DA spacing allows us to realize significantly reduced k(nr) and improved device voltage. This points out a new research direction for breaking the performance bottleneck of organic solar cells.