Navigating the relationship between voltage losses and efficiency in organic solar cells

Abstract

In recent years, organic solar cells (OSCs) have made significant progress, with power conversion efficiencies over 20 %, mainly due to advances in non-fullerene acceptors. Nevertheless, high voltage losses— especially those caused by non-radiative recombination — remain a major obstacle to further efficiency improvements. In contrast to traditional fullerene-based OSCs, state-of-the-art non-fullerene OSCs enable efficient charge generation and extraction even at small energy offsets (<0.3 eV) between the HOMO levels of the donor and the acceptor materials (ΔHOMO). However, achieving low voltage loss while maintaining high fill factor and short-circuit current (Jsc) is a major challenge due to inherent trade-offs. In this study, we systematically investigate the relationship between voltage losses and the other two parameters in nine non-fullerene OSC devices with ΔHOMO in the range of about 0–0.5 eV. Both radiative and non-radiative voltage losses show a clear increase with ΔHOMO. However, no general correlation was found between the voltage loss, Jsc and FF. Only poorly performing IDTBR-based devices show clear increase of Jsc with ΔHOMO, which, according to the transient absorption data, is caused by inefficient charge generation at low ΔHOMO attributable to high exciton binding energy or inferior molecule packing. At the same time, the opposite trend is observed for well-performing blends. These features suggest that variations of other blend parameters rather ΔHOMO cause variations of Jsc and FF overwhelming dependence on ΔHOMO.