Investigation of low-bandgap nonfullerene acceptor-based polymer solar cells with very low photovoltage loss

Polymer solar cells (PSCs) have seen great progress in recent years, with power conversion efficiencies of over 15%. However, PSCs suffer from larger energy losses than inorganic and perovskite solar cells, leading to lower open-circuit voltage (V^OC). The main factors that hinder the V^OC improvements include (i) relatively large nonradiative recombination losses and thus low electroluminescence quantum efficiency (EQE^EL) in PSCs and (ii) the existence of a charge transfer state at the interface of donor and acceptor. For efficient charge separation in state-of-the-art PSCs, empirically, the driving force for exciton dissociation is considered to be at least 0.3 eV. The large driving force could lead to large voltage losses and thus hinder the PSC performance. In this study, we report using wide bandgap material PB3T as electron donor and low bandgap material IEICO-4F as electron acceptor for nonfullerene PSCs with very small driving forces, which, however, show a decent maximum external quantum efficiency (EQE) of nearly 40%. Moreover, we demonstrate a nonfullerene PSC with high EQE^EL up to 5.1 × 10^{- 4}, corresponding to very low nonradiative recombination losses of 0.20 eV and overall photovoltage energy losses of 0.46 to 0.52 eV, derived from different bandgap (E^gap) determination methods, which can now be comparable to those in perovskite solar cells and inorganic solar cells.