Quantifying and Understanding Voltage Losses Due to Nonradiative Recombination in Bulk Heterojunction Organic Solar Cells with Low Energetic Offsets

Open-circuit voltage (V-OC) losses in organic photovoltaics (OPVs) inhibit devices from reaching V-OC values comparable to the bandgap of the donor-acceptor blend. Specifically, nonradiative recombination losses ( increment V-nr) are much greater in OPVs than in silicon or perovskite solar cells, yet the origins of this are not fully understood. To understand what makes a system have high or low loss, an investigation of the nonradiative recombination losses in a total of nine blend systems is carried out. An apparent relationship is observed between the relative domain purity of six blends and the degree of nonradiative recombination loss, where films exhibiting relatively less pure domains show lower increment V-nr than films with higher domain purity. Additionally, it is shown that when paired with a fullerene acceptor, polymer donors which have bulky backbone units to inhibit close pi-pi stacking exhibit lower nonradiative recombination losses than in blends where the polymer can pack more closely. This work reports a strategy that ensures increment V-nr can be measured accurately and reports key observations on the relationship between increment V-nr and properties of the donor/acceptor interface.