Nongeminate Radiative Recombination and <i>V</i> _oc in Organic Solar Cells Enhanced by a Charge Transporter/Absorber Interface Change

The fluorescence quantum yield (FQY) defines the ultimate open-circuit voltage (<i>V</i> _oc) and efficiency limit in single-junction solar cells. However, enhancing solar cell efficiency by increasing the fluorescence is rarely approached provided in the majority of solar cells, the <i>V</i> _oc is assumed to be essentially limited by a large fraction of nonradiative recombination when compared to the radiative one. Here, we show that in completed PM6:Y6 organic solar cells (OSCs), the FQY is 1 order of magnitude higher than in the blend alone. Such an FQY is further enhanced by 18% through the incorporation of an ultrathin LiF layer between the blend and the ZnO electron transport layer (ETL), which reduces interfacial oxygen molecules acting as electron traps. The FQY enhancement leads to an increase in quasi-Fermi level splitting (QFLS), resulting in a <i>V</i> _oc gain which we measured to be as large as 12 mV. Using the two-diode model, we confirm that this pathway for enhancing QFLS has the potential to be applicable to other single-junction solar cells where radiative recombination is present at the interface between charge selective contacts and the absorber material.