Charge recombination in polythiophene: non-fullerene acceptor solar cells with IE offsets exceeding 1 eV

In organic solar cells the energetic landscape of the donor–acceptor heterojunction determines the efficiency of charge generation and charge recombination processes, and thereby the device performance. Here, we present a study on a series of 15 donor–acceptor bulk heterojunctions (BHJs) consisting of either the donor polymer poly(3-hexylthiophene) (P3HT) or poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (pBTTT-C14) and selected non-fullerene acceptors (NFAs), spanning a wide range of interfacial energetics. We demonstrate that the internal quantum efficiency (IQE) is limited by geminate and non-geminate recombination processes and, importantly, decreases with the energy difference between the donor's ionization energy (IE) and the acceptor's electron affinity (EA), in other words, the diagonal bandgap, specifically if less than 1 eV, regardless of the interfacial IE offset. The dependence of charge recombination on the diagonal bandgap can be explained in the framework of the energy gap law. Our results provide further insight into the importance and impact of interfacial energetics in donor:NFA blends with large IE offsets.