Increasing the Ionization Energy Offset to Increase the Quantum Efficiency in Non-Fullerene Acceptor-Based Organic Solar Cells: How Far Can We Go?

Molecular engineering of organic semiconductors provides a virtually unlimited number of possible structures, yet only a handful of combinations lead to state-of-the-art efficiencies in photovoltaic applications. Thus, design rules that guide material development are needed. One such design principle is that in a bulk heterojunction consisting of an electron donor and lower bandgap acceptor an offset (ΔIE) of at least 0.45 eV is required between both materials ionization energies to overcome energy level bending at the donor–acceptor interface, in turn maximizing the charge separation yield and the cell's internal quantum efficiency. The present work studies energy losses associated with ΔIE and, based on 24 blends, finds that losses are minimal up to a ΔIE of 0.6 eV. Electroluminescence spectroscopy shows that low energy losses are achieved when the charge transfer state energy (E_CT) is similar to the acceptor's optical bandgap (E_g^A). Further ΔIE increase lowers E_CT with respect to E_g^A, thus decreasing V_OC. Within that 0.45–0.6 eV ΔIE sweet range, the fill factor FF, hence the power conversion efficiency, increases only marginally as the FF is often already close to maximal for ΔIE = 0.45 eV. The results are extended to 76 binary and ternary blends.