Elementary processes in ternary solar cells†

Abstract

The insertion of a third component in bulk heterojunction solar cells has led to enhanced power conversion efficiencies (PCEs). However, the rationale beyond the superior performance of ternary solar cells (TSCs) is still a matter of debate and device design is usually based on qualitative considerations. Herein, we present an exhaustive analysis of the kinetics of interfacial charge and energy transfer elementary processes occurring in an archetypal ternary blend, composed of two donors (FG3 and FG4) and one acceptor (Y6). Using molecular dynamics (MD) simulations to generate realistic blend morphologies, coupled with a full quantum mechanical approach to compute reaction rates, we provide insights into the factors contributing to the final PCE of TSCs. Our results indicate that, for the system under study, the presence of two donors allows for more effective solar spectrum coverage, while Förster resonance energy transfer plays a key role in funneling the energy absorbed by FG3 towards a more kinetically efficient FG4:Y6 donor–acceptor pair. Indeed, the FG3:Y6 combination is hampered by slower charge transfer rates, primarily due to energy loss pathways. These findings indicate that even small differences between donor molecules (such as FG3 and FG4) can lead to dramatically different charge transfer kinetics, suggesting that the improved PCE observed in TSCs cannot be easily anticipated through qualitative assessments alone. Instead, device performance is highly sensitive to the intricate interplay of charge and energy transfer processes, highlighting the need for theoretical modeling to accurately predict outcomes. In this respect, we show that our protocol can provide useful elements for a deeper understanding of the physical effects concurring to determine the final PCE of a device, thus enabling a rational design of novel blends for organic solar cells.