Safa Shoaee1
University of Potsdam1
Organic solar cells (OSCs) have rapidly advanced, reaching state-of-the-art efficiencies of above 19 % for single-junction solar cells [1]. Yet, these cells suffer from non-radiative recombination, which limits their FF and VOC. It is generally believed that free charge carrier recombination in OSCs proceeds primarily via reformation and decay of charge transfer (CT) states, involving radiative and non-radiative pathways. The latter typically dominates the decay rate and is, therefore, responsible for the VOC loss. The situation becomes more complex when the energy of the CT state, ECT, approaches the energy of the lowest singlet exciton, the local exciton of the component dominating the optical gap. In this case, reformation of the singlet exciton from the CT state becomes more likely, documented e.g. by an increased singlet exciton emission intensity in electroluminescence (EL) [2]. As singlet excitons are generally more emissive than CT states, this causes a significant reduction of the non-radiative voltage loss. It has, therefore, been proposed that OSCs benefit from a strongly-emitting singlet exciton [3]. In contrast, we have recently shown for the prototypical PM6:Y6 blend, that predominant emission from the S_1 state does not automatically mean that free charge recombination proceeds through this state [4]. Understanding the interplay between these two states and how they determine the voltage losses in relation to ΔE(S1-CT) is therefore of great interest.<br/>We combine PM6 with three different acceptors which have similarly low offsets while leaving the S1 energy of the polymer unaffected. We show that the emission of the blends becomes dominated by the S1 of the acceptor in all cases, but that the CT state properties determine the non-radiative processes and the VOC in all devices. Importantly, the high oscillator strength of the singlet exciton reduces the non-radiative voltage loss not because it is more emissive, but because its higher absorption as compared to the CT state lowers the radiative VOC limit. We suggest that it’s only for very low CT state oscillator strengths and a fairly high radiative efficiency of the CT state that the VOC benefits from the radiative properties of the singlet excitons.