Energy Level Alignment in Ternary Organic Solar Cells

Ternary organic solar cells (TOSC) are currently under intensive investigation, recently reaching a record efficiency of 17.1%. The origin of the device open-circuit voltage (VOC), already a multifaceted issue in binary OSC, is even more complex in TOSCs. Herein, we investigate two ternary systems with one donor (D) and two acceptor materials (A1, A2) including fullerene and non-fullerene acceptors. By varying the ratio between the two acceptors, we find the VOC to be gradually tuned between those of the two binary systems, D:A1 and D:A2. To investigate the origin of this change, we employ ultra-violet photoemission spectroscopy (UPS) depth profiling, which is used to estimate the photovoltaic gap in the ternary systems. Our results reveal an excellent agreement between the estimated photovoltaic gap and the VOC for all mixing ratios, suggesting that the energetic alignment between the blend components varies depending on the ratio D:A1:A2. Furthermore, our results indicate that the sum of radiative and non-radiative losses in these ternary systems is independent of the blend composition. Finally, we demonstrate the superiority of UPS over X-ray photoemission spectroscopy (XPS) depth profiling in resolving compositional profiles for material combinations with very similar chemical, but dissimilar electronic structures, as common in TOSCs.