Energy Level Alignment in Ternary Organic Solar Cells

Abstract

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, two ternary systems are investigated with one donor (D) and two acceptor materials (A1, A2) including fullerene and nonfullerene acceptors. By varying the ratio between the two acceptors, VOC is found to be gradually tuned between those of the two binary systems, D:A1 and D:A2. To investigate the origin of this change, ultraviolet photoemission spectroscopy (UPS) depth profiling is employed, which is used to estimate the photovoltaic gap in the ternary systems. The 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, the results indicate that the sum of radiative and nonradiative losses in these ternary systems is independent of the blend composition. Finally, the superiority of UPS over X-ray photoemission spectroscopy (XPS) depth profiling is demonstrated in resolving compositional profiles for material combinations with very similar chemical, but dissimilar electronic structures, as common in TOSCs.