Traps, Tail States and Their Consequences on the Open-circuit Voltage in Organic Solar Cells

Abstract

All that remains to reliably beat the 20% efficiency hurdle in organic solar cells are the relatively low open-circuit voltages (V_OC). A still-needed step toward solving this problem is to shed light on the mechanisms behind these losses, and herein it is focused on understanding the roll-off of V_OC at low temperatures, which has been linked to various detrimental processes. Here, a light intensity sweep is added to the temperature-dependent measurements and the resulting trends are compared with a kinetic analytical model that not only incorporates all previously suggested explanations for the temperature dependence of V_OC, but, importantly, also includes carrier density contributions to the electron and hole mobilities by treating the density of states (DOS) of the active layer as a two-level system. It is found that this description is sufficient to quantitatively explain V_OC roll-off in terms of charges getting trapped in intrinsic tail states of the Gaussian DOS without having to assume the presence of extrinsic traps.