Oxygen-Induced Degradation in C-60-Based Organic Solar Cells: Relation Between Film Properties and Device Performance

Abstract

Fullerene-based molecules are the archetypical electron accepting materials for organic photovoltaic devices. A detailed knowledge of the degradation mechanisms that occur in C-60 layers will aid in the development of more stable organic solar cells. Here, the impact of storage in air on the optical and electrical properties of C-60 is studied in thin films and in devices. Atmospheric exposure induces oxygen-trap states that are 0.19 eV below the LUMO of the fullerene C-60. Moreover, oxygen causes a 4-fold decrease of the exciton lifetime in C-60 layers, resulting in a 40% drop of short-circuit current from optimized planar heterojunction solar cells. The presence of oxygen-trap states increases the saturation current of the device, resulting in a 20% loss of open-circuit voltage. Design guidelines are outlined to improve air stability for fullerene-containing devices.