- Stabilities of ITO-containing and ITO-free organic solar cells were investigated under simulated AM 1.5 G illumination and under concentrated natural sunlight. In both cases ITO- free devices exhibit high stability, while devices containing ITO show degradation of their photovoltaic performance. The accelerated degradation under concentrated sunlight (of up to 20 suns) in ITO-containing devices was found to be reversible. Dark exposure of degraded samples can partly restore performance. A possible underlying mechanism for such a phenomenon is discussed.