Surface States and Interface States: Two Fundamental Sources of Photodegradation in Organic Bulk Heterojunction Devices

In this paper we present a systematic study of evolution of surface and interface states in organic bulk heterojunction devices under prolonged illumination. Identical solar cells were prepared with and without calcium as cathode buffer layer (CBL) to study the effect of anaerobic photodegradation. The device with only Al (without CBL) underwent significant degradation in Voc (~35%) and Isc (~26%) during the initial illumination time of 2000 min. Electrical characterization such as dark current, capacitance spectroscopy, and bias modulated photocurrent measurements suggested that in the absence of any CBL, the surface state density at organic-metal cathode interface increased by ~5× leading to increase in reverse saturation current and consequently significant drop in Voc and I sc. It was experimentally demonstrated that increase in both surface state density (metal-organic interface by ~5×) and interface state density (donor-acceptor interface by ~3×) contributes to the overall increase in reverse saturation current by ~15× times in devices without CBL. Presence of Ca as surface passivating layer greatly reduces the amount of photodegradation under inert atmosphere.