Modeling of degradation in normal and inverted OSC devices

Abstract

Degradation of organic solar cells (OSC) remains a problem which has prevented its commercial exploitation. While there are several experimental studies pointing towards different reasons behind the atmospheric degradation of these devices, yet a microscopic understanding is lacking. Here, we present a combined experimental and modeling study to understand the degradation in the electrical characteristics of OSC in normal and inverted architectures, further correlated with the interfacial degradation. Our investigations show that degradation of current density – voltage (J-V) characteristics of normal devices is extremely fast i.e. 50% in 9 h as against 21 days for inverted devices. Interestingly, degradation in normal devices is driven primarily by a decrease in short circuit current density (Jsc) without appearance of S-shape in J-V characteristics while degraded inverted devices exhibit S-shaped J-V characteristics. Modeling of J-V data along with time dependent capacitance-voltage (C-V) analysis suggests that the degradation in normal OSC devices can be modeled using degradation induced reduced effective area theory supporting the formation of Al2O3 pockets at the cathode-active layer interface which grow exponentially with storage time eventually forming a continuous layer. In contrast, inverted devices degrade much slowly, via deterioration of hole transporting MoO3 layer resulting in increased anode barrier height and reduced surface recombination velocity (Sp) at anode causing a S shaped J-V curve due to decreased carrier extraction rate. The study clearly highlights the differences between the way two differences architectures and elucidates the underlying reasons.