Origin of the S-Shape upon Aging in Standard Organic Solar Cells with Zinc Oxide as Transport Layer

Abstract

In standard organic solar cells, an electron transporting layer (ETL) is often inserted between the active layer and the cathode in order to achieve good performances. In this work, a zinc oxide (ZnO) ETL is used with an aluminum cathode, yielding functional fresh cells. However, upon storage in the dark and inert atmosphere, an S-shape quickly appears in the J–V curves, leading to a sharp decline of the solar conversion efficiency. By building cells with various ETL sequences, we show that the S-shape stems from a modification of the ZnO/Al interface. Further experiments using Kelvin probe force microscopy (KPFM) and X-ray photoelectron spectroscopy (XPS) suggest that the source of this phenomenon is the appearance of an aluminum oxide layer at the ZnO/Al interface, which could result from a red–ox reaction between the two materials.