Metal ions diffusion at heterojunction chromium Oxide/CH3NH3PbI3 interface on the stability of perovskite solar cells

The stability of perovskite solar cell (PSC) is still a major challenge although it has recently demonstrated high power conversion efficiencies (PCEs) of over 22%. Here, the PCE degradation resulting from the chromium ions diffusion has been investigated in the inverted structured PSC by X-ray photoelectron spectroscopy. Elemental depth profiles indicate that chromium ions diffuse at CH3NH3PbI3/CrOx interface to change its p-type semiconducting properties with the content alteration of chromium in CrOx film, resulting in the instability of CrOx based device. With copper doping, the formation of CuCrO2 could suppress the diffusion of metal ions and the formation of hexavalent chromium ions and surface hydrate phase CrO(OH)/Cr(OH)3, and then decrease the chance to react with CH3NH3PbI3 (or its degradation product). Consequently, the stability is improved for the Cu:CrOx-based device. The direct evidence helps us to understand metal ionic diffusion phenomena in hole-transfer layer of PSC, which is very important for the long-term stability and performance of PSC.