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/CrO_x interface to change its p-type semiconducting properties with the content alteration of chromium in CrO_x film, resulting in the instability of CrO_x 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:CrO_x-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.