Degradation of Co-evaporated Perovskite Thin Films

Methylammonium lead halide perovskites have been developed as highly promising materials to fabricate efficient solar cells in the past few years. We have investigated degradation of co-evaporated CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$ films using x-ray photoelectron spectroscopy (XPS), small angle x-ray diffraction (XRD), and atomic force microscopy (AFM). The CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$ films have an excellent atomic ratio and crystal structure. The films were exposed to oxygen, air and water, respectively. The results indicate that CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$ film is not sensitive to oxygen and dry air. The XPS results of H$_{\mathrm{2}}$O exposure are similar to those of ambient exposure except for the higher intensity of C and O. The XRD results indicate that the perovskite turned to PbI$_{\mathrm{2}}$ after ambient exposure. The AFM measurements reveal that the morphology of the film changed drastically from smooth to rough by ambient exposure. The experiment indicated that H$_{\mathrm{2}}$O plays a dominated role in the degradation of CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$ films. The degradation can be characterized by almost complete removal of N, substantial reduction of I, residual of PbI$_{\mathrm{2}}$, C, O, and I compounds on the surface.