Temperature Dependent Diffusion of DMSO in CH3NH3PbI3 Precursor Films During Layer Formation and Impact on Solar Cells

The temperature dependent out-diffusion of dimethyl sulfoxide (DMSO) from CH3NH3PbI3 precursor layers was investigated by analyzing the S/Pb molar ratio in the layers by high-resolution continuum source absorption spectroscopy (HR-CSAS) and the evolution of the S=O vibrational mode in the layers with infrared spectroscopic ellipsometry (IRSE). The diffusion coefficients were extracted by applying a diffusion model in a homogeneous layer. At 100 degrees C, for example, the diffusion coefficient of DMSO in CH3NH3PbI3 amounted to about 10(-11) cm(2)/s. The diffusion constant was thermally activated by two processes with activation energies of 0.6 and 1.8 eV, respectively. The lower and higher activation energies can be explained by decomposition of DMSO complexes and by the activation of DMSO incorporated in the perovskite lattice structure. A strong influence of the S/Pb molar ratio on the fill factor and its standard deviation was observed for solar cells with CH3NH3PbI3 layers. With regard to the performance of solar cells with high efficiency, it seems that some residual DMSO is useful for the preparation of homogeneous CH3NH3PbI3 layers and for passivation of defect states in the material.