Concentration gradient-controlled growth of large-grain CH3NH3PbI3 films and enhanced photovoltaic performance of solar cells under ambient conditions

Abstract

We report a new method to prepare high-quality CH₃NH₃PbI₃ perovskite films under ambient conditions via two-step concentration gradient-controlled reactions. X-ray diffraction and scanning electron microscopy were used to determine their degree of crystallization and microstructures. The results showed that the reactions between CH₃NH₃I solutions of different concentrations and PbI₂ films yielded perovskite films with various degrees of crystallization and surface morphologies. The first-step reaction between a low concentration CH₃NH₃I solution (6 mg ml⁻¹) and PbI₂ films resulted in the formation of large-scale perovskite grains with some PbI₂ residue. In addition, the subsequent second-step reaction with a high concentration MAI solution (30 mg ml⁻¹) achieved good crystallization and full coverage of the perovskite films on the substrates. The average grain size of the CH₃NH₃PbI₃ perovskite films prepared from this concentration gradient method reached 1.4 μm, and they also showed reduced overall grain boundaries. Enhanced spectral absorption in the long-wavelength region was also observed for the large-grain perovskite films due to the grain size-dependent light scattering. Moreover, the comparison between AM1.5G sunlight and the spectral absorption indicated that it might benefit the efficient power conversion of solar cells. Even though processed under ambient conditions with a high humidity of ca. 75%, the solar cells fabricated from these films exhibited enhanced photovoltaic performance compared with those constructed using the single solution method, reaching a power conversion efficiency of 11.48%.