A PbI_2-xCl_x seed layer for obtaining efficient planar-heterojunction perovskite solar cells via an interdiffusion process

Despite the previous reports on the fabrication of CH₃NH₃PbI_3-xCl_x films via sequential deposition, the positioning and formation of PbI₂ in MAPbI_3-xCl_x perovskite films made from the seed layer containing PbI₂ and PbCl₂ in different ratios have not yet been addressed. In this study, the PbI₂ content in a perovskite absorber layer is controlled by changing the PbCl₂ ratio in a PbI_2-xCl_x seed layer. The addition of PbCl₂ in the seed layer facilitates PbI₂ generation and affects the morphology of the perovskite film. By integrating a perovskite absorber via the PbI_2-xCl_x seed-layer into a solar cell, we investigated the effects of the correlation between the chlorine and PbI₂ contents on the device performance through intensity-modulated photocurrent spectroscopy and intensity-modulated photovoltage spectroscopy. Elemental depth profiling analyses confirm that not only was the formed PbI₂ preferentially located near the metal-oxide layer, but residual chlorine was adsorbed at the TiO₂ layer. Our findings demonstrate that the geometric features of the formed PbI₂ affected the perovskite solar cells according to the chlorine content, likely because of the elemental gradient induced by annealing. The PbI_2-xCl_x-derived planar-heterojunction perovskite solar cells exhibited maximum power-conversion efficiencies of 17.56% at reverse scan and 17.21% at forward scan, suppressed current density-voltage hysteresis, and good performance distributions.