A PbI2−xClx seed layer for obtaining efficient planar-heterojunction perovskite solar cells via an interdiffusion process†
Despite the previous reports on the fabrication of CH3NH3PbI3−xClx films via sequential deposition, the positioning and formation of PbI2 in MAPbI3−xClx 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 PbI2−xClx 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 PbI2−xClx 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 PbI2−xClx-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.