Brominated zinc porphyrin assisted grain boundary defects passivation in carbon-based planar perovskite solar cells

Despite the great promotion of perovskite solar cells, the presence of defect trap states at the surfaces and grain boundaries has a negative influence on photovoltaic functionality, primarily due to the polycrystalline nature of perovskite film formation. Motivated by porphyrin molecules excellent charge mobility and various optical transitions in the visible region, we introduce a simple zinc(II)beta-tetra-bromo-meso-tetra-phenyl-porphyrin as an additive in the perovskite layer to improve the performance and stability of planar perovskite solar cells. The modified electron density distribution of the molecule developed on electronegative Br sites enables the passivation of charged defect states. Its compatible energy levels and high charge mobility lead to better electronhole transport through the perovskite layer and defect passivation in the grain boundaries. Moreover, it also has a positive effect on the perovskite film formation quality. As a result, the efficiency of the best device is up to 18.5 %, a factor of 15 % increase to that of the reference cell with a value of 16.1 %, which is superior in planar device structure with copper indium disulfide (CIS) as a hole transport material and carbon as back contact. Furthermore, enhanced hydrophobicity and crystalline quality improve the stability of devices, and the modified device maintained 96 % of its initial efficiency after 40 days in comparison with the control device with 38 % drop in its performance.