Key role of residual lead iodide in two-step processed perovskite layer for high performance perovskite solar cells

The delicate control of the crystallization behavior of the perovskite layer is critical for achieving high power conversion efficiency (PCE) and longer-term stability of perovskite solar cells (PSCs). Residual lead iodide (PbI2) was usually evitable in the two-step processed perovskite film due to incomplete reaction; however, its exact effects on the perovskite layer and resulting PSCs were not well elucidated so far. Here, the lead halide perovskite films were grown by the two-step process, where the residual PbI2 can be fine controlled by the concentration of organic salt solution. Results indicated that the properties of the perovskite layer may be varied greatly depending on the amount of residual PbI2, and a series of beneficial effects for PSCs can be achieved via suitable management of residue PbI2, i.e., prolonged carrier diffusion capacity, reduced defect density, inhibited ion migration, and interfacial charge recombination. Accordingly, an impressive PCE of 22.4% was achieved from the PSCs with optimized PbI2. Furthermore, the PSCs also displayed fairly good operational stability and retained 68% of its initial PCE after 400 h (at continuous irradiation under 1-sun illumination). This work may inspire solutions for further enhancing the performance of PSCs via fine controlled residual PbI2 in the perovskite layer.