Organic-inorganic lead halide perovskite solar cells (PSCs) have attracted much attention as renewable energy conversion technologies due to their low cost, long exciton diffusion length and superior light absorption. Despite recent breakthroughs in power conversion efficiency (PCE) exceeding 22%, the PSCs still have limitations in terms of their processing temperature, hysteresis, reproducibility and lab-scale processing method. Up to now, most of the high performance PSCs were produced by a one-step anti-solvent dropping method (one-step method) during spin-coating process for uniform perovskite layer.
However, the PSC devices using one-step method indispensably require annealing and also show large deviations of PCE because the film quality depends on anti-solvent dropping time and substrate size. Although the two-step process is potentially more beneficial than one-step method for large area device, it demands long reaction time for conversion of metal halide to perovskite, which is undesirable for mass production. Therefore, developing new processing method for ensuring fast reaction and uniform film is highly imperative.
Here, we report an innovative two-step method controlling the crystallinity and uniformity of metal halide (PbI2) via the treatment of various solvent on top of PbI2 layer. Unlike a typical two-step sequential deposition, the solvent-treated PbI2 is completely converted to perovskite film within 20 seconds after dropping methylammonium iodide (MAI). The PSC devices using our two-step method were not affected by the rate of voltage sweep and showed a deviation of less than 3%. Above all, there is no hysteresis feature.
Furthermore, our universal method is successfully applied to the mixed-cation-halide perovskite (MA1-xFAxPbI3-yBry) as well as methylammonium lead iodide (MAPbI3), achieving above 18% even without any annealing process for each layers. Our novel processing method will be valuable for the commercialization of perovskite electronic applications.