Effect of solvent on the perovskite thin film morphology and crystallinity

Due to their high efficiency and low cost, organic-inorganic hybrid perovskite solar cells are attracting growing interest recently. For the most commonly studied perovskite CH3NH3PbI3, optimization of the morphology and crystallinity of CH3NH3PbI3 thin films can greatly improve the efficiency of perovskite solar cells. A homogenous and uniform perovskite film can prevent direct contact between the hole transport layer and the electron transport layer, and thus can significantly reduce charge recombination. And the high crystallinity perovskite film facilitates fast charge transportation and injection. Various studies have proved that solvent has a critical influence on both the morphology and the crystallinity of perovskite thin films. In this work, we thoroughly studied the influence of the normally used N, N-Dimethylformamide (DMF) and r-butyrolactone (GBL) solvents on perovskite morphology, crystallinity, as well as the solar cells efficiency. When using DMF as the solvent, the efficiency is only 2.8%, while the efficiency of the cell obtained based on GBL can reach 10.1%. SEM and HRTEM are employed to study the morphology and crystallinity of these two kinds of perovskite films. The perovskite film prepared using solvent DMF shows a rough capping layer consisting of strip-like perovskite crystals, and the filling of meso-TiO2 is poor. Compared with DMF, the GBL perovskite film shows a better capping layer structure consisting of large perovskite domains, and the filling of meso-TiO2 is improved as well. This great difference in capping layer morphology and meso-TiO2 filling is one reason for the different performance. Besides morphology, different defect concentrations in these two kinds of perovskite films are another crucial issue. By Combined XRD and UV techniques, the mechanisms how perovskite precipitats from DMF and GBL solutions can be disclosed. In DMF, because of its low spoiling point of 153 ℃, most of DMF solvent volatilize by spin-coating, and an intermediate MOF structure of PbI2: MAI: xDMF is formed. During thermal annealing, the unstable MOF structure breaks down and a large amount of dislocations form in perovskite films, which highly restrict the charge transport. However, the spoil point of GBL (206 ℃) is higher than that of DMF, which makes it hard to be fully volatilized by spin-coating. During the following thermal treatment, the solubility of perovskite is lowered with increasing temperature. So perovskite crystallites precipitate from the GBL first and then gradually grow up with the volatilization of the excess solvent. We finally find that coordination between the solvent and the PbI2 plays a big role on the morphology and the crystallinity of the solution-processed perovskite film, and this is responsible for the difference of the device performance.