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Thermal annealing plays multiple roles in fabricating high quality perovskite films. Generally, it might result in large perovskite grains by elevating annealing temperature, but might also lead to decomposition of perovskite. Here, we study the effects of annealing temperature on the coarsening of perovskite grains in a temperature range from 100 to 250 °C, and find that the coarsening rate of the perovskite grain increase significantly with the annealing temperature. Compared with the perovskite films annealed at 100 °C, high quality perovskite films with large columnar grains are obtained by annealing perovskite precursor films at 250 °C for only 10 s. As a result, the power conversion efficiency of best solar cell increased from 12.35% to 16.35% due to its low recombination rate and high efficient charge transportation in solar cells.

Compositional engineering of mixed halides is an effective approach to fabricate highly efficient perovskite solar cell. The mixed halide perovskite of MAPbI3-xClx attracts a great of attentions due to its excellent optoelectronic properties. However, the role of chlorine in MAPbI3-xClx is still under discussion by now. Here we introduce PbCl2 as additives into PbI2/DMF solution to study its effects on the formation of MAPbI3-xClx film, which is fabricated from Lewis acid-base adducts through a modified two step method. We demonstrate that the chloride ions have positive roles in fabrication of high quality perovskite MAPbI3-xClx films, characterized by high crystallization, strong grain orientation, large grain size and lower trap densities. Compared to perovskite solar cells without chloride incorporation, the performance of solar cells based on MAPbI3-xClx improves significantly due to its high carrier mobility, efficient carrier transportation and low recombination in device.

Generally, residual solvent is embedded in perovskite precursor films fabricated from the Lewis adduct method. Most of the research focus on the ligand function of the solvent in forming a solvate complex for fabricating high quality perovskite films. However, little attention has been paid to the latent function of the solvent in the perovskite precursor films during the annealing process due to its fast extravasation at high temperature. Here, we develop a sandwich configuration of substrate/perovskite precursor films/PC61BM to retard the extravasation of solvent during annealing. We find that the restrained solvent induces an obvious solvent-mediated dissolution–recrystallization process, leading to high quality perovskite films with large columnar grains. There is mass transportation from small grains to large grains in the dissolution–recrystallization process, which follows the Ostwald ripening model. Inverted planar solar cells are fabricated on the basis of this annealing method. The photovoltaic ...

The solvent embedded in the intermediate phase is widely observed in the fabrication of perovskite films. The perovskite precursor films obtained from Lewis adducts through molecular exchange contain some residue solvent. It has an intrinsic solvent annealing effect during the annealing process. Here, we pre-deposit a protective layer on the perovskite precursor films to retard the escape of solvent during the annealing process. The restricted solvent strengthens the solvent annealing effect during the formation of perovskite films. As a result, the perovskite quality, including grain size and crystallization, is improved significantly, which leads to efficient charge transportation, low recombination rate, and enhancement of the photovoltaic performance of the corresponding perovskite solar cells.

A two-step method is widely used to fabricate highly efficient perovskite solar cells. In a typical two-step method, CH3NH3PbI3 perovskites are fabricated from PbI2 films through an intercalation reaction between PbI2 and CH3NH3I, which usually have a rough surface and residual PbI2 due to a compact PbI2 structure. Here, we introduce some strong Lewis bases into PbI2/DMF solutions in the first step, which is helpful in controlling the morphology of PbI2 films with a mesoporous structure. The mesopores in the PbI2 films provide not only space for accommodating the volume expansion during the reaction between PbI2 and CH3NH3I, but also channels for CH3NH3I solution to diffuse into the PbI2 films, which helps eliminate the residual PbI2 and the dissolution and recrystallization process. As a result, smooth perovskite films without residual PbI2 are easily obtained. The power conversion efficiency, stability and reproducibility of the perovskite solar cells fabricated from the optimized mesoporous PbI2 films improved simultaneously due to the improvement in perovskite films. This work reveals the key roles of solvents (boiling point, Lewis basicity) in controlling the microstructure of the PbI2 films.

High-quality perovskite films can be fabricated from Lewis acid–base adducts through molecule exchange. Substantial work is needed to fully understand the formation mechanism of the perovskite films, which helps to further improve their quality. Here, we study the formation of CH3NH3PbI3 perovskite films by introducing some dimethylacetamide into the PbI2/N,N-dimethylformamide solution. We reveal that there are three key processes during the formation of perovskite films through the Lewis acid–base adduct approach: molecule intercalation of solvent into the PbI2 lattice, molecule exchange between the solvent and CH3NH3I, and dissolution–recrystallization of the perovskite grains during annealing. The Lewis base solvents play multiple functions in the above processes. The properties of the solvent, including Lewis basicity and boiling point, play key roles in forming smooth perovskite films with large grains. We also provide some rules for choosing Lewis base additives to prepare high-quality perovskite fi...