Effect of Solvent Polarity on Antisolvent Bathing of Halide Perovskite Film for Large-Scale Solar Cells

Perovskite solar cells (PSCs) have achieved comparable lab efficiency to c-Si solar cells but still elude commercialization due to non-scalable process. Spin coating method with antisolvent dripping for perovskite layer deposition works well on small scale but loses uniformity when scaled up. An alternative approach for deposition of uniform perovskite layer is inkjet printing (IJP). However, this approach needs an extra step to remove the solvents from as-deposited film. Most IJP studies of PSCs have relied on vacuum flashing (or vacuum quenching) for solvent extraction and intermediate phase formation.

In this work we examined a method called “antisolvent bathing” that is compatible with a general wet chemical process of the perovskite solar cells and does not require isolating the films to a high vacuum level. The facile “antisolvent” bathing rapidly removes solvents from a wet film and induces the formation of an intermediate phase. The “bathing” refers to the process of submerging the as-deposited perovskite films in an antisolvent bath. The antisolvent replaces original solvents in the film through diffusion without dissolving the perovskite solutes. In contrast to antisolvent dripping as a part of the spin coating step, the antisolvent bathing is simple and compatible to a role-to-role process which is suitable for the manufacturing of large-scale PSCs. The quality of final perovskite film depends on the choice of solvent (polarity), time of bathing and temperature of antisolvent.

We systematically studied the effect of the polarity difference between the solvent and the antisolvent on the crystallization and grain growth of the film. The use of low polarity antisolvent resulted in a well crystallized perovskite film without any detrimental second phases and pinholes. We attribute this enhanced crystallinity and microstructure to the large difference in polarity of antisolvent and solvents. During the antisolvent bathing, the perovskite solutes and DMSO precipitate to form an intermediate phase (IMP, sometimes expressed as MAI-PbI₂-DMSO) that converted to pristine MAPbI₃ by thermal annealing. We were able to isolate the IMP phase and observe the peaks through XRD. Longer antisolvent bathing time or antisolvents with high polarity (less polarity difference between antisolvent and solvent) resulted in direct conversion of precursor materials to the perovskite phase without going through the IMP phase. Higher power conversion efficiency (PCE) was achieved when an IMP mediated the crystallization into the perovskite phase through thermal annealing. Further we studied the role of bath temperature during bathing process. We decreased the temperature of antisolvent by using a cold DE bath. This cold bath reduced the diffusion of antisolvent and solvent across the film and indirectly reduced the nucleation rate. This results in the formation of large grains from fewer nucleation sites. XRD resulted showed improved crystallization with I₁₁₀/I₃₁₀ peak ratio increasing from 5.09 for room temperature bathing to 9.12 for cold bathing.

In this presentation, detailed crystallization mechanisms and their effect on the performance on the solar cells will be reported in connection with different IJP techniques.