Highly efficient perovskite-based optoelectronic devices require the preparation of high-quality perovskite thin films. However, detailed understanding and rationalization of the solution-based perovskite processing are still lacking. Detailed process rationalization will lead to reliability and reproducibility in perovskite thin-film preparation, also on a large scale. Thus, here we rationalize the influence of the precursor solution concentration on the formation kinetics, which determines, e.g., the process window during the preparation process.

To rationalize the influence of the precursor solution concentration on the formation kinetics, the series of 0.5 M, 0.8 M, and 1.2 M Cs_0.05FA_0.85MA_0.10PbBr₃ (3CatPbBr₃) solutions is spin-coated. A home-built optical in-situ setup^[1] is utilized to follow the formation process of this series via in-situ UV-vis and PL measurements. A bromide-based perovskite composition is chosen since those form directly from solution without intermediate steps.^[2]

In-situ UV-vis measurements show an increase in the transflectance signal over the entire wavelength range instead of a clear absorption edge expected to form during spin-coating. However, during in-situ PL measurements, a defined PL peak increases fast in intensity while the peak position shifts to a longer wavelength. Over time the PL signal vanishes. For each concentration, the signal onsets correlate in time for both in-situ measurements, and their appearance is delayed increasing the concentration. Thus, complementary in-situ UV-vis and PL measurements reveal a strong concentration dependence of the 3CatPbBr₃ perovskite formation kinetics. Namely, a delayed crystallization onset accompanied by slowed-down crystallite growth is determined for higher concentrated solutions.

In addition, the concentration-dependent chemical characteristics of the solution concentration series are investigated by SAXS, NMR, and UV-Vis measurements. Concentration-dependent changes in the solution chemistry reason those slowed-down formation kinetics. Namely, the colloidal size, their interaction, and the individual chemical surrounding of the Pb²⁺ ion modify with the solution concentration.

Thus, this application example demonstrates the significance and importance of in-situ measurements during film formation. Overall, this connected study investigating the precursor solution chemistry and the corresponding formation kinetics identifies that slight modifications in the precursor solution impact the perovskite formation dramatically. Hence, even for slight variations in the precursor solution, preparation procedures need to be adjusted to achieve high-quality thin films. Thus, for complete process control, each detail influencing the perovskite formation, such as the precursor solution concentration, needs to be understood to fabricate high-quality thin-film by, e.g., aimed induction of the crystallization during an open process window.