Tailoring of CsPbIBr2 perovskite crystallization via phenylthiourea for stable and efficiency perovskite solar cells*

Inorganic CsPbIBr2 perovskite has been considered as a promising light-absorbing material for solar cells due to its high stability and suitable bandgap. Although the remarkable improvement of CsPbIBr2 PSC has been achieved, the efficiency of this cell is still lower than those of other analogues and far below its theoretical limit. This is mainly due to the serious charge recombination in the as-fabricated CsPbIBr2 cells derived from the poor-quality CsPbIBr2 perovskite film with a large quantity of defects and numerous grain boundaries. Therefore, fabricating high-quality CsPbIBr2 perovskite film is a key factor for the further efficiency improvement of CsPbIBr2 PSCs. Herein, phenylthiourea (PTU) additive is introduced into the CsPbIBr2 precursor to tailor the crystallization of CsPbIBr2 perovskite for fabricating high-quality CsPbIBr2 perovskite. The C=S group of PTU can coordinate with PbBr2 in the precursor owing to the lone-pair electrons on S and the empty orbits of Pb2+. The strong interaction between PTU and the CsPbIBr2 precursor components can form PTU & BULL;Pb & BULL;& BULL;& BULL;Br(I) intermediate phase in the precursor upon PTU introduction. The PTU & BULL;Pb & BULL;& BULL;& BULL;Br(I) intermediate phase can reduce the nucleation rate of perovskite and modulate the perovskite crystal growth because the extra energy is required to break the strong coordination bond in the intermediate phase, resulting in a low crystallization rate of CsPbIBr2 perovskite. Such a retardation of perovskite crystallization is conducive to the formation of high crystallinity perovskite film with smooth surface, large crystal grains, high crystallization, and low density of defect. Meanwhile, the decomposition of PTU during thermal annealing makes the S2- inserted into interstitial of CsPbIBr2 crystal lattice, which greatly enhance the stability of CsPbIBr2 perovskite. The carbon-based PSCs with a normal n-i-p structure of FTO/compact-TiO2 layer/meso-TiO2 layer/perovskite film/carbon layer are fabricated, and their photovoltaic performances are measured under a simulated AM1.5 illumination (100 mW & BULL;cm-2). The PSC based on PTU-CsPbIBr2 perovskite delivers a high power conversion efficiency of 10.09%, which is much higher than that of the control device. This great improvement of photovoltaic performance can be attributed to the largely promoted perovskite quality, which enhances the charge collection and suppresses the charge recombination in the device. In addition, the unencapsulated device preserves 82% of the initial efficiency after being stored under ambient condition for 35 days, suggesting excellent stability. Therefore, this work provides an effective complementary strategy for effectively improving the performance of inorganic PSCs.