Unveiling the Relationship of the Iodoplumbate Species in the Perovskite Precursor and the Resulting Film

The power conversion efficiency (PCE) of solution-processed (e.g., spin coating, blade coating, and spray coating) organic–inorganic halide perovskite (OIHP) solar cells has been drastically improved. Representatively, since the emergence of OIHP photovoltaics in 2012, the PCE of the photovoltaics exceeds 25% nowadays. Despite this considerable progress, systematic research on precursor solution chemistry and its effects on the film morphology and photovoltaic parameters has been limited thus far.<br/>In typical halide perovskite precursor solutions, Group 14 transition metal ions (mainly Pb²⁺ and Sn²⁺) form bonds with other ions or solvent because the d orbitals and s orbitals of transition metals are filled with electrons. Therefore, depending on the position of an anion or a solvent near the empty bond of metal ions, various complexes can be formed in a solution. When lead iodide (PbI₂) is dissolved in the solution, iodide ions (I^–) and solvent (S) coordinate around the lead (Pb²⁺) center, forming iodoplumbate complexes, i.e., PbIS₅⁺, PbI₂S₄, PbI₃S₃^–, PbI₄S₂²^–, PbI₅S³^–, and PbI₆^4–.<br/>Herein, we report on the tracking of changes in chemical species in a MAPbI₃ precursor solution under solar illumination as an imposed energy and investigate the correlation between the iodoplumbate species development in the solution and the corresponding perovskite film formation. According to the solution Raman spectroscopy and UV-vis spectroscopy, the illuminated OIHP precursors display a higher density of high-valent iodoplumbate along with the detachment of dimethylformamide (DMF) from Pb. As DMF contributes to uncontrolled crystal growth and that higher iodoplumbate species result in crystalline perovskite with fewer defects, we analyzed trap density and electrical properties with conductive AFM (c-AFM). We revealed that trap density of illuminated groups was reduced, and electrical conductivity was significantly enhanced.<br/>Subsequently, we compared the photovoltaic parameters of the control and illuminated groups. The reproducibility significantly increased in the illuminated groups with highly concentrated fill factor and increased short circuit voltage (J_SC). Increased photocurrent can also be observed in mixed cation OIHP solar cells, which indicates that analysis of the perovskite precursor chemistry can also contribute to the fabrication of high-quality perovskite thin films in the mixed perovskite system. This discovery shows robust control of perovskite precursor solutions from a simple treatment and suggests that the resulting uniform film may be applicable to various halide perovskite-based devices.