Isoelectronic Cations: A Toolset to Study Hydrogen-Halide Interactions in Lead Halide Perovskite Excluding Other Variables

Cations in lead halide perovskite (LHP), though at first considered to merely charge balance the PbX₆ octahedra, have been revealed to have significant impacts on device efficiency and stability. Cs ion, methylammonium, and formamidinium are the common cations in LHP materials. These cations have different sizes and varied numbers of hydrogen atoms interacting with the surrounding PbX₆ octahedra, and so far there are no experimental studies that investigate how LHP properties in correlate with hydrogen-halide interaction independently. To bridge this knowledge gap and provide more fundamental understanding of LHP materials, we designed two ligands based on guanidinium (C(NH₂)₃⁺) and uronium (C(NH₂)(OH)⁺), and studied their differences in efficacy and mechanism in LHP thin-film passivation. Guanidinium and uronium are a pair of isoelectronic cations and therefore of nearly identical sizes, while guanidinium has one more hydrogen atom interacting with the PbX₆ octahedra. In addition, these two ligands share the same counter ion (methanesulfonic group) and the same alkyl chain attached to cations (-C₁₂H₂₅). These deliberate designs allowed us to exclusively investigate the effects of hydrogen-halide interaction in LHP without other inferences.<br/><br/>After passivating wide-bandgap LHP films ((Cs_0.17,FA_0.83)Pb(I_0.75Br_0.25)₃) with the alkylated guanidinium ligand (GA), we observed a 12-time increase in photoluminescence (PL) intensity along with the PL lifetime improved from 134 ns to 337 ns compared to the untreated film. On the other hand, the passivation efficacy of the alkylated uronium ligand (UA) is less pronounced, with a 5-time enhancement in the PL intensity and a slight improvement of the PL lifetime (134 ns to 176 ns). In solar cell devices, the GA increased fill factors of the solar cells from 72.37 % to ~80 % and power conversion efficiency from 15.48 % to 17.71% compared to the untreated devices, while the UA passivated devices showed little improvement. These drastically different results indicate that guanidinium and uronium adopt different mechanisms in LHP thin-film passivation despite their nearly identical sizes.<br/><br/>X-ray diffraction and atomic force microscopy suggested a formation of a crystalline guanidinium-rich 3D perovskite phase at the original grain boundary after GA treatment, supported by the (110) diffraction peak shifting to lower angles and an increase in Young’s modulus in the GA-treated film. Compared to the original amorphous phase at the grain boundary, the newly formed crystalline phase significantly reduces surface trap density and facilitates charge extraction, leading to a boost in fill factors. On the other hand, the lower passivation efficacy of the UA suggested UA passivates LHP thin films via filling cation vacancies at the amorphous grain boundary. In addition, we observed a slightly reduced open circuit voltage in the GA-treated device, indicating a reduced bandgap of the guanidinium-rich phase, which is consistent with previous theoretical simulations that show hydrogen-halide interactions reduce LHP bandgaps through enhancing spin-orbit interaction and modulating octahedral tilting. The interaction between hydrogen and the PbX₆ octahedral was further confirmed by nuclear magnetic resonance, as we observed all hydrogen signals from guanidinium broaden after filling in the cation sites in the LHP lattice.<br/><br/>In conclusion, by using a pair of isoelectronic organic cations, we excluded other variables to exclusively study how the hydrogen-halide interactions between cations and PbX₆ octahedra affect the surface passivation and the optoelectronic properties of LHP materials, meanwhile providing a novel approach for LHP passivation via surface recrystallization. This project also underlines the value of in-depth interdisciplinary collaborations between the organic and inorganic communities in unfolding the fundamental aspects of LHP materials.