Molecular Design of Three-Dimensional Metal-Free A(NH4)X3 Perovskites for Photovoltaic Applications

The intense research activities on the hybrid organic-inorganic perovskites (HOIPs) have led to the greatly improved light absorbers for solar cells with high power conversion efficiency (PCE). However, it is still challenging to find an alternative lead-free perovskite to replace the organohalide lead perovskites to achieve high PCE. This is because both previous experimental and theoretical investigations have shown that the Pb²⁺ cations play a dominating role in contributing the desirable frontier electronic bands of the HOIPs for light absorbing. Recent advances in the chemical synthesis of three-dimensional (3D) metal-free perovskites, by replacing Pb²⁺ with NH₄ ⁺, have markedly enriched the family of multifunctionalized perovskites (Ye et al., Science2018, 361, 151-155). These metal-free perovskites possess the chemical formula of A(NH₄)X₃, where A is divalent organic cations and X denotes halogen atoms. Without involving transition-metal cations, the metal-free A(NH₄)X₃ perovskites can entail notably different frontier electronic band features from those of the organohalide lead perovskites. Indeed, the valence and conduction bands of A(NH₄)X₃ perovskites are mainly attributed by the halogen atoms and the divalent A²⁺ organic cations, respectively. Importantly, a linear relationship between the bandgaps of A(NH₄)X₃ perovskites and the lowest unoccupied molecular orbital energies of the A²⁺ cations is identified, suggesting that bandgaps can be tailored via molecular design, especially through a chemical modification of the A²⁺ cations. Our comprehensive computational study and molecular design predict a metal-free perovskite, namely, 6-ammonio-1-methyl-5-nitropyrimidin-1-ium-(NH₄)I₃, with a desirable bandgap of ∼1.74 eV and good optical absorption property, both being important requirements for photovoltaic applications. Moreover, the application of strain can further fine-tune the bandgap of this metal-free perovskite. Our proposed design principle not only offers chemical insights into the structure-property relationship of the multifunctional metal-free perovskites but also can facilitate the discovery of highly efficient alternative, lead-free perovskites for potential photovoltaic or optoelectronic applications.