Improving the stability of hybrid perovskite FAPbI3 by forming 3D/2D interfaces with organic spacers.

Interfaces composed of three-dimensional (3D) and 2D organic-inorganic hybrid formamidinium lead iodide (FAPbI3) linked by organic spacers (OSs) are studied using first-principles calculations. The OS cations with aromatic rings, like phenylethylammonium and anilinium (AN), are found to be more favourable for enhancing the stability of the 3D/2D interface than butylammonium with aliphatic chains. The AN-based interface shows the highest resistance to penetration of water molecules.

Enhancing the Photocatalytic Hydrogen Evolution Performance of the CsPbI3/MoS2 Heterostructure with Interfacial Defect Engineering.

Developing highly efficient photocatalysts for the hydrogen evolution reaction (HER) by solar-driven water splitting is a great challenge. Here, we study the atomistic origin of interface properties and the HER performance of all-inorganic iodide perovskite ß-CsPbI3/2H-MoS2 heterostructures with interfacial vacancy defects using first-principles calculations. Both CsI/MoS2 and PbI2/MoS2 heterostructures have strong binding and dipole moment, which are enhanced by interfacial iodine vacancies (VI). Because of the nature of type II heterojunctions, photogenerated electrons on the CsPbI3 side are promptly transferred to the MoS2 side where HER occurs, and sulfur vacancies (VS) spoil this process, acting as surface traps. The formation energies of various defects are calculated by applying atomistic thermodynamics, identifying the growth conditions for promoting VI and suppressing VS formation. The HER performance is enhanced by forming interfaces with lower ΔGH values for hydrogen adsorption on the MoS2 side, suggesting PbI2/MoS2 with VI to be the most promising photocatalyst.

First-principles study on the elastic, electronic and optical properties of all-inorganic halide perovskite solid solutions of CsPb(Br1-x Cl x )3 within the virtual crystal approximation.

All-inorganic halide perovskites have drawn significant attention for optoelectronic applications such as solar cells and light-emitting diodes due to their excellent optoelectronic properties and high stabilities. In this work, we report a systematic study on the material properties of all-inorganic bromide and chloride perovskite solid solutions, CsPb(Br1-x Cl x )3, varying the Cl content x from 0 to 1 with an interval of 0.1 by applying the first-principles method within the virtual crystal approximation. The lattice constants of the cubic phase are shown to follow the linear function of mixing ratio x, verifying that Vegard's law is satisfied and the pseudopotentials of the virtual atoms are reliable. We calculate the band structures with the HSE06 hybrid functional with and without spin-orbit coupling, yielding band gaps in good agreement with experimental results, and find that the band gap increases along the quadratic function of the Cl content x. With increasing Cl content x, the elastic constants and moduli increase linearly, the effective mass of the electron and hole increase, while mobilities decrease linearly, the static dielectric constant decreases linearly, and exciton binding energy increases quadratically. We calculate the photo-absorption coefficients and reflectivity, predicting the absorption peaks shift to the ultraviolet region from bromide to chloride.

Twofold rattling mode-induced ultralow thermal conductivity in vacancy-ordered double perovskite Cs2SnI6.

We report a first-principles study of lattice vibrations and thermal transport in Cs2SnI6, the vacancy-ordered double perovskite. Twofold rattlers of Cs atoms and SnI6 clusters in Cs2SnI6, being different from CsSnI3 with only Cs atom rattlers, largely scatter heat-carrying acoustic phonons strongly coupled with low-lying optical phonons and lower phonon group velocity. Using renormalized phonon dispersions at finite temperatures, we reveal that anharmonicity and twofold rattling modes induce an ultralow thermal conductivity at room temperature.

First-principles study on structural, electronic and optical properties of perovskite solid solutions KB1-x Mg x I3 (B = Ge, Sn) toward water splitting photocatalysis.

Perovskite materials have been recently attracting a great amount of attention as new potential photocatalysts for water splitting hydrogen evolution. Here, we propose lead-free potassium iodide perovskite solid solutions KBI3 with B-site mixing between Ge/Sn and Mg as potential candidates for photocatalysts based on systematic first-principles calculations. Our calculations demonstrate that these solid solutions, with proper Goldschmidt and octahedral factors for the perovskite structure, become stable by configurational entropy at finite temperature and follow Vegard's law in terms of lattice constant, bond length and elastic constants. We calculate their band gaps with different levels of theory with and without spin-orbit coupling, revealing that the hybrid HSE06 method yields band gaps increasing along the quadratic function of Mg content x. Moreover, we show that the solid solutions with 0.25 ≤ x ≤ 0.5 have appropriate band gaps between 1.5 and 2.2 eV, reasonable effective masses of charge carriers, and suitable photoabsorption coefficients for absorbing sunlight. Among the solid solutions, KB0.5Mg0.5I3 (B = Ge, Sn) is found to have the most promising band edge alignment with respect to the water redox potentials with different pH values, motivating experimentalists to synthesize them.

Interfacial Enhancement of Photovoltaic Performance in MAPbI3/CsPbI3 Superlattice.

Perovskite solar cells have continued to fascinate over the past decade due to fast increasing power conversion efficiency and very low fabrication cost but still suffered from poor stability. Interface engineering is evolved to be one of the most promising solutions to the instability problem. In this work, we perform a first-principles study on the MAPbI3/CsPbI3 interface system, aiming at clarifying the underlying mechanism of interfacial enhancement of solar cell performance. We devise the atomistic modeling of superlattices as increasing the number of included unit cells and carry out structural optimizations, revealing that the binding strength between the perovskite layers becomes stronger while the band gap decreases as the supercell size increases. Using enough large supercells of the interface system, we further estimate the formation energies of the interfacial vacancy defects and activation barriers for vacancy-mediated I atom migrations. Our calculations show the shallow transition states for most of the defects and the higher activation barriers for I atom migrations across the interface, providing an evidence of performance enhancement by the interface formation. By giving an insightful understanding of the MAPbI3/CsPbI3 heterojunction, this work definitely contributes to the design of interface systems for high-performance solar cells.