Electronic and geometrical parametrization of the role of organic/inorganic cations on the photovoltaic perovskite band gap

Electronic and structural properties of mixed-cation hybrid perovskites studied using an efficient spin-orbit included DFT-1/2 approach

Fundamental understanding and optimization of the emerging mixed organic-inorganic hybrid perovskites for solar cells require multiscale modeling starting from ab initio quantum mechanics methods. Particularly, it is important to correctly predict the structural and electronic properties such as phase stability, lattice parameters, band gaps, and band structures. Although density functional theory is the method of choice to address these properties and generate the input for subsequent multiscale, high-throughput, and data-driven approaches, standard exchange correlation functionals fail to reproduce the bandgap, particularly if spin-orbit coupling (SOC) is correctly taken into account. While many SOC-included hybrid functionals suffer from low transferability between different molecular ions and are computationally costly, we propose an efficient multistep simulation protocol based on the DFT-1/2 method. We apply this approach to APbI3 with A: FA, MA, Cs, and systems with mixed cations and show how the choice of the A-cation modifies the Pb-I scaffold and the hydrogen bonding and discuss their interplay with structural stability. Furthermore, band gaps, band structures, Rashba band splitting, Born effective charges as well as partial density of states (PDOS) are compared for different cases w/wo the SOC effect and the DFT-1/2 approach.

DFT-1/2 and shell DFT-1/2 methods: electronic structure calculation for semiconductors at LDA complexity

It is known that the Kohn-Sham eigenvalues do not characterize experimental excitation energies directly, and the band gap of a semiconductor is typically underestimated by local density approximation (LDA) of density functional theory (DFT). An embarrassing situation is that one usually uses LDA+Ufor strongly correlated materials with rectified band gaps, but for non-strongly-correlated semiconductors one has to resort to expensive methods like hybrid functionals orGW. In spite of the state-of-the-art meta-generalized gradient approximation functionals like TB-mBJ and SCAN, methods with LDA-level complexity to rectify the semiconductor band gaps are in high demand. DFT-1/2 stands as a feasible approach and has been more widely used in recent years. In this work we give a detailed derivation of the Slater half occupation technique, and review the assumptions made by DFT-1/2 in semiconductor band structure calculations. In particular, the self-energy potential approach is verified through mathematical derivations. The aims, features and principles of shell DFT-1/2 for covalent semiconductors are also accounted for in great detail. Other developments of DFT-1/2 including conduction band correction, DFT+A-1/2, empirical formula for the self-energy potential cutoff radius, etc, are further reviewed. The relations of DFT-1/2 to hybrid functional, sX-LDA,GW, self-interaction correction, scissor's operator as well as DFT+Uare explained. Applications, issues and limitations of DFT-1/2 are comprehensively included in this review.

Insight into the Interface Engineering of a SnO2/FAPbI3 Perovskite Using Lead Halide as an Interlayer: A First-Principles Study

The interfacial properties of the perovskite photovoltaic layer and electron transport layer (ETL) are critical to minimize energy losses of perovskite solar cells (PSCs) induced by interfacial recombination. Herein, the interface engineering of the SnO₂/FAPbI₃ perovskite using PbX₂ (X = Cl, Br, or I) as an interlayer is extensively studied using first-principles calculations. The results reveal that the thickness of the PbI₂ interlayer needs to be finely controlled, which may limit charge transport if there is a large amount of PbI₂ precipitation at the interface. The high lattice mismatch of the PbBr₂ with the SnO₂/FAPbI₃ interface makes PbBr₂ an unfavorable passivation material. Due to the strong coupling of the PbCl₂ with both SnO₂ and FAPbI₃, an efficient electron transport pathway could be built after applying PbCl₂ as an interlayer. Meanwhile, the PbCl₂ interlayer could also effectively passivate interface defects, therefore lowering the energy losses of PSCs.