Designing A Novel Lead-Free Perovskite Composition Using First-Principles Quantum Mechanical Modeling

Overview

Perovskites have long been touted as a superior alternative to pure-silicon solar panels, which often have lower conversion efficiencies. However, many of these high-performing materials contain toxic lead (Pb), posing environmental threats and imposing obstacles to commercial production. Meanwhile, most Pb-free alternatives, namely tin (Sn) based perovskites, have traditionally performed below 20%. Thus, there is a need to develop and evaluate a new perovskite composition which can achieve a high PCE without utilizing lead. To accomplish this, we altered a high-performing composition introduced by Restat et al., implementing dual-halide structuring (which has seen promising results in improving the performance of perovskites) and replacing the weaker iodine element with bromine and chlorine (Br, Cl). The resulting composition, Cs0.2FA0.8Sn(Br0.5Cl0.5)3, was tested using Density Functional Theory (DFT) and SCAPS-1D. DFT, commonly used to perform first-principles quantum mechanical calculations, computed the ground state energy level of a composition based on its atomic parameters and yielded the optical parameters necessary to determine the PCE: the band structure, density of states (DOS), formation energy, and light absorption. A four-step DFT methodology was formulated: relaxation, followed by a clean self-consistent calculation (SCF), a non-self-consistent calculation (NSCF), and an epsilon calculation. Relaxation (vc-relax) takes the preliminary atomic positions and cell parameters of a material and minimizes the energy levels, forces, and pressure. Our composition converged with a force threshold of 0.01 and pressure of 0.5, with the resulting parameters being fed into SCF/NSCF. The SCF and NSCF calculations served as an extension of the relaxation, providing the band structure and DOS to be used by epsilon. And epsilon, the final DFT step, calculated the dielectric function of the composition. This function, which has a real and imaginary part, is where the composition’s optical parameters (absorption coefficient, bandgap, refraction index) can be calculated via a series of equations. These parameters, when plugged into SCAPS-1D, yielded a PCE of 30.21%, substantially higher than most Sn-based perovskites and comparable with Pb-based perovskites, proving there is a path forward for efficient non-toxic energy production.