[Paper Review] A First-principles study on ABBr3 (A = Cs, Rb, K, Na; B = Ge, Sn) halide perovskites for photovoltaic applications

Dibyajyoti Saikia, Mahfooz Alam+4 more|arXiv (Cornell University)|May 3, 2022

Perovskite Materials and Applications4 citations

TL;DR

This first-principles study investigates ABBr3 (A = Cs, Rb, K, Na; B = Ge, Sn) halide perovskites as lead-free candidates for photovoltaic applications. It reveals all compounds exhibit direct bandgaps between 1.10 and 1.97 eV and strong optical absorption (>10⁵ cm⁻¹) across the UV-Vis spectrum, indicating strong potential for efficient solar energy conversion.

ABSTRACT

In recent years, halide perovskite-based solar cells have received intensive attention, and demonstrated power conversion efficiency as high as 25.8%. With regard to the toxicity of Pb and the instability of organic elements, all inorganic lead-free perovskites (ILPs) have been extensively studied to achieve comparable or greater photovoltaic performance. In order to develop ILPs as an alternative for solar cell applications, we performed first-principles calculations of ABBr3 perovskites (A = Cs, Rb, K, and Na, and B = Sn, and Ge). Structural, electronic, and optical properties were systematically studied to probe the potentiality in photovoltaic applications. All these ILPs exhibited a direct bandgap in the range of 1.10 to 1.97 eV, highly beneficial for absorbing solar energy. Furthermore, these ILPs demonstrated significant optical absorption (over 105 cm-1) in the whole UV-Vis spectrum. These results will be helpful for designing highly efficient lead-free perovskite solar cells.

Motivation & Objective

• To evaluate the structural, electronic, and optical properties of all-inorganic lead-free ABBr3 perovskites for photovoltaic use.
• To address the limitations of lead-based perovskites, including toxicity and instability, by identifying viable Pb-free alternatives.
• To systematically assess the impact of A-site (Cs, Rb, K, Na) and B-site (Ge, Sn) cation substitutions on optoelectronic performance.
• To identify promising candidates with suitable bandgaps and strong light absorption for high-efficiency solar cells.

Proposed method

• Employed density functional theory (DFT) calculations with the generalized gradient approximation (GGA) and the DFT+U method to correct for self-interaction error.
• Performed structural optimization to determine equilibrium lattice parameters and atomic positions.
• Calculated electronic band structures and density of states to analyze bandgap nature and electronic characteristics.
• Computed optical absorption coefficients using the dielectric function derived from electronic structure calculations.
• Analyzed the direct or indirect nature of the bandgap based on the band structure at high-symmetry points.
• Evaluated the absorption edge and intensity across the UV-Vis spectrum to assess solar energy harvesting potential.

Experimental results

Research questions

• RQ1 What are the structural stability and equilibrium lattice parameters of ABBr3 perovskites with different A and B cations?
• RQ2 Do these ABBr3 perovskites exhibit direct or indirect bandgaps, and what is the range of their bandgap energies?
• RQ3 How strong is the optical absorption across the solar spectrum, particularly in the UV-Vis region?
• RQ4 Which A-site and B-site combinations yield the most favorable optoelectronic properties for photovoltaic applications?
• RQ5 Can these all-inorganic, lead-free perovskites match or exceed the performance of lead-based counterparts in key photovoltaic metrics?

Key findings

• All investigated ABBr3 perovskites exhibit a direct bandgap, with values ranging from 1.10 eV to 1.97 eV, which is highly suitable for efficient solar energy absorption.
• The optical absorption coefficient exceeds 10⁵ cm⁻¹ across the entire UV-Vis spectrum, indicating strong photon absorption capability.
• CsSnBr₃ and RbSnBr₃ show particularly favorable bandgaps near 1.10–1.20 eV, approaching the ideal range for single-junction solar cells.
• The Ge-based perovskites (e.g., CsGeBr₃) exhibit wider bandgaps (1.60–1.97 eV), making them suitable for tandem or top-cell applications.
• The electronic structure analysis confirms the presence of strong hybridization between Br-p and B-site p-orbitals, contributing to favorable charge transport properties.
• The results demonstrate that these all-inorganic, lead-free perovskites are promising candidates for next-generation photovoltaic devices due to their optimal bandgaps and high absorption coefficients.