Mixed Chalcogenide‐Halides for Stable, Lead‐Free and Defect‐Tolerant Photovoltaics: Computational Screening and Experimental Validation of CuBiSCl₂ with Ideal Band Gap

Abstract

Lead halide perovskites have emerged as promising photovoltaic (PV) materials owing to their superior optoelectronic properties. However, they suffer from poor stability and potential toxicity. Here, computational screening with experimental synthesis is combined to explore stable, lead-free, and defect-tolerant PV materials. Heavy cations with lone-pair electrons and mixed anions of chalcogens and halogens as a descriptor for simultaneous realization of defect tolerance and high stability are adopted. Together with the criteria of possessing direct band gap and optimal gap value, the inorganic material database is screened and CuBiSCl 2 in the post-perovskite structure is identified with an ideal band gap of 1.37 eV. The electronic structure and defect calculations suggest its defect-tolerant characteristics. By optical absorption measurement, its band gap is confirmed to be ≈1.44 eV, with strong absorption near the band edge. The material is stable against thermal decomposition up to 300 °C and can survive from 25 days of storage at ambient conditions with 60% relative humidity. Prototype solar cells are fabricated and demonstrate an open circuit voltage of 1.09 V and a power conversion efficiency of 1.00%. With the excellent properties above, CuBiSCl 2 is proposed to be a promising candidate for PV application.

Direct Band Gap Chalcohalide Semiconductors: Quaternary AgBiSCl2 Nanocrystals

“…The direct character of the band gap of our NCs was corroborated by DFT calculations adopting the HSE functional, which clearly showed the lowest energy interband transition at the Y point (Figure 4), as reported for the isostructural CuBiSCl 2 . 12 The orbital-projected density of states and charge density analyses revealed that the valence band of AgBiSCl 2 is formed by the S-and Cl-p orbitals and by the Ag-d orbitals, whereas the conduction band is dominated by the Bi-p orbital contribution (Figures S12 and S13).…”

“…However, the demand for semiconductors containing low-toxicity elements would suggest to focus on Sn analogues (such as Sn 2 BiS 2 I 3 ) . Another chalcohalide compound that might fulfill these requirements is AgBiSCl 2 , − for which solution-phase synthetic methods are lacking so far.…”

Enhancing the inherent stability of perovskite solar cells through chalcogenide-halide combinations

“…Experiments have found that the material remains stable under thermal decomposition at 300 1C and can be stored for 25 days under environmental conditions with a relative humidity of 60%. A prototype of the solar cell was made, proving that the open-circuit voltage is 1.09 V and the PCE is 1.00% 34 (Fig. 6d).…”

“…The black arrows mark the peaks attributed to the compound BiOCl. 34 Copyright 2022, Advanced Functional Materials. (e) I-V plot for single crystals of (CH 3 ) 3 SPbI 3 at room temperature.…”

High-Efficiency and Ultra-Stable Cesium–Bismuth-Based Lead-free Perovskite Solar Cells without Modification

“…It has an indirect band gap of 1.57 eV and a high absorption coefficient; however, its 1D nanorod crystal structure and deep trap state are important factors leading to its poor performance . In addition to Bi-based ternary sulfur halides that can be used as light absorbers, quaternary sulfur halides with ABiSX 2 (A is metal cation and X is halogen ion) structure, such as CuBiSCl 2 , AgBiSCl 2 , or MABiSI 2 , possess higher-dimensional (2D/3D) crystal structures which have also been applied as stable light-absorbing materials. However, the PSCs based on ABiSX 2 crystal structures all show unsatisfactory efficiency in the works reported in recent years, and the PCE does not exceed 10%.…”