Transformation between the Dark and Bright Self-Trapped Excitons in Lead-Free Double-Perovskite Cs₂NaBiCl₆ under Pressure

Abstract

Understanding the relationships between the structure and the properties in lead-free double perovskites is significant for their applications in the optoelectronic field. Here the nonluminous Cs2NaBiCl6 crystal exhibits an unexpected broadband dual-color emission as the external pressure is increased to 6.77 GPa. The emission intensity is remarkably enhanced with further compression to 8.50 GPa. By analyzing the results of in situ high-pressure experiments and the density functional theory, we conclude that the dual-color emission is attributed to singlet self-trapped excitons (STEs) and triplet STEs, respectively. This phenomenon originates from the tilting and twisting of [BiCl6]3– caused by the transition of cubic Cs2NaBiCl6 to the tetragonal phase. Notably, the transformation between the dark and bright STEs in the Cs2NaBiCl6 crystal is demonstrated by ultrafast transient absorption experiments under different pressures. This work not only offers deep insight into the structure–property relationship in lead-free double perovskites but also opens the door for the design of new lead-free double perovskites.

Direct Electron Transfer Enables Highly Efficient Dual Emission Modes of Mn2+-Doped Cs2Na1–xAgxBiCl6 Double Perovskites

“…14−16 In Mn 2+ -doped double perovskite systems, most of the energy transfer processes usually take place from the STE state of the double perovskite to Mn 2+ rather than directly from free electrons to Mn 2+ , 14,17,18 which would generate a pronounced energy loss caused by the inevitable defects between the host and doped states. 19,20 Therefore, realizing the direct energy transfer between the host and dopant is crucial for enhancing the PL efficiency of double perovskites.…”

Mechanism of Pressure-Driven Band Gap Evolutions in Lead-Free Halide Double Perovskites

“…For example, the two Pb 2+ was replaced into [Na + + Bi 3+ ] and [Ag + + Bi 3+ ], generating two cubic double perovskites Cs 2 NaBiCl 6 and Cs 2 AgBiCl 6 (Figure 1). 42,43 First, the DFT calculations confirm the intricate band gap evolutions of cubic Cs 2 NaBiCl 6 and cubic Cs 2 AgBiCl 6 observed in photoluminescence spectra under pressure 29,30 (Figure 2). As shown in Figure 2c and d, the DFT band gap of Cs 2 NaBiCl 6 gradually increases to 0.03 eV while pressure changes from 1 atm to 5 GPa, whereas the band gap of Cs 2 AgBiCl 6 decreases to as large as 0.37 eV in the same pressure range.…”

“…28 By applying pressure, recent studies successfully reduced the band gap of Cs 2 AgBiBr 6 from ∼2.2 to 1.7 eV 27 and regulated the photoluminescence emission range in Cs 2 AgBiCl 6 and Cs 2 NaBiCl 6 . 29,30 Interestingly, the band gap of Cs 2 NaBiCl 6 increases with pressure up to ∼5 GPa, whereas the band gaps of Cs 2 AgBiCl 6 /Cs 2 AgBiBr 6 decrease with pressure in the studied range (<5 GPa for Cs 2 AgBiCl 6 or <3 GPa for Cs 2 AgBiBr 6 ). Then, they transform into a tetrahedral structure at higher pressure.…”

“…Despite the enormous attention and progress in recent years, these materials exhibit wide band gaps and low luminescence, which severely restrict their applications in the optoelectronic field. − To solve this problem, many efforts have been devoted, including compositional variation, ,, chemical substitution, , and high-pressure technique. − Especially, high pressure is a powerful tool that can effectively modify the structural and electronic properties of materials without chemically changing the materials . By applying pressure, recent studies successfully reduced the band gap of Cs 2 AgBiBr 6 from ∼2.2 to 1.7 eV and regulated the photoluminescence emission range in Cs 2 AgBiCl 6 and Cs 2 NaBiCl 6 . , Interestingly, the band gap of Cs 2 NaBiCl 6 increases with pressure up to ∼5 GPa, whereas the band gaps of Cs 2 AgBiCl 6 /Cs 2 AgBiBr 6 decrease with pressure in the studied range (<5 GPa for Cs 2 AgBiCl 6 or <3 GPa for Cs 2 AgBiBr 6 ). Then, they transform into a tetrahedral structure at higher pressure.…”

Celebrating the 10th Anniversary of the Youth Innovation Promotion Association, Chinese Academy of Sciences: Emerging Young Scientists in Physical Chemistry

“…The Letter by Li et al + ions in the growth of single ice crystals. The Letter by Jiang et al 12 reports the luminescence properties of lead-free double-perovskite Cs 2 NaBiCl 6 crystals under high pressure. The Letter by Wang et al 13 reports the relationship between the delocalization of metal cluster cores in tin clusters as a potential third-order nonlinear optical (NLO) material.…”