Thermochromic Lead‐Free Halide Double Perovskites

Abstract

Lead‐free halide double perovskites with diverse electronic structures and optical responses, as well as superior material stability show great promise for a range of optoelectronic applications. However, their large bandgaps limit their applications in the visible light range such as solar cells. In this work, an efficient temperature‐derived bandgap modulation, that is, an exotic fully reversible thermochromism in both single crystals and thin films of Cs2AgBiBr6 double perovskites is demonstrated. Along with the thermochromism, temperature‐dependent changes in the bond lengths of AgBr (RAgBr) and BiBr (RBiBr) are observed. The first‐principle molecular dynamics simulations reveal substantial anharmonic fluctuations of the RAgBr and RBiBr at high temperatures. The synergy of anharmonic fluctuations and associated electron–phonon coupling, and the peculiar spin–orbit coupling effect, is responsible for the thermochromism. In addition, the intrinsic bandgap of Cs2AgBiBr6 shows negligible changes after repeated heating/cooling cycles under ambient conditions, indicating excellent thermal and environmental stability. This work demonstrates a stable thermochromic lead‐free double perovskite that has great potential in the applications of smart windows and temperature sensors. Moreover, the findings on the structure modulation‐induced bandgap narrowing of Cs2AgBiBr6 provide new insights for the further development of optoelectronic devices based on the lead‐free halide double perovskites.

From Pb to Bi: A Promising Family of Pb‐Free Optoelectronic Materials and Devices

“…This value is 8.2% reduction compared to the original Cs 2 AgBiBr 6 (2.3 eV). In addition to pressure, the thermochromic effect was observed in both single crystals and thin films . The color of Cs 2 AgBiBr 6 single crystals changes from red to black and that of the thin film changes from yellow to red with increasing the temperature from room temperature to 250 °C.…”

Understanding the Effect of Crystalline Structural Transformation for Lead‐Free Inorganic Halide Perovskites

“…[ 22 ] The peaks at 134 and 178 cm −1 for Cs 2 AgBiBr 6 are attributable to the stretching vibrations arising from the AgBr 6 octahedral. [ 23 ] After Ag incorporation, the vibrations of BiBr 6 are submerged, which can be attributed to the stronger bonding strength of the AgBr bond than that of BiBr bond due to bond compression. [ 20 ]…”

Reversible Thermochromism and Strong Ferromagnetism in Two‐Dimensional Hybrid Perovskites

“…Differential scanning calorimetry (DSC) analysis from 290 Kt o3 80 Kf or (PED)CuCl 4 indicates that no structural phase transition occurs up to the decomposition (Figure S11). [38,39] To understand the mechanisms of the thermochromism in (PED)CuCl 4 and (BED) 2 CuCl 6 perovskites,w e performed single-crystal X-ray structures at 150, 200, 300, and 380 K. For( PED)CuCl 4 ,a ll structures were solved in the monoclinic space group P2 1 /c. When the temperature changed from 150 Kt o3 80 K, crystallographic data and refinement parameters show several important changes of the crystal structures.F irst, the Cu À Cl 1 bond length (see Fig-ure S12 for atom numbering scheme) elongates from 2.2348 to 2.2375 ( Figure S12b), while the other Cu À Cl bonds show at rend of decrease (Table S3) over the same temperature range.S econd, the in-plane Cl-Cu-Cl angle [25] (i.e., in-plane buckling of the octahedra, q in-plane )d ecreases from 97.178 8 to 97.008 8 ( Figure S12c).…”