Thermochromic Lead-Free Halide Double Perovskites

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 Cs ₂ AgBiBr ₆ double perovskites is demonstrated. Along with the thermochromism, temperature-dependent changes in the bond lengths of AgBr (R _AgBr ) and BiBr (R _BiBr ) are observed. The first-principle molecular dynamics simulations reveal substantial anharmonic fluctuations of the R _AgBr and R _Bi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 Cs ₂ AgBiBr ₆ 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 Cs ₂ AgBiBr ₆ provide new insights for the further development of optoelectronic devices based on the lead-free halide double perovskites.