High-Contrast\nThermochromism in Room-Temperature\nTransparent Layered Perovskite PEA<sub>2</sub>PbBr<sub>4</sub> with\na High Temperature-Induced Bandgap Change Rate of 0.8 meV/K

Two-dimensional\norganic–inorganic hybrid layered perovskites\nhave emerged as a new generation of optoelectronic materials. However,\nthe thermochromism in organic–inorganic hybrid layered perovskites\nhas been rarely explored in depth. A further understanding of the\nmechanism is necessary and favorable for the application. Here, transparent\ncentimeter-sized single crystals of the organic–inorganic hybrid\nlayered perovskite (C<sub>6</sub>H<sub>5</sub>C<sub>2</sub>H<sub>4</sub>NH<sub>3</sub>)<sub>2</sub>PbBr<sub>4</sub> (PEA<sub>2</sub>PbBr<sub>4</sub>) were synthesized using an improved evaporation method. As\na typical organic–inorganic hybrid layered perovskite, the\nPEA<sub>2</sub>PbBr<sub>4</sub> single crystal shows high-contrast\nand progressive thermochromism exhibiting a change from colorlessness\nand transparency to lemon yellow in a wide temperature range of 200–450\nK. Based on the calculation through the Varshni equation, the temperature-induced\nbandgap change rate directly associated with the high-contrast thermochromism\nof PEA<sub>2</sub>PbBr<sub>4</sub> reaching 0.8 meV/K. This value\nis higher than that of many three-dimensional perovskites and traditional\nIV–III semiconductors. Furthermore, the temperature-dependent\n193 nm photoluminescence spectra suggest that this high temperature-induced\nbandgap change rate of PEA<sub>2</sub>PbBr<sub>4</sub> is a result\nof the competitive interaction between lattice thermal expansion and\nelectron–phonon coupling (Fröhlich coupling coefficient\nΓ<sub>LO</sub> = 2.215). Based on the characteristics introduced\nabove, PEA<sub>2</sub>PbBr<sub>4</sub> as an organic–inorganic\nhybrid layered perovskite has a better performance in achieving the\nbalance between high-contrast and high room-temperature transmittance.\nTherefore, PEA<sub>2</sub>PbBr<sub>4</sub> is a material with great\npotential in applications like temperature-indicating labels. This\nwork provides valuable insights into the thermochromism of layered\nperovskites, offering a new material system and approach for developing\nthermochromic materials with higher sensitivity and efficiency.