Excitons in CsPbBr₃ Halide Perovskite

Abstract

Excitons in Bridgman grown halide\nperovskite CsPbBr₃ single crystals were examined using\nphotoluminescence (PL) spectroscopy\nto determine the nature of the electronic states. The photoluminescence\nintensity was strongly temperature-dependent and depended upon the\nspecific exciton band. At low temperatures intrinsic disorder and\nits related shallow below bandgap tail states determine the emission\nproperties. Photoluminescence at low temperature revealed the presence\nof several strong bands at the band edge that is attributed to free\nor trapped/bound excitons. This PL emission results from strong electron–phonon\ncoupling with an average phonon energy <i>E</i>_ph of 6.5 and 27.4 meV for the emissions, comparable to that observed\nin other perovskites. The Huang–Rhys parameter <i>S</i> was calculated to be 3.81 and 1.51, indicating strong electron–phonon\ncoupling. The interactions between electrons and phonons produce small\npolarons that tend to bind charge carriers and result in trapped/bound\nexcitons. The transient photoluminescence response of each specific\nband was studied, and the results indicated a multiphonon recombination\nprocess. Average PL lifetimes of ∼17 ns for free excitons and\n∼38 ns for trapped/bound excitons were determined. The observed\nedge states could be associated with native defects such as vacancies\nand interstitials, as well as twinning due to the cubic-to-tetragonal\nphase transition in CsPbBr₃. Elimination of the trapping\nsites for binding excitons could lead to improved charge transport\nmobilities, carrier lifetimes, and detector properties in this system.