Exciton-Phonon Interactions in Halide Perovskite Quantum Dots

Recently, lead halide perovskites (APbX₃, A = Cs, MA (CH₃NH₃), FA (HC(NH₂)₂), X = Cl, Br, I) have attracted much attention as a new class of semiconductor materials because of their defect-tolerance structures and outstanding optical and electronic properties [1,2]. In addition, colloidal halide perovskite nanocrystal quantum dots (QDs) show almost 100% photoluminescence (PL) quantum yields at room temperature. Because of their unique electronic band-edge structures, PL dynamics of halide perovskite QDs are governed by the formation and recombination of excitons, trions, and biexcitons [3,4]. Furthermore, for ionic semiconductors, electron-phonon interactions strongly affect the PL spectra and dynamics of halide perovskites [5]. For example, at room temperature, efficient anti-Stokes PL clearly appears [6], and the relaxation dynamics of hot carriers is manipulated by direct phonon excitation [7]. At low temperatures, individual QDs show very narrow PL linewidths, and single QD spectroscopy is a powerful tool to investigate the exciton-exciton and exciton–phonon interactions in perovskite QDs. Deep understanding of exciton-phonon interactions in QDs is desirable for design for light sources with high color purity, narrow linewidth, and long exciton coherence time. Here, we report exciton-related multipeak PL structures of single perovskite QDs at low temperatures and discuss their origins and electron-phonon interactions in QDs.<br/>In this work, we prepared three types of colloidal perovskite QD samples, CsPbBr₃, CsPbI₃, and FAPbBr₃, and studied their PL spectra of single QDs at low temperatures. All QD samples show strong exciton PL, and several PL peaks originating a trion and a biexciton appear in the low energy side of the strong exciton peak. PL spectra of all the samples were very similar to each other. However, the PL linewidths of all inorganic CsPbBr₃ and CsPbI₃ QDs were narrower than those of organic-inorganic hybrid FAPbBr₃ QDs, suggesting the stronger exciton–phonon coupling because of large organic cations. In addition, LO-phonon side bands of an exciton, a trion, and a biexciton are observed. We clarified the QD size dependence of the Huang–Rhys factors, i.e., the strength of the exciton–phonon coupling, in three different perovskites. We found that in all samples, the Huang–Rhys factors increase with a decrease of the QD size, but the LO-phonon energies are independent of the QD size [8-11]. Moreover, PL spectra clearly show the positive binding energies of trions and biexcitons and the attractive exciton–exciton Coulomb interactions in perovskite QDs. We found that the size dependence of the binding energies follows a universal scaling curve regardless of chemical composition [11]. These findings of the size-dependent optical responses provide new insights into understanding of the photophysics of halide perovskite QDs and the design of QD-based light sources.<br/>Part of this work was supported by JST-CREST (JPMJCR21B4) and NEDO-GI (JPNP21016).<br/><br/>References<br/>1. Y. Kanemitsu, <i>J. Mater. Chem.</i> C <b>5</b>, 3427 (2017).<br/>2. Y. Yamada, T. Yamada, Y. Kanemitsu, <i>Bull. Chem. Soc. Jpn.</i> <b>90</b>, 1129 (2017).<br/>3. Y. Kanemitsu, <i>J. Chem. Phys.</i> <b>151</b>, 170902 (2019).<br/>4. G. Yumoto, Y. Kanemitsu, <i>Phys. Chem. Chem. Phys.</i> <b>24</b>, 22405 (2022).<br/>5. Y. Yamada, Y. Kanemitsu, <i>NPG Asia Mater.</i> <b>14</b>, 48 (2022).<br/>6. T. Yamada, T. Aharen, Y. Kanemitsu, <i>Phys. Rev. Mater.</i> <b>3</b>, 024601 (2019).<br/>7. F. Sekiguchi <i>et al.</i>, <i>Phys. Rev. Lett.</i> <b>126</b>, 077401 (2021).<br/>8. S. Masada <i>et al</i>., <i>Nano Lett.</i> <b>20</b>, 4022 (2020).<br/>9. K. Cho <i>et al</i>., <i>Nano Lett.</i> <b>21</b>, 7206 (2021).<br/>10. K. Cho <i>et al</i>., <i>Nano Lett</i>. <b>22</b>, 7674 (2022).<br/>11. K. Cho <i>et al</i>., <i>ACS Nano</i> <b>18</b>, 5723 (2024).