Structural and Size Effects on CsPbI₃ Nanocrystals Luminescence—A Temperature and Pressure Dependent Study

Metal halide perovskite nanocrystals (NCs) are promising for photovoltaic and light emitting applications thanks to the tunability of their optoelectronic properties and high photoluminescence quantum yield [1,2]. NCs’ size plays a critical role not only by affecting the optical and electronic characteristics through quantum confinement, but also by changing the surface-to-volume ratio and consequently the structural properties[3]. The soft nature of the metal halide lattice can result in a wide range of structural distortions and defects that also impact the material optoelectronic properties [4], [5], particularly for CsPbI₃ where the perovskite g-phase is only stable at room temperature due to the strain induced by the small NC size. Therefore, accurate size-dependent study of perovskite NCs is required to single-out the complex contribution of particle size, quantum confinement, structural distortions, defect losses and phonon scattering, retrieving fundamental structure-property relationships that allow a more precise material’s design.<br/>The size-dependent investigation of perovskite NCs optical and structural properties are limited by the poor size selectivity of the standard hot injection synthesis [3]. We employ the hot injection synthetic method and adapt a size-selective precipitation strategy [6] to controllably isolate CsPbI₃ NCs with sizes ranging between the strong and weak quantum confinement regime (7-17 nm), and combine photoluminescence (PL) spectroscopy and X-Ray diffraction (XRD) characterization to elucidate their size-dependent structural and photophysical properties under different thermodynamic conditions controlled by temperature and pressure.<br/>Using temperature dependent PL measurements, we identify the role of exciton-phonon coupling and surface defect passivation by the organic ligands: we find that the level of surface coverage not only directly affects the availability of nonradiative loss channels but also the NPs luminescence through more subtle phonon coupling effects, stressing the need for the careful consideration of the chosen ligands. Pressure on the other hand is an ideal post-synthesis method to investigate the structure-properties relationship by adjusting interatomic distances, lattice deformation and band to band electronic overlap, controlling these parameters independently from NC size [7]. We perform pressure dependent PL and XRD measurements up to 2.5 GPa, observing changes in recombination dynamics and PL quenching at high pressures, as well as a size dependent solid-solid phase transition from the γ-phase to the non-perovskite δ-phase. The relationship between size dependent pressure effect and PL quenching further demonstrate that the lattice deformation mechanism strongly affects the material’s bandgap and recombination dynamics, stressing the importance of the structural engineering of these class of soft semiconductors. By highlighting the key role of particles’ size, our findings shine light on a fundamental relationship between structural and optoelectronic properties of CsPbI₃ NCs that can likely be applicable to a wide range of perovskite systems.<br/><br/>[1] Fu, Y. <i>et al., </i> <i>Nat. Rev. Mater.</i> <b>4</b>, 169–188 (2019).<br/>[2] Huang, H. <i>et al.</i>, <i>Nat. Commun.</i> <b>8</b>, (2017).<br/>[3] Beimborn, J. C., Walther, L. R., Wilson, K. D. & Weber, J. M., <i>J. Phys. Chem. Lett.</i> <b>11</b>, 1975–1980 (2020).<br/>[4] Zhang, L., Wang, K., Lin, Y. & Zou, B., <i>J. Phys. Chem. Lett.</i> <b>11</b>, 4693–4701 (2020).<br/>[5] Cortecchia, D. <i>et al.</i>, <i>J. Am. Chem. Soc.</i> <b>139</b>, 39–42 (2017).<br/>[6] Zhao, Q. <i>et al.</i>, <i>ACS Energy Lett.</i> <b>5</b>, 238–247 (2020).<br/>[7] Yesudhas, S. <i>et al.</i>, <i>Chem. Mater.</i> <b>32</b>, 785–794 (2020).