Semi-empirical approach to assess externally-induced photoluminescence linewidth broadening of halide perovskite nanocrystals with particle-size distribution

Colloidal nanocrystals (NCs) can be used to prepare high-color-purity metal halide perovskites (MHPs) for light-emitting displays. However, the NCs have a finite particle-size distribution, which broadens the linewidth of photoluminescence spectra under strong quantum confinement, and thereby degrades the color purity of the MHPs. This paper presents a simple method to quantify this externally-introduced broadening of linewidth $${{{{{{\boldsymbol{\Gamma }}}}}}}_{{{{{{\bf{SD}}}}}}}$$ by combining experimental size-distribution histogram with size-dependent photoluminescence-wavelength (PL- $${{{{{\boldsymbol{\lambda }}}}}}$$ ) curve. We develop a semi-empirical method to estimate the other three contributions to the experimentally-measured full-width at half-maximum, $${{{{{{\boldsymbol{\Gamma }}}}}}}_{{{{{{\bf{EXP}}}}}}}$$ . Namely: the intrinsic linewidth $${{{{{{\boldsymbol{\Gamma }}}}}}}_{{{{{{\bf{LO}}}}}}}$$ caused by exciton-longitudinal optical (LO) phonon Fröhlich coupling; the inhomogeneous linewidth $${{{{{{\boldsymbol{\Gamma }}}}}}}_{{{{{{\bf{o}}}}}}}$$ caused by imperfections-related scattering, and the broadening $${{{{{{\boldsymbol{\Gamma }}}}}}}_{{{{{{\bf{QC}}}}}}}$$ due to the quantum-confinement effect. We show that $${{{{{{\boldsymbol{\Gamma }}}}}}}_{{{{{{\bf{LO}}}}}}}$$ of a nanocrystal decreases together with the particle size, disappearing at 1.6 nm radius. Finally, we show that $${{{{{{\boldsymbol{\Gamma }}}}}}}_{{{{{{\bf{LO}}}}}}}$$ is correlated with the degree of Fröhlich-polaron formation, hence proportional to the long-range LO-phonon-electron coupling. Colloidal nanocrystals are used to prepare high-color-purity metal halide perovskites for LEDs, but the color purity is disrupted by the broadening of the photoluminescent spectrum. The authors present a method to quantify the distinct effects causing such broadening and identify ideal conditions to remove the broadening due to exciton-phonon coupling.