Pure White Emission with 91.9% Photoluminescence Quantum Yield of [(C 3 H 7 ) 4 N] 2 Cu 2 I 4 out of Polaronic States and Ultra-High Color Rendering Index.

Recently, cuprous halide perovskite-type materials have drawn tremendous attention for their intriguing optical properties. Here, a zero-dimensional (0D) Cu(I)-based compound of [(C 3 H 7 ) 4 N] 2 Cu 2 I 4 ([C 3 H 7 ) 4 N] + = tetrapropylammonium cation) was synthesized by a facile solution method, a monoclinic system of P 2 1 / n symmetry with a Cu 2 I 4 2- cluster as the confined structure. The as-synthesized [(C 3 H 7 ) 4 N] 2 Cu 2 I 4 exhibits bright dual-band pure white emission with a photoluminescence quantum yield (PLQY) of 91.9% and CIE color coordinates of (0.33, 0.35). Notably, this compound also exhibits an ultrahigh color rendering index (CRI) of 92.2, which is comparable to the highest value of single-component metal halides reported recently. Its Raman spectra provide a clear spectral profile of strong electron-phonon interaction after [(C 3 H 7 ) 4 N] + incorporation, favoring the self-trapped exciton (STE) formation. [(C 3 H 7 ) 4 N] 2 Cu 2 I 4 can give dual-STE bands at the same time because of the Cu-Cu metal bond in a Cu 2 I 4 2- cluster, whose populations could be scaled by temperature, together with the local dipole orientation modulation of neighboring STEs and phase transition related emission color coordinate change. Particularly, the outstanding chemical- and antiwater stability of this compound was also demonstrated. This work illustrates the potential of such cuprous halide perovskite-type materials in multifunctional applications, such as lighting in varied environments.