Highly Efficient Cool-White Photoluminescence of (Gua)CuI Single Crystals: Formation and Optical Properties.

Zero-dimensional lead-free organic-inorganic hybrid metal halides have drawn attention as a result of their local metal ion confinement structure and photoelectric properties. Herein, a lead-free compound of (Gua)CuI (Gua = guanidine) with a different metal ion confinement has been discovered, which possesses a unique [CuI] face-sharing tetrahedral dimer structure. First-principles calculation demonstrates the inherent nature of a direct band gap for (Gua)CuI, and its band gap of ∼2.98 eV was determined by experiments. Worthy of note is that (Gua)CuI exhibits a highly efficient cool-white emission peaking at 481 nm, a full-width at half-maximum of 125 nm, a large Stokes shift, and a photoluminescence quantum efficiency of 96%, originating from self-trapped exciton emission. More importantly, (Gua)CuI single crystals have a reversible thermoinduced luminescence characteristic due to a structural transition scaled by the electron-phonon coupling coefficients, which can be converted back and forth between cool-white and yellow color emission by heating or cooling treatment within a short time. In brief, as-synthesized (Gua)CuI shows great potential for application both in single-component white solid-state lighting and sensitive temperature scaling.