Progress in Lead-free Hybrid Organic-Inorganic Metal Halides for Optoelectronic Applications

Lead-based perovskites have advanced tremendously over the last decade, producing solar cells with PCE > 24% and LEDs with EQE over 20 % for both red and green. However, the toxicity and instability of lead halide perovskites will likely limit their use in many instances. Here we have synthesized a novel class of copper-based micro- and nanomaterials that have high QY, up to 100%, and are long lived when packaged under nitrogen. We show recent improvements in copper-based self-trapped excitonic (STE) emitters, to produce highly luminescent, long lived emitters with red to near-IR emission peaks. In particular, we report alkali copper(I) iodide (ACuI)-based (A = Na, K, Rb, and Cs) organic-inorganic hybrid materials with different structures and emission peaks by adjustment of the alkali atoms and organic molecules. We can also substitute bromide and chloride for pure and mixed halides, such as red emissive lead-free hybrid organic-inorganic copper halides A₆(C₄H₈OS)₁₂[Cu₈X₁₃][Cu₄X₄(OH)(H₂O)] (ACX-THTO, A = K, Rb, and Cs; X = Cl, Br; THTO = C₄H₈OS). These compounds possess strong photoluminescence with emission peaks in the range of 530-660 nm with a highest PLQY of nearly 100%. In addition to copper based compounds we examined one-dimensional (1D) Na₃SbBr₆(C₂H₆OS)₆ and Na₃SbBr₆(C₄H₈OS)₆ single crystals, which exhibit bright yellow and orange emission with PL peaks at 610 nm and 664 nm, respectively, and a high photoluminescence quantum yield (PLQY) of 86% was obtained for Na₃SbBr₆(C₂H₆OS)₆. These two compounds can be reversibly converted into each other by the removal and addition of the organic components. In general, the technique for producing hybrid organic inorganic copper halides can be extended to several members of the family, with increasing stability, in order to synthesize a wide array of emissive species, covering the visible spectrum. Green, yellow, and red down-conversion light emitting devices (LEDs) can be fabricated by using these materials, and a white LED based on all-copper iodide phosphors is demonstrated with a high color rendering index (CRI) of 93 by utilizing a mixture of blue-emissive Cs₃Cu₂I₅ and yellow-emissive Rb₂Cu₂I₄(C₄H₈OS)₃powders. With high brightness and large absorption cross sections in the UV, these materials are ideal for white light arrays, and could be used as well in displays where visible wavelength transparency is more critical than color purity of emission.