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The outstanding photovoltaic characteristics of lead halide perovskites have made them a potential class of materials for upcoming optoelectronic applications. However, a major obstacle to the practical use of lead halide perovskites is the inherent toxicity and low environmental stability of lead. The exploration and development of nontoxic and stable alternatives for lead halide perovskites has therefore become an urgent and critical goal in the field of photovoltaics. In this paper, blue lead-free Cs3Cu2I5 nanocrystals (NCs) with bright emission were prepared by using KI as a metal additive through an improved thermal injection strategy. A breakthrough in the previous photoluminescence quantum (PLQY) yield of 97.2% was achieved when the content of KI was optimized to be 8%. Detailed experimental and theoretical studies have also been combined to conclude that the KI additive acts to passivate surface defects and that the broad-spectrum blue emission originates from self-trapped excitons. When combined with commercial indium tin oxide (ITO), the device may achieve broad-spectrum blue electroluminescence. Its low production cost and simple structure are its main features. Furthermore, we were able to improve the utilization of high-energy ultraviolet energy and overcome the weak absorption of short-wavelength silicon-based solar cells by using nanocrystals as down-conversion luminescent materials in conjunction with silicon-based solar cells. This increased the photoelectric conversion efficiency (PCE) for the solar cells through approximately 0.5%. The result was achieved through the principle of down-conversion. Therefore, fully inorganic metal halide Cs3Cu2I5: K+ nanocrystals have great potential for future electroluminescence and photovoltaic applications. [ABSTRACT FROM AUTHOR]