Achieving High Quantum\nEfficiency in Cs<sub>3</sub>Cu<sub>2</sub>I<sub>5</sub> Nanocrystals\nby the A‑Site Ion\nSubstitution for Flexible Blue Electroluminescence Devices and Enhanced\nPhotovoltaic Cells

The outstanding photovoltaic characteristics of lead\nhalide perovskites\nhave made them a potential class of materials for upcoming optoelectronic\napplications. However, a major obstacle to the practical use of lead\nhalide perovskites is the inherent toxicity and low environmental\nstability of lead. The exploration and development of nontoxic and\nstable alternatives for lead halide perovskites has therefore become\nan urgent and critical goal in the field of photovoltaics. In this\npaper, blue lead-free Cs<sub>3</sub>Cu<sub>2</sub>I<sub>5</sub> nanocrystals\n(NCs) with bright emission were prepared by using KI as a metal additive\nthrough an improved thermal injection strategy. A breakthrough in\nthe previous photoluminescence quantum (PLQY) yield of 97.2% was achieved\nwhen the content of KI was optimized to be 8%. Detailed experimental\nand theoretical studies have also been combined to conclude that the\nKI additive acts to passivate surface defects and that the broad-spectrum\nblue emission originates from self-trapped excitons. When combined\nwith commercial indium tin oxide (ITO), the device may achieve broad-spectrum\nblue electroluminescence. Its low production cost and simple structure\nare its main features. Furthermore, we were able to improve the utilization\nof high-energy ultraviolet energy and overcome the weak absorption\nof short-wavelength silicon-based solar cells by using nanocrystals\nas down-conversion luminescent materials in conjunction with silicon-based\nsolar cells. This increased the photoelectric conversion efficiency\n(PCE) for the solar cells through approximately 0.5%. The result was\nachieved through the principle of down-conversion. Therefore, fully\ninorganic metal halide Cs<sub>3</sub>Cu<sub>2</sub>I<sub>5</sub>:\nK<sup>+</sup> nanocrystals have great potential for future electroluminescence\nand photovoltaic applications.