Unraveling the Phase Transition and Luminescence Tuning of Pb-Free Cs–Cu–I Perovskites Enabled by Reaction Temperature and Polar Solvent

Ternary Pb-free Cs–Cu–I perovskites have attracted widespread attention because of their excellent optical properties and environmentally friendly advantages. Herein, two different Pb-free ternary Cs 3 Cu 2 I 5 nanocrystals (NCs) and CsCu 2 I 3 microrods (MRs) were synthesized via a heating method. The phase and morphology transition from blue emission of Cs 3 Cu 2 I 5 NCs to yellow emission of CsCu 2 I 3 MRs could be tuned effectively by manipulating the reaction temperature, decreasing the maximum photoluminescence quantum yields (PLQYs) from 82.7% to ∼10%. More interestingly, the Cs 3 Cu 2 I 5 NCs could self-assemble into stacking chains, which exhibited a strong dependence on the polarity of solvents. In addition, it was demonstrated that the rapid phase transition and luminescence tuning between Cs 3 Cu 2 I 5 and CsCu 2 I 3 films took only a few seconds by direct heating or exposure to the polar solvent. This work may deepen the understanding of the phase transition process in Cu-based perovskites and provide a fluorescence material with a short switching time for anticounterfeiting applications.