Photoelectric Properties of Ultrastable Carbon Quantum Dots and TiO ₂ -Composited CsPb(Cl/Br) ₃ Perovskite Nanocrystals

The composites of CsPb(Cl/Br)3 incorporating carbon quantum dots (CQDs) were prepared via a thermal injection method. Enhanced-stability CsPb(Cl/Br)3@CQDs@TiO2 composites were fabricated from CsPb(Cl/Br)3@CQDs precursors through tetrabutyl titanate hydrolysis. The transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analyses confirm the successful formation of a TiO2 coating on the composite. The transient photoluminescence (TRPL) measurements reveal lifetimes of 3.89, 4.2, and 2.3 ns for CsPb(Cl/Br)3, CsPb(Cl/Br)3@CQDs, and CsPb(Cl/Br)3@CQDs@TiO2, respectively. The increased lifetime in CsPb(Cl/Br)3@CQDs suggests that CQDs may contribute to surface passivation of CsPb(Cl/Br)3, suppressing nonradiative recombination. The shortened lifetime observed after TiO2 coating indicates further modification of interfacial carrier relaxation dynamics. Temperature-dependent PL analysis shows that CsPb(Cl/Br)3@CQDs@TiO2 exhibits a higher exciton binding energy (65.7 meV) compared with CsPb(Cl/Br)3@CQDs (36.2 meV), indicating an enhanced excitonic state in the TiO2-coated composite structure. The electrochemical impedance spectroscopy (EIS) and photocurrent measurements indicate reduced charge transfer resistance and enhanced photocurrent density (0.6 μA cm–2), consistent with improved interfacial carrier transport. The environmental stability tests further confirm that CsPb(Cl/Br)3@CQDs@TiO2 exhibits significantly improved stability compared with CsPb(Cl/Br)3@CQDs. These results highlight the synergistic roles of CQD-induced passivation and TiO2 interfacial engineering in modulating interfacial carrier behavior, leading to improved optoelectronic performance and environmental stability.