Interfacial Photophysics and Solvent‐triggered Phase Reconstruction in Cs 4 PbBr 6 Quantum Dots for Optical/Electrical Dual‐Mode Sensing

While all‐inorganic lead halide perovskite quantum dots offer tunable pathways toward programmable optoelectronics, the origin of stimulus‐activated phase transformation and its underlying photophysics remain controversial. Here, by constructing a composition‐tunable dual‐phase Cs 4 PbBr 6 ‐CsPbBr 3 quantum dots platform with direct interfacial contact, we have identified key factors that regulate the emission behavior through careful structural and spectroscopic analyses, including interfacial strain and localized‐states‐induced changes of carrier trapping and recombination pathways. Furthermore, in situ solvent treatment uncovers the hydroxyl‐triggered, polarity‐modulated CsBr extraction mechanism that drives the localized conversion from non‐emissive Cs 4 PbBr 6 to green emissive CsPbBr 3 . Such solvent‐programmable optical activation is further converted into an optical/electrical dual‐mode readout using a planar photoconductor. The dual‐mode response enables discrimination between hydroxyl‐containing and hydroxyl‐free solvents, identification of various protic solvents, and semi‐quantitative analysis of water content in ethanol‐water mixtures. This work provides synthetic and mechanistic strategies for regulating the structure‐property relationships of interphase perovskite quantum dots and establishes a dual‐mode sensing platform that couple photoluminescence activation with photoconductive signatures for solvent identification and discrimination.