Insight into the effect of ligand-exchange on colloidal CsPbBr3 perovskite quantum dot/mesoporous-TiO2 composite-based photodetectors: much faster electron injection

Abstract

In the current work, we fabricate an all-inorganic perovskite quantum dot-based photodetector with bifunctional linker molecules, which is highly superior to its counterpart without the linker. The ultrahigh on/off ratio of 10⁵ is two orders of magnitude higher than that of the device without the ligand treatment. Moreover, the responsivity increases from ∼2.2 A W⁻¹ to ∼24.5 A W⁻¹ and the detectivity from 3.5 × 10¹² Jones to 8.9 × 10¹³ Jones. To gain further insight into the effect of ligand-exchange by bifunctional linker molecules besides surface passivation, we divide the electron transfer process into two parts, the electron transfer from quantum dots (QDs) to the linker and from the linker to TiO₂. Injection of photoexcited electrons from colloidal CsPbBr₃ QDs into TiO₂ nanoparticles by bifunctional linker molecules is found to be much faster. Continuous-wave and time-resolved fluorescence spectra exhibit strong quenching for CsPbBr₃ QDs assembled by the linker or coupled to TiO₂ nanoparticles, which is consistent with electron transfer for QDs. From the results of the transient absorption measurement and fluorescence lifetime, the electron transfer time of QDs–TiO₂ linked by the bifunctional molecules is calculated to be within 40 ns, while that from QDs to TiO₂ particles directly is estimated to be ∼290 ns. This indicates that much faster electron injection occurs under the effect of the electron linker. In addition, the linker molecule modification can improve the adsorption of mp-TiO₂ to capture more QDs.