Quantitative Analysis of Trap-State-Mediated Exciton Transport in Perovskite-Shelled PbS Quantum Dot Thin Films Using Photocarrier Diffusion-Wave Nondestructive Evaluation and Imaging

Perovskite-shelled colloidal quantum dots (CQDs) with low trap-state density are promising candidates for large-scale, low-cost, and lightweight solar cell applications. However, even minimal trap states can significantly limit CQD-based solar cell efficiency. We reported trap-state-mediated exciton transports in methylammonium lead triiodide (MAPbI3) perovskite-passivated PbS CQD thin films. Excitation power-dependent photocarrier radiometry (PCR) intensity study demonstrated the free (electrons/holes)-to-bound (acceptors/donors) and trap-state-related transition-induced nonlinear response of CQD thin films to excitation. The existence of shallow trap states (activation energy: 33.8–40.7 meV) was characterized using photothermal emission spectra at different modulation frequencies. CQD thin films were imaged, for the first time, by InGaAs-camera-based nondestructive homodyne and heterodyne lock-in carrierographies (LIC; spectrally gated dynamic photoluminescence imaging), clearly showcasing photocarrier density diffusion-wave inhomogeneities that stem from defect-associated multiple effective exciton lifetimes. PCR frequency scans, coupled with the trap-state-mediated exciton transport model, extracted multiple material and carrier transport parameters such as effective exciton lifetime τE, hopping diffusivity Dh, dark and bright state separation energy ΔE, and carrier trapping rate NT. Camera-based contactless homodyne and heterodyne LIC imaging of QD thin films was found to be a promising nondestructive characterization technique for monitoring optoelectronic qualities of QD materials and devices.