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

Abstract

Perovskite-shelled colloidal quantum dots (CQDs) with low trap-state density are promising candidates for largescale, 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 (MAPbI 3 ) perovskitepassivated 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 τ E , hopping diffusivity D h , dark and bright state separation energy ΔE, and carrier trapping rate N T . 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.

Perovskite-quantum dots interface: Deciphering its ultrafast charge carrier dynamics

“…The TA spectra of the MAPbI3/QDs and Consequently, the blue shift in the PB2 band of MAPbI3/QDs is due to changes in the electronic and/or crystallographic structure of MAPbI3 in presence of the QDs. Such modifications were observed after passivation by the QDs due to the lattice mismatch between the two materials [18,21,44,45]. The interpretation of the blue shift also agrees with reported band gap expansion of MAPbI3 related to structural deformation [46].…”

“…Thirdly, electrons can be transferred and subsequently trapped in perovskite/QDs interface. Indeed, it has been reported that, in the presence of QDs, the crystallographic structure of the MAPbI3 lattice might be different inside the crystallite and at the interface with the QDs, thus, new trap states will appear [18,21,44].…”

Colloidal quantum dot solar cell power conversion efficiency optimization using analysis of current‐voltage characteristics and electrode contact imaging by lock‐in carrierography

“…A better electrode coating with lower interface states leads to low S ( d ) that results in higher CQDSC performance. To investigate the CQD/Au interface effects on CQDSC performance, nondestructive imaging (NDI) of carrier population distributions and key photovoltaic parameters was conducted using LIC, a spectrally gated dynamic frequency‐domain photoluminescence‐based transport property imaging method . The LIC images revealed the complexity and inhomogeneity of the electrode‐coating‐associated surface recombination in our experimental CQDSCs.…”

“…Furthermore, the calibration for spectral mismatch was conducted using a reference solar cell (Newport). Finally, following our previously reported method, LIC imaging of the CQDSCs was performed in a room‐temperature nitrogen environment under 10 Hz modulation frequency.…”

“…Colloidal quantum dot solar cells (CQDSCs) are presently attracting immense research interest on a global scale due to the meteoric rise of their solar to electric power conversion efficiency (PCE) from 3% to 13.4% within a period of only 7 years . Intensive efforts are underway to boost CQDSC PCE through device architecture engineering, surface materials chemistry, synthesis methodologies, charge carrier dynamics, and theoretical modeling . However, no comprehensive device efficiency optimization strategies have been reported aiming at achieving higher PCE, specifically for CQDSCs.…”