Lock-in and Heterodyne Carrierographic Imaging Characterization of Industrial Multicrystalline Silicon Solar Cells

et al. 2016

J. Phys. Chem. C

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.

Section: Theoretical Methodsmentioning

confidence: 99%

Section: Can Be Expressed Bymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Yang

et al. 2016

J. Phys. Chem. C

“…To overcome the abovementioned limitations, a heterodyne scheme has been proposed . The idea is to construct a slow enough beat frequency component through frequency mixing.…”

Section: Introductionmentioning

confidence: 99%

Camera‐based high frequency heterodyne lock‐in carrierographic (frequency‐domain photoluminescence) imaging of crystalline silicon wafers

Sun

Melnikov

Physica Status Solidi (a)

2015

A heterodyne lock‐in imaging scheme is introduced to overcome NIR camera speed limitations and poor signal‐to‐noise ratios, thereby generating high‐frequency carrierographic imaging up to 10 kHz. A theoretical model was established to investigate the heterodyne signal generation mechanism as well as to extract the major carrier transport parameters by best‐fitting the entire modulation frequency dependence. Good agreement was found between the theoretically predicted and experimentally measured heterodyne amplitude dependence on excitation intensity. A contrast inversion feature in the high‐frequency heterodyne amplitude images was observed. High‐frequency information presents a method for resolving local near‐surface carrier transport parameters from the lumped effective lifetime, thereby providing ac‐diffusion‐length‐controlled depth‐selective/resolved imaging. Heterodyne lock‐in carrierography (LIC) amplitude images of a crystalline silicon wafer at different frequencies.

“…The inspection of the entire solar cell area or some specific regions can also fulfill various other purposes such as overall device performance, shading effects, electrical, and/or mechanical defects. Solar cell large‐area imaging has been used intensively with perovskite solar cells using camera‐based photoluminescence, series resistance imaging using electroluminescence, lock‐in thermography (LIT), lock‐in carrierography (LIC) characterization of Si wafers, and solar cells . However, to the best of our knowledge, no imaging studies of CQDSCs have been reported in efforts to acquire an insightful physical picture of defect or contact effects on key solar cell performance parameters.…”

Section: Introductionmentioning

confidence: 99%

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

Liu

Progress in Photovoltaics

et al. 2017

Although the power conversion efficiency (PCE) of colloidal quantum dot solar cells (CQDSCs) has increased sharply, researchers are struggling with the lack of comprehensive device efficiency optimization strategies, which retards significant progress in CQDSC improvement. This paper addresses this critical issue through analyzing the impact of colloidal quantum dot (CQD) carrier hopping mobility, bandgap energy, illumination intensity, and electrode/CQD interface on device performance to develop a guiding criterion for CQDSC PCE optimization. This general strategy has been used for the successful fabrication of high-efficiency CQDSCs yielding certified PCEs as high as 11.28 %. A major experimental finding of this work is that the widely used constant photocurrent density (J ph ) assumption is invalid as J ph is external-voltage dependent due to the low carrier hopping mobility. Furthermore, the theoretical model developed herein predicts the nonmonotonic dependence of CQDSC PCE on carrier hopping mobility and bandgap energy, which were also demonstrated with the high-efficiency CQDSCs. These results constitute a revision basis of the widespread belief that higher mobility and lower bandgap energy correspond to a higher CQDSC efficiency. Furthermore, electrode/CQD interfacedependent surface recombination velocities were investigated in the framework of our abovementioned theoretical model using lock-in carrierography, a contactless, large-area frequency-domain photocarrier diffusion-wave imaging methodology that elucidated the carrier collection process at the electrodes through open-circuit voltage distribution imaging.Lock-in carrierography eliminates the limitations of today's widely used small-spot (<0.1 cm 2 ) testing methods which, however, raise questionable overall solar cell performance and stability estimations. KEYWORDS bandgap energy, colloidal quantum dot solar cell, electrode-semiconductor interface, hopping mobility, large-area imaging, lock-in carrierography 1 | INTRODUCTION 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. 1 Intensive efforts are underway to boost CQDSC PCE through device architecture engineering, 2-4 surface materials chemistry, 5-7 synthesis methodologies, 8,9 charge carrier dynamics, 10-17 and theoretical modeling. 18-20 However, no comprehensive device efficiency optimization strategies have been reported aiming at achieving higher PCE, specifically for CQDSCs.Researchers use common sense approaches instead, trying to improve CQDSC efficiency through pursuing higher carrier mobility using disparate surface passivation materials and increasing quantum dot size for lower bandgap energy to harvest the solar spectrum in a wider wavelength range. This universal strategy, however, is typically valid for conventional solar cells of high carrier mobility such as Si solar cells, rather t...