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

Abstract

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...

Recombination Dynamics in PbS Nanocrystal Quantum Dot Solar Cells Studied through Drift–Diffusion Simulations

“…17,18 For example, identifying trapassisted Shockley−Read−Hall (SRH) recombination as the dominant recombination process in PbS NC solar cells, and determining the impact of the NC band gaps and mobilities on recombination dynamics, was possible through fitting to temperature-dependent current−voltage (I−V) measurements. 19,20 While such approaches can provide insights into the dominant recombination mechanism, they only offer limited insights into where recombination occurs spatially within the device. Numerical drift diffusion simulations can be used to gain further insights into the spatial distribution of charge and the recombination mechanisms under different operating conditions.…”

“…Further improvement in the device performance can be accelerated with in depth knowledge of the recombination processes occurring in the solar cells under operating conditions. Fitting analytical models to experimental data provides important insights. , For example, identifying trap-assisted Shockley–Read–Hall (SRH) recombination as the dominant recombination process in PbS NC solar cells, and determining the impact of the NC band gaps and mobilities on recombination dynamics, was possible through fitting to temperature-dependent current–voltage ( I – V ) measurements. , While such approaches can provide insights into the dominant recombination mechanism, they only offer limited insights into where recombination occurs spatially within the device.…”

Surface recombination velocity on wet-cleaned silicon wafers using heterodyne lock-in ca rrierography imaging: measurement uniqueness investigation

“…Since LIC employs the lock-in algorithm, it has a better signal-to-noise ratio (SNR) than PL and can measure kinetic parameters in semiconductors as a dynamic (non-static) modality. It has been used to characterize electronic transport parameters (bulk lifetime, diffusion coefficient and SRV) [8][9][10] and electrical parameters (saturation current density, open-circuit voltage, fill factor and so on) in semiconductor materials and devices [11][12][13].…”

Simultaneous determination of effective carrier lifetime and resistivity of Si wafers using the nonlinear nature of photocarrier radiometric signals

“…Photocarrier radiometry (PCR) [16] and its imaging counter part lock-in carrierography [17], a frequency-domain (FD) photoluminescence (PL) based quantitative characterization technique that measures photocarrier density distributions, has been demonstrated to be capable of characterizing carrier recombination properties [18][19][20], mobility/diffusivity [21,22], ion implantation dose [23], junction properties [24,25], and trap states/activation energy [26,27] in various semiconductor materials and devices. However, PCR shares the same nonlinearity issues as other dynamic optical techniques, which may substantially compromise its theoretical rigor and measurement self-consistency and reliability.…”