Light Soaking Test Research Articles

Accelerating the evaluation of operational lifetimes of perovskite solar cells and modules

Compared with the power conversion efficicency, the operational stability of perovskite solar cells (PSCs) remains a major challenge hampering its commercialization. However, conducting a light soaking test under 1 sun illumination to get a long lifetime is time-consuming and experimentally inefficient. Here, we report an accelerated stability test protocol by aging PSCs under high-intensity light illumination to accelerate the evaluation of their operation stability. It is found that the efficiency degradation rate of a typical inverted PSC is almost linearly dependent on the light intensity within the range of 1 to 4 suns regardless of the encapsulations. The results prove that it can save the light-soaking time by at least 4 times to predict the operation lifetime on the basis of the equivalent light irradiation dose.

Improvement of Encapsulation Technique of Organic Photovoltaics by UV-Curable Adhesive

This study develops a low-cost UV adhesive as a sealant and the associated encapsulation architecture, highly improving the stability of organic photovoltaic (OPV) device. In detail, this present study demonstrates how the composition and additive in the UV adhesive as a sealant affect the OPV stability. The face sealing and UV adhesive enable the OPV device to successfully meet three industrial criteria of accelerated tests of OPV stability: (1) T80 lifetime is more than 1000 h under the damp heat test of 65 0C/65% relative humidity (RH) environment, (2) T80 lifetime is more than 1000 h under the damp heat test of 85 0C/85% RH environment, and (3) T80 lifetime is larger than 1000 h under the continuous AM 1.5G solar illumination (i.e., light-soaking test).

Design and characterization of a continuous solar simulator for photovoltaic modules with automatic I-V curve acquisition system

This paper presents the development, characterization, and testing of a continuous large-area solar simulator for PV modules with an automatic I-V curve acquisition system and the results of a CIGS PV module over 25 h of light soaking. The simulator was developed to perform the IEC 61215 stabilization test and automatic characterization of PV modules during indoor testing. The equipment records the I-V curves of the PV module at programmed time intervals so that the change in the electrical characteristics of the PV modules can be tracked over time. The test plane has an area of 1.2 × 2 m and an irradiance of 800 W/m2. The simulator was installed in a container and consists of 16 metal halide lamps of 1 kW each. The simulator is characterized by temporal instability, spatial non-uniformity, and spectral mismatch, resulting in CCB classification. The I-V curves of the tested PV module were measured every 30 s, with 200 I-V points per curve. The power change of the tested module occurs mainly in the first 12 min and stabilizes after about 5 h under the conditions selected for the test. The developed device can save significant resources compared to commercial continuous solar simulators. The ability to automatically track changes in PV module electrical parameters during light-soaking tests is presented in detail and provides important information for future work related to the development and characterization of solar simulators of this type.

A universal multi-additive strategy to enhance efficiency and stability in inverted perovskite solar cells

Perovskite solar technology has become a trend topic in the last decade, reaching promising efficiencies up to 25.7 %. Researchers mainly focused on obtaining high performance rather than caring for stability under accelerated stress conditions, such as thermal and light soaking tests. For this reason, we studied a standard triple cation perovskite (∼1.58 eV) and wider bandgap perovskite (∼1.63 eV) with the scope of finding a common strategy to build a robust device stable over time independently of the perovskite used. We use a combination of additives inside the perovskite ink: ionic liquid 1-Butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF4), alkylamine ligands oleylamine (OAm) and benzylhydrazine hydrochloride (BHC). Our work reveals that the combination of these additives helps to improve the efficiency and stability of the entire device, reaching a power conversion efficiency up to 21.3 % and over 20 % for both types of perovskite and stability beyond 1000 h under continuous light soaking. The universal applicability of this method was further applied to a robust methylammonium free perovskite, leading to an impressive stability both under light soaking and under 85 °C, showing T90 > 1500 h and T80 > 8658 h, respectively: the solar cells modified with the additive mixture retained 95 % of their efficiency after 1000 h under light soaking at 45 °C and 100 % of their efficiency after 1500 h of stress at 85 °C.

Dramatic improvement in the stability and mechanism of high-performance inverted polymer solar cells featuring a solution-processed buffer layer.

In this study, we demonstrate inverted PTB7:PC71BM polymer solar cells (PSCs) featuring a solution-processed s-MoO3 hole transport layer (HTL) that can, after thermal aging at 85 °C, retain their initial power conversion efficiency (PCE) for at least 2200 h. The T80 lifetimes of the PSCs incorporating the novel s-MoO3 HTL were up to ten times greater than those currently reported for PTB7- or low-band-gap polymer:PCBM PSCs, the result of the inhibition of burn-in losses and long-term degradation under various heat-equivalent testing conditions. We used X-ray photoelectron spectroscopy (XPS) to study devices containing thermally deposited t-MoO3 and s-MoO3 HTLs and obtain a mechanistic understanding of how the robust HTL is formed and how it prevented the PSCs from undergoing thermal degradation. Heat tests revealed that the mechanisms of thermal inter-diffusion and interaction of various elements within active layer/HTL/Ag electrodes controlled by the s-MoO3 HTL were dramatically different from those controlled by the t-MoO3 HTL. The new prevention mechanism revealed here can provide the conceptual strategy for designing the buffer layer in the future. The PCEs of PSCs featuring s-MoO3 HTLs, measured in damp-heat (65 °C/65% RH; 85 °C per air) and light soaking tests, confirmed their excellent stability. Such solution-processed MoO3 HTLs appear to have great potential as replacements for commonly used t-MoO3 HTLs.

Enhancing the efficiency and stability of p-i-n triple cation perovskite solar cells using benzyl cyanoacetate passivation agent

It is extremely difficult to realize high-performance perovskite solar cells (PeSCs) due to a large number of defects of perovskite. Thus, perovskite defects should be passivated as much as possible. In this study, effective cyano (CN), benzene ring, and carbonyl (CO) functional groups were newly introduced to benzyl cyanoacetate (BCA) to prepare a multifunctional BCA passivation agent. The influence of functional group of BCA passivation molecule on perovskite and PeSCs was systemically studied using cyanoethyl acrylate (CA) with CN without benzene ring and benzyl acrylate (BA) with benzene ring without CN as reference molecules. Through various analyses, we found that the novel BCA passivation agent with both CN and benzene ring passivated perovskite defects the most efficiently. Thus, the combination of each effective functional group can synergistically passivate more perovskite defects. As a result, PeSCs created using BCA showed improved power conversion efficiency (PCE) from 18.38 % to 20.23 %. Additionally, PeSC with BCA retained 70 % of the initial PCE after ∼2034 h of air exposure test, 80 % of the initial PCE after ∼193 h of heat aging test, and 60 % of the initial PCE after ∼72 h of light-soaking test, demonstrating better stability than PeSC without BCA in air/heat/light-soaking stability.

Scale-up fabrication and characteristic study of oligomer-like small-molecule solar cells by ambient halogen-free sheet-to-sheet and roll-to-roll slot-die coating

The upscale fabrication of the inverted small-molecule organic solar cell (SMOSC) from spin coating to large-area slot-die coating using halogen-free solvent under the ambient air environment is developed. This work successfully adopts thermal annealing treatment to tune the bulk heterojunction (BHJ) structure for two advantages of compatibility to mass production and good photo-stability. By tuning the respective optimum film structure, the power conversion efficiency (PCE) values under AM 1.5G light and indoor light (T5 lamp with 500 lx) illuminations for the ambient spin-coated device achieve 8.5% and 13.26%, respectively. The photo-bleaching and light-soaking tests under the AM 1.5G illumination show that the photo-stability of the thermally-annealed DRCN5T:PC71BM device is much better than PTB7-Th:PC71BM device. The tunable BHJ film morphologies of large-area ITO/glass-based sheet-to-sheet (S2S) and flexible roll-to-roll (R2R) slot-die coatings of DRCN5T:PC71BM SMOSCs are based on different dry-film formation and crystallization mechanisms. The structural control during the drying-film process for S2S and R2R coatings is a critical key to determine the optimum performance. The PCEs of the ITO/glass-based and flexible ITO/PET-based SMOSCs achieve 7.6% and 7.3%, respectively. The PCE of the ambient flexible R2R slot-die-coated SMOSC using halogen-free solvent is much higher than that of the currently flexible R2R coated SMOSCs. The PCEs of S2S slot-die-coated cell device of 1 × 5 cm2 and module achieve 5.1%. We conduct a characteristic, structural and systematic study of how to scale-up SMOSC via slot-die coatings, demonstrating the relationship among processing control, performance and structure for different coatings.

Comparative performances of nature-based dyes extracted from Baobab and Shea leaves photo-sensitizers for dye-sensitized solar cells (DSSCs)

Dye-sensitized solar cells (DSSCs) were fabricated using dye extracts from Baobab and Shea leaves as photo-sensitizers. The photoanode was made from spin-coating of titanium dioxide (TiO2) and annealed at 450°C for 1 ​h. The TiO2 shows good optical absorption capability in the ultraviolet region with a band gap of 3.3 ​eV and surface morphology that revealed grains that are spherical and randomly oriented with average particle size of (26±4.0) nm. The Shea and Baobab dyes have shown absorption at the blue-yellow region of the visible light with energy band gap of 1.66 ​eV and 1.89 ​eV respectively. The Shea-based DSSC (SDSSC) yielded power conversion efficiency (PCE) of (0.25±0.13) % with short current density, Jsc, open-circuit voltage, Voc and fill factor, FF of (2.66±1.61)mAcm−2, (0.158±0.0877) V and 0.596 ​± ​0.389, respectively while Baobab-based DSSCs (BDSSCs) gave lower PCE of (0.11±0.028) % with Jsc, Voc and FF of (0.286±0.0483)mAcm−2,0.676±0.0622 V and 0.592±0.0339. Light soaking tests show that SDSSCs are more stable in photoelectric performance than BDSSCs for 5 ​h. Hence, preferred as a sensitizer for better performance in dye-sensitized solar cells than Baobab dye extract.

Improving the Photostability of Small-Molecule-Based Organic Photovoltaics by Providing a Charge Percolation Pathway of Crystalline Conjugated Polymer.

Photostability of small-molecule (SM)-based organic photovoltaics (SM-OPVs) is greatly improved by utilizing a ternary photo-active layer incorporating a small amount of a conjugated polymer (CP). Semi-crystalline poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT) and amorphous poly[(2,5-bis(2-decyltetradecyloxy)phenylene)-alt-(5,6-dicyano-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2CNBT) with similar chemical structures were used for preparing SM:fullerene:CP ternary photo-active layers. The power conversion efficiency (PCE) of the ternary device with PPDT2FBT (Ternary-F) was higher than those of the ternary device with PPDT2CNBT (Ternary-CN) and a binary SM-OPV device (Binary) by 15% and 17%, respectively. The photostability of the SM-OPV was considerably improved by the addition of the crystalline CP, PPDT2FBT. Ternary-F retained 76% of its initial PCE after 1500 h of light soaking, whereas Ternary-CN and Binary retained only 38% and 17% of their initial PCEs, respectively. The electrical and morphological analyses of the SM-OPV devices revealed that the addition of the semi-crystalline CP led to the formation of percolation pathways for charge transport without disturbing the optimized bulk heterojunction morphology. The CP also suppressed trap-assisted recombination and enhanced the hole mobility in Ternary-F. The percolation pathways enabled the hole mobility of Ternary-F to remain constant during the light-soaking test. The photostability of Ternary-CN did not improve because the addition of the amorphous CP inhibited the formation of ordered SM domains.

Photovoltaic efficiency enhancement of polycrystalline silicon solar cells by a highly stable luminescent film

Si-based solar cells have dominated the entire photovoltaic market, but remain suffering from low power conversion efficiency (PCE), partly because of the poor utilization of ultraviolet (UV) light. Europium(III) (Eu3+) complexes with organic ligands are capable of converting UV light into strong visible light, which makes them ideal light converter to increase the efficiency of solar cells. However, the low stability of such complexes seriously hampers their practical applications. In this work, we report a highly stable and luminescent ethylene-vinyl acetate (EVA) copolymer film consisting of a Eu3+ complex as a down-shift material, Eu(ND)4CTAC (ND = 4-hydroxy-2-methyl-1,5-naphthyridine-3-carbonitrile, CTAC = hexadecyl trimethyl ammonium chloride), coating of which onto the surface of large area polycrystalline silicon solar cells (active area: 110 cm2) results in an increase of PCE from 15.06% to 15.57%. Remarkable stability of the luminescent film was also demonstrated under light-soaking test for 500 h, and no obvious luminescence degradation can be observed. The remarkable enhancement of the conversion efficiency by 0.51% absolute on such a large active area, together with the high stability of the luminescent film, demonstrates a prospect for the implementation of the films in photovoltaic industry.

Overall analysis of change in power generation with outdoor exposure of photovoltaic modules installed at AIST Kyushu Center

In this paper, the current status of the power generation capacity of installed photovoltaic modules, which include monocrystalline Si, multicrystalline Si, thin-film multijunctions consisting of hydrogenated amorphous Si (a-Si:H) and hydrogenated microcrystalline Si, copper indium gallium selenide, thin-film single-junction a-Si:H, passivated emitter and rear cell (PERC) monocrystalline Si, Si heterojunction (SHJ), and interdigitated back contact (IBC) modules, at the National Institute of Advanced Industrial Science and Technology is reported. The degradation of PERC modules was confirmed by indoor measurement and a light soaking test. In the case of SHJ modules installed in 2016, the output power was unstable under the initial light irradiation conditions. In the case of IBC modules, potential-induced degradation was observed for one type within only 6 months of outdoor exposure, and for the other type, it was observed within four years of outdoor exposure. A formula for estimating the amount of power generation from the degradation rate was proposed.

The state of external circuit affects the stability of dye-sensitized solar cells

We found that the electrical state in which dye solar cells operate affect their ageing. Three states were analysed: open-circuit (OC), short circuit (SC), and under maximum power point (MPP) / load. OC and SC are more or less atypical states, which are relevant while storing cells or in the event of malfunction, whereas the MPP/load corresponds to real life operation of the cells. Our results indicate that keeping the cells at OC or near the MPP lead to practically identical stability, whilst the cells at SC degraded much faster in a 1000 h light soaking test. The underlying cause for the degradation of all the cells was the loss of tri-iodide (i.e. limiting charge carriers) in the electrolyte. While the degradation mechanism appears to be the same, the loss rate of tri-iodide was about five times faster with SC than with OC and MPP cells. In the SC cells, the loss of tri-iodide decreased both the short-circuit current and fill factor resulting in a 36% efficiency loss by the end of the test. In contrast, the efficiency of the OC and load cells remained quite stable throughout the test. Since OC is the most commonly used state in aging tests, it is good news for ageing studies that the real life MPP state and the OC state yield roughly similar results. The identification of the degradation pathway, the loss of charge carriers, and the related degradation rate were used to estimate the remaining lifetime of cells which did not degrade during the 1000 h test. Based on the degradation rate related to the charge carrier loss, full degradation of the OC and MPP cells is expected in approximately 3000 h of operational time.

Vapor Annealing Controlled Crystal Growth and Photovoltaic Performance of Bismuth Triiodide Embedded in Mesostructured Configurations.

Low stability of organic-inorganic lead halide perovskite and toxicity of lead (Pb) still remain a concern. Therefore, there is a constant quest for alternative nontoxic and stable light-absorbing materials with promising optoelectronic properties. Herein, we report about nontoxic bismuth triiodide (BiI3) photovoltaic device prepared using TiO2 mesoporous film and spiro-OMeTAD as electron- and hole-transporting materials, respectively. Effect of annealing methods (e.g., thermal annealing (TA), solvent vapor annealing (SVA), and Petri dish covered recycled vapor annealing (PR-VA)) and different annealing temperatures (90, 120, 150, and 180 °C for PR-VA) on BiI3 film morphology have been investigated. As found in the study, grain size increased and film uniformity improved as temperature was raised from 90 to 150 °C. The photovoltaic devices based on BiI3 films processed at 150 °C with PR-VA treatment showed power conversion efficiency (PCE) of 0.5% with high reproducibility, which is, so far, the best PCE reported for BiI3 photovoltaic device employing organic hole-transporting material (HTM), owing to the increase in grain size and uniform morphology of BiI3 film. These devices showed stable performance even after 30 days of exposure to 50% relative humidity, and after 100 °C heat stress and 20 min light soaking test. More importantly, the study reveals many challenges and room (discussed in the details) for further development of the BiI3 photovoltaic devices.

Light-soaking tests of zinc oxide photoanodes sensitized with an indoline dye on different transparent conductive substrates

Dye-sensitized solar cells (DSCs) were prepared using porous zinc oxide (ZnO) films on aluminum-doped zinc oxide (AZO) and fluorine-doped tin oxide (FTO) transparent conductive glass substrates. X-ray diffraction measurements revealed that, the porous ZnO films were crystallographically oriented differently on the two transparent substrates. The two DSCs were prepared using metal-free indoline dye as the sensitizer and a liquid electrolyte as the hole conductor. Measurements of the power conversion efficiency of the two DSCs over a period of time showed deterioration in the conversion efficiency of the DSCs with the deterioration being faster in ZnO/FTO than ZnO/AZO. The deterioration is attributed mainly to the decrease in light-harvesting ability of the sensitizer and recombination of photo-excited electrons resulting in the decrease in the short-circuit photocurrent densities and the open-circuit voltages in both DSCs during the light-soaking process.

Reducing Trap-Assisted Recombination in Small Organic Molecule-Based Photovoltaics by the Addition of a Conjugated Block Copolymer.

The performance of organic photovoltaics (OPVs) based on the small-molecule organic semiconductor p-DTS(FBTTh2 )2 is greatly improved by the addition of a conjugated block copolymer composed of difluoroquinoxaline and thienopyrrolodione blocks (D130). The power conversion efficiency (PCE) of the p-DTS(FBTTh2 )2 -based OPV is improved from 5.08% to 6.75% by the addition of 5 wt% D130 to the photoactive layer, which is composed of p-DTS(FBTTh2 )2 and a fullerene derivative. Current-voltage and grazing incidence wide-angle X-ray scattering analyses revealed that the addition of D130 significantly reduces the trap density of the device and changes the packing orientation of p-DTS(FBTTh2 )2 from mostly edge-on to partially face-on. These changes greatly improve the charge carrier mobility of the OPV, indicating that D130 is highly compatible with p-DTS(FBTTh2 )2 . Furthermore, the addition of D130 improve the photostability of the OPV by reducing the burn-in loss under a light soaking intensity of 1 sun. The D130-based OPV maintained 34% of its initial PCE after a light soaking test for 858 h. In contrast, the PCE of the OPV without D130 reduced to 14% of its initial efficiency in the same time period.

Atmospherically Processed and Stable Cs-Pb Based Perovskite Solar Cells

ABSTRACTIn this work, a planar heterojunction superstrate n-i-p device based on Zn(O,S) electron transport layer and CsPbI2Br absorber material at 1.93 eV bandgap is presented. The CsPbI2Br films are deposited using a 2-step atmospheric solution deposition process and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-vis spectroscopy and photoluminescence (PL). Best device with an efficiency of 12.34 % and 11.94% in reverse and forward scans respectively and stabilized power output of 12.14 mW/cm2 has been demonstrated via atmospheric solution processing with minimal hysteresis between forward and reverse scans. The devices show voltage dependent current collection as well as light-dark crossover in forward bias. Light soaking tests at 65 °C and 1-sun at Voc, resulted in open-circuit voltage and fill-factor degradation. Electroluminescence (EL) after 100 hours of light soaking shows a reduction in overall EL intensity as well a shift in emission to lower wavelength. The devices exhibit a positive temperature coefficient of about 0.14 %/°C. It is found that Zn(O,S) is a viable alternative electron transport layer to replace TiO2. By replacing methylammonium cation with cesium and addition of Br has improved the stability of the perovskite phase.

Gel Electrolytes with Polyamidopyridine Dendron Modified Talc for Dye-Sensitized Solar Cells.

Organic-inorganic hybrid layered materials are proposed as additives in a quasi-solid gel electrolyte for dye-sensitized solar cells. Talcs could provide a low-cost and environmentally friendly, as well as abundant, option as gelators. Here, talcs were prepared by functionalizing an organotalc with three polyamidopyridine dendron generations, PAMPy-talc-Gn (n = 1, 2 and 3). PAMPy dendrons grow parallel to the lamellae plane and form an organized structure by intermolecular interactions. In addition, polyiodide-dendron charge-transfer complexes were prepared onto the organotalc by adsorption of iodine. In this work, the effect of the dendron generation of PAMPy-talc and the influence of polyiodide intercalation on solar cell performance and stability were investigated. The best results were reached with the use of lowest-generation PAMPy-talc (η = 4.5 ± 0.3%, VOC = 710 ± 19 mV, Jsc = 10.4 ± 0.9 mA cm-2, and FF = 61 ± 2%): 15% higher efficiency compared to similar liquid devices. While some previously studied talcs illustrate very strong absorption of the iodide from the electrolyte, in the case of PAMPy-talc such interfering effects were absent: In a 1000 h light soaking test, the PAMPy-talc cells both with and without polyiodide intercalation demonstrated stable performances. Furthermore, the color analysis of the electrolyte indicated that the color of the electrolyte remained stable after an initial period of stabilization, which is a good indication of the compound being stable and not absorbing charge carriers from the electrolyte. The performance and stability results indicate that PAMPy-talc has potential as a gelling method for electrolytes for dye solar cells.

Current Induced Stabilization (CIS) of Large Area Commercial Amorphous Silicon Photovoltaic Module and Its Performance Variation

To reduce the demand for fossil fuel energy, one of the solutions is using an renewable energy sources, such as wind and solar. Solar energy is increasingly becoming the hottest issue in recent years because of its merits in installation and operation. In 2015, about 50.7 GW photovoltaic (PV) systems were installed around world. Many renewable energy organizations expect that the total PV system installation will be 60 GW in the coming year.[1] To sustain the long-term stability of the PV system, several institutes have performed tests based on IEC standards, such as IEC 61215 for silicon crystalline PV modules and IEC 61646 for thin film PV modules and IEC 61730-1&2 for safety tests. In the case of amorphous silicon thin film modules,[2,3] its performance can be lowered because its natural characteristics under sun light called, the Staebler-Wronski effect.[4,5] Therefore, some companies conduct light soaking tests to guarantee stable electric performance before shipment. On the other hand, in the case of having high degradation characteristics at the maximum power, this test could take much time for production and lead to an increase in product cost. To solve this technical difficulty and obtain actual stable performance power of silicon-based amorphous solar modules, the current-induced stabilization (CIS) technique was proposed and assessed.

Improving the stability of CdS quantum dot sensitized solar cell using highly efficient and porous CuS counter electrode

Thin films from copper sulfide (CuxS) are the most commonly used electrocatalyst counter electrodes (CEs) for high-efficiency quantum dot sensitized solar cells (QDSSCs) because of its superior electrocatalytic activity in the presence of polysulfide electrolytes. In addition to the stability issues, the CuxS CEs are usually prepared by complicated, costly, time consuming, and less productive methods, which are inadequate for practical applications of QDSSCs. In this work, we present a simple approach for fabricating an efficient and stable CE for QDSSCs using pure covellite phase CuS nanoparticles (NPs) pre-prepared via a cheap, fast, and scalable chemical method. The catalyst ink was obtained by mixing the as-prepared CuS NPs with polyvinylidene fluoride, as a polymeric binder, which was then directly applied to a conductive fluorine-doped tin oxide substrate without any further high temperature post treatment. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and Tafel polarization measurements were used to investigate the electrocatalytic activity of the CuS NPs CE. The power conversion efficiency of 2.6% was achieved from CdS QDSSC assembled with CuS NPs CE, which was higher than 1.57% for conventional Cu2S/brass and 1.33% for Pt CEs under one-sun illumination. The CdS QDSSC with CuS NPs CE was also able to supply a constant photocurrent value without any obvious decrease under light soaking test, in contrast to the devices with Cu2S/brass and Pt CEs, which showed inferior stability. This remarkable photovoltaic performance was attributed to the nanoporous morphology and the excellent electrocatalytic activity of CuS NPs CE.

One-step fabrication of copper sulfide nanoparticles decorated on graphene sheets as highly stable and efficient counter electrode for CdS-sensitized solar cells

Quantum-dot-sensitized solar cells (QDSSCs) are thin-film photovoltaics and highly promising as next-generation solar cells owing to their high theoretical efficiency, easy fabrication process, and low production cost. However, the practical photoconversion efficiencies (PCEs) of QDSSCs are still far below the theoretically estimated value owing to the lack of an applicable design of the materials and electrodes. In this work, we developed a highly stable and efficient counter electrode (CE) from copper sulfide nanocrystals and reduced graphene oxide (CuxS@RGO) for QDSSC applications. The CuxS@RGO electrocatalyst was successfully prepared by a facile one-pot hydrothermal method, then directly applied to a fluorine-doped tin oxide (FTO)-coated glass substrate by the simple drop-casting technique. Owing to the synergistic effect between CuxS nanocrystals and conductive RGO sheets, the CuxS@RGO CE showed high electrocatalytic activity for polysulfide electrolyte reduction. A CdS QDSSC based on the CuxS@RGO CE yielded a high and reproducible PCE of 2.36%, exceeding those of 1.57 and 1.33% obtained with the commonly used Cu2S/brass and Pt CEs, respectively. Moreover, the QDSSC with the CuxS@RGO CE showed excellent photostability in a light-soaking test without any obvious decay in the photocurrent, whereas the cell based on the Cu2S/brass CE was severely degraded.