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Third-generation solar cells, including dye-sensitized solar cells (DSSCs) and quantum dot-sensitized solar cells (QDSSCs), have been associated with low-cost material requirements, simple fabrication processes, and mechanical robustness. Hence, counter electrodes (CEs) are a critical component for the functionality of these solar cells. Although platinum (Pt)-based CEs have been dominant in CE fabrication, they are costly and have limited market availability. Therefore, it is important to find alternative materials to overcome these issues. Transition metal chalcogenides (TMCs) and transition metal dichalcogenides (TMDs) have demonstrated capabilities as a more cost-effective alternative to Pt materials. This advantage has been attributed to their strong electrocatalytic activity, excellent thermal stability, tunability of bandgap energies, and variable crystalline morphologies. In this study, a comprehensive review of the major components and working principles of the DSSC and QDSSC are presented. In developing CEs for DSSCs and QDSSCs, various TMS materials synthesized through several techniques are thoroughly reviewed. The performance efficiencies of DSSCs and QDSSCs resulting from TMS-based CEs are subjected to in-depth comparative analysis with Pt-based CEs. Thus, the power conversion efficiency (PCE), fill factor (FF), short circuit current density (Jsc) and open circuit voltage (Voc) are investigated. Based on this review, the PCEs for DSSCs and QDSSCs are found to range from 5.37 to 9.80% (I−/I3− redox couple electrolyte) and 1.62 to 6.70% (S−2/Sx− electrolyte). This review seeks to navigate the future direction of TMS-based CEs towards the performance efficiency improvement of DSSCs and QDSSCs in the most cost-effective and environmentally friendly manner. [ABSTRACT FROM AUTHOR]

• Composite MXene/CoS based CE is first time used in QDSSCs. • The novel MXene/CoS CE shows better charge transfer and catalytic properties.. • The QDSSC with MXene/CoS CE exhibit a high efficiency of 8.09%. • A better photovoltaic performance than most of reported similar composite CEs. Low electrocatalytic activity and/or high charge transfer resistance of counter electrodes (CEs) in quantum-dot-sensitized solar cells (QDSSCs) are the main reasons for the low photovoltaic performance. Herein, the CoS nanoparticles are anchored on the MXene (Ti 3 C 2 T x) via a simple in-situ hydrothermal reaction process. It is the first attempt to use the MXene/CoS as a highly electrocatalytic CE for QDSSCs. The QDSSCs with a MXene/CoS CE shows a significant enhancement in cell performances and yields a promising power conversion efficiency (PCE) of 8.09% compared with that of the QDSSCs with bare CoS (5.77%) and bare MXene (4.25%) CEs. The significant enhancement of PCE value is mainly attributed to the synergistic effects of the unique layered morphology of conductive MXene nanosheets and its cocatalysis with CoS nanoparticles. The coupling of layered MXene with small CoS nanoparticles (~15 nm) provides abundant catalytic active sites. The improved mesoporous morphology contributes a high specific surface area and fast electron transfer channels for S x 2− reduction. Consequently, the MXene/CoS nanocomposite can be utilized as an effective agent to boost up the photovoltaic performance of QDSSCs. [ABSTRACT FROM AUTHOR]

In this work, we report an electrodeposition technique for fabricating chalcopyrite phase CuInSe 2 (CISe) thin films and demonstrate the great prospect of CISe films for serving as efficient counter electrodes (CEs) in quantum dot sensitized solar cells (QDSCs). The as-prepared CISe materials, directly deposited onto conducting substrates, display long-range uniformity with high crystallinity and nanopillar shapes. A CISe CE-based CdS/CdSe QDSC delivers a considerable power conversion efficiency of 2.92%, coupled with excellent device stability. In contrast to the conventional noble Pt electrodes, CISe CEs exhibit much higher catalytic activity for electrocatalysing polysulfide electrolyte regeneration. This work offers a promising CE with superior catalytic capability for constructing highly efficient and stable QDSCs. [ABSTRACT FROM AUTHOR]

The electrochemical performance of counter electrodes (CEs) plays an important role for achieving high efficient quantum dot sensitized solar cells (QDSSCs). Considering the optical band gap energy of semiconductor materials and the V oc of the QDSSCs is determined by the energy difference between the quasi-Fermi levels ( q E f ) of the anode and cathode, so in this work, we synthesized the different metal sulfides and their composites including CuS/CoS, CuS/NiS, CuS, CoS and NiS by chemical bath deposition (CBD) method and used as CEs of QDSSCs. The effect of different CEs on performance of cells were investigated, and the best power conversion efficiency (η) of 5.22% was achieved by CuS/CoS CE, which is higher than that of CuS/NiS, CuS, CoS and NiS (η = 2.56%, 4.73%, 2.23% and 1.62%). The improved cell efficiency was attributed to the excellent electrical conductivity and catalytic activity of CuS and CoS, respectively. The result indicated t hat the combination of different metal sulfide composites could increase open-circuit voltage (V oc ) value without sacrificing the short-circuit current (J sc ), and leaving good perspectives for significantly higher solar cell performances. [ABSTRACT FROM AUTHOR]

Currently, most of CuS counter electrodes (CEs) used in quantum dot-sensitized solar cells (QDSSCs) are provided with a thickness of hundreds of nanometers or even several microns. Considering the CE with low thickness having many advantages, thin CuS films with thickness ranging from 47 nm below to 115 nm have been synthetized in this paper via chemical bath deposition (CBD) method with different bath concentrations. A power conversion efficiency (PCE) of 3.25% has been achieved utilizing CuS thin films a thickness of only 64 nm as CEs in QDSSCs without any structural optimization, which is higher than the value of the cell employing CuS CE with thickness of 2.8 μm. Electrochemical impedance spectroscopy, Tafel polarization, and two-point current-voltage measurements are used to investigate the electrocatalytic and conductive performance of CuS CEs with different thickness. Owing to the highest electrocatalytic capacity and good conductivity of the 64 nm-thick CuS CE, QDSSC assembled with this CE has reached relatively high PCE under one sun illumination (100 mW cm −2 , AM 1.5). In addition, cyclic voltammetry measurements indicate that the thin CuS CE has a good stability against the polysulfide electrolyte. [ABSTRACT FROM AUTHOR]

A low-cost, stable and efficient FeS thin film counter electrode (CE) was fabricated for quantum dot-sensitized solar cells (QDSSCs) by a facile chemical bath deposition method using different acidic hydroxyl solvents (ethanol, isopropanol, and tert-butanol) and triethanolamine. The choice of solvents greatly affected the surface morphology and thickness of FeS CEs. Apart from surface morphology, the increase in the atomic percentage of sulfur in tert-butanol and triethanolamine-mediated FeS also plays a crucial role in increasing the electrocatalytic activity of the CE. The QDSSC’s power conversion efficiency was 2.15%, which is attributed to the high fill factor (FF) of 0.443. The obtained PCE is far better than the previously reported value of 1.76% for FeS in QDDSC’s. Excellent stability of the photoelectric performance of the FeS CE is also demonstrated. The enhanced electrochemical performance of the interwoven FeS networks can be attributed to their unique morphology, which possesses a shorter diffusion length of the ions and easier transportation of the electrons. [ABSTRACT FROM AUTHOR]

A series of novel PbS/TiO 2 nanotubes was fabricated as counter electrodes (CEs) through a combination of anodic oxidation and successive ionic-layer adsorption and reaction methods. The photovoltaic-conversion efficiency of ZnO nanorods sensitized with CdS quantum dots using the optimized PbS(6)/TiO 2 CE increased by nearly 440.8% and 227.5% compared with the efficiency of nanorods using planar Pt and bare TiO 2 CE, respectively. Thus, the novel PbS/TiO 2 composite CE was a promising catalytic electrode with excellent electrocatalytic activity for the reduction of charge carriers in polysulfide electrolyte. [ABSTRACT FROM AUTHOR]

Copper sulfide (Cu 2 S) films were deposited on F-doped SnO 2 (FTO) substrates via the electrodeposition (ED) of copper (Cu) nanoparticles followed by sulfurization. The Cu nanoparticles were deposited on FTO substrates for various ED times ranging from 10 to 30 min at a constant −0.4 V. The FTO/Cu films consisted of flower-like nanoparticles comprised of randomly-clustering nanoflakes. The Cu nanoparticles electrodeposited for 10 min (FTO/Cu (10 min)) were dispersed sparsely over the FTO substrate, whereas the FTO/Cu (20 and 30 min) provided increased coverage. Unlike FTO/Cu 2 S (10 min), the FTO/Cu 2 S (20 and 30 min) consisted of vertically-standing large Cu 2 S nanosheets with numerous small nanosheets in between. This was attributed to the sufficient number of Cu seed nanoflakes, which not only facilitate ion transport of the redox couple but also increased the surface area, leading to significantly enhanced electrocatalytic activity. The quantum dot-sensitized solar cell (QD-SSC) with FTO/Cu 2 S (20 min) exhibited a significantly improved cell efficiency of 4.58%, compared to those with Pt and FTO/Cu 2 S (10 min). The QD-SSC with the FTO/Cu 2 S (30 min) showed similar cell efficiency to that with the FTO/Cu 2 S (20 min), despite the larger surface area because of its amorphous crystallographic structure offsetting the electrocatalytic activity. [ABSTRACT FROM AUTHOR]

An important component of quantum dot sensitized solar cells (QDSSC) is the counter electrode which mediates the regeneration of oxidized quantum dots by reducing the polysulphide electrolyte. However, design and synthesis of an efficient counter electrode material is a challenging task. Herein, we report the synthesis of a unique Cu 2 S nanohusks directly on FTO coated glass substrates by electrodeposition and used as a counter electrode in QDSSC. When these electrodes are used for the reduction of polysulfide electrolyte in QDSSC, they exhibit higher catalytic activity and photovoltaic performance as compared to the Platinum counter electrode. The power conversion efficiency of about 4.68% has been achieved by optimizing the deposition time of Cu 2 S. [ABSTRACT FROM AUTHOR]

In this paper, highly efficient nano peas like structure CuS film has been successfully employed in quantum dot sensitized solar cells (QDSSCs) for its highest catalytic activity at minimal cost. The CuS thin film electrode was deposited on fluorine-doped tin oxide (FTO) substrate by chemical bath deposition technique using urea at low deposition temperatures. This electrode elevated the short circuit current and fill factor in comparison to the frequently used Pt electrode. Moreover, electrochemical measurement data disclosed higher electrocatalytic activity toward polysulfide reduction. The CuS film exhibited an average particle size of 180–270 nm and film thickness of 825 nm. It also revealed superior J sc (13.87 mA/cm 2 ) and conversion efficiency of 4.01% which is remarkably higher than that of the Pt-based cell (1.07%). In addition, stability test conducted for both CuS and Pt-based cells for about 900 min at working conditions affirmed that the long-term stability of the CuS film decreased by 16.66% (4.02–3.35%), while that of the Pt based cell got elevated by 24.29% until 700 min, and consequently diminished by 8.27% from 700 to 900 min. [ABSTRACT FROM AUTHOR]

Nickel sulfide (NiS) thin film has been deposited on a fluorine-doped tin oxide substrate by a hydrothermal method using 3-mercaptopropionic acid and used as an efficient counter electrode (CE) for polysulfide redox reactions in quantum dot-sensitized solar cells (QDSSCs). NiS has low toxicity and environmental compatibility. In the present study, the size of the NiS nanoparticle increases with the hydrothermal deposition time. The performance of the QDSSCs is examined in detail using polysulfide electrolyte with the NiS CE. A TiO 2 /CdS/CdSe/ZnS-based QDSSC using the NiS CE shows enhanced photovoltaic performance with a power conversion efficiency (PCE) of 3.03%, which is superior to that of a cell with Pt CE (PCE 2.20%) under one sun illumination (AM 1.5, 100 mW cm −2 ). The improved photovoltaic performance of the NiS-based QDSSC may be attributed to a low charge transfer resistance (5.08 Ω) for the reduction of polysulfide on the CE, indicating greater electrocatalytic activity of the NiS. Electrochemical impedance spectroscopy, cyclic voltammetry, and Tafel-polarization measurements were used to investigate the electrocatalytic activity of the NiS and Pt CEs. [ABSTRACT FROM AUTHOR]

Vacuum thermal evaporated Cu x S (VTE-Cu x S) film on fluorine-doped tin oxide substrate has been investigated as counter electrode (CE) for quantum dot-sensitized solar cells (QDSCs) with polysulfide electrolyte. The photovoltaic parameters of QDSCs show an obvious dependence on the annealing time of Cu x S film, and the maximum power conversion efficiency of 3.16 ± 0.05% under one sun illumination (100 mW cm −2 , AM 1.5 G) was obtained when the VTE-Cu x S CE was annealed at 270 °C for 300 s (VTE-Cu x S-300s CE). Electrochemical impedance spectroscopy, Tafel polarization, and cyclic voltammetry measurements have been employed to investigate the electrocatalytic activity of VTE-Cu x S-300s CE. The electrocatalytic activity of the VTE-Cu x S-300s CE is much higher than that of Pt CE, and is slightly higher than that of Cu 2 S CE in situ prepared on brass sheet. In particular, VTE-Cu x S-300s CE holds high diffusion velocity of S 2− /S n 2− in polysulfide electrolyte, and the stability of its electrocatalytic activity in polysulfide electrolyte is better than that of Pt and Brass-Cu 2 S CEs obviously. [ABSTRACT FROM AUTHOR]

Abstract: The Cu2ZnSnS4 (CZTS) thin films with uniform and porous surface feature with the pore size of 100-200nm were successfully prepared by pyrolysis procedure using the methanol solution containing copper chloride dihydrate (0.06mol dm−3), zinc chloride (0.03mol dm−3), stannous chloride dihydrate (0.03mol dm−3), thiourea (0.48mol dm−3) and deionized water (1.92mol dm−3) as precursor solution at 380°C in air atmosphere. Electrochemical impedance spectroscopy was applied to evaluate the electrochemical catalytic activity of the pyrolysis CZTS thin films for redox couple of Sn 2-/S2− and the charge transfer resistance was 64.08Ω using the pyrolysis CZTS thin film obtained by repeating the procedure of dipping the FTO substrate into the precursor solution and heating at 380°C for 7 cycles. The assembled quantum dot-sensitized solar cells gave an open-circuit photovoltage of 0.52V, a short-circuit photocurrent density of 12.96mAcm−2, and a fill factor of 0.38, corresponding to the photoelectric conversion efficiency of 2.56%. Because the pyrolysis procedure was a facile, low cost and vacuum-free process, and had the advantage of obtaining various porous microstructure thin films and allowing deposition over large areas, the pyrolysis CZTS thin films can serve as an effective counter electrode for QDSCs. [Copyright &y& Elsevier]

[Display omitted] • A new way of preparation of rGO-Cu 2 S counter electrode using electrochemical co-deposition. • Low cost, minimal time and simple procedure for the fabrication process. • Reduced R ct and R s values of rGO-Cu 2 S CE for QDSSCs application. • Cell efficiency significantly improved by enhancement of current density and fill factor. Reduced graphene oxide (rGO)-Cu 2 S as a counter electrode (CE) is synthesized by a novel electrochemical method for quantum-dot-sensitized solar cell (QDSSC) applications. One-step electrochemical co-reduction using cyclic voltammetry for various cycles (3, 5, and 7), followed by sulfurization, is employed to obtain rGO-Cu 2 S CEs on the surface of fluorine-doped tin oxide. Compared with Pt and Cu 2 S CEs, rGO-Cu 2 S CEs (5) display lower charge transfer resistance (R ct) and higher electrocatalytic activity. This is because the introduction of rGO along with Cu 2 S in the rGO-Cu 2 S surface enhances the surface area and charge transfer and improves the electrical contact between the CEs and substrate surface. An optimum power conversion efficiency (η) of 4.26% is recorded for rGO-Cu 2 S (5) in CdS/CdSe/ZnS QDSSCs using a polysulfide electrolyte. QDSSCs with rGO-Cu 2 S (5) showed the highest short-circuit current density (J sc) of 17.2 mA cm−2, open circuit voltage (V oc) of 0.57 V, and fill factor of 44%. The obtained results are superior to those for Cu 2 S (3.77%) and Pt (2.44%) CEs. Additionally, the preparation method of rGO-Cu 2 S CEs is more scientific, less time-consuming, economical, and easy. [ABSTRACT FROM AUTHOR]

• PbS nanoparticles were grown by SILAR technique on electrochemically synthesized Cu 2 S nanosheets. • FTO/Cu 2 S/PbS CE has the stepwise energy level configuration which enhances the fast charge transportation. • FTO/Cu 2 S/PbS CE has enhanced electrocatalytic activity as compared to the FTO/Cu 2 S CE. • FTO/Cu 2 S/PbS exhibited excellent electrochemical stability in polysulfide electrolyte. • QDSSC with the FTO/Cu 2 S/PbS (3 SILAR cycles) CE showed the highest efficiency. A new strategy has been successfully developed for highly efficient copper sulfide/lead sulfide (Cu 2 S/PbS) counter electrodes (CEs) for quantum-dot-sensitized solar cells (QDSSCs). PbS nanoparticles were grown by successive ionic layer adsorption and reaction (SILAR) technique for various cycles (from 1 to 4) on electrochemically deposited Cu 2 S nanosheets. All the Cu 2 S and Cu 2 S/PbS CEs showed petal-like morphology and nanosized PbS nanoparticles were deposited over the Cu 2 S nanosheets. All FTO/Cu 2 S/PbS CEs exhibited superior electrocatalytic activity than FTO/Pt and FTO/Cu 2 S CEs. In particular, QDSSCs with FTO/Cu 2 S/PbS CE (3 SILAR cycles) exhibited extremely high short-circuit current density (18.08 mA cm−2) and fill factor (53.55%), resulting in a significantly enhanced power conversion efficiency as high as 5.28%. This is because the FTO/Cu 2 S/PbS CE not only exhibits the cascaded, stepwise energy level configuration, which enhances the fast charge transportation, but also serves as a blocking layer to prevent electrons from returning from the electrolyte to the Cu 2 S. Furthermore, FTO/Cu 2 S/PbS exhibited excellent electrochemical stability unlike FTO/Cu 2 S, owing to the passivation of PbS. [ABSTRACT FROM AUTHOR]