Controlled growth of a nanoplatelet-structured copper sulfide thin film as a highly efficient counter electrode for quantum dot-sensitized solar cells

Abstract

An ideal counter electrode (CE), with high-level electrocatalytic activity, high performance stability, and applicable fabrication simplicity, is significant to convey the advantages of quantum-dot-sensitized solar cells (QDSSCs). We report CuS nanoplatelets that are cohered on a conductive FTO substrate using an inexpensive and facile one-step low-temperature chemical bath deposition technique with different concentrations of acetic acid and can be employed as CE without any post-treatments for QDSSCs. The acetic acid affects the morphology, density of nanostructure, thickness, and Cu vacancies with increased S composition of CuS. The TiO₂/CdS/CdSe/ZnS QDSSC with the optimized CuS CE under one sun illumination (AM 1.5G, 100 mW cm⁻²) exhibits an energy conversion efficiency (η) of 5.15%, which is higher than that of a platinum (Pt) CE (1.25%). This enhancement is mainly attributed to the optimized CuS nanostructure, which exhibits a lower charge transfer resistance (7.89 Ω) at the interface of the CE/electrolyte and superior electrochemical catalytic ability. A preliminary stability test reveals that the CuS CE exhibits good stability with no degradation for 20 h. Therefore, the easily prepared CuS is very promising as a stable and efficient CE for QDSSCs.

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1. Introduction

In recent years, researchers have made several efforts for preserving the environment and lowering energy utilization. These efforts have led to a greater focus on renewable energy resources. Solar power is regarded as one of the most advantageous energy conversion devices for electrical energy because of its abundance, safety, cleanliness, and high economic value. Dye-sensitized solar cells (DSSCs) have a promising future for third-generation solar cell applications owing to their ease of fabrication, low production cost, reasonably high conversion efficiency, earth-abundant materials, and use in a variety of applications.^1–3 The superior efficiency of DSSCs relies on effective separation and transportation of the charges generated in dye molecules. However, the energy conversion efficiency of DSSCs is still lower than that in conventional Si solar cells, and we need to discover an alternative material due to the low absorption coefficient and photo degradation in DSSCs.^4–6

Quantum dot-sensitized solar cells (QDSSCs) are gaining tremendous attention as a promising approach to develop third-generation solar cells due to the versatile advantages of QD sensitizers in comparison with conventional dye sensitizers.^7,8 QDs have attractive properties like the potential to achieve efficiency beyond the Shockley–Queisser limit (31%) through multiple exciton generation,^9,10 higher extinction coefficients,¹¹ an energy band gap that can be tuned by changing the size,¹² large intrinsic dipole moments,^13,14 low cost, facile synthesis, and direct hot carrier collection to multiply the current generation.¹⁵ In contrast to the conventional dyes of DSSCs, the theoretical maximum photovoltaic power conversion efficiency of a QD solar cell could reach 44%.

Normally, QDSSCs are composed of a TiO₂ photo anode, sensitizer, redox electrolyte, and counter electrode (CE). To improve the performance of the QDSSCs, intensive efforts should be focused on developing photoanodes with more efficient light absorption and faster electron injection and CEs with better electrocatalytic activity and faster charge transfer. The role of a CE in a solar cell is to collect electrons from external circuits and to regenerate the holes by catalyzing the reduction of the oxidized species in the electrolyte and keep the cell working. Consequently, superior electrocatalytic activity, vast surface area, outstanding electrical conductivity, chemical stability, low production cost, easy preparation, and ideal efficiency are a part of the fundamental prerequisites for efficient CEs. Therefore, the optimization of CEs plays a significant role in the improvement of QDSSCs.

Conventional planar platinum (Pt) is stable and an effective CE for DSSCs, with superior electrocatalytic activity and perfect stability in the iodide/triiodide (I⁻/I₃⁻) redox couple electrolyte. The I⁻/I₃ redox couple is not a suitable candidate for QDSSCs because it causes photocorrosion to the QDs.¹⁶ Polysulfide (S²⁻/S_n²⁻) is more preferred for QDSSCs over I⁻/I₃⁻ to minimize photocorrosion.^17,18 Pt has very high conductivity, but the photovoltaic performance of QDSSCs in polysulfide electrolyte (S²⁻/S_n²⁻) is poor due to chemisorption of sulfur-containing compounds on the Pt CE surface. This causes catalytic poisoning and results in poor electrocatalytic activity and high over-potential. This hinders the reduction of the oxidized redox species of the electrolyte and rapidly reduces the cell's performance.^19,20 Also, Pt is an expensive metal and has extremely low abundance in nature, which limits the application in solar cells. Therefore, it is a major hurdle to find an effective CE that is cost-effective, abundant, and efficient for attaining high FF, high photocurrent density (J_SC), and high power conversion efficiency (η) for QDSSCs.

Thus, to improve the performance of a QDSSC, in recent times, several investigations have been done to introduce a new material with novel structures for a potential CE for polysulfide electrolyte. These include CuS,^20,21 NiS,^20,22 CoS,^20,23 PbS,²⁴ Cu₂ZnSnS₄,²⁵ and carbon²⁶ films. These are found to be cost-effective alternative materials to Pt CEs, and the best results were reported for metal sulfide electrodes. CuS is one of the transition metal chalcogenides and is a suitable CE material since it not only yields high efficiency but also is cost-effective and has superior photocatalytic, electrocatalytic, and conductivity properties.^20,21,23,27 Some issues include high overpotential for polysulfide reduction at the CE, large internal resistance, and stability. Very few studies have focused on the precise design and control of the structure and morphology of a CE to facilitate the charge collection at the CE/electrolyte interface to enhance the performance of QDSSCs.²⁸ Therefore, it is necessary to continuously research low-cost CEs with high catalytic activity and satisfactory stability.

In the present work, CuS was deposited in situ on FTO glass substrates by a facile and low-temperature process of chemical bath deposition (CBD) using various concentrations of acetic acid and the thin film was used as a CE without any post-treatment. The morphology-controlled CuS nanoplatelets and Cu : S ratio affect the electrocatalytic activity of CuS films used in a QDSSC as a CE. Benefitting from the unique features of optimized nanostructured CuS as CEs, the QDSSC exhibited a high open circuit voltage (V_OC) of 0.596 V, a high J_SC of 17.43 mA cm⁻², and enhanced η of 5.15%, which is 312% higher than that obtained using a Pt CE. The physicochemical properties of the synthesized catalysts were investigated through scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray diffraction (XRD) measurements. The electrocatalytic activities of a CuS CE in QDSSCs were evaluated based on the device photovoltaic performance, cyclic voltammetry (CV), electrochemical impedance spectra (EIS), and Tafel polarization measurements.

2. Experimental section

2.1 Materials

Copper sulfate pentahydrate (CuSO₄·5H₂O), thioacetamide (C₂H₅NS), acetic acid (CH₃COOH), cadmium acetate dehydrate [Cd(CH₃COO)₂·2H₂O], sodium sulfide (Na₂S), sodium thiosulfate (Na₂S₂O₃), selenium (Se), zinc acetate dehydrate [Zn(CH₃COO)₂·2H₂O], sulfur (S), potassium chloride (KCl), and TiO₂ paste (Ti-nanoxide HT/SP) were supplied by Solaronix. All other chemicals were commercially available and of analytical grade.

2.2 Preparation of CuS and Pt CEs

Morphology-controlled CuS thin films were deposited on well-cleaned fluorine-doped tin oxide (FTO) glass substrates with a sheet resistance of 7 Ω cm⁻² (Hartford Glass) by the facile CBD method. Prior to deposition, FTO substrates were cleaned ultrasonically with acetone, ethanol, and distilled (DI) water for 10 min each. In 25 mL of DI water, 0.1 M CuSO₄·5H₂O and 1 M C₂H₅NS were dissolved and vigorously stirred for 20 min. Under constant stirring, 0.2 M of CH₃COOH was added, and the reaction mixture was stirred vigorously for 20 min.

The as-prepared CuS substrates were immersed into the growth solution vertically and placed in a hot air oven. The deposition was carried out at a constant temperature of 60 °C for 60 min. The CH₃COOH concentration was varied to 0.4 M, 0.6 M, and 0.8 M, and the experimental procedure was repeated. After 60 min of deposition, the electrodes were cleaned with DI water and ethanol. The CuS films obtained at CH₃COOH concentrations of 0.2 M, 0.4 M, 0.6 M, and 0.8 M were labeled as 0.2 M-CuS, 0.4 M-CuS, 0.6 M-CuS, and 0.8 M-CuS, respectively. To fabricate the Pt electrode, well-cleaned FTO glass substrate was coated with a Pt paste (Pt-catalyst T/SP, Solaronix) using the doctor blade method and sintered at 450 °C in air for 10 min.

2.3 Preparation of TiO₂/CdS/CdSe/ZnS photoanode

The FTO substrates were cleaned ultrasonically and used for the fabrication of a photoanode. FTO substrates were cleaned ultrasonically with acetone, ethanol, and DI water and used for the fabrication. Porous TiO₂ films were coated on FTO from commercially available TiO₂ paste with a particle size of 20 nm (Ti-nanoxide HT/S, Solaronix) by the doctor blade method with an active area of 0.27 cm². It was then annealed at 450 °C for 30 min, producing a thickness of 7.5 μm after solvent evaporation. The SILAR process was used to deposit the CdS and CdSe QDs on the surface of TiO₂ according to our previous work.²⁹

For the deposition of CdS QDs, TiO₂ porous films were immersed into an aqueous solution of 0.1 M Cd(CH₃COO)₂·2H₂O for 5 min and rinsed with DI water and ethanol. They were then immersed into an aqueous solution of 0.1 M Na₂S for 5 min, rinsed with DI water and ethanol, and dried with a dryer. The process was conducted at room temperature and repeated five times. The as-prepared electrodes are named as TiO₂/CdS. For the deposition of CdSe QDs, the TiO₂/CdS electrodes were immersed into an aqueous solution of 0.1 M Cd(CH₃COO)₂·2H₂O for 5 min at room temperature, followed by rinsing with DI water and ethanol and then immersing into an aqueous solution of Na₂SeSO₃ for 5 min at 50 °C. This was followed by rinsing with DI water and ethanol and drying with the dryer. The process was repeated eight times. The as-prepared samples were named as TiO₂/CdS/CdSe. Finally, TiO₂/CdS/CdSe electrodes were immersed into an ethanol solution of 0.1 M Zn(CH₃COO)₂·2H₂O for 2 min and rinsed with DI water and ethanol. They were then immersed into an aqueous solution of 0.1 M Na₂S for 2 min, followed by rinsing with DI water and ethanol and then drying with the dryer. The as-prepared samples are called TiO₂/CdS/CdSe/ZnS.

2.4 Fabrication of QDSSC and symmetrical dummy cells

The as-prepared photoanodes and CEs were used to fabricate a QDSSC with polysulfide electrolyte. The electrolyte (2 M S, 1 M Na₂S, and 0.1 M KCl) was prepared by dissolving S in a solution of methanol and water (at a ratio of 7 : 3) with Na₂S and KCl. The CdS/CdSe/ZnS sensitized photoelectrodes, and CuS-based CEs were assembled with a 60 μm Surlyn spacer (Solaronix) into full cells. The polysulfide electrolyte was then injected into the cells by a vacuum backfilling technique. For cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Tafel measurements, symmetric dummy cells were fabricated by assembling two identical CEs face to face and filling the polysulfide electrolyte in the same manner.

2.5 Characterization

The surface morphology, thickness, and elemental compositions of the electrodes were investigated using a field emission scanning electron microscope (FE-SEM, S-2400, Hitachi) equipped with energy-dispersive X-ray spectroscopy (EDX) operated at 15 kV. X-ray diffraction (XRD) analysis was performed on a D8 ADVANCE with a DAVINCI diffractometer (Bruker AXS) using Cu Kα radiation operated at 40 kV and 40 mA. X-ray photon spectroscopy (XPS) was performed using a VG Scientific ESCALAB 250 with monochromatic Al-Kα radiation of 1486.6 eV and an electron take-off angle of 90°. The surface roughness of the substrates prepared using CBD (CuS) was characterized by atomic force microscope (JPK NanoWizard II AFM, JPK instruments, Berlin, Germany) with a scan rate of 0.8 Hz in contact mode.

Tafel polarization with a scan rate of 10 mV s⁻¹, EIS, and cyclic voltammetry (CV) analysis were performed using a BioLogic potentiostat/galvanostat/EIS analyzer (SP-150, France) for the symmetrical dummy cells with CuS/CuS and Pt/Pt CEs in dark conditions in a frequency range of 100 mHz to 500 kHz. The current–voltage (J–V) characteristics of QDSSCs were examined under one sun illumination (AM 1.5G, 100 mW cm⁻²) using an ABET Technologies (USA) solar simulator with an irradiance uniformity of ±3%. The incident photon-to-current conversion efficiency (IPCE) spectra were measured using an Oriel® IQE-200™. EIS was conducted on the QDSSCs using a BioLogic potentiostat/galvanostat/EIS analyzer (SP-150, France) under one sun illumination at frequency ranging from 100 mHz to 500 kHz. The applied bias voltage and AC amplitude were set to V_OC of the QDSSCs and 10 mV, respectively. The electrical impedance was characterized using Nyquist and Bode plots. To perform the stability test, QDSSCs based on CuS and Pt CEs were continuously irradiated with AM 1.5G radiation at 100 mW cm⁻² in working conditions, and the J–V curves were tested every 1 h for 10 h.

3. Results and discussion

Fig. 1 shows the SEM image of the CuS thin film samples deposited with different concentrations of acetic acid using the CBD method over FTO glass substrate. The pH of the solution for 0.2 M, 0.4 M, 0.6 M, and 0.8 M of acetic acid were 3, 2.8, 2.3, and 2.1, respectively. The pH of the solution varied with the addition of acetic acid. The pH from the acetic acid changes the surface morphology of the CuS thin films. The dependence of the morphology of films under various pH values is indicated in Fig. 1. The SEM image in Fig. 1(a) shows that the film consists of densely packed nanoplatelet structures with size of ∼238 nm and thickness of ∼445 nm (Fig. S1†).

Fig. 1 SEM images of (a) 0.2 M-CuS, (b) 0.4 M-CuS, (c) 0.6 M-CuS, and (d) 0.8 M-CuS thin films deposited on FTO substrates using facile CBD method.

The thickness, density, and sizes of the nanoplatelets increase with the acetic acid concentration from 0.2 M (pH 3) to 0.8 M (pH 2.1). The nanoplatelets in 0.2 M-CuS and 0.4 M-CuS change into an extended network of leaf-like nanoplatelets in 0.6 M-CuS and 0.8 M-CuS with increased density. As the acetic acid concentration increases, the space between the nanoplatelets is reduced with denser and bigger nanoplatelets. The thicknesses of the 0.4 M-CuS, 0.6 M-CuS, and 0.8 M-CuS films are 480, 512, and 556 nm, respectively (Fig. S1†). The change in thickness may alter the electrocatalytic activity.²¹ The improved morphology of nanoplatelets with uniform pores and optimized thickness provides many opportunities to alter the structural and electronic properties of the CE, which improves the catalytic activity and performance of the CE by promoting electron production.

The surface morphology of the CuS films was greatly influenced by the acetic acid concentration and pH value. The reaction is as follows:

CuSO₄ → Cu²⁺ + SO₄²⁻ (1)

Thioacetamide reacts with H₂O and dissociates to give H₂S.

CH₃–CS–NH₂ + H₂O → CH₃–CO–NH₂ + H₂S (2)

H₂S + H₂O → SH⁻ + H₃O⁺ (3)

SH⁻ + H₂O → S²⁻ + H₃O⁺ (4)

Cu²⁺ + S²⁻ → CuS (5)

Here, acetic acid plays a dual role of acting as a complexing agent and enriching the S²⁻ concentration against S atoms. The thermodynamic equilibria for the metal sulfide formation suggest that the sulfur concentration is dominated by the pH of the solution.³⁰

Energy-dispersive X-ray spectroscopy (EDX) analysis was used to evaluate the elemental compositions of CuS thin films on FTO substrate, and the results are shown in Fig. S2.† The S²⁻ ions generated through the hydrolysis of C₂H₅NS are readily attracted by Cu²⁺ ions. The supply of S²⁻ ions is enriched by acetic acid which also acts as a stabilizing agent.³¹ The EDX data show that the atomic ratios of Cu and S are 37.59 : 32.42, 34.65 : 33.29, 30.08 : 34.73, and 33.77 : 32.96 in the 0.2 M-CuS, 0.4 M-CuS, 0.6 M-CuS, and 0.8 M-CuS electrodes, respectively. The sulfur content in 0.2 M-CuS was 32.42%, and it increased to 34.73% when the concentration of acetic acid increased from 0.2 M to 0.6 M. This is because acetic acid acts as a reducing agent to Cu(II) ions and enriches S²⁻, resulting in more sulfur ions. However, when increasing the amount of acetic acid from 0.6 M to 0.8 M, the Cu content increased from 30.08% to 33.77% and the S percentage decreased from 34.73% to 32.96%.

In contrast, the formation of Cu is decreased from 37.59% to 30.08% when the concentration of acetic acid is increased from 0.2 M to 0.6 M, and the Cu percentage is increased with the addition of more acetic acid (0.6 M to 0.8 M). Apart from surface morphology, the increase in the atomic ratio of S in Cu : S might also play a vital role in increasing the electrocatalytic activity of the CE. The S content in 0.6 M-CuS is the highest at 30.08 : 34.73 (Cu : S), and it has well-arranged nanoplatelet structure, giving the electrolyte the ability to diffuse more freely. On the other hand, the electrocatalytic activity of CuS films may decrease as the amount of Cu in the CuS films becomes progressively richer.

Fig. 2(a) shows the X-ray diffraction pattern of CuS thin films on the FTO substrate with different concentrations of acetic acid. Some of the peaks of the CuS thin films were overlapping with the peaks arising from the FTO. The peaks centered around 31.8, 32.8, 44.5, 60.8, and 77.8° were assigned to the (103), (006), (008), (204), and (212) planes corresponding to hexagonal CuS and match well with ICDD file [no. 079-2321]. The strong diffraction of FTO is mainly due to the low thickness of the as-synthesized sample without any heat treatment.

Fig. 2 (a) X-ray diffraction patterns of CuS thin films deposited on FTO substrates. XPS spectra of the as-prepared 0.6 M-CuS: (b) survey spectra, (c) high-resolution spectra of S 2p and (d) high-resolution spectra of Cu 2p.

XPS was used to investigate the chemical composition and purity of the prepared sample and to identify the chemical status of Cu in the 0.6 M-CuS thin film on the surface of FTO substrate, as shown in Fig. 2(b)–(d). Fig. 2(b) exhibits the XPS full survey spectrum of 0.6 M-CuS film, and the presence of peaks of Cu 2p and S 2p can clearly be observed. In Fig. 2(c), the high-resolution survey in the S 2p region shows the presence of two peaks at 160.8 eV and 162.0 eV. These are assigned to the binding energies of S 2p_3/2 and S 2p_1/2 of CuS, respectively. Fig. 2(d) shows the high-resolution spectrum of the Cu 2p region, revealing the presence of two strong peaks. It is obvious that the two strong peaks separated by 19.9 eV at 932.3 eV and 952.2 eV for Cu 2p_3/2 and Cu 2p_1/2, respectively, are essentially identical binding energies for the Cu 2p orbital in accord with Cu(II).³² These results are in agreement with the reported values of the binding state of the elements of CuS.³³ Therefore, the XPS results suggest that the CuS is successfully deposited on the surface of FTO.

To study the surface roughness of the films, two-dimensional (2D) and three-dimensional (3D) micrographs of CuS thin films were recorded using AFM, as shown in Fig. 3. The AFM images indicate that by varying the acetic acid concentration, the grain size and roughness are changed. Therefore, based on the AFM micrographs, it seems that one may control the grain size and roughness by choosing the concentration of acetic acid. The root mean square roughness (R_rms) of the CuS film deposited with 0.2 M acetic acid is 24.9 nm. This increases to 32.95 nm with the acetic acid increasing to 0.4 M. At 0.6 M, the R_rms increases to 51.88 nm. This indicates that the films become much rougher with increasing acetic acid concentration. Further addition of acetic acid to 0.8 M decreases R_rms to 18.61 nm. The roughest film of 0.6 M-CuS offers more electrocatalytically active sites for the reduction of the polysulfide redox couple in the electrolyte. A CE with a high R_rms value would have enhanced electrocatalytic activity for a polysulfide electrolyte system and also reduce the charge-transfer resistance at the CE/electrolyte interface, resulting in improved photovoltaic performance in a QDSSC.³⁴

Fig. 3 2-Dimensional (2D) and 3-dimensional (3D) AFM images of (a) 0.2 M-CuS, (b) 0.4 M-CuS, (c) 0.6 M-CuS, and (d) 0.8 M-CuS thin films on FTO substrate.

Fig. 4 shows the structure of a QDSSC based on a CdS sensitizer on TiO₂ and the CuS CEs with polysulfide electrolyte. The photoanode absorbs the incoming photons, and the CE plays a vital role in reduction of polysulfide electrolyte. The redox behavior of the polysulfide system is successful in the QDSSCs due to the slow recombination of generated electrons between TiO₂/oxidized QDs and redox polysulfide, leading to longer electron lifetimes. The slower recombination and faster QD regeneration rates of the redox electrolyte and the transfer of electrons from the CE during the reduction of polysulfide also result in better QDSSC performance. The route to lower recombination is optimizing the electrical conductivity and catalytic activity of the CE. The major barriers for commercialization of efficient QDSSCs are cost, ease of manufacturing, and large-area production. The low charge transfer resistance (R_ct) at the CuS CE/electrolyte interface indicates good electrocatalytic activity of a CE for the reduction of the oxidized redox couple.

Fig. 4 QDSSC prepared using a TiO₂/CdS/CdSe/ZnS photoelectrode and a CuS CE in presence of polysulfide electrolyte.

To understand the electrocatalytic activity of the CuS electrodes towards the reduction of polysulfide electrolyte was verified by cyclic voltammetry (CV), symmetrical cells were made and measured at a scan rate of 100 mV s⁻¹, and the obtained CV are shown in Fig. 5(A). From the CV curve, the negative and positive peaks correspond to the reduction of S_n²⁻ and the oxidation of nS²⁻, respectively. The high current peak indicates active electrocatalytic reaction of the electrode for the S²⁻/S_n²⁻ redox couples in the polysulfide electrolyte.

Fig. 5 (A) Cyclic voltammograms (CV) of the symmetrical cells based on CuS electrodes at a scan rate of 100 mV s⁻¹. (B) Nyquist plots of the CuS symmetrical dummy cells toward polysulfide electrolyte with a recipe of 2 M S, 1 M Na₂S, and 0.1 M KCl in a solution of methanol and water, which were present at a ratio of 7 : 3. The frequency range was set from 100 mHz to 500 kHz, and the amplitude of the alternating current was set to 10 mV in the impedance measurements. (C) High impedance range of Pt. The equivalent circuit in the inset of (C) is used to simulate the EIS curves. (D) Tafel curves of symmetrical cells fabricated with two identical electrodes of (a) 0.2 M-CuS (b) 0.4 M-CuS, (c) 0.6 M-CuS, (d) 0.8 M-CuS, and (e) Pt.

The current density obtained for the CuS CEs is larger than of the Pt CE. The lower performance of the Pt CE is due to the lower electrocatalytic activity in the presence of the polysulfide electrolyte as a result of its irreversibility and overpotential due to physisorption. The 0.6 M-CuS electrode shows the highest current density, which accounts for its better electrocatalytic activity and best photovoltaic performance in a QDSSC. This results from its improved surface morphology, higher surface roughness, optimized thickness, and better carrier concentration that is enriched by the sulfur ions. The 0.4 M-CuS electrode system shows moderate current density, but the 0.2 M-CuS exhibits much less electrocatalytic activity. Therefore, CV suggests that the 0.6 M-CuS electrode effectively acts as a catalyst in the reduction of the S²⁻/S_n²⁻ redox couple.

The charge transfer resistance at the interface between the CE and the electrolyte were analyzed using EIS measurements for the symmetrical cells fabricated with two identical electrodes. The Nyquist plots were obtained for the frequency range of 100 mHz to 500 kHz under dark conditions. In Fig. 5(B) and (C), the high frequency intercept on the real axis represents the series resistance (R_s). The semicircle in the middle frequency region denotes the charge-transfer resistance (R_ct) and the corresponding constant phase angle element (C_μ) at the CE/electrolyte interface, while the low frequency region provides the Warburg diffusion impedance (Z_W) of the polysulfide electrolyte.^35,36

The typical Nyquist plots are shown in Fig. 5(B), and the inset in Fig. 5(C) contains the equivalent circuit used to fit the Nyquist plots. The R_s values of the 0.2 M-CuS, 0.4 M-CuS, 0.6 M-CuS, 0.8 M-CuS, and Pt are 10.54, 7.11, 6.16, 7.16, and 12.14 Ω, respectively. The 0.6 M-CuS CE has the smallest R_s value, which reflects the good bonding strength between CuS and FTO substrate, which in turn promotes the collection of more electrons from the external circuit.^37,38 The R_s value greatly affects the FF of the solar cell, with lower FF resulting from larger R_s.³⁴ The R_ct values of 0.2 M-CuS, 0.4 M-CuS, 0.6 M-CuS, 0.8 M-CuS, and Pt CEs were found to be 21.17, 8.04, 7.89, 11.74, and 205.01 Ω, respectively. The value of R_ct decreases as the acetic acid concentration increases from 0.2 M to 0.6 M. 0.2 M-CuS CE exhibits the largest R_ct value (21.17 Ω), suggesting the catalytic activity is very poor in polysulfide electrolyte due to insufficient deposition of CuS. As the acetic acid concentration in CuS CE increases to 0.4 M and 0.6 M, the density of the CuS catalyst increases, and the R_ct value decreased strikingly to 8.04 Ω and 7.89 Ω, respectively. On further increasing the acetic acid amount to 0.8 M, R_ct of the 0.8 M-CuS CE becomes larger compared to that of 0.6 M-CuS.

The Pt CE has the highest R_ct value, indicating lower electrocatalytic activity compared to CuS CEs. The lower value of R_ct usually corresponds to an improvement in the electrocatalytic activity of the CE, which results in acceleration of the electron transfer process at the electrolyte/CE interface. The R_ct values suggest that the catalytic activity follows the order of Pt < 0.2 M-CuS < 0.8 M-CuS < 0.4 M-CuS < 0.6 M-CuS. The Z_W values of the 0.2 M-CuS, 0.4 M-CuS, 0.6 M-CuS, 0.8 M-CuS, and Pt CEs were measured to be 21.97, 8.12, 6.45, 9.73, and 36.63 Ω, respectively. The lower Z_W value of the CE indicates more electrolyte diffusion, resulting in improved performance of the QDSSC.

Tafel-polarization measurements were conducted with the dummy cells used in the EIS experiments. Fig. 5(D) shows the logarithmic current density (log J) as a function of the potential (V) for the oxidation/reduction of polysulfide electrolyte. The Tafel plot was used to investigate the interfacial charge transfer properties of the polysulfide electrolyte couple on the symmetrical cell configuration using different CEs. The Tafel equations for both the anodic and cathodic reactions in a corroding system can be combined to produce the Butler–Volmer eqn (6):³⁹

where I_corr is the corrosion current in amps, E is the electrode potential, E_OC is the corrosion potential in volts, and β_a and β_c are the anodic and cathodic Tafel constants, respectively.

In the plot, β_c indicates the reduction of S_n²⁻ to S²⁻ ions, whereas β_a denotes the oxidation of S²⁻ to S_n²⁻ ions. The electrocatalytic activities of the CEs can be extracted from the exchange current density (J₀), which can be measured by extrapolating the intercepts of two branches of the corresponding Tafel curves to the zero overpotential. Thus, the steep gradient of the cathodic and anodic branches was related to J₀. J₀ for the as-prepared electrodes was in the order of 0.6 M-CuS > 0.4 M-CuS > 0.8 M-CuS > 0.2 M-CuS > Pt. J₀ is inversely proportional to R_ct.⁴⁰

where R_ct is the charge-transfer resistance calculated from the EIS measurement, R is the gas constant, n is the number of electrons involved in the reduction of polysulfide electrolyte, and F is Faraday's constant.

In addition, the limiting diffusion current density (J_lim) obtained from the intersection of the cathodic branch with the Y-axis is a parameter that depends on the diffusion coefficient (D_n) of polysulfide redox couples at the interface of the CE and electrolyte, and J_lim is proportional to D_n.⁴¹

where D_n is the diffusion coefficient of the polysulfide, l is the electrolyte thickness, n is the number of electrons involved in the reduction of disulfide at the CE, F is the Faraday constant, and C is the polysulfide concentration. The J_lim value of the 0.6 M-CuS electrode is higher than that of the 0.2 M-CuS, 0.4 M-CuS, 0.8 M-CuS, and Pt electrodes, which induces a higher diffusion coefficient. The Tafel plot confirmed that the 0.6 M-CuS CE shows superior electrocatalytic activity in the reduction of polysulfide electrolyte. This shows that the CuS CEs have great potential for the fabrication of highly efficient QDSSCs.

To evaluate the photoelectrochemical performance of CuS as the CEs of the QDSSCs, we employ TiO₂/CdS/CdSe/ZnS as the working electrodes. Under the illumination of 100 mW cm⁻², J–V curves for the CuS CE based on QDSSCs were recorded and are shown in Fig. 6, which presents the J–V curves of the CuS and Pt CEs in QDSSCs using a TiO₂/CdS/CdSe/ZnS photoanode. Table 1 summarizes the derived photovoltaic parameters including the short-circuit current density (J_SC), open-circuit voltage (V_OC), fill factor (FF), and photovoltaic conversion efficiency (η). The QDSSC based on a Pt CE shows a very low photoelectrochemical performance with η of about 1.25%. The low performance is due to the very low electrocatalytic activity of Pt in polysulfide electrolyte resulting from the strong absorption of S²⁻ on the surface, which reduces the surface activity of Pt. As a result, a relative low FF (0.259) is achieved by the QDSSC based on Pt.

Fig. 6 Current density–voltage (J–V) curves of TiO₂/CdS/CdSe/ZnS QDSSCs assembled with (a) 0.2 M-CuS (b) 0.4 M-CuS, (c) 0.6 M-CuS, (d) 0.8 M-CuS, and (e) Pt CEs under one sun illumination (AM 1.5G, 100 mW cm⁻²).

Table 1 Solar cell parameters of TiO₂/CdS/CdSe/ZnS QDSSCs with various CuS CEs and resistances of CuS electrodes obtained from EIS analysis

Cell	VOC (V)	JSC (mA cm^−2)	FF	η%	Rs (Ω)	RCE (Ω)	Rct (Ω)	Cμ (μF)	ZW (Ω)	τe (ms)
0.2 M-CuS	0.571	14.00	0.477	3.82	10.78	0.79	3.10	729	1.54	0.19
0.4 M-CuS	0.587	16.88	0.489	4.85	10.11	0.71	2.43	856	1.48	0.28
0.6 M-CuS	0.596	17.43	0.495	5.15	8.32	0.52	2.4	912	0.77	0.35
0.8 M-CuS	0.586	15.64	0.471	4.32	10.45	0.77	3.07	837	1.52	0.25
Pt	0.531	9.14	0.259	1.25	10.55	17.07	7.8	163	7.15	0.047

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The cell with the 0.2 M-CuS CE showed low photovoltaic performance with η of 3.82%. The low η value arose from the low J_SC and FF values, which originated from the lack of electrocatalytic activity toward polysulfide reduction. As acetic acid increased to 0.4 M (0.4 M-CuS), more CuS catalyst was deposited, so the catalytic activity of the CE was enhanced and the performance was boosted (4.85%). When further increasing the acetic acid concentration to 0.6 M, the cell with the 0.6 M-CuS CE exhibited the highest η of 5.15%, with V_OC, J_SC, and FF values of 0.596 V, 17.43 mA cm⁻², and 0.495, respectively. However, further increasing the amount of acetic acid to 0.8 M reduced cell performance because J_SC and FF deteriorated due to the lower atomic percentage of sulfur,⁴² but the overall η (4.32%) was still much higher than that with the reference Pt CE.

The enrichment of the atomic ratio of S in 0.6 M-CuS plays a key role in increasing the electrocatalytic activity of the CE, resulting in improved performance. The enhancement of the conversion efficiency and performance of 0.6 M-CuS is also due to the influence of the CuS thin film layer by acetic acid on the FTO. This produces a higher charge exchange rate at the CE/electrolyte interface corresponding to higher electrocatalytic activity than the other electrodes. These results are in good agreement with the EIS, Tafel, and CV results.

EIS was employed to determine the charge transfer kinetics of QD-sensitized solar cells, and the obtained Nyquist plots for the frequency range of 100 mHz to 500 kHz are shown in Fig. 7(A). The equivalent circuit in the inset of Fig. 7(A) was used to fit the Nyquist plots with Z-View software. In the equivalent circuit, R_s is the non-zero intercept on the real axis of the impedance plot. In the high frequency region, R_CE is the electron transfer resistance at the CE/electrolyte interface, which is parallel to the corresponding capacitance (C_CE). In the middle frequency region, R_ct is the charge transfer resistance, which is parallel to the chemical capacitance (C_μ) at the photoanode/electrolyte interface. The diffusion resistance Z_W in the low frequency region was obtained directly from the fit.

Fig. 7 (A) Nyquist and (B) Bode plots for TiO₂/CdS/CdSe/ZnS QDSSCs against CuS and Pt based CEs with a S²⁻/S_n²⁻ couple as the redox electrolyte, under the under one sun illumination at open-circuit voltage and a frequency range of 100 mHz to 500 kHz. The inset of (A) shows the high impedance range for the Pt CE and equivalent circuit used to fit the experimental data.

The parameters obtained from the EIS analysis are summarized in Table 1. In this study, R_ct is not discussed because the value is not directly influenced by the choice of CE materials. The result in Fig. 7(A) shows that the QDSSCs fabricated using the 0.6 M-CuS CE exhibit the lowest R_CE value of 0.52 Ω, which is lower than the values for 0.2 M-CuS (0.79 Ω), 0.4 M-CuS (0.71 Ω), 0.8 M-CuS (0.77 Ω), and Pt CEs (17.07 Ω), indicating faster charge transport at the interface of the CE and electrolyte. The high C_μ of the 0.6 M-CuS QDSSC (912 μF) is mainly related to the collection of more photo-excited electrons into the conduction band of the photoanode due to low recombination.⁴³ The low Z_W value of the 0.6 M-CuS CE (4.72 Ω) indicates elevated electrolyte diffusion, facilitates faster mass transport of the electrons, and improves the performance of the QDSSC by increasing J_SC.

Fig. 7(B) shows the Bode phase plots of QDSSCs based on CuS CEs with different concentrations of acetic acid. The lifetime (τ_e) of electrons in the device based on CuS CEs can be measured from the maximum frequency of the peak:

The τ_e values obtained for the 0.2 M-CuS, 0.4 M-CuS, 0.6 M-CuS, 0.8 M-CuS, and Pt CEs were measured to be 0.19, 0.28, 0.35, 0.25, and 0.047 ms, respectively. The improved electron lifetime supports the reduction in the recombination rate of injected electrons with the polysulfide electrolyte. The larger recombination of the injected electrons with the polysulfide electrolyte in the Pt CE leads to low electron lifetime (with fast back reaction with the electrolyte). Among the investigated samples, 0.6 M-CuS exhibits a lower charge-transport resistance and higher electron lifetime, which contributes to the higher photovoltaic performance of the QDSSCs.

The differences of these CEs were further investigated by the OCVD technique. OCVD measurements were performed by monitoring the V_OC transient during relaxation from an illuminated quasi-equilibrium state to the dark equilibrium. When the visible light illumination on a QDSSC in an open circuit is interrupted, the excess electrons are removed due to recombination, and the photovoltage decay rate is directly related to the electron lifetime. Fig. 8 shows the OCVD curves measured for CuS and Pt CEs in a QDSSC. It appears that the 0.6 M-CuS has a slower voltage decay rate compared to other CEs, which suggests a longer electron lifetime. The slower electron recombination kinetics indicates that more electrons survive from the reverse reaction, which supports the enhancement of V_OC and J_SC. This result indicates that the recombination rate of photogenerated electrons with polysulfide electrolyte-oxidized species is the slowest in the QDSSCs fabricated based on the 0.6 M-CuS CE. With the addition of acetic acid from 0.2 M to 0.8 M, the electron lifetime increased and then decreased, which is in agreement with the J–V analysis.

Fig. 8 Open-circuit voltage decay (OCVD) characteristics of the CuS and Pt CEs.

Stability is important when evaluating a CE. The stability of QDSSCs based on 0.6 M-CuS and Pt CEs was investigated under continuous AM 1.5G illumination for 20 h. Fig. 9 shows the extracted photovoltaic parameters (V_OC, J_SC, FF, and PCE). The initial increase in performance is due to the capillary effect involving slow electrolyte solution permeation into the TiO₂ pores and the improved ionic transport due to heating of the electrolyte.⁴⁴ In the Pt CE, there is a decline in V_OC (0.531 to 0.503 V), J_SC (9.14 to 8.80 mA cm⁻¹) and FF (0.259 to 0.245) in the course of 20 h of illumination. Interestingly, in the CuS CE, there was a slight improvement of V_OC from 0.596 to 0.600 V, while J_SC (17.43 to 17.44 mA cm⁻²) and FF (0.495 to 0.492) were maintained at constant values in the 20 h of illumination. As a result, the PCE of the Pt decreased from 1.25% to 1.08%, and the 0.6 M-CuS showed a constant PCE of 5.15% in 20 h. Efficient photoelectron supply from the CE reduces the rate of recombination and causes electron accumulation in the conduction band of TiO₂, thereby maintaining almost constant V_OC, J_SC, and FF in the CuS CE. These accelerated aging results revealed that the QDSSC based on the CuS CE is more stable than the Pt CE, which is caused by the improved stability of the CuS CE in the presence of polysulfide electrolyte.

Fig. 9 Temporal evolution of photovoltaic parameter values, V_OC (a), J_SC (b), FF (c), PCE (d), for the 0.6 M-CuS and Pt CEs in TiO₂/CdS QDSSCs under continuous illumination of 100 mW cm⁻², in the course of 20 h.

The better photovoltaic performance of the 0.6 M-CuS CE is attributed to the enhanced electrocatalytic activity due to (i) improved surface morphology with nanoplatelet structure; (ii) the increase in S content, which increased Cu vacancies with increased conductivity, enabling better reduction of polysulfide electrolyte because of its readiness to react electrochemically due to the weak van der Waals forces between S–S layers;⁴⁵ and (iii) the optimized thickness of CE results in a lower electrical resistance at the interface of the CE and electrolyte. Overall, the structural and electronic features of the CuS CE can increase the overall performance of polysulfide electrolyte-based QDSSCs to the next level.

4. Conclusions

This study has reported the synthesis of a CuS thin film on FTO substrate made by a facile CBD method using acetic acid. This film can serve as a robust, high-performance CE material for TiO₂/CdS/CdSe/ZnS QDSSCs without any post-treatments. The acetic acid concentration changes the surface morphology of CuS from nanoplatelets to leaf-like nanoplatelets (0.2 M to 0.8 M). In-depth characterization combined with electrochemical analyses revealed for the first time that the subtle difference in the Cu/S ratios played a crucial role in dictating the electrocatalytic activity toward polysulfide reduction. The electrocatalytic activity of CuS films decreased as the amount of Cu in the CuS films became progressively richer. The 0.6 M-CuS CE exhibits the lowest R_ct value of 7.89 Ω at the CE/electrolyte interface, which is an order lower than the values of the 0.2 M-CuS (21.17 Ω), 0.4 M-CuS (8.04 Ω), 0.8 M-CuS (11.74 Ω), and Pt (205.01 Ω) CEs. As a result, the TiO₂/CdS/CdSe/ZnS QDSSCs with the 0.6 M-CuS CE present 5.15% efficiency under AM 1.5 illumination of 100 mW cm⁻², which is superior to that of the 0.2 M-CuS (3.82%), 0.4 M-CuS (4.85%), 0.8 M-CuS (4.32%), and Pt (1.25%) CEs. The improved QDSSC performance obtained by the 0.6 M-CuS CE is attributed to the high and consistent electrocatalytic activity toward the reduction of polysulfide species. The stability test of sealed TiO₂/CdS/CdSe/ZnS QDSSCs with the 0.6 M-CuS CE over 20 h proves that the 0.6 M-CuS CE is highly stable in the QDSSCs. The procedure for obtaining the CuS nanostructures is quite simple, environmentally benign, and cost-effective, and it has potential for application in future QDSSCs. The exploration of different QD sensitization techniques is currently in progress.

Conflict of interest

The authors declare no competing financial interest. The samples were analyzed by scanning electron microscope (SEM, Hitachi S-4200) with a 4k × 4k CCD camera (Ultra Scan 400SP, Gatan corp.) at the Busan KBSI, South Korea.

Acknowledgements

This work was financially supported for two years by the Pusan National University research grants.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra08375g

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