Stacked Cu_1.8S nanoplatelets as counter electrode for quantum dot-sensitized solar cell

Abstract

It is found that the electrocatalytic activity of Cu_2−xS thin films used in quantum dot-sensitized solar cells (QDSSCs) as counter electrode (CE) for the reduction of polysulfide electrolyte depends on the surface active sulfide and disulfide species and the deficiency of Cu. The preferential bonding between Cu²⁺ and S²⁻, leading to the selective formation of a Cu_1.8S stacked platelet-like morphology, is determined by the cetyl trimethyl ammonium bromide surfactant and deposition temperature; the crab-like Cu–S coordination bond formed dictates the surface area to volume ratio of the Cu_1.8S thin films and their electrocatalytic activity. The Cu deficiency enhances the conductivity of the Cu_1.8S thin films, which exhibit near-infrared localized surface plasmon resonance due to free carriers, and UV-vis absorption spectra show an excitonic effect due to the quantum size effect. When these Cu_1.8S thin films were employed as CEs in QDSSCs, a robust photoconversion efficiency of 5.2% was obtained for the film deposited at 60 °C by a single-step chemical bath deposition method.

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1. Introduction

Quantum dot-sensitized solar cells (QDSSCs) emerged as an alternative to dye-sensitized solar cells (DSSCs), which fail due to photodegradation in spite of being cost effective.¹ To overcome this, inorganic quantum dots (QDs) are used as photosensitizers, due to the generation of multiple excitons through impact ionization with single photon absorption, in addition to band gap tunability.^2–4 QDs incorporated into the photoanode absorb incident light over a wide range of spectral wavelengths with a flexible tandem arrangement.^1,5 Even though QDSSCs outperform organometallic dye-based DSSCs in terms of stability against photodegradation, their photovoltaic performance is still very low.^6–9 Many efforts have been made to improve the efficiency by employing cascade layers of FTO/TiO₂ with different QDs, such as PbS,¹⁰ CdS,¹¹ CdSe¹² and CdS/CdSe hetero structures,¹³ as efficient sensitizers in QDSSCs with enhanced charge separation due to the large intrinsic dipole moment.¹³ In QDSSCs, the counter electrode (CE) is an equally important component since it plays the crucial role of electrolyte reduction. In comparison to several CEs, platinum (Pt) has both high electrocatalytic activity and very low resistance for the iodide/triiodide redox electrolyte reduction when employed in DSSCs;¹⁴ however, it has failed to perform well in QDSSCs with a sulfide/polysulfide (S²⁻/S_n²⁻) redox couple electrolyte, since it readily adsorbs S²⁻ on to the surface, which increases the sheet resistance and hence electrocatalytic activity is reduced.^15,16 In this regard, it is highly essential to find a suitable CE which can alleviate the aforementioned problem with a concurrent improvement in the efficiency of the solar cells, since the regeneration rate of the QDs depends on the redox rate of the electrolyte.¹⁷ In view of this, several materials have been employed as alternative CEs, from carbon-based candidates like carbon nanotubes¹⁸ to transition metal sulphides.^19–21 The carbon-based materials lack both catalytic activity and effective charge conduction, even though they provide a large surface and uniform pores.²² Among metal sulfides, group IB-VIA metal chalcogenides (MC) such as Cu_2−xS is found to be a well-suited CE due to its self-doped p-type semiconducting nature, with tunable phase and surface morphologies depending on the preparation method.²³ The surface morphology and chemical species present at the surface are often the fingerprints of the material’s properties, to suit the need and, more specifically, the electrocatalytic properties of Cu_2−xS nanostructures.²⁴ Therefore, synthesizing Cu_2−xS with tailored properties becomes both inevitable and challenging for the development of QDSSCs with high photoconversion efficiency.²⁵ In order to synthesize such well-defined structures, a bottom-up process with controlled nucleation is very handy. Therefore, several experimental parameters, such as concentration, temperature, reaction time and concentrations of surfactants or capping agents or structure-directing agents (SDAs), are to be fine-tuned to obtain both size- and shape-controlled Cu_2−xS nanostructures.²⁴ Cetyl trimethyl ammonium bromide (CTAB) is one such SDA that binds well with the Cu_2−xS coordination formed in the initial stage, and as the reaction proceeds it determines the shape and morphology of the Cu_2−xS thin films to be synthesized.

Moreover, it is essential to optimize the molarity, quantity of precursor solutions, temperature and position of the substrate inside the growth vial in order to assure the quality of the films grown by chemical bath deposition (CBD) methods. In this study, growth of Cu_2−xS thin film was not initiated below 55 °C and the deposited films peeled off from the substrate above 65 °C. The Cu_2−xS thin films grown at optimized temperature were characterized with respect to the phase, composition, surface morphology and electrochemical properties associated with QDSSCs. We strongly believe that our method is very simple and that the clearly-demonstrated growth method would allow researchers to further tailor the properties of Cu_2−xS thin films to suit their needs. In this report, we have presented a one-step CTAB-assisted synthetic route to fabricate stacked nanoplatelet-like Cu_2−xS thin films using simple chemical bath deposition (CBD), by varying the deposition temperature for a period of 2 hours. It was found that both the temperature and the SDA influence the shape and surface morphology. The Cu_2−xS thin films synthesized possess a stacked nanoplatelet-like structure with a large surface to volume ratio, terminated with different surface active sulfide species, disulfides and oxides. This in turn influences their electrocatalytic activity, and particularly enhances the redox reaction of the sulfide/polysulfide (S²⁻/S_n²⁻) redox couple electrolyte when the Cu_2−xS thin films are employed as CEs for QDSSCs. A QDSSC fabricated using Cu_2−xS thin film synthesized at 60 °C gave a photoconversion efficiency of up to 5.2% with consistent stability. Based on X-ray photoelectron spectroscopy (XPS), impedance spectroscopy and Tafel polarization, a detailed exploratory investigation of the electrocatalytic behavior of the CE is discussed and presented here.

2. Experimental

2.1 Materials

Cu_2−xS stacked nanoplatelets were successfully deposited by CBD, using water as the solvent, on well cleaned fluorine-doped tin oxide (FTO) of resistance 7 Ω cm⁻² (Hartford Glass). All the precursors used for the synthesis were analytical grade and purchased from Sigma Aldrich, and the synthesis was carried out without further purification.

2.2 Preparation of photoanode and counter electrode

A typical synthesis was carried out by dissolving 0.1 M of copper chloride (CuCl₂·2H₂O) in 50 ml of deionized water and adding 1.0 M of thioacetamide (CH₃CSNH₂) as a S source, followed by adding 0.85 M of acetic acid (CH₃COOH) in drops and stirring continuously. To the above solution, 0.25 M of CTAB ((C₁₆H₃₃)N(CH₃)₃Br) was added and stirred vigorously for 25 minutes to make a homogeneous solution. The previously cleaned FTO glass substrates were immersed and kept horizontally in the growth solution, and chemical bath deposition was carried out for 2 hours at 55 °C, 60 °C and 65 °C; the samples were labelled CE55, CE60 and CE65, respectively. Above 65 °C, the deposited Cu_2−xS films peeled off from the substrate and hence were not suitable for use in CEs. The CBD-synthesized stacked Cu_2−xS nanoplatelets were then rinsed with deionized water and 99% ethanol and purged with N₂ gas.

The photoanodes were prepared by coating commercially-available TiO₂ paste of particle size 20 nm (Ti-Nanoxide HT/S, Solaronix) on the FTO using the doctor blade method, and were sintered at 450 °C for 30 minutes to get good crystallinity. The QD photosensitizers were coated on the TiO₂ using a successive ionic layer adsorption and reaction method (SILAR) with optimized conditions already reported elsewhere.²⁶ For CdS quantum dots, 0.025 M cadmium acetate dihydrate (Cd(CH₃COO)₂·2H₂O) and 0.2 M sodium sulfide (Na₂S) were prepared in 50 ml of deionized water separately, for cation and anion sources, respectively, and 5 cycles of SILAR were carried out. For CdSe QDs, the cation source was prepared by dissolving 0.025 M of Cd(CH₃COO)₂·2H₂O in 50 ml of deionized water. The anion source containing Se was prepared from aqueous 0.2 M selenium (Se) and 0.4 M Na₂SO₃ taken in a round bottom flask. This solution was refluxed at 125 °C for a period of two hours to get sodium selenosulfate (Na₂SeSO₃), as selenium is not soluble in water and cannot react in pristine form. Using the above prepared cation and anion precursors, 8 SILAR cycles of CdSe QDs were deposited, while the temperature of the precursors was kept at 55 °C for better adsorption. Finally, to avoid corrosion due to the polysulfide electrolyte and back electron transfer into the electrolyte, two SILAR cycles of a zinc sulfide (ZnS) passivation layer were coated using 0.2 M of zinc nitrate hexahydrate (ZnNO₃·6H₂O) and 0.2 M of Na₂S solutions.

2.3 Fabrication of QDSSCs and symmetric cells

The QDSSCs were assembled by sandwiching the SILAR-processed tandem layered TiO₂/CdS/CdSe/ZnS photoanodes and chemical-bath-deposited Cu_2−xS and Pt CEs with a 25 μm hot-melt sealing sheet (SX 1170-25, Solaronix) in between them. The setup was heated at 110 °C in a hotplate for 45 seconds. Then, the internal space of the cell was filled with the sulfide/polysulfide (S²⁻/S_n²⁻) redox couple electrolyte, consisting of 1 M of Na₂S, 2 M of S and 0.2 M of KCl, by capillary action. In the case of symmetric cells, the photoanode was replaced with a CE itself, such as Cu_2−xS/sulfide–polysulfide (S²⁻/S_n²⁻)/Cu_2−xS, and Pt in place of Cu_2−xS, which served as a reference; these were sandwiched by a 25 μm hot-melt sealing sheet, and the space between them was filled with polysulfide electrolyte.

2.4 Characterization

The phase purity and crystallinity of the Cu_2−xS thin films were analyzed using X-ray diffraction (XRD; Bruker D8 Advance) with a Cu Kα radiation (l = 1.54056 Å) source, operated at 40 kV and 30 mA, in the range of 10–80°. The surface morphology of the thin films was analyzed using FE-SEM (Hitachi, model SU-70). UV-vis spectroscopic analysis was carried out using an Optizen 3220 UV-vis spectrophotometer. X-ray photo electron spectroscopy (XPS) was performed using a VG Scientific ESCALAB250 instrument with monochromatic Al Kα radiation of 1486.6 eV, with an electron take-off angle of 90°. The survey spectrum was scanned in the binding energy (BE) range of 0.0–1400 eV in steps of 1 eV. The binding energy values reported here are relative to the carbon C 1s core level at 284.6 eV. The pressure of the chamber was maintained at 10⁻¹⁰ Torr throughout the measurement. The current–voltage characteristics of the QDSSCs were studied under 1 sun illumination (AM 1.5G 100 mW cm⁻²) with a San-ei Electric (XES-301S, Japan) solar simulator, with the irradiance uniformity of ±3%. Electrochemical impedance spectroscopy (EIS) was performed using a BioLogic potentiostat/galvanostat/EIS analyzer (SP-150, France) under 1 sun illumination.

3. Results and discussion

3.1 XRD analysis

X-ray diffraction patterns of the CBD-deposited Cu_1.8S thin films on FTO substrates is shown in Fig. 1. The diffracted peaks of all the films are found to be along the (111), (200), (220), (400) and (313) planes, corresponding to 2θ values of 26.465, 31.175, 44.125, 65.479 and 70.741, matching the cubic phase of Cu_1.8S (ICDD file no. 01-075-2241).

Fig. 1 X-ray diffraction patterns of the CBD-deposited Cu_1.8S thin films on FTO substrates.

In the case of Cu_2−xS, identifying and attributing the correct phase is difficult, since many phases exist for bulk Cu_2−xS with compositions close to the ratio of Cu/S = 2, like anilite, chalcocite, digenite and djurleite, which all have the maximum intensity diffraction peak at a 2θ value of 46 degrees. However, in our case it is easy to identify the phase, since the diffraction peak at 44.5 degrees occurs only for the cubic phase of Cu_1.8S, and hence it is referred to as such hereafter instead of Cu_2−xS. The grain sizes were calculated from Scherrer’s method and are 112, 167 and 152 nm respectively for CE55, CE60 and CE65. The slight decrease in grain size for CE65 might be due to the reduced thickness of the nanoplatelets. The intensity of the (111) diffraction peak increases with increasing deposition temperature, while the (400) peak appears for CE60. Even though there is not much remarkable change in the phase of the Cu_1.8S thin films, little changes in the orientation of the nanoplatelets might have caused the disappearance of the (400) peak for CE65, evidenced from the SEM images.

3.2 Surface morphological studies and growth mechanism

Fig. 2(A)–(C) show the SEM images of the Cu_1.8S thin films deposited at 55 °C (CE55), 60 °C (CE60) and 65 °C (CE65), respectively, while Fig. 2(D)–(F) display the cross-sections of the respective films. The surface of the CE55 exhibits a uniform arrangement of stacked nanoplatelets with lower density, while raising the deposition temperature to 60 °C gives evenly-distributed Cu_1.8S of dense nanoplatelets, with an increased surface area to volume ratio. However, the surface morphology of the film deposited at 65 °C shows nanoplatelets with reduced thickness. The uniform surface morphology and high crystallinity of the Cu_1.8S is greatly influenced by CTAB, which allows the system to bypass self-agglomeration and provides the Cu_1.8S nanoplatelets with capping and surface transformation. CTAB is basically a strong cationic surfactant which can form micelles in the solution. Based on the solution conditions, CTAB forms micelles of different shapes, like cylindrical, spherical or high-order lamellar, to give crystals of different shapes.²⁷ The S²⁻ ions generated through the hydrolysis of thioacetamide are readily attracted by Cu²⁺ ions and form a crab-like Cu–S coordination bond.²⁶ The supply of S²⁻ ions is enhanced by acetic acid which also acts as a stabilizing agent. When cationic CTAB is introduced, it forms a micelle and is attracted by S atoms present in the Cu_1.8S. Apart from the prevention of self-agglomeration by CTAB, π–π bond interactions and weak van der Waals attractions direct the reaction towards the formation of stable Cu_1.8S nanoplatelets (Fig. 3).^28,29 The preferential bonding between Cu²⁺ and S²⁻ leads to the selective formation of Cu_1.8S. Due to the three-dimensional surface of the thin films, the thickness of the films varies from place to place, as revealed in the cross-sections of the films shown in Fig. 2(D)–(F).

Fig. 2 SEM images of stacked Cu_1.8S nanoplatelets, (A) CE55, (B) CE60 and (C) CE65, and cross sectional images of (D) CE60, (E) CE60, (F) CE65 deposited on FTO substrates.

Fig. 3 Schematic representation of the CTAB-assisted formation of Cu_1.8S stacked nanoplatelets on FTO substrates.

3.3 Optical studies

Fig. 4 compares the absorption spectra of the Cu_1.8S thin films fabricated via CBD. All the films show absorption at the UV/visible region, which is red-shifted as the deposition temperature is increased due to a quantum confinement exciton effect, and the absorption in the NIR region corresponds to localized surface plasmon resonance (LSPR) due to free carrier density.

Fig. 4 UV absorption spectra of stacked Cu_1.8S nanoplatelets deposited on FTO substrates.

This is due to the presence of free holes (Cu vacancies) in the valence band that can act as self-dopants, leading to resonance absorption.²³ The absorption maximum in the UV/vis spectra is at 410, 417 and 475 nm respectively for the CE55, CE60 and CE65 samples, with increased absorption. These high-energy absorption peaks arise from the 1S_h–1S_e excitonic transition found in semiconductor nanoparticles.³⁰ From the absorption spectrum, the band gaps of the Cu_1.8S thin films were calculated to be 2.2, 2.1 and 1.75 eV respectively for CE55, CE60 and CE65. The reduction in the band gap is due to the formation of uniform crystals of increased size showing a quantum confinement effect. In the longer wavelength region, non-stoichiometric Cu_2−xS (x > 0) develops a LSPR in the NIR region and shifts to the blue region with strong absorption, due to higher free carrier density from increased copper vacancies.^31,32 The LSPR properties are always influenced by the surrounding medium, and an increase of the refractive index red-shifts the NIR absorption of NCs. Here, the Cu_1.8S films synthesized exhibit significant LSPR features with only air as the surrounding medium, and hence the blue shift might come from the Cu deficiency.^23,33,34 The absorption of light in the NIR spectral window is also suitable to harvest the residual light penetrating from the tandem layered photoanode. This p-type semiconductor material can also contribute to the increase of the photovoltage with its photoactive nature, through the auxiliary tandem junction.¹

3.4 XPS analysis

The XPS analysis was employed to infer the ionization states, elemental composition and to ensure the purity of the Cu_1.8S thin film compounds. The samples were not subjected to any pre- or post-heat treatment. The presence of Cu, S, C and O elements was confirmed from the XPS survey spectra presented in Fig. 5. From the Auger line of Cu LMM at the binding energy of 569.0 eV, the presence of bivalent Cu is evidenced. The atomic percentages of Cu : S : O were 54.62 : 34.86 : 10.52, 62.18 : 36.3 : 1.45 and 60.18 : 35.96 : 3.86 respectively for CE55, CE60 and CE65. CE55 shows very low S content due to the slow sulfidation rate at low temperature, and has more oxygen. On the other hand, both CE60 and CE65 have an almost equal atomic percentage of S. In order to elucidate the nature of the bonding of the surface elements, the individual core level spectra of S 2p and Cu 2p were measured at a higher resolution rate. XPS data were fitted with Gaussian–Lorentzian (30% Gaussian) functions and a Shirley-type background, using the software CasaXPS. Four constraints were applied to fit S peak components such as the spin-orbit splitting (1.18 eV between 2p_3/2 and 2p_1/2), the peak area ratio (2p_3/2 : 2p_1/2 = 2 : 1) and equal full width at half maximum, and the binding energy position of S 2p_3/2 was fixed at 161.2 eV. Fig. 6 represents the individual S 2p peaks with clear distinction of their bonding. Cu does not show remarkable changes in oxidation state, and all spectra have satellite peaks due to Cu vacancies, which caused the LSPR in the optical spectra.²³

Fig. 5 XPS survey spectra of Cu_1.8S stacked nanoplatelets deposited on FTO substrates.

Fig. 6 XPS spectra for the S 2p of Cu_1.8S nanoplatelets.

The S 2p (S²⁻) doublet peaks (161.2 and 162.38 eV) of Cu_1.8S are attributed to the presence of S 2p_3/2 and S 2p_1/2 states. CE55 has both sulfide (SO₃²⁻) and sulfates (SO₄²⁻) at the surface, at 165.5 and 168 eV, respectively, which are absent in CE60 and CE65. This clearly shows that oxidation is much higher for low S content samples prepared at low temperature. The peak found at 163.3 eV is assigned to bridging sulfur (S–S), which is at a lower binding energy state (less oxidation) in CE60; however, there are Cu–S, disulfide (S₂²⁻) and sulfide (S²⁻) peaks at 163.7, 162 and 161.2 eV, respectively.^35,36 The presence of S in the form of SO₄²⁻ at the surface of the films is in a residual form, which is further confirmed from the O 1s peak located at 532.2 eV in the survey spectrum.³⁷ These clearly show that the surface bonding is controlled by the deposition temperature.

3.5 IV characteristics and electrochemical characterization

Fig. 7(A) shows the J–V characteristics of the QDSSCs fabricated with TiO₂/CdS/CdSe/ZnS as the photoanode, CBD-synthesized Cu_1.8S and Pt (reference) as the CE and polysulfide as the electrolyte. The photovoltaic parameters are summarized in Table 1. The best photoconversion behavior was observed for CE60 with V_oc = 0.606 V, J_sc = 19.079 mA cm⁻², FF = 45.05 and η = 5.16%, and the Pt CE showed V_oc = 0.559 V, J_sc = 6.671 mA cm⁻², FF = 31.71 and η = 1.45%. The affinity of the Pt CE towards S²⁻ accounts for its poor performance, which slows down the electrocatalytic activity of Pt with a rapid decrease in the current density, and fails to reduce the redox electrolyte on its surface.^38,39 All the Cu_1.8S CEs exhibited better performance than Pt, and among the Cu_1.8S CEs, the performance of CE60 was better than the rest. The surface active S species are very crucial for the electrolyte reduction. However, bridging S–S is more resistive than the Cu–S bonds present in CE60. Hence, the improvement might have come from the surface active species in addition to the surface morphology influenced by CTAB, which plays the role of structure-directing agent to improve the shape and surface morphology and, in turn, the photoconversion ability. The stacked platelet-like structure possesses an increased surface to volume ratio that provides a larger area of contact for the electrolyte. The interfaces between the stacked layers also offer space for the electrolyte to flow. Therefore the redox reaction of the electrolyte is enhanced. Even though the efficiency is much higher than in earlier reports,^23,26 the FF is much lower, which might be due to higher charge transfer resistance (R_CT) of the QDSSC.

Fig. 7 (A) I–V behavior for TiO₂/CdS/CdSe/ZnS QDSSCs based on stacked Cu_1.8S nanoplatelets and Pt counter electrodes. (B) Nyquist plot for Cu_1.8S and Pt symmetrical cells; the inset shows the equivalent circuit. (C) Tafel polarization plot for Cu_1.8S and Pt symmetrical cells. (D) Nyquist plot of TiO₂/CdS/CdSe/ZnS QDSSCs for Cu_1.8S and Pt counter electrodes. The inset shows the equivalent circuit.

Table 1 Photovoltaic and electrochemical impedance spectroscopic parameters of QDSSCs

Cell	Composition Cu : S (atomic%)	EIS for symmetric cell		Voc (V)	Jsc (mA cm^−2)	FF (%)	η (%)	EIS parameters for QDSSCs
		RS (Ω)	RCE (Ω)					RS (Ω)	RCE (Ω)	RCT (Ω)	ZW (Ω)
55 °C	52.12 : 47.88	8.71	71.91	0.58	14.3	46.15	3.84	9.39	2.13	36.13	1.61
60 °C	51.93 : 48.07	8.26	3.26	0.60	19.1	45.05	5.16	9.22	1.7	24.19	1.17
65 °C	51.60 : 48.40	6.44	38.69	0.59	15.4	47.07	4.28	7.86	1.39	41.50	2.10
Pt		8.83	576.23	0.56	06.7	31.71	1.19	9.86	12.32	418.47	10.24

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The scavenging behaviour of the sulfide ions (S²⁻), over the photogenerated holes at the photoanode and the oxidized ions (S_x²⁻) to reach the CE in order to combine with the electron, must be enhanced by the CE.¹ Therefore, the charge transfer resistance (R_CT) of the QDSSC, which determines the electrocatalytic activity of the counter electrode, has to be studied by using electrochemical impedance spectroscopy (EIS). Fig. 7(B) shows the Nyquist plots of the symmetrical cells. The equivalent circuit (inset) is made with a series resistance (R_S) which represents a high-frequency non-zero intercept on the real axis, the resistance at the counter electrode/electrolyte interface (R_CE) and the respective capacitance (C_PE), with the diffusion impedance (Z_W) represented by a saturated semicircle usually observed at low frequency. The symmetrical cells were made of two identical Cu_1.8S and Pt CEs, respectively, with an active area of 1 cm², and the polysulfide electrode in between them. The measurements were carried out in the frequency range of 0.1 Hz to 500 kHz. The obtained impedance data were fitted using the equivalent circuit given in the inset, and the parameters extracted are presented in Table 1. The value of R_S for the Cu_1.8S CEs decreases as the deposition temperature is increased, indicating better adhesion of the Cu_1.8S thin films on to the FTO substrate. The extent of R_CE determines the charge transfer between the CE and the electrolyte towards the redox reaction of S²⁻/S_n²⁻. It was found that the Pt CE has the highest R_CE value of 576.23 Ω, against the very small values for the Cu_1.8S CEs; among the Cu_1.8S CEs, CE60 has the lowest value of 3.26 Ω while CE55 and CE65 have larger R_CE values than CE60, and therefore this accounts for the enhanced performance of CE60. The diffusion impedance (Z_W) is yet another yardstick to determine the ability of a CE in reducing the redox polysulfide electrolyte. The Pt CE has the highest value of 102.58 Ω, suggesting a poor ability and strong resistance to reduce the electrolyte as catalytic poisoning hinters.^40,41 On the contrary, the Cu_1.8S CEs with very negligible values outperformed the Pt CE (CE55 = 1.49 Ω, CE60 = 0.34 Ω and CE65 = 0.76 Ω (not shown in the table)).

In order to throw light on the better electrocatalytic activity and the interfacial charge transfer properties of the S²⁻/S_n²⁻ redox couple, Tafel polarization measurement was carried out using the symmetrical cells. Fig. 7(C) shows the logarithmic current density (log J) as a function of voltage (V) for the redox reaction of the polysulfide electrolyte redox couple (S²⁻/S_n²⁻). By extrapolating the linear region of the curve towards the zero overpotential and from the intercept, the exchange current density (J₀) was obtained. The exchange current density (J₀) and limiting current density (J_lim) determine the electrochemical activity of the CE. As seen in Fig. 7(C), all the CEs except Pt show almost even anodic and cathodic slopes. CE60 shows a higher slope both on the anodic and cathodic sides, with zero corrosion potential. Its highest limiting current density (J_lim), highest corrosion current of 3245.61 μA and balanced oxidation and reduction rate is confirmed from its equal cathodic (β_c) (298.0 mV) and anodic slopes (β_a) (300.1 mV), further supporting its best performance, yielding an efficiency of 5.16%. CE55 and CE65 stay behind CE60, with lower corrosion currents (I_corr) of 2778.95 μA and 2285.68 μA, respectively. CE65 has a slightly predominant cathodic slope (β_c) (299.7 mV) over its anodic slope (β_a) (286.4 mV), with a corrosion potential (E_corr) of −1.843 mV, and therefore a slight shift from the equilibrium reduces the catalytic activity. CE55, with its lowest limiting current density (J_lim) and high corrosion potential (E_corr) of −7.512 mV is the lowest-performing counter electrode. In addition to that, its uneven cathodic and anodic slopes (β_c = 283.4 mV, β_a = 310.6 mV) also make it inferior among the Cu_1.8S counter electrodes. In the case of the Pt CE, it not only has uneven cathodic and anodic slopes (β_c = 290.00 mV, β_a = 330.2 mV) and the lowest limiting current density (J_lim), but also has a negative corrosion potential (E_corr) of −18.16 mV. The Pt CE’s large negative corrosion potential (E_corr) means that when all the Cu_1.8S CEs can readily reduce the polysulfide electrolyte, the Pt CE has to attain equilibrium (null point) prior to its commencement of reduction of the polysulfide electrolyte, at the expense of photoconversion ability.²⁰ To support the above-stated reasons for the better electrocatalytic activity of CE60, the internal charge transfer kinetics must be explained from the EIS point of view. EIS measurements for the QDSSCs were carried out at open circuit voltage (V_oc) (light condition). The impedance data were fitted using the equivalent circuit given in the inset in Fig. 7(D). In the circuit, R_CE represents electron transfer at the counter electrode/polysulfide electrolyte interface, R_CT represents charge transfer resistance at the photoanode/polysulfide electrolyte interface and C_PE is the chemical capacitance related to the amount of photoexcited charge carriers available in the conduction band of the photoanode which result from minimum recombination; Z_W is the diffusion resistance. The QDSSC assembled with CE60 gives an R_CE of 1.7 Ω and Z_W = 1.17 Ω, showing a better reduction rate of the electrolyte and electrolyte diffusion. On the contrary, the Pt CE with a high R_CE = 12.32 Ω and Z_W = 10.24 Ω, as it is subjected to adsorption of S²⁻, is poor at reducing the electrolyte with the supply of electrons. Therefore, the Pt CE fails to have a better photoconversion ability and reduced FF, while all the Cu_1.8S CEs have high J_sc values and photoconversion abilities. The best photoconversion efficiency demonstrated by CE60 is due to (i) its low resistance and (ii) its zero corrosion potential, implying that the system is already at equilibrium to readily reduce the electrolyte, unlike Pt, whose negative corrosion potential must be balanced with an additional sacrifice of energy.⁴²

For the best-performing QDSSC, a stability test was conducted by exposing the cell for 20 continuous hours under light illumination, and the results are presented in Fig. 8. It is found that the cell showed consistent photoconversion efficiency, with a minimum decrement of only 2.1% in the efficiency. The photoconversion efficiency was found to increase after sufficient exposure to the light illumination, since the heating of the electrolyte improves the ionic mobility, with good penetration of the electrolyte into the pores of TiO₂ because of the capillary effect.^43,44 Therefore, Cu_1.8S can be a suitable cost-effective substitute for Pt as a counter electrode material with superior electrocatalytic properties, high stability and commendable longevity.

Fig. 8 Comparison of the variation of I–V parameters with aging time for the QDSSC assembled with CE60: (a) open circuit voltage, (b) fill factor, (c) short current density and (d) efficiency.

4. Conclusion

Stacked nanoplatelets of Cu_1.8S were synthesized using CTAB as surfactant. CTAB is found to influence the surface morphology of the Cu_1.8S thin films and to yield highly crystalline Cu_1.8S. The electrocatalytic behavior of the Cu_1.8S CEs is greatly influenced by the surface morphology and surface active sulfide species. All the QDSSC-assembled with Cu_1.8S CEs exhibited over 3.87% photoconversion efficiency; the highest photoconversion efficiency of 5.16% was achieved for the film synthesized at 60 °C, with very low R_S, R_CT, R_CE and Z_W values and zero corrosion potential, against the poor electrocatalytic behavior of Pt owing to catalytic poisoning. The free carrier concentration due to Cu vacancies and the Cu–S bonding at the surface of the counter electrode was found to play a key role in the redox reaction. The electrocatalytic features of the Cu_1.8S make it a suitable cost-effective alternative to Pt as a counter electrode.

Conflict of interest

The authors declare no competing financial interests.

Acknowledgements

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

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