Zinc oxide/copper sulfide nanorods as a highly catalytic counter electrode material for quantum dot sensitized solar cells

Abstract

Copper sulfide deposited ZnO nanorods (ZnO NRs/CuS) have been applied as a new counter electrode material with high electrocatalytic activity towards polysulfide electrolyte, which results in the formation of a highly efficient counter electrode for QDSSCs. It was observed from the current density–voltage (J–V) characteristics that the short-circuit current density (J_sc), power conversion efficiency (PCE), and fill factor (FF) were enhanced from 7.63 mA cm⁻² to 14.48 mA cm⁻², 1.59% to 4.18%, and 0.29 to 0.38, respectively, when a bare CuS counter electrode was changed to a ZnO NRs/CuS counter electrode. Electrochemical impedance spectroscopy (EIS), Tafel polarization and cyclic voltammetry (CV) measurements demonstrate that the higher PCE value obtained using the ZnO NRs/CuS counter electrode is due to its superior photoelectrochemical performance and electrocatalytic activity. Another reason for this higher PCE might be the reduction in charge transfer resistance at the counter electrode/polysulfide interface.

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1. Introduction

Recently, widespread research has been carried out on quantum dot-sensitized solar cells (QDSSCs). This type of solar cell is based on a semiconductor photoanode, usually TiO₂ or ZnO, treated with QD sensitizers.^1–3 For enhancement of the performance and power conversion efficiency (PCE) of QDSSCs, an enormous amount of work has been carried out.^4–9 QDs have a high extinction coefficient, leading to good electron transfer to the semiconductor.¹⁰ Electron transfer from the QDs to the semiconductor and electron transport in the semiconductor is a very significant subject in QDSSCs. The ZnO semiconductor has a higher electronic mobility and diffusion coefficient than the TiO₂ semiconductor, which causes better injection of the electrons into the ZnO conduction band (CB).¹¹ Also, ZnO has numerous benefits over TiO₂ such as higher thermal stability, practicable growth, and lower synthesis cost, making it a suitable alternative to TiO₂.¹² The counter electrode plays an important role in charge transfer to the electrolyte. Therefore, a high catalytic activity, large surface area, high electrical conductivity, chemical durability, and low cost are several of the basic requirements for the applied counter electrode.¹³ The Pt-based counter electrode is the most commonly applied in QDSSCs. However, it shows reduced activity in polysulfide electrolyte, probably due to the decreased charge transfer rate at the counter electrode/electrolyte interface. Also, the Pt-based counter electrode is not compatible with polysulfide electrolytes due to the precipitation of sulfur atoms on the Pt surface, which causes the oxidation–reduction reactions at the polysulfide electrolyte to be reduced. Moreover, counter electrodes using noble metals are expensive and thus could be a problem in the commercialization of QDSSCs.

Researchers recommend metal-free materials for substituting Pt as electrocatalysts, including carbon, Cu₂S, CoS, PbS, CoS/CuS, conducting polymers, Cu₂S/reduced graphene oxide, etc.^14–20 Also, in previous work, we have applied a CuS/PbS counter electrode instead of bare CuS.²¹ The results showed that the recombination rate in the structure was reduced by changing the counter electrode from CuS to CuS/PbS. Among these counter electrodes, Cu₂S and CuS are efficient and cost-effective catalysts and are reported commonly in top level QDSSCs. However, the adhesion between CuS and FTO glass is typically poor, which that leads to low coverage, and hence the low catalytic activity of the CuS counter electrode.²² To enhance the coverage and raise the catalytic activity, porous materials with excellent electron transport properties are deposited on the FTO surface. ZnO NRs provide a higher surface area than bare FTO for the deposition of CuS and easy availability for the polysulfide electrolyte.²² Also, ZnO NRs can act as an excellent electron channel for electron transport from the external circuit to the CuS sites. For this work, ZnO NRs were first grown on a conducting fluorine-doped tin oxide (FTO) substrate by chemical bath deposition (CBD). Then, CuS was decorated onto the ZnO NRs using the successive ion layer adsorption and reaction (SILAR) technique. This CuS/ZnO NR counter electrode displays good catalytic activity towards the polysulfide electrolyte. Compared with bare CuS, ZnO NRs offer an extremely high surface area for the enormous deposition of CuS and easy accessibility for the polysulfide electrolyte.

2. Experimental

ZnO NR/CdS/CdSe photoanodes were fabricated using a solution method that has been described in previous work.²¹ For preparation of the counter electrodes, first, ZnO NRs were grown on FTO by the CBD method as explained in ref. 23. Then, the CuS catalyst was deposited onto the ZnO NRs by the SILAR method. For the deposition of CuS, the ZnO NR deposited fluorine tin oxide (FTO) substrates are dipped into an aqueous solution containing 0.5 M Cu(NO₃)₂ for 1 min, washed with ethanol, and then dipped for another 1 min into a 0.5 M Na₂S methanol solution and washed with methanol. This two-step dipping procedure is repeated 2, 4, 6, and 8 times, and ZnO NRs/CuS is produced.

The ZnO NRs/CdS/CdSe photoanodes were assembled with ZnO NRs/CuS as the counter electrode. The photoanodes and counter electrodes are sealed by a 50 μm thermoplastic spacer. A polysulfide electrolyte consisting of 1 M Na₂S and 1 M S is injected into the cells.

The photocurrent density–voltage (J–V) characteristics of the solar cells are measured under one sun (AM1.5G, 100 mW cm⁻²) illumination using a solar simulator (Sharif solar simulator). Electrochemical impedance spectroscopy (EIS) of the CEs was recorded using an IVIUM stat potentiostat/galvanostat (XRE model) and performed on dummy cells with a symmetric sandwich-like structure between two similar electrodes, that is, CE/electrolyte/CE. The measured frequency ranged from 10⁻² to 10⁶ Hz and the amplitude was set at 10 mV. A Tafel polarization measurement was carried out on the electrochemical analyzer in the dummy cell used in the EIS measurement. The morphologies of the CuS and ZnO NRs were obtained by field emission scanning electron microscopy (FESEM-S4160 Hitachi). X-ray diffraction (XRD, Expert-Philips) was used to characterize the structure of the ZnO NR/CuS.

3. Results and discussion

For characterization of the counter electrodes, FESEM and XRD were used. Fig. 1(a) displays the morphology of the ZnO NRs on FTO, which has a smooth structure. After SILAR deposition of the CuS, the nanoparticles are recognizable on the surface of the ZnO NRs. Fig. 1(b) indicates that when the cycle number reaches 2, the deposition of CuS onto the ZnO NRs is partial and a few sparsely dispersed CuS nanoparticles are observed. It is perceived from Fig. 1(c) that with a deposition of 6 SILAR cycles of CuS on the ZnO NRs, a rough surface area is produced which is critical for high electrocatalytic activity. The ZnO NRs/CuS counter electrode is further studied by XRD (Fig. 2), where peaks of the SnO₂, ZnO and CuS are indexed. No peaks for other compounds are observed in the spectra except for SnO₂, which is derived from the FTO substrate. For the bare ZnO NRs the diffraction peaks are marked by a hash symbol in the figure and they can be indexed to hexagonal phase ZnO (JCPDS file no. 36-1451). After decoration with CuS, some new peaks appear. The XRD pattern clearly shows peaks at (102), (105), (106), and (116) as in accordance with the JCPDS file (06-0464) indicating covellite phase CuS. These peaks are in good agreement with other reports,²⁴ and they clearly illustrate that ZnO NRs/CuS has been formed.

Fig. 1 SEM images of (a) ZnO NRs, (b) ZnO NRs/CuS (2 cycles), and (c) ZnO NRs/CuS (6 cycles).

Fig. 2 XRD pattern of the FTO/ZnO NRs/CuS (6 cycles) layer.

SEM imaging and absorption spectra were applied for the characterization of the photoanodes. Fig. 3 presents the morphology of the ZnO seed layer and the ZnO NRs. The image of the ZnO seed layer on the FTO substrate (Fig. 3(a)) indicates that the seed layer is homogeneously deposited on the FTO. Fig. 3(b) shows that the ZnO NR arrays are grown on the FTO substrate with a diameter of 30–55 nm. The figure also shows that the NRs are hexagonal in form and are approximately vertically grown on the FTO substrate. In addition, Fig. 3(c) shows that a 2 μm layer of ZnO NRs with a diameter of 120–200 nm has been deposited on the ZnO NR arrays in order to form the light scattering layer of the photoanode.

Fig. 3 SEM image of (a) the ZnO seed layer on the FTO substrate, (b) ZnO NR arrays grown on the ZnO seed deposited FTO substrate, and (c) a layer of ZnO NRs deposited on the ZnO NR arrays.

Absorption spectra are studied to elucidate the absorption properties of the ZnO NRs, ZnO NRs/CdS, and ZnO NRs/CdS/CdSe photoanodes as shown in Fig. 4. It is observed from the figure that the absorption edge of the layer is changed from 360 nm to 550 nm and from 550 nm to 600 nm after decoration of the CdS QDs and CdSe QDs on the ZnO NRs, respectively. This shift of the absorption edge increases the light absorption in the cell which suggests the generation of an electron–hole couple and a resulting enhancement of current density in the cell. Also, it is observed that the absorption of ZnO/CdS/CdSe and ZnO/CdS are lower than that of ZnO in the 300–400 nm wavelength range. According to the band gap of the ZnO (about∼ 3.2 eV), light absorption in the 300–400 nm wavelength range is extremely raised in the bare ZnO. With the deposition of CdS and CdSe QDs on the ZnO, the ZnO can’t absorb in the wavelength range between 300–400 nm due to the absorption by QDs, and the lack of light getting to the ZnO, and thus the absorption of ZnO/CdS/CdSe and ZnO/CdS is lower than that of ZnO in the 300–400 nm wavelength range.

Fig. 4 Absorption spectra of the ZnO NRs, ZnO NRs/CdS, and ZnO NRs/CdS/CdSe photoanodes.

The EIS is measured to study R_ct at the counter electrode/polysulfide interfaces. The R_ct is a significant parameter to determine the fill factor (FF), open-circuit voltage (V_oc) and short-circuit current density (J_sc) of a cell.²⁵ CuS(6 cycles):CuS(6 cycles) and ZnO NR/CuS(X = 2, 4, 6 and 8 cycles):ZnO NR/CuS(X = 2, 4, 6 and 8 cycles) symmetric cells are prepared for the EIS measurement, and polysulfide electrolyte is used as the electrolyte. The R_ct is found from the semicircle in the Nyquist plots shown in Fig. 5. R_ct is obtained by the Z-view software with the equivalent circuit models. Table 1 displays the R_ct values for bare CuS and ZnO NRs/CuS (X = 2, 4, 6 and 8 cycles) counter electrode based symmetric cells. R_ct has a value of 80 Ω cm² for the ZnO NR/CuS (6 cycles) counter electrode, while the R_ct of the bare CuS counter electrode is 300 Ω cm². For SILAR cycles of CuS which is more than 6 on the ZnO NRs, the thickness of ZnO NRs/CuS is increased, leading to poor reduction in the polysulfide electrolyte and increasing R_ct. Also, the results from Table 1 illustrate that the R_ct value for the ZnO NRs/CuS counter electrode with 2 SILAR cycles of CuS is extremely increased. As the SEM images (Fig. 1(b)) show, the ZnO NR surfaces are purely uncovered by CuS crystals for 2 SILAR cycles of CuS, which causes ZnO NRs and the polysulfide electrolyte to come in direct interaction. Sine ZnO NRs isn’t a sufficient electro-catalyst, cannot reduce polysulfide electrolyte as well as CuS crystals, charge transfer between the counter electrode and the electrolyte is decreased and the R_ct is considerably increased.

Fig. 5 Nyquist plots of the symmetric cells with bare CuS(6 cycles) and ZnO NRs/CuS(2, 4, 6, and 8 cycles) counter electrodes.

Table 1 R_ct of the symmetric cells using polysulfide electrolyte

Counter electrode	Rct (Ω cm^2)
CuS (6 cycles)	300
ZnO NRs/CuS (2 cycles)	5478
ZnO NRs/CuS (4 cycles)	151.8
ZnO NRs/CuS (6 cycles)	80
ZnO NRs/CuS (8 cycles)	324

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The J–V characteristics of the QDSSCs with bare CuS and ZnO NRs/CuS counter electrodes for different SILAR cycles of CuS are measured at 100 mW cm⁻² light intensity. The J–V curves are shown in Fig. 6(a), and the corresponding photovoltaic parameters are listed in Table 2. The results indicate that the V_oc, J_sc, and FF of the QDSSC of 0.72 V, 7.63 mA cm⁻², and 0.29 to 0.76 V, 14.48 mA cm⁻², and 0.38, respectively, are enhanced with altering counter electrode from bare CuS (6 cycles) to ZnO NRs/CuS (6 cycles). This statement is in agreement with the EIS results shown in Fig. 5, which shows that the R_ct of the electrolyte/counter electrode interfaces is reduced from 300 Ω cm² to 80 Ω cm² with changing the counter electrode from bare CuS (6 cycles) to ZnO NRs/CuS (6 cycles). The reduction of the R_ct of the electrolyte/counter electrode interfaces causes the exchange current in the cell to be enhanced and consequently, J_sc is increased. The cell with the ZnO NRs/CuS (2 cycles) counter electrode shows the worst performance with a maximum PCE of 1.39% and both J_sc and FF are the lowest among the compared samples, which is caused by a deficit of electrocatalytic activity towards polysulfide reduction due to the lacking amount of CuS. As the SILAR cycle number is increased to 6, more CuS is decorated on the ZnO NRs, and the electrocatalytic activity of the counter electrode is enhanced and therefore the performance is greatly boosted (4.18%). These results show that the bare CuS and insufficient CuS deposited ZnO NRs as counter electrodes are not efficient for the reduction of the polysulfide electrolyte as they involve a high R_ct value at the counter electrode/electrolyte interface.

Fig. 6 The J–V characteristics (a) under light conditions, and (b) V_oc decay characteristics.

Table 2 The J–V results of the QDSSCs with bare CuS and the ZnO NRs/CuS counter electrode for different numbers of SILAR cycles of CuS

Counter electrode	Photoanode	Jsc (mA cm^−2)	Voc (V)	PCE (%)	FF
CuS (6 cycles)	ZnO NRs/CdS/CdSe	7.63	0.72	1.59	29
ZnO NRs/CuS (2 cycles)	ZnO NRs/CdS/CdSe	7.64	0.73	1.39	25
ZnO NRs/CuS (4 cycles)	ZnO NRs/CdS/CdSe	10.01	0.74	2.29	31
ZnO NRs/CuS (6 cycles)	ZnO NRs/CdS/CdSe	14.48	0.76	4.18	38
ZnO NRs/CuS (8 cycles)	ZnO NRs/CdS/CdSe	10.89	0.74	2.82	35

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The significant improvement in charge transfer at the counter electrode/polysulfide interface can be attributed to the reduction of internal resistance, and increasing recombination resistance in the cell. These parameters strongly affect the cell performance in terms of the J_sc, V_oc and FF. The enhancement of V_oc with varying the counter electrode from CuS to ZnO NRs/CuS can be the consequence of a more positive potential of the ZnO NRs/CuS in the polysulfide electrolyte.¹⁶ In addition, the QDSSC with the ZnO NRs/CuS counter electrode has a higher FF than the QDSSC with the bare CuS counter electrode, indicating a high charge transfer rate at the counter electrode/polysulfide electrolyte interface. Also, it is observed that with increasing the number of SILAR cycles to 8, all the parameters of the cell start to reduce. However, the overall PCE (2.82%) is still much higher than that with the bare CuS (6 cycles) counter electrode. This result is in agreement with the R_ct of the electrolyte/counter electrode interface (Fig. 5), which shows that the R_ct value is raised from 80 Ω cm² to 324 Ω cm² with increasing the number of CuS SILAR cycles from 6 to 8. This phenomenon is attributed to increasing the thickness of the counter electrode and the reduction of electron transport to the polysulfide electrolyte. Since ZnO NRs have weak electrocatalytic activity for the reduction of the polysulfide electrolyte, the charge transfer resistance at the counter electrode/polysulfide interface is extremely increased, which causes the cell FF to reduce. It is suggested that the reduction of FF leads to the declining efficiency of QDSSCs with bare ZnO as the counter electrode. As can be seen from the J–V results, the efficiency of the QDSSC with a ZnO NRs/CuS (2 cycles) counter electrode is 1.39%, which is lower than the bare CuS (1.59%), which is attributed to the insufficient coverage of CuS crystals on the ZnO NRs and direct contact of the polysulfide and the ZnO NRs. Therefore, the efficiency of QDSSCs with bare ZnO as the counter electrode should be lower than QDSSCs with the ZnO NRs/CuS (2 cycles) counter electrode.

Fig. 6(b) shows the V_oc decay curves of the QDSSCs for different counter electrodes. In this analysis, the rate of the voltage reduction indicates the recombination rate in the cell. It is seen that the V_oc of the cells with the ZnO/CuS (6 cycles) counter electrode decay more slowly than that of cells with a bare CuS (6 cycles) counter electrode, which suggests a lower recombination rate than that of QDSSCs with a bare CuS counter electrode. Besides, it is observed that by increasing the number of SILAR cycles of CuS to 6, the electron recombination rate is decreased and increased afterward. Also, it is found that the electron recombination rate for the ZnO/CuS (2 cycles) counter electrode is extremely increased, which is in agreement with the EIS results.

EIS measurements of cells with photoanodes and counter electrodes were made to investigate the charge transfer kinetics of the QDSSCs. The selected frequency is in the range of 10⁻² to 10⁶ Hz with an AC amplitude of 10 mV. Fig. 7 shows the obtained Nyquist plots from the EIS results of the QDSSCs with bare CuS and ZnO NRs/CuS (2, 4, 6, and 8 cycles) counter electrodes under dark conditions. One distinct arc is observed in the low frequency area. The low-frequency arc contains the chemical capacitance of the photoanode and recombination resistance (R_rc) between the photoanode and the polysulfide electrolyte as R_rc has a direct relation with the arc diameter of the Nyquist plot and inversely with the recombination rate at the photoanode/electrolyte interface. The results illustrate that the ZnO/CuS (6 cycles) counter electrode has the highest R_rc value, which suggests that the recombination rate at the photoanode/electrolyte interface is reduced. Moreover, the reduction in electron recombination rates would shift up the electron Fermi level of the ZnO NRs,²² and hence, V_oc could also be increased because V_oc is determined by the difference between the electron Fermi level of the ZnO NRs and the redox potential of the polysulfide electrolyte.^26,27 Also, it is found that the R_rc value is lowest for the ZnO/CuS (2 cycles) counter electrode, suggesting that the recombination rate is higher than for other counter electrodes, which caused the FF to be diminished. This higher recombination rate of the ZnO/CuS (2 cycles) counter electrode compared with the ZnO/CuS (4, 6, and 8 cycles) counter electrodes suggests a downwards shift in the electron Fermi level of the ZnO NRs, and hence a reduction in V_oc that is in agreement with the J–V results in Table 2.

Fig. 7 Nyquist plots from the EIS results of the QDSSCs with bare CuS and ZnO NRs/CuS (2, 4, 6, and 8 cycles) counter electrodes at V_oc conditions.

To evaluate the interfacial charge-transfer property of the polysulfide electrolyte on the counter electrode surface, Tafel plots for the bare CuS (6 cycles) and ZnO/CuS (2, and 6 cycles) counter electrodes are measured. Fig. 8(a) illustrates the current density as a function of applied voltage of the symmetrical dummy cells. The catalytic activities of the counter electrodes can be recognized from the exchange current density (J₀), that could be acquired by extrapolation of the cathodic branch of every curve to the zero over potential.²² As a result, larger slopes are observed for the ZnO NRs/CuS (6 cycles) than for the bare CuS (6 cycles) counter electrode, which indicated the larger J₀ value for the electrode surface. This result proves that the electrocatalytic activities of the ZnO NRs/CuS (6 cycles) is higher than the activity of the bare CuS (6 cycles), and the other ZnO/CuS counter electrodes.

Fig. 8 (a) Tafel curves of the symmetrical dummy cells fabricated with bare CuS and ZnO NRs/CuS (2, and 6 cycles) counter electrodes. (b) Cyclic voltammetry of bare CuS and ZnO NRs/CuS (2, and 6 cycles) counter electrodes.

Additional investigation of the redox processes at the electrolyte/counter electrode interface is carried out by studying the cyclic voltammetry (CV) measurements. In this QDSSC, photoelectrons from the QDs are transferred into the ZnO CB. The oxidized QDs are reduced by S²⁻ ions in the polysulfide, sulfur in the electrolyte reacts with S²⁻ leading to the formation of polysulfide, and the produced S_x²⁻ ions are ultimately reduced at the counter electrode.²⁸ Fig. 8(b) displays the CV of the counter electrodes with bare CuS (6 cycles) and ZnO NRs/CuS (2, and 8 cycles). As shown in Fig. 8(b), the ZnO NRs/CuS (6 cycles) counter electrodes have higher redox current densities than the bare CuS (6 cycles) counter electrode, suggesting the better electrocatalytic ability of the ZnO NRs/CuS (6 cycles) counter electrodes for reducing the S_x²⁻ ions than the bare CuS counter electrode. Furthermore, the higher electrocatalytic ability of the ZnO NRs/CuS (6 cycles) counter electrode led to a higher FF and lower R_ct as compared to the cell with a bare CuS (6 cycles) counter electrode, which is in agreement with the J–V and EIS results. Also, the ZnO NRs/CuS (2 cycles) counter electrode has the lowest electrocatalytic ability, which is attributed to the poor covering of CuS on the ZnO NRs and the direct contact of the ZnO NRs with the polysulfide.

4. Conclusion

This work presents the improvement in efficiency of a ZnO nanorod-based QDSSC by employing ZnO NRs/CuS as the counter electrode. The results showed that with a change of counter electrode from bare CuS (6 cycles) to ZnO NRs/CuS (6 cycles), the J_sc, V_oc, FF and PCE values shift from 7.64 mA cm⁻² to 14.48 mA cm⁻², 0.72 V to 0.76 V, 0.29 to 0.38 and 1.59% to 4.18%, respectively. The enhanced performance of the QDSSCs can be attributed to the reduction of R_ct with the ZnO NRs/CuS (6 cycles) counter electrode in comparison with the bare CuS (6 cycles) counter electrode, which caused the recombination rate to reduce in the structure. Moreover, the ZnO NRs/CuS (6 cycles) counter electrodes had higher redox current densities than the bare CuS (6 cycles) counter electrode, indicating the better electrocatalytic ability of the ZnO NRs/CuS (6 cycles) counter electrodes for reducing the S_x⁻² ions as compared with the bare CuS (6 cycles) counter electrode. Also, the results showed that the NRs/CuS (2 cycles) counter electrode had the lowest electrocatalytic activity, which can be attributed to the poor covering of CuS on the ZnO NRs and the direct contact with the polysulfide.

Note added after first publication

This article replaces the version published on 27th May 2016, in which the designation of the fourth author as a corresponding author was omitted.

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