Performances of some low-cost counter electrode materials in CdS and CdSe quantum dot-sensitized solar cells
© Jun et al.; licensee Springer. 2014
Received: 11 November 2013
Accepted: 24 January 2014
Published: 10 February 2014
Different counter electrode (CE) materials based on carbon and Cu2S were prepared for the application in CdS and CdSe quantum dot-sensitized solar cells (QDSSCs). The CEs were prepared using low-cost and facile methods. Platinum was used as the reference CE material to compare the performances of the other materials. While carbon-based materials produced the best solar cell performance in CdS QDSSCs, platinum and Cu2S were superior in CdSe QDSSCs. Different CE materials have different performance in the two types of QDSSCs employed due to the different type of sensitizers and composition of polysulfide electrolytes used. The poor performance of QDSSCs with some CE materials is largely due to the lower photocurrent density and open-circuit voltage. The electrochemical impedance spectroscopy performed on the cells showed that the poor-performing QDSSCs had higher charge-transfer resistances and CPE values at their CE/electrolyte interfaces.
As the world population grows, the demand for energy consumption will also increase in tandem. In order to meet the growing demand, there is a need to use renewable energy source as an alternative source for fossil fuels. One of the renewable energy routes is solar cells. Of all the solar cell technologies, quantum dot-sensitized solar cells (QDSSCs) have emerged as a widely researched topic in recent years [1–4]. The high interest in this field is due to the attractive properties of the quantum dots (QDs), namely ease of synthesis, ability to tune the band gap energy and possibility of attaining multiple exciton generation (MEG) [3–5]. Some examples of QDs include but not limited to Ag2S , CdS , CdSe , PbS  and CuInS2. Recently, QDs based on organometallic perovskites such as CH3NH3Pbl3 have shown impressive efficiencies .
In QDSSCs, the working principle is almost similar to that of the dye-sensitized solar cell (DSSC) . Upon light irradiation, the electrons in the QD will be excited into the conduction band (CB) leaving holes in the valence band (VB). The electrons will then get injected into the CB of the wide band gap semiconductor (usually TiO2), percolate through the TiO2 network and reach the substrate. The electrons reach the counter electrode (CE) by passing through the external load and reduce the redox mediators which donate electrons to fill the holes in the QDs. Thus, current is produced continuously as long as light is present without the consumption or production of any chemicals.
In order to obtain a high-performing QDSSC, material selection plays a major role . The type of QD sensitizers, CE materials and electrolyte composition could affect the overall performance in one way or another. Among the prominent materials for QD sensitizers, CdS and CdSe are widely used due to their easy preparation. The QDSSCs based on them usually employ polysulfide-based liquid electrolytes. For CE, the usual choice is platinum even though other materials such as gold, Cu2S and reduced graphene oxide (RGO) are possible [14–16].
In this work, alternative low-cost CE materials were used in CdS and CdSe QDSSC assembly to understand the effect of CE materials towards the solar cell performance. The materials for the CEs used were commercially obtained or prepared economically at lab scale. Two different optimized polysulfide liquid electrolytes were used in the CdS and CdSe QDSSCs. Photoelectrochemical performance of the cells was investigated to assess the effect of the CE materials. The behaviour of the QDSSCs was also investigated using electrochemical impedance spectroscopy (EIS). This study was undertaken to explore the best low-cost and easy-to-prepare CE material for CdS and CdSe QDSSCs. To the author's best knowledge, there is no report in the literature on the performance of easy-to-prepare low-cost graphite, carbon soot and RGO used as CEs in QDSSCs.
Titanium dioxide (TiO2) paste (18NR) was obtained from JGC C&C, Kawasaki City, Kanagawa, Japan. Fluorine-doped tin oxide (FTO) conducting glasses (8 Ω/sq sheet resistance) purchased from Solaronix, Aubonne, Switzerland were used as electrode substrates. The di-isopropoxytitanum bis(acetylacetonate) needed for the TiO2 compact layer was procured from Sigma-Aldrich, St. Louis, MO, USA. Cadmium nitrate tetrahydrate, selenium dioxide, sodium borohydride, potassium chloride, sulfur and guanidine thiocyanate (GuSCN) were all purchased from Sigma-Aldrich while sodium sulfide nonahydrate was procured from Bendosen, Hamburg, Germany.
Preparation of TiO2 film working electrode
A compact layer of TiO2 was first prepared by spin coating 0.38 M ethanolic solution of di-isopropoxytitanum bis(acetylacetonate) on the FTO surface of the substrate at 3,000 rpm for 10 s. The coated FTO glass was then sintered at 450°C for 30 min. The acquired TiO2 compact layer not only enhances the adhesion of TiO2 particles to the substrate but also provides a larger TiO2/FTO contact area ratio and minimizes electron recombination by reducing the contact between the electrolyte and the FTO surface . The doctor blade method was used to spread the TiO2 paste on the compact layer in order to form the mesoporous network of TiO2. The newly deposited layer was also sintered at 450°C for 30 min in order to remove organic residues and moisture for obtaining a mesoporous TiO2 layer.
Fabrication of CdS and CdSe QD-sensitized electrodes
Both CdS and CdSe QDs were prepared using the successive ionic layer adsorption and reaction (SILAR) deposition method. To fabricate CdS QDs, the TiO2-coated electrode was successively dipped into 0.1 M Cd(NO3)2 ethanolic solution for 5 min and into 0.1 M Na2S methanol solution for another 5 min. The electrode was rinsed with alcohol and allowed to dry in between the dipping process. This two-step dipping is considered as 1 SILAR cycle. Four SILAR cycles were used to prepare a CdS QD-sensitized TiO2 electrode.
For CdSe QDs, preparation process was performed in a glove box filled with argon gas . TiO2-coated electrode was first dipped into 0.03 M Cd(NO3)2 ethanolic solution for 30 s followed by ethanol rinsing and drying. Then, it was dipped into Se2- solution for 30 s followed by ethanol rinsing and drying. Se2- solution was prepared by reacting 0.03 M SeO2 ethanolic solution with 0.06 M NaBH4. The mixture was stirred for about an hour before it was used for SILAR dipping process. Seven SILAR cycles were used to prepare a CdSe QD-sensitized TiO2 electrode.
Preparation of CEs
Five types of CE materials were used: platinum, graphite, carbon, Cu2S and RGO. Platinum layer was prepared by spin coating a thin layer of commercial platinum solution (Plastisol from Solaronix) on the conducting glass surface and sintering at 450°C for 30 min. Graphite layer was obtained by rubbing pencil lead on the conducting glass surface. To obtain carbon layer, the conducting glass was placed over a candle flame for a few seconds so that black carbon soot formed readily on the surface. Cu2S electrode was prepared according to the procedure given in the literature . In this procedure, a brass electrode was immersed in hydrochloric acid at 70°C for 5 min, and then, the treated brass was dipped into polysulfide aqueous solution containing 1 M Na2S and 1 M S for 10 min. Upon the solution treatment, Cu2S would be formed on the brass surface as a thin black layer. To prepare counter electrode with RGO, RGO powder (Timesnano) was mixed in the N-methyl-2-pyrrolidone (NMP) solution with 10 wt.% of polyvinylidene difluoride (PVDF). The suspension was then cast on the conducting glass and allowed to dry at 70°C.
Assembly of QDSSCs
Solar cell was fabricated by clamping the QD-sensitized TiO2 electrode with a selected CE. Parafilm (130 μm thickness) was used as a spacer between the two electrodes. The spacer also prevented the liquid electrolyte from leaking. Prior to the cell assembly, few drops of polysulfide electrolyte were dropped onto the surface of QD-sensitized TiO2 film until the active surface area was covered with the electrolyte. Different polysulfide liquid electrolytes were selected for CdS and CdSe QDSSCs based on previous optimization reports [20, 21]. The polysulfide electrolyte solution for CdS QDSSCs was prepared from 0.5 M Na2S, 2 M S and 0.2 M KCl in water/methanol = 3:7 (v/v) . For CdSe QDSSCs, the polysulfide electrolyte contained 0.5 M Na2S, 0.1 M S and 0.05 M GuSCN in water/ethanol = 2:8 (v/v) . An effective cell area of 0.25 cm2 was used for the solar cell performance investigations.
Photoresponse and EIS measurements
where JSC is the short-circuit photocurrent density, VOC is open-circuit voltage, FF is the fill factor and Pin is the intensity of the incident light. Measurement on each cell was repeated three times to ensure the consistency of the data.
The EIS study was performed using an Autolab potentiostat/galvanostat (Utrecht, The Netherlands). Measurement was performed on cells under dark and illuminated conditions. Light illumination was provided by a xenon lamp at the intensity of 1,000 W m-2. The EIS measurements were made on cells biased at potentials given and explained in the ‘Results and discussion’ section with a 15-mV RMS voltage perturbation in the frequency range 106 to 0.01 Hz. EIS results were fitted with ZSimWin software to obtain the series resistance, RS and charge-transfer resistance at the CE/electrolyte interface, RCE.
Results and discussion
CdS and CdSe QDSSCs have been fabricated with QD-sensitized TiO2 layers prepared via SILAR method and selected liquid electrolytes. Both CdS and CdSe QD-sensitized TiO2 layers were assembled with the five different types of CE materials including platinum. The cell with platinum as the CE was used as the reference cell. The J-V curves for both types of QDSSCs showed that solar cell performance is considerably influenced by the choice of CE materials.
Performance parameters of CdS QDSSCs with various CEs
CdSe QDSSC performance parameters with various CEs
EIS results of CdS QDSSCs
CPE2-T (μS.s n )
CPE2-P (0 < n < 1)
From the EIS results, it can be seen that the CdS QDSSC with Cu2S as CE has the lowest series resistance, RS. This is reasonable considering the highly conductive brass metal involved in comparison to the usual FTO layer used. RS is the resistance corresponding to the transport resistance of the conducting substrate. In this study, charge-transfer resistance at the QD-sensitized TiO2/electrolyte interface (Rr) is not discussed as the value is not directly influenced by the choice of counter electrode materials. Under dark condition, the charge-transfer resistance at the CE/electrolyte interface, RCE is high in all the cells. When the cells were tested under illumination, the RCE value reduced substantially for most of the cells due to more charge transfer taking place in the system. It is observed that the low RCE gives rise to higher open-circuit voltage of the cell as seen in the case of QDSSCs with carbon soot and platinum as their CEs. However, this is not the case for Cu2S as its photocurrent density is few times lower than that of the cell with platinum as CE. The low RCE could be due to the excessive potential bias applied (0.45 V) to the cell as its open-circuit voltage is only 0.28 V. This high potential bias could have provided a more conductive state for the charge transfer. The overall low performance of the cell could be attributed to the low catalytic activity at the Cu2S/electrolyte interface which implies a slow reduction rate for polysulfide S x 2- species. For the high-efficiency CdS QDSSCs having platinum, graphite or carbon soot as CEs, the good performance is due to low constant phase element (CPE) values. This translates to low true capacitance at the CE/electrolyte interface which could imply a better electrocatalytic activity.
EIS results of CdSe QDSSCs
CPE2-T (μS.s n )
CPE2-P (0 < n < 1)
Since the polysulfide electrolyte could impair the platinum CE surface as reported by Mora-Sero et al., the performance of the cell with platinum CE could deteriorate over the long run . Ultimately, the charge-transfer resistance will increase. Therefore, Cu2S appears to be a good candidate for CE material for the CdSe QDSSCs. Nevertheless, the high performance as observed in both CdS and CdSe QDSSCs with platinum CE suggests the detrimental effect from polysulfide electrolyte might not be that serious at the early stage of operation. Based on the EIS response, should a multilayered CdS/CdSe QDSSC be prepared, a composite between carbon and Cu2S could be the best material for the CE. Similar conclusion has been made by Deng et al. . It is to be noted that the different EIS parameter values obtained for both CdS and CdSe QDSSCs with similar CE materials can be partly attributed to the different choice of electrolytes used as well. Therefore, further optimization is necessary to improve the efficiencies of the cells.
The efficiencies reported in this work are somewhat lower than the values reported in the literature for similar QDSSCs. It should be noted the present study was undertaken with standard TiO2 layer sensitized with a single QD layer and standard electrolytes to explore the best CE materials, which resulted in lower efficiencies. A different type of wide band gap semiconducting layer such as ZnO or Nb2O5 could perhaps produce different results. Nevertheless, the efficiencies of the TiO2-based cells can be improved considerably with optimization of all the components involved in the QDSSC and by using passivation layers at the photoanode to reduce the charge recombination losses.
Low-cost CEs have been prepared from graphite, carbon soot, Cu2S and RGO to study their effect on the performance of CdS and CdSe QDSSCs. Carbon-based materials were found to be a good CE material for CdS QDSSCs and such a cell with graphite as CE produced the best efficiency value of 1.20% with the highest photocurrent density. For CdSe QDSSCs, although cell with platinum CE showed a relatively good performance, Cu2S could be the alternative choice for CE. EIS measurements on both CdS and CdSe QDSSCs showed that low RCE and CPE values for the CE/electrolyte interface are the key criteria for selecting good-performance CE materials. Further optimization of the cell is possible for achieving higher efficiencies.
The authors would like to thank University of Malaya for the IPPP grant no. PV094-2012A. H.K. Jun thanks University of Malaya for the Fellowship Scheme Scholarship.
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