Low-Cost Flexible Nano-Sulfide/Carbon Composite Counter Electrode for Quantum-Dot-Sensitized Solar Cell
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Cu2S nanocrystal particles were in situ deposited on graphite paper to prepare nano-sulfide/carbon composite counter electrode for CdS/CdSe quantum-dot-sensitized solar cell (QDSC). By optimization of deposition time, photovoltaic conversion efficiency up to 3.08% was obtained. In the meantime, this composite counter electrode was superior to the commonly used Pt, Au and carbon counter electrodes. Electrochemical impedance spectra further confirmed that low charge transfer resistance at counter electrode/electrolyte interface was responsible for this, implied the potential application of this composite counter electrode in high-efficiency QDSC.
KeywordsQuantum dot Sensitized solar cell Composite Flexible Carbon electrode Cu2S CdS/CdSe
The quantum-dot-sensitized solar cell has aroused great research interests due to the superior properties of semiconductor quantum dots (QDs) in recent years [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. The merits of QDs include higher extinction coefficient in visible light spectrum , multiple excitons generation through impact ionization  and readily tuned bandgap by size control . Therefore, various semiconductor QD sensitizers, such as CdS [1, 4, 5], CdSe [6, 7], CdTe , InAs , InP , and their linking to the photoanode [12, 13, 14] have been widely studied for QDSCs. Meanwhile, as another important part of the sandwich-type QDSC, more attention was also paid to the research of counter electrode (CE) lately [14, 15, 16]. Bisquert et al.  found that Pt CE constituted a limiting factor for the cell performance because the sulfides (S2−, Sx2− ions) can adsorb onto Pt surface and impair its electrocatalytic activity. Lee et al.  verified this result and proved that Au CE was more immune to the sulfur ions with high energy conversion efficiency up to 4.22% for CdS/CdSe QDSC.
As we know, for all kinds of solar cells, the cost reduction is crucial for their future development all the time. Two typical strategies in cost cutting include the introduction of (a) easily handled preparation methods and (b) inexpensive alternative materials into the fabrication of solar cells, such as low-cost electrochemical etching method to prepare silicon nanocrystallites  and various conducting polymers [18, 19] or carbon materials [20, 21]. In dye-sensitized solar cells (DSCs), as promising low-cost replacements of Pt CE, carbon CEs have been widely investigated [20, 21, 22, 23, 24, 25, 26, 27]. For QDSCs, latest research revealed that carbon electrode exhibited much higher activity beyond Pt in polysulfide redox system (S2−/Sx2−) and the cell efficiency of 1.47% was obtained . Another cheap material Cu2S also exhibited possible application as CE for QDSCs in virtue of its high electrocatalytical activity reported by Hodes et al. . A newly published result showed that with the CE of Cu2S made from brass in CdSe QDSC, the fill factor (FF) was increased remarkably and photovoltaic efficiency was improved to 1.83% . Here, in order to further increase the cell efficiency, conductive graphite with noticeable activity was associated with the highly active Cu2S by the in situ deposition of Cu2S nanoparticles on flexible graphite paper. This nano-sulfide/carbon composite electrode was introduced into CdS/CdSe QDSC for the first time and superior energy conversion efficiency of 3.08% was achieved.
Preparation of Counter Electrode
The flexible graphite paper used as conductive substrate was the same as our previous research . In order to increase the functionalized sites for Cu2S adhesion on the surface of graphite, the substrate was annealed under 450 °C for 30 min in the air before use. Solvent thermal method was employed to deposit Cu2S nanoparticles. The procedures were described as follows : 20 mM Cu(CH3COO)2 and 10 mM thiourea were dissolved in diethylene glycol (DEG) sequentially and transferred to Teflon autoclave. Then, the annealed graphite paper was immersed and the autoclave was sealed and maintained at 180 °C for 2–12 h. After the reaction, the treated graphite was washed with deionized water three times and dried at 60 °C under vacuum overnight to get rid of water and residual DEG. This kind of CE was referred as nano-Cu2S/C in the following. For comparison, the annealed graphite paper, the thermally decomposed Pt electrodes on F-doped SnO2 conducting glass (FTO, 15 Ω/□) and annealed graphite paper, carbon counter electrodes on these two substrates  as well as Au electrode on FTO glass by evaporative deposition were also used as CEs for QDSCs.
CdS/CdSe-sensitized photoanode was fabricated by a previously published chemical bath method [30, 31]. CdS was pre-deposited onto TiO2 nanoporous film in the aqueous solution of 20 mM CdCl2, 66 mM NH4Cl, 140 mM thiourea and 230 mM ammonia, followed by the CdSe deposition in a mixture with the composition of 80 mM sodium selenosulphate (Na2SeSO3, prepared by dissolving 0.2 M Se powder in a 0.5 M Na2SO3 solution at 80 °C), 80 mM CdSO4 and 160 mM nitrilotriacetic acid tripotassium salt (NTA, N(CH2COOK)3). Surface passivation with ZnS was realized by dipping the sensitized photoanode alternatively into 0.1 M Zn(CH3COO)2 and 0.1 M Na2S solution for 1 min twice [7, 12]. Polysulfide electrolyte with 1 M Na2S and 1 M S aqueous solution was used as redox agent. Electrolytes were dropped on the sensitized photoanode, and counter electrode was clipped firmly to make a sandwich structure QDSC. A 50-μm silicone film was used as spacer. The active area of the cell was 0.15 cm2.
The morphology of the nano-Cu2S/C CE was investigated by a scanning electron microscope (SEM) (FEI, XL30 S-FEG). The X-ray diffraction (XRD, M18X-AHF, MAC Science) pattern was recorded with Cu Kα radiation source for the dried powders from the reaction autoclave. The cells were irradiated by simulated AM1.5 irradiation (Oriel, 91192). Current–photovoltage (I–V) characteristics were recorded by a potentiostat (Princeton Applied Research, Model 263A). Electrochemical impedance spectroscopy (EIS) measurements were carried out on electrochemical workstations (Zahner, IM6ex) under illumination. During the measurement, the cell was biased with open-circuit voltage under sinusoidal perturbation of 10 mV with the frequency scanning range 10−1–105 Hz. EIS results were fitted with Z-view to obtain the charge transfer resistance (Rct) at the CE/electrolyte interface.
Results and Discussion
Photovoltaic parameters of tested QDSCs, together with R□ and Rct value of various counter electrodes
Cu2S nanoparticles were deposited on the surface of graphite paper to obtain a composite counter electrode for CdS/CdSe-sensitized solar cell. With the cell parameters of Jsc = 10.68 mA cm−2, Voc = 497 mV, FF = 0.581 and η = 3.08%, QDSC with nano-Cu2S/C composite CE exhibits superior performance to the Pt, Au and carbon CEs. Electrochemical impedance spectroscopy measurement indicates that the Rct at CE/electrolyte was very low and made the composite CE an excellent candidate for high-efficiency QDSCs.
We gratefully acknowledge the support of the National Science Fund for Distinguished Young Scholars under Grant No. 20725311, the National Natural Science Foundation of China under Grant No. 20673141, 20703063 and 20873178, Strategic China-Japan (NSFC-JST) Joint Research Program under Grant No. 20721140647, the National Basic Research Program of China (“973”) under Grant No. 2006CB202606, the National High Technology Research and Development Program (“863”) under Grant No. 2006AA03Z341 and the 100-Talents Project of Chinese Academy of Sciences. Part of this work was supported by JST PRESTO program and by a Grant-in Aid for Scientific Research (No.21310073) from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government.
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