Improving the efficiency of cadmium sulfide-sensitized titanium dioxide/indium tin oxide glass photoelectrodes using silver sulfide as an energy barrier layer and a light absorber
© Chen et al.; licensee Springer. 2014
Received: 2 October 2014
Accepted: 19 October 2014
Published: 7 November 2014
Cadmium sulfide (CdS) and silver sulfide (Ag2S) nanocrystals are deposited on the titanium dioxide (TiO2) nanocrystalline film on indium tin oxide (ITO) substrate to prepare CdS/Ag2S/TiO2/ITO photoelectrodes through a new method known as the molecular precursor decomposition method. The Ag2S is interposed between the TiO2 nanocrystal film and CdS nanocrystals as an energy barrier layer and a light absorber. As a consequence, the energy conversion efficiency of the CdS/Ag2S/TiO2/ITO electrodes is significantly improved. Under AM 1.5 G sunlight irradiation, the maximum efficiency achieved for the CdS(4)/Ag2S/TiO2/ITO electrode is 3.46%, corresponding to an increase of about 150% as compared to the CdS(4)/TiO2/ITO electrode without the Ag2S layer. Our experimental results show that the improved efficiency is mainly due to the formation of Ag2S layer that may increase the light absorbance and reduce the recombination of photogenerated electrons with redox ions from the electrolyte.
Dye-sensitized photoelectrodes consisting of a wide band gap semiconductor film and a dye form the basis of many applications in photocatalytic, optoelectronic, and photovoltaic devices [1–10]. In photovoltaic applications, the photoelectrodes are typically titanium dioxide (TiO2) films, which are sensitized by an organic or inorganic dye [7, 9, 11]. In dye-sensitized photoelectrodes, the dye plays an important role in light absorption and charge transfer. Compared with organic dyes, semiconductor nanocrystals (i.e., inorganic dyes) with their size-tunable absorption and high molar extinction coefficient [12, 13] are superior in thermal and photochemical stability. Due to these advantages of semiconductor nanocrystals, theoretically, semiconductor nanocrystal-sensitized solar cells may have a maximum efficiency of 44%, which is much higher than that of organic dye-sensitized solar cells .
So far, various types of inorganic nanocrystals such as CdS [15–17], CdTe [15, 16], CuInS2[18, 19], Ag2S [20–24], and PbS [25, 26] have been incorporated on TiO2 photoelectrodes as sensitizers to enhance the light absorption of the TiO2 photoelectrodes in the visible light region. Among single nanocrystal-sensitized TiO2 photoelectrodes, CdS-sensitized TiO2 photoelectrodes show a better photoelectric conversion performance. The efficiency of over 4% has been reported for CdS-sensitized TiO2 nanotube array photoelectrodes. However, it is still much lower than that of organic dye-sensitized TiO2 photoelectrodes [27–30]. The low efficiency is mostly caused by the serious charge recombination between the electrolyte and photoelectrodes . Thus, to increase the conversion efficiency of the semiconductor nanocrystal-sensitized TiO2 photoelectrodes, considerable efforts have been made to suppress the charge recombination between the electrolyte and electrode. One common method for decreasing the charge recombination is to interpose an intermediate layer, such as a ZnS coating, between the inorganic nanocrystals and the electrolyte. Besides, another effective method, interposing an energy barrier layer between the TiO2 and electrolyte, has been recently reported. For example, a ZnO layer was deposited on the TiO2 photoelectrodes to significantly decrease the charge recombination in CdSe , CdS , and Ag2S  nanocrystal-sensitized TiO2 photoelectrodes. Similarly, a CuInS2 nanocrystal film was formed between the TiO2 photoelectrode and CdS to suppress the charge recombination in CuInS2-sensitized TiO2 photoelectrodes . Among these reported nanocrystals, Ag2S has a narrow band gap of 0.9 to 1.05 eV and a larger absorption coefficient, which makes it an important material for photovoltaic application [33–35]. Furthermore, for these reported nanocrystals, the most commonly employed synthetic methods include solution synthesis [18, 36, 37], chemical bath deposition (CBD) [17, 33], and successive ionic layer absorption and reaction (SILAR) . For example, the Ag2S [20, 33] and CdS [17, 19] nanocrystals are commonly prepared by a CBD method.
Cadmium chloride (CdCl2, 98.0%) was purchased from Kanto Chemical Co. Inc. Titanium tetrachloride (TiCl4, 99.995%), nitric acid (HNO3, 70%), hydrochloric acid (HCl, 37%), ethyl cellulose (CAS 9004-57-3), silver acetate (AgOAc, 99%), thiourea (≥99.0%), terpineol (≥96%), Ti(OCH2CH2CH2CH3)4 (Ti(OBu)4, 97%), 1-butylamine (99.5%), and 1-propionic acid (≥99.5%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). All the reagents were used without further purification. Indium tin oxide coated glass slides (ITO, ≤15 Ω/sq, Wuhu Token Sci. Co., Ltd., Wuhu, China) were cleaned by successive sonication in deionized water, acetone, and isopropyl alcohol and then dried with nitrogen gas.
Formation of TiO2 nanocrystalline film on ITO substrate
First, a TiO2 dense layer was introduced on the cleaned ITO substrate by spin coating a TiO2 sol-gel precursor at 3,000 rpm for 60 s. The procedure for the preparation of TiO2 sol-gel has been reported previously . Briefly, 10 ml Ti(OBu)4 was dissolved in 60 ml ethanol and stirred about 5 min at room temperature. After that, 5 ml acetyl acetone was added and stirring was continued for 15 min. Then, a solution composed of 30 ml ethanol, 10 ml deionized (DI) water, and 2 ml HCl with a density of 0.28 mol/l was added dropwise under vigorous stirring. The final mixture was stirred at room temperature for 2 h to obtain a TiO2 sol-gel precursor. The substrates were annealed at 450°C for 2 h in a muffle furnace.
Secondly, the TiO2 nanocrystalline film was deposited on the prepared TiO2 dense layer. The solution-processed nanocrystalline titanium (TiO2) film was prepared as follows. A total of 2 g of titanium nanoparticles (TiO2 P25, Degussa, Frankfurt, Germany) was initially dissolved in 100 mL HNO3 solution (0.1 mol/L) and stirred for 12 h at 200°C. Afterward, the obtained solution was centrifuged at 7,000 rpm for 3 min to collect the TiO2 nanoparticles. To remove the remaining water and acid, the obtained TiO2 nanoparticles were re-dispersed in DI water and then the mixture was centrifuged at 7,000 rpm for 3 min. This washing step was repeated three times. The final product was dried at room temperature to get dried TiO2 nanoparticles. After that, the TiO2 paste consisting of 11.6% dried TiO2 nanoparticles and 5% ethyl cellulose in terpineol was prepared, which was spin cast on the TiO2 dense layer at 2,000 rpm. Then, the samples were annealed at 500°C for 30 min in a muffle furnace to obtain the TiO2/ITO films.
Synthesis of Ag2S and CdS nanocrystals
Ag2S nanocrystals were synthesized through a method of using a molecular-based precursor solution. First, AgOAc (0.1 mmol) and thiourea (0.2 mmol) were dissolved in a mixture of 1-butylamine (0.7 mL) and 1-propionic acid (45 μL) under a nitrogen atmosphere in a glove box (O2 < 0.1 ppm, H2O <0.1 ppm). The mixture was then stirred for 3 min; after which, the obtained Ag2S precursor solution was then spin cast onto the prepared TiO2/ITO substrates at 1,500 rpm for 30 s. The obtained films were calcined at 150°C for 10 min and then heated to 250°C and held 15 min at this temperature to obtain the Ag2S/TiO2/ITO films. The CdS nanocrystals were synthesized through the same process. Briefly, the prepared CdS precursor solution composed of 0.1 mmol CdCl2 and thiourea (0.3 mmol) was spin cast onto the prepared Ag2S/TiO2/ITO films at 1,500 rpm for 30 s and then the films were calcined. Such a spinning-drying cycle was repeated several times to increase the thickness of the CdS film. The CdS/Ag2S/TiO2/ITO film after n cycles of the CdS deposition was denoted as CdS(n)/Ag2S/TiO2/ITO. The schematic diagram of CdS(n)/Ag2S/TiO2/ITO electrode is shown in Figure 1b. For comparison, the CdS(n)/TiO2/ITO films without Ag2S were also fabricated by the same process.
Characterization and photovoltaic measurements
The structural and optical analyses of the prepared films were studied by X-ray diffractometer (XRD; DX-2500, Dandong Fangyuan Instrument Co., Ltd., Dandong, China) and UV-VIS-NIR spectrophotometer (UV-2550, Shimadzu Corporation, Kyoto, Japan), respectively. The surface morphologies were observed by scanning electron microscopy with energy dispersive X-ray analysis (EDX) (SEM, JSM-7001 F, Japan Electron Optics Laboratory Co., Ltd., Tokyo, Japan). Photoelectrochemical experiments were performed using an electrochemical workstation (CHI660E, Shanghai Chenhua Instruments Co., Ltd., Shanghai, China) using a three-electrode configuration with the as-prepared samples as working electrode, a Pt foil counter electrode, and a saturated Ag/AgCl reference electrode, and devices were illuminated with a calibrated AM 1.5 solar light simulator (Newport Inc., Irvine, CA, USA) operating at an intensity of 100 mW cm−2. The light intensity was calibrated with a monocrystalline Si reference cell. The electrolyte was 1.0 M Na2S aqueous solution. The photocurrent responses of the working electrodes with a surface area of 0.5 cm−2 were recorded during a voltage sweep from −1.4 to 0.2 V.
Results and discussion
Figure 2c,d shows the top-view SEM images of the Ag2S/TiO2/ITO film. Figure 2c shows that, after the deposition of Ag2S, the number of the pinholes in the Ag2S/TiO2/ITO film is significantly reduced and the surface of the Ag2S/TiO2/ITO film became more flat compared to that of the TiO2/ITO film, which might be due to the filling of the Ag2S precursor solution in the low surface regions of the TiO2/ITO film. After the calcinations, more Ag2S nanocrystals that resulted from the Ag2S precursor solution aggregated in the low surface regions of the Ag2S/TiO2/ITO film and therefore improved the flatness of the film surface. This explanation can be supported by the higher magnification SEM image (Figure 2d) of the Ag2S/TiO2/ITO film. Figure 2d shows that the Ag2S nanocrystals appear to be fused together. In particular, the Ag2S nanocrystals in the low surface regions become fused together to form solid blocks.
Figure 2e,f shows the top-view SEM images of the CdS(3)/Ag2S/TiO2/ITO film. As shown in Figure 2e, after introduction of three cycles of CdS deposition, a large amount of CdS nanocrystals are deposited on the surface of the CdS(3)/Ag2S/TiO2/ITO film, and these deposited CdS nanocrystals are further fused together, which causes a large reduction in the number of pinholes. In addition, some lumps appear on the surface of the CdS(3)/Ag2S/TiO2/ITO film. Obviously, these lumps should be CdS that resulted from the residual CdS precursor solution on the surface of the film after the calcinations at 250°C. The corresponding high-magnification SEM image of the CdS(3)/Ag2S/TiO2/ITO film shown in Figure 2f further reveals that the CdS nanocrystals become fused together, which is similar to the case of Ag2S nanocrystals in the Ag2S/TiO2/ITO film.
To improve the efficiencies of CdS-sensitized TiO2/ITO electrodes, the CdS/Ag2S/TiO2/ITO electrodes were prepared by the interposition of Ag2S nanocrystalline film between the CdS and TiO2 nanocrystals as an energy barrier layer and a light absorber, in which both Ag2S and CdS nanocrystals were synthesized through a spin coating method of using a molecular-based precursor solution. The deposited Ag2S nanocrystals not only enhance the light absorption of the CdS-sensitized TiO2/ITO electrodes but also reduce the charge recombination, which resulted in the improved efficiencies of the CdS/Ag2S/TiO2/ITO electrodes. The maximum efficiency of the CdS(4)/Ag2S/TiO2/ITO electrode is 3.46%, which is about 2.5 times that (1.39%) of the CdS(4)/TiO2/ITO electrode without a Ag2S layer. Our research results indicate that the approach may provide a strategy to improve the efficiency of QSSCs.
This work was supported by the Henan University Distinguished Professor Startup Fund, the Natural Science Foundation of Henan University (2013YBZR046), the National Natural Science Foundation of China-Talent Training Fund of Henan (U1404616), and the Seed Fund of Young Scientific Research Talents of Henan University (0000A40540).
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