Improved conversion efficiency of Ag2S quantum dot-sensitized solar cells based on TiO2 nanotubes with a ZnO recombination barrier layer
© Chen et al; licensee Springer. 2011
Received: 5 April 2011
Accepted: 21 July 2011
Published: 21 July 2011
We improve the conversion efficiency of Ag2S quantum dot (QD)-sensitized TiO2 nanotube-array electrodes by chemically depositing ZnO recombination barrier layer on plain TiO2 nanotube-array electrodes. The optical properties, structural properties, compositional analysis, and photoelectrochemistry properties of prepared electrodes have been investigated. It is found that for the prepared electrodes, with increasing the cycles of Ag2S deposition, the photocurrent density and the conversion efficiency increase. In addition, as compared to the Ag2S QD-sensitized TiO2 nanotube-array electrode without the ZnO layers, the conversion efficiency of the electrode with the ZnO layers increases significantly due to the formation of efficient recombination layer between the TiO2 nanotube array and electrolyte.
Keywordsquantum dots TiO2 nanotube Ag2S solar cells
In recent years, dye-sensitized solar cells (DSSCs) have attracted much attention as a promising alternative to conventional p-n junction photovoltaic devices because of their low cost and ease of production [1–4]. A high power conversion efficiency of 11.3% was achieved . The conventional DSSCs consist of dye-sensitized nanocrystalline TiO2 film as working electrode, electrolyte, and opposite electrode. In DSSCs, the organic dyes act as light absorbers and usually have a strong absorption band in the visible. Various organic dyes such as N719 and black dye have been applied for improving the efficiency, light absorption coverage, stability, and reducing the cost. However, the organic dyes have a weak absorbance at shorter wavelengths. Materials that have high absorption coefficients over the whole spectral region from NIR to UV are needed for high power conversion efficiency. During the last few years, instead of organic dyes, the narrow band gap semiconductor quantum dots (QDs) such as CdS [6, 7], CdSe [7–9], PbS [10, 11], InAs , and InP  have been used as sensitizers. The unique characteristics of QDs over the organic dyes are their stronger photoresponse in the visible region, tunable optical properties, and band gaps simply by controlling the sizes. The QD-sensitized solar cells (QDSSCs) have been considered the next-generation sensitizers . In either DSSCs or QDSSCs, the nanoparticle porous film electrode plays a key role in the improvement of power conversion efficiency. Recently, to improve the properties of TiO2 film electrode, one-dimensional nanostructure arrays as working electrodes, including nanowires and nanotubes, have been proposed and studied. Compared with the nanoparticle porous films, aligned one-dimensional nanostructure arrays can provide a direct pathway for charge transport and superior optical absorption properties. Therefore, more and more studies focus on QDSSCs based on one-dimensional nanomaterials, such as the TiO2 nanotubes (TNTs) [15–17].
Among QDs, Ag2S is an important material for photocatalysis [18–20] and electronic devices [21–24]. Ag2S has a large absorption coefficient and a direct band gap of 0.9 to 1.05 eV, which makes Ag2S an effective semiconductor material for photovoltaic application. In the past several years, although there are some reports on the photovoltaic application of Ag2S [10, 25], few studies on Ag2S QDSSCs based on TNTs are reported. In this work, we report on the synthesis of Ag2S QD-sensitized TNT photoelectrode combining the excellent charge transport property of the TNTs and absorption property of Ag2S. Besides, to improve the efficiency of as-prepared photoelectrodes, we interpose a ZnO recombination barrier layer between TNTs and Ag2S QDs to reduce the charge recombination in Ag2S QDSSCs because the ZnO layer can block the recombination of photoinjected electrons with redox ions from the electrolyte. Recently, we have reported the improved conversion efficiency of CdS QD-sensitized TiO2 nanotube array using ZnO energy barrier layer . Similar method has been used by Lee et al. to enhance the efficiency of CdSe QDSSCs by interposing a ZnO layer between CdSe QDs and TNT . Our results show that Ag2S QD-sensitized TiO2 nanotube-array photoelectrodes were successfully achieved. The more important thing is that the conversion efficiency of the Ag2S-sensitized TNTs is significantly enhanced due to the formation of ZnO on the TNTs.
Titanium foil (99.6% purity, 0.1 mm thick) was purchased from Goodfellow (Huntingdon, England). Silver nitrate (AgNO3, 99.5%) and glycerol were from Junsei Chemical Co. (Tokyo, Japan). Ammonium fluoride (NH4F), sodium sulfide nonahydrate (Na2S, 98.0%), and zinc chloride (ZnCl2, 99.995+%) were available from Sigma-Aldrich (St. Louis, MO, USA).
Synthesis of TNTs
Vertically oriented TNTs were fabricated by anodic oxidation of Ti foil, which is similar to that described by Paulose et al. . Briefly, the Ti foils were first treated with acetone, isopropanol, methanol, and ethanol, followed by distilled (DI) water and finally drying in a N2 stream. Then, the dried Ti foils were immersed in high-purity glycerol (90.0 wt.%) solution with 0.5 wt.% of NH4F and 9.5 wt.% DI water and anodic oxidized at 60 V in a two-electrode configuration with a cathode of flag-shaped platinum (Pt) foil at 20°C for 25 h. After oxidation, the samples were washed in DI water to remove precipitation atop the nanotube film and dried in a N2 stream. The obtained titania nanotube film was annealed at 450°C in an air environment for 2 h.
Synthesis of Ag2S-sensitized plain TNT and ZnO/TNT electrodes
The surface morphology of the as-prepared electrodes was monitored using a scanning electron microscope (SEM) (Nova230, FEI Company, Eindhoven, Netherland). The mapping and crystal distribution of the samples were done using a scanning transmission electron microscope (TEM) (Tecnai G2 F30, FEI Company Eindhoven, Netherland) to which an Oxford Instruments (Abingdon, Oxfordshire, UK) energy dispersive X-ray spectroscopy (EDS) detector was coupled. The surface compositions of the samples were analyzed using EDS. The crystalline phase and structure were confirmed by using X-ray diffraction (XRD) (Rigaku D/MAX 2500 V diffractor; Rigaku Corporation, Tokyo, Japan). The UV-visible (UV-vis) absorbance spectroscopy was obtained from a S-4100 spectrometer with a SA-13.1 diffuse reflector (Scinco Co., Ltd, Seoul, South Korea).
The photoelectrochemical measurements were performed in a 300-mL rectangular quartz cell using a three-electrode configuration with a Pt foil counter electrode and a saturated SCE reference electrode, and the electrolyte was 1.0 M Na2S. The working electrode, including the TNTs, ZnO/TNTs, Ag2S(n)/TNTs, and Ag2S(n)/ZnO/TNTs (n = 2, 4, and 8), with a surface area of 0.5 cm2 was illuminated under UV-vis light (I = 100 mW cm-2) with a simulated solar light during a voltage sweep from -1.4 to 0 V. The simulated solar light was produced by a solar simulator equipped with a 150-W Xe lamp. The light intensity was measured with a digital power meter.
Results and discussion
Morphology of the TNTs
Characterization of the Ag2S QD-sensitized ZnO/TNT (and TNTs) electrodes
Figure 2a shows the surface SEM image of the Ag2S(4)/TNT film. It can be clearly seen from Figure 2b that Ag2S is deposited as spherical nanoparticles on the TNTs and the wall thickness of the Ag2S(4)/TNTs is similar to that of the plain TNTs. In addition, a uniform distribution of the Ag2S nanoparticles with diameters of approximately 10 nm is also observed.
For a comparison, the surface SEM image of the ZnO/TNTs covered by Ag2S after four CBD cycles (i.e., the Ag2S/ZnO/TNT electrode) is shown in Figure 2c. It is found that after the formation of the ZnO thin layer on the TNTs, the diameter and distribution of Ag2S nanoparticles did not change much. However, the diameter of the ZnO-coated TNTs increased slightly compared to that of the plain TNTs shown in Figure 2b. These results are similar to previous reports [26, 27].
To determine the composition of the nanoparticles, the corresponding energy dispersive x-ray (EDX) spectrum of the Ag2S(4)/ZnO/TNTs was carried out in the HR-TEM as seen in Figure 3c. The characteristics peaks in the spectrum are associated with Ag, Ti, O, Zn, and S. The quantitative analysis reveals the atomic ratio of Ag and S is close to 2:1, indicating the deposited materials are possible Ag2S.
In order to determine the structure of the Ag2S(4)/ZnO/TNTs, the crystalline phases of the Ag2S(4)/ZnO/TNTs and the corresponding TNTs were characterized by XRD, as shown in Figure 3d. The XRD pattern shows peaks corresponding to TiO2 (anatase), ZnO (hexagon), and Ag2S (acanthite). The observed peaks indicate high crystallinities in the TNTs, ZnO, and Ag2S nanoparticles, consistent with the SEM results shown in Figure 2. The results further confirm that the obtained films are composed of TiO2, ZnO, and Ag2S.
Optical and photoelectrochemistry properties of Ag2S QD-sensitized TNT electrodes in the presence of ZnO layers
In conclusion, Ag2S quantum dot-sensitized TiO2 nanotube array photoelectrodes were successfully achieved using a simple sequential chemical bath deposition (CBD) method. In order to improve the efficiencies of as-prepared Ag2S quantum dot-sensitized solar cells, the Ag2S quantum dot-sensitized ZnO/TNT electrodes were prepared by the interposition of a ZnO energy barrier between the TNTs and Ag2S quantum dots. The ZnO thin layers were formed using wet-chemical process. The formed ZnO energy barrier layers over TNTs significantly increase the power conversion efficiencies of the Ag2S(n)/ZnO/TNTs due to a reduced recombination.
This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea Ministry of Education, Science and Technology (MEST) (no. 2010-0026150).
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