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.
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).
- Grätzel M: Dye-sensitized solid-state heterojunction solar cells. MRS Bull 2005, 30: 23–27. 10.1557/mrs2005.4View ArticleGoogle Scholar
- Wei D: Dye sensitized solar cells. Int J Mol Sci 2010, 11: 1103–1113. 10.3390/ijms11031103View ArticleGoogle Scholar
- Fan SH, Wang KZ: Recent advances on molecular design of ruthenium (II) sensitizers in dye-sensitized solar cells. Chinese J Inorg Chem 2008, 24: 1206–1212.Google Scholar
- Grätzel M: Dye-sensitized solar cells. J Photoch Photobio C 2003, 4: 145–153. 10.1016/S1389-5567(03)00026-1View ArticleGoogle Scholar
- Gao F, Wang Y, Shi D, Zhang J, Wang MK, Jing XY, Humphry-Baker R, Wang P, Zakeeruddin SM, Grätzel M: Enhance the optical absorptivity of nanocrystalline TiO 2 film with high molar extinction coefficient ruthenium sensitizers for high performance dye-sensitized solar cells. J Am Chem Soc 2008, 130: 10720–10728. 10.1021/ja801942jView ArticleGoogle Scholar
- Vogel R, Pohl K, Weller H: Sensitization of highly porous, polycrystalline TiO 2 electrodes by quantum sized CdS. Chem Phys Lett 1990, 174: 241–246. 10.1016/0009-2614(90)85339-EView ArticleGoogle Scholar
- Niitsoo O, Sarkar SK, Pejoux C, Ruhle S, Cahen D, Hodes G: Chemical bath deposited CdS/CdSe-sensitized porous TiO 2 solar cells. J Photochem Photobiol A: Chem 2006, 181: 306–313. 10.1016/j.jphotochem.2005.12.012View ArticleGoogle Scholar
- Diguna LJ, Shen Q, Kobayashi J, Toyoda T: High efficiency of CdSe quantum-dot-sensitized TiO 2 inverse opal solar cells. Appl Phys Lett 2007, 91: 023116. 10.1063/1.2757130View ArticleGoogle Scholar
- Lόpez-Luke T, Wolcott A, Xu LP, Chen SW, Wcn ZH, Li JH, De La Rosa E, Zhang JZ: Nitrogen-doped and CdSe quantum-dot-sensitized nanocrystalline TiO 2 films for solar energy conversion applications. J Phys Chem C 2008, 112: 1282–1292. 10.1021/jp077345pView ArticleGoogle Scholar
- Vogel R, Hoyer P, Weller H: Quantum-sized PbS, CdS, Ag2S, Sb2S3, and Bi2S3 particles as sensitizers for various nanoporous wide-bandgap semiconductors. J Phys Chem 1994, 98: 3183–3188. 10.1021/j100063a022View ArticleGoogle Scholar
- Lee H, Leventis HC, Moon SJ, Chen P, Ito S, Haque SA, Torres T, Nuesch F, Geiger T, Zakeeruddin SM, Grätzel M, Nazeeruddin MK: PbS and CdS quantum dot-sensitized solid-state solar cells: "Old Concepts, New Results". Adv Funct Mater 2009, 19: 2735–2742. 10.1002/adfm.200900081View ArticleGoogle Scholar
- Yu PR, Zhu K, Norman AG, Ferrere S, Frank AJ, Nozik AJ: Nanocrystalline TiO 2 solar cells sensitized with InAs quantum dots. J Phys Chem B 2006, 110: 25451–25454. 10.1021/jp064817bView ArticleGoogle Scholar
- Zaban A, Micic OI, Gregg BA, Nozik AJ: Photosensitization of nanoporous TiO 2 electrodes with InP quantum dots. Langmuir 1998, 14: 3153–3156. 10.1021/la9713863View ArticleGoogle Scholar
- Nozik AJ: Quantum dot solar cells. Physica E: Low-Dimensional Systems & Nanostructures 2002, 14: 115–120. 10.1016/S1386-9477(02)00374-0View ArticleGoogle Scholar
- Roy P, Kim D, Lee K, Spiecker E, Schmuki P: TiO 2 nanotubes and their application in dye-sensitized solar cells. Nanoscale 2010, 2: 45–59. 10.1039/b9nr00131jView ArticleGoogle Scholar
- Xu CK, Shin PH, Cao LL, Wu JM, Gao D: Ordered TiO 2 nanotube arrays on transparent conductive oxide for dye-sensitized solar cells. Chem Mater 2010, 22: 143–148. 10.1021/cm9027513View ArticleGoogle Scholar
- Uchida S, Chiba R, Tomiha M, Masaki N, Shirai M: Application of titania nanotubes to a dye-sensitized solar cells. Electrochemistry 2002, 70: 418–420.Google Scholar
- Xie Y, Heo SH, Kim YN, Yoo SH, Cho SO: Synthesis and visible-light-induced catalytic activity of Ag 2 S-coupled TiO 2 nanoparticles and nanowires. Nanotechnology 2010, 21: 015703. 10.1088/0957-4484/21/1/015703View ArticleGoogle Scholar
- Neves MC, Nogueira JMF, Trindade T, Mendonca MH, Pereira MI, Monteiro OC: Organic dyes with a novel anchoring group for dye-sensitized solar cell applications. J Photochem Photobiol A 2009, 204: 168–173. 10.1016/j.jphotochem.2009.03.014View ArticleGoogle Scholar
- Kryukov AI, Stroyuk AL, Zińchuk NN, Korzhak AV, Kuchmii SY: Optical and catalytic properties of Ag 2 S nanoparticles. J Mol Catal A Chem 2004, 221: 209–221. 10.1016/j.molcata.2004.07.009View ArticleGoogle Scholar
- Morales-Masis M, van der Molen SJ, Fu WT, Hesselberth MB, van Ruitenbeek JM: Conductance switching in Ag 2 S devices fabricated by in situ sulfurization. Nanotechnology 2009, 20: 095710. 10.1088/0957-4484/20/9/095710View ArticleGoogle Scholar
- Reid M, Punch J, Ryan C, Franey J, Derkits GE, Reents WD, Garfias LF: The corrosion of electronic resistors. IEEE Tran Components and Packaging Technologies 2007, 30: 666–672.View ArticleGoogle Scholar
- Wang HL, Qi LM: Controlled synthesis of Ag 2 S, Ag 2 Se, and Ag nanofibers by using a general sacrificial template and their application in electronic device fabrication. Adv Funct Mater 2008, 18: 1249–1256. 10.1002/adfm.200700953View ArticleGoogle Scholar
- Kitova S, Eneva J, Panov A, Haefke H: Infrared photography based on vapor-deposited silver sulfide thin films. J Imaging Sci Technol 1994, 38: 484–488.Google Scholar
- Tubtimtae A, Wu K, Tung H, Lee M, Wang GJ: Ag 2 S quantum dot-sensitized solar cells. Electrochem Commun 2010, 12: 1158–1160. 10.1016/j.elecom.2010.06.006View ArticleGoogle Scholar
- Chen C, Xie Y, Ali G, Yoo SH, Cho SO: Improved conversion efficiency of CdS quantum dots-sensitized TiO 2 nanotube array using ZnO energy barrier layer. Nanotechnology 2011, 22: 015202. 10.1088/0957-4484/22/1/015202View ArticleGoogle Scholar
- Lee W, Kang SH, Kim JY, Kolekar GB, Sung YE, Han SH: TiO 2 nanotubes with a ZnO thin energy barrier for improved current efficiency of CdSe quantum-dot-sensitized solar cells. Nanotechnology 2009, 20: 335706. 10.1088/0957-4484/20/33/335706View ArticleGoogle Scholar
- Paulose M, Shankar K, Yoriya S, Prakasam HE, Varghese OK, Mor GK, Latempa TA, Fitzgerald A, Grimes CA: Anodic growth of highly ordered TiO 2 nanotube arrays to 134 μm in length. J Phys Chem B 2006, 110: 16179–16184. 10.1021/jp064020kView ArticleGoogle Scholar
- Roh SJ, Mane RS, Min SK, Lee WJ, Lokhande CD, Han SH: Achievement of 4.51% conversion efficiency using ZnO recombination barrier layer in TiO 2 based dye-sensitized solar cells. Appl Phys Lett 2006, 89: 253512. 10.1063/1.2410240View ArticleGoogle Scholar
- Sun WT, Yu Y, Pan HY, Gao XF, Chen Q, Peng LM: CdS quantum dots sensitized TiO 2 nanotube-array photoelectrodes. J Am Chem Soc 2008, 130: 1124–1125. 10.1021/ja0777741View ArticleGoogle Scholar
- Chi CF, Lee YL, Weng HS: A CdS-modified TiO 2 nanocrystalline photoanode for efficient hydrogen generation by visible light. Nanotechnology 2008, 19: 125704. 10.1088/0957-4484/19/12/125704View ArticleGoogle Scholar
- Gerischer H: Charge transfer processes at semiconductor-electrolyte interfaces in connection with problems of catalysis. Surf Sci 1969, 18: 97–122. 10.1016/0039-6028(69)90269-6View ArticleGoogle Scholar
- Marcus RA: On the theory of oxidation-reduction reactions involving electron transfer. J Chem Phys 1956, 24: 966–978. 10.1063/1.1742723View ArticleGoogle Scholar
- Marcus RA: Chemical and electrochemical electron-transfer theory. Ann Rev Phys Chem 1964, 15: 155–196. 10.1146/annurev.pc.15.100164.001103View ArticleGoogle Scholar
- Feng Y, Badaeva E, Gamelin DR, Li XS: Excited-state double exchange in manganese-doped ZnO quantum dots: a time-dependent density-functional study. J Phys Chem Lett 2010, 1: 1927–1931. 10.1021/jz100402qView ArticleGoogle Scholar
- Lei Y, Liu H, Xiao W: First principles study of the size effect of TiO 2 anatase nanoparticles in dye-sensitized solar cell. Modelling Simul Mater Sci Eng 2010, 18: 025004. 10.1088/0965-0393/18/2/025004View ArticleGoogle Scholar
- Abayev H, Zaban A, Kytin VG, Danilin AA, Garcia-Belmonte G, Bisquert J: Properties of the electronic density of states in TiO 2 nanoparticles surrounded with aqueous electrolyte. J Solid State Electronchem 2007, 11: 647–653. 10.1007/s10008-006-0220-1View ArticleGoogle Scholar
- Sun S, Xia D: An abinitio calculation study on the super ionic conductors α-AgI and Ag 2 X (X = S, Se) with BCC structure. Solid State Ionics 2008, 179: 2330–2334. 10.1016/j.ssi.2008.09.028View ArticleGoogle Scholar
- Khan SUM, Shahry MA, Ingler WBJ: Efficient photochemical water splitting by a chemically modified n-TiO 2 . Science 2002, 297: 2243–2245. 10.1126/science.1075035View ArticleGoogle Scholar
- Xu Y, Schoonen MMA: The absolute energy positions of conduction and valence bands of selected semiconducting minerals. American Mineralogist 2000, 85: 543–556.Google Scholar
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