The influence of anatase-rutile mixed phase and ZnO blocking layer on dye-sensitized solar cells based on TiO2nanofiberphotoanodes
© Ding et al.; licensee Springer. 2013
Received: 29 November 2012
Accepted: 27 December 2012
Published: 3 January 2013
High performance is expected in dye-sensitized solar cells (DSSCs) that utilize one-dimensional (1-D) TiO2 nanostructures owing to the effective electron transport. However, due to the low dye adsorption, mainly because of their smooth surfaces, 1-D TiO2 DSSCs show relatively lower efficiencies than nanoparticle-based ones. Herein, we demonstrate a very simple approach using thick TiO2 electrospun nanofiber films as photoanodes to obtain high conversion efficiency. To improve the performance of the DSCCs, anatase-rutile mixed-phase TiO2 nanofibers are achieved by increasing sintering temperature above 500°C, and very thin ZnO films are deposited by atomic layer deposition (ALD) method as blocking layers. With approximately 40-μm-thick mixed-phase (approximately 15.6 wt.% rutile) TiO2 nanofiber as photoanode and 15-nm-thick compact ZnO film as a blocking layer in DSSC, the photoelectric conversion efficiency and short-circuit current are measured as 8.01% and 17.3 mA cm−2, respectively. Intensity-modulated photocurrent spectroscopy and intensity-modulated photovoltage spectroscopy measurements reveal that extremely large electron diffusion length is the key point to support the usage of thick TiO2 nanofibers as photoanodes with very thin ZnO blocking layers to obtain high photocurrents and high conversion efficiencies.
KeywordsDye-sensitized solar cell Titanium dioxide nanofiber photoanode Anatase-rutile mixed phase Zinc oxide blocking layer Atomic layer deposition method
Due to their cost-effectiveness, ease of manufacturing, and suitability for large-area production, dye-sensitized solar cells (DSSCs) have attracted much attention. Typically, the photoanode of a DSSC is made of a TiO2 nanoparticle film (10-μm thickness) adsorbed with a monolayer Ru-based complex dye. Although the certified energy conversion efficiency of DSSCs has exceeded 12%, electrons generated from photoexcited dyes injected into the conduction band of TiO2 will pass through the grain boundaries and interparticle connections, which are strongly influenced by the surface trapping/detrapping effect, leading to slow electron transport. One-dimensional (1-D) nanostructures have superior electron transport characteristics compared to nanoparticle-based systems[3, 4]. Several methods have been established for the preparation of 1-D structured TiO2, including nanowires[5, 6], nanotubes[7–10] and nanofibers. Among the methods for preparing 1-D TiO2 nanostructures, electrospinning provides a versatile, simple, and continuous process[11–13]. However, even though extremely fast electron transport is available in the 1-D nanostructures, these 1-D TiO2-based DSSCs usually show relatively lower efficiencies than nanoparticle-based ones, mainly because of low dye adsorption. To solve this problem of TiO2 electrospun nanofiber DSSCs, some attempts have been done, such as applying mechanical pressure to break the outer sheaths of nanofibers to increase surface area[14, 15], calcination of nanofibers with a hot pressing pre-treatment to obtain multi-core cable-like nanofibers. However, these methods destroy continuous 1-D nanostructures. In view of the excellent electron transport characteristic, which will result in a large diffusion length, it is feasible to increase the thickness of 1-D nanostructure photoanodes to improve dye adsorption and, consequently, to enhance the conversion efficiency of cells. Unfortunately, the lengths of TiO2 nanowires or nanorods are usually several micrometers[5, 6], and it is a very difficult or time-consuming mission to enlarge their length, so the conversion efficiency is limited. Long TiO2 nanotube can be formed by anodization of titanium foils. However, backside-illumination mode of anodized TiO2 nanotube-based solar cells is an obstacle for realizing a high efficiency since the redox electrolyte containing the iodine species has an absorption in near UV spectrum and platinum-coated fluorine-doped SnO2 (FTO) partially and inevitably reflects light[17, 18]. On the contrary, it is very easy within a short period of process to enlarge the thickness of TiO2 electrospun nanofiber photoanode on FTO substrates for front illumination.
On the other hand, superior performance of anatase-rutile mixed-phase TiO2 nanoparticle DSSCs with a small amount of rutile to pure phase ones was claimed[19, 20]. Different from nanoparticles, it is relatively difficult for nanowires or nanotubes to control their crystalline phase, so there are little researches on anatase-rutile mixed-phase 1-D TiO2 DSSCs. Besides, it has been proven effective to block electron recombination by introduction of a compact layer, such as TiO2[21–25], Nb2O5, and ZnO[27, 28] between the FTO and porous TiO2. Nb2O5 is an expensive material for compact film. For ZnO, not only electron transmission is faster than that in TiO2 but also its conduction band edge is a little more negative than that of TiO2, which will introduce an energy barrier at the interface of FTO/TiO2. The energy barrier will be favorable to suppress the back electron transfer from FTO to electrolytes. However, the thickness of the reported ZnO blocking layers deposited by sputtering methods[27, 28] was around 150 nm to get the highest conversion efficiency. Thick blocking layers will reduce transmittance of FTO substrates and consequently decrease the absorption of visible light. Meanwhile, it probably retards the transport of injected electrons from TiO2 conduction band to FTO, resulting in a low photocurrent. Atomic layer deposition (ALD) technique can produce continuous, angstrom-level-controlled, and defect-free films, which is very suitable to deposit ultrathin compact film.
In this paper, to make the best of excellent electron transport characteristic of 1-D nanostructures, thick TiO2 nanofiber films were used as photoanodes to fabricate DSSCs. Meanwhile, anatase-rutile mixed-phase TiO2 nanofibers obtained by increasing sintering temperature and very thin ZnO compact layers deposited by ALD method were first adopted in the TiO2 nanofiber DSSC fabrication to further improve photocurrent and conversion efficiency. Combining the above two steps, a short-circuit current density of 17.3 mAcm−2 and a conversion efficiency of 8.01% were achieved for the DSSC using approximately 40-μm-thick TiO2 nanofiber film as photoanode. Intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) were used to investigate the dynamic response of charge transfer and recombination in TiO2 nanofiber DSSCs.
TiO2 nanofiber synthesis
The polyvinylpyrrolidone (PVP)-TiO2 nanofibers were fabricated using electrospinning technique. Typically, the precursor solution for electrospinning was made from 0.45 g of PVP (with a molecular weight of 1,300,000; Sigma-Aldrich Corporation, St. Louis, MO, USA), 7 ml of ethanol, 2 ml of acetic acid, and 1 g of titanium (IV) isopropoxide (Sigma-Aldrich). In a typical electrospinning procedure, the precursor solution was loaded into a syringe equipped with a 24 gauge silver-coated needle. The needle was connected to a high-voltage power supply. The electric voltage of 16 kV was applied between the metal orifice and the Al collector at a distance of 10 cm. The spinning rate was controlled by the syringe pump at 60 μl min−1. After the electrospinning procedure, the PVP-TiO2 fiber composite films were then heated at a rate of 4°C min−1 up to 500°C, 550°C, 600°C, and 700°C, respectively, and then sintered at this temperature for 2 h to obtain pure TiO2-based nanofibers.
Preparation of ultrathin ZnO blocking layers by ALD method
Before deposition, the reaction chamber was pumped down from 1 to 2 Torr. The operating environment of ZnO deposition was maintained at 3 Torr and 200°C. Each deposition cycle consisted of four steps, which included DEZ reactant, N2 purge, H2O reactant, and N2 purge. The typical pulse time for introducing DEZ and H2O precursors was 0.5 s, and the purge time of N2 was 10 s. The deposition rate of ZnO film at the above conditions approached 0.182 nm/cycle. Thus, the deposition cycles of 22, 55, 83, and 110 were chosen to produce ZnO layers with thicknesses of 4, 10, 15, and 20 nm.
Solar cell fabrication and characterization
First, the substrates were coated with TiO2 nanofiber films using a mixed solution (20 ml ethanol and 2 ml titanium (IV) isopropoxide) for enhancing the adhesion between TiO2 films and substrates, and were subsequently sintered at 500°C for 30 min to ensure good electrical contact between the TiO2 nanofiber films and the FTO substrates. Then, the TiO2 electrodes were immersed into the N-719 dye solution (0.5 mM in ethanol) and were held at room temperature for 24 h. The dye-treated TiO2 electrodes were rinsed with ethanol and dried under nitrogen flow. For the counter electrodes, the FTO plates were drilled and coated with a drop of 10 mM H2PtCl6 (99.99%, Sigma-Aldrich) solution and were then heated at 400°C for 20 min. The liquid electrolyte was prepared by dissolving 0.6 M of 1-butyl-3-methylimidazolium iodide, 0.03 M of iodine, 0.1 M of guanidinium thiocyanate, and 0.5 M of 4-tert-butylpyridine in acetonitrile/valeronitrile (85:15 v/v). Finally, dye-coated TiO2 films and Pt counter electrodes were assembled into sealed sandwich-type cells by heating with hot-melt films used as spacers. The typical active area of the cell was 0.25 cm2.
The crystallographic structure of the nanofiber was analyzed by X-ray diffraction (XRD) (D/MAX Ultima III, Rigaku Corporation, Tokyo, Japan) using Cu Kα radiation. The morphology was determined by scanning electron microscopy (SEM). Specific surface areas of the nanofibers in powder form were measured with a Quantachrome Autosorb-3b static volumetric instrument (Quantachrome Instruments, Boynton Beach, FL, USA). UV-visible (UV–vis) spectra were carried out on a Hitachi U-3010 spectrophotometer (Hitachi, Ltd., Chiyoda, Tokyo, Japan). The thicknesses of the films were obtained using an α-Step 500 surface-profile measurement system (KLA-Tencor Corporation, Milpitas, CA, USA). Photovoltaic characteristics were measured using a Keithley 2400 source meter (Keithley Instruments Inc., Cleveland, OH, USA). A solar simulator (500-W Xe lamp) was employed as the light source, and the light intensity was adjusted with a Si reference solar cell for approximating AM 1.5 global radiation. IMPS and IMVS spectra were measured on a controlled intensity-modulated photospectroscopy (Zahner Co., Kansas City, MO, USA) in ambient conditions under illumination through the FTO glass side, using a blue light-emitting diode as the light source (BLL01, λmax = 470 nm, spectral half-width = 25 nm; Zahner Co.) driven by a frequency response analyzer, and the light intensity (incident photon flux) of the DC component was controlled at 2.5 × 1016 cm−2 s−1. During the IMVS and IMPS measurements, the cell was illuminated with sinusoidally modulated light having a small AC component (10% or less of the DC component).
Results and discussion
Characterization of TiO2 nanofibers
Characterization of ultrathin ZnO layers deposited by ALD method
Performance of DSSCs
The influence of sintering temperature of TiO2 nanofiber photoanodes on the performance of TiO2 nanofiber cells
Photocurrent density-voltage characteristics of TiO 2 nanofiber cells sintered at 500°C, 550°C, and 600°C
The influence of ZnO blocking layer on the performance of TiO2 nanofiber cells
Photocurrent density-voltage characteristics of TiO 2 nanofiber cells
ZnO thickness (nm)
In summary, thick electrospun TiO2 nanofibers sintered at 500°C to 600°C were used as photoanodes to fabricate DSSCs. The remarkable electron diffusion length in TiO2 nanofiber cells is the key point that makes it feasible to use thick photoanode to obtain high photocurrent and high conversion efficiency. Besides, at sintering temperature of 550°C, a small rutile content in the nanofiber (approximately 15.6%) improved conversion efficiency, short-circuit current, and open-circuit voltage of the cell by 10.9%, 7.4%, and 1.35%, respectively. Moreover, it is demonstrated that ultrathin ZnO layer prepared by ALD method could effectively suppress the electron transfer from FTO to electrolytes by IMVS measurements, and its suppression effect of back reaction was stronger than the potential barrier effect of electron transfer from TiO2 to FTO by IMPS measurements. A large ratio of electron diffusion length to photoanode thickness (Ln/d) was obtained in the approximately 40-μm-thick TiO2 nanofiber DSSC with a 15-nm-thick ZnO blocking layer, which is responsible for short-circuit current density of 17.3 mA cm−2 and conversion efficiency of 8.01%. The research provides a potential approach to fabricate high-efficient DSSCs.
atomic layer deposition
dye-sensitized solar cells
intensity-modulated photocurrent spectroscopy
intensity-modulated photovoltage spectroscopy
- J sc :
- J-V :
photoelectric conversion efficiency
scanning electron microscope
- V oc :
- τ d :
- τ n :
This work was supported by the National High Technology Research and Development Program 863 (2011AA050511), Jiangsu ‘333’ Project, and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
- Yella A, Lee HW, Tsao HN, Yi C, Chandiran AK, Nazeeruddin MK, Diau EWG, Yeh CY, Zakeeruddin SM, Grätzel M: Porphyrin-sensitized solar cells with cobalt (II/III)-based redox electrolyte exceed 12% efficiency. Science 2011, 334: 629–634. 10.1126/science.1209688View ArticleGoogle Scholar
- Lagemaat JVD, Park NG, Frank AJ: Influence of electrical potential distribution, charge transport, and recombination on the photopotential and photocurrent conversion efficiency of dye-sensitized nanocrystallineTiO2 solar cells: a study by electrical impedance and optical modulation techniques. J Phys Chem B 2000, 104: 2044–2052. 10.1021/jp993172vView ArticleGoogle Scholar
- Zhu K, Neale NR, Miedaner A, Frank AJ: Enhanced charge-collection efficiencies and light scattering in dye-sensitized solar cells using oriented TiO2 nanotubes arrays. Nano Lett 2007, 7: 69–74. 10.1021/nl062000oView ArticleGoogle Scholar
- Kang SH, Choi SH, Kang MS, Kim JY, Kim HS, Hyeon T, Sung YE: Nanorod-based dye-sensitized solar cells with improved charge collection efficiency. Adv Mater 2008, 20: 54–58. 10.1002/adma.200701819View ArticleGoogle Scholar
- Limmer SJ, Cao GZ: Sol–gel electrophoretic deposition for the growth of oxide nanorods. Adv Mater 2003, 15: 427–431. 10.1002/adma.200390099View ArticleGoogle Scholar
- Miao Z, Xu DS, Ouyang JH, Guo GL, Zhao XS, Tang YQ: Electrochemically induced sol–gel preparation of single-crystalline TiO2 nanowires. Nano Lett 2002, 2: 717–720. 10.1021/nl025541wView ArticleGoogle Scholar
- Kasuga T, Hiramatsu M, Hoson A, Sekino T, Niihara K: Titania nanotubes prepared by chemical processing. Adv Mater 1999, 11: 1307–1311. 10.1002/(SICI)1521-4095(199910)11:15<1307::AID-ADMA1307>3.0.CO;2-HView ArticleGoogle Scholar
- Chen Q, Zhou WZ, Du GH, Peng LM: Trititanate nanotubes made via a single alkali treatment. Adv Mater 2002, 14: 1208–1211. 10.1002/1521-4095(20020903)14:17<1208::AID-ADMA1208>3.0.CO;2-0View ArticleGoogle Scholar
- Zwilling V, Darque-Ceretti E, Boutry-Forveille A, David D, Perrin MY, Aucouturier M: Structure and physicochemistry of anodic oxide films on titanium and TA6V alloy. Surf Interface Anal 1999, 27: 629–637. 10.1002/(SICI)1096-9918(199907)27:7<629::AID-SIA551>3.0.CO;2-0View ArticleGoogle Scholar
- Zhao JL, Wang XH, Sun TY, Li LT: In situ templated synthesis of anatase single-crystal nanotube arrays. Nanotechnology 2005, 16: 2450–2454. 10.1088/0957-4484/16/10/077View ArticleGoogle Scholar
- Krishnamoorthy T, Thavasi V, Subodh GM, Ramakrishna S: A first report on the fabrication of vertically aligned anatase TiO2 nanowires by electrospinning: preferred architecture for nanostructured solar cells. Energ Environ Sci 2011, 4: 2807–2812. 10.1039/c1ee01315gView ArticleGoogle Scholar
- Lee BH, Song MY, Jang SY, Jo SM, Kwak SY, Kim DY: Charge transport characteristics of high efficiency dye-sensitized solar cells based on electrospun TiO2 nanorod photoelectrodes. J Phys Chem C 2009, 113: 21453–21457. 10.1021/jp907855xView ArticleGoogle Scholar
- Dong ZX, Kennedy SJ, Wu YQ: Electrospinning materials for energy-related applications and devices. J Power Sources 2011, 196: 4886–4904. 10.1016/j.jpowsour.2011.01.090View ArticleGoogle Scholar
- Song MY, Ahn YR, Jo SM, Kim DY, Ahn JP: TiO2 single-crystalline nanorod electrode for quasi-solid-state dye-sensitized solar cells. Appl Phys Lett 2005, 87: 113113. 10.1063/1.2048816View ArticleGoogle Scholar
- Kim ID, Rothschild A, Lee BH, Kim DY, Jo SM, Tuller HL: Ultrasensitive chemiresistors based on electrospun TiO2 nanofibers. Nano Lett 2006, 6: 2009–2013. 10.1021/nl061197hView ArticleGoogle Scholar
- Kokubo H, Ding B, Naka T, Tsuchihira H, Shiratori S: Multi-core cable-like TiO2 nanofibrous membranes for dye-sensitized solar cells. Nanotechnology 2007, 18: 165604–6. 10.1088/0957-4484/18/16/165604View ArticleGoogle Scholar
- Mohamed AE, Rohani S: Modified TiO2 nanotube arrays (TNTAs): progressive strategies towards visible light responsive photoanode, a review. Energ Environ Sci 2011, 4: 1065–1086. 10.1039/c0ee00488jView ArticleGoogle Scholar
- Shankar K, Mor GK, Prakasam HE, Yoriya S, Paulose M, Varghese OK, Grimes CA: Highly-ordered TiO2 nanotube arrays up to 220 μm in length: use in water photoelectrolysis and dye-sensitized solar cells. Nanotechnology 2007, 18: 1–11.View ArticleGoogle Scholar
- Li GH, Richter CP, Milot RL, Cai L, Schmuttenmaer CA, Crabtree RH, Brudvig GW, Batista VS: Synergistic effect between anatase and rutile TiO2 nanoparticles in dye-sensitized solar cells. Dalton Trans 2009, 45: 10078–10085.View ArticleGoogle Scholar
- Yun TK, Park SS, Kim D, Shim JH, Bae JY, Huh S, Won YS: Effect of the rutile content on the photovoltaic performance of the dye-sensitized solar cells composed of mixed-phase TiO2 photoelectrodes. Dalton Trans 2012, 41: 1284–1288. 10.1039/c1dt11765cView ArticleGoogle Scholar
- Cameron PJ, Peter LM: Characterization of titanium dioxide blocking layers in dye-sensitized nanocrystalline solar cells. J Phys Chem B 2003, 107: 14394–14400. 10.1021/jp030790+View ArticleGoogle Scholar
- Yu H, Zhang SQ, Zhao HJ, Will G, Liu PR: An efficient and low-cost TiO2 compact layer for performance improvement of dye-sensitized solar cells. Electrochim Acta 2009, 54: 1319–1324. 10.1016/j.electacta.2008.09.025View ArticleGoogle Scholar
- Hattori R, Goto H: Carrier leakage blocking effect of high temperature sputtered TiO2 film on dye-sensitized mesoporous photoelectrode. Thin Solid Films 2007, 515: 8045–8049. 10.1016/j.tsf.2007.03.079View ArticleGoogle Scholar
- Ahn KS, Kang MS, Lee JW, Kang YS: Effects of a surfactant-templated nanoporous TiO2 interlayer on dye-sensitized solar cells. J ApplPhys 2007, 101: 084312.View ArticleGoogle Scholar
- Peng B, Jungmann G, Jager C, Haarer D, Schmidt HW, Thelakkat M: Systematic investigation of the role of compact TiO2 layer in solid state dye-sensitized TiO2 solar cells. Coordin Chem Rev 2004, 248: 1479–1489. 10.1016/j.ccr.2004.02.008View ArticleGoogle Scholar
- Xia J, Masaki N, Jiang K, Yanagida S: Sputtered Nb2O5 as a novel blocking layer at conducting glass/TiO2 interfaces in dye-sensitized ionic liquid solar cells. J PhysChem C 2007, 111: 8092–8097.Google Scholar
- Perez-Hernandez G, Vega-Poot A, Perez-Juarez I, Camacho JM, Ares O, Rejon V, Pena JL, Oskam G: Effect of a compact ZnO interlayer on the performance of ZnO-based dye-sensitized solar cells. Sol Energ Mat Sol C 2012, 100: 21–26.View ArticleGoogle Scholar
- Liu YM, Sun XH, Tai QD, Hu H, Chen BL, Huang N, Sebo B, Zhao XZ: Influences on photovoltage performance by interfacial modification of FTO/mesoporous TiO2 using ZnO and TiO2 as the compact film. J Alloy Compd 2011, 509: 9264–9270. 10.1016/j.jallcom.2011.07.018View ArticleGoogle Scholar
- Zhang HZ, Banfield JF: Understanding polymorphic phase transformation behavior during growth of nanocrystalline aggregates: insights from TiO2. J Phys Chem B 2000, 104: 3481–3487.View ArticleGoogle Scholar
- Kruger J, Plass R, Gratzel M, Cameron PJ, Peter LM: Charge transport and back reaction in solid-state dye-sensitized solar cells: a study using intensity-modulated photovoltage and photocurrent spectroscopy. J Phys Chem B 2003, 107: 7536–7539. 10.1021/jp0348777View ArticleGoogle Scholar
- Bandic ZZ, Bridger PM, Piquette EC, McGill TC: Electron diffusion length and lifetime in p-type GaN. Appl Phys Lett 1998, 73: 3276. 10.1063/1.122743View ArticleGoogle Scholar
- Wang M, Chen P, Humphry-Baker R, Zakeeruddin SM, Gratzel M: The influence of charge transport and recombination on the performance of dye-sensitized solar cells. Chemphyschem 2009, 10: 290–299. 10.1002/cphc.200800708View ArticleGoogle Scholar
- Gregg BA, Hanna MC: Comparing organic to inorganic photovoltaic cells: theory, experiment, and simulation. J Appl Phys 2003, 93: 3605–3614. 10.1063/1.1544413View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.