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.
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