The influence of anatase-rutile mixed phase and ZnO blocking layer on dye-sensitized solar cells based on TiO2nanofiberphotoanodes

1556-276X-8-9 1556-276X Nano Express The influence of anatase-rutile mixed phase and ZnO blocking layer on dye-sensitized solar cells based on TiO2nanofiberphotoanodes DingJianningdingjn@cczu.edu.cn LiYanliyanlydialee@163.com HuHongweihwhu1989@163.com BaiLim13616129348@163.com ZhangShuaishuaizhang@cczu.edu.cn YuanNingyinyyuan@cczu.edu.cn

Center for Low-dimensional Materials, Micro-nano Devices and System, Changzhou University, Changzhou 213164, China

Jiangsu Key Laboratory for Solar Cell Materials and Technology, Changzhou 213164, China

Nanoscale Research Letters

Regular submissions

1556-276X 2013 8 1 9 http://www.nanoscalereslett.com/content/8/1/910.1186/1556-276X-8-923286741 2911201227122012312013 2013Ding et al.; licensee Springer.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. Dye-sensitized solar cell Titanium dioxide nanofiber photoanode Anatase-rutile mixed phase Zinc oxide blocking layer Atomic layer deposition method

Abstract

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.

Background

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% 1, 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 2. 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 8 9 10 and nanofibers. Among the methods for preparing 1-D TiO₂ nanostructures, electrospinning provides a versatile, simple, and continuous process 11 12 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 16. 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 17. 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 22 23 24 25, Nb₂O₅ 26, 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 28. 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.

Methods

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

ZnO layers were deposited on FTO-coated glass substrates (25 Ω/sq) by ALD method. FTO glass plates were first cleaned in a detergent solution using an ultrasonic bath for 15 min and were then rinsed with water and ethanol. Diethylzinc (DEZ; Zn(C₂H₅)₂) and deionized water were used as precursors for ZnO deposition on the cleaned FTO plates. Pure N₂ gas (99.999%) was used to carry and purge gas. The reaction was carried out as follows:

Zn C 2 H 5 2 + H 2 O → Zn O + 2 C 2 H 6

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 H₂PtC_l6 (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, λ_max = 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

The surface morphologies of as-spun TiO₂-PVP composite and sintered TiO₂ nanofibers were characterized by SEM as shown in Figure 1. It is found that the network structure of the former is maintained after calcinations in air to remove PVP, forming a porous TiO₂ membrane. The higher sintering temperature, the thinner is TiO₂ fiber. The average fiber diameter of the composite nanofibers is 290 ± 90 nm which decreases to 210 ± 60 nm, 180 ± 70 nm, and 140 ± 80 nm after sintering at 500°C, 550°C, and 600°C, respectively. It is known that crystalline grains of anatase TiO₂ are spherical, while rutile ones are of rod structure. With the increase of the sintering temperature, some anatase TiO₂ grains will transform to rutile ones, which may result in the thinning of the fibers. Moreover, transformation of anatase TiO₂ grains to rutile ones will introduce stress in the fibers, which will cause the fibers to become brittle and even fracture. The insets in Figure 1b, c, d are high-magnification photos of nanofibers, which indicate that the surfaces of TiO₂ nanofibers sintered at 500°C and 550°C are rather smooth, while become a little rough when sintering temperature increases to 600°C. Figure 2 shows the XRD patterns of TiO₂ nanofibers. All the peaks of the TiO₂ nanofibers sintered at 500°C are indexed for anatase TiO₂ with dominant (101) peaks. The mean grain size determined from the XRD pattern using the Scherrer formula is around 16 nm. The nanofibers sintered at 550°C, 600°C, and 700°C are observed to contain both anatase and rutile phases. The phase composition can be determined from XRD results according to the following equation 29:

W R = A R / 0.884 × A A + A R

where W_R, A_A, and A_R represent rutile weight percentage, integrated intensity of anatase (101) peak, and rutile (110) peak, respectively 29. The calculated rutile contents in the above three mixed-phase nanofiber samples are approximately 15.6, 87.8, and 90.5 wt.%, and the mean grain sizes are 22, 30, and 42 nm, respectively. The XRD results indicate that with the increase of sintering temperature, the grain size is gradually increased; however, rutile content is sharply increased in the temperature range of 550°C to 600°C.

Figure 1

SEM images of electrospun nanofibers

SEM images of electrospun nanofibers. As-spun TiO₂-PVP nanofibers (a), TiO₂ nanofibers after calcination at 500°C (b), 550°C (c), and 600°C (d). The insets in b, c, and d are high-magnification photos of single nanofibers.

Figure 2

XRD patterns of TiO₂ nanofibers sintered at 500 °C, 550 °C, 600 °C, and 700 °C

XRD patterns of TiO₂nanofibers sintered at 500°C, 550°C, 600°C, and 700°C. The diffractions of anatase and rutile phase are labeled in the figure as ‘A’ and ‘R’, respectively.

Characterization of ultrathin ZnO layers deposited by ALD method

To detect the crystallographic structure and thickness of ZnO layers, except FTO substrates, glass substrates were also used to deposit ZnO layers. XRD patterns for ZnO layers deposited on glass substrate are shown in Figure 3a. A 4-nm-thick ZnO layer does not show any diffraction peak, whereas peaks corresponding to hexagonal phase ZnO are observed for the thickness of 10 or 20 nm, which indicates that the deposited ZnO layers by ALD method are polycrystalline. Figure 3b shows the UV–vis transmission spectra for the FTO substrates without ZnO layers and with ZnO layers of different thicknesses. The absorbance edges for the bare FTO and ZnO are around 315 and 380 nm, respectively, from which the bandgap of ZnO is calculated to be 3.2 to 3.3 eV. In the visible range, the transmittance of the FTO covered with ZnO decreases slightly with the increase of ZnO film thickness. For instance, it decreases to approximately 95% of the transmittance of the bare FTO for 20-nm-thick ZnO. Therefore, the presence of ZnO layer with the thickness less than 20 nm will not obviously influence the harvest of light. The inset in Figure 3b is the SEM photo of FTO substrate covered with 15-nm-thick ZnO film, which shows that the ZnO film deposited by ALD method keeps the surface morphology of FTO substrate very well.

Figure 3

XRD patterns

XRD patterns. ZnO layers deposited on glass substrate (a) and UV–vis transmission spectra for the FTO substrate without and with ZnO layers (b). The inset in b is the SEM photo of FTO substrate covered with 15-nm-thick ZnO film.

Performance of DSSCs

The influence of sintering temperature of TiO₂ nanofiber photoanodes on the performance of TiO₂ nanofiber cells

Cells I to III are TiO₂ nanofiber cells (sintered at 500°C, 550°C, and 600°C) on the bare FTO substrates. Based on the above photocurrent-voltage(JV) measurement results, a thickness of approximately 40 μm was set to fabricate cells I to III. Figure 4 illustrates the JV characteristics of TiO₂ nanofiber cells under AM 1.5 irradiation of 100 mW cm⁻². The photovoltaic properties such as short-circuit current density (J_sc), open-circuit voltage (V_oc), fill factor (FF), and photoelectric conversion efficiency (PCE) of the cells are listed in Table 1. Cell I has a J_sc of 15.1 mA cm⁻², PCE of 6.39%, V_oc of 0.814 V, and fill factor (FF) of 0.52. When sintering temperature increased from 500°C to 550°C, cell II gave an improvement of J_sc and V_oc about 1.2 mA cm⁻² and 11 mV, respectively, resulting in an efficiency of 7.12%. However, the further increase of sintering temperature decreased J_sc, V_oc, and PCE of cell III to 14.1 mA cm⁻², 0.818 V, and 6.11%, respectively. According to the XRD data, rutile contents of TiO₂ nanofibers are approximately 0, 15.6, and 87.8 wt.% in cells I, II, and III, respectively. The JV measurement results demonstrate that the anatase-rutile mixed-phase TiO₂ nanofiber with a low rutile content is good for enhancing efficiencies of the DSSCs, whereas a high rutile content is detrimental to the efficiencies, which is similar to the reported DSSCs based on mixed-phase TiO₂ nanoparticles 19 20.

Figure 4

Photocurrent-voltage characteristics of cells I to III under AM 1.5 irradiation of 100 mW cm⁻²

Photocurrent-voltage characteristics of cells I to III under AM 1.5 irradiation of 100 mW cm⁻². Based on TiO₂ nanofibers sintered at 500°C, 550°C, and 600°C.

Table 1

Cell

Temperature (°C)

J_sc (mA/cm²)

V_oc (V)

η (%)

τ_d (ms)

τ_n (ms)

L_n (μm)

TiO₂ nanofiber cells on the bare FTO substrates, the transit time (τ_d) and electron lifetime (τ_n), and diffusion length (L_n).

500

15.1

0.814

0.52

6.39

3.36

55.3

74.2

550

16.3

0.825

0.53

7.12

1.88

107.7

138.3

III

600

14.1

0.818

0.53

6.11

6.47

86.9

67.1

In this study, specific surface areas were measured to be 28.5, 31.7, and 34.2 m² g⁻¹ for TiO₂ nanofibers sintered at 500°C, 550°C, and 600°C, respectively, which indicate that thinner rough nanofibers sintered at a higher temperature is favorable to increase the specific surface areas. UV–vis absorption spectra (Figure 5) of the sensitized TiO₂ nanofiber film show that the absorption edges are successfully extended to the visible region for all the three samples. In contrast with pure anatase phase (sintered at 500°C), mixed-phase TiO₂ nanofibers (sintered at 550°C and 600°C) after N719 sensitization absorb a greater portion of the visible light, which should be the result of joint contribution of large specific surface area and mixed phase. Because anatase phase TiO₂ has the greatest dye absorption ability, while rutile phase TiO₂ possesses excellent light scattering characteristics due to its high refractive index (n = 2.7) 25 26, dye-sensitized anatase-rutile mixed-phase TiO₂ with a proper proportion will have an enhanced light absorption.

Figure 5

UV–vis absorption spectra

UV–vis absorption spectra. Sensitized TiO₂ nanofiber films (approximately 60-μm thick) sintered at 500°C, 550°C, and 600°C.

The IMPS and IMVS plots of cells I to III display semicircles in the complex plane as shown in Figure 6. The transit time (τ_d) and electron lifetime (τ_n) can be calculated using the equations τ_d = 1/(2πf_IMPS_min) and τ_n = 1/(2πf_IMVS,min), respectively, where f_IMPS,min and f_IMVS,min are the frequencies at the minimum imaginary component in the IMPS and IMVS plots 30. The estimated electron lifetimes of the three cells follow the trend τ_{n II} > τ_{n III} > τ_{n I}, suggesting a reduction in recombination of electrons at the interface between TiO₂ and electrolyte in the presence of rutile phase, while transit times vary in the order τ_{d II} > τ_{d I} > τ_{d III}, indicating that the variation in electron transport rate is dependent on the amount of rutile phase. The competition between collection and recombination of electrons can be expressed in terms of the electron diffusion length. The electron collection efficiency is determined by the effective electron diffusion length, L_n, 31:

L n = d τ n / 2.35 τ d 1 / 2

where d is the thickness of the photoanode. The calculated L_n/d (as shown in Table 1) of TiO₂ nanofiber cell is large and follows the sequence L_{n II}/d_II > L_{n I}/d_I > L_{n III}/d_III. A remarkable large value of 4.9 is found for cell II. A large electron diffusion length is the key point to support the usage of thick TiO₂nanofibers as photoanodes to obtain high photocurrents and high conversion efficiencies. The largest L_n/d_II of cell II with 15.6% rutile content is speculated to be caused by the interaction of rutile and anatase phases which prompts electron transfer from anatase to rutile due to their different conduction band edge and decreased charge recombination rate. However, if the amount of rutile phase is too high in TiO₂ nanofibers, such as 87.8% in cell III, the property of rutile phase will play a leading role in the cell. A large transit time shows a slow electron transport in cell III, which leads to a decrease in electron diffusion length for cell III. From the above analysis, it is concluded that the superior J_sc of cell II is a consequence of more efficient electron collection and light harvesting. As far as V_oc is concerned, it is known that V_oc corresponds to the energy difference between the quasi-Fermi level of the electrons in the TiO₂ under illumination and the redox potential. If the electron recombination is retarded, the electron density in the conduction band of TiO₂ will be increased, which will result in a negative shift in quasi-Fermi level, thereby V_oc will be increased 32. Thus, the higher V_oc of cell II is ascribed to the reduced electron recombination rate. For cell III, in spite of the largest absorbance of visible light, a relatively low J_sc is produced because of an inefficient electron collection. The comparison of cells I to III highlights the existence of a synergistic effect between the anatase and rutile phases in TiO₂ nanofiber DSSCs, as well as suggests a sintering temperature of approximately 550°C which is optimal for enhancing the performance of nanofiber DSSCs.

Figure 6

IMPS (a) and IMVS (b) plots of cells I to III

IMPS (a) and IMVS (b) plots of cells I to III. Based on TiO₂ nanofibers sintered at 500°C, 550°C, and 600°C.

The influence of ZnO blocking layer on the performance of TiO₂ nanofiber cells

Based on the above results, cell II was chosen as the reference cell to study the influence of ZnO blocking layer on the performance of TiO₂ nanofiber cells. ZnO layers with thicknesses of 4, 10, 15, and 20 nm were deposited by ALD method on FTO substrates to fabricate cells IV, V, VI, and VII, respectively. JV curves of cells II and IV to VII are shown in Figure 7, and the photovoltaic characteristics of these cells are summarized in Table 2. Compared with cell II, the performances of the cells with the ZnO layer are significantly improved. With the ZnO layer thickness increased from 0 to 15 nm, J_sc of the cells is monotonously boosted, but when decreased obviously at 20 nm, it is still larger than that without the ZnO layer. It is noticed that enhancement in V_oc and FF is very small. The largest J_sc of 17.3 mA cm⁻² is obtained from cell VI with 15-nm-thick ZnO layer, resulting in the highest PCE of 8.01%, in contrast with 16.3 mA cm⁻² and 7.12% of reference cell II. This phenomenon indicates that the charge collection of the cells is improved by the blocking function of ZnO layer on interfacial recombination, which is very different from the reported decrease of J_sc caused by thick ZnO blocking layers 30.

Figure 7

Photocurrent-voltage curves of TiO₂ nanofiber cells (sintered at 550 °C and approximately 60-μm thick)

Photocurrent-voltage curves of TiO₂nanofiber cells (sintered at 550°C and approximately 60-μm thick). With ZnO layer of different thicknesses (4 to 20 nm).

Table 2

Cell

ZnO thickness (nm)

J_sc (mA/cm²)

V_oc (V)

η (%)

τ_d (ms)

τ_n (ms)

L_n (μm)

TiO₂ nanofiber cells with ZnO layer of different thicknesses, the transit time (τ_d) and electron lifetime (τ_n), and diffusion length (L_n).

14.5

0.825

0.53

6.34

1.88

107.7

138.3

15.0

0.828

0.54

6.71

1.43

119.5

166.9

16.5

0.833

0.54

7.42

1.21

154.3

206.4

17.3

0.842

0.55

8.01

1.08

179.7

235.7

VII

14.8

0.825

0.53

6.47

4.62

354.5

159.9

The schematic view of electron transfer with ZnO layer is shown in Figure 8. The interfacial processes involved in charge transportation in the cell are depicted in Figure 8b. As exciton dissociation occurs, electrons injected into the TiO₂ conduction band will transport to the FTO by diffusion 33. Because the conduction band edge of ZnO is a little more negative than that of TiO₂, an energy barrier is introduced at the interface of FTO/TiO₂, in which ultrathin ZnO layer can effectively suppress the back electron transfer from FTO to electrolytes or may block injected electron transfer from TiO₂ to FTO. The back reaction was studied using IMVS measurements. The electron lifetime τ_n obtained from IMVS (as shown in Table 2) is 107.7 ms for the cell without ZnO layer but is significantly increased from 119.5 to 354.5 ms with ZnO layer thickness increasing from 4 to 20 nm. The striking increase in the lifetime shows direct evidence that ultrathin ZnO layers prepared by ALD method successfully suppress the charge recombination between electrons emanating from the FTO substrate and I₃⁻ ions in the electrolyte. The transit times of electrons calculated from IMPS measurements reflect charge transport and back reaction. Although an energy barrier is induced by introduction of ZnO layer between the TiO₂ and FTO, the electron transit time estimated from IMPS measurement is decreased from 1.88 to 1.08 ms for cells with ZnO layer thickness increasing from 0 to 15 nm. However, when the thickness of ZnO layer further increases, the change trend is reverse, and electron transit time for the cell with 20-nm-thick ZnO layer is markedly increased to 4.62 ms. It is put forward that relative to the cell without ZnO blocking layer, the electron transport in the cells with ZnO layers is determined by the two competition roles of the suppression effect of recombination with I₃⁻ and potential barrier blocking effect. The increased electron lifetime has verified that ultrathin ZnO layer effectively slows the back recombination of electrons at the interface of FTO/electrolyte, so the decreased electron transit time reveals that the suppression effect is stronger than the potential barrier effect when the ZnO layer thickness is smaller than 20 nm. The obtained values of L_n/d of cells IV to VII are shown in Table 2, which are all larger than that of the reference cell without ZnO layer, with the largest value of 8.4 for cell VI with a 15-nm-thick ZnO blocking layer. As a consequence, J_sc's of the four cells are significantly improved and reaches the largest value of 17.3 mA cm⁻² for cell VI. No matter significant improvement of J_sc's for the four cells, little variation in V_oc is found for cells with and without ZnO layers, manifesting no electrons accumulate at the interface between ZnO and TiO₂, which is in good agreement with the rapid transport of injected electrons in TiO₂ conduction band to FTO substrates through ZnO layers.

Figure 8

Schematic view of electron transfer with ZnO layer

Schematic view of electron transfer with ZnO layer. TiO₂ nanofiber DSSC with an ultrathin ZnO layer (a). Illustration of the interfacial charge-transfer processes occurring in the DSSC (b). Also shown is the blocking function of ZnO blocking layer on interfacial recombination as described in this paper.

Conclusions

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 (L_n/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.

Abbreviations

ALD: atomic layer deposition; DSSCs: dye-sensitized solar cells; FF: fill factor; FTO: fluorine-doped SnO₂; IMPS: intensity-modulated photocurrent spectroscopy; IMVS: intensity-modulated photovoltage spectroscopy; J_sc: short-circuit current; J-V: photocurrent-voltage; PCE: photoelectric conversion efficiency; PVP: polyvinylpyrrolidone; SEM: scanning electron microscope; V_oc: open-circuit voltage; XRD: X-ray diffraction; 1-D: one-dimensional; τ_d: transit time; τ_n: electron lifetime.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

JND and NYN conceived and designed the experiments, and wrote the paper. YL carried out the experiments and took part in writing. HH and LB participated in the experiments. SZ participated in the discussion and correction of the paper. All authors read and approved the final manuscript.

Acknowledgements

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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