Electron Transport and Recombination in Photoanode of Electrospun

Jan 8, 2013 - Applied Biological Sciences, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701, United States. ABSTRACT: The ...
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Electron Transport and Recombination in Photoanode of Electrospun TiO2 Nanotubes for Dye-Sensitized Solar Cells Xiaoxu Wang,† Guangfei He,‡ Hao Fong,*,†,‡,§ and Zhengtao Zhu*,†,‡,§ †

Program of Nanoscience and Nanoengineering, ‡Program of Materials Engineering and Science, and §Department of Chemistry and Applied Biological Sciences, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701, United States ABSTRACT: The technique of coaxial electrospinning has been adopted to prepare the TiO2 nanotubes with average inner diameter of ∼275 nm and wall thickness of ∼115 nm. The electrospun TiO2 nanotubes possess the anatase type of crystalline structure, welldefined tubular morphology, and large aspect ratio. The dyesensitized solar cell (DSSC) based on the TiO2 nanotubes alone has an efficiency of 3.33% with open-circuit voltage (Voc) of 800 mV, short-circuit current (Jsc) of 6.01 mA cm−2, and fill factor (FF) of 68.5%. Intriguingly, the addition of 20 wt % TiO2 nanoparticles in the TiO2 nanotubes improves the DSSC efficiency significantly. The charge transport and recombination in the fabricated DSSCs are characterized by the dark current, photovoltage as a function of light intensity, and transient photovoltage and photocurrent measurements. The results indicate that electrospun TiO2 nanotubes have large diffusion coefficient and slow recombination compared to the mesoporous film of TiO2 nanoparticles. The enhanced electron transport properties of the TiO2 nanotubes combined with the facile and scalable preparation technique of electrospinning suggest that the electrospun TiO2 nanotubes could be promising as photoanode material for low-cost and highefficiency DSSCs.

1. INTRODUCTION Dye-sensitized solar cell (DSSC),1 with reported efficiency over 12%,2 has been considered as a promising low-cost alternative to conventional silicon solar cells. In a typical DSSC, the photoanode composed of the mesoporous film of sintered TiO2 nanoparticles is sensitized with dye molecules. Upon light illumination, electrons from the photoexcited dye molecules are injected and then transported through the mesoporous TiO2 film to the anode electrode. Thereafter, these electrons are collected at the counter electrode through an external load and shuttled back to the oxidized dye molecules via reactions with I−/I3− redox couple in electrolyte. Unlike a p−n junction solar cell, DSSC is a “majority carrier” device with no internal longrange electric field gradient for electron drifting in the photoanode; thus, the electron transport in the TiO2 film follows a diffusion mechanism.3−5 The mesoporous TiO2 film provides a large surface area for dye adsorption, which is crucial to efficient light harvesting. On the other hand, the sintered TiO2 nanoparticles have a large number of interfacial contacts and grain boundaries, consequently introducing a high density of surface states near the conduction band of the TiO2 film. These surface states may become the limiting factor for charge collection in DSSC because trapping and detrapping of electrons from the surface states to the TiO2 conduction band occur during the electron diffusion/transport process. Additionally, a large number of electrons within the surfaces states may recombine with I3− ions in the electrolyte. Both trapping and recombination processes reduce the overall energy-conversion efficiency of DSSC.6,7 © 2013 American Chemical Society

To improve the electron transport in photoanode of DSSC, one-dimensional (1D) nanostructures of TiO2 (e.g., nanofibers, nanowires, and nanotubes) have been investigated.8−16 Compared to nanoparticles, these 1D nanostructures contain less grain boundaries and provide direct pathways for electron transport, leading to substantial improvement of the charge collection efficiency.12−16 However, the surface areas of these nanomaterials are usually lower than that of the nanoparticles, and such a situation results in reduction of the dye loading (i.e., the amount of dye adsorbed on the TiO2 surface). The light harvesting and overall device efficiency are thus limited in the DSSCs based on these 1D nanomaterials. Recently, TiO2 nanotube arrays produced by anodic oxidation have attracted attention due to the simple synthesis route, the high surface area, and the good charge transport property.11,15−25 In general, DSSCs based on TiO2 nanotube arrays have energy conversion efficiencies in the range 3−4%.11 One disadvantage of such TiO2 nanotube arrays is the difficulty to grow the nanostructures on the conductive glass such as FTO glass. Instead, the nanotubes are generally prepared on the titanium film, and the film with the nanotubes is used as the anode electrode. The resulting DSSC therefore has to be illuminated through the transparent counter electrode. Additionally, the anodic oxidation method requires expensive Received: November 28, 2012 Revised: January 7, 2013 Published: January 8, 2013 1641

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2.3. Cell Fabrication and Photovoltaic Measurement. A paste of TiO2 nanotubes (NTs) was prepared by adding 0.2 g of sonicated electrospun TiO2 NTs into a solution of 0.2 g of ethanol, 0.5 g of water, 0.1 g of acetic acid, and 0.1 g of PEO. The paste was sonicated for 1 h to allow complete and uniform dispersion. A paste of TiO2 nanoparticles (NPs) was prepared following the same recipe. The paste containing 80 wt % NTs and 20 wt % NPs (NT-80) was prepared by mixing 0.8 g of NT paste and 0.2 g of NP paste together followed by sonication for 1 h. The DSSC fabrication and dye loading test were carried out by procedures slightly modified from our previous report.9 FTO glass substrates were cleaned in a sequence of detergent solution, deionized water, acetone, and isopropanol for 15 min each under sonication. A thin compact TiO2 layer was coated on the conductive side of FTO glass via spray pyrolysis of 0.2 mM titanium di-isopropoxide bis(acetylacetonate). The TiO2 photoanode film was made by doctor blading of the as-prepared TiO2 paste on the FTO glass substrate followed by being sintered at 100 °C for 30 min and then at 450 °C for 45 min. After cooled down to room temperature, the photoanode film was immersed in a freshly prepared 40 mM TiCl4 aqueous solution at 70 °C for 30 min and then was rinsed with deionized water and dried with N2 flow. Thereafter, the TiCl4treated photoanode was sintered at 100 °C for 30 min and 450 °C for 45 min. Immediately after the photoanode was cooled down to room temperature, it was immersed in 0.5 mM N719 dye solution in dark for 48 h at room temperature. The dye solution was prepared by dissolving N719 dye in a mixture solvent of acetonitrile and tert-butanol with 1:1 volume ratio. The counter electrode was prepared by annealing a platinum precursor (Platisol T) on an FTO glass with two drilled holes at 385 °C for 15 min. The photoanode and the counter electrode were sealed together using Parafilm at 100 °C; subsequently, the Iodolyte AN-50 electrolyte was injected through the drilled hole on the counter electrode glass. Finally, the drilled holes were sealed with Parafilm. To measure the dye loading, the dye molecules sensitized on the photoanode were first deadsorbed in 0.1 M NaOH water/ ethanol solution (1:1 volume ratio). The dye concentration in the NaOH solution was quantified by UV/vis absorption spectroscopy (PerkinElmer Lambda 650) using the Beer− Lambert law

starting materials such as Ti foil, which deviates from the intention of fabricating cost-effective solar cells.26 The technique of electrospinning is facile, low-cost, and scalable for preparation of the 1D nanomaterials including polymer, ceramic, and carbon/graphite fibers with diameters typically ranging from tens to hundreds of nanometers.27 The DSSCs based on electrospun TiO2 nanofibers as photoanode have shown substantial improvement in electron transport.8,13,14 Herein, we report characterization and electron transport studies of DSSCs based on electrospun TiO2 nanotubes. The DSSC devices based on these TiO2 nanotubes exhibited a moderate energy conversion efficiency of 3.33% with opencircuit voltage (Voc) of 800 mV, short-circuit current (Jsc) of 6.01 mA cm−2, and fill factor (FF) of 68.5% under AM 1.5 illumination. The electron transport properties of the photoanode based on the TiO2 nanotubes were studied using transient photovoltage and photocurrent measurement. The results indicated that the electrospun TiO2 nanotubes could significantly suppress the electron recombination and enhance the electron transport. To the best of the authors’ knowledge, this is the first report on the DSSC devices based on the TiO2 nanotubes prepared via the electrospinning technique.

2. EXPERIMENTAL SECTION 2.1. Materials. Titanium(IV) butoxide (TNBT), polyvinylpyrrolidone (PVP, Mw = 1 300 000), poly(ethylene glycol) (PEO, Mw = 35 000), titanium(IV) oxide (P25 TiO2) nanoparticles, titanium di-isopropoxide bis(acetylacetonate), titanium(IV) chloride (TiCl4), and all solvents were purchased from the Sigma-Aldrich Chemical Co. The FTO glasses (Fdoped SnO2, 8 Ω/□) were provided by the Hartford Glass Co. The dye of cis-diisothiocyanatobis(2,2′-bipyridyl-4,4′dicarboxylato)ruthenium(II) bis(tetrabutylammonium) (N719), the platinum precursor (Platisol T), and the electrolyte (Iodolyte AN-50) were purchased from Solaronix (Switzerland). All of the chemicals/materials were used as received without further purifications. 2.2. Preparation and Characterization of TiO2 Nanotubes. The TiO2 nanotubes were prepared using a modified technique of coaxial electrospinning.28 The precursor solution containing 5 g of TNBT and 1 g of PVP in 10 mL of ethanol was stirred at room temperature for 24 h. The solution was then filled in a syringe connected to the outer capillary of a coaxial two-capillary spinneret. The other syringe containing paraffin oil was connected to the inner capillary of the spinneret. The flow rates for the inner (paraffin oil) and outer (TNBT and PVP solution) capillaries were set to 0.08 and 0.6 mL/h, respectively. During the electrospinning process, a dc voltage of 15 kV was applied to the metallic needle of the spinneret, and the product was collected on an electrically grounded roller covered with aluminum foil. The as-electrospun product on the aluminum foil was kept in air for 48 h to allow complete hydrolysis of TNBT; subsequently, the product was heated to 500 °C at the rate of 10 °C/min in a tube furnace (Lindberg 54453, TPS Co.). The furnace temperature was maintained at 500 °C for 6 h with constant air flow before naturally cooling down to the ambient temperature. The morphology of the final product was characterized by a Zeiss Supra 40VP field-emission scanning electron microscope (SEM), a JEOL JEM-2100 high-resolution transmission electron microscope (TEM), and a Rigaku Ultima Plus X-ray diffractometer (XRD).

c=

A εl

where c is the dye concentration, A is the absorbance, ε is the extinction coefficient, and l is the length of the optical path. The absorbance at the wavelength of 512 nm was used for calculation. The average value of ε was calculated from the absorbance of three solutions with known dye concentrations. The concentration of dye deadsorbed from the photoanode was then calculated using the Beer−Lambert law. The concentration was converted to the amount of dye adsorbed on the unit area of TiO2 film (t) using the equation cv t= s where v is the volume of the dye solution and s is the area of the photoanode film. The performance of DSSC was characterized using the Keithley 2612 sourcemeter. A 150 W solar simulator (Newport Co.) was used to simulate 100 mW cm−2 sunlight. The light 1642

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intensity was adjusted using a Hamamatsu S1133 reference cell calibrated by the National Renewable Energy Laboratory. 2.4. Transient Photovoltage and Photocurrent Measurement. The recombination lifetime (τe) and the diffusion coefficient (Deff) of the electrons in the photoanode of DSSC were measured using the transient photovoltage and photocurrent setup.13 In brief, under open-circuit conditions, a white light emitting diode (LED) array was used to establish a bias open-circuit photovoltage of the device. A red LED pulse with 500 μs width was used to generate a small rise of photovoltage in the cell. The decay of such photovoltage perturbation was recorded via an oscilloscope. The recombination lifetime of the electrons under specified bias light intensity was then obtained by fitting the decay curve with a single exponential decay function. For the Deff measurement, the cell was in a shortcircuit condition, and the current decay over a 50 Ω resistor in series with the cell was recorded after the light pulse. The current decay was fitted via a biexponential decay function, and the fast decay component was chosen as the electron transport time (τtrans).8 The diffusion coefficient was calculated using the equation of Deff = L2/ τtrans, where L is the TiO2 film thickness.12

nanotubes. The average length of TiO2 nanotubes after sonication is ∼6.5 μm, determined by averaging the lengths of 100 randomly selected nanotubes from the SEM image. Figure 1c shows a cross-section view of the typical photoanode film composed of TiO2 nanotubes on FTO glass. The thickness values of the photoanodes based on NT, NT-80, and NP films are 18, 18, and 11 μm, respectively. The TiO2 NTs are randomly packed to form an interconnecting network. Figure 1d shows the comparison between the pure NT film and the NT-80 (i.e., 80 wt % NT and 20 wt % NP) film. It is evident that the overall morphology of the film is maintained after blending the TiO2 NTs with 20 wt % TiO2 NPs. It is worth noting that the morphology of the photoanode based on electrospun TiO2 nanotubes is different from that of the TiO2 nanotubes prepared by anodic oxidation of a Ti-metal sheet. The electrospun TiO2 nanotubes are packed randomly, while the TiO2 nanotubes prepared by anodic oxidation are ordered arrays perpendicular to the substrate. The current density−voltage (J−V) curves of the three devices are shown in Figure 2. The solar cell parameters of the

3. RESULTS AND DISCUSSION Figure 1a is an SEM image showing the typical electrospun TiO2 nanotubes with average inner diameter of ∼275 nm and

Figure 2. J−V characteristics of TiO2 NT, NT-80, and NP based DSSCs in dark and under AM 1.5 illumination.

devices based on TiO2 NT, NT-80, and NP are summarized in Table 1. The TiO2 NT based DSSC has an overall efficiency of Figure 1. SEM image (a) and HR-TEM image (b) of electrospun TiO2 nanotubes (NTs) with the inset SAED pattern showing the anatase crystalline structure. (c) Cross-sectional view of TiO2 NT anode film on FTO glass substrate. (d) Plane view of DSSC anode films of (left) pure TiO2 NTs and (right) TiO2 NTs blended with 20 wt % P25 TiO2 NPs (NT-80).

Table 1. Characteristics of DSSCs with Different Photoanode Materials

wall thickness of ∼115 nm. The nanotubes have well-defined tubular structures and large aspect ratios. The high-resolution TEM image (Figure 1b) of a representative TiO2 nanotube shows that the wall of the nanotube is polycrystalline and composed of TiO2 grains with an average size of ∼15 nm. The lattice fringes in each grain can be readily identified, indicating that the grains are single crystalline. The anatase crystalline structure of the TiO2 nanotubes is confirmed by the SAED pattern (the inset in Figure 1b) and the XRD measurement (not shown). Although the long nanotubes as shown in Figure 1a might be favorable for electron transport, it is difficult to assemble them into a dense photoanode for DSSC. Hence, these nanotubes have been sonicated into shorter TiO2

TiO2

η/%

Jsc/ mA cm−2

Voc/ mV

FF/%

dye loading/× 10−8 mol cm−2

NT NT-80 NP

3.33 4.26 5.98

6.01 7.91 11.5

800 780 742

68.5 68.7 69.9

6.5 7.7 15

3.33% with Voc of 800 mV, Jsc of 6.01 mA cm−2, and FF of 68.5%. This moderate cell efficiency of the NT based DSSC is similar to those of DSSCs based on TiO2 nanotubes prepared from anodic oxidation.11 The control device (i.e., the DSSC based on TiO2 NPs) has an efficiency of 5.98% with Voc of 742 mV, Jsc of 11.5 mA cm−2, and FF of 69.9%. The DSSC with photoanode being NT-80 (i.e., 80 wt % NTs with 20 wt % NPs) has the device efficiency of 4.26% with Voc of 780 mV, Jsc of 7.91 mA cm−2, and FF of 68.7%. In general, the efficiency of DSSC is determined by three factors: light harvesting efficiency (ηlh), electron injection 1643

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the charge collection efficiency ηcol, which may account for the higher Jsc values than expected from the dye loading results in the NT and NT-80 based devices. The dark current represents the recombination of electrons with I3− ions in the electrolyte. Figure 2 shows the dark current of the NT, NT-80, and NP based devices. The small dark current for the NT based device suggests that the recombination has been suppressed in the cells based on electrospun TiO2 nanotubes. The result is consistent with the trend of open-circuit voltages in the three devices (Table 1). The open-circuit voltages of NT, NT-80, and NP based DSSCs are 800, 780, and 742 mV, respectively. The reduced recombination leads to high Voc of the NT based DSSC.33 Figure 4 plots Voc as a function of log(Φ0). The linear fitting of Voc vs log(Φ0) gives the slope (dVoc/d log Φ0) of ∼104 mV/

efficiency (ηinj), and electron collection efficiency (ηcol). Jsc is the integration of ηlh, ηinj, ηcol, and spectral photon flux of the light source over the device absorption range.15 ηinj is expected to be very similar in all three devices.15 Since ηlh depends linearly on the amount of dye molecules attached on the surface of the TiO2 mesoporous film,7,29,30 we have measured the dye loading of each device. The result is listed in Table 1. The dye loading of the NT film is 6.5 × 10−8 mol cm−2, which is ∼40% of the dye loading of the NP film; the Jsc of the cell based on the NT film is ∼53% of the Jsc of the device based on the NP film. The addition of 20 wt % of NPs in the photoanode film increases the dye loading to 7.7 × 10−8 mol cm−2, about 51% of the dye loading of the NP film; meanwhile, the Jsc of the cell based on NT-80 increases to ∼69% of the Jsc of the cell based on NP. Therefore, the poor dye loading leads to inferior ηlh and Jsc in the NT based DSSC. However, the Jsc values in the photoanode containing TiO2 nanotubes are higher than expected from the dye loading estimation. These results indicate that other factors may play important roles in determining Jsc although the dye loading is the major factor responsible for the relatively moderate efficiency in the NT based DSSC. One factor may be the improved light scattering in the NT based film. Our previous work has suggested that electrospun nanofibers have good light scattering capability.9 Similar light scattering effects could be expected for the nanotubes, which helps the light harvesting of the device. Additionally, ηcol may be improved due to the one-dimensional morphology of the electrospun nanotubes. The electron transport within the three devices is investigated using the transient photocurrent method. The diffusion coefficient (Deff), obtained from the photocurrent decay at short-circuit condition, is plotted as a function of Jsc in Figure 3. For the NT based DSSC, the value of Deff decreases

Figure 4. Voc of the DSSCs as a function of light intensity (Φ0).

decade for all three devices. Based on a model that considers several recombination mechanisms (including recombination of electrons in both conduction band and trapping states), the slope of the linear fit in Figure 4 can be described using the equation

dVoc 2.3 kT = d log Φ0 β q where β is the ideality factor related to recombination involving the band gap surface states, Φ0 is the light intensity, k is the Boltzmann constant, q is the elementary charge, and T is the absolute temperature.34−36 In the ideal case (β = 1), only electrons in the conduction band recombine with I3− ions, and the slope is 59 mV/decade; in a case where electrons from the surface trapping states are involved in the recombination reaction, β is deviated from the value of 59 mV/decade. In our devices based on NT, NT-80, and NP, the slope (dVoc/d log Φ0) is ∼104 mV/decade. This value is similar to that of DSSCs based on TiO2 nanotubes prepared by anodic oxidization, giving the ideality factor β of 0.57.36 The result suggests that the recombination mechanism in all three devices based NT, NT-80, and NP is dominated by the electrons in the surface trapping states.36,37 To further understand the electron recombination in the three DSSC devices, we have derived the electron recombination lifetime (τe) by the transient photovoltage measurement. The electron recombination depends on the incident light intensity and, more specifically, on the quasi-Fermi level within

Figure 3. Electron diffusion coefficient (Deff) as a function of Jsc.

with the increase of Jsc; for the NP based DSSC, Deff increases with the increase of Jsc. The Deff of the cell based on NT-80 is similar to that of the cell based on NT. It is noteworthy that the Deff values of all devices are approaching a fixed value around 10−6 cm2 s−1 at high current density, indicating that these devices are likely “RC”-limited.31,32 The Deff value of the NT based DSSC is larger than that of the NP based device under the same current density, suggesting faster electron transport within the NT network. The fast electron transport improves 1644

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the TiO2 band gap under illumination.38 Since Voc is determined by the difference between the quasi-Fermi level under illumination and the energy level of the redox couple,29 the logarithm of τe as a function of Voc is shown in Figure 5. For

4. CONCLUSIONS In conclusion, the TiO2 nanotubes (NTs) prepared via electrospinning followed by pyrolysis are used as the photoanodes for DSSCs. The device based on the TiO2 NTs alone has an efficiency of 3.33% with Voc of 800 mV, Jsc of 6.01 mA cm−2, and FF of 68.5%. The addition of 20 wt % TiO2 nanoparticles in the NT based photoanode improves the cell performance significantly. The limiting factor in the DSSC based on electrospun TiO2 nanotubes is the poor dye loading, which decreases the Jsc of the cell substantially. The transient photocurrent study on the electron diffusion coefficient indicates that the TiO2 nanotubes have superior electron transport property. Furthermore, Voc in the DSSC based on the TiO2 nanotubes is 800 mV, which is ∼60 mV higher than the Voc of the DSSC based on the nanoparticles. The results from the dark current, photovoltage vs light intensity, and the transient photovoltage measurement indicate that the recombination is significantly suppressed in the NT based DSSC. The reduced recombination and enhanced electron transport suggest that the electrospun TiO2 nanotube could be promising for developing low-cost DSSCs with high efficiency.

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Figure 5. Electron recombination lifetime (τe) as a function of opencircuit voltage (Voc).

AUTHOR INFORMATION

Corresponding Author

*Tel (605) 394-2447, Fax (605) 394-1232, e-mail Zhengtao. [email protected] (Z.Z.); Tel (605) 394-1229, Fax (605) 3941232, e-mail [email protected] (H.F.).

all three samples, the linear relationship between log(τe) and Voc has been observed. The slopes of the fitting are −7.97 ± 0.14, −7.36 ± 0.15, and −4.72 ± 0.06 for the devices based on NT, NT-80, and NP, respectively. The observed increase of τe with decrease of Voc is characteristic of DSSCs. It is evident that the electron lifetime of the NT based DSSC is significantly longer than that of the NP based DSSC under the same Voc condition, consistent with the suppressed recombination in the NT based DSSC. Such a suppression of recombination could be attributed to less surface states in the electrospun TiO2 NTs due to the reduced interfacial contacts and fewer grain boundaries.39,40 It is interesting that the slopes of the NT and NP based devices in Figure 5 are different. A similar phenomenon has been observed in other DSSCs based on 1D TiO2 nanomaterials.38,39 The reason for the slope difference is not clear and under further investigation. One possible explanation is that the order of the recombination reaction could be different between devices based on nanotubes and nanoparticles due to the differences in the number of the surface states and distribution of the trapping states. The long electron lifetime and the large diffusion coefficient lead to improvement on the charge collection efficiency, which may partially compensate the limited dye loading in the DSSC devices based on electrospun TiO2 NTs. This explains that the efficiency of the device based on NTs is ∼53% of that of the device based on NPs, although the dye loading in the cell based on NTs is only ∼40% of the value in the cell based on NPs. The addition of 20 wt % NP into the NT photoanode significantly improves the cell efficiency with a moderate improvement in dye loading. The results suggest that the photoanodes based on 1D nanostructures with high surface areas may be desirable for the development of DSSCs with high efficiency. Furthermore, the electrospun TiO2 nanotubes may be more suitable for the solid state DSSCs because of their enhanced charge transport and collection efficiency. Work along this direction is in progress.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Science Foundation (Grant EPS-0903804), the National Aeronautics and Space Administration (Cooperative Agreement NNX10AN34A), the Research Corporation Cottrell College Science Award (Award 10597), and the State of South Dakota.

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dx.doi.org/10.1021/jp311725g | J. Phys. Chem. C 2013, 117, 1641−1646