Dye-Sensitized Solar Cells Based on Porous Hollow ...

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Dye-Sensitized Solar Cells Based on Porous Hollow Tin Oxide Nanofibers Sudhan Sigdel, Hytham Elbohy, Jiawei Gong, Nirmal Adhikari, Krishnan Sumathy, Hui Qiao, Qufu Wei, Muhammad Hassan Sayyad, Jiantao Zai, Xuefeng Qian, and Qiquan Qiao Abstract— Porous hollow tin oxide (SnO2 ) nanofibers and their composite with titanium dioxide (TiO2 ) particles (Degussa P25) were investigated as a photoanode for dye-sensitized solar cells. Incorporation of TiO2 particles in porous hollow SnO2 fibers enhanced the power conversion efficiency (η) from 4.06% to 5.72% under 100-mW/cm2 light intensity. The enhancement of efficiency was mainly attributed to increase in current density ( Jsc ) and improvement in fill factor (FF). Increase in Jsc was caused by higher dye loading as indicated by UV–Vis absorption spectra and the improvement in FF was attributed to faster charge transport time as obtained from transient analysis. The microstructure of SnO2 fibers was studied using transmission electron microscope, scanning electron microscope, and X-ray diffraction. The electron transfer and recombination life times were studied using transient analysis, whereas interfacial charge transfer was studied using electrochemical impedance spectroscopy. Index Terms— Dye-sensitized solar cells (DSSCs), hollow, nanofibers, porous, tin oxide (SnO2 ).

I. I NTRODUCTION

A

DYE-SENSITIZED solar cell (DSSC) is a promising cost-effective third-generation solar cell technology. A lot of research is being conducted on DSSC to improve the power conversion efficiency and stability, so that it can

Manuscript received December 12, 2014; revised March 18, 2015; accepted March 27, 2015. This work was supported in part by the U.S.–Pakistan Joint Science and Technology through National Academy of Science, in part by the National Natural Science Foundation of China under Grant 21201083, and in part by the Cooperative Innovation Fund-Prospective Project of Jiangsu Province under Grant BY2014023-29 and Grant BY2014023-23. The review of this paper was arranged by Editor A. G. Aberle. (Corresponding authors: Hui Qiao, Qufu Wei, Jiantao Zai, Xuefeng Qian, and Qiquan Qiao.) S. Sigdel, H. Elbohy, N. Adhikari, and Q. Qiao are with the Center for Advanced Photovoltaics, Department of Electrical Engineering and Computer Sciences, South Dakota State University, Brookings, SD 57007 USA (e-mail: [email protected]; [email protected]; [email protected]; [email protected]). J. Gong is with the Center for Advanced Photovoltaics, Department of Electrical Engineering and Computer Sciences, South Dakota State University, Brookings, SD 57007 USA, and also with the Department of Mechanical Engineering, North Dakota State University, Fargo, ND 58102 USA (e-mail: [email protected]). K. Sumathy is with the Department of Mechanical Engineering, North Dakota State University, Fargo, ND 58102 USA (e-mail: [email protected]). H. Qiao and Q. Wei are with the Key Laboratory of Eco-textiles, Ministry of Education, Jiangnan University, Wuxi 214122, China (e-mail: [email protected]; [email protected]). M. H. Sayyad is with the Ghulam Ishaq Khan Institute of Engineering Sciences and Technology, Topi 23640, Pakistan (e-mail: [email protected]). J. Zai and X. Qian are with the State Key Laboratory of Metal Matrix Composites, Shanghai Electrochemical Energy Devices Research Center, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2015.2421475

be used as an alternative to the conventional silicon-based solar cell. The DSSC technology has attracted interest among the researchers because of its low cost and simple fabrication process [1]–[6]. A DSSC typically consists of photoanode, counter electrode (CE), and electrolyte in between them. The photoanode consists of metal oxides, e.g., TiO2 nanoparticles deposited on fluorine-doped tin oxide (FTO)-glass substrate and the CE is a FTO-glass substrate coated with platinum (Pt) or carbon nanostructures [7]–[9]. Metal oxides, such as TiO2 , tin oxide (SnO2 ), zinc oxide (ZnO), and niobium oxide, have been investigated for photoanode materials in DSSCs [10]–[12]. Although TiO2 is found to be the most efficient material for photoanode, its stability under UV light is a concern. Due to the generation of electron-hole pair by UV light absorption, the dye attached on a TiO2 surface degrades slowly with time [13]. This is because of the low bandgap of TiO2 (3.2 eV). The higher bandgap of SnO2 (3.6 eV) results in fewer number of oxidative holes in the valence band of SnO2 under UV illumination, which can help for long-term stability of the DSSCs [13]–[16]. In addition, SnO2 has the advantage of higher electron mobility (100–200 cm2 V−1 s−1 ) than TiO2 (0.1–1 cm2 V−1 s−1 ) [15]. However, the power conversion efficiency is low compared with nanocrystalline TiO2 . The lower efficiency of SnO2 has been attributed to low short-circuit current due to less adsorption of dye molecules such as N719 because of its lower isoelectric point (iep, at pH 4–5) than anatase TiO2 (iep, at pH 6–7) [17], [18] and low open-circuit voltage due to a 300 meV positive shift of the conduction band edge of SnO2 with respect to that of nanocrystalline TiO2 [19]. To improve these problems, surface modification has been done by coating a thin layer of TiO2 , ZnO, Al2 O3 , or MgO [20]–[22]. On the other side, research on different morphologies in SnO2 , such as nanoparticles [23], nanofibers [18], [24], nanotubes [25], nanorods [13], nanobeans [26], nanoflowers [16], nanosheets [27], and urchin structure [28], is still going on. Nanoparticles have the advantage of more dye attachment due to its large surface area. However, the electron transport from one particle to another is hindered because of numerous grain boundaries, surface defects, and trap states. Nanofibers and nonorods can enhance charge transport by offering direct electron pathways, but these structures have disadvantage of insufficient surface area for dye attachment [3]. Nanosheets composing of self-organized nanoparticles offer large specific surface area for dye absorption as well as exhibit good interconnectivity for efficient charge transport [23]. Similarly, urchin structures improve light absorption efficiency due to enhanced

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light trapping by scattering [28]. TiCl4 treated different morphologies of SnO2 , such as SnO2 octahedral with efficiency 6.8% [22], SnO2 hollow nanospheres with efficiency 6.02% [29], urchin like SnO2 with efficiency 6.05% [28], SnO2 nanotubes of efficiency 5.11% [25], SnO2 nanofibers with efficiency 4.63% [18], SnO2 nanoflower with efficiency 5.6% [16], SnO2 nanobeans with efficiency 5.6% [26], and SnO2 nanorods with efficiency 4.67% [13], have been reported. Similarly, DSSCs based on SnO2 nanofibers/TiO2 composite with efficiency 6.17% [18], SnO2 nanoparticles/SnO2 nanofibers double-layered anode with efficiency 6.31% [24], SnO2 nanorods-TiO2 with efficiency 8.61% [10], and SnO2 nanoparticles/ZnO nanotetrapods with efficiency 6.31% [23] have been reported. In this paper, we report the synthesis of porous hollow SnO2 nanofibers and their application in DSSC as photoanode after TiCl4 posttreatment. We prepared this material for enhancing both electron transport (nanofibers structure) in the SnO2 film and dye attachment due to more surface area (porous structure), thus taking the advantages of both nanoparticles and nanofibrous SnO2 structure. In addition, P25-TiO2 incorporated SnO2 in the ratio of 1:1 by weight was investigated as photoanode in DSSCs. Incorporation of TiO2 particles in porous hollow SnO2 fibers enhanced the power conversion efficiency (η) from 4.06% to 5.72% under 100 mW/cm2 light intensity. II. E XPERIMENTAL D ETAILS A. Preparation of Porous Hollow SnO2 Nanofibers The electrospinning solution was prepared by adding SnCl4 ·5H2 O into 10 wt% Polyvinylpyrrolidone (PVP) in ethanol/Dimethylformamide (DMF) solvent mixture. The weight ratio of SnCl4 ·5H2 O to polymer intermediate was fixed at 1:1. The solution was stirred by magnetic stirring at ambient temperature. Subsequently, electrospinning was carried out with this solution. The PVP/SnCl4 ·5H2 O precursor was ejected from a stainless steel needle under a high voltage of 17 kV, and formed fibrous nonwoven mats on the collector. The flow rate used was 1 mL/h, and the needle-to-collector distance was 21 cm. The electrospun nanofiber mats were calcinated at 600 °C for 4 h with a heating rate of 0.5 °C/min. B. Preparation of Porous Hollow SnO2 Nanofiber Paste Ethyl cellulose (0.1 g) was mixed with 5 ml of ethanol using magnetic stirrer for 30 min. Then, 0.1 gm of grinded porous hollow SnO2 nanofibers and 0.5 mL of terpineol were added to the above solution. The resulting solution was ultrasonicated for 15 min and left for magnetic stirring for 3 h. After that, the magnetic stirring was done on a hot plate at 75 °C until the paste obtained was suitable for doctor blading. In addition, the paste of SnO2 /P25-TiO2 was prepared by adding SnO2 (0.05 g) and P25-TiO2 (0.05 g) in the weight ratio 1:1 by following the above-mentioned procedures. C. Fabrication and Evaluation of DSSCs 1) Preparation of Anodes: A photoanode was prepared by spin coating TiO2 compact layer on an FTO-glass

substrate. Then, ∼6.5–7-μm-thick SnO2 layer was doctor bladed, followed by deposition of 4-μm-thick layer of light scattering layer (Solaronix Ti-Nanooxide R/SP) and treatment in 0.2 M of TiCl4 aqueous solution (optimum concentration) at 80 °C for 30 min. After each step, the anode was sintered at 115 °C for 15 min and 475 °C for 30 min. Then, it was immersed in dye solution containing 0.3-mm Ruthenizer 535-bisTBA dye (N-719) in acetonitrile/t-butanol (volume ratio: 1/1) for 30 h at room temperature. Finally, the anode was kept in acetonitrile for 3 h to remove any excess dye. In addition, reference anode using 14-μm-thick nanocrystalline TiO2 as main layer (Solaronix Ti-Nanoxide HT/SP) was prepared. The above-mentioned thicknesses were the optimum thicknesses that had the highest efficiencies. 2) Preparation of Platinized FTO-Glass Counter Electrodes: Platinum (Pt) precursor solution (0.02 M H2 PtCl6 ·6H2 0 in anhydrous ethanol) was spin coated on an FTO-glass substrate and heated at 400 °C for 15 min in the air to obtain a CE. 3) Assembly of the DSSCs: An electrolyte containing 0.03 M I2 , 0.60-M 1-butyl-3-methylimidaz-olium, 0.10-M ganidine thiocyanate, and 0.5-M tert-butylpyridin in acetonitrile and valeronitrile (85:15 by volume) was then injected into the cells through the reserved channel. The channel was made in the thermoplastic sealant kept in between the photoanode and the CE. 4) Dye Desorption From Photoanode: Each different photoanode were dipped in 2 mL of 10-mM NaOH solution at room temperature for 24 h, and 10-mM NaOH solution was prepared by dissolving NaOH in the mixture of ethanol and water at 1:1 ratio by volume. Then, UV–Vis absorption spectroscopy was done to measure and compare the amount of dyes desorbed from different photoanodes. 5) Characterization: J –V characteristics of the cells were tested under AM 1.5 illumination at a light intensity of 100 mW/cm2 . For this measurement, Xenon lamp (Newport 67005) with AM 1.5 filter was used as light source. Electrochemical impedance spectroscopy (EIS) was conducted using a computer-controlled Ametek VERSASTAT3-200 potentiostat with frequency analysis module. The ac signal of amplitude 10 mV in the frequency range from 0.1 to 105 Hz at a certain dc bias voltage (open-circuit voltage) under dark condition was used [30]. Transient photocurrent (TPC) and transient photovoltage (TPV) were done using OBB’s Model OL-4300 nitrogen laser. The morphology of porous hollow SnO2 nanofibers was studied using Hitachi S4800 field-emission scanning electron microscopy (SEM) and a JEOL JEM-2100 transmission electron microscopy (TEM) unit at an accelerating voltage of 120 kV. The crystal structures were analyzed by Bruker D8 Advance X-ray diffractometer using Cu-Kα irradiation (λ = 1.5406 Å) over Bragg angles from 10° to 70° with the scanning speed of 4° min−1 . III. R ESULTS AND D ISCUSSION Fig. 1 shows SEM image of pure porous hollow SnO2 nanofibers. The hollow nanofibers have the diameter

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Fig. 1.

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SEM image of porous hollow SnO2 nanofibers.

Fig. 4. (a) Current density-voltage (J –V ) curves for DSSCs for TiCl4 treated SnO2 nanofibers, SnO2 /P25-TiO2 , and nanocrystalline TiO2 under AM 1.5 illumination at the light intensity 100 mW/cm2 . (b) IPCE spectral action responses of the DSSCs based on TiCl4 treated SnO2 nanofibers, SnO2 /P25-TiO2 , and nanocrystalline TiO2 photoanodes. Fig. 2.

TEM image of porous hollow SnO2 nanofibers. TABLE I C OMPARISON OF P ERFORMANCE PARAMETERS OF TiCl4 T REATED SnO 2 F IBERS , SnO 2 /P25-TiO 2, AND N ANOCRYSTALLINE TiO 2 -BASED DSSCs

Fig. 3.

XRD patterns of porous hollow SnO2 nanofibers.

of 100–150 nm with pores on it. The distribution of pores in the fibers can be seen in TEM image in Fig. 2. Fig. 3 shows the powder X-ray diffraction (XRD) patterns from the prepared porous hollow SnO2 nanofibers. The diffraction peaks can be indexed to tetragonal rutile type of SnO2 (JCPDS card no 41–1445). No other impurity peaks were observed in the SnO2 fibers, indicating high purity of the final product.

Fig. 4(a) shows the current density versus voltage ( J –V ) curves of TiCl4 treated SnO2 fibers, SnO2 /P25-TiO2 , and nanocrystalline TiO2 -based cells under AM 1.5 illumination with a light intensity of 100 mW/cm2 . As shown in Table I, the power conversion efficiencies (η%) of SnO2 fibers (∼6.5–7-μm thick), SnO2 /P25-TiO2 (∼6.5–7-μm thick), and nanocrystalline TiO2 (∼14-μm thick) based cells were 4.06%, 5.72%, and 7.12%, respectively. After adding P25-TiO2 in SnO2 fibers, short-circuit current density of SnO2 /P25-TiO2 increased from 10.75 to 11.97 mA/cm2 and the fill factor (FF) improved to 0.60 as compared with 0.50 in case of SnO2 . The open-circuit voltage (VOC ) was



Fig. 5. UV−Vis normalized absorbance spectra from the solutions of dyes that were desorbed from TiCl4 treated SnO2 fibers, SnO2 /P25-TiO2 , and nanocrystalline TiO2 photoanodes used in the DSSCs.

0.75 V for SnO2 fibers and 0.8 V for SnO2 /P25-TiO2. Since both types of cells were posttreated with TiCl4 , this layer acted as a recombination barrier layer giving higher values of VOC . Nanocrystalline TiO2 -based cells had JSC , VOC , and FF of 12.78 mA/cm2 , 0.79 V, and 0.71, respectively. Fig. 4(b) shows the incident photon-to-current conversion efficiency (IPCE) spectral action responses of these cells. The JSC values obtained using integrating IPCE were almost same as the JSC values obtained from J –V curves. The highest IPCE observed for TiCl4 treated SnO2 fibers, SnO2 /P25-TiO2, and nanocrystalline TiO2 -based cells were 62%, 63%, and 68%, respectively. Fig. 5 shows the UV−Vis normalized absorbance spectra from the solutions of dyes that were desorbed from TiCl4 treated SnO2 fibers, SnO2 /P25-TiO2, and nanocrystalline TiO2 anodes. This spectra show the dye absorption in SnO2 /P25-TiO2 was higher than that in SnO2 fibers. P25 increased the surface area of the SnO2 fibers leading to more dye absorption. To understand the improvement in FF after addition of P25-TiO2 in SnO2 fibers, the charge transport in the bulk of the anode films was studied using TPC measurement. Fig. 6(a) shows the normalized TPC decay of TiCl4 treated SnO2 fibers, SnO2 /P25-TiO2 , and treated nanocrystalline TiO2 -based DSSCs. The charge transport time (τ ) obtained from these exponentially decaying curves were 4.4 ms for SnO2 and 1.31 ms for SnO2 /P25-TiO2 . This shows that bulk of the SnO2 /P25-TiO2 offers less resistance to the electron flow within the film. The reason for faster charge transport time in P25-TiO2 incorporated SnO2 may be because of the presence of P25 between the gaps of nanofibers resulting in smooth flow of electron in the bulk. The above-mentioned charge transport time in SnO2 and SnO2 /P25-TiO2 justifies the higher value of FF in SnO2 /P25-TiO2 as compared with that of SnO2 . Moreover, the charge transport time in nanocrystalline TiO2 -based cells was found to be lowest, 0.58 ms, suggesting the lowest bulk resistance in TiO2 film, thus the highest FF, 0.71 as compared with SnO2 and SnO2 /P25-TiO2 -based cells. Fig. 6(b) shows the normalized TPV decay of TiCl4 treated SnO2 fibers, SnO2 /P25-TiO2 , and nanocrystalline

Fig. 6. Normalized transient (a) photocurrent and (b) photovoltage decay of TiCl4 treated SnO2 nanofibers, SnO2 /P25-TiO2 , and nanocrystalline TiO2 -based DSSCs.

TiO2 -based DSSCs. The recombination lifetime in SnO2 fibers, SnO2 /P25-TiO2 , and treated nanocrystalline TiO2 -based cells were found to be 55.77, 26.97, and 18 ms, respectively. The ratios of recombination lifetime to charge transport time were found to be 12.68, 20.58, and 31 for SnO2 fibers, SnO2 /P25-TiO2 , and nanocrystalline TiO2 -based DSSCs, respectively. Considering these values, the ratio of recombination process is slower as compared with charge transport time in these three different anodes, which justifies the values of VOC above 0.75 V. In all these cases, the energy barrier of ∼300 meV created by post TiCl4 treatment prevents back charge transfer from the main layer to the electrolyte or dye. Moreover, lower value of recombination time-to-charge transport time in SnO2 than that in SnO2 /TiO2 can lead to the lower VOC in SnO2 fibers-based DSSCs than that in SnO2 /P25-TiO2 -based DSSCs. To analyze the interfacial charge transfer, EIS was performed at dc bias voltage (VOC ) and 10 mV ac with frequency from 0.1 Hz to 100 kHz. Fig. 7(a) shows the Nyquist plots of a complete cell using TiCl4 treated SnO2 fibers, SnO2 /P25-TiO2 , and treated nanocrystalline TiO2 as anodes. Using a small ac signal model, any interface can be represented by a resistance and a capacitor in parallel combination. Fig. 7(b) shows the equivalent circuit diagram used to fit

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greater the VOC . Considering this fact, SnO2 should have the highest VOC . However, we believe that although SnO2 had the highest recombination resistance, the longer charge transport time (TPC analysis) may increase the probability of recombination resulting in lower VOC . IV. C ONCLUSION Porous hollow SnO2 nanofibers with half the thickness of nanocrystalline TiO2 have been demonstrated as an efficient novel anode for DSSCs. In addition, with the thickness about half of that nanocrystalline TiO2 , P25-TiO2 incorporated porous hollow SnO2 nanofibers have been demonstrated as a promising photoanode for DSSCs. The efficiencies achieved using porous hollow SnO2 fibers and SnO2 /P25-TiO2 (1:1 by weight) were 4.06 % and 5.72%, respectively. R EFERENCES

Fig. 7. (a) Nyquist plots of TiCl4 treated SnO2 nanofibers, SnO2 /P25-TiO2 , and nanocrystalline TiO2 -based DSSCs measured at a dc bias voltage (VOC ) under dark condition and from 0.1 Hz to 100 KHz with an amplitude of 10 mV. (b) Equivalent circuit of a full DSSC for EIS measurement. Rs : series resistance at the electrodes. CPE: constant phase element. RCT : charge transfer resistance at electrolyte/CE interface. RCR : charge recombination resistance at anode/electrolyte interface. TABLE II F ITTED PARAMETERS E XTRACTED F ROM N YQUIST P LOTS OF TiC L4 T REATED SnO 2 F IBERS , SnO 2 /P25-TiO 2, AND N ANOCRYSTALLINE TiO 2 -BASED DSSCs

the impedance spectra. The equivalent circuit was fitted to extract the parameters from first two arcs. Rs represents the series resistance of the electrodes, RCT represents the charge transfer resistance at the electrolyte/CE interface, and RCR represents the back charge transfer from photoanode to electrolyte [30]. The high frequency arc (left-hand side) represents the interfacial charge transfer at electrolyte/Pt interface and the low-frequency arc (middle arc) represents the − electrolyte. back charge transfer from photoanode to I− 3 /I The lower frequency arc (right arc) represents the finite Warburg impedance of tri-iodide in electrolyte [30]. Since, − I− 3 /I electrolyte was used in all cells, the arcs representing the finite Warburg impedance of tri-iodide in electrolyte were almost equal. Fitted parameters obtained from the Nyquist plots (first two arcs) are summarized in Table II. Since Pt is used as CE in all three different cells and the electrolyte is the same, the RCT is found to be almost equal, 12–13 . The charge recombination resistances (RCR ) in SnO2 , SnO2 /P25-TiO2, and nanocrystalline TiO2 were found to be 188.4, 163.4, and 121 , respectively. In general, it is considered that higher the recombination resistance,

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