Hierarchical Structured TiO 2 Photoanodes for Dye-Sensitized Solar Cells

Copyright 2012 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 12, 1 7, 2012 Hierarchical Structured TiO 2 Photoanodes for Dye-Sensitized Solar Cells Yen-Chen Shih, Ann-Kuo Chu, and Wen-Yao Huang Department of Photonics, National Sun Yat-sen University, Kaohsiung 80424, Taiwan A novel approach has been developed to fabricate hills-like hierarchical structured TiO 2 photoanodes for dye-sensitized solar cells (DSSCs). The appropriately aggregated TiO 2 clusters in the photoanode layer could cause stronger light scattering and higher dye loading that increases the efficiency of photovoltaic device. For detailed light-harvesting study, different molecular weights of polyvinyl alcohol (PVA) were used as binders for TiO 2 nanoparticles (P-25 Degussa) aggregation. A series of TiO 2 films with dissimilar morphology, the reflection of TiO 2 films, absorbance of attached dye, amount of dye loading, and performance of fabricated DSSC devices, were measured and investigated. An optimized device had energy conversion efficiency of 4.47% having a higher dye loading and good light harvesting, achieving a 23% increase of short-circuit current J sc in DSSCs. Keywords: Hierarchical Structure, TiO 2 Photoanode, Dye-Sensitized Solar Cell, Light Scattering. 1. INTRODUCTION Recently, dye-sensitized solar cells (DSSCs) have attracted much attention 1 9 owing to their low production cost and high conversion efficiency. In general, the photoanode of a DSSC is a porous layer with singular-size, around 20 nm TiO 2 particles coated on transparent conductive oxide (TCO) glass, and a monolayer of the charge transfer dye adheres to the nanoparticles. Under illumination, electrons from the photoexcited dye molecules inject into the conduction band of TiO 2, diffuse through the porous layer and are subsequently collected and transported to the external load by the TCO. In determining the performance of DSSC, the porous TiO 2 electrode plays an important role. It is essential to have a TiO 2 layer with a high surface area for large dye adsorption and for it to be internally light scattering to enhance light harvesting, gain short-circuit current J sc and improve the power conversion efficiency of DSSC devices. 10 The research results of Wang et al. 11 demonstrated that the surface area of the TiO 2 films is important. The authors also demonstrated that, as the TiO 2 nano particles decrease in size, the surface area increases, and hence the amount of dye is increased; while large particles are required to enhance absorption of red light through internal light scattering. It is difficult to simultaneously increase surface area and light scattering because they oppose each other. So far, some groups have indicated Author to whom correspondence should be addressed. that aggregating TiO 2 nanoparticles to form clusters was beneficial to improve the performance of DSSC. 12 13 Some studies showed better efficiency of electrolyte diffusion or penetration and high dye loading in such structured TiO 2 films; moreover, there seemed to be some scattering benefits in the clustered structure. However, the influence of dye amount and the light-scattering effect of light harvesting were not particularly discussed. As shown in Mie theory, light-scattering intensity is relevant to the size of the illuminated object. That is, the scattering cross-section increases when a larger sized scattering object is used, and the application is practical for light harvesting of longer wavelengths in solar cells. 14 16 In this study, we developed a novel method to get a film with a high surface area and good light scattering at the same time. Furthermore, in order to find out the turning point of these two factors that influence light harvesting, we utilized polyvinyl alcohol (PVA) polymers with different molecular weights as binders to form a series of hierarchical structured TiO 2 films with different degrees of aggregation as shown in Figure 1. A series of TiO 2 films with dissimilar morphology, the reflectance of TiO 2 films, absorbance of attached dye, amount of dye loading, and performance of fabricated DSSC devices were measured and investigated. As a result, an optimized device with higher dye loading and good light harvesting was achieved in our experiment at the same time as achieving a 23% increase of short-circuit current J sc in DSSC. J. Nanosci. Nanotechnol. 2012, Vol. 12, No. xx 1533-4880/2012/12/001/007 doi:10.1166/jnn.2012.5840 1

Hierarchical Structured TiO 2 Photoanodes for Dye-Sensitized Solar Cells Shih et al. ruthenium(ii)bis-tetrabutylammonium (D719 dye, Eversolar) in the tert-butanol and acetonitrile co-solvent, for 20 hours at 25 C. Then the dye absorbed TiO 2 films were rinsed with ethanol and dried at 85 C for 10 minutes. Pt-coated ITO as counter electrode and dye-absorbed TiO 2 film were sealed with hot-melt adhesive (30 m thickness, Surlyn, Tripod), and a liquid electrolyte consisting of I 2 (0.1 M), LiI (0.1 M), 1-propyl-2,3-dimethylimidazolium iodide (DMPII, 0.6 M), and 4-tert-butylpyridine (TBP, 0.25 M, Aldrich) in 3-Methoxypropionitrile (MPN, Fluka) was injected into the interspace of the cells. For comparison, a conventional TiO 2 film was prepared without adding the PVA, and the device as a standard was fabricated equally with the above methods. 2.3. Characterization and Measurement Fig. 1. Schematic diagram of conventional film and hierarchical structured film. 2. EXPERIMENTAL DETAILS 2.1. Preparation of the TiO 2 Particles Paste for Hierarchical Structured Photoanodes To prepare a hierarchical structured TiO 2 paste, we used PVA (CCP) as binders to aggregate the nanoparticles, and attempted to control the size of aggregations by altering the molecular weight (MW 31,000 99,000) of the polymers. First, 1 g PVA water solution was stirred at 65 C overnight, and then dissolved in mixed solvent with 1.0 ml H 2 O and 0.8 ml ethanol. Then 0.6 g TiO 2 powder (P-25, 21 nm Degussa) was added into the above solution. The suspension was put into the ultrasonic cleaner for 30 minutes, then 0.05 ml Triton X-100 and 0.05 ml acetyl acetone of surfactants were added and the suspension was put into the ultrasonic cleaner for 30 minutes again. It was stirred for 24 hours subsequently. The surface morphology of the TiO 2 porous films was obtained by using a field emission gun-scanning electron microscopy (JEOL JSM-6700F). Reflectance and absorbance spectra were measured by UV-vis spectrometer (Perkin Elmer UV-Vis Lambda35) equipped with an integrating sphere. The photocurrent-voltage measurements of DSSCs were performed using a Keithley 2400 analyser under AM 1.5 simulated illuminations with an intensity of 100 mw/cm 2. A 300 W xenon lamp (MODEL 6258) was used as the light source with a filter for approximating one sunlight spectrum. The incident photo to current conversion efficiency (IPCE) spectra were measured by a self-designed IPCE system consisting of a xenon lamp (Xe Light source 70051), a monochromator (Cornerstone 13074004), a power measurement (2935-C), a source measurement unit (M3500A) and a power detector (818-SL). The amount of dye absorption was evaluated by the following method. The dye molecules were first desorbed from the TiO 2 layer by rinsing with 0.1 M NaOH aqueous solution. Then the absorbance at 511 nm of dye solution was measured by UV-vis spectrometer to identify the concentration. 2.2. Fabrication of DSSCs A dense TiO 2 layer of about 45 nm was spin-coated on indium tin oxide (ITO) glass (Merck) as a blocking layer to avoid recombination from the I 3 grabbing photoelectrons on the conducting substrate surface. Thereafter a TiO 2 porous layer was formed by the spin-coating process with an active area of 1.3 0.8 cm 2. In order to evaporate the solvent and decompose the PVA of TiO 2 films, ensuring electrical contact and mechanical adhesion on the glass, the coated films were baked at 100 C for 10 minutes, and calcined subsequently at 450 C for about 30 minutes in air. The average thickness of the TiO 2 films was controlled to be about 10 m measured by an -step surface profiler. For dye absorbing, the calcined films were immersed in a 5 10 4 M ruthenium dye solution, cisbis(isothiocyanato) bis(2, 2 -bipyridyl-4, 4 -dicarboxylato) 3. RESULTS AND DISCUSSION 3.1. Morphologies of TiO 2 Films In order to figure out the optical properties of TiO 2 films with different particle aggregation degrees, we used PVA polymers with different molecular weights as binders. We expected potential variation of light reflectance and light absorbance with dissimilar morphology of these films. To verify that those aggregations were actually formed in the paste, first we diluted the paste and identified it by SEM, as shown in Figure 2. In these pictures, we found that there were really many micro-scale TiO 2 nanoparticle aggregations, with different molecular weights of treated PVA. The size of aggregation increased when the molecular weight increased. We supposed that the interactions of PVA, TiO 2 particles and surfactant were under the static equilibrium. 2 J. Nanosci. Nanotechnol. 12, 1 7, 2012

Shih et al. Hierarchical Structured TiO 2 Photoanodes for Dye-Sensitized Solar Cells (c) (d) (e) (f) Fig. 2. TiO 2 aggregations SEM images from diluted paste., Mw 31,000; (c), (d) Mw 40,000; (e), (f) Mw 54,000. In this steady situation, many TiO 2 aggregations with uniform size were formed in the paste, and size was proportional to the molecular weight of PVA because of stronger interaction between TiO 2 and PVA. Thus, in order to clarify our supposition on the aggregation mechanism, we kept the same weight of molecules and tried to vary the amount of PVA from 0.2 g to 0.3 g in the past for the fabrication of hierarchical structured photoanodes. Indeed, the size of aggregations almost remained unchanged as the amount of PVA in the paste increased. This phenomenon could be attributed to the hydroxyl groups in the PVA molecules interacting with the hydroxyl groups on the TiO 2 particles and forming the clusters in the processing paste. Owing to surfactant restraint the TiO 2 -PVA clusters did not aggregate continuously and remained at a certain size as time went by. When the PVA molecular weight increased, there were more hydroxyl groups in each single PVA molecule. It represented that the interaction between PVA molecules and TiO 2 particles would become stronger; thus bigger aggregations were formed in the paste. However, when we added more PVA molecules into the paste, the hydroxyl groups in a single PVA molecule remained the same, so adding more PVA did not enlarge the size of the aggregation but just increased the aggregation amount. Figure 3 shows the surface SEM images of conventional and PVA treated films. As depicted in Figure 3, the conventional film was formed by uniformly stacking the nanoparticles, and we found that the TiO 2 aggregation size and porosity were proportional to the molecular weight of the PVA. From film A to film D, the size of aggregation increased from about 100 nm to 300 nm. This suggested that, as higher molecular weights of PVA were treated, J. Nanosci. Nanotechnol. 12, 1 7, 2012 3

Hierarchical Structured TiO2 Photoanodes for Dye-Sensitized Solar Cells (c) (d) Shih et al. (e) Fig. 3. Surface SEM images of conventional TiO2 film, PVA treated TiO2 films with Mw 31,000, (c) Mw 40,000, (d) Mw 54,000, and (e) Mw 99,000. larger TiO2 -PVA aggregations formed in the paste. Bigger TiO2 aggregations still existed after coating the substrate and after the polymers had decomposed during the calcining process. The side views of SEM as shown in Figure 4 show a very smooth top surface on the conventional film. However, we clearly observed that the surface of PVA treated film was hill-like, and that these hills were probably formed by those micro-scale TiO2 clusters. So, we called this micro-nano aggregation a hierarchical structured TiO2 film, and expected that the light-scattering effect could be enhanced in this structure. 3.2. Optical Properties of the TiO2 Films and Photoanodes To realize the light-scattering effect in each TiO2 film, the reflectance was measured, as shown in Figure 5. It 4 was apparent that the reflectance of was lower than the others, because those uniformly stacking nanoparticles could not scatter incident light well. However, by PVA treating, those nanoparticles aggregated and formed the hierarchical structured film, obtaining higher reflectance. Moreover, Figure 5 shows the normalized reflectance spectra, in which we found that these reflectance curves were not similar. At the region of longer wavelengths from 600 nm to 800 nm, our results showed that if aggregation size increased, the reflectance comparatively increased. The phenomenon corresponds to Mie scattering theory. So far, several studies have shown that light reflectance is related to the scattering effect of TiO2 films.15 17 However, we could not ensure that those reflected photons were actually well scattered in such porous films. As shown in the schematic diagram (Fig. 1), the incident light is scattered J. Nanosci. Nanotechnol. 12, 1 7, 2012

Shih et al. Hierarchical Structured TiO 2 Photoanodes for Dye-Sensitized Solar Cells Reflectance (R%) 70 60 50 40 30 Film A Film B Film C Film D 20 10 1.0 Normalized Reflectance 0.8 0.6 0.4 400 500 600 700 800 Film A Film B Film C Film D 0.2 Fig. 4. Side views SEM images of conventional film and PVA treated film. by those nanoparticles and subsequently reflects backward or traverses forward through the porous film. If we only measure the reflectance, we neglect the photons that were scattered in the film and traverse out. Moreover, placing a detector in the film to recognize the actual scattering intensity is also a difficult task. Here, in order to improve the evidence of the light-scattering effect, we tried to measure the unit of dye absorbance, so that we could understand how much light in the film was actually absorbed by each dye molecule and characterize the intensity of the lightscattering effect thereafter. The absorbance spectra of each dye molecule in TiO 2 films are shown in Figure 6. These spectra were taken by deducting the absorbance of TiO 2 films from photoanodes (TiO 2 with dye), and then eliminating the dye amount of each film. In Figure 6, we obviously noticed that units of dye molecule harvested more light in the PVA treated films, and that the light absorbance of each dye increased when the degree of aggregation size increased. It represented that the light-scattering effect was really stronger in the higher aggregation film. 400 500 600 700 800 Fig. 5. Reflectance spectra of TiO 2 films Normalized reflectance spectra. devices performance with light-scattering effect below. Table I summarizes the device characteristics and the lightscattering effect. The devices A to D in the table were fabricated with the TiO 2 films A to D respectively. We noticed that when the size of aggregation increased, the dye amount decreased, probably owing to there being less Normalized Absorbance Film A Film B Film C Film D 3.3. Performance of DSSCs We have studied the optical properties of TiO 2 films with different morphologies above. Here, we discuss the 400 500 600 700 800 Fig. 6. Absorbance spectra of each dye molecule in TiO 2 films. J. Nanosci. Nanotechnol. 12, 1 7, 2012 5

Hierarchical Structured TiO 2 Photoanodes for Dye-Sensitized Solar Cells Shih et al. Table I. Peformance of DSSCs. Adsorbed Scattering dye ( 10 8 effect J sc V oc Sample mol/cm 2 ) (525 nm) (ma/cm 2 ) (V) FF (%) 6.35 9 50 0.58 0.69 3 79 Device A 6.46 11 65 0.59 0.65 4 47 Device B 5.92 9 41 0.59 0.65 3 64 Device C 5.56 9 49 0.57 0.68 3 71 Device D 5.81 10 63 0.58 0.66 4 08 surface area of the films, and the scattering effect increased owing to a higher degree of size aggregation. The shortcircuit current from devices A to D did not decrease directly. We found that there was a turning point of the competition between dye amount and light-scattering effect at device C. Moreover, although film D had less dye amount 5.81 10 8 mol/cm 2 than conventional film 6.35 10 8 mol/cm 2, the short-circuit and the power conversion efficiency of device D were higher than the device, which was made from conventional film. This suggested that in device D the influence of the device performance, light-scattering effect was stronger than the dye amount. We obtained the same results from the incident photon to the current efficiency (IPCE) spectra, as shown in Figure 7. The quantum efficiencies of, device A and device D with D719 dye were all maximized around 550 nm. For device D, the peak quantum efficiency was almost the same as with, but it was apparently higher in longer wavelength regions of about 560 700 nm. The higher efficiency could be caused by a stronger scattering effect in longer wavelengths of bigger TiO 2 particle aggregations. Since device A had the largest amount of dye and the best scattering effect, it had the highest quantum efficiency over the whole spectral range and the highest power conversion efficiency of 4.47%, enhanced by a 23% increase of J sc compared with. Quantum Efficiency (%) 50 40 30 20 10 0 Device A Device D 500 600 700 800 Fig. 7. Incident photo to current conversion efficiency spectra of DSSCs. In addition, the quantum efficiency in the shorter wavelength region of device D was lower than the other devices in the IPCE spectra. We supposed that this was owing to the higher porosity and bigger pores in the film D, which was derived from a larger size of TiO 2 aggregations. When the incident light strikes on the film, longer wavelengths of light are scattered for sure, but shorter wavelengths of light could not be properly scattered by such a high porosity structure. However, these photons of the shorter wavelength regions could be finally reflected forward because of enough thickness of the film; in other words, there were enough chances to be reflected. This corresponded to why device D had lower quantum efficiency but a reflectance that was not lower than the other films in shorter wavelength regions. 4. CONCLUSIONS In this work, a series of hierarchical structured TiO 2 photoanodes were prepared and characterized. We found that J sc was increased owing to the stronger scattering effect in larger TiO 2 film aggregations. In our study, we obtained higher J sc with better light scattering in photoanodes, although the amount of the absorbed dye was less. As a result, an optimized device A, derived from PVA 31,000 treated film, had higher dye loading and good light harvesting at the same time as achieving a 23% increase of J sc and producing the highest power conversion efficiency of 4.47%. Acknowledgment: We wish to thank the National Science Council of Taiwan, under Grant No. NSC 100-2221-E-110-046, for supporting this research. References and Notes 1. B. O Regan and M. Grätzel, Nature 353, 737 (1991). 2. Y. S. Jin, K. H. Kim, S. J. Park, H. H. Yoon, and H. W. Choi, J. Nanosci. Nanotechnol. 11, 10971 (2011). 3. W. Y. Huang, C. C. Lee, S. G. Wang, Y. K. Han, and M. Y. Chang, J. Electrochem. Soc. 157, B1336 (2010). 4. W. Y. Huang, C. C. Lee, and T. L. Hsieh, Sol. Energy Mater. Sol. Cells 93, 382 (2009). 5. W. Y. Huang, P. T. Huang, Y. K. Han, C. C. Lee, T. L. Hsieh, and M. Y. Chang, Macromolecules 41, 7485 (2008). 6. J. Qian, P. Liu, Y. Xiao, Y. Jiang, U. Cao, X. Ai, and H. Yang, Adv. Mater. 21, 3663 (2009). 7. S. Bhattacharya, G. Mandal, M. Dutta, D. Basak, and T. Ganguly, J. Nanosci. Nanotechnol. 11, 7735 (2011). 8. M. Grätzel, Acc. Chem. Res. 42, 1788 (2009). 9. J. Yu, J. Fan, and L. Zhao, Electrochim. Acta 55, 597 (2010). 10. U. O. Krašovec, M. Berginc, M. Hočevar, and M. Topič, Sol. Energy Mater. Sol. Cells 93, 379 (2009). 11. Z. Wang, H. Kawauchi, T. Kashima, and H. Arakawa, Coord. Chem. Rev. 248, 1381 (2004). 12. Y. Zhao, J. Zhai, S. Tan, L. Wang, L. Jiang, and D. Zhu, Nanotechnology 17, 2090 (2006). 13. Y. J. Kim, M. H. Lee, H. J. Kim, G. Lim, Y. S. Choi, N. Park, K. Kim, and W. I. Lee, Adv. Mater. 21, 3668 (2009). 6 J. Nanosci. Nanotechnol. 12, 1 7, 2012

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