Toward a Quantitative Correlation between Microstructure and DSSC

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Toward a Quantitative Correlation between Microstructure and DSSC Efficiency: A Case Study of TiO2 xNx Nanoparticles in a Disordered Mesoporous Framework Kumarsrinivasan Sivaranjani,† Shruti Agarkar,‡ Satishchandra B. Ogale,*,‡ and Chinnakonda S. Gopinath*,†,§ †

Catalysis Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India Physical and Materials Chemistry Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India § Center of Excellence on Surface Science, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India ‡

bS Supporting Information ABSTRACT: The efficiency (η) of a dye-sensitized solar cell (DSSC) depends on various parameters, the critical factors being a fast charge carrier transport and a slow rate of electron hole recombination. The present article describes a simple combustion synthesis method to prepare TiO2 xNx with following four important features that directly influences η: (1) a disordered mesoporous structural framework with high surface area to give high dye-loading and a small diffusion length for charge carriers allowing rapid movement to the surface; (2) electrically interconnected nanocrystalline TiO2 xNx particles with good necking and predominant (101) anatase facets to minimize electron hole recombination; (3) low charge storage capacity in the titania framework; and (4) surface unsaturation assisting all the above factors. The pseudo-three-dimensional nature of mesoporous TiO2 xNx with the above features demonstrates the importance of textural features, and porosity allows faster diffusion of charge carriers to surface and their utilization to generate power. A quantitative correlation between interconnected nanoparticles over larger distances in a mesoporous framework and η is demonstrated. This study also demonstrates an inexpensive and rapid method of producing the photoanode material with high η in about 10 min.

1. INTRODUCTION All energy consumption forecasts predict an increasing trend globally, despite some temporary economic downturn in significant parts of the globe. It is increasingly indispensable to tap all the energy resources, especially green/pollution-free energy resources, such as sunlight, to meet the present and future energy needs.1,2 Photovoltaics is perhaps the most attractive mode of light harvesting because of the electrically driven power systems, devices, and gadgets of the modern world. Traditionally, the vehicle for photovoltaic device is silicon; however, the same is very expensive and has availability issues. More recently attention is, therefore, focused on newer and novel alternatives to siliconbased solar cells (SC). Among these is the dye-sensitized solar cell (DSSC) using titania, which represents a potentially scalable and economically viable option. This concept was introduced by Gr€atzel in 1991 and has achieved an overall photovoltaic efficiency (η) of around 11.5%.3,4 The efficiency of carrier transport and the rate of electron hole recombination are the two major competing factors3,4 that determine the η of any SC. Since grain boundaries hinder transport and enhance recombination,5 it is highly desirable to have an electrically interconnected nanoparticulate (EINP) material in a high surface area framework, such as a mesoporous system, to render high efficiency. Compared to ordered mesoporous materials, r 2011 American Chemical Society

such as SBA-15 and MCM-41,6 disordered wormhole mesoporous framework greatly decreases the diffusional barriers,7,8 aiding rapid transport of photogenerated electrons to the anode surface. This is because of the smaller mesochannel depth (20 nm). This is characteristic of the mesoporous materials due to the agglomeration of nanoparticles with slit shaped pores. Both pore size and number of pores increases simultaneously for TU10. Among all, TU10 shows the highest surface area (234 m2/g) and pore volume (0.44 cc/g) due to well developed mesoporosity and smaller average crystallite size. High surface area usually enhances the light harvesting ability of the photocatalyst.21 UV visible absorption spectra of as prepared TUx materials are given in the Supporting Information, Figure S3. Among all, TU9 shows significantly higher visible light absorption. In TU10 and TU12, though they have more nitrogen content (Table 1), smaller particle size leads to a blue shift due to quantum size effects. It is unlikely that visible light absorption due to N-content might have any influence on efficiency. Figure 3a shows the photocurrent voltage (J V) characteristics of the DSSCs made of different photoanodes prepared with anatase TUx materials and compared with Degussa P25 and

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Figure 3. (a) J V characteristics of DSSC based on photoanodes prepared from TUx and compared with P-25. Efficiency (η) values are given in parentheses. (b) Electrochemical impedance spectra of DSSC made from P-25 and TUx materials. (c) Current perpendicular to plane (CPP) transport data for TUx materials.

mixed-phase TU5. Photovoltaic properties (open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF), and η) are given in Table 1. TU9, 10, and 12 materials show η > 5% indicating the efficacy of SCM. Mesoporosity with high surface area is the major reason for the higher η value of TUx (x g 9). In addition, high Jsc and marginally higher Voc values of TU10 help to achieve the highest η, compared to those of TU9 and TU12. This is attributed to the EINP and the highest crystallinity with TU10, apart from mesoporosity. Better textural properties present in TU10 2584

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The Journal of Physical Chemistry C enhance dye diffusion throughout the interior surfaces too and increases dye adsorption significantly. The relatively lower surface area of TU9 and lower crystallinity of TU12 decreases η. The η of DSSC based on TU10 is significantly better, by about 47% compared to P25. The higher crystallinity (or low defect concentration) of TU10 clearly minimizes carrier recombination leading to fast electron conduction and maximum charge collection. Superior textural properties of TU10 allow interior and exterior surfaces to be accessible to the electrolyte, leading to higher Voc and Jsc. In contrast to the above, TU5 shows η = 2.9%, which is much lower than P-25. This is attributed to the low surface area and especially the predominant rutile phase (80%) with large crystallite size (Table 1). Even though P-25 and TU5 contain rutile and anatase phases, the high anatase percent (70%) contribution in P-25 is responsible for η = 4.9% compared to the high rutile percent (80%) contribution in TU5. In addition, there is no interconnectivity between the particles observed on TU5, mainly due to large rutile content with large crystallite size. In short, most of the favorable factors found in TU10 toward high η, such as high surface area, EINP, are not present in TU5. Impedance measurements were made to explore the resistance values for different interfaces in the DSSC assembly, and the resulting Nyquist plots30 are shown in Figure 3b. As the electrolyte and counter electrode are the same in all the materials, our primary interest is in the second semicircle (Z2), which focuses on the TiO2|dye|electrolyte interface. The resistance Rk, i.e., the diameter of the central arc, which gives the charge transfer resistance including recombination of electrons with I3 at the TiO2| electrolyte interface, is minimum (10.05 Ω) for TU10, while 15.9, 46.8, 17.74, and 18.15 Ω for P-25, TU5, TU9, and TU12, respectively. Con value is also calculated, which describes the charge transport resistance and its recombination rate along the entire thickness of the working electrode and is equal to RkLkeff,31 where L is thickness of the film. Con value for TU5, TU9, and TU12 is 31.9, 6.8, and 10.1 Ω cms 1, respectively. This value is substantially low for TU10, which is 5.6 Ω cms 1. This all indicates that TU10 has the property of efficient charge transfer within its network as compared to TU9 and TU12. In other words, charge storage capacity has decreased in the case of TU10 and not in all others (height of Z2, Figure 3b). This also suggests a good necking between the individual particles in TU10 confirming EINP, as seen in TEM images. The number of defect states in other materials are relatively high, and they act as trapping sites for electrons thereby enhancing the capacitance and decreasing the Jsc (Table 1). It is to be noted that the thickness of meso-TUx films prepared for photovoltaic measurements is ∼12 μm, and the EINP extending, at least up to 1 μm, demonstrates a minimum defect with TU10. Very high Con and Rk values measured for TU5 suggesting a poor connectivity between crystallites, and the bigger crystallite size rutile apparently hinders the charge transfer and hence an overall low η is obtained. Current perpendicular-to-plane (CPP) conductivity measurements were performed on TUx films before dye loading (i.e., after annealing at 450 °C) using aluminum top electrode. The voltage was swept from 0 to 10 V with a step size of 0.5 V, and a delay time of 0.5 s. The results of these measurements are summarized in Figure 3c. It is observed that the current is significantly higher for TU10 film than the TU12 and TU9 films. It is mainly due to the presence of better connectivity between the TU10 nanoparticles. This observation confirms the proposed EINP nature of the TU10 material. Very poor CPP conductivity

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Figure 4. Cartoon model to show the connectivity between nanocrystalline particles and the same extending to micrometer scale length to enhance electron transfer capacity between particles. Introduction of yellow color in the latter two cartoons is to emphasize the increase in electron density.

Figure 5. XPS spectra collected from (a) Ti 2p and (b) O 1s core levels of meso-TUx materials. XPS results of TU10 recorded at normal and grazing angle are similar. Inset in panel b shows the results from N 1s core level. An unambiguous increase in the electron density on TU10 is evident from the above results.

observed for TU5 further reiterates the interpretation that large rutile content with low surface area and missing EINP do not favor high η. A series of cartoons shown in Figure 4 illustrates the EINP nature of meso-TiO2 xNx from the single nanocrystalline particle level to micrometer length. Fast electron transfer occurs through the EINP network upon illumination on partially dye-coated titania; this is mainly to emphasize the importance of effective charge transfer from the point of origin to the collecting electrode. We believe that such intercrystallite connectivity is essential to minimize the loss of electrons in secondary processes. Among TUx materials, TU10 exhibits all the four important properties, namely, (a) maximum EINP with (101) facets, (b) highest surface area, (c) porosity with small depth mesochannels, and (d) hence high dye loading; all these factors helps for fast charge conduction. Absence of the above factors in TU5 leads to the lowest η. TU9 and TU12 shows η between the above two values. Although many favorable factors (high surface area and comparable dye loading to that of TU10) are found with TU9 and 12, a missing EINP character decreases the η. In general, all the above four factors in a single material improves the DSSC efficiency, and it is worth exploring on this line of research to accommodate all four factors in other materials and correlate with efficiency. 2585

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The Journal of Physical Chemistry C X-ray photoelectron spectroscopy (XPS) analysis was carried out to explore the nature of surfaces of TUx materials (Figure 5). Binding energy (BE) for the Ti 2p3/2 core level in TU10 is observed at 457.7 eV, in contrast to 458.6 eV for TU9 and TU12 or 459 eV for P2532,33 (Figure 5a). Further, the normal as well as grazing angle XPS of TU10 match with each other highlighting very similar electronic states of surface layer and top 5 nm of the crystallites. This BE value indicates that TU10 along with high crystallinity and better porosity differs electronically also from other TUx materials. A substantial decrease in the BE value of Ti 2p3/2 in TU10 shows that there is an increase in electron density around Ti. The O 1s core level also displays similar changes. TU10 exhibits O 1s peak at 528.7 eV, while other TUx show the same at 529.8 eV (Figure 5b). This is attributed to two factors. Primarily the unsaturation occurs on TU10, compared to TU9 and TU12, due to the EINP and textural properties discussed earlier leading to an increase in the effective electron density on the surface.34,35 A secondary effect due to the substitution of less electronegative N in the TiO2 lattice also might contribute to higher charge density. N 1s BE observed at 398.3 eV on TU10 material is attributed to the N Ti O linkage corresponding to anion like nitrogen in the TiO2 xNx lattice.28,32,33 These two factors might increase the effective electron transfer between particles in the mesoporous framework of TU10. Although the properties associated with TU10 (and other TUx) has been reproduced several times in our laboratory, we are yet to understand the origin of unique properties associated with TU10. Upon illumination of DSSC with 1 sun AM 1.5 light, electron transfer from dye to electron-rich TU10 nancrystallites occurs, which creates an artificial potential difference with near neighbor environment. This potential difference would help for fast charge transfer toward the TCO through the TU10 network (Figure 4).

4. CONCLUSIONS The pseudo-3D nature of mesoporous TiO2 xNx with 5 10 nm mesochannels in the present set of materials leads to faster diffusion of charge carriers to surfaces. Mesoporosity with a large number of bigger pores and electrically interconnected nanosized, but crystalline, particles lead to a higher efficiency in DSSC, as in TU10. Generally, the efficiency is higher when the nanocrystalline (101) faceted particles are interconnected in a mesoporous framework, and it is quantitatively shown. The energy conversion efficiency of DSSC based on TU10 is significantly higher, by about 47% compared to that of P25. The present study throws light on the importance of textural properties for fast electron transfer and hence higher efficiency of DSSC. More studies in this direction, such as further optimization of textural properties, is likely to increase the efficiency further. It is expected that such electrically interconnected nanocrystallite titania would be potential candidates21,36 for water splitting with a noble metal. Further studies are in progress to improve efficiency through surface modifications by different preparation strategies and adopting light harvesting protocols as well as for water splitting by photocatalysis. ’ ASSOCIATED CONTENT

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Supporting Information. SAED pattern of TU5 and TU10 (Figure S1), N 2 adsorption desorption isotherm studies (Figure S2), and UV vis spectra (Figure S3) for TUx materials.

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’ AUTHOR INFORMATION Corresponding Author

*Phone: 0091-20-2590 2043. E-mail: [email protected]. Website: http://www.ncl.org.in/csgopinath.

’ ACKNOWLEDGMENT We thank Mr. R. K. Jha for surface area analysis. K.S. thanks CSIR, New Delhi, for a research fellowship. We thank Director, NCL, for supporting and funding through an in-house project. We thank CSIR’s TAPSUN program for financial support. ’ REFERENCES (1) Hoffert, M. I.; Caldeira, K.; Benford, G.; Criswell, D. R.; Green, C.; Herzog, H.; Jain, A. K.; Kheshgi, H. S.; Lackner, K. S.; Lewis, J. S.; Lightfoot, H. D.; Manheimer, W.; Mankins, J. C.; Mauel, M. E.; John Perkins, L.; Schlesinger, M. E.; Volk, T.; Wigley, T. M. L. Science 2002, 298, 981. (2) Green, M. A. Power to the People: Sunlight to Electricy Using Solar Cells; University of New South Wales Press: Sydney, Australia, 2000. (3) O’Regan, B.; Gr€atzel, M. Nature 1991, 353, 737. (4) Gr€atzel, M. Nature 2001, 414, 338. (5) Snaith, H. J. Adv. Funct. Mater. 2010, 20, 13. (6) Wang, Y.; Tang, X.; Yin, L.; Huang, W.; Hacohen, Y. R.; Gedanken, A. Adv. Mater. 2000, 12, 1183. (7) (a) Maity, N.; Rajamohanan, P. R.; Ganapathy, S.; Gopinath, C. S.; Bhaduri, S.; Lahiri, G. K. J. Phys. Chem. C 2008, 112, 9428. (b) Basu, S.; Mapa, M.; Gopinath, C. S.; Bhaduri, S.; Lahiri, G. K. J. Catal. 2006, 239, 154. (8) Zhang, Z.; Pinnavaia, T. J. Am. Chem. Soc. 2002, 124, 12294. (9) He, X.; Antonelli, D. Angew. Chem., Int. Ed. 2001, 41, 214. (10) Kulkarni, D. G.; Murugan, A. V.; Viswanath, A. K.; Gopinath, C. S. J. Nanosci. Nanotechnol. 2009, 9, 371. (11) Hou, K.; Tian, B.; Li, F.; Bian, Z.; Zhao., D.; Huang, C. J. Mater. Chem. 2005, 15, 2414. (12) Li, L.; Liu, C. J. Phys. Chem. C 2010, 114, 1444. (13) Chen, D.; Huang, F.; Cheng, Y.-B.; Caruso, R. A. Adv. Mater. 2009, 21, 2206. (14) Zhang, Y.; Xie, Z.; Wang, J. ACS Appl. Mater. Interfaces 2009, 12, 2789. (15) Ghadiri, E.; Taghavinia, N.; Zakeeruddin, S. M.; Gr€atzel, M.; Moser, J.-E. Nano Lett. 2010, 10, 1632. (16) Wei, M.; Konishi, Y.; Zhou, H.; Yanagida, M.; Sugihara, H.; Rakawa, H. J. Mater. Chem. 2006, 16, 1287. (17) Muduli, S.; Game, O.; Dhas, V.; Yengantiwar, A.; Ogale, S. B. Energy Environ. Sci. 2011, 4, 2835. (18) Hegde, M. S.; Madras, G.; Patil, K., C. Acc. Chem. Res. 2009, 42, 704. (19) Mapa, M.; Gopinath, C. S. Chem. Mater. 2009, 21, 351. (20) Mapa, M.; Sivaranjani, K.; Bhange, D. S.; Saha, B.; Chakraborty, P.; Viswanath, A. K.; Gopinath, C. S. Chem. Mater. 2010, 22, 565. (21) Sivaranjani, K.; Gopinath, C. S. J. Mater. Chem. 2011, 21, 2639. (22) Mapa, M.; Thushara, K. S.; Saha, B.; Chakraborty, P.; Janet, C. M.; Viswanath, R. P.; Madhavan Nair, C.; Murti, K. V. G. K.; Gopinath, C. S. Chem. Mater. 2009, 21, 2973. (23) Gholap, S.; Badiger, M. V.; Gopinath, C. S. J. Phys. Chem. B 2005, 109, 13941. (24) Mathew, T.; Shylesh, S.; Devassy, B. M.; Vijayaraj, M.; Satyanarayana, C. V.; Rao., B. S.; Gopinath, C. S. Appl. Catal., A 2004, 273, 35. (25) Mathew, T.; Tope, B. B.; Shiju, N. R.; Hegde, S. G.; Rao, B. S.; Gopinath, C. S. Phys. Chem. Chem. Phys. 2002, 4, 4260. 2586

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