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International Journal of Photoenergy
Volume 2012, Article ID 638571, 7 pages
http://dx.doi.org/10.1155/2012/638571
Research Article

Influence of the Sol-Gel pH Process and Compact Film on the Efficiency of -Based Dye-Sensitized Solar Cells

1Laboratory of Photochemistry and Energy Conversion (USP), Instituto de Química, Universidade de São Paulo, Avenue Prof. Lineu Prestes 748, 05508-900 São Paulo, SP, Brazil
2Instituto de Química, Universidade Federal de Uberlândia 38400-902 Uberlândia, MG, Brazil
3Departamento de Química, Universidade Federal de Minas Gerais 31270-010 Belo Horizonte, MG, Brazil
4Departamento de Física, Universidade Federal de Minas Gerais 31270-010 Belo Horizonte, MG, Brazil

Received 2 December 2011; Revised 27 January 2012; Accepted 27 January 2012

Academic Editor: Leonardo Palmisano

Copyright © 2012 A. O. T. Patrocinio et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The influence of pH during hydrolysis of titanium(IV) isopropoxide on the morphological and electronic properties of TiO2 nanoparticles prepared by the sol-gel method is investigated and correlated to the photoelectrochemical parameters of dye-sensitized solar cells (DSCs) based on TiO2 films. Nanoparticles prepared under acid pH exhibit smaller particle size and higher surface area, which result in higher dye loadings and better short-circuit current densities than DSCs based on alkaline TiO2-processed films. On the other hand, the product of charge collection and separation quantum yields in films with TiO2 obtained by alkaline hydrolysis is c.a. 27% higher than for the acid TiO2 films. The combination of acid and alkaline TiO2 nanoparticles as mesoporous layer in DSCs results in a synergic effect with overall efficiencies up to 6.3%, which is better than the results found for devices employing one of the nanoparticles separately. These distinct nanoparticles can be also combined by using the layer-by-layer technique (LbL) to prepare compact TiO2 films applied before the mesoporous layer. DSCs employing photoanodes with 30 TiO2 bilayers have shown efficiencies up to 12% higher than the nontreated photoanode ones. These results can be conveniently used to develop optimized synthetic procedures of TiO2 nanoparticles for several dye-sensitized solar cell applications.

1. Introduction

Dye-sensitized solar cells (DSCs) are one of the new photovoltaic technologies capable of directly and efficiently converting solar light to electrical current at reasonable costs [16]. Inspired on natural photosynthesis, sensitization of wide bang-gap semiconductors by selected dyes allows the separation of light harvesting and electron transport functions, which reduces the necessity of high-purity materials to reach reasonable conversion efficiencies [79].

Among oxide semiconductors, titanium dioxide, TiO2, is by far the most commonly used semiconductor in DSCs. It is an inert, nontoxic, cheap, and readily available material with wide band gap ( = 3.2 eV) and high refractive index, which are very suitable properties for solar cell applications [1012]. Several studies have shown that the methodology and the experimental conditions to prepare TiO2 nanoparticles influence their morphologic and electronic properties and, consequently, the overall performance of DSCs [1321].

Hore and coworkers [17] reported the influence of hydrolysis pH of titanium(IV) isopropoxide on the interfacial electron transfer kinetics of TiO2 thin films sensitized by Ru(II) polypyridyl complexes. Their results showed that DSCs with TiO2 nanoparticles synthesized under alkaline conditions exhibit slower interfacial recombination losses and higher open-circuit voltages than solar cells with oxide particles prepared by acid peptization.

In this paper, we provide new insights on the influence of the hydrolysis pH on the overall efficiency of DSCs, based on morphological characterization of TiO2 nanoparticles synthesized under acid or alkaline conditions. Bare and sensitized TiO2 films were analyzed by N2 adsorption-desorption isotherms, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Photoelectrochemical parameters determined for DSCs with nanoparticles obtained in different conditions were rationalized based on their morphological characterization. Additionally, the influence of TiCl4 after treatment on mesoporous oxide surface is investigated, as well as the characteristics of DSCs using, simultaneously, acid and alkaline TiO2 nanoparticles.

2. Experimental

All chemicals were used as received with the exception of 3-methyl-2-oxazolidinone (Aldrich), which was purified by distillation under reduced pressure. The N3 dye, cis-[Ru(dcbH2)2(NCS)2], dcbH2 = 4,4′-dicarboxylic acid-2,2′-bipyridine, was synthesized as previously reported [22].

TiO2 nanoparticles were prepared by the sol-gel method [23] employing acid or alkaline hydrolysis conditions. In acid route, 12 mL of titanium(IV) isopropoxide (Strem, 98%) was slowly added to 70 mL of 0.1 mol L−1 HNO3 aqueous solution under vigorous stirring. In alkaline hydrolysis, the same volume of titanium(IV) isopropoxide was slowly dropped into 70 mL of 0.1 mol L−1 ammonia solution. Both, acid and alkaline, mixtures were left under stirring and heating (80°C) for 8 hours. Aliquots of the resulted sols, named acid and alkaline TiO2, respectively, were used for deposition of layer-by-layer (LbL) compact films. The remaining aliquots were autoclaved for 8 hours at 200°C in a nonstirred titanium pressure vessel (Parr, 4750 Series), concentrated to ~200 mg mL−1, and stabilized with Carbowax 20 M (Supelco) to yield a paste employed for mesoporous TiO2 layer.

TiO2 compact films, used as blocking and contact layers, were deposited onto cleaned FTO substrates (Pilkington, TEC-15, 15 Ω per square) using the LbL technique, as described elsewhere [24]. The substrate was immersed alternately for 5 min in a 10 mg mL−1 suspension of the nonautoclaved acid TiO2 at pH 2 and in a 10 mg mL−1 suspension of the nonautoclaved alkaline TiO2 at pH 8. In this approach, acid and alkaline TiO2 nanoparticles were used as cations and anions, respectively.

Mesoporous TiO2 films were deposited directly over FTO substrates or on the compact film by the painting technique [23]. The films were dried at room temperature and sintered at 450°C for 30 minutes. The sensitization was achieved by immersion of electrodes in an N3 saturated ethanolic solution, and the amount of adsorbed dye was determined spectrophotometrically by desorption with NaOH 10−4 mol L−1 aqueous solution. The surfaces of some TiO2 films were treated by 0.05 mol L−1 TiCl4 solution prior to sensitization, as described elsewhere [25]. Sintered electrodes were immersed into the solution and kept at 70°C for 30 minutes. After treatment, they were washed with water and resintered at 450°C for 30 minutes.

Dye-sensitized solar cells with 0.25 cm2 active area were assembled in a sandwich-type arrangement [23] using the sensitized TiO2 photoanode and a transparent Pt-covered FTO as a counterelectrode. A solution of 0.05 mol L−1 I2/0.5 mol L−1 LiI/0.5 mol L−1 pyridine in 90 : 10 mixture of acetonitrile (Aldrich) and 3-methyl-2-oxazolidinone was used as an electrolyte mediator.

UV-Vis absorption spectra were obtained by using an 8453 UV-Vis spectrophotometer (Hewlett Packard). The size of TiO2 nanoparticles and the morphology of the films were evaluated by field-emission scanning electron microscopy (FESEM) using a JSM 7401F (JEOL) microscope. Morphological parameters such as surface area and porosity were obtained by using nitrogen adsorption-desorption data acquired in an NOVA 2200e surface area analyzer (Quantachrome Instruments). The data were modeled by the Brunauer-Emmett-Teller (BET) equation [26]. XPS analyses of TiO2 films were carried out in an ESCALAB 220ixL spectrometer (VG Scientific) equipped with a hemispherical electron energy analyzer using Mg-Kα radiation (h = 1487 eV). Photoelectron spectra were recorded in constant analyzer energy (CAE) mode. The binding energies were measured in reference to the C-1s peak at 284.6 eV.

Photoelectrochemical characterization of DSCs was carried out by current-potential measurements using a PAR 270 galvanostat/potentiostat (EG&G Instruments) system at simulated AM 1.5 solar radiation (100 mW cm−2) provided by a solar simulator (Newport/Oriel), as previously described [24, 27]. Photocurrent action spectra were obtained by using an Oriel system comprised by a 400 W Xe lamp coupled to a 0.25 m Czerny-Turner monochromator as described earlier [28]. All parameters were determined from the average values measured with at least three individual cells of each type of photoanode.

3. Results and Discussion

3.1. Characterization of TiO2 Nanoparticles

TiO2 nanoparticles synthesized by acid and alkaline routes were analyzed by N2 adsorption-desorption isotherms using the BET method. The main morphological parameters are shown in Table 1.

tab1
Table 1: Morphological parameters determined by the BET method for TiO2 nanoparticles synthesized in different hydrolysis conditions.

TiO2 nanoparticles obtained by acid route showed a larger surface area and slightly higher porosity than the ones obtained at alkaline medium. In fact, acid pH hydrolysis results in a very transparent sol, different from the film obtained by alkaline route, which is opaque due to the presence of larger particles and aggregates. Scanning electron micrographs of TiO2 films deposited over FTO substrates confirm that the alkaline route yields larger nanoparticles, Figure 1. Similar effect was reported by Barbé et al. by changing the pH of hydrothermal treatment of nanoparticles produced after an acid hydrolysis [13].

fig1
Figure 1: Scanning electron micrographs of TiO2 nanoparticles obtained by acid (a) or alkaline (b) routes on FTO.

Typically, acid or alkaline-promoted hydrolysis of Ti(IV) alkoxides occurs as follows [2931]:

Hydrolysis is followed by a condensation step to yield Ti–O–Ti chains as follows:

Continuous hydrolysis/condensation of alkoxyl groups will lead to three-dimensional polymeric Ti–O–Ti chains, which yield TiO2 particles with different shapes and sizes, depending on experimental conditions (temperature, condensing agents, templates, etc.) [19, 29, 32].

In acid promoted hydrolysis, it is expected that the superficial Ti–OH sites will be constantly protonated during peptization, which decreases the condensation rate and results in smaller particles with positive surface charges that avoid aggregation and increase the porosity of the resulted film. On the other hand, in alkaline catalysis, the superficial Ti–OH Ti–O + H+ equilibrium is shifted to the dissociated form, which facilitates the condensation with other Ti–OH or Ti–OR sites to yield larger nanoparticles. Moreover, during the hydrothermal step in alkaline conditions, the Ostwald ripening effect or coarsening is more effective than in acid medium enhancing the particle growth as discussed by Barbé and coworkers [13]. In the coarsening growth mechanism, larger particles are formed at expense of dissolution of small ones [33]. The growth rate is proportional to the particle solubility and solid-liquid interfacial tension [34]. In alkaline medium, the solubility of TiO2 is increased, likely due to the complexation of generated anionic species by ions [13], consequently, the coarsening rate constant is enhanced. The resulted TiO2 nanoparticles are larger than the particles obtained in acid medium, as can be seen in Figure 1.

Both acidic or alkaline processed, TiO2 pastes result in 6 to 8 μm thickness films, with considerable different optical properties. The acid TiO2 films are highly transparent with transmittance of 58% at 500 nm, while the alkaline TiO2 films exhibit only 40% transmittance at 500 nm.

Adsorption of N3 on both TiO2 films was carried out in ethanolic solutions without any other reagent. In this condition, the N3 surface concentration on films with nanoparticles obtained by the alkaline pathway (2.6 × 10−8 mol cm−2) is c.a. 3 times lower than that determined for acid TiO2 nanoparticles (8.6 × 10−8 mol cm−2). Higher surface area as well as concentration of hydroxyl groups on the surface of TiO2 nanoparticles synthesized under acid conditions justifies higher dye loadings.

XPS analyses of sensitized films have shown similar adsorption modes for both acid and alkaline TiO2 films. O-1s and Ru-3d5/2 signals, Figures 2(a) and 2(b), respectively, are very similar in both films, which indicates that the chemical environments around Ru(II) and O2− ions are the same. In Figure 2(a), one can also observe a broadening of the O-1s peak at the higher binding energy side, which is caused by the presence of free and bounded carboxylate groups from N3 species on TiO2 surface, as previously discussed [23].

fig2
Figure 2: O-1s (a) and Ru-3d5/2 (b) peaks of N3-sensitized TiO2 films prepared by acid (—) or alkaline (- - -) routes.
3.2. Photoelectrochemical Behavior of DSCs Employing Acid- or Alkaline-Hydrolyzed TiO2 Nanoparticles

Photoelectrochemical parameters determined for DSCs prepared with acid or alkaline TiO2 nanoparticles are shown in Table 2. Some solar cells were prepared with films further treated with TiCl4 solutions in order to increase the quantum yield of charge separation at TiO2/dye interface, enhancing the photocurrent [25, 35].

tab2
Table 2: Photoelectrochemical parameters determined for DSCs using TiO2 obtained by the acid or alkaline routes. (100 mW cm−2, AM 1.5).

One can observe that DSCs prepared with TiO2 nanoparticles obtained by the acid pathway exhibited conversion efficiencies c.a. 40% higher than the ones using the particles obtained by the alkaline route. Despite their better open-circuit voltages () and fill factor (ff), the solar cells using alkaline TiO2 have shown short-circuit current densities () 50% lower than the acid TiO2-based DSCs, which is the main reason for the difference observed in the overall efficiency.

In DSCs, the short-circuit current () can be defined by (3) [35], in which is the incident photon flux in Coulomb s−1, is the light-harvesting efficiency, is the quantum yield for charge separation at TiO2/dye interface, and is the charge collection efficiency. is dependent on the nature of the sensitizer and its concentration on the surface, whereas and depend basically on the morphological and electronic properties of the semiconductor metal oxide film

The reduction of photocurrent observed for the DSCs employing alkaline TiO2 nanoparticles can be justified by the decrease of , since the amount of N3 adsorbed in this film is three times lower than adsorbed on acid TiO2. In fact, is only dependent on the optical and excited state properties of the selected sensitizer. Thus, it can be expected that the observed photocurrent will be proportional to the N3 concentration in each TiO2 film. However, while the photocurrent of DSCs using the alkaline TiO2 decreases 40% in relation to the ones using acid TiO2 films, the N3 concentration in the different photoanodes varies 70%, that is, the photocurrent is not proportional to the N3 concentration.

These results evidence that TiO2 nanoparticles synthesized under alkaline hydrolysis conditions exhibit better charge separation and charge collection efficiencies. From photoelectrochemical parameters determined in this work, it can be estimated that the product is c.a. 27% higher for alkaline TiO2 in relation to the nanoparticles synthesized in acid conditions. This analysis agrees with previous discussion carried out by Hore and coworkers based on transient absorption data, in which the different dependence of charge recombination rates on applied bias for acid and alkaline TiO2 films evidences that the alkaline hydrolysis results in a decrease of charge recombination rate [17]. Additional evidence for the better electronic properties of TiO2 nanoparticles synthesized by alkaline route is given by the lower dark current observed for the DSCs prepared with their films, as in Figure 3.

638571.fig.003
Figure 3: Dark current-voltage curves of DSCs using TiO2 obtained by the acid (—) or alkaline (- - -) routes.

The surface posttreatment of alkaline TiO2-based films with acidified TiCl4 solutions resulted in an increase of c.a. 7% of adsorbed N3, similarly to what is observed for acid TiO2-based films and to earlier reported works [25, 35, 36]. As expected, the photocurrent observed for DSCs with both treated films increases linearly with the N3 concentration, as in Table 2. Photoaction spectra of DSCs show that the TiCl4 treatment improves the light-to-current conversion efficiency in all visible region, as in Figure 4.

638571.fig.004
Figure 4: Photocurrent action spectra of DSCs prepared with acid-route TiO2 films treated (- -■- -) and nontreated (- -□- -) by TiCl4 solutions.

Photoanodes comprised by an acid TiO2 film deposited over an alkaline TiO2 layer were prepared in an attempt to integrate the better electronic properties of TiO2 synthesized by the alkaline route with the higher dye loading of the acid-hydrolyzed particles. The resulting solar cells have shown intermediate properties in relation to those with individual acid and alkaline TiO2 films and, in this new setup, the maximum overall efficiency achieved 6.3%, as shown in Figure 5.

638571.fig.005
Figure 5: Current-voltage curve for a DSC with a photoanode having an acid TiO2 film deposited over an alkaline TiO2 layer (AM 1.5 illumination; 100 mW cm−2).

Another effective strategy is the use of a compact oxide layer on the FTO surface beneath the mesoporous semiconductor layer to aggregate the desired morphological and electronic properties of the distinct TiO2 nanoparticles. Positive charged nonautoclaved acid TiO2 are attracted by negative charged alkaline ones to yield a compact film over FTO. This procedure leads to self-assembled TiO2 films more stable to the sintering temperature than those obtained using organic polyanions [37]. Thus, the produced films address the two main requisites to be an efficient compact blocking layer in DSCs [37]: low porosity and temperature stability. This compact film is an elegant solution to prevent the physical contact between the electrolyte and the FTO surface, decreasing the charge recombination at this interface and, consequently, improving the conversion efficiency [3840].

A mesoporous layer of acid TiO2 was deposited over the FTO substrate modified by 15 or 30 bilayers of the LbL TiO2 compact film. The use of resulted electrodes as photoanodes in DSCs increases the solar cell overall efficiency by 6% and 12%, respectively. Thus, the compact film produced with acid and alkaline TiO2 nanoparticles effectively avoids the charge recombination at FTO/electrolyte interface and improves the electrical contact of FTO with the mesoporous TiO2 layer [24, 37]. However, these compact films have also led to a decrease of 16% and 23%, respectively, on the transmittance of the FTO substrate. Therefore, when photoelectrochemical data are corrected, the increase on the overall efficiency is, respectively, 26% and 46%.

4. Conclusions

In this work, the influence of pH during hydrolysis of titanium(IV) isopropoxide on the morphologic and electronic properties of TiO2 thin films is discussed and correlated to the solar light-to-current conversion efficiency of DSCs based on such films. TiO2 particles hydrolyzed under alkaline conditions exhibit higher diameters, lower porosity, and lower dye loading than the nanoparticles synthesized by the acid route. On the other hand, when N3-sensitized thin films based on alkaline TiO2 are applied as photoanode in DSCs, one can observe a better charge separation as well as higher charge collection efficiencies, which evidence that electron transport among alkaline TiO2 nanoparticles is more efficient than in acid TiO2-based films.

Based on these experimental observations, two new strategies were proposed in order to improve the DSC efficiency. In a first attempt, DSCs were prepared with a photoanode comprised by acid TiO2 nanoparticles over an alkaline TiO2-based film, and higher dye loadings are achieved as well as high charge separation and collection efficiencies, which result in a better overall conversion efficiency than DSCs using the nanoparticles separately.

This synergic effect is also achieved in compact layers prepared by the LbL technique using acid TiO2 as cations and alkaline TiO2 as anions before the mesoporous layer. The resulted compact film physically prevents the electron recombination at FTO/electrolyte interface and also improves the electrical contact between the conductive substrate and the mesoporous layer. Such a strategy has resulted in an improvement of 12% in the conversion efficiency of solar cells.

Acknowledgments

The authors thank the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the financial support. Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG) is acknowledged for supporting XPS analyses. They also thank Pilkington Glass Company for supplying FTO glasses.

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