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Surfactant influence in the performance of titanium dioxide photoelectrodes for dye-sensitized solar cells
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Solar Energy 91 (2013) 263–272 www.elsevier.com/locate/solener
Surfactant influence in the performance of titanium dioxide photoelectrodes for dye-sensitized solar cells Ana I. Maldonado-Valdivia, Emilio G. Galindo, Marı´a J. Ariza, Marı´a J. Garcı´a-Salinas ⇑ Complex-Fluids Physics Group, Department of Applied Physics, University of Almerı´a, E-04120 Almerı´a, Spain Received 31 July 2012; received in revised form 16 November 2012; accepted 12 February 2013 Available online 22 March 2013 Communicated by: Associate Editor Sam-Shajin Sun
Abstract The semiconductor thin film of dye-sensitized solar cells (DSCs) photoelectrodes is commonly fabricated from a paste containing TiO2 nanoparticles and whatever surfactant as a thickener or stabilizer. The aim of this paper is to study the influence of the type and quantity of the paste surfactant on the photoelectrode film features and cell performance. To this end, identical techniques and materials, except the film paste surfactant, will be used. Seven pastes are prepared with three surfactants in different quantities: Triton X-100Ò (T), polyethylene glycol 2000 (PEG) and ethyl cellulose (EC). The effects on the mesoporous film (film thickness and surface features like evenness, presence of cracks or aggregates, etc.) are studied because they are expected to play a major role in the performance of the DSC. The main finding is that the thickness of the TiO2 film strongly depends on the surfactant type and concentration. Fully operating DSCs have been manufactured using a natural dye as sensitizer and a liquid electrolyte, and their energy conversion efficiencies have been measured in environmental conditions. Higher efficiencies are obtained for thicker TiO2 films, thus showing the influence of the paste surfactant on the cell performance. Although the film surface characteristics are also influenced by the surfactant type and quantity, no clear correlation has been found between surface morphology and cell performance. Ó 2013 Elsevier Ltd. All rights reserved. Keywords: Surfactant; Dye-sensitized solar cell; TiO2 photoelectrode; Film thickness; Efficiency; Natural dye
1. Introduction Nowadays dye-sensitized solar cells (DSCs) (O’Regan and Gra¨tzel, 1991) are still being extensively studied because of their relatively high energy conversion efficiency, simple fabrication process and low production cost (Halme et al., 2010; Ito et al., 2008; Longo and De Paoli, 2003; Nayak et al., 2011). DSCs are sandwich-type electrochemical cells essentially composed by three elements: a photoelectrode, an electrolyte solution, usually containing iodide/triiodide ions as redox couple, and a platinized fluo⇑ Corresponding author. Address: Departamento de Fı´sica Aplicada, Ed. CITE II-A, 2.150, Universidad de Almerı´a, 04120 Almerı´a, Spain. Tel.: +34 950015913; fax: +34 950015477. E-mail address: [email protected] (M.J. Garcı´a-Salinas).
0038-092X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2013.02.009
rine-doped tin oxide conducting glass (FTO) as a counter electrode (Gra¨tzel, 2001, 2003). The photoelectrode is a dye-sensitized mesoporous TiO2 film deposited on a transparent fluorine-doped tin oxide conducting glass, providing a large surface area for the adsorption of the dye molecules and allowing the electrical connection with the redox electrolyte. Regarding dye molecules, synthetic dyes such as Ru complexes have yielded the highest conversion efficiencies (>10%) (Gra¨tzel, 2005), but their preparation normally requires multi-step procedures, use of organic solvents and, in most cases, time consuming chromatographic purification methods. Natural dyes extracted from fruits, vegetable or flowers can be an alternative low cost option despite the low cell efficiencies, around 1% (Calogero et al., 2012; Furukawa et al., 2009). The successful combination of the materials involved in cell fabrication, as well as the
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precise methods used, will determine the final cell performance. In this paper we analyze some of the factors affecting the working photoelectrode. It is well known that porosity and morphology of the mesoporous semiconductor film significantly affect the cell efficiency, so the film fabrication method is crucial to obtain highly efficient cells (Ito et al., 2003; Wang et al., 2004), and the usual way to improve film quality is by adding a suitable surfactant or binder to the precursor paste. The type and content of surfactant is crucial to optimize the properties of the paste, affecting the viscoelastic properties during film formation and topological structure after sintering. Despite the increasing number of papers in this field, only a few works deal with a systematic analysis of the influence of paste surfactant on film morphology. We focus in this work on the effect of the surfactant type and concentration. Three surfactants widely used in the literature are polyethylene glycol (Barbe´ et al., 1997; Furukawa et al., 2009; Lee et al., 2006; Liu et al., 2005; Park and Hong, 2008; Wan et al., 2012), Triton X-100Ò (Burnside et al., 1998; Xu et al., 2011; Zhang et al., 2008a) and ethyl cellulose (Dhungel and Park, 2010; Ito et al., 2006, 2008; Li et al., 2010; Muniz et al., 2011; Wang et al., 2004). These surfactants will be named from now on as PEG, T, and EC respectively. Lee et al. (2006) studied the influence of the surface morphology of TiO2 coating on DSC performance. They added 30 wt.% poly(ethylene glycol), PEG, of two different molecular weights, and found higher film porosity for the higher molecular weight binder. The highest efficiency was for a double-coated cell with both pastes. Other work by Furukawa et al. (2009) found that the efficiency of a cell from a paste with a higher molecular weight PEG binder was twice the efficiency of the cells prepared using a low molecular weight one. However, Liu et al. (2005) concluded that cells fabricated using a paste with no binder, were better than those using 40 wt.% PEG binder. It seems that PEG residual carbon introduces impurity states which trap the photo-generated electrons, and even plant oils have been proposed as substitute binders (Park and Hong, 2008), obtaining films with large surface roughness and pore size comparing with PEG, and thus higher final cell efficiency. The possible influence of Triton X 100Ò, T, surfactant has been also seen in some papers, from the early work by Burnside et al. (1998), in which they showed that the addition of different surfactants had very little effect upon the ordering phenomena or self-organization in TiO2 thin films, to the recently published work by Xu et al. (2011). In the latter, the quantities of ethanol, acetyl acetone and Triton X 100Ò in the paste were varied and the cross-influence in the DSC efficiency was described. The authors concluded that the efficiency improved with increasing amount of acetyl acetone or Triton X 100Ò. The thickness of the films tested ranged from 6 to 14 lm, but no correlation was established with the amount of surfactant. Zhang et al. (2008a) also studied the influence of added T surfac-
tant and found good performance solar cell for the paste containing 0.8 ml Triton X-100 and 3 g TiO2 P25. Another commonly used surfactant is ethyl cellulose (EC), due to its physical properties that make it an ideal temporary binder in pastes for DSC. Recently, Dhungel and Park (2010) varied the proportion of EC in the TiO2 paste and found an optimal EC percentage which gave the highest efficient DSC. Higher EC content resulted in poor film adhesion and lower film density. The EC influence has been also studied in ZnO pastes (Li et al., 2010), finding that a 10 wt.% in the paste reduces micro-cracks in the films. Despite the works mentioned above, the influence of the surfactant on the film structure is still an open subject. In this work, TiO2 photoelectrodes are prepared using three non-ionic surfactants in different quantities as paste stabilizers and thickeners: Triton X-100Ò (T), polyethylene glycol (PEG), and ethyl cellulose (EC). Photoelectrode films and DSCs are made using identical techniques and materials except the film paste surfactant, in order to study itseffects on the cell performance. The correlation among surface morphology of the mesoporous film (evenness, aggregates and cracks), film thickness and photochemical parameters of the concomitant DSCs using a liquid electrolyte and a natural dye is investigated. Despite low efficiencies are expected with the natural dye, it is a low-cost easy-made option and efficiency results can enrich data base of natural dyed solar cells. Some N719-sensitized standard cells are also prepared and measured to check the whole DSC fabrication process. 2. Materials and methods 2.1. Materials Nanocrystalline TiO2 (P-25, Degussa), nitric acid (65%, Panreac), H2PtCl6 (Aldrich), Ethanol absolute (J.T. Baker), Iodine (99.9%, Aldrich), LiI (99.99%, Aldrich), Triton X-100 (Merck), polyethylene glycol 2000 (Aldrich), ethyl cellulose (Aldrich), 4-tert-butylpyridine (96%, Aldrich) and 3-methoxypropionitrile (P98%, Aldrich) and other chemicals were used as received. N719 (Aldrich) was diluted in ethanol to 3 10 4 M. Conducting glass plates (FTO) (F-doped SnO2, sheet resistance 11–13 O/sq, Nippon Sheet Glass) were used as substrates for fabricating the TiO2 mesoporous films. H2O was purified by a membrane system from Atapa up to <3 lS/cm. 2.2. Measurements and methods 2.2.1. FTO electrical resistance The Van der Pauw method (see Price, 1972; Ramadan et al., 1994 and references therein) was applied to obtain the sheet resistance, RS, of the FTO conducting glass substrates in order to check the effects of cleaning and thermal treatments on them. The values obtained were Rs = (12.57 + 0.24) X/sq for as received FTOs,
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Rs = (13.49 + 0.05) X/sq for 70% isopropanol washed FTOs and Rs = (13.47 + 0.07) X/sq for FTOs washed and activated by a thermal treatment (420 °C for 30 min). Therefore, the washing and activation treatments slightly increase the sheet resistance just over the manufacturer’s nominal values. This ensures that transparent electrodes based on FTO are adequate for application in DSC, because calcination does not increase the glass-FTO resistivity or the sheet resistance, which could cause an increase in the series resistance in the DSC, decreasing its performance (Longo and De Paoli, 2003).
2.2.2. Natural dye extraction In order to obtain the natural dye, local bougainvillea red leaves were harvested, dried, crushed and stored in dark. Two different solvent solutions, 0.1 M HCl in ethanol and a 31.8 vol.% acetone aqueous solution, were used to extract the colored molecules and the resulting extract was tested as dye. Two grams of crushed leaves were stirred at 800 rpm in 45 ml of the solvent during 24 h and then centrifuged to remove any solid residue. If properly stored, protected from direct sunlight and refrigerated at about 4 °C, the natural dye solutions are stable for several months. The coloration of the extract is due to the combination of two main betalain dyes: betacyanins (red–purple) an indicaxanthin (yellow–orange), both having carboxylic groups facilitating TiO2 surface binding (Oprea et al., 2012; Zhang et al., 2008b). The dye UV–VIS absorption spectra were recorded using a high resolution spectrophotometer HR4000 (Ocean Optics). Fig. 1 compares the absorption spectra of three dyes: HCl-extracted natural dye, acetone-extracted natural dye and the N719 ruthenium complex dye. As can be observed, the absorption spectra of both extracted dyes are similar in the UV region, showing an intense peak broader than that of the N719 dye. In the visible range, the HCl-extract shows less absorption without peaks than
Fig. 1. UV–VIS absorption spectra of bougainvillea dyes extracted with the solutions: acetone–water (red dashed–dotted line) and HCl–ethanol (blue dashed line); N719 ruthenium complex dye spectrum (green solid line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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ruthenium complex – N719. However, the absorption spectrum of the acetone-extracted natural dye shows absorption peaks in the visible range as intense as N719. These peaks correspond to betacyanins-type pigments, which exhibit absorption maxima around 535 nm, and indicaxanthin dye, with a maximum absorption for 482 nm (Zhang et al. (2008b)). Therefore, the acetone-extracted natural dye was selected to fabricate the DSCs. 2.2.3. Photoelectrode fabrication and DSC assembly Seven TiO2 nanoparticles pastes were prepared as detailed in Table 1. TiO2 nanoparticles (15 wt.%) were stirred in ethanol and pH 3–4 diluted nitric acid for 12 h at room temperature to obtain a paste with 150 g of solid per solvent liter. The surfactant was added and the paste was stirred 12 h more. Following the procedure proposed by Ito and coworkers (Ito et al., 2003, 2008), the edges of a transparent conducting glass substrate (FTO) were covered with a 50 lm thickness adhesive tape (Scotch) as a masking material, leaving a 0.49 cm2 square free surface. A drop of the paste was poured into one edge of the substrate and flattened with a glass rod by sliding over the tape-covered edges as in a doctor-blade technique. The spread layer was dried for 1 h at room temperature in ethanol atmosphere and then calcined at 450 °C for 1 h. The morphology and thickness of the mesoporous TiO2 layers were characterized by optical (Leyca DM IRB) and scanning electron microscopy (SEM; Hitachi, S-3500). SEM images were analyzed with commercial image processing software. To adsorb the dye onto the TiO2, the mesoporous TiO2 film was placed upwards in a Petri dish and the dye solution was added ensuring a full coverage of the TiO2 area. This was kept in dark at room temperature for 18–24 h and finally water washed to eliminate the non-adsorbed dye. The counter electrode was platinized by coating with a drop of H2PtCl6 solution (0.01 M in ethanol) on the FTO glass and calcined in air at 380 °C for 20 min. Then, the Pt-counter electrode was placed on the top of the dye-sensitized TiO2 film. The gap between the two electrodes was sealed by a thermal adhesive film (Surlyn, Dupont). Finally, the electrolyte was introduced in the gap by capillary action from a hole made on the counter electrode, which was sealed by the same thermal adhesive film. The liquid electrolyte is a widely used charge mediator: the iodide/triiodide redox couple (Longo and De Paoli, 2003), in this case prepared as follows: 0.05 M I2, 0.5 M LiI, and 0.5 M 4-tert-butylpyridine in 3-methoxypropionitrile. The active electrode area of DSC was 0.36 cm2. At least three cells of each paste type were performed to confirm the reproducibility of the results. These cells will be referenced from now on with the name of the TiO2 paste used in the corresponding photoelectrode. For comparison, an ethanol N719 solution was used to make a reference cell (named RC) using the thicker film EC-2.
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Table 1 Batch composition of the TiO2 pastes used in this work. Sample* a b c d e f g *
EC-1 EC-2 PEG–EC PEG-1 PEG-2 PEG-3 T
wt.% Water diluted HNO3 (pH 3–4)
wt.% Ethanol
Surfactant
24 19 19 24 19 19 24
61 63.75 63.75 61 65.25 63.75 61
<0.06 wt.% EC 2.25 wt.% EC 0.56 wt.% of EC + 1.69 wt.% of PEG <0.02 wt.% of PEG 0.75 wt.% of PEG 2.25 wt.% of PEG <0.2 wt.% of Triton X-100
All the pastes were prepared with 15 wt.% of TiO2 nanoparticles.
2.2.4. Cell performance Photocurrent–voltage measurements of DSC (I–V curves) were carried out under direct sun radiation (summer, shiny days, noon time), working with similar light irradiation in all tests: 800 W/m2 (admitting 5% error tolerance). I–V curves of the reference cell were recorded in winter with a solar irradiation of 500 W/m2. A calibrated high performance thermopile-based pyranometer (kipp & Zonnen, CMP 11) was used as a reference. Digital source meters were used to measure the I–V curves without any external bias. At least three cells fabricated using photoelectrodes from each paste were measured to check the reproducibility and the results averaged. 3. Results and discussion 3.1. Morphology and thickness of the TiO2 thin film The mesoporous electrodes were firstly examined by optical microscopy (10 and 40 magnification objectives) to obtain a general vision of the main features and irregularities of the TiO2 films. A more detailed exam was carried out using scanning electron microscopy. Fig. 2 shows some of the surface SEM images (500) taken for each film type. Cracks open to the surface (dark lines in the images) clearly appear, and with different appearance, in all the samples containing PEG and in EC-1, while EC-2 and T samples are free of open cracks (see Fig. 2h at higher magnification). On the other hand, PEG-2 seems the roughest film, with small cracks, while EC-2 shows a grainy surface. Therefore, there is a strong surfactant influence on the film microstructure. Image processing software was applied to obtain quantitative indicators of these surface features: open cracks, surface evenness, and presence of aggregates and their size distribution. Although SEM images have no information of the height (roughness), these indicators will facilitate comparison between samples. The cracks percentage (see Table 2) is obtained after thresholding, filtering and analyzing histograms to “count” the dark pixels in the 500 images, and gives the percentage of pixels belonging to cracks in the whole sample. Fig. 3b shows an example of one processed image. A similar procedure is used to select pixels with gray levels clearer than the average (Fig. 3c), therefore belonging to irregularities in the surface (aggre-
gates, grainy or bumpy areas, crinkles). From both procedures, a final percentage of pixels is obtained indicating the absence of surface irregularities, named surface homogeneity (Homo in Table 2). Finally, the presence, size and number of aggregates were also studied in a set of 100 images, and the number of aggregates per square millimeter is also listed in Table 2. As can be also observed in Fig. 2, the %cracks quantitative indicator shows that EC-1 has the highest cracks percentage value. However, EC-2 sample is free of surface cracks but slightly grainy, which shows the significant influence of the surfactant concentration on the paste preparation. When some EC is substituted by PEG the cracks become visible again in the PEG–EC sample, but they are less and smaller than in EC1 sample. In the PEG series, the cracks percentages also decrease when increasing the PEG content. The T sample is free of cracks. Films may shrink with evaporation of the solvent, resulting in the typical formation of cracks during the drying process of nanoparticles pastes. Since the drying conditions were systematically the same in all samples, the differences must be attributed to the type and concentration of the surfactant in the TiO2 paste. Regarding particle aggregates, very few were detected in the T sample. The highest number of aggregates is found for PEG–EC samples, probably due to PEG addition, because samples with this additive present more aggregates when increasing PEG content. The surfaces are homogeneous in general, giving T the highest indicator, and PEG 2 and PEG 3 the lowest but with very small differences, thus confirming the visual appearance. As a first conclusion, regarding film morphology, the features studied depend more on the quantity, than on the type of surfactant. Triton surfactant seems to be a priori a good option as a binder, because the film has a smooth surface without granular features nor cracks and the lowest number of particle clusters, showing the highest surface homogeneity. The influence of the surfactant on the film thickness is shown in Fig. 4. The thickness was measured on 10,000 SEM images of film cross-section using commercial software. About 40 measures were taken of each sample, using several images and applying statistical criteria to obtain the thickness values listed in Table 2. A main result can be con-
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(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
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T
5um EC-2
100um
Fig. 2. SEM images at 500 of the surface of TiO2 films prepared with different surfactants in the paste: (a) EC-1, (b) EC-2, (c) PEG–EC, (d) PEG-1, (e) PEG-2, (f) PEG-3, (g) T, (h) SEM images at 10,000 of T (top) and EC-2 (bottom) films.
cluded: the thickness of the TiO2 layer is clearly related to the type and quantity of the surfactant used in the preparation of the paste. Pastes containing EC (surfactant with a more complex molecular structure) produce higher values of film thickness, while pastes with PEG (simpler molecular structure) give the thinner films. The inner structure of the TiO2 layer is also observed in the film cross-section (Fig. 4). The texture of films obtained using PEG additive is smoother and more homogeneous than that of EC films,
which seem more porous, and internal porosity is clear especially in EC-2 sample. The paste containing both PEG and EC additives gives a double-layered film structure on the glass substrate with an smother top layer (like PEG samples) and a rougher bottom layer (like EC samples). The T sample looks similar to the EC-1 sample. As a second conclusion, regarding film thickness, varying the surfactant the thickness can be widely changed (from 1 to 6 lm). Previous works have used multilayer
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Table 2 Morphological parameters of the TiO2 films: cracks percentage and surface homogeneity measured on 500 processed images, number of aggregates measured on 100 images, and thickness measured on 10,000 SEM images. Sample
%Cracks
%Homo
Clusters/mm2
Thickness (lm)
EC-1 EC-2 PEG–EC PEG-1 PEG-2 PEG-3 T
6.0 ± 0.3 No cracks 3.3 ± 0.3 4.0 ± 0.6 2.21 ± 0.15 1.30 ± 0.14 No cracks
92.2 ± 0.7 98.3 ± 0.1 94.7 ± 0.7 94.6 ± 0.8 95.7 ± 0.4 96.5 ± 0.5 99.3 ± 0.1
94 ± 4 88 ± 5 129 ± 6 71 ± 3 97 ± 2 100 ± 1 33 ± 3
3.920 ± 0.019 6.18 ± 0.04 2.986 ± 0.016 1.370 ± 0.018 1.106 ± 0.013 2.06 ± 0.04 2.05 ± 0.13
deposition to increase film thickness (Ito et al., 2003; Lee et al., 2006), however, the results shown in this work open a new way to control this parameter. Among the pastes used in this work, EC seems to be the best option for paste formulation, as the films obtained are more porous and thicker than the others. In any case, all the results in this section point out that the morphology and thickness of the TiO2 film strongly depend on the type and quantity of surfactant in the initial paste. The question is whether they influence the cell performance. 3.2. DSC characterization Fig. 5 shows average I–V curves of the natural dyed solar cells fabricated with each one of the proposed TiO2 pastes. The characteristic electrical parameters (open circuit voltage, VOC; short circuit current, ISC; fill factor,
FF; and efficiency) obtained averaging measures from at least three DSCs, are shown in Table 3. The last row labeled RC (Reference Cell) shows results obtained for cells made as EC-2, the thickest film and most efficient cell, but using N719 dye. Comparing EC-2 and RC cells, N719 dye improves efficiency more than 40 times, reaching values close to 10%, as expected according to bibliography for Ru-complex dyes (Wang et al., 2004; Ito et al., 2008). Therefore our whole DSC fabrication process has been validated. Since our objective is to study the effect of the surfactant of the coating paste on the performance of the DSC photoelectrodes and not to improve cell efficiency by additional procedures (Go´mez-Ortı´z et al., 2010; Hernandez-martinez et al., 2011; Ito et al., 2008, 2009; Kashyout et al., 2010) we selected the cheaper and less tested natural dye. Values of the conversion efficiency with the natural dye are very low, but they are consistent with previous works by other authors using natural dyes. Efficiency values about 0.03–0.57% have been reported (Zhou et al., 2011) using different natural dyes and extraction methods. More to the point, not pre-treated titania-based DSC with betalain pigments gave efficiencies between 0.05% and 0.19% (Zhang et al., 2008b) while DSC with betalain pigments extracted from bougainvillea with different purification processes, have given values from 0.31% to 0.49% for the efficiency (Hernandez-martinez et al., 2011). Additional information can be found in a recent review of natural photosensitizers for DSC (Narayan, 2012). Comparing the electrical parameters of the different cells, fill factor values were similar in all cases, being higher
Fig. 3. Original SEM (a) and filtered images to count very dark pixels in cracks (b) and very clear pixels for aggregates and wrapping (c). The cracks percentage is calculated from (b). The surface homogeneity is calculated from (b + c).
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Fig. 4. SEM images at 10,000 x magnification of the cross-section of (a) EC-1, (b) EC-2, (c) PEG–EC, (d) PEG-1, (e) PEG-2, (f) PEG-3 and (g) T TiO2 films.
in EC-2 and T samples and lower in all PEG samples. Regarding VOC and ISC, again the use of PEG additive gives lower values. This leads to the same trend in the cell performance, and so DSCs made with EC pastes presented the highest values of efficiency, in contrast with DSC from
PEG samples that presented the smallest efficiencies. DSC from T samples gave intermediate efficiencies. Interestingly, the highest efficiency is obtained for the DSCs prepared with the film paste using EC in 2.25 wt.%. This is the same percentage that gave the max-
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Fig. 5. I–V curves for each one of the DSC manufactured with photoelectrodes made from each paste.
Table 3 Electrical parameters of the DSC fabricated with each one of the TiO2 photoelectrodes. RC stand for the N719 reference cell (see text). Sample
VOC (mV)
ISC (mA)
FF
Efficiency (%)
EC-1 EC-2 PEG–EC PEG-1 PEG-2 PEG-3 T RC
527 ± 1 550 ± 1 544 ± 1 361 ± 1 499 ± 1 483 ± 1 535 ± 1 811 ± 17
0.213 ± 0.001 0.191 ± 0.001 0.170 ± 0.001 0.056 ± 0.001 0.070 ± 0.001 0.087 ± 0.001 0.121 ± 0.001 3.8 ± 0.3
0.517 ± 0.005 0.6100 ± 0.0017 0.515 ± 0.004 0.483 ± 0.015 0.489 ± 0.013 0.468 ± 0.008 0.547 ± 0.009 0.50 ± 0.05
0.206 ± 0.017 0.228 ± 0.011 0.170 ± 0.020 0.035 ± 0.009 0.061 ± 0.022 0.070 ± 0.003 0.126 ± 0.001 8.9 ± 0.9
Fig. 6. Energy conversion efficiency versus TiO2 film thickness. Dash line is to guide the eye.
imum efficiency in the work by Dhungel and Park (2010), although both pastes do not have exactly the same composition. They claim that this amount of EC binder provides optimal film adhesion to FTO and porosity after sintering. The photoelectrodes fabricated from EC pastes were thicker and more porous (as seen by SEM) than those from PEG pastes. Thicker and more porous TiO2 films enhance dye absorption and thus the light-harvesting capability of the photoelectrode. Note that we modify the film thickness and porosity only by the addition of a suitable surfactant in the coating paste.
To correlate the electrical and morphological parameters, Fig. 6 plots the cell conversion efficiency versus the TiO2 film thickness. The dashed line is a guide to the eye. The efficiency increases with increasing thickness, as has been previously reported (Ito et al., 2008, 2009). The thickness differences are very significant (from about 1 to 6 lm) and they probably mask the effect of mesoporosity and cracks of the TiO2 film on the cell performance. However, PEG-3 and T samples have the same thickness and the cell efficiency corresponding to the former is about a half of the efficiency of the latter, so the influence of film morphology can be checked. The T TiO2 film had no cracks, with a highly homogeneous surface quite free of particles aggregates, and the internal texture of the film (cross-section) looks similar to the EC samples. In contrast, the PEG-3 sample showed a small percentage of cracks open to the surface, three times more aggregates than the T sample and a smoother texture in the cross-section, which suggest a less porous structure than the T sample. A higher porosity increases the photoelectrode specific surface and should enhance electrolyte and dye diffusivity into the TiO2 nanostructure. These other aspects of surfactant influence on the inner structure of the film (porosity and pore size, number and type of particle contacts, diffusion of the dye and ionic species, etc.) and then on the cell performance, are a rather complex subject that needs to be dealt with in another work focused on the fabrication of films with similar thickness from pastes with all the same components except surfactant. On the other hand, EC-1 and EC-2 samples have different thickness (4 and 6 lm, respectively) but the corresponding cells have very similar efficiencies (0.21 and 0.23, respectively). Both thicknesses are below reported thickness for maxima efficiency: about 15 lm (Ito et al., 2009), or about 20–25 lm (Galindo et al., in preparation). Therefore, following the upward trend of efficiency in T, PEG– EC and EC-1 sample (Fig. 6), it would be expected a higher increase of the cell efficiency for the EC-2 sample. The fact that this higher efficiency is not reached could be explained in two different ways. First, the maximum in efficiency is between 4 and 6 lm for the experimental conditions used in this work, below the values in the literature. Or second, the film surface morphology counterbalances the increase in thickness. The differences in surface features for EC-1 (6% cracks, 92% surface homogeneity), and EC-2 (0% cracks, 98% surface homogeneity), samples suggest that open-to-the surface cracks on the TiO2 films like that showed in Fig. 2a increase the film external surface and might facilitate dye absorption during the cell fabrication process and electrolyte diffusion to regenerate the dye. The absence of this enhanced available surface for EC-2 sample may decrease the expected cell performance. 4. Conclusions In this work, TiO2 mesoporous semiconductor thin films for dye-sensitized solar cells photoelectrodes have been pre-
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pared from pastes with different surfactants, analyzed, and used in fully operating cells with a natural dye as sensitizer and a liquid electrolyte. In order to detect only the influence of the surfactant, identical techniques and materials, except the paste surfactant, have been used. The type and quantity of surfactant in the TiO2 nanoparticles pastes used to prepare the films determine the film thickness and morphology (cross-section appearance, surface homogeneity, open-to-the surface cracks and particle clusters). Using ethyl cellulose, more porous and thicker films are obtained, which are better candidates to fabricate the solar cell. The Triton X-100 surfactant, when used in similar weight percentage, produces also good quality films but thinner than the EC. The energy conversion efficiencies of the cells have been measured in environmental conditions, finding higher efficiencies for thicker TiO2 films, showing the influence of the paste surfactant on the cell performance. Although the efficiency depends mainly on the TiO2 film thickness, we have also found some signs of the influence of the film morphology. However, no clear correlation has been found between these other film features and cell performance, except the possible contribution of the open-to-the-surface cracks to the increase in available surface area and efficiency enhancement. Acknowledgements This work was supported by the Junta de Andalucı´a and Feder under Project P09-FQM-4938. References Barbe´, C.J., Arendse, F., Comte, P., Jirousek, M., Lenzmann, F., Shklover, V., Gra¨tzel, M., 1997. Nanocrystalline titanium oxide electrodes for photovoltaic applications. J. Am. Ceram. Soc. 80 (12), 3157–3171. Burnside, S.D., Shklover, V., Barbe´, C., Comte, P., Arendse, F., Brooks, K., Gra¨tzel, M., 1998. Self-organization of TiO2 nanoparticles in thin films. Chem. Mater. 10, 2419–2425. Calogero, G., Yumb, J., Sinopoli, A., Di Marco, G., Gra¨tzel, M., Nazeeruddin, M.K., 2012. Anthocyanins and betalains as lightharvesting pigments for dye-sensitized solar cells. Sol. Energy 86, 1563–1575. Dhungel, S.K., Park, J.G., 2010. Optimization of paste formulation for TiO2 nanoparticles with wide range of size distribution for its application in dye sensitized solar cells. Renew. Energy 35, 2776–2780. Furukawa, S., Iino, H., Iwamoto, T., Kukita, K., Yamauchi, S., 2009. Characteristics of dye-sensitized solar cells using natural dye. Thin Solid Films 518, 526–529. Galindo, E.G., Garcı´a-Salinas, M.J., Ariza, M.J., in preparation. Effect of the multilayer coating on the photoelectrode morphology for dye solar cells. Go´mez-Ortı´z, N.M., Va´zquez-Maldonado, I.A., Pe´rez-espadas, A.R., Mena-Rejo´n, G.J., Azamar-Barrios, J.A., Oskam, G., 2010. Dyesensitized solar cells with natural dyes extracted from achicote seeds. Sol. Energy Mater. Sol. Cells 94, 40–44. Gra¨tzel, M., 2001. Photoelectrochemical cells. Nature 414, 338–344. Gra¨tzel, M., 2003. Dye-sensitized solar cells. J. Photo. Photobiol. 4, 145– 153.
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Gra¨tzel, M., 2005. Solar energy conversion by dye-sensitized photovoltaic cells. Inorg. Chem. 44 (20), 6841–6851. Halme, J., Vahermaa, P., Miettunen, K., Lund, P., 2010. Device physics of dye solar cells. Adv. Mater. 22, E210–E234. Hernandez-Martinez, A.R., Estevez, M., Vargas, S., Quintanilla, F., Rodriguez, R., 2011. New dye-sensitized solar cells obtained from extracted bracts of bougainvillea glabra and spectabilis betalain pigments by different purification processes. Int. J. Mol. Sci. 12, 5565–5576. Ito, S., Kitamura, T., Wada, Y., Yanagida, S., 2003. Facile fabrication of mesoporous TiO2 electrodes for dye solar cells: chemical modification and repetitive coating. Sol. Energy Mater. Solar Cells 76, 3–13. Ito, S., Zakeeruddin, S.M., Humphry-Baker, R., Liska, P., Charvet, R., Comte, P., Nazeeruddin, M.K., Pe´chy, P., Takata, M., Miura, H., Uchida, S., Gra¨tzel, M., 2006. High-efficiency organic-dye-sensitized solar cells controlled by nanocrystalline-TiO2 electrode thickness. Adv. Mater. 18, 1202–1205. Ito, S., Murakami, T.N., Comte, P., Liska, P., Gra¨tzel, C., Nazeeruddin, M.K., Gra¨tzel, M., 2008. Fabrication of thin film dye sensitized solar cells with solar to electric power conversion efficiency over 10%. Thin Solid Films 516, 4613–4619. Ito, S., Nazeeruddin, M.K., Zakeeruddin, S.M., Pe´chy, P., Comte, P., Gra¨tzel, M., Mizuno, T., Tanaka, A., Koyanagi, T., 2009. Study of dye-sensitized solar cells by scanning electron micrograph observation and thickness optimization of porous TiO2 electrodes. Int. J. Photoenergy (Article ID 517609). Kashyout, A.B., Soliman, M., Fathy, M., 2010. Effect of preparation parameters on the properties of TiO2 nanoparticles for dye sensitized solar cells. Renew. Energy 35, 2914–2920. Lee, K., Suryanarayanan, V., Ho, K., 2006. The influence of surface morphology of TiO2 coating on the performance of dye-sensitized solar cells. Sol. Energy Mater. Solar Cells 90, 2398–2404. Li, H., Xie, Z., Zhang, Y., Wang, J., 2010. The effects of ethyl cellulose on PV performance of DSSC made of nanostructured ZnO pastes. Thin Solid Films 518, e68–e71. Liu, Z., Pan, K., Liu, M., Zhang, Q., Li, J., Liu, Y., Lu¨, Q., Li, J., Wang, D., Bai, Y., Li, T., 2005. Influence of the binder on the electron transport in the dye-sensitized TiO2 electrode. Thin Solid Films 484, 346–351. Longo, C., De Paoli, M.A., 2003. Dye-sensitized solar cells: a successful combination of materials. J. Braz. Chem. Soc. 14 (6), 889–901. Muniz, E.C., Go´es, M.S., Silva, J.J., Varela, J.A., Joanni, E., Parra, R., Bueno, P.R., 2011. Synthesis and characterization of mesoporous TiO2 nanostructured films prepared by a modified sol–gel method for application in dye solar cells. Ceram. Int. 37, 1017–1024. Narayan, M.R., 2012. Dye sensitized solar cells based on natural photosensitizers. Renew. Sust. Energy Rev. 16, 208–215. Nayak, P.B., Bisquert, J., Cahen, D., 2011. Assessing possibilities and limits for solar cells. Adv. Mater. 23, 2870–2876. Oprea, C.I., Dumbrava, A., Enache, I., Georgescu, A., Girtu, M.A., 2012. A combined experimental and theoretical study of natural betalain pigments used in dye-sensitized solar cells. J. Photochem. Photobiol. A: Chem. 240, 5–13. O’Regan, B., Gra¨tzel, M., 1991. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 353, 737–740. Park, K.H., Hong, C.K., 2008. Morphology and photoelectrochemical properties of TiO2 electrodes prepared using functionalized plant oil binders. Electrochem. Commun. 10, 1187–1190. Price, W.L.V., 1972. Extension of van der Pauw’s theorem for measuring specific resistivity in discs of arbitrary shape to anisotropic media. J. Phys. D: Appl. Phys. 5, 1127–1132. Ramadan, A.A., Gould, R.D., Ashour, A., 1994. On the Van der Pauw method of resistivity measurements. Thin Solid Films 239, 272–275. Wan, J., Lei, Y., Zhang, Y., Leng, Y., Liu, J., 2012. Study on TiO2 photoelectrode to improve the overall performance of dye-sensitized solar cells. Electrochim. Acta 59, 75–80. Wang, Z., Kawauchi, H., Kashima, T., Arakawa, H., 2004. Significant influence of TiO2 Photoelectrode morphology on the energy conver-
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sion efficiency of N719 dye-sensitized solar cell. Coordin. Chem. Rev. 248, 1381–1389. Xu, S., Zhou, C., Yang, Y., Hu, H., Sebo, B., Chen, B., Tai, Q., Zhao, X., 2011. Effects of ethanol on optimizing porous films of dye-sensitized solar cells. Energy Fuels 25, 1168–1172. Zhang, Y., Cai, N., Zhao, Y., Zhao, D.-W., Liu, G.-L., Ji, W.-W., Yang, R.-X., 2008a. Influence of triton X-100 on the performance of dyesensitized solar cell. Yingxiang Kexue yu Guanghuaxue/Imaging Sci. Photochem. 26, 125–130.
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Solar Energy 91 (2013) 263–272 www.elsevier.com/locate/solener
Surfactant influence in the performance of titanium dioxide photoelectrodes for dye-sensitized solar cells Ana I. Maldonado-Valdivia, Emilio G. Galindo, Marı´a J. Ariza, Marı´a J. Garcı´a-Salinas ⇑ Complex-Fluids Physics Group, Department of Applied Physics, University of Almerı´a, E-04120 Almerı´a, Spain Received 31 July 2012; received in revised form 16 November 2012; accepted 12 February 2013 Available online 22 March 2013 Communicated by: Associate Editor Sam-Shajin Sun
Abstract The semiconductor thin film of dye-sensitized solar cells (DSCs) photoelectrodes is commonly fabricated from a paste containing TiO2 nanoparticles and whatever surfactant as a thickener or stabilizer. The aim of this paper is to study the influence of the type and quantity of the paste surfactant on the photoelectrode film features and cell performance. To this end, identical techniques and materials, except the film paste surfactant, will be used. Seven pastes are prepared with three surfactants in different quantities: Triton X-100Ò (T), polyethylene glycol 2000 (PEG) and ethyl cellulose (EC). The effects on the mesoporous film (film thickness and surface features like evenness, presence of cracks or aggregates, etc.) are studied because they are expected to play a major role in the performance of the DSC. The main finding is that the thickness of the TiO2 film strongly depends on the surfactant type and concentration. Fully operating DSCs have been manufactured using a natural dye as sensitizer and a liquid electrolyte, and their energy conversion efficiencies have been measured in environmental conditions. Higher efficiencies are obtained for thicker TiO2 films, thus showing the influence of the paste surfactant on the cell performance. Although the film surface characteristics are also influenced by the surfactant type and quantity, no clear correlation has been found between surface morphology and cell performance. Ó 2013 Elsevier Ltd. All rights reserved. Keywords: Surfactant; Dye-sensitized solar cell; TiO2 photoelectrode; Film thickness; Efficiency; Natural dye
1. Introduction Nowadays dye-sensitized solar cells (DSCs) (O’Regan and Gra¨tzel, 1991) are still being extensively studied because of their relatively high energy conversion efficiency, simple fabrication process and low production cost (Halme et al., 2010; Ito et al., 2008; Longo and De Paoli, 2003; Nayak et al., 2011). DSCs are sandwich-type electrochemical cells essentially composed by three elements: a photoelectrode, an electrolyte solution, usually containing iodide/triiodide ions as redox couple, and a platinized fluo⇑ Corresponding author. Address: Departamento de Fı´sica Aplicada, Ed. CITE II-A, 2.150, Universidad de Almerı´a, 04120 Almerı´a, Spain. Tel.: +34 950015913; fax: +34 950015477. E-mail address: [email protected] (M.J. Garcı´a-Salinas).
0038-092X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2013.02.009
rine-doped tin oxide conducting glass (FTO) as a counter electrode (Gra¨tzel, 2001, 2003). The photoelectrode is a dye-sensitized mesoporous TiO2 film deposited on a transparent fluorine-doped tin oxide conducting glass, providing a large surface area for the adsorption of the dye molecules and allowing the electrical connection with the redox electrolyte. Regarding dye molecules, synthetic dyes such as Ru complexes have yielded the highest conversion efficiencies (>10%) (Gra¨tzel, 2005), but their preparation normally requires multi-step procedures, use of organic solvents and, in most cases, time consuming chromatographic purification methods. Natural dyes extracted from fruits, vegetable or flowers can be an alternative low cost option despite the low cell efficiencies, around 1% (Calogero et al., 2012; Furukawa et al., 2009). The successful combination of the materials involved in cell fabrication, as well as the
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precise methods used, will determine the final cell performance. In this paper we analyze some of the factors affecting the working photoelectrode. It is well known that porosity and morphology of the mesoporous semiconductor film significantly affect the cell efficiency, so the film fabrication method is crucial to obtain highly efficient cells (Ito et al., 2003; Wang et al., 2004), and the usual way to improve film quality is by adding a suitable surfactant or binder to the precursor paste. The type and content of surfactant is crucial to optimize the properties of the paste, affecting the viscoelastic properties during film formation and topological structure after sintering. Despite the increasing number of papers in this field, only a few works deal with a systematic analysis of the influence of paste surfactant on film morphology. We focus in this work on the effect of the surfactant type and concentration. Three surfactants widely used in the literature are polyethylene glycol (Barbe´ et al., 1997; Furukawa et al., 2009; Lee et al., 2006; Liu et al., 2005; Park and Hong, 2008; Wan et al., 2012), Triton X-100Ò (Burnside et al., 1998; Xu et al., 2011; Zhang et al., 2008a) and ethyl cellulose (Dhungel and Park, 2010; Ito et al., 2006, 2008; Li et al., 2010; Muniz et al., 2011; Wang et al., 2004). These surfactants will be named from now on as PEG, T, and EC respectively. Lee et al. (2006) studied the influence of the surface morphology of TiO2 coating on DSC performance. They added 30 wt.% poly(ethylene glycol), PEG, of two different molecular weights, and found higher film porosity for the higher molecular weight binder. The highest efficiency was for a double-coated cell with both pastes. Other work by Furukawa et al. (2009) found that the efficiency of a cell from a paste with a higher molecular weight PEG binder was twice the efficiency of the cells prepared using a low molecular weight one. However, Liu et al. (2005) concluded that cells fabricated using a paste with no binder, were better than those using 40 wt.% PEG binder. It seems that PEG residual carbon introduces impurity states which trap the photo-generated electrons, and even plant oils have been proposed as substitute binders (Park and Hong, 2008), obtaining films with large surface roughness and pore size comparing with PEG, and thus higher final cell efficiency. The possible influence of Triton X 100Ò, T, surfactant has been also seen in some papers, from the early work by Burnside et al. (1998), in which they showed that the addition of different surfactants had very little effect upon the ordering phenomena or self-organization in TiO2 thin films, to the recently published work by Xu et al. (2011). In the latter, the quantities of ethanol, acetyl acetone and Triton X 100Ò in the paste were varied and the cross-influence in the DSC efficiency was described. The authors concluded that the efficiency improved with increasing amount of acetyl acetone or Triton X 100Ò. The thickness of the films tested ranged from 6 to 14 lm, but no correlation was established with the amount of surfactant. Zhang et al. (2008a) also studied the influence of added T surfac-
tant and found good performance solar cell for the paste containing 0.8 ml Triton X-100 and 3 g TiO2 P25. Another commonly used surfactant is ethyl cellulose (EC), due to its physical properties that make it an ideal temporary binder in pastes for DSC. Recently, Dhungel and Park (2010) varied the proportion of EC in the TiO2 paste and found an optimal EC percentage which gave the highest efficient DSC. Higher EC content resulted in poor film adhesion and lower film density. The EC influence has been also studied in ZnO pastes (Li et al., 2010), finding that a 10 wt.% in the paste reduces micro-cracks in the films. Despite the works mentioned above, the influence of the surfactant on the film structure is still an open subject. In this work, TiO2 photoelectrodes are prepared using three non-ionic surfactants in different quantities as paste stabilizers and thickeners: Triton X-100Ò (T), polyethylene glycol (PEG), and ethyl cellulose (EC). Photoelectrode films and DSCs are made using identical techniques and materials except the film paste surfactant, in order to study itseffects on the cell performance. The correlation among surface morphology of the mesoporous film (evenness, aggregates and cracks), film thickness and photochemical parameters of the concomitant DSCs using a liquid electrolyte and a natural dye is investigated. Despite low efficiencies are expected with the natural dye, it is a low-cost easy-made option and efficiency results can enrich data base of natural dyed solar cells. Some N719-sensitized standard cells are also prepared and measured to check the whole DSC fabrication process. 2. Materials and methods 2.1. Materials Nanocrystalline TiO2 (P-25, Degussa), nitric acid (65%, Panreac), H2PtCl6 (Aldrich), Ethanol absolute (J.T. Baker), Iodine (99.9%, Aldrich), LiI (99.99%, Aldrich), Triton X-100 (Merck), polyethylene glycol 2000 (Aldrich), ethyl cellulose (Aldrich), 4-tert-butylpyridine (96%, Aldrich) and 3-methoxypropionitrile (P98%, Aldrich) and other chemicals were used as received. N719 (Aldrich) was diluted in ethanol to 3 10 4 M. Conducting glass plates (FTO) (F-doped SnO2, sheet resistance 11–13 O/sq, Nippon Sheet Glass) were used as substrates for fabricating the TiO2 mesoporous films. H2O was purified by a membrane system from Atapa up to <3 lS/cm. 2.2. Measurements and methods 2.2.1. FTO electrical resistance The Van der Pauw method (see Price, 1972; Ramadan et al., 1994 and references therein) was applied to obtain the sheet resistance, RS, of the FTO conducting glass substrates in order to check the effects of cleaning and thermal treatments on them. The values obtained were Rs = (12.57 + 0.24) X/sq for as received FTOs,
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Rs = (13.49 + 0.05) X/sq for 70% isopropanol washed FTOs and Rs = (13.47 + 0.07) X/sq for FTOs washed and activated by a thermal treatment (420 °C for 30 min). Therefore, the washing and activation treatments slightly increase the sheet resistance just over the manufacturer’s nominal values. This ensures that transparent electrodes based on FTO are adequate for application in DSC, because calcination does not increase the glass-FTO resistivity or the sheet resistance, which could cause an increase in the series resistance in the DSC, decreasing its performance (Longo and De Paoli, 2003).
2.2.2. Natural dye extraction In order to obtain the natural dye, local bougainvillea red leaves were harvested, dried, crushed and stored in dark. Two different solvent solutions, 0.1 M HCl in ethanol and a 31.8 vol.% acetone aqueous solution, were used to extract the colored molecules and the resulting extract was tested as dye. Two grams of crushed leaves were stirred at 800 rpm in 45 ml of the solvent during 24 h and then centrifuged to remove any solid residue. If properly stored, protected from direct sunlight and refrigerated at about 4 °C, the natural dye solutions are stable for several months. The coloration of the extract is due to the combination of two main betalain dyes: betacyanins (red–purple) an indicaxanthin (yellow–orange), both having carboxylic groups facilitating TiO2 surface binding (Oprea et al., 2012; Zhang et al., 2008b). The dye UV–VIS absorption spectra were recorded using a high resolution spectrophotometer HR4000 (Ocean Optics). Fig. 1 compares the absorption spectra of three dyes: HCl-extracted natural dye, acetone-extracted natural dye and the N719 ruthenium complex dye. As can be observed, the absorption spectra of both extracted dyes are similar in the UV region, showing an intense peak broader than that of the N719 dye. In the visible range, the HCl-extract shows less absorption without peaks than
Fig. 1. UV–VIS absorption spectra of bougainvillea dyes extracted with the solutions: acetone–water (red dashed–dotted line) and HCl–ethanol (blue dashed line); N719 ruthenium complex dye spectrum (green solid line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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ruthenium complex – N719. However, the absorption spectrum of the acetone-extracted natural dye shows absorption peaks in the visible range as intense as N719. These peaks correspond to betacyanins-type pigments, which exhibit absorption maxima around 535 nm, and indicaxanthin dye, with a maximum absorption for 482 nm (Zhang et al. (2008b)). Therefore, the acetone-extracted natural dye was selected to fabricate the DSCs. 2.2.3. Photoelectrode fabrication and DSC assembly Seven TiO2 nanoparticles pastes were prepared as detailed in Table 1. TiO2 nanoparticles (15 wt.%) were stirred in ethanol and pH 3–4 diluted nitric acid for 12 h at room temperature to obtain a paste with 150 g of solid per solvent liter. The surfactant was added and the paste was stirred 12 h more. Following the procedure proposed by Ito and coworkers (Ito et al., 2003, 2008), the edges of a transparent conducting glass substrate (FTO) were covered with a 50 lm thickness adhesive tape (Scotch) as a masking material, leaving a 0.49 cm2 square free surface. A drop of the paste was poured into one edge of the substrate and flattened with a glass rod by sliding over the tape-covered edges as in a doctor-blade technique. The spread layer was dried for 1 h at room temperature in ethanol atmosphere and then calcined at 450 °C for 1 h. The morphology and thickness of the mesoporous TiO2 layers were characterized by optical (Leyca DM IRB) and scanning electron microscopy (SEM; Hitachi, S-3500). SEM images were analyzed with commercial image processing software. To adsorb the dye onto the TiO2, the mesoporous TiO2 film was placed upwards in a Petri dish and the dye solution was added ensuring a full coverage of the TiO2 area. This was kept in dark at room temperature for 18–24 h and finally water washed to eliminate the non-adsorbed dye. The counter electrode was platinized by coating with a drop of H2PtCl6 solution (0.01 M in ethanol) on the FTO glass and calcined in air at 380 °C for 20 min. Then, the Pt-counter electrode was placed on the top of the dye-sensitized TiO2 film. The gap between the two electrodes was sealed by a thermal adhesive film (Surlyn, Dupont). Finally, the electrolyte was introduced in the gap by capillary action from a hole made on the counter electrode, which was sealed by the same thermal adhesive film. The liquid electrolyte is a widely used charge mediator: the iodide/triiodide redox couple (Longo and De Paoli, 2003), in this case prepared as follows: 0.05 M I2, 0.5 M LiI, and 0.5 M 4-tert-butylpyridine in 3-methoxypropionitrile. The active electrode area of DSC was 0.36 cm2. At least three cells of each paste type were performed to confirm the reproducibility of the results. These cells will be referenced from now on with the name of the TiO2 paste used in the corresponding photoelectrode. For comparison, an ethanol N719 solution was used to make a reference cell (named RC) using the thicker film EC-2.
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Table 1 Batch composition of the TiO2 pastes used in this work. Sample* a b c d e f g *
EC-1 EC-2 PEG–EC PEG-1 PEG-2 PEG-3 T
wt.% Water diluted HNO3 (pH 3–4)
wt.% Ethanol
Surfactant
24 19 19 24 19 19 24
61 63.75 63.75 61 65.25 63.75 61
<0.06 wt.% EC 2.25 wt.% EC 0.56 wt.% of EC + 1.69 wt.% of PEG <0.02 wt.% of PEG 0.75 wt.% of PEG 2.25 wt.% of PEG <0.2 wt.% of Triton X-100
All the pastes were prepared with 15 wt.% of TiO2 nanoparticles.
2.2.4. Cell performance Photocurrent–voltage measurements of DSC (I–V curves) were carried out under direct sun radiation (summer, shiny days, noon time), working with similar light irradiation in all tests: 800 W/m2 (admitting 5% error tolerance). I–V curves of the reference cell were recorded in winter with a solar irradiation of 500 W/m2. A calibrated high performance thermopile-based pyranometer (kipp & Zonnen, CMP 11) was used as a reference. Digital source meters were used to measure the I–V curves without any external bias. At least three cells fabricated using photoelectrodes from each paste were measured to check the reproducibility and the results averaged. 3. Results and discussion 3.1. Morphology and thickness of the TiO2 thin film The mesoporous electrodes were firstly examined by optical microscopy (10 and 40 magnification objectives) to obtain a general vision of the main features and irregularities of the TiO2 films. A more detailed exam was carried out using scanning electron microscopy. Fig. 2 shows some of the surface SEM images (500) taken for each film type. Cracks open to the surface (dark lines in the images) clearly appear, and with different appearance, in all the samples containing PEG and in EC-1, while EC-2 and T samples are free of open cracks (see Fig. 2h at higher magnification). On the other hand, PEG-2 seems the roughest film, with small cracks, while EC-2 shows a grainy surface. Therefore, there is a strong surfactant influence on the film microstructure. Image processing software was applied to obtain quantitative indicators of these surface features: open cracks, surface evenness, and presence of aggregates and their size distribution. Although SEM images have no information of the height (roughness), these indicators will facilitate comparison between samples. The cracks percentage (see Table 2) is obtained after thresholding, filtering and analyzing histograms to “count” the dark pixels in the 500 images, and gives the percentage of pixels belonging to cracks in the whole sample. Fig. 3b shows an example of one processed image. A similar procedure is used to select pixels with gray levels clearer than the average (Fig. 3c), therefore belonging to irregularities in the surface (aggre-
gates, grainy or bumpy areas, crinkles). From both procedures, a final percentage of pixels is obtained indicating the absence of surface irregularities, named surface homogeneity (Homo in Table 2). Finally, the presence, size and number of aggregates were also studied in a set of 100 images, and the number of aggregates per square millimeter is also listed in Table 2. As can be also observed in Fig. 2, the %cracks quantitative indicator shows that EC-1 has the highest cracks percentage value. However, EC-2 sample is free of surface cracks but slightly grainy, which shows the significant influence of the surfactant concentration on the paste preparation. When some EC is substituted by PEG the cracks become visible again in the PEG–EC sample, but they are less and smaller than in EC1 sample. In the PEG series, the cracks percentages also decrease when increasing the PEG content. The T sample is free of cracks. Films may shrink with evaporation of the solvent, resulting in the typical formation of cracks during the drying process of nanoparticles pastes. Since the drying conditions were systematically the same in all samples, the differences must be attributed to the type and concentration of the surfactant in the TiO2 paste. Regarding particle aggregates, very few were detected in the T sample. The highest number of aggregates is found for PEG–EC samples, probably due to PEG addition, because samples with this additive present more aggregates when increasing PEG content. The surfaces are homogeneous in general, giving T the highest indicator, and PEG 2 and PEG 3 the lowest but with very small differences, thus confirming the visual appearance. As a first conclusion, regarding film morphology, the features studied depend more on the quantity, than on the type of surfactant. Triton surfactant seems to be a priori a good option as a binder, because the film has a smooth surface without granular features nor cracks and the lowest number of particle clusters, showing the highest surface homogeneity. The influence of the surfactant on the film thickness is shown in Fig. 4. The thickness was measured on 10,000 SEM images of film cross-section using commercial software. About 40 measures were taken of each sample, using several images and applying statistical criteria to obtain the thickness values listed in Table 2. A main result can be con-
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(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
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T
5um EC-2
100um
Fig. 2. SEM images at 500 of the surface of TiO2 films prepared with different surfactants in the paste: (a) EC-1, (b) EC-2, (c) PEG–EC, (d) PEG-1, (e) PEG-2, (f) PEG-3, (g) T, (h) SEM images at 10,000 of T (top) and EC-2 (bottom) films.
cluded: the thickness of the TiO2 layer is clearly related to the type and quantity of the surfactant used in the preparation of the paste. Pastes containing EC (surfactant with a more complex molecular structure) produce higher values of film thickness, while pastes with PEG (simpler molecular structure) give the thinner films. The inner structure of the TiO2 layer is also observed in the film cross-section (Fig. 4). The texture of films obtained using PEG additive is smoother and more homogeneous than that of EC films,
which seem more porous, and internal porosity is clear especially in EC-2 sample. The paste containing both PEG and EC additives gives a double-layered film structure on the glass substrate with an smother top layer (like PEG samples) and a rougher bottom layer (like EC samples). The T sample looks similar to the EC-1 sample. As a second conclusion, regarding film thickness, varying the surfactant the thickness can be widely changed (from 1 to 6 lm). Previous works have used multilayer
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Table 2 Morphological parameters of the TiO2 films: cracks percentage and surface homogeneity measured on 500 processed images, number of aggregates measured on 100 images, and thickness measured on 10,000 SEM images. Sample
%Cracks
%Homo
Clusters/mm2
Thickness (lm)
EC-1 EC-2 PEG–EC PEG-1 PEG-2 PEG-3 T
6.0 ± 0.3 No cracks 3.3 ± 0.3 4.0 ± 0.6 2.21 ± 0.15 1.30 ± 0.14 No cracks
92.2 ± 0.7 98.3 ± 0.1 94.7 ± 0.7 94.6 ± 0.8 95.7 ± 0.4 96.5 ± 0.5 99.3 ± 0.1
94 ± 4 88 ± 5 129 ± 6 71 ± 3 97 ± 2 100 ± 1 33 ± 3
3.920 ± 0.019 6.18 ± 0.04 2.986 ± 0.016 1.370 ± 0.018 1.106 ± 0.013 2.06 ± 0.04 2.05 ± 0.13
deposition to increase film thickness (Ito et al., 2003; Lee et al., 2006), however, the results shown in this work open a new way to control this parameter. Among the pastes used in this work, EC seems to be the best option for paste formulation, as the films obtained are more porous and thicker than the others. In any case, all the results in this section point out that the morphology and thickness of the TiO2 film strongly depend on the type and quantity of surfactant in the initial paste. The question is whether they influence the cell performance. 3.2. DSC characterization Fig. 5 shows average I–V curves of the natural dyed solar cells fabricated with each one of the proposed TiO2 pastes. The characteristic electrical parameters (open circuit voltage, VOC; short circuit current, ISC; fill factor,
FF; and efficiency) obtained averaging measures from at least three DSCs, are shown in Table 3. The last row labeled RC (Reference Cell) shows results obtained for cells made as EC-2, the thickest film and most efficient cell, but using N719 dye. Comparing EC-2 and RC cells, N719 dye improves efficiency more than 40 times, reaching values close to 10%, as expected according to bibliography for Ru-complex dyes (Wang et al., 2004; Ito et al., 2008). Therefore our whole DSC fabrication process has been validated. Since our objective is to study the effect of the surfactant of the coating paste on the performance of the DSC photoelectrodes and not to improve cell efficiency by additional procedures (Go´mez-Ortı´z et al., 2010; Hernandez-martinez et al., 2011; Ito et al., 2008, 2009; Kashyout et al., 2010) we selected the cheaper and less tested natural dye. Values of the conversion efficiency with the natural dye are very low, but they are consistent with previous works by other authors using natural dyes. Efficiency values about 0.03–0.57% have been reported (Zhou et al., 2011) using different natural dyes and extraction methods. More to the point, not pre-treated titania-based DSC with betalain pigments gave efficiencies between 0.05% and 0.19% (Zhang et al., 2008b) while DSC with betalain pigments extracted from bougainvillea with different purification processes, have given values from 0.31% to 0.49% for the efficiency (Hernandez-martinez et al., 2011). Additional information can be found in a recent review of natural photosensitizers for DSC (Narayan, 2012). Comparing the electrical parameters of the different cells, fill factor values were similar in all cases, being higher
Fig. 3. Original SEM (a) and filtered images to count very dark pixels in cracks (b) and very clear pixels for aggregates and wrapping (c). The cracks percentage is calculated from (b). The surface homogeneity is calculated from (b + c).
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Fig. 4. SEM images at 10,000 x magnification of the cross-section of (a) EC-1, (b) EC-2, (c) PEG–EC, (d) PEG-1, (e) PEG-2, (f) PEG-3 and (g) T TiO2 films.
in EC-2 and T samples and lower in all PEG samples. Regarding VOC and ISC, again the use of PEG additive gives lower values. This leads to the same trend in the cell performance, and so DSCs made with EC pastes presented the highest values of efficiency, in contrast with DSC from
PEG samples that presented the smallest efficiencies. DSC from T samples gave intermediate efficiencies. Interestingly, the highest efficiency is obtained for the DSCs prepared with the film paste using EC in 2.25 wt.%. This is the same percentage that gave the max-
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Fig. 5. I–V curves for each one of the DSC manufactured with photoelectrodes made from each paste.
Table 3 Electrical parameters of the DSC fabricated with each one of the TiO2 photoelectrodes. RC stand for the N719 reference cell (see text). Sample
VOC (mV)
ISC (mA)
FF
Efficiency (%)
EC-1 EC-2 PEG–EC PEG-1 PEG-2 PEG-3 T RC
527 ± 1 550 ± 1 544 ± 1 361 ± 1 499 ± 1 483 ± 1 535 ± 1 811 ± 17
0.213 ± 0.001 0.191 ± 0.001 0.170 ± 0.001 0.056 ± 0.001 0.070 ± 0.001 0.087 ± 0.001 0.121 ± 0.001 3.8 ± 0.3
0.517 ± 0.005 0.6100 ± 0.0017 0.515 ± 0.004 0.483 ± 0.015 0.489 ± 0.013 0.468 ± 0.008 0.547 ± 0.009 0.50 ± 0.05
0.206 ± 0.017 0.228 ± 0.011 0.170 ± 0.020 0.035 ± 0.009 0.061 ± 0.022 0.070 ± 0.003 0.126 ± 0.001 8.9 ± 0.9
Fig. 6. Energy conversion efficiency versus TiO2 film thickness. Dash line is to guide the eye.
imum efficiency in the work by Dhungel and Park (2010), although both pastes do not have exactly the same composition. They claim that this amount of EC binder provides optimal film adhesion to FTO and porosity after sintering. The photoelectrodes fabricated from EC pastes were thicker and more porous (as seen by SEM) than those from PEG pastes. Thicker and more porous TiO2 films enhance dye absorption and thus the light-harvesting capability of the photoelectrode. Note that we modify the film thickness and porosity only by the addition of a suitable surfactant in the coating paste.
To correlate the electrical and morphological parameters, Fig. 6 plots the cell conversion efficiency versus the TiO2 film thickness. The dashed line is a guide to the eye. The efficiency increases with increasing thickness, as has been previously reported (Ito et al., 2008, 2009). The thickness differences are very significant (from about 1 to 6 lm) and they probably mask the effect of mesoporosity and cracks of the TiO2 film on the cell performance. However, PEG-3 and T samples have the same thickness and the cell efficiency corresponding to the former is about a half of the efficiency of the latter, so the influence of film morphology can be checked. The T TiO2 film had no cracks, with a highly homogeneous surface quite free of particles aggregates, and the internal texture of the film (cross-section) looks similar to the EC samples. In contrast, the PEG-3 sample showed a small percentage of cracks open to the surface, three times more aggregates than the T sample and a smoother texture in the cross-section, which suggest a less porous structure than the T sample. A higher porosity increases the photoelectrode specific surface and should enhance electrolyte and dye diffusivity into the TiO2 nanostructure. These other aspects of surfactant influence on the inner structure of the film (porosity and pore size, number and type of particle contacts, diffusion of the dye and ionic species, etc.) and then on the cell performance, are a rather complex subject that needs to be dealt with in another work focused on the fabrication of films with similar thickness from pastes with all the same components except surfactant. On the other hand, EC-1 and EC-2 samples have different thickness (4 and 6 lm, respectively) but the corresponding cells have very similar efficiencies (0.21 and 0.23, respectively). Both thicknesses are below reported thickness for maxima efficiency: about 15 lm (Ito et al., 2009), or about 20–25 lm (Galindo et al., in preparation). Therefore, following the upward trend of efficiency in T, PEG– EC and EC-1 sample (Fig. 6), it would be expected a higher increase of the cell efficiency for the EC-2 sample. The fact that this higher efficiency is not reached could be explained in two different ways. First, the maximum in efficiency is between 4 and 6 lm for the experimental conditions used in this work, below the values in the literature. Or second, the film surface morphology counterbalances the increase in thickness. The differences in surface features for EC-1 (6% cracks, 92% surface homogeneity), and EC-2 (0% cracks, 98% surface homogeneity), samples suggest that open-to-the surface cracks on the TiO2 films like that showed in Fig. 2a increase the film external surface and might facilitate dye absorption during the cell fabrication process and electrolyte diffusion to regenerate the dye. The absence of this enhanced available surface for EC-2 sample may decrease the expected cell performance. 4. Conclusions In this work, TiO2 mesoporous semiconductor thin films for dye-sensitized solar cells photoelectrodes have been pre-
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pared from pastes with different surfactants, analyzed, and used in fully operating cells with a natural dye as sensitizer and a liquid electrolyte. In order to detect only the influence of the surfactant, identical techniques and materials, except the paste surfactant, have been used. The type and quantity of surfactant in the TiO2 nanoparticles pastes used to prepare the films determine the film thickness and morphology (cross-section appearance, surface homogeneity, open-to-the surface cracks and particle clusters). Using ethyl cellulose, more porous and thicker films are obtained, which are better candidates to fabricate the solar cell. The Triton X-100 surfactant, when used in similar weight percentage, produces also good quality films but thinner than the EC. The energy conversion efficiencies of the cells have been measured in environmental conditions, finding higher efficiencies for thicker TiO2 films, showing the influence of the paste surfactant on the cell performance. Although the efficiency depends mainly on the TiO2 film thickness, we have also found some signs of the influence of the film morphology. However, no clear correlation has been found between these other film features and cell performance, except the possible contribution of the open-to-the-surface cracks to the increase in available surface area and efficiency enhancement. Acknowledgements This work was supported by the Junta de Andalucı´a and Feder under Project P09-FQM-4938. References Barbe´, C.J., Arendse, F., Comte, P., Jirousek, M., Lenzmann, F., Shklover, V., Gra¨tzel, M., 1997. Nanocrystalline titanium oxide electrodes for photovoltaic applications. J. Am. Ceram. Soc. 80 (12), 3157–3171. Burnside, S.D., Shklover, V., Barbe´, C., Comte, P., Arendse, F., Brooks, K., Gra¨tzel, M., 1998. Self-organization of TiO2 nanoparticles in thin films. Chem. Mater. 10, 2419–2425. Calogero, G., Yumb, J., Sinopoli, A., Di Marco, G., Gra¨tzel, M., Nazeeruddin, M.K., 2012. Anthocyanins and betalains as lightharvesting pigments for dye-sensitized solar cells. Sol. Energy 86, 1563–1575. Dhungel, S.K., Park, J.G., 2010. Optimization of paste formulation for TiO2 nanoparticles with wide range of size distribution for its application in dye sensitized solar cells. Renew. Energy 35, 2776–2780. Furukawa, S., Iino, H., Iwamoto, T., Kukita, K., Yamauchi, S., 2009. Characteristics of dye-sensitized solar cells using natural dye. Thin Solid Films 518, 526–529. Galindo, E.G., Garcı´a-Salinas, M.J., Ariza, M.J., in preparation. Effect of the multilayer coating on the photoelectrode morphology for dye solar cells. Go´mez-Ortı´z, N.M., Va´zquez-Maldonado, I.A., Pe´rez-espadas, A.R., Mena-Rejo´n, G.J., Azamar-Barrios, J.A., Oskam, G., 2010. Dyesensitized solar cells with natural dyes extracted from achicote seeds. Sol. Energy Mater. Sol. Cells 94, 40–44. Gra¨tzel, M., 2001. Photoelectrochemical cells. Nature 414, 338–344. Gra¨tzel, M., 2003. Dye-sensitized solar cells. J. Photo. Photobiol. 4, 145– 153.
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