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Performance enhancement of dye sensitized solar cells #based TiO2 thick mesoporous photoanodes by #morphological manipulation # Reza Keshavarzi, Valiollah Mirkhani, Majid Moghadam, Shahram Tangestaninejad, and Iraj Mohammadpoor-Baltork Langmuir, Just Accepted Manuscript • Publication Date (Web): 30 Sep 2015 Downloaded from http://pubs.acs.org on October 1, 2015

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Performance enhancement of dye sensitized solar cells based TiO2 thick mesoporous photoanodes by morphological manipulation Reza Keshavarzi, Valiollah Mirkhani,* Majid Moghadam,* Shahram Tangestaninejad, Iraj Mohammadpoor-Baltork Department of Chemistry, Catalysis Division, University of Isfahan, Isfahan 81746-73441, Iran

ABSTRACT- This study is an attempt to give an account of the preparation of mesoporous TiO2 thick templated films of non-similar pore architecture and their use in dye sensitized solar cells (DSSCs). Highly crystallized mesoporous titania thick templated films with different four morphologies including hexagonal, worm-like, cubic and grid-like mesostructure, have been successfully synthesized through evaporation induced self assembly (EISA) route followed by a layer-by-layer deposition. Stabilization, followed by each coating, and calcinations, carried out after every five layers, were used for having crack-free thick films. These mesoporous templated titanium dioxide samples were characterized by TEM, XRD, SEM, BET and UV-vis measurements, used as photo-electrode material in DSSCs. The mesostructured films with thickness of about 7µm demonstrated a better performance in comparison to nanocrystalline TiO2 films (NC-TiO2) at the film thickness of 13 µm – as the most typical films utilized in DSSCs. The findings reveal that a surfactant/Ti ratio change undergone for developing cubic mesostructures can enhance the crystallinity and roughness factor and therefore increase the energy conversion efficiency of DSSC. The cell performances derived from these mesofilms were enhanced compared with the efficiencies reported thus far. The best photovoltaic performance of 8.73% came from DSSC using the cubic mesoporous TiO2 photoelectrod with the

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following properties: open circuit voltage of 743 mV, short circuit photocurrent density of 16.35 mA/cm2, and a fill factor of 0.72. INTRODUCTION Dye sensitized solar cells have proved a reasonable alternative to the conventional silicon-based solar cells due to their better processing capabilities, processing speed as well as economical advantage.1-4 Usually, a dye-adsorbed nanocrystalline TiO2 layer with a thickness of 10-15 µm deposited on a transparent conductive oxide (TCO), a redox electrolyte and a Platinum counter electrode constitute main components of DSSCs.5, 6 Among the many structures applied for DSSC, 7-13 mesoporous titania templated films have received extensive attention because of their high surface area and uniform pore structure.14-19 Most mesoporous surfactant-templated films are produced using an evaporation-induced self assembly (EISA) process.20-22In this process usually an acidic solution composed of an inorganic precursor and an amphiphilic organic template is obtained in a volatile solvent containing some water. The solution is spin or dip-coated onto a substrate. The solvent evaporation in particular conditions of humidity and temperature causes the spontaneous association of individual components into an organised structure or pattern to form a periodic inorganic–organic composite. After a thermal treatment and calcination at a temperature above 300 ºC, block copolymer can be completely removed from the gradually solidified film, and mesoporous TiO2 films are thus obtained.23 The mesoporous structure of the film is influenced by many experimental parameters such as Surfactant/Ti ratio, water content and relative humidity during coating and/or ageing which can affect the mesoporous morphology.24-26 As morphology can exert a profound effect on the DSSCs performance, the ability to tune the pore architecture of the mesoporous templated titania could be of great advantage. In fact, different conditions can develop different structures and thus different crystallinity and surface area, which in 2


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turn crucially exert influence in the performance of DSSC: the higher the surface area and crystallinity, the higher the DSSCs photovoltaic performance. In fact, mesoporous titania templated film refers to a semicrystalline network with the coexistence of anatase nanocrystals and a significant amount of amorphous titania. The low crystallinity always decreases the solar conversion efficiencies.20 Moreover, the films bearing higher surface area adsorbs higher amounts of dye, and therefore, increases the light harvesting and cell performance. Meanwhile, as all of the pores in mesoporous templated films are regularly interconnected, electrolyte diffusion will be highly efficient and TiO2 templated film can be considered as a potential candidate for photoanode in DSSCs.17, 27 First, Zukalova et. al. prepared the ordered mesoporous thin templated films with only one morphology and calcined them after each coating. This synthetic method led to a high crystallinity with a decrease in the roughness factor after 3-5 layers, and therefore, the photovoltaic performances reached a plateau.28, 29 Their 1-µm-thick mesoporous films showed higher solar conversion efficiency by about 50% compared to the device made from conventional random porous nanocrystalline TiO2 film (NC-TiO2) with the similar thickness. In order to increase the crystallinity and to preserve the open porous structure, phosphor-doping agents have been used. However, the solar conversion efficiency of these films reached a plateau after eight layers with 2.3 µm in thickness due to the electron-hole recombination.30 Alternatively, Zhang et. al. prepared worm-like and cubic titania mesostructures by EISA using spin coating method and applied them in DSSCs. Each deposition cycle was followed by a stabilization step performed by heating the fresh film at moderate temperature on a hot plate during a few minutes. When the desired number of layers (60 layers) was coated, the calcination step was performed to overcome the surface area limitations induced by repeated calcinations.31 Appearance of macro-cracks upon increasing the films thickness (thicker than ~3.5µm) was a disadvantage of this work. 3


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Dewalque and coworkers reported a dip-coating multilayer deposition following a similar heat treatment route,27 in which fewer layers needed to obtain a thick film (up to 4µm). Despite the increasing dye loading and roughness factor (RF), the conversion efficiency levels off after 9 layers of titania which entail the worm-like structure. This arises from the decline in open circuit voltage (Voc) values due to the low film crystallinity resulting from the method applied. Recently, we have reported highly efficient dye sensitized solar cells based on worm-like and grid like mesoporous titania thick templated films prepared from constant ratios of surfactant/Ti by spin and dip coating methods. Stabilization, carried out after each coating, and calcinations, carried out after every five layers, led to crack-free thick templated films.32 The sol gel templating approach through the EISA process provides a highly suitable platform to study the dependence of cell performance on the film morphology of TiO2 mesolayers as it can yield a wide range of controlled morphologies within the same material system. The present study reports a successful synthesis of crack-free thick templated films (up to 7µm) with four different types of mesostructure morphologies including worm-like, hexagonal, cubic, and gridlike structures through layer by layer deposition. Various surfactant/Ti ratio and thermal treatments were used in the study to develop the four pore architectures in the study. Finally, these titania meso structures were used in DSSC devices and their photovoltaic performances were investigated. These meso-films, illustrated a complex interplay of factors determining the cell performance. The findings reveal that a surfactant/Ti ratio change undergone for developing cubic mesostructures can enhance the crystallinity and roughness factor at the same time. The photovoltaic performances derived from thick templated films were enhanced compared with the efficiencies reported thus far. Mesoporous films in this study provided an efficiency of 8.73% in solar cells; so far the highest efficiency reported for mesoporous TiO2 templated photoanodes.

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EXPERIMENTAL SECTION Preparation of the sol precursors for coating Three mother solutions were prepared as following: Concentrated HCl (Merck, 36% wt, 4.85g) was added to tetraethyl orthotitanate (Merck, 6.35g) under vigorous stirring in an ice bath. Block copolymer Pluronic P123 (3 g, 2 g or 1.4 g) were dissolved in 1-butanol (Aldrich, 18.15g) to respectively develop hexagonal, worm-like, and cubic morphologies. Under vigorous stirring, the solutions of P123, dissolved in 1-butanol, were gradually added to the HCl/Ti(OEt)4 solutions. The worm-like and hexagonal solutions were aged by stirring at ambient temperature for at least 3 h and the cubic solution was aged for 15 min before coating. Titania film with hexagonal mesostructure In order to develop the "hexagonal" mesostructure film, we used the solution containing 3 g of surfactant. The film was prepared by dip coating (withdrawal rate 0.8 mm/s) of the solution onto glass slides for the film mesostructure characterization or on FTO conducting glass (Dyesol, 15 Ω sq-1) for the photovoltaic measurements. The relative humidity (RH) was set at 25% at 20 ◦C for the film in the electronic dip coating chamber. The as-prepared films were directly transferred into an electronic ageing chamber with a controlled relative humidity of 75% for 24 h at 20 ºC. The deposition of several layers was carried out by the insertion of a stabilization step between each coating cycle. This stabilization was performed by heating the fresh film for 15 min on a hot plate pre-heated at 300 ºC. Evaporation of solvent and water as well as partial condensation of the inorganic network, which prevent the re-dissolution of the film in the next dip coating step, take place during the heat treatment.27, 32 After stabilizing of each five layers, the film was calcined under air at 350 ºC for 2 h (heating rate: 1 ºC/min). Thicker films were prepared by repeating the described procedure. Final calcination of prepared thick films was done at 450 ºC for 1 h (heating rate: 1 ºC/min) to fully condense the inorganic network, increase the nanocrystallinity of the mesoporous TiO2 films, crystallize the 5


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anatase phase and burn out all surfactant residues, leading to an anatase mesoporous structure. A thorough discussion of the issue is presented elswhere.32All prepared thick samples were crack free and optically transparent. Titania film with worm-like mesostructure The worm-like mesoporous film was prepared from the solution bearing 2 g of surfactant after dip coating (withdrawal rate of 0.8 mm/s) at 25% at 20 ºC and stabilization on a hot plate at 300 ºC. For preparing the meso film with this type of morphology, the as obtained film did not undergo any humidity ageing after coating. A similar heat treatment was applied as that for the hexagonal film (stabilizing after each coating for 15min and calcination after every five layers at 350 ºC and finally, calcination at 450 ºC for 1 h). Titania film with cubic mesostructure The steps for synthesizing the cubic mesoporous titania films were adapted from Alberius et al.25 The only difference is using 1-buthanol instead of ethanol in our study. The steps are as follows: The solution containing 1.4 g P123 template and aged for 15 min was dip coated with a withdrawal rate of 0.8 mm/s on the substrate at 90±2% RH and 10 ºC. The coated layer was aged at 10 ºC and 85% RH for at least 24 h after deposition. The film was stabilized on a hot plate under air at 300 ºC for 15 min. After stabilizing of every five layers, the film was calcined under air at 350 ºC for 2 h (heating rate: 1 ºC/min). Repeating of this procedure leads to preparation of thicker films. Finally, the crack free thick film was calcined at 400 ºC for 1 h (heating rate: 1 ºC/min). Titania film with grid-like mesostructure The synthetic procedure of the grid-like film was similar to cubic film, except that temperature of 450 ºC was applied for final calcination instead of 400 ºC. Fig. 1 presents the schematic route for synthesis of the templated films. For comparison, a standard non-organized nanocrystalline TiO2 (NCTiO2) photoanode was synthesized according to the method reported by Mallouk and coworkers.8 6


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Figure 1. Synthetic route for cubic, grid-like, worm-like and hexagonal thick templated films. Fabrication of DSSCs After the TiO2 films coated on FTO substrates were cooled to 150 °C, they were taken out of the furnace, and then sensitized. The TiO2/FTO samples were slowly submerged into a 0.3 mM solution of N719 ruthenium dye (solaronix) in ethanol and sensitized for 2 dayes. The dye-adsorbed photoanode electrode was then rinsed thoroughly with ethanol and dried in the air. The excess amount of sensitized TiO2 is scraped off the FTO substrate to make an active area of 0.25 cm2 (0.5 cm x 0.5 cm). For 7


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preparation of platinum counter electrode, a drop of 0.5 mM H2PtCl6 solution in anhydrous isopropanol was poured on an FTO glass (solaronix, 7 Ω sq-1) and then heated at 385 °C for 15 min in air before cell assembly. The redox couple/electrolyte solution was composed of 0.05 M I2, 0.1 M lithium iodine (LiI, Aldrich Chemical), 0.6 M 1,2-dimethyl-3-propylimidazolium iodide, (98%, Ionic Liquids Technology), and 0.5 M 4-tert-butylpyridine (TBP, Aldrich Chemical Company) and 0.1 M guanidium thiocyanate dissolved in acetonitrile (Aldrich Chemical Company). Stretched parafilm was used as a 20∼30 µm spacer between the anode and platinum counter electrode. A drop of the redox electrolyte was placed on top of the active area and the platinum counter electrode was placed on it and clamped firmly using binder clips. Characterization techniques For observation of the morphology of the templated films by TEM (Phillips-CM200), the films were scratched off the substrate and were dispersed in ethanol under ultrasound. Finally they were deposited on carbon-coated copper grids. In order to identify crystalline phase and crystallinity of titania present in the mesoporous films, XRD technique was used by a X-ray diffractometer (Bruker D8 Advance, Germany) with monochromated Cu Kα (l = 1.54 Å) in a scan rate of 0.03 (2θ/s). The film texture, morphology and thickness were studied using KYKY-EM3200 and field emission Tescan Mira3 scanning electron microscopes (SEM). The specific surface area of TiO2 templated films was evaluated by using an adsorption/desorption BET (Belsorp-mini II system) with nitrogen as the adsorbate. The amount of dye absorbed on the different titania films was measured using UV-vis spectrophotometer (Cary 500 Scan Spectrophotometers, Varian) by the reported method.27 Photovoltaic device testing was done by a solar simulator (Luzchem, v1.2) coupled with a µAutolab type III (Ecovchemie, Utrecht, the Netherlands) controlled by a microcomputer with Nova 1.7 8


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software. Photocurrent-voltage measurements of the cells were performed under a simulated irradiation (global AM 1.5, 100 mW/cm2). The electrode active area of the cells were set to 0.25 cm2 with a black mask (aperture area: 0.58 cm × 0.58 cm). Incident photon to current conversion efficiency (IPCE) was measured with spectral response measuring equipment (CEP-1500). Electrochemical impedance spectra of DSSCs were recorded with an Autolab-302 at open-circuit potential under AM1.5G one Sun irradiation. The frequency range for the analysis was from 10-1 Hz to 106 Hz and the amplitude of the alternating signal was 10 mV. The obtained impedance spectra were fitted with Z-view software.

RESULTS AND DISCUSSION Structural Analysis of Mesoporous Titania Films Fig. 2 presents the TEM images of the crack-free mesoporous titania templated films with different morphologies. Fig. 2a shows a common cubic arrangement of discrete pores which is developed under the lowest surfactant/Ti ratio at aging sol time of 15min, coating temperature of 10 ºC, relative humidity of 90% and calcined at 400 ºC. Fig. 2b illustrates the titania grid like mesostructure, which is obtained via a similar condition to yield the cubic mesostructure yet calcined at a higher temperature (450 ºC). The cubic structure transforms into a grid-like structure at the higher temperature.

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Figure 2. TEM images of (a) cubic, (b) grid-like, (c) worm-like, and (d) hexagonal mesoporous templated films. The pore network becomes interconnected as the heating continues, which can be attributed to the crystallization into anatase of the inorganic walls. Such a structural modification arises from the diffuse sintering of nanocrystallites following the thermal induced nucleation-growth of anatase. As could be seen in the TEM micrographs, titania films prepared by the highest surfactant/Ti ratio at aging sol time of 3h and under coating temperature of 20 ºC, relative humidity of 75% and calcined at 450 ºC demonstrate well-defined channel-type pore architecture known as hexagonal (Fig. 2d). Although we have already studied the evolution of 15 layers of titania worm-like mesostructure film,32 our findings are repeated here to have provided a clear and accurate account. Additionally, characterization and photovoltaic performance of the 18 layers of that were investigated in this research. Fig. 2c presents the worm-like morphology of the titania film. The worm-like structure is defined as an isotropic and vermicular arrangement of two interpenetrating phases (walls and pores).

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Figure 3. FE-SEM top view images of (a) cubic, (b) grid-like, (c) worm-like, and (d) hexagonal titania mesoporous thick templated films.

Fig. 3 represents the top view field-emission scanning electron microscope (FE-SEM) images of titania mesoporous templated films with non-similar four morphologies. As it can be observed, SEM imaging reveals crack free open pores mesoporous films with a well-defined and regular porous structure for the cubic, gridlike and hexagonal samples and a disordered structure for the wormlike tenplated film. Fig. 3a shows that the cubic films have highly ordered discrete spherical open pores at the top surface. From the top view SEM image of the gridlike film calcined at 450 °C (Fig. 3b) can be observed that the TiO2 pore walls are composed of spherical nanocrystallites and the pores network has 11


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been interconnected due to the temperature-induced growth of anatas nanocrystalline TiO2. Meanwhile, although the mesoporous gridlike film shown in Fig. 3(b) is not as well ordered or as regularly shaped as the cubic film shown in Fig. 3(a), its ordered mesostructure is retained at a higher temperature treatment. At the top surface of the wormlike film in Fig. 3(c), mesopores can be seen clearly. Nevertheless, the poor alignment of the pores and disordered arrangment of the pore are noticable. Fig. 3(d) shows a clear mesoscopic order arranged in a hexagonal pattern, in which the pore channels are well ordered.

Figure 4. Nitrogen adsorption-desorption spectra of (a) grid-like, (b) cubic, (c) worm-like, and (d) hexagonal mesoporous thick templated films. Fig. 4 shows the nitrogen adsorption/desorption isotherms of mesoporous titania thick templated films with different morphologies. All of the samples present a type-IV isotherm, which is representative of mesoporous solids.36, 37 The hexagonal and grid-like titania mesoporous films (Fig. 4a and 4d) shows a H1 hysteresis loop that exhibit parallel and nearly vertical branches between the adsorption and desorption isotherms.32, 38, 39 The H1 hysteresis typically is a characteristic of the porous materials with well-defined cylindrical-like pore geometry or agglomerates of approximately uniform spheres.40 As illustrated in Fig. 4b, the cubic mesoporous film shows a hysteresis loop similar to the hexagonal and grid-like titania films as well, albeit with a larger hysteresis loop. This is in line with 12


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other studies.39,

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As shown in Fig. 4c, the worm-like mesoporous film exhibits an H2 type

hysteresis loop with a triangular shape which often is a characteristic of disordered materials.32 The BET specific surface areas for the hexagonal, worm-like, cubic and grid-like mesoporous titania templated films were calculated to be 153.95, 138.65, 123.32, 110.65 m2g-1, respectively, and total pore volume of 0.266, 0.233, 0.209 and 0.218 cm3g-1, respectively. The BET surface area of the meso-TiO2 films in the present study are much larger than the surface areas of P25 powder (50 m2/g) and NC-TiO2 prepared by sol gel method (69.5 m2/g) as reported in the literature.17, 43, 44 As can be seen, the meso films containing a greater template content enjoy a larger surface area and pore volume and vice versa i.e., the increase in organic template content has been resulted in the increase in the surface area and pore volume after decomposition of organic template by the heat treatments. Meanwhile, when the pore volume increases, the mass of titania decreases. The mesoporous titania templated film with less pore volume thus possesses a higher titania weight and vice versa, which is in accord with the literature.31 It can be noted that, the grid-like meso-film has smaller BET surface area and higher pore volume compared to the cubic titania meso film with the same organic template content. This arises due to a growth in the crystallinity as well as the pore merging resulting from the sintering at a higher temperature, which leads to a boost in the pore volume as well as a shrink in the BET surface area. 34, 45 The nanocrystalline phase and crystallite size of the mesoporous titania templated films with different morphologies were determined by XRD technique, as shown in Fig. 5. The remarkable peaks are attributed to (101), (200), (105), (211) and (204) reflections of anatase phase, respectively. As calculated from the Scherrer equation using the (101) diffraction peak, the crystallite sizes of anatase nanocrystals were 14.3, 12.9, 10.7 and 9 nm for the grid-like, cubic, worm-like and hexagonal mesoporous films, respectively. These crystallite sizes are relatively high compared to the ones mentioned in the literature.30-32 We compared the crystallinity of the mesoporous titania templated films

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with some of those reported elsewhere.32 Crystallinity is an important factor for DSSCs. As mentioned, the low crystallinity decreases the solar conversion efficiencies (< 1.25%).15

Figure 5. XRD patterns of: (a) FTO substrate, (b) hexagonal, (c) worm-like, (d) cubic, and (e) grid-like mesoporous thick templated films. A better crystallinity improves the open-circuit voltage (Voc) and also electron transfer into the film and transports it towards the collecting electrode, which in turn leads to increase in the short-circuit current density (Jsc).27-32 According to the literature, there are many experimental parameters including surfactant/Ti ratio,31 relative humidity,23, 32 calcination temperature,18, 27, 32 humidity ageing time23, 32 and number of calcinations,15, 32 which exert influence on crystallinity. As shown in Fig. 5d and 5e, the cubic and grid-like mesoporous films have a stronger (101) anatas peak than the worm-like mesostructure films (Fig. 5c), and the worm-like mesostructure film has a stronger (101) anatas peak than the hexagonal mesostructure films (Fig. 5b). In other words, the gridlike and cubic films are more crystalline than the worm-like films and the worm-like mesostructure 14


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film is more crystalline than hexagonal films. The crystallite sizes calculated by Scherrer equation also follow the same order. The variation in crystallite size with morphology can be related to the different level of template content employed. As the grid-like and cubic mesoporous titania films were synthesized under a lower template content, they possess a higher titania content and therefore thicker titania walls than the other films. The thicker titania walls are capable of embracing relatively larger nanocrystals. On the other hand, the hexagonal mesoporous film has lowest crystallinity than other due to highest template and lowest titania contents. The higher crystallinity for grid like and cubic mesoporous films may also be attributed to the higher relative humidity in ageing chamber. In fact, higher RH can lead to more hydrolysis and polycondensation of inorganic species in the hybrid framework and therefore creation of larger TiO2 nanocrystallites after calcinations.23, 32.It should be noted that, the grid-like meso film is more crystalline than cubic meso film due to higher calcination temperature to obtain the grid-like structure. Actually, at the higher temperature and upon further heating, a grid-like structure encompassing an interconnected pore network results from the cubic structure, which is also evident in the TEM images. This could be attributed to the crystallization into anatase of the inorganic walls and crystal growth of anatase. Other studies confirm the same finding as they reported the transformation of a cubic to grid-like mesostructure by a higher temperature.34, 35.The worm-like titania films containing a higher content of titania prepared at a lower relative humidity show a higher crystallinity than hexagonal meso-films bearing a lower titania content obtained at a higher relative humidity i.e., the crystallinity enhancement resulting from the higher titania content overcome the crystallinity decline caused by the lower relative humidity in worm-like films. Moreover, the cubic meso film is more crystalline owing to its higher titania content and probably higher RH although it was obtained at a lower calcination temperature (400 ºC) compared to the worm-like and hexagonal meso films. To the best of our knowledge, the crystallite size of 14.3 nm is the largest crystallite size so far reported for titania templated films used in DSSCs, which could be attributed to 15


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the high calcination numbers, high relative humidity, high calcination temperature, and high titania content required for the preparation of the grid-like meso film. As evident, many experimental parameters exert influence on the films crystallinity; however, it seems that surfactant/Ti ratio plays the most crucial role in this respect. Surface area of the films, expressed by the roughness factor, crystallinity, and quality of crystal interconnects affect the efficiency of a DSSC made of tiatania photoanode.27-32 usually, the films having higher surface area adsorb higher amounts of dye, and therefore, increases the light harvesting and cell performance.27-30, 32 As presented in the literature, the dye loading follows the same order as the specific surface area and the roughness factor. Intrestingly in this study, although the specific surface area of mesoporous TiO2 templated films decrease, dye uptake becomes more efficient. The disagreement between the specific surface area and dye loading variation can be attributed to the TiO2 weight growth since the roughness factor can be obtained as specific surface area multiplied by the TiO2 weight:31,

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as an instance,

despite the decline in the specific surface area in the cubic meso film, there is enough of the TiO2 weight enhancement resulting from the content of titania to allow for an increment in the roughness factor and accordingly the dye loading. On the other hands, hexagonal film with highest specific surface area has the lowest dye loading due to the lowest titania content, i.e., there is enough TiO2 weight drop resulting from the lowest titania content allowing for a decline in the roughness factor although there is a large surface area. By the same token, the content of titania plays a crucial role in RF and dye loading determination as the crystallinity determination. As for the grid-like meso film which has the same titania content as the cubic film, the RF and accordingly the dye loading follows the same order as the specific surface area. In other words, in the grid-like film with decrease in surface area, RF and dye loading also decrease.

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Figure 6. Dye loading (expressed in moles of N719 dye per mm2 of projected electrode area) as a function of the number of layers for the: (a) hexagonal, (b) worm-like, (c) grid-like and (d) cubic mesoporous films. Thicker multilayer films were manufactured via layer by layer deposition, and their dye loading values were measured (Fig. 6). It is obvious that the thicker the film, the greater the dye loading for the four types of mesoporous template films. Nevertheless, this conclusion is valid only for the mesoporous films composed of 1-18 layers But a dye loading plateau is reached for higher number of layers. It can be related to the number of calcination steps. As the authors have already shown in their earlier study,32 when the number of calcination steps exceeds four times, RF and therefore dye loading decreases significantly. This is also in line with the findings reported previously.29,

30, 45

The amount of

‘‘amorphous’’ TiO2 in the mesoporous film can be minimized by thermal treatment. However, prolonged calcination also causes collapse of the mesopore morphology and the loss of active electrode area (roughness factor).29 In fact, to obtain the titania mesoporous templated films with a thickness of about 7µm, we need 18 layers with 4 calcination steps. As such, the roughness factor will increase definitely in such conditions. Meanwhile, the cubic and grid-like mesoporous films with the highest 17


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titania content have the highest dye loading and the hexagonal mesoporous film with the lowest titania content has the lowest dye loading. Owing to an increase in the roughness factor, the dye uptake shows an increment for all the mesoporous titania templated films as the film thickness increases (Fig. 6). Increasing the anatas cystallinity by heat treatment in the mesopoous films has been the object of the majority of the studies conducted so far; however, there are many disadvantages to the method including structure collapse and decline in dye uptake, RF, and surface area. As the present study shows, the decrease in organic template/titania ratio leads to an increase in both the dye loading and cystallinity in a mesoporous titania templated films. Fig. 7 represents the cross-sectional SEM micrographs of the cubic mesostructure film obtained by multilayer deposition and the NC-TiO2 film (13 µm) prepared by the Doctor-blading method commonly used in DSSCs. As can be clearly seen, the layer by layer deposition results in an increase in the film thickness. As Fig. 7b illustrates, 18 layers are needed to prepare a cubic film with a thickness of 6.9 µm. The same thickness was observed in other mesoporous temlated films of the same layer number.

Figure. 7. Cross sectional SEM micrographs of: (a) 13µm NC-TiO2 and (b) cubic mesoporous 6.9 µm thick titania films Photovoltaic performances of thick templated films 18


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The prepared mesoporous thick templated films with hexagonal, worm-like, cubic and grid-like morphologies and similar thicknesses (7 µm) were used in DSSCs. Their photovoltaic performances and the current density-voltage curves for fabricated DSSCs by these films are shown in Table 1 and Fig. 8, respectively. These data were compared with a cell made from 13 µm NC-TiO2 film commonly used in DSSCs. As can be seen from data in Table 1, the crystallite size and dye loading values have a significant influence on the device performance. Increasing the dye loading and crystallinity increase the Jsc and Voc values. The Jsc parameter is highly dependent on the dye loading, and thus the roughness factor and pore connectivity. As shown in the Table 1 and also Fig. 8, the cubic mesostructure film has the highest dye loading values, and therefore, Jsc parameter. Table 1. Photovoltaic performances of the cells made by titania thick templated films (7 µm) with different morphologies and NC-TiO2 film (13 µm). Device Hexagonal Worm-like Grid-like Cubic NC-TiO2

Crystallite size/nm 9 10.7 14.3 12.9 19

Dye loading/ mol mm-2

VOC/V

Jsc/mAcm-2

FF

ɳ (%)

13.27×10-10

0.722

15.59

0.67

7.49

-10

0.725 0.757 0.743 0.712

16.07 16.20 16.35 14.78

0.69 0.70 0.72 0.66

7.98 8.63 8.73 6.96

13.70×10 13.86×10-10 14.05×10-10 12.46×10-10

The short-circuit current density Jsc of the cell made by meso-TiO2 films with thickness of about 7 µm is higher than that from NC-TiO2 photoanode with 13µm in thickness. Lower Jsc of NC-TiO2 based solar cell can be related to the fact that the lower electron density of photoelectrons are generated owing to its smaller which allows less amount of adsorbed dye compared with the meso-TiO2 films. This could also be attributed to the high crystallinity and good interconnected grains in meso-TiO2, which might have favoured increased diffusion length for a facile charge transport.17, 33 The incident photon to current conversion efficiency (IPCE) of the solar cells made of the mesoporous titania 19


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templated films (Fig. 8) clearly shows that DSSCs based on templated films have significantly higher IPCE over the whole spectral range of 400-800 nm compared to the DSSC made of NC-TiO2 film. The maximum IPCE of the mesoporous templated titania solar cells was 71.7% at 530 nm related to the cubic film as photoanode with the highest dye loading and the minimum was related to the heaxagonal device with lowest dye loading among the mesoporous films (67.7%).

Figure 8. Current-voltage and IPCE curves of the DSSCs made of: (a) NC-TiO2, (b) hexagonal, (c) worm-like, (d) grid-like and (e) cubic meso-film.

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The maximum values of the IPCE spectra for the NC-TiO2, worm-like and grid-like mesopoous films at 530 nm are 64.1%, 69.9% and 70.7%, respectively. Obviously, the IPCE curves follow the loading of dye. In other words, the enhancement of IPCE reflects the increased dye uptake amount in the photoanodes. In fact, IPCE is expressed by the product of light harvesting efficiency (LHE), charg injection efficiency (CIE), and charge collection efficiency (CCE). Fig. 9 shows the UV-visible absorption spectra of the dye adsorbed TiO2 films. The light harvesting efficiencies (LHE) were calculated according to equation LHE= 1-10-A, where A stands for the absorbance of dye-loaded TiO2 films. The values of A, LHE, and IPCE at 530nm are listed in the table 2. As can be seen, the LHE data at 530 nm is 98.41% and 99.09% for dye loaded NCTiO2 film and cubic film, respectively. On the other hand, the IPCE at 530 nm is 64.1% for NC-TiO2 and 71.7% for cubic meso DSSCs. The increase in LHE from the NC-TiO2 to cubic TiO2 electrode was calculated to be 0.68%, which is much smaller than the increase (7.7%) in IPCE at 530 nm for the DSSC based NC-TiO2 and cubic film. Other films follow this rule as well. Obviously, either CIE or CCE or both must also be responsible for the improvement in IPCE.

Figure 9. UV-visible absorption spectra of the N719 dye adsorbed: (a) NC-TiO2 film and (b) hexagonal, (c) worm-like, (d) gridlike, and (e) cubic mesoporous TiO2 thick templated films. 21


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Table 2. The values of absorbance and LHE percent of N719 dye-loaded TiO2 films and also IPCE percent of the DSSCs made of these films at 530 nm. Dye adsorbed film NC-TiO2 hexagonal wormlike gridlike cubic

A at 530 nm 1.80 1.92 1.98 2.00 2.04

LHE(%) 98.41 98.79 98.95 99.00 99.09

IPCE(%) 64.1 67.7 69.9 70.7 71.7

As shown in Fig.8 and Table 1, all mesoporous photoanode cells have higher Voc values than NCTiO2 cell. Such result has been observed in other literatures.17, 32, 47 The higher open circuit voltage is usually an indication of less defect in TiO2 and smaller charge recombination. The higher Voc in templated films can be attributed to the fact that, pore connectivity and inorganic wall connectivity in mesoporous films are much higher than NC-TiO2 films.17,

33,

51

The consequence of this

interconnectivity is a faster electron transport in meso films i.e., the disconnected nanocrystallites in NC-TiO2 film give rise to a slowdown in electron transport as well as an increase in recombination rate which results in a fall in Voc.17, 33, 51 Fig. 10 shows the impedance spectra (Nyquist plots) for the NCTiO2 and Meso-TiO2 DSSC cells. The lifetime of electrons within the TiO2 photoanode (τe) was estimated based on the relation τe = (ωmax)-1, where ωmax is the maximum angular frequency of the larger impedance semicircle in the Nyquist plots. The calculated electron lifetimes for the NC-TiO2, hexagonal, worm-like, cubic and grid-like mesopoous films were 0.14s, 0.18s, 0.19s, 0.30s and 0.40s, respectively.

22


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Figure 10. Electrochemical impedance spectra of the DSSCs based on NC-TiO2 and mesoporous TiO2 templated films with different morphologies. The maximum angular frequencies (ωmax) have been marked at the top of the larger semicircles which are attributed to charge transfer resistance at the TiO2/electrolyte interface. The DSSCs made from mesoporous titania films exhibit efficient electron transport due to the suppression of charge recombination, longer effective life time of electrons, which in turn improves its Voc compared to the cell based NC-TiO2 layer. In fact, the increased τe value suggests the retardation of the recombination or back reaction during the transport of electrons through TiO2 layer. Voc is known to be highly dependant on the recombination or back reactions which happen on the TiO2 and electrolyte interface, and by suppressing those reactions a larger Voc value in DSSCs based mesoporous films can be obtained.14 Actually, improving charge collection results in both increasing recombination resistance and shifting the Fermi level in the TiO2 closer to the conduction band of TiO2 due to smaller population of charges in the deeper defect states and as a result of this shift we will get improved Voc value. As the current-voltage curves of mesoporous templated cells in Fig. 8 show, the grid-like cell has the highest Voc and the hexagonal cell has the lowest Voc. This could be explained by the fact that since 23


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the recombination rate is proportional to the surface area, electrodes in the larger surface area often show a lower Voc. A Faster recombination entails a lower Voc: the open circuit photovoltage drops since a larger surface area implies a higher dark current.8, 46 This is confirmed by current-voltage curves of DSSCs in dark as shown in Fig. 8 as well as by electron lifetimes obtained from electrochemical impedance spectra (EIS) analysis. Therefore, the hexagonal cell has the lowest Voc among the cells made from mesoporous films due to the largest surface area, and the grid like meso film has the highest Voc due to the lowest surface area. Also, as mentioned earlier, the higher crystallinity of films exerts a positive effect on the Voc value; better electron injection and transport in the semiconductor phase as well as fewer recombination losses on grain boundaries.27,

32

Hence, This can also account for the

highest Voc in grid-like cells i.e., the hexagonal film has the lowest anatas crystallite size among the mesoporous templated films and thus the lowest Voc, and the grid-like film has the greatest crystallit size and Voc. The same is true for other mesoporous templated films such as the worm-like and cubic films. The nanocrystalline titania film with a thickness of 13 µm provides 6.96% conversion efficiency while the lower thickness (6.9 µm) cubic mesoporous photoanode provides 8.73% conversion efficiency, the highest thickness and efficiency reported for mesoporous templated films in DSSCs to date. Meanwhile, although the grid-like film has a higher crystallinity, it has a lower efficiency than the cubic cell (8.63%). Thus, the improvement in Voc value due to the better crystallinity of the grid-like film does not overcome the Jsc decrease due to its lower dye loading. In general, conversion efficiencies in all mesoporous devices are higher than the NC-TiO2 device. This is mainly related to the higher roughness and surface area, higher pores connectivity, better dye loading and electrolyte impregnation in mesostructured films. Unfortunately, our devices did not show more efficiency due to a plateau arising in dye loading. However, it is worth pointing out that the observed efficiencies were achieved without applying any treatment of TiO2 electrode with TiCl4, which is known to improve

24


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current density48-50 and without employing a scattering layer with a large particle size to trap more light and as a reflective coating to reduce transmission losses.50 Various methods have been reported for synthesis of mesoporous films in order to apply them in DSSCs. There is controversy among different authors regarding the solar conversion efficiencies of the cells made from these films that can be attributed to the different conditions and materials used in DSSC assembly such as coating methods, dyes, electrolytes, surfactant etc. Thus, it is difficult to compare the efficiencies as an absolute criterion. A novelty of the present study lies in the use of mesoporous templated films with four different morphologies in DSSCs while in others just one or two morphology was evaluated. Zukalova and co-workers built DSSC devices by organized mesoporous titania films with only one morphology as photoelectrodes with 3-5 layers.28, 29 Their film was calcined at 350 ºC for 2 h after each coating and finally calcined at 450 ᵒC for 30 min. This method cannot be appropriate because prolonged calcination causes collapse of the mesopore morphology and the loss of active electrode area (roughness factor).29 They also reported a solar conversion efficiency of 5% for a 2.3 µm thick phosphor-doped templated-film.30 The chemical doping is not appropriate because it could induce electron–hole recombination. Furthermore, they used a dye bearing high molar extinction coefficient (N945 dye) and a different electrolyte to improve the Voc. Zhang et al. prepared 5-6 µm worm-like and cubic thick films by spin coating method using F127 as surfactant with a conversion efficiency of 67%.31 But their solar cell efficiency is weakened when the film thickness was above 3.5 µm because of the serious cracks in the film. While, no cracks was observed in our films and also they required 60 layers to make a 5.08 µm film but in our synthetic protocol, 18 layers were sufficient for obtaining a 7 µm thickness by dip coating method. Moreover, P123 copolymer was used in this work which do not show any pore filling after coatings due to pre-existing P123 micelles in the precursor solution unlike F127 surfactant that has not this advantage.27, 32 They also used one calcination step at the temperature 25


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of 450 ºC for 1h after the desired film thickness was reached while we used four calcinations steps for 18 layers (one calcination step after every five layers). This method increases the crystallinity and therefore solar conversion efficiency. Chi et al. also prepared a DSSC with 6.01% conversion efficiency from mesoporous titania films with thickness up to 3.5 µm after 21 cycles by dip coating method,52 Whereas, we built 7 µm thick films by our synthetic protocol and with 18 layers. A solar conversion efficiency about 6.1% was reported by Dewalque et al. for a 4 µm (15 layers) worm-like thick templated film obtained by dip coating method with a final calcination at the temperature of 400 ºC for 2 h and sensitized with N719 dye.27 Sun's group built a solar cell device with a bifunctional photoanode consist of a 30-layers mesoporous titania film (4.15 µm in thickness and 3-5 nm in crystallite size) and a Degussa P25 TiO2 light-scattering top-layer (4 µm). Their samples spin coated at 4000 rpm for 30 seconds and calcined at 400 ᵒC for 6 h showed 5.18% solar conversion efficiency.47 Compared to reported films in the literature, our TiO2 templated films showed excellent photovoltaic performance without addition of any doping agent or scattering layer. We had already prepared the worm-like and grid-like films with constant ratios of surfactant/Ti by spin coating and dip coating methods.32 A solar conversion efficiency of 8.33% was achieved from the DSSC made of the grid-like mesoporous film and prepared by the dip coating method. While in the current project, the cubic, worm-like, grid-like, and hexagonal films were prepared with different surfactant/Ti ratios and used in DSSCs. In our earlier study, the mesoporous films with higher crystallinity showed a smaller surface area and dye loadings. However, the meso-films showed a different behavior in the present study. The lowest dye loading and crystallinity as well as the highest surface area are the features of the hexagonal film with highest surfactant/Ti ratio. Besides, the lowest surface area, and the highest dye loading and crystallinity came from the cubic and grid like meso films with the lowest surfactant/Ti ratio. Ultimately, a conversion efficiency of 8.73% was achieved by cubic film in DSSC with no TiCl4

26


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treatment, scattering layer, or doping. This is the highest efficiency of mesoporous titania templated devices reported to date to the best of the our knowledge. CONCLUSION Worm-like, hexagonal, cubic, and grid-like films were prepared by EISA process with different surfactant/Ti ratios and applied in DSSCs. In this study, the meso-films exhibited a complex interplay of factors which determine the cell performance. Based on the findings, the hexagonal film with the highest surfactant/Ti ratio had the lowest dye loading and crystallinity as well as the largest surface area. Furthermore, the highest dye loading and crystallinity as well as the smallest surface area came from the cubic and grid like meso films with the lowest surfactant/Ti ratio. In addition, the worm-like meso film with a moderate surfactant content had a moderate dye loading and crystallinity. The findings indicated that a decline in the surfactant/Ti ratio to produce cubic mesostructures can cause a concurrent increase in crystallinity and roughness factor due to the increase in titania content. The mesostructured thick films (about 7µm) were compared in DSSCs with nanocrystalline TiO2 films (NC-TiO2) at the film thickness of 13 µm – as commonly used in DSSCs – and proved a superior performance. We could enhance the conversion efficiency of the cells made from the mesoporous templated films up to 8.73% - higher than the other efficiencies reported thus far -using cubic templated film without using any doping agent, TiCl4 treatments or scattering layer, which can be a significant improvement in this field. ACKNOWLEDGEMENT. The financial support of this work by the University of Isfahan is acknowledged. Also, the authors greatly thank Prof. Thomas E. Mallouk and his collaborator, Dr. Seung-Hyun Anna Lee, from the Pennsylvania State University, USA, for their useful scientific guidance and insightful comments. Authors also thank Prof. Geoffrey A. Ozin and Dr. Navid Soheilnia, from the University of Toronto, Canada, and Dr. Mohsen Khosravi-Babadi from the University of Isfahan for their invaluable scientific discussions. 27


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Graetzel, M. Organized

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(51) Park, J. T.; Chi, W. S.; Kim, S. J.; Lee, D.; Kim, J. H. Mesoporous TiO2 Bragg Stack Templated by Graft Copolymer for Dye-sensitized Solar Cells. Scientific Reports 2014, 4, 5505. (52) Chi, B.; Zhao, L.; Li, J.; Pu, J.; Chen, Y.; Wu, C.; Jin, T. TiO2 Mesoporous Thick Films with Large-Pore Structure for Dye-Sensitized Solar Cell. J. Nanosci. Nanotechnol. 2008, 8, 3877 -3882.

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Performance Enhancement of Dye-Sensitized Solar Cells Based on

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