High Conversion Efficiency of Dye-Sensitized Solar Cells Based on

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High Conversion Efficiency of Dye-sensitized Solar Cells Based on Corallike TiO2 Nanostructured Films: Synthesis and Physical Characterization Alireza Bahramian Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 24 Sep 2013 Downloaded from http://pubs.acs.org on September 25, 2013

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Industrial & Engineering Chemistry Research

High Conversion Efficiency of Dye-sensitized Solar Cells Based on Coral-like TiO2 Nanostructured Films: Synthesis and Physical Characterization Alireza Bahramian* Department of Chemical Engineering, Hamedan University of Technology, Hamedan, Iran ∗ [email protected], [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Coral-like TiO2 nanostructured films were chemically synthesized through the solgel method for fabrication of dye-sensitized solar cell (DSSC). The influence of experimental parameters such as precursor hydrolysis rate, reaction time, type and concentration of acid, and annealing temperature were studied through analysis of the surface structure of the films. The coral-like TiO2 film has excellent light scattering property and a mesoporous structure with fairly large specific surface area of 164 m2.g-1. The resulting DSSC, which consists of a dense, corallike TiO2 nanostructured film and the dye N719 in an electrolyte, shows a better performance compared to a fabricated DSSC by using commonly-used TiO2. A photocurrent value of approximately16.1 mA.cm-2, a fill factor of 77.6%, and a conversion efficiency of 9.4% were obtained. The low cost and the possibility of controlling the morphology of the prepared film, make this method as an interesting candidate for using it in fabricating photovoltaic devices.


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INTRODUCTION Searching for clean and green energy resources would be one of the most notable challenges in the next 100 years for any sustainable development in human civilization that could be potentially solve by renewable energy technology. The specific physical properties of titanium dioxide (TiO2) as an important inorganic material have been extensively noticed over the years mainly because of its potentiality for many devices applications. It has been used in photovoltaic cells,1-2 photocatalysis,3 dye-sensitized solar cells (DSSCs),4-6 gas sensors,7 and lithium ion batteries.8-9 TiO2 nanostructured films are also often used as various optical coatings for its high refractive index, wide band gap (3.02 and 3.23 eV for rutile and anatase phase, respectively) and suitable chemical stability.10-11 In most potential applications, the quality and surface structure of the films will undoubtedly play the pivotal role in determining their acts. The performances of TiO2-based devices are affected by shape, size, specific surface areas, pore structure, and thickness of the nanostructure layers.12-13 Based on this fact, research on the intrinsic morphology and particle size has caused a need for synthetic mechanism in controllable thin film designs. It was showed that two-dimensional (2D) structures such as nanosheets and three-dimensional (3D) nanostructures such as flower-like ZnO, SnO2, and even TiO2 clusters have superior photoelectron-catalytic and photovoltaic properties and entirely novel reaction pathways when they are compared to the nanoparticles with smooth surfaces.14-16 Despite the above-mentioned successful demonstrations, novel methods for the synthesis of new shapes of TiO2 nanostructures is essential to get a high specific surface area. Thus, flake-like, flower-like and cauliflower-like hierarchical TiO2 structures formed as 1D, 2D or 3D nanostructure units are found to be attractive photovoltaic and photocatalyst materials.10,17-18-19 They own the advantages of enhanced light absorption, because of reflection in the many voids of the inorganic metal oxides, thus usually showing higher photo efficiency. But, the assembled nanostructures provide much


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more exposed surface region, well helping the adsorption and photoreaction of reactants. For instance, experiments showed that the photocatalytical activity of flower-like anatase TiO2 nanostructures synthesized by a low-temperature process is almost two times greater than that of commercial P25 titania.20, 21 Various coating methods have been used during the two last decades for preparation of the TiO2 nanostructured films, such as reactive thermal deposition,22 chemical vapor deposition,23 chemical spray pyrolysis,24 pulsed laser deposition25 and sol–gel dip-coating method.26-28 In comparison with other methods, the dip-coating method has some advantages such as controllability, reliability, reproducibility, low cost and available to use on an industrial scale, which can be selected for preparation of thin film solar cells.29-30 Usually, TiO2 nanostructures synthesized by the hydrothermal process are in regular shapes with smooth surface, but this method is expensive and complex because of rapid hydrolysis rate of Ti-containing precursor in aqueous solution.31-33 Therefore, it is required to develop a simple and effective method for synthesis of new shapes of TiO2 nanostructured films under low-temperature conditions. In this work, we report a new and simple method for synthesis of coral-like TiO2 nanostructures under low-temperature conditions. The key innovation in the present study is to control the hydrolysis rate of the precursor by changing the type of acid and its concentration, temperature and reaction time. The method proposed in this work allows clarification of the specific morphology with excellent light scattering property and high surface area for fabrication of the assembled dye-sensitized solar cells with high conversion efficiency. EXPERIMENTAL SECTION Synthesis of coral-like TiO2 nanostructures. Coral-like TiO2 nanostructures were chemically

synthesis through the sol-gel technique. At first, an aqueous titanium tetra-isopropoxide [TTIP]


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(97.0% Sigma Aldrich) solution as a precursor was slowly added into ethanol under stirring in a drop wise manner. Later, a series of solutions were prepared with acids that is, HCl, HNO3, and HF dissolved in deionized water under stirring at room temperature. Then, the TTIP solution was slowly added into last one under stirring for 30 min at the temperature of 90 oC in a three-necked refluxing pot. During refluxing, temperature of the solution was controlled by inserting manually adjustable thermocouple in the refluxing pot. The solution was allowed to react for 4 h at the above temperature while the volumes of aqueous solution reach to the half of first solution. After stirring and refluxing, a homogenous milky solution was formed. The deionized water was slowly added to milky solution until its volume reach to the initial value. The obtained solution was allowed to stir slowly during 1 to 42 h at 40-60 oC until hydrolysis reaction could be finished and a light bluish resulting sol could be formed. The molar ratios of the reagents were adjusted as follows: TTIP:HCl:C2H5OH:H2O = 1:5:18:340 (denoted as R1); TTIP:HNO3:C2H5OH:H2O = 1:5:18:340 (denoted as R2); TTIP:HF:C2H5OH:H2O = 1:0.5:18:340 (denoted as R3); TTIP:HF:C2H5OH:H2O = 1:1:18:340 (denoted as R4); TTIP:HF:C2H5OH:H2O = 1:2:18:340 (denoted as R5). Preparation of TiO2 film. Soda lime glasses as a substrate were cleaned in an ultrasonic bath

containing ethanol for 10 min and treated in ultraviolet cleaner system for 15 min. The TiO2 nanostructured films were obtained by immersion of cleaned substrates into TiO2 solution (corresponding to samples R1-5) using a dip-coater with minimum withdrawal velocity of 0.5 cm.min-1 to reach a homogeneous film. The prepared films were preheated at the temperature of 125 oC in an electric oven and cooled down to room temperature. The same process was repeated two times, then the prepared film annealed from 200 to 400 oC for 1 h using a furnace in vacuum condition with temperature rates of 2-3 oC min-1.


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DSSC fabrication. Conventional DSSCs (Grätzel cells) consist of a self-assembled monolayer of

molecular dye at the interface between a mesoporous wide-band gap semiconductor such as titanium dioxide and an electrolyte.1, 34-35 The most commonly used redox couple is iodide/triiodide (I–/I3–) in an organic liquid electrolyte. DSSC fabricated in this work comprised of two layers of TiO2 film. Under-layer was composed of dense TiO2 nanometer-sized film, while corallike TiO2 nanostructured film was coated on the under-layer. Dye-sensitized solar cell fabricated in this work comprised of two layers of TiO2 film. Underlayer was composed of dense TiO2 nanometer-sized film, while coral-like TiO2 nanostructured film was coated on the under-layer. To make a dense TiO2 sandwiched layer, a cleaned fluorinedoped SnO2 glass substrate (FTO, Pilkington, TEC 8 glass, 8 Ω for each square, 2.3 mm thick) was immersed in HNO3-ethanol mixed aqueous solution (corresponding to sample R2) using the dip-coating method. At first, the obtained film was preheated at 125 oC for 10 min. After the coated film was cooled down to the room temperature, the same process was repeated two times. The coated FTO glasses with dense TiO2 film were heated at 400 oC for 1 h, which also improve the crystallinity of the TiO2. The thickness of dense TiO2 film was measured to 100±5 nm by Dektak profilometer. On the prepared dense TiO2 blocking layer, the coral-like TiO2 nanostructured film was deposited and the prepared film was annealed at 400 oC for 1 h. For this propose, the FTO/denseTiO2 film was immersed into the solution of R4 by dipping withdrawing cycles at room temperature. Thickness of the prepared film was adjusted to an average value of 2.0 µm, using a micrometer adjustable film applicator.30 The TiO2 film was then dipped in an ethanol solution containing purified 3×10-4 M [cis-di isothiocyanato-bis (2,2- bipyridyl-4,4dicaboxylato) ruthenium (II) bis(tetrabutylammonium)] (dye N719, Solaronix, Switzerland) for 18 h at room temperature, followed by rinsing with ethanol and drying process under vacuum. Finally TiO2 electrode were employed to assemble a sandwich-type cell with a liquid electrolyte


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[containing 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPImI, Solaronix), 50 mM I2, 0.5 M LiI, and 0.5 M 4-tert-butylpyridine in Acetonitrile], and a Pt foil as counter electrode 1, 5, 34, 36-39

according to procedure reported elsewhere.

. Figure 1 illustrates the solution-

processed DSSC architecture.

Pt

Electrolyte

TiO2

Dye

FTO

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Figure 1. Solution-processed DSSC architecture. [The adsorbed dye molecule absorbs a photon from a light source and forming an excited state (dye*). This excited dye transfers an electron to the TiO2 and a hole to the electrolyte. This separates the electron-hole pair leaving the hole on the dye (dye*+). The hole is filled by an electron from an iodide ion. The reaction is: 2dye*+ + 3I- → 2dye + I3-]. Physical characterization. The morphology and microstructure of the coral-like TiO2

nanostructures were examined by scanning electron microscopy (SEM) using a Cam Scan MV2300 microscope. The thickness of dense and coral-like nanostructured film was measured by Dektak Stylus profilometer (Bruker) and micrometer adjustable film applicator. The crystalline phase of TiO2 nanostructures were determined by X-ray diffraction (XRD) method using a Philips PW 1800 diffractometer with Cu Kα radiation. Spectroscopic analyses of TiO2 films were performed by a UV–vis spectrophotometer (Hitachi 3140) operating in the 300–1000 nm region. The surface area was calculated by the Brunauer-Emmett-Teller (BET) method using a Micromeritics ASAP 2020 surface area analyzer. Pore size distribution was calculated by the Barrett -Joyner - Halenda (BJH) method by the adsorption branch of N2-sorption isotherms. The


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incident photon-to-current conversion efficiency (IPCE) spectrum for the solar cell was measured at AC mode on an IPCE measuring system (PV measurement Inc.). A 75 W xenon lamp as an original light source for monochromatic beam and a 75W–12 V halogen lamp were used as a bias light source. The photovoltaic performance of the cell was evaluated under intensity of 100 mW.cm-2 standard Air Mass 1.5 global (AM 1.5G) sunlight simulation with a class A solar cell analyzer (Spectra Nova Tech.). The intensity was calibrated by using KG5 filtered Si reference solar cell. RESULTS AND DISCUSSION Figure 2 presents SEM images of the TiO2 nanostructures obtained under different synthesis conditions. Figure 2a shows the surface morphology of TiO2 film obtained by an HCl-ethanol mixed aqueous solution (corresponding to sample R1). It can be seen that the smooth surface comprises of single nanorods with mean diameter of 150 nm and length of about 900 nm. When HCl replaced by HNO3-ethanol mixed aqueous solution (corresponding to sample R2), the spherical particles with a non-uniform size distribution are given in Figure 2b with an average diameter of 80 nm. But, the TiO2 nanostructures synthesized in samples R1 and R2 were not homogenous and intact structures, and reproducibility was not good. These results are in good agreement with the results obtained by other researchers.36, 40 It can be noted that the formation of the two different morphologies and crystal structures depends on the presence of ions Cl− or NO3− during synthesis. When HCl medium was used during the synthesis, the free chlorine ion with weaker affinity to titanium compounds had an anisotropic effect with small changes in the phase diagram of particles. When HNO3-ethanol mixed aqueous solution was used during the synthesis, the strong affinity of NO3− ion to titanium compounds inhibits the structural rearrangement and, then, irregular nanospheres were obtained on the film surface. In the last


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conditions, the spherical assembly can significantly reduce the total free energy. Generally, if the surface energy is isotropic, the obtained crystal should be nearly spherical, but if the surface energy is anisotropic, the special limitations can contribute to the energy minimizing shape. Figure 2c shows low-magnification SEM images of TiO2 nanostructured film obtained by HFethanol mixed aqueous solution (sample R3). Large quantities of coral-like TiO2 nanostructures were formed on the film surface, which may undergo a preferential coarsening of the nanoparticles as governed by the surface charges and surface energy. The growth and morphology of the coral-like TiO2 structures may be explained as follows. First mechanism is dissolution of smaller, more soluble particles, accompanied by the growth of larger particles, which is called Ostwald ripening process.41-42 In this process, semispherical nanoparticles are produced in the fast nucleation stage. At the beginning of the coarsening period, nanoparticles with cashew-like forms can be seen on the surface of the coral-like structures. Later, complete crystalline coral-like structures are formed by smoothing their surfaces through Ostwald ripening. Second proposed mechanism may be agglomeration of primary particles to form small particles with fairly spherical shape. In this mechanism, the surface of the semi-spheres acts as nucleation sites for formation of small blisters of TiO2. As the reaction proceeds, regarding the energy level, it is more favorable for the small TiO2 blisters to grow, which occurs through shrinkages and protuberances in crystalline coral-like structures. It is difficult to determine the exact size of coral-like structures as they are aggregated. However, it may be estimated that the most coral-shape structures are in the range of 200-600 nm. These aggregates seem to be formed by super-positing several layers, which make interesting structures for their unique shape. After magnification, it was found that the surface of the corallike structures was composed of many nanoparticles with irregular shape and sharp tips. The size of coral-like TiO2 structures differs spatially from upper part to the lower one. For instance, the


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typical size of one particle is 24.61-40.37 nm as shown in Figure 2d. Nevertheless, Figure 2d suggests that size distributing particles is fairly uniform.

a

b

c

d

Figure 2. SEM images of TiO2 nanostructures. Low-magnification images of sample (a) R1, (b) R2, (c) R3, and (d) High-magnification image of R3 Further characterization of the TiO2 films obtained from the samples R4 and R5 are provided by the SEM images shown in Figure 3. Comparing Figures 2c, 3a and 3b (low-magnification SEM images), it can be seen that with a rising molar ratio of HF in aqueous solution, corresponding to samples R4 and R5, the sizes of TiO2 coral-like structures decreased. In addition, as shown in Figures 3c and 3d, the typical size of one particle decreased from 18.0025.99 nm (sample R4) to 10.07-15.00 nm (sample R5). The TiO2 nanoparticles formed on the coral-like structures that obtained from samples R4 and R5 have heterogeneously distributed particle size and non-uniform shapes appearing in cashew-like forms. The nanostructures prepared with low molar ratio of HF (corresponding to sample R3) had more homogenously distributed particle size and more uniform shapes than those synthesized in high molar ratio of HF (sample R5).


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a

b

c

d

Figure 3. SEM images of TiO2 coral-like structures obtained from samples: (a) R4 and (b) R5. [(a-b): Low-magnification images; (c-d): High-magnification images] From the SEM image shown in Figures 4, we can observe that some non-uniformly grown crystals of TiO2 are trapped among the coral-like nanostructures, when the molar ratio of TTIP: HF corresponds to sample R3. Also, it can be seen from the Figure 4a that the film surface consisted of two layers. The upper layer comprises of coral-like aggregates made of randomly aligned nanoparticles (Figure 4b), while lower layer is formed by nanocrystals contributing to high specific surface area (Figure 4c). a

b

c

Figure 4. SEM images of TiO2 nanostructured film obtained from sample R3 (a) Lowmagnification image of film surface, (b) High-magnification image of coral-like aggregates, (c) High-magnification of nanocrystals


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To determine optimized reaction time for reaching the best morphology, various timedependent experiments were performed. The molar ratio of TTIP: HF was kept at 1:1 (sample R4), while the reaction and annealing temperatures were 50 and 300 °C, respectively. The lowmagnification SEM images are given in Figures 5a, b, c, d, e, and f corresponding to reaction times of 1, 6, 12, 24, 36, and 42 h, respectively. During the early stage of the reaction (1 h), non-uniform surface morphologies was formed (Figure 5a). Nevertheless, by raising the reaction time (6 h), the surface morphology of the resultant film was changed and a fairly uniform distribution of nanoparticles was formed on the film surface (Figure 5b). However, there are some blurred points on the film surface. With a rise in the reaction time (12 h), the nanoparticles start to stick together partly and form the smooth coral-like structures (Figure 5c). By rising further the reaction time to 24 h, the precipitation in the interstices of smooth structures considerably higher and fairly rough structures consisting of cumulative aggregates were obtained (Figure 5d).On the other hand, for the reaction times between 12 to 24 h, the coral-like structures further grew, which induce large size structures. As a result, the voids between coral-like structures become more evident. By extending further the reaction time to 36 h, the particles started to adhere and assemble in somewhat hierarchical corallike structures with the sizes of several hundred nanometers to fine micrometric size (Figure 5e). As shown in Figure 5e, some of the collapsed particles from assembled non-uniform spheres are still loose. It can be expected that, when reaction time is extended to 48 h (Figure 5f), the TiO2 structures continuously grow, which gradually self-assemble into irregular hierarchical structures under the help of van der Waals interactions. The high-magnification SEM images of samples synthesized with reaction times of 6, 12, and 24 h are presented in Figures 5g, h, and i respectively. Figure 5g shows uniform distribution of particles with size of around 30-50 nm that was formed on the film surface. With a longer reaction time (12 h), premature structures were


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precipitated on the film surface (Figure 5h). In the aging period, gel dissolution and reprecipitation occurred, driven by differences in solubility between surfaces with different radii of curvature. This led to structures with lower defects and vacant sites. As a result, during the calcinations step of the films whose aging time were 12 h, the diffusion pathways for nucleation and crystal growth would be largely obstructed, leading to smooth coral-like structures consisting of nanoparticles. When the aging time is extended from 12 to 24 h, the size of coral-like structures rises roughly from 200-500 to 400-600 nm (Figures 5h and i). It can be seen that with rising the aging time to 24 h, precipitating of TiO2 particles was raised, which fills the voids between adjacent nanoparticles formed on the surface of coral-like structures. As a result, it was found that the best aging time to obtain the coral-like TiO2 structures is 24 h.

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Figure 5. SEM images of the TiO2 nanostructures synthesized at different reaction time of (a) 1 h, (b and g) 6 h, (c and h) 12 h, (d and i) 24 h, (e) 36 h, and (f) 42 h [a-f: low-magnification and g-i: high-magnification SEM images] All samples calcinated at 300 oC As for changing the aging temperature from 40 to 60 oC, there were no significant changes in the morphology and shape of TiO2 nanostructures obtained from the samples R3 to R5. The mentioned temperatures were chosen in the experiments to ensure that the alcoholic solution stays below its boiling point. This may be caused by complete polymerization of the titanium precursor leading to better dispersion of TiO2 nanoparticles. The effect of annealing temperature on the surface morphology of final films was analyzed with three annealing temperature of 200, 300 and 400 oC. Low-magnification SEM images in Figure 6a-c show the morphological evolution of the as-prepared TiO2 nanostructured films. The TiO2 film obtained at temperature of 200 oC as shown in Figure 6a was form an amorphous phase, however, the small amount of the anatase crystalline phase can be found in the amorphous matrix. After calcinations treatment at 300 and 400 oC (Figures 6b-c), the as-prepared amorphous TiO2 films were transferred into crystalline ones, which well-preserve the coral-like structures. High-magnification SEM images of the coral-like TiO2 films obtained from the solution synthesized at reaction time of 24 h and at annealing temperatures of 200, 300, and 400 oC are presented in Figures 6d, e and f, respectively. It seem that, at lower calcination temperature bigger particle sizes of about 17.58-28.95 nm are induced (Figure 6d), while higher calcination temperature (corresponding to 400 oC) induces smaller particle size of around 11.72-16.07 nm (Figure 6f).


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a

b

c

d

e

f

Figure 6. SEM images of the TiO2 thin films at annealing temperatures of 200 oC (a and d), 300 o

C (b and e), and 400 oC (c and f) [a-c: low-magnification and d-f: high-magnification SEM

images] All films obtained from the solution synthesized at reaction time of 24 h XRD is an essential technique in determining the crystallinity, crystal phase, and in estimating of the crystal grain sizes according to Debye-Scherrer formula. Crystallite size is determined by measuring broaden of a particular peak in a diffraction pattern associated with a particular planar reflection from within the crystal unit cell. It is inversely related to the full width at halfmaximum (FWHM) of an individual peak, the narrower the peak - the larger the crystallite size. If the crystals are randomly arranged, the result will be a broader peak. Figure 7 shows XRD patterns of coral-like TiO2 nanostructured film obtained from sample R4 under annealing temperatures of 200, 300 and 400 oC. X-ray diffraction peaks ascribed to (101), (004), (200), (105) and (211) planes of TiO2. The TiO2 film annealed at 200 oC shows the weak anatase phase peak according to (101) plane. It can be seen that anatase is the dominant phase in TiO2 film, which was annealed at 300 oC, but the weak rutile phase diffraction (110) was detected for the film annealed at 400 oC. However, the phase transformation temperature in air from anatase to


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rutile TiO2 is between 600 to 750 oC. The crystalline size of (101), (004), or (200) planes were estimated from their FWHM related peaks to be 12.3, 19.2, or 16.6 nm, respectively. Crystallite size of (004) plane was larger than that of the others. Difference in the crystallite size showed anisotropic crystal growth along the c-axis to form a coral-like morphology.

A (101)

80

200 ooC C 300 ooC C

A (204)

A (211)

A (105)

R (110)

50 40

400 ooC C

A (004)

60

A (200)

70

Intensity (a.u.)

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30 20 10 0 20

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2 Teat

Figure 7. XRD patterns for the coral-like TiO2 nanostructured films obtained from sample R4 under annealing temperatures of 200- 400 oC As seen in the X-ray diffraction patterns presented in Figure 7, rising the annealing temperature from 200 to 400 oC leads to a rise in the intensity of TiO2 peaks. As showed by the XRD patterns of film which was calcinated at 200 oC, there is a hump at 2θ between 21 to 36o. It demonstrates the existence of carbon in the film, and it is in agreement with the observation of light matte gray product. Carbon is produced from decomposition of alkoxide groups in the precursor during hydrolysis process. Besides, the broad peak implies low crystallinity, small crystal size or long range ordering. Carbon can be removed by a simple calcination treatment. After calcination at 300 oC, the size of hump becomes smaller, and at temperature of 400 oC the hump completely disappears, confirming the removal of carbon impurity from the film and/or high crystallization. In this condition, observation shows that the color of the film is white. The diffraction peak for


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calcinated film at 400 oC is broader than those calcinated at 300 oC, suggesting a higher crystallization rate at higher temperature. Optical properties of the TiO2 films were evaluated with UV-vis spectroscopy. Figure 8a shows transmittance spectra of the dense nanostructured TiO2 film obtained from the sample R2 at annealing temperatures of 200, 300 and 400 oC in the wavelength range of 300-1000 nm. The thickness of all films was 100±5 nm, which was coated on the FTO glass. The spectra can be divided into two regions. Transparencies rise suddenly with a rise of wavelength in the range of 395-440 nm and reach to a fairly stable value because of the interference between the film and the glass substrate in high wavelengths. The fast decrease approximately below 395 nm is due to absorption of light caused by the excitation of electrons from the valence band to the conduction band of TiO2 nanostructured films. According to Figure 8a, it can be seen that there is an important difference in transmittance spectra between annealing temperatures. This big difference is caused by carbon impurity in the as-prepared films, which provide copious micropores to the materials. After calcinations, carbon and organic compounds are removed and the products are phase-pure mesoporous TiO2 nanostructured films. Therefore, a maximum transmittance (approximately 80%) is seen for the film that was annealed

at 400 oC, while a low transmittance (about 68%) with more fluctuation is obtained for the film annealed at 200 oC. The bulk band gap structures are known as direct-transition for anatase and indirect-transition for rutile phase in titanium dioxide, respectively. The band gap, Eg, is usually estimated from the absorption edge wavelength of the interband transition according to the following equation43: 1

α hυ = A ( hυ − Eg ) n

(1)

where α is the absorption coefficient, hυ is the discrete photon energy, A is the proportionality constant, and n equals 2 for direct transition and 1/2 for indirect transition.43


17


The band gap of the TiO2 nanostructured film annealed at 400 oC was also estimated to be 3.22 ev (395 nm) by assuming direct transition (Figure 8a, inset). It was similar to that of TiO2 nanostructures such as anatase TiO2 nanowalls (3.2 ev (388 nm))9, 44, TiO2 hierarchical nanostructures (3.02 ev (409 nm))45, single-layered TiO2 nanosheet with a thickness less than 1 nm (3.15 ev (394 nm))46, multineedle TiO2 nanostructures (3.2 ev (388 nm))10 and anatase TiO2 films (3.2 ev (388 nm)) 47. The small difference in the band gap of TiO2 particles reported by other researchers is caused by ratio of anatase to rutile phase in a mixture of particles, because the band gap of anatase (3.2 ± 0.1 ev) is larger than that of rutile phase (3.0 ± 0.1 ev).47

90

o

300 C

70

o

200 C

60 50 30 20 10

b

o

400 C

80

40

1

a

Uncoated glass

α2 3.22 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4

o

Transmittance (%)

100

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 32

400 C

0.8

200 oC

0.6 0.4 0.2

Photon energy (ev)

0

0 300 400 500 600 700 800 900 1000

300 400 500 600 700 800 900 1000

Wavelength (nm)

Wavelength (nm)

Figure 8. UV-vis spectra of (a) dense TiO2 nanometer-sized film and (inset) band gap estimation, (b) coral-like TiO2 nanostructured film with thickness of 2 µm Figure 8b shows the UV-vis spectra of coral-like TiO2 nanostructured film with thickness of 2 µm prepared on FTO glass/dense TiO2 film at annealing temperatures of 200 and 400 oC. Curves show the transmittance of coral-like TiO2 film showed in Figure 8b is less than 1%, suggesting that most incident light can be scattered It clearly shows that the TiO2 film with coral-like


18

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structure has excellent light scattering performance that can also yield desirable performance for solar cells. N2 adsorption-desorption isotherms of the coral-like TiO2 nanostructures are shown in Figure 9. Surface area of the as-prepared films obtained from the solution of R4 at three reaction times of 12, 24 and 36 h, and calcinated films at two annealing temperature of 300 and 400 oC are given in Figure 9 (left). There are two distinct capillary condensation steps in the N2 adsorptiondesorption isotherms curves A and B in Figure 9. The first hysteresis loop is at 0.4 < P/Po < 0.52, corresponding to the filling of the mesopores formed between intra-agglomerated primary nanoparticles. The second hysteresis loop is at 0.92 < P/Po < 1, corresponding to the filling of macropores produced by interaggregated secondary particles. The hysteresis loop at 0.68 < P/Po < 1 in curve C reveals the existence of hierarchical coral-like TiO2 nanostructures. From the Figure 9 (curve D), it can be found that the film calcinated at 400 oC has smaller specific surface area, 131 m2.g-1. This difference is caused by carbon impurity remained in the as-prepared films, which provides copious micropores to the materials. Carbon impurities make more mesopores on the films caused by high specific surface area that were removed after calcination at 400 oC. 0.05

250

Desorption

0.04

Adsorption

200

150

dV/dD (cc/nm.g)

Volume adsorbed (cc/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


D

100

C

0.03 D

0.02

C

B

50

B

0.01

A

A

0

0

0

0.1

0.2

0.3

0.4

0.5 0.6

0.7

0.8

0.9

1

1

Relative Pressure (P/Po)


10

100

Pore diameter (nm)

19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 32

Figure 9. N2 adsorption-desorption isotherms (left) and pore size distribution plots (right) of the coral-like TiO2 nanostructures. Sample of R4 synthesized for (A) 12 h, (B) 24 h, and (C) 36 h at annealing temperature of 300 oC; (D) Sample B calcinated at 400 oC. The specific surface areas of films calcinated at 300 and 400 oC are summarized in Table 1. The coral-like TiO2 nanostructured film had an maximum specific surface area of 164 m2.g-1, which is higher than that reported for TiO2 nanoparticles such as Aeroxide P90 (BET ≈ 90-100 m2.g-1, 14 nm in diameter, anatase 90%+ rutile 10%, Degussa)10 and Aeroxide P25 (BET ≈ 45 m2.g-1, 21 nm in diameter, anatase 80%+ rutile 20%, Degussa).48-49 This high surface area suggests that these thin films should have excellent performance as electron carriers for fabricating of photovoltaic devices. Table 1: Specific surface areas of films calcinated at temperatures of 300 and 400 oC Reaction time (h)

specific surface area (m2.g-1) 300 oC

400 oC

12

142

131

24

164

149

36

91

86

Pore size distribution of the as-prepared films obtained from the sample of R4 is given in Figure 9 (right). Curves A, B, and C represent the film calcinated at 300 oC obtained from the solution synthesized at reaction times of 12, 24, and 36 h, respectively. Curve D represent the film calcinated at 400 oC, which was obtained from the solution synthesized at reaction time of 24 h. The pores are mainly resulted from a variety of aggregated voids among the TiO2 nanoparticles. The pore sizes for the samples calcinated at 300 oC are about 21 nm (A), 14 nm (B) and 17 nm (C). The product calcinated at 400 oC shows pore size of about 12 nm (D). Thus,


20

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it can be found that the higher annealing temperature induces the smaller pore size with the growth of the crystallites and shrinkage of the aggregates, resulting in a right shift of intraaggregated pores and a left shift of inter-aggregated pores.12 To explore the possibilities for photovoltaic applications, the performance of a DSSC assembled based on submicron-thick film consist of coral-like TiO2 nanostructures synthesized based on the sample R4 was evaluated (Figure 10). Figure 10a shows the current density−voltage (J−V) curve for device and reference Pt counter electrodes measured under 100 mW.cm−2 AM 1.5G illumination. The largest power output (Pmax) is determined by the point where the product of voltage and current is maximized (denotes as VMPP and IMPP in Figure 10a). Division of Pmax by the product of the short-circuit current density (Jsc) and open-circuit voltage (Voc) yields the fill factor (FF). For the present solar cell, Jsc = 16.12 mA.cm-2, Voc = 0.752 V, FF = 77.63 %, and conversion efficiency, η, of 9.4%, (η = Voc × Jsc × FF). Figure 10b compares the IPCE as a function of incident wavelength for the present cell and the Grätzel cell in the test range of 400700 nm. The IPCE spectrum is a measure of the light response of photovoltaic devices, which is directly related to the short-circuit current. In the visible region from about 480 to 630 nm, a strong and fairly constant photo-response is observed and the efficiency lies between approximately 70-80% with a nearly flat profile. In the 570-670 nm spectral range, this cell produces a higher and broader photocurrent density in the external circuit under monochromatic illumination (for each photon flux). Note that the upper limit of our IPCE measurement setting is 700 nm, resulting in a sharp drop about 640 nm.


21


100

a

b

90 80 70

IPCE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 32

60 50 40 30 20

Gratzel cell

10

DSSC based on coral-like TiO2

0 400 450 500 550 600 650 700

Wavelength (nm)

Figure 10. Photovoltaic characteristics of DSSC fabricated based on the coral-like TiO2 nanostructures synthesized based on the sample R4. (a) Photocurrent density-voltage curve. (b) IPCE spectrum as a function of incident wavelength.

The photovoltaic characteristics of DSSCs fabricated based on the different TiO2 samples (R 35) are summarized at Table 2. According to this table and based on the different TiO2 structures, the following points should be noted: (1) The current density of the DSSC fabricated based on the coral-like TiO2 nanostructures is larger than that of a reported nanostructured film consisting of Degussa P25 TiO2 powder (Jsc ≈ 15.7 mA.cm-2).38 The coral-like TiO2 structures can be attributed to the larger surface area of the prepared film, which enables more dye molecules to be adsorbed caused by fairly suitable dye loading. (2) The relatively high Voc in this cell may be explained by the electron recombination at the interface of the TiO2 film and the loss of light upon passage through the electrolyte.38 The present cell exhibits an increase in conversion efficiency in compared with the solid-state solar cells consist of CsSnI3/nanoporous TiO2 film (η = 3.72%), CsSnI2.95F0.05+ 2% SnF2/nanoporous TiO2 film (η = 6.81%)29 and CsSnI2.95F0.05+ 5% SnF2/nanoporous TiO2 film (η = 9.28%)50 and close to that of the (CH3NH3)PbI3 perovskite cell


22

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(η = 9.7%),30 CsSnI2.95F0.05 + 5% SnF2 with ZnO (η = 10.2%)51 and highest reported performance N719-dye-containing Grätzel cell (η ≈ 11%).51 (3) The FF values for DSSC based on the corallike TiO2 nanostructured films are higher than those obtained using TiO2 nanoparticles (60-70 %). This is attributed to fairly low recombination between photo-excited carriers in the photoanodes and tri-iodide ions in the electrolyte. (4) The conversion efficiency rises as the surface structure of TiO2 film changes from coral-like aggregates to the relatively smooth coral-like structures because the effective surface area increases.

Table 2. Photovoltaic characteristics of DSSCs fabricated based on the different TiO2 samples Sampl e

Reaction time

Structure

Jsc

Voc

FF

(mA.cm-2)

(V)

(%)

(h)

R3

R4

R5

Efficienc y (%)

12

coral-like aggregates

15.38

0.743

74.16

8.5

24

fairly smooth coral-like

15.89

0.750

76.40

9.1

36

smooth coral-like

15.72

0.745

75.37

8.8

12

smooth coral-like

15.73

0.748

75.44

8.9

24

Coral-like

16.12

0.752

77.63

9.4

36

hierarchical coral-like

15.65

0.731

75.22

8.6

12

Large aggregates

15.24

0.735

74.10

8.3

24

hierarchical coral-like

15.59

0.737

75.05

8.6

36

irregular hierarchical

14.97

0.743

73.88

8.2

The increase in efficiency can be explained by considering a combination of several effects. First, the enhanced photon absorption associated with the augmented surface area results in higher dye loading and correspondingly to larger JSC. Second, mesoporous structures can increase the electron diffusion length because the micropores provide more direct conduction paths for electron transport between the film material–electrolyte–counter electrode interfaces within the DSSC fabricated based on the coral-like TiO2 nanostructured films. Third, randomly aligned


23


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Page 24 of 32

nanoparticles with cashew-like shapes formed on the coral-like TiO2 structures promote enhanced light-dye interaction without sacrificing efficient electron transport.

CONCLUSION In summary, novel coral-like TiO2 nanostructures were synthesized through the sol-gel method at low reaction temperature for fabrication of DSSCs with high conversion efficiency. The influence of experimental parameters such as precursor hydrolysis rate, reaction time, type and concentration of acid, and reaction and annealing temperatures were investigated through analysis of the surface morphology of the prepared films. It was found that the surface of the coral-like structures was composed of nanoparticles with cashew-like shapes, which result in a mesoporous structure with excellent light scattering property and high specific surface area of 164 m2.g-1. This structure is a better electron carrier and as a result exhibits better performance than commonly-used TiO2 film for fabricated DSSC. A photocurrent value of about16.1 mA.cm2

, a fill factor of 77.6%, and a conversion efficiency of 9.4% were obtained. The low cost and the

possibility of controlling and adjusting the morphology suggest this method as an interesting candidate for using it in fabricating photovoltaic devices. ACKNOWLEDGMENT This work was supported by Hamedan University of Technology under Grant number of HUTCE-082010.


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(51) Lee, B.; Buchholz, D. B.; Guo, P. J.; Hwang, D. K.; Chang, R. P. H. Optimizing the performance of a plastic dye-sensitized solar cell. J. Phys. Chem. C 2011, 115, 9787.


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For TOC only

TiO2 Nanostructures

High Efficiency DSSC

Cauliflower Coral Reefs


32

High Conversion Efficiency of Dye-Sensitized Solar Cells Based on

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