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Effects of compression at elevated temperature for electrophorically deposited TiO2-based dye-sensitized solar cell

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Japanese Journal of Applied Physics 55, 01AE13 (2016) http://doi.org/10.7567/JJAP.55.01AE13

Effects of compression at elevated temperature for electrophorically deposited TiO2-based dye-sensitized solar cell Md. Shamimul Haque Choudhury1,2*, Naoki Kishi1, and Tetsuo Soga1 1

Department of Frontier Material, Nagoya Institute of Technology, Nagoya 466-8555, Japan International Islamic University Chittagong, Chittagong 4203, Bangladesh

2

*E-mail: [email protected] Received April 30, 2015; accepted August 25, 2015; published online December 11, 2015 In this investigation, dye-sensitized solar cells (DSSCs) were prepared by electrophoretic deposition (EPD) of commercially available nanometersized titanium oxide (TiO2) nanoparticles (anatase, ST01) on fluorine-doped tin oxide (FTO) glass substrates. The rate of cathodic electrophoretic deposition of TiO2 nanoparticle agglomerates and the density of the obtained films were explored as a function of the applied electric field, keeping optimized suspension compositions, such as the particle concentration and the type of solvent. Optimized deposition conditions were found to result in homogeneous, well-controlled, mesoporous TiO2 thick-film photoanodes. Compression of the prepared glass substrate TiO2 photoanode at elevated temperature was commenced as a promising postdeposition surface treatment. The photovoltaic performance characteristics of DSSC prepared by this method were found to be considerably improved compared with those of DSSCs prepared by high-temperature postannealing and compression at room temperature. Surface morphologies were observed by scanning electron microscopy (SEM) and significant improvement was observed after compression as well as compression at elevated temperature. © 2016 The Japan Society of Applied Physics

1.

Introduction

Most solar cells are fabricated using Si-based materials because of their higher photovoltaic performance.1) However, dye-sensitized solar cells (DSSCs) are now being considered as an alternative to silicon-based solar cells.2–4) The use of DSSCs can be a credible cost-effective solution for the present renewable energy demand, making them a hot research theme nowadays. DSSCs show outstanding power conversion efficiencies because of using metal oxide nanoparticle films for the photoanode. TiO2 is stable and nontoxic, and has a relatively high transmittance in the visible spectrum, making it favorable for DSSC application. The band gaps of rutile and anatase-phase TiO2 are 3.0 and 3.2 eV, respectively. The anatase phase has good photocatalytic properties and wide direct band gap suitable for this application.5,6) A DSSC has three main components: i) the dye that absorbs solar energy to generate excitons,7,8) ii) a metal oxide photoanode that accepts electrons from the dye,9,10) and iii) the redox electrolyte that supplies electrons and holes between the dye and the counter electrode.11,12) Therefore, the photovoltaic performance is solely influenced by the transparent conductive oxide (TCO) used as the photoanode, sensitizer dye, and electrolyte. The preparation of a homogeneous layer of metal oxide photoanode is one of the main requirements of the DSSC. The conventional method of preparing porous nanocrystalline TiO2 electrodes is doctor blade coating.2,13) In order to improve the photovoltaic performance of the DSSC, some other techniques such as screen printing,14–17) sol gel,18–22) hydrothermal deposition,23,24) spray coating,25,26) and liquidphase crystal deposition (LPCD)27) methods have been comprehensively studied for preparing TiO2 thin films. In these methods, organic additives are used to obtain uniform crack free layers of metal oxides. High-temperature postannealing of the films at about 450 to 550 °C is used in these methods to remove the organic additives and to sinter the nanoparticles together, in order to obtain a better electrically connected network. Nowadays, electrophoretic deposition of metal oxides on a conductive glass or plastic substrate is one of the popular

low-temperature fabrication methods that has been investigated by many researchers.28–31) Researchers are showing interest in this deposition method as it offers a much more controlled deposition of metal oxides and it requires very simple arrangements.32) It is a very effective method for the preparation of thick binder-free films on conductive substrates in a much shorter time compared with other coating techniques.33) Cathodic electrophoretic deposition using P25 and P50 was performed by Grinis et al. in 2008.34) They investigated the photovoltaic performance of the DSSC by compressing the photoanode and by changing the layer thickness. The effect of the layer thickness of the electrophorically deposited TiO2 photoanode on the performance of the DSSC was also investigated.34,35) Later, the effect of suspension composition, such as the concentration of P25, and the effects of different types of solvent on the surface morphology and photovoltaic performance of DSSCs were also investigated.36) The effect of postdeposition chemical treatment on the electrophoretically deposited TiO2 film was reported by another group.37) Mechanical compression is one of the effective postdeposition surface treatments generally used for flexible substrates and is rarely used for conductive glass substrates to prepare DSSCs.38,39) The effect of heating during compression was initially investigated by our group for a TiO2 photoanode on a flexible poly(ethylene terephthalate) (PET) substrate.40) However, the effect of compression at an elevated temperature was not investigated for glass-substrate photoanodes. In this investigation, DSSCs were prepared by electrophoretic deposition under optimized conditions and the effectiveness of compressing the deposited FTO glass substrate at an elevated temperature was studied. 2.

Experimental methods

2.1 Electrophoretic deposition and postdeposition surface treatment

TiO2 suspension for electrophoretic deposition was prepared in two steps. TiO2 (anatase, ST01, 7 nm) at 20 wt % and acetylacetone at 0.1 vol % were added into ethanol. Acetylacetone was used to break the aggregates. The suspension was then dispersed with the homogenizer at 70% amplitude for 10 min. Before electrophoretic deposition, acetone (8 ml

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M. S. H. Choudhury et al.

Fabrication and characterization of flexible DSSC Postdeposition-treated TiO2 photoanodes were then dipped into an ethanol solution of the Ru-complex dye N719 at a concentration of 0.5 mM. Pt-coated glass was used as a counter electrode. A polymer film (Himilan, 50 µm) was attached between two electrodes to provide space as well as to encapsulate the electrolyte. Then, the iodide=triiodide redox (Solaronix Iodolyte AN-50) electrolyte was carefully inserted into that space between the two electrodes by capillary action using a glass tube injector. Both electrodes were attached using two ordinary paper clips. The area of the photoanode was about 4 cm2 (20 × 20 mm2) and its thickness was 12–14 µm depending on the amount of compression, as measured by alpha step 500. The photovoltaic properties were measured using a solar simulator (100 mW·cm−2, AM 1.5 illumination) in air with the area of 0.16 cm2 restricted by a still mask. The photoanode surface morphology was observed by scanning electron microscopy (SEM). 2.2

3.

Results and discussion

The effects of deposition voltage on layer thickness, rate of deposition, and surface morphology were examined, keeping all other parameters (e.g., ratio of the components of the solution and distance between the electrodes) constant. The deposition voltage was varied from 20 to 100 V for 3 min. Layer thickness and amount of deposition were increased with deposition voltage. In this study, the distance between the electrodes was 2.5 cm. For a single deposition voltage, the deposition rate was observed at each successive minute (shown in Fig. 1). At a low voltage, the deposition rate was almost constant at each successive minute. Up to 60 V, the rate of deposition was almost constant over the entire deposition time. When the deposition voltage was more than 60 V, the deposition rate changed for each minute of deposition. At higher deposition voltages, the rate of

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Fig. 1. Rate of deposition of TiO2 at each minute with changing deposition voltage.

Thickness (μm)

per 100 ml solution) and H2O (4 ml per 100 ml solution) were added to charge the solution. The solution was then dispersed with the homogenizer at 70% amplitude for 2 min. Before deposition, the substrates were cleaned with acetone two times and methanol one time (5 min each time). All the substrates were dried with flowing nitrogen gas. Then, the substrates were cleaned by UV–O3 cleaning. To obtain a uniform crack-free layer of TiO2 photoanodes, conditions such as the deposition time, voltage and distance were optimized. The deposition distance was optimized to 2.5 cm and the deposition voltage to 60 V dc. After deposition, the samples were dried at 80 °C for 1 min. The FTO glass substrate (2 × 2 cm2) was used as the cathode and a titanium sheet (2 × 2 cm2) was used as the anode. Before each sample deposition, the solution was dispersed with the homogenizer for 1 min. To maintain the same deposition rate, the solution was changed after each deposition. The total deposition time was 3 min. After the electrophoretic deposition, the samples were dried for 30 min at room temperature. One of the samples was sintered at 450 °C for 1 h. The second sample was compressed at 40 MPa for 5 min. A teflon sheet was inserted in between the photoanode and the compression plate in accordance with Ref. 21. For the third sample, the temperature of the compression plate was changed to 70 °C and was also compressed at 40 MPa for 5 min.

Deposition rate (mg/cm2)

Jpn. J. Appl. Phys. 55, 01AE13 (2016)

110

Fig. 2. Effects of deposition voltage on TiO2 film thickness and deposition current density.

deposition was higher. Therefore, the concentration of TiO2 inside the solution decreased rapidly after each minute of deposition. To maintain a uniform deposition rate, the deposition voltage of 60 V was selected and the deposition time was 3 min. The deposition current and the thickness of the film were also observed by changing the deposition voltage from 20 to 100 V with a 3 min deposition time. The deposited film thickness and current density increased with deposition voltage, as shown in Fig. 2. When the layer thickness was less than 12 µm (for deposition voltage of less than 60 V and deposition time of 3 min), the cracks were not visible [Fig. 3(a)]. For the thickness of about 16 µm, cracks were more evident [Fig. 3(b)], and in the case of 20 µm layer thickness, a nonhomogeneous layer was visible [Fig. 3(c)]. Therefore, in this investigation, films were deposited at 60 V for 3 min only to obtain a crack-free uniform layer. To investigate the effectiveness of annealing temperature during compression, three samples were prepared by three kinds of postdeposition surface treatments. Firstly, sample A was prepared with conventional postannealing at 450 °C. The second sample (sample B) was prepared with 40 MPa compression of the photoanode at room temperature, and the third sample (sample C) was prepared with 40 MPa compression and 70 °C heating without any high-temperature postannealing. The photovoltaic performance was measured with a solar simulator (100 mW·cm−2, AM 1.5 illumination) in air with the area of 0.16 cm2 restricted by a steel mask. The photocurrent–voltage (I–V) characteristic curve is presented in Fig. 4, and the I–V data along with thickness are shown in Table I. The thickness was 14 µm for the sample prepared with conventional high-temperature sintering (sample A). A

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Current density, JSC (mA/cm2)

Fig. 3. (Color online) SEM images of the TiO2 photoanodes of different thicknesses: (a) 12, (b) 16, and (c) 20 µm.

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Table I. Thickness and photovoltaic performance of the DSSCs prepared with various post-treatments. Sample A prepared with 450 °C sintering without compression, sample B prepared with compression at 40 MPa at room temperature without sintering, and sample C prepared with 40 MPa compression with 70 °C annealing without sintering.

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Fig. 4. (Color online) I–V characteristic curve of TiO2 (anatase) solar cells prepared under different postdeposition surface treatment conditions.

short-circuit current density of 5.86 mA=cm2 and an efficiency of 2.31% were obtained in this sample. After compression at room temperature, the layer thickness became 13 µm (sample B). From this, it was evident that the layer became more compact after compression. Thus, the particleto-particle distance was reduced, and as a result, a more transparent layer was obtained with compression. Therefore, the series resistance is expected to be reduced with a possibility of better conduction of carriers. The short-circuit current density and efficiency of sample B were both enhanced considerably to 7.67 mA=cm2 and 2.75%, respectively. After compression at an elevated temperature (sample C), the thickness of the film was about 12.5 µm, which was 0.5 µm less than that of the sample prepared with

only compression (sample B). The short-circuit current density of this sample was found to be about 9.42 mA=cm2 and the efficiency was about 3.04%. It was observed that the fill factors of samples B and C were somewhat lower than that of sample A. The application of compression as well as compression at an elevated temperature caused a few minor defects owing to the delamination on the photoanode surface. This in turn might cause the leakage current, resulting in the lowering of the shunt resistance as well as the fill factor of the device. The surface morphologies of these three samples were observed by SEM and are shown in Fig. 5. Without compression, the surface was rough and nonuniform with air gaps between the particles. As a result, a thicker nontransparent film was obtained. From the thickness measurement, it was evident that the thickness was reduced by increasing the compression pressure. After compression, the interparticle distance was reduced, and the air gaps

a)

b)

c)

Fig. 5. SEM of photoanodes prepared with (a) conventional post-annealing at 450 °C, (b) compression at 40 MPa (room temperature), and (c) 40 MPa compression at 70 °C (without sintering).

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seemed to be minimized. Moreover, the surface became smooth and uniform. Nevertheless, the films were relatively transparent and more adherent to the conductive glass substrates. As a result, lower series resistance and higher short-circuit current density are expected. Furthermore, the uniform compact layers are suitable for an increased level of dye loading and higher charge transport, which result in better photovoltaic performance. From the SEM image of sample C, it was evident that the surface is more smooth and compact than those to samples A and B. Therefore, reduced interparticle distance, and as a result, lower series resistance are expected. Here, the mechanical compression was found to be a better postdeposition surface treatment than the conventional high-temperature sintering method. 4.

Conclusions

In this investigation, a TiO2 DSSC was successfully prepared by electrophoretic deposition. High-temperature compression was found to be an efficient and prominent surface treatment method. The performance of the heated and compressed sample was found to be better than those of the sample prepared with the conventional postannealing method and the sample prepared with compression at room temperature. From this result, it is evident that compression with annealing is suitable for low-temperature fabrication of flexible DSSCs.

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