TiO2 solar efeito da espessura

TiO2 solar efeito da espessura

The effects of the thickness of TiO2 films on the performance of dye-sensitized solar cells

M.C. Kao a,⁎, H.Z. Chen a, S.L. Young b, C.Y. Kung c, C.C. Lin cDepartment of Electronic Engineering, Hsiuping Institute of Technology, Taichung 412, TaiwanDepartment of Electrical Engineering, Hsiuping Institute of Technology, Taichung 412, TaiwanDepartment of Electrical Engineering, National Chung Hsing University, Taichung 250, Taiwan abstractarticle i nfo

Keywords: TiO Short-circuit photocurrent Open-circuit voltage Dye-sensitized solar cell

Nanocrystalline anatase TiO2 thin films with different thicknesses (0.5–2.0 μm) have been deposited on ITO- coated glass substrates by a sol–gel method and rapid thermal annealing for application as the work electrode for dye-sensitized solar cells (DSSC). From the results, the increases in thickness of TiO2 films can increase adsorption of the N3 dye through TiO2 layers to improve the short-circuit photocurrent (Jsc) and open-circuit voltage (Voc), respectively. However, the Jsc and Voc of DSSC with a TiO2 film thickness of 2.0 μm (8.5 mA/cm2 and 0.61 V) are smaller than those of DSSC with a TiO2 film thickness of 1.5 μm (9.2 mA/cm2 and 0.62 V). It could be due to the fact that the increased thickness of TiO2 thin films also resulted in a decrease in the transmittance of TiO2 thin films thus reducing the incident light intensity on the N3 dye. An optimum power conversion efficiency (η) of 2.9% was obtained in a DSSC with the TiO2 film thickness of 1.5 μm.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Recently, nanocrystalline titanium oxide (TiO2), well known as a metal oxide semiconductor, has been extensively studied in many applications. For example, it is used for photo-electrodes, photocatalysts and dye-sensitized solar cell (DSSC) applications with high performance due to its fine physical, chemical and optical properties [1–3]. The DSSC consists of sensitizing dye, nanoporous metal oxide film, electrolyte and opposite electrode. The metal oxide film plays a key role in the enhancement of photoelectric conversion efficiency of DSSC, and many studies focus on the relation between film structure and photocatalytic activity as well as the power conversion efficiency of DSSC [4–6]. A high incident photon to current conversion efficiency can be expected for TiO2 films with better crystallization and higher specific surface area. Therefore, the control of the film structure is very important for applications of TiO2 films.

TiO2 thin films have been prepared by many growth techniques, such as RF-magnetron sputtering [7], pulsed laser deposition (PLD)

[8], metal–organic chemical vapor deposition (MOCVD) [9] and sol– gelprocess[10].Amongthesemethods,sol–gelmethodshavedrawna considerable amount of attention in scientific and technological fields because of their advantages of generally low temperature processing conditions, easy composition control and homogeneity, and easy fabrication of thin films with large area and low cost [1,12].

In this paper, we report the preparation of high-quality TiO2 nanocrystalline thin films on ITO-coated glass substrates, using a sol– gel method. The effect of the film thickness of nanocrystalline TiO2 films on the photovoltaic performance of DSSC was investigated.

2. Experimental

Titanium diisoprop-oxide bis (2,4-pentanedionate) (TIAA), Ti

(OC3H7)2 (CH3COCHCOCH3)2 (Alfa, 9.9%+ purity), was used as a precursor and 2-methoxyethanol, C3H8O2 (Fluka, 9.9%+ purity) was used as a solvent. The gravimetrically assayed Ti(OC3H7)2 (CH3COCH-

COCH3)2 reagents were dissolved in a mixture of 2-methoxyethanol solution at room temperature. The mixture was refluxed at 120 °C for

1 h under ambient atmosphere and then cooled to 80 °C for 2 h to promote its homogeneity. A stock solution of ~1 M concentrationwith a golden color was obtained by this procedure.

The stock solutions were spin-coated on ITO-coated glass substrates (2 cm×2.5 cm) at a spin rate of 3000 rpm for 30 s using a commercial photoresist spinner. The precursor solutions were deposited onto the substrates via a 0.2 μm syring filter, thus avoiding particulate contamination. After each coating step, the gel films were pyrolyzed on the hot plate at 300 °C for 2 min before final annealing. The averagethickness of a single-coated as-fired layer, measured byan α-step surface profiler, was found to be about 0.1 μm. After multi- coating, TiO2 thin films were annealed at 700 °C for 2 min by the rapid thermal annealing (RTA) in an oxygen atmosphere. The desired TiO2 thin film thicknesses of 0.5–2.0 μm were achieved by repeating the spin-coating and annealing cycles.

E-mail addresses: kmc@mail.hit.edu.tw (M.C. Kao), hzc@mail.hit.edu.tw (H.Z. Chen), slyoung@mail.hit.edu.tw (S.L. Young).

Contents lists available at ScienceDirect

Thin Solid Films journal homepage: w w.elsevier .com/locate/tsf

Thermal analysis of precursor gels using differential thermal analysis (DTA) and thermo gravimetric analysis (TGA) was carried out to identify the major evaporation, decomposition and phase crystallization steps during pyrolysis. The gels were predried at 100 °C for 10 h, and then analysed under an annealing rate of 5 °C/min. The thicknessof the TiO2 film obtainedwasmeasured byanα-step surface profiler. The crystallization and microstructures of the thin films were analysed by X-ray diffraction (XRD) with Cu–Ka radiation and scanning electron microscopy (SEM), respectively. In order to sensitize the

of RuL2(NCS)2 (Solaronix, N3 dye) in ethanol for 24 h at room temperature. The electrolyte was composed of 0.5 M lithium iodide (LiI)/

0.05 M iodine (I2) in acetonitrile. The short-circuit photocurrent (Jsc) and open-circuit voltage (Voc) were measured using a Keithley Model 2400 source measure unit underan illumination of 100 mW/cm2 (AM

1.5 simulated light radiation) from a 1000 W xenon lamp. Incident monochromatic photo-to-current conversion efficiency (IPCE) measurements were carried out using small band-pass filters to generate monochromatic light. The measured wavelength was controlled from 300 to 800 nm.

3. Results and discussion

The thermolysis behavior of the TiO2 gel predried at 100 °C is shown in Fig. 1. In the TGA analysis, the gel exhibited approximately

50% weight loss at temperatures ranging from 100 °C to 300 °C. This result is due to the elimination of adsorbed water, solvent and the decomposition of organic by-products. The DTA data indicate that two of exothermic peaks are associated with the former weight loss at temperatures between 100 °C and 300 °C. The final peak which appeared at about 500 °C is associated with a weight loss that is probably due to crystallization of TiO2. This indicates that the TiO2 gel decomposes and crystallizes at a temperature as low as 500 °C.

Fig. 2 shows the XRD patterns of TiO2 thin films deposited on ITO- coated glass substrates at 700 °C with different thicknesses of 0.5–

2.0 μm, respectively. TiO2 films show an anatase structure with the preferred (101) orientation. As the film thickness was increased, the peaks of the (101)-plane for samplesbecame sharper indicating better crystallinity with increasing film thickness. The (101) peaks obtained for TiO2 films indicated a strong (101)-axis orientation tothe substrate surface. These results demonstrate that the structure of TiO2 films is improved when the film thickness is increased. The average grain size t was estimated from the half-width of the X-ray diffraction peaks using Scherrer's equation:

t = kλ βcosθ ; ð1Þ where θ is the diffraction angle, λ is the average wavelength of X-ray, k is the shape factor, and β is taken as half-maximum line breadth. The calculated grain size of TiO2 films (1.5 μm) annealed at 700 °C was 30 nm.

A cross-sectional SEM micrograph of the TiO2 thin film (1.5 μm) at an annealingtemperatureof 700 °C is shown in Fig. 3. Noevidence of a second-phase interfacial layer between the TiO2 layer and the bottom ITO layer is found. The SEM micrograph also reveals that the film was uniform, smooth and crack-free on the surface. According to the SEM results, the mean sizes of the TiO2 crystallites were estimated to be about 30–35 nm from Fig. 3. The grain size estimated from SEM observations was consistent with that done by means of Scherrer's equation. The evolution of grain size in the TiO2 thin film with other thicknesses was similar to that for the TiO2 thin film with a thickness of 1.5 μm. In addition, the TiO2 thin film exhibits a

Fig. 1. Thermo gravimetric analysis (TGA) and the corresponding DTA for gel of TiO .

Fig. 2. XRD results as a function of various thicknesses of TiO thin films. Fig. 3. A cross-sectional photograph of TiO thin film with a film thickness of 1.5 μm.

nanocrystalline and nanoporous structure which is composed of interconnected nanoparticles. Fig. 4(a)–(d) show the UV–visible optical transmittance spectra of the TiO2 films with different thicknesses between 300 and 800 nm in wavelength. It can be seen that all films have high transmittance and the absorption edge is at about 300 nm. In addition, the transmittance of the films becomes lower as the films actually become thicker. The average transmittance of the film (a), (b) (c) and (d) in the visible range was about 85%, 80%, 75% and 60%, respectively. Fig. 5 shows the

I–V curve for DSSC with different TiO2 film thicknesses (0.5–2.0 μm). Table 1 also shows the various parameters of DSSC with different film thicknesses. From these results, the Jsc and Voc of DSSC increase with the increase of the TiO2 thin film thickness from 0.5 μmt o 1.5 μm. It is due to the larger film thickness of TiO2 thin film that it results in a higher adsorption of the N3 dye through the TiO2/RuL2(NCS)2 layers. However, the Jsc and Voc of DSSC with a TiO2 film thickness of 2.0 μm

(8.5 mA/cm2 and 0.61 V) are smaller than those of DSSC with a TiO2 film thickness of 1.5 μm (9.2 mA/cm2 and 0.62 V), respectively. This can be explained by the lower transmittance of the TiO2 thin filmwith a thickness of 2.0 μm to reduce the incident light intensity on the N3 dye (Fig. 4). The optimum η of 2.9% with Jsc and Voc of 9.2 mA/cm2 and

0.62 V, respectively, was obtained by the TiO2 film with a thickness of 1.5 μm.

Fig. 6 shows the incident monochromatic photo-to-current con- version efficiency (IPCE) of DSSC with different TiO2 film thicknesses

(0.5–2.0 μm) as a function of wavelength. The IPCE is defined as the ratio of the number of electrons generated by light in the external circuit to the number of incident photons, as follows:

The photocurrent density was determined at the short-circuit state. With the increase of thickness of TiO2 thin films from 0.5 μmt o 1.5μm,themaximumIPCEvalueofDSSCincreasesfrom60%upto80% at 510 nm. However, the maximum IPCE value of DSSC with the TiO2 thin film thickness of 1.5 μm (86%) was larger than that of DSSC with theTiO2 thin film thicknessof 2.0 μm (80%)at 510 nm.The higher IPCE value in the DSSC with the TiO2 thin film thickness of 1.5 μmi s attributed to the better transmittance of TiO2 thin films to increase the incident light intensity on the N3 dye.

4. Conclusions

The influence of film thickness on the performance of DSSC with

TiO2 filmswasstudied. WithincreasingthethicknessofTiO2 thinfilms from 0.5 μmt o 1.5 μm, the values of Jsc and Voc of DSSC increased from 6.8 to 9.2 mA/cm2 and from 0.59 to 0.62 V, respectively. However, the

Jsc and Voc of DSSC with the TiO2 film thickness of 2.0 μm (8.5 mA/cm2 and 0.61 V) are smaller than those of DSSC with the TiO2 film thickness of 1.5 μm (9.2 mA/cm2 and 0.62 V), respectively. This can be explained by the lower incident light intensity from the lower trans- mittance of the TiO2 thin film with a thickness of 2.0 μm. The cor- responding results show that the obtained DSSC with the TiO2 thin film thickness of 1.5 μm exhibited excellent photovoltaic properties.

Fig. 4. Optical transmittance spectra of TiO films with film thicknesses of, (a) 0.5 μm, (b) 1.0 μm, (c) 1.5 μm and (d) 2.0 μm.

Fig. 5. I–V curve for DSSC with different TiO film thicknesses.

Table 1 The photovoltaic performances of DSSC fabricated with various thicknesses of TiO thin films.

Thickness of the TiO films (μm) J

(mA/cm ) V(V) Fill factor(%)

Fig. 6. Wavelength dependence of photocurrent density for DSSC with different TiO film thicknesses.

Acknowledgement

This study was supported by the National Science Council,.R.O.C., under contrast nos. NSC 96-2112-M-164-002-MY2, NSC 97-2112-M- 164-002-MY2 and NSC 97-2112-M-164-003-MY2.

References

[3] T. Kitamura, M. Ikeda, K. Shigaki, T. Inoue, N.A. Anderson, X. Ai, T. Lian, S. Yanagida,

[5] M.V. Jorge, F.R. Claudio, S. Calixto, B. Pedro, H.L. Victor, Mater. Charact. 58 (2007) 233. [6] D. Yoo, I. Kim, S. Kim, C.H. Hahn, C. Lee, S. Cho, Appl. Surf. Sci. 253 (2007) 3888.

[8] L. Kavan, M. Grätzel, Elecyrochimica Acta. 40 (1995) 643.

Comentários