Structural Properties of Tio2 Thin Films Investigated

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[1] this system has aroused a lot of interest over the last decade ... many researchers have paid much attention to the ... due to the fact that the film consists of tiny.

Vol 2 | Issue 6 | Spring Edition | DOI : February 2014 | Pp 674-676 | ISSN 2279 – 0381

Structural Properties of Tio2 Thin Films Investigated for Low Cost High Efficiency Solar Cells T.Ranganayaki*a , M.Venkatachalam b, T.Vasuki

b

, S. Shankarb

a

b

Department of computer science, Erode Arts and Science College(Autonomous),Erode, Tamil Nadu, India. Department of Electronics, Thin film research Centre, Erode Arts and Science College(Autonomous), Erode, Tamil Nadu, India. * e-mail : [email protected]

Keywords: Sol-gel, composition, Amorphous-crystalline phase.

Film

Abstract: Research for alternate source of energy explores lot of technology in the field of material science .Harvesting UV light is immediate alternate source of energy.Composites of organic and inorganic material inferences reveals lot of inherent structural properties. Solutions TTIP:CH3 COOH:C2 H5 OH (1.5:0.1:10) prepared by sol- gel method. By changing the proportion of substances different films were obtained. Fabrication of TiO 2 thin films on glass substrates can be done by Doctoral blade method. The specimens were dried at 250°c for 5minutes. The films obtained were annealed and then cooled, inheriting the high adherence to the substrates transparency, Chemical and mechanical stability. The surface morphology, film composition, structure and optical properties were evaluated from SEM, XRD. Wave length of the refractive index and absorption co-efficient in the UV VIS-near IR regions as well as the optical band gap were inferred from optical transmittance measurements. The characteristics of the investigated thin films are interpreted in terms amorphous-crystalline phase. The obtained results were conformed with already reported values. Introduction Since the first report of dye-sensitized solar cell ((DSSC) in 1991by O’ Regan and Gratzel [1] this system has aroused a lot of interest over the last decade due to its high efficiency, low cost, and simple preparation procedure[2][3][4].In general, a porous TiO2 nano particle thin film is used as an electron transport medium in DSSC[5].A novel approach is explored to improve the photovoltaic performance of DSSC by using TiO2 nano materials [6][7]. The DSSCs are composed of a dyeadsorbed nano porous TiO2 layer on a glass substrate redox electrolytes and a counter electrode. A unidirectional charge flow with no www.indiasciencetech.com

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electron leakage at the interfaces is essential for high energy conversion efficiency [8]. Recently many researchers have paid much attention to the glass/TiO2 interface [9][10]. Photovoltaic cells containing organic semiconductors have attracted considerable attention[11][12][13] because their electrical and optical properties can be varied widely, and generally they can be fabricated more easily than inorganic photovoltaic cells. The randomly porous structure of the TiO 2 electrode gives raise to several undesirable characteristics. These include a low conductivity, which is due to the fact that the film consists of tiny crystals measuring 10-30nm in diameter [14]. The small size of the crystals does not support the formation of a space charge region. This is thought to enhance the recombination rate of the photo-injected electrons due to the absence of an energy barrier at the electrode/electrolyte interface. Sol-gel processing of titanium dioxide has been extensively investigated, and modern processes have been developed to refine and control the stability, as well as the phase formation of the colloidal precursors [15]. The hydrolysis of the titanium tetra Isopropoxide combined with acetic acid and ethanol with subsequent annealing has led to new materials characterized by their controlled porosity and strong adhesion to the substrate [16]. 2.EXPERIMENTAL 2.1. Materials Titanium tetra Isopropoxide(TTIP, Aldrich, 97%), the precursor was used without further purification. Standard ethanol solution (M=46.07g/mol, purity ≈ 8%)was purchased from Fluka Chemical. Acetic acid (M = 36.5 g/mol) was supplied from Merk.TiO2 Nano powder was anatase in crystalline form and it has a surface

Journal of NanoScience and NanoTechnology | Vol 2 | Issue 6 | Spring Edition | ISSN 2279 – 0381 D=Kλ/βcosθ The average grain size obtained for sol-gel(TTIP:CH3COOH:C2H5OH, 1.5:0.1:10) derived and applied on glass substrate by Doctoral blade method annealed at 600°c is 27.55nm. The very broad peak at about 2θ = 25.37 (which is identified as the most intensive peak(101) for the anatase TiO2 ) was observed. The corresponding crystallite size of the anatase phase TiO2 was observed.

TTIP :Aceticacid :C2H5OH 1.5 : 0.1

3. RESULTS AND DISCUSSION 3.1 XRD Characterization of thin film The structure and crystallie size were determined by powder x-ray diffraction(Brucker D8 Advanced x-ray diffractometer CuKα radiation(1.542A); scan rate (0.03 2θ-1 ). The strongest peaks of TiO2 corresponding to anatase(101) βwere selected to evaluate the crystallinity of the samples. The mean crystal size ‘d’ was determined from the broadening β of the most intense line , for each polymorph in the x-ray diffraction pattern based on the Scherrer’s equation

20

40

2

60

(107)A

(116)A

(213)A

(105)A (200)R

(200)A

(111)R (210)R

(101)R (004)A

Titanium Tetra Isopropoxide(TTIP) thin films with anatase phase have been prepared by sol-gel method for three different amount of solvent with Acetic acid mixed together . After stirring this solution for 30min, additional 10ml of ethanol was added and the stirring was continued for 3hrs at room temperature. The relative volume ratio of each chemical in the precursor sol was TTIP:CH3COOH:C2H5OH=1.5:0.1:10.resultant solution was stable for a period of 24 hours followed by occurrence of gelation. Figure shows XRD patterns of an as deposited TiO2 thin film by Doctoral blade method and annealed at 600°c.The matching of the observed and standard ‘d’ values conforms that the deposited films are of phase-pure anatase TiO2 with tetragonal structure. The strongest peaks for film deposited using Acetic acid and Ethanol was characterized , the estimated grain size,27.55nm, annealed at 600°c reveals decrease in grain size. Sol-Gel TiO2 thin films grown on glass substrates evaluated using SEM,XRD.

Intensity(a.u)

2.3 PREPARATION OF THIN FILM

: 10

(101)A (110)R

area about 190-290m2/g and particle size of 15nm was obtained from Aldrich. 2.2 PREPARATION OF Sol-gel Different combinations of TiO2 sol were prepared by altering the molar ratio of TTIP:CH3COOH:C2H5OH. The typical combinations reported here is 1.5:0.1:10. Using sol-gel technique, TiO2 thin film was prepared by mixing TTIP, Glacial acetic acid, Ethanol. In order to investigate the influence of amount of solvent, the amount of acetic acid added was varied . In this technique, TTIP was used as a precursor, glacial acetic acid as a chelating agent, ehanol as a solvent. The mixture was stirred for 3hrs to avoid precipitation.

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Fig shows the x-ray diffraction pattern of the TiO2 thin films prepared using 0.1,0.2,0.3 acetic acid concentrations and annealed at 600°c. The XRD pattern of the TiO2 thin film with acetic acid concentration 0.1mol reveals a prominent anatase peak at 25.3°(101) and 48.2°c(200). With the catalyst concentration we discovered the crystallized(101) direction of the anatase phase appeared. The other peaks when TiO2 thin film with various molar concentrations of acetic acid that diffraction pattern displays the coexistence of both amorphous and crystalline TiO2 regions showing the simultaneous presence of the broad hump in the low 2θ region demonstrating short range order and amorphicity. The diffraction pattern of film with strongest peak shows the formation of phase pure anatase TiO2 thin film crystallites.

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Journal of NanoScience and NanoTechnology | Vol 2 | Issue 6 | Spring Edition | ISSN 2279 – 0381 It is suggested that the increase in surface area and porocity of the TiO 2 anatase phase is mainly because of acetic acid. 3.2 SEM CHARACTERIZATION

anatase phase with increasing catalyst concentration. The grain size of the TiO 2 film was observed to be 27.55 with 0.1 mol catalyst concentration. The TiO2 thin film prepared had a high transparency in the visible range(400 to 800nm). conversion efficiency of the DSSC was observed for films grown at a catalyst concentration of 0.1 mol. TiO2 film prepared at 0.1 mol had high quality anatase crystallites and an extensive surface specific area. References [1] [2] [3]

[4]

Fig.2. SEM image of the TiO2 films with molar ratios TiO2 : CH3COOH : C2H5OH=1.5:0.1:10 The grains of the TiO2 film are spherical. The SEM image of the TiO2 thin film with 0.1 mol acetic acid reveals the grain size 27.55nm. From the above observed grain size the surface morphology of the thin film due to the crystal phase transformation ,as shown in the XRD results[17]. The SEM image also reveals that the thin film had a uniform, smooth, crack free surface.

[5]

[6]

[7]

3.0

Absorbance(%)

2.5 2.0

[8] 1.5

[9]

1.0 0.5 0.0 200

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[10]

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Wavelength(nm)

UV Vis CHARACTERIZATION Spectral measurements have been performed using an UV Vis near IR Spectrometer (JACOV570) revealed that the optical transmittance of the TiO2 thin film was monitored in the range 200-800nm. From the optical transmittance spectra, the wave length dependence of the refractive index and optical absorption coefficient was inferred. 4. CONCLUSION A TiO2 sol was prepared by using sol-gel method. The film were coated using the Doctoral blade method. The TiO2 films grew into an www.indiasciencetech.com

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[11] [12] [13] [14] [15] [16] [17]

O’Rezelgan B, Gratzel M.A low cost,high efficiency solar cell based on dye-sensitized TiO2 films. Nature. 1991;353:737-739. Gratzel M Photoelectrochemical cells. Nature. 2001;414:338-344. Wu JH, lan Z, l in JM, et al. A novel thermosetting gel electrolyte for stable quasisolid state dye –sensitized solar cells. Adv Mater 2007;19;4006-4011. Wu J, Hao S, Lan Z, ei al. An all-solid-state dye-sensitized solar cell-based poly(N-alkyl-4VINY L-PRIDINE IODIDE) electrolyte with efficiency of 5.64%.J Am Chem Soc. 2008;130:11568-11569. Ito S, Murakami TN, Comte P, et al. Fabrication of thin film dye-sensitized solae cells with solar to electric power conversion efficiency over 10%. Thin solid films. 2008;516:4613-4619. Kopidakis N, Benkstein KD, Van de Lagemaat J, Frank AJ. Transport-limited recombination of photocarriers in dye-sensitized nanocrystalline TiO2 solar cells. J Phys Chem B. 1999;107:11307-II315. Song MY, Ahn YR, Jo SM, Kim DY. TiO 2 single crystalline nanorod electrode for quasisolid-state dye-sensitized solar cells. Appl Phys Lett. 2005;87:113113. J. Xia, N. Masaki, K. Jiang, Y. Wada and S. Yanagida, Chem. Lett. 35, 252(2006). S.Ito, P. Liska, P. Comte, R. Charvet, P.Pechy, U. Bach, L. Schmidt-Mende, S. M. Zakeeruddin, A. Kay, M. K. Nazeeruddin and M. Gratzel, Chem. Commun. 34, 4351(2005). B. Peng, G. Jungmann, C. Jager, D. Haarer, HW. Scjmidt and M. Thelakkat, Coord. Chem. Rev. 248, 1479(2001). S. Sariciftci, J.C. Hummelen, Adv. Funct. Mater. 3 (2000) 15. W.U. Huyah, J.J. Dittmer, A.P. Alivisator, Science 295 (2002) 2425. S.E. Shaheen, C.J. Brabee, N.S.SARICIFTCI, Appl. Phys. Lett. 78 (2001) 841. M. Gratzel, Prog. Photovoltaics Res. Appl. 8, 171 (2000). P. A. Venz, R. L. Frost, J. R. Bartlett and J. L. Woolfrey, Spectrochim. Acta, Part, A 53, 969 (1997). P. Falaras, A. Hugot-Le Goff, M. C. Bernard and A. Xagas, Sol. Energy Mater. Sol. Cells 64 167 (2000). E.J. Kim and S. H. Hahn,Mater.Lett.49,244 (2001)Mat Syst. Str. 3 (1993) 380-95.

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