Structural and optical properties of ITO/TiO2 anti-reflective films for solar cell applications
© Ali et al.; licensee Springer. 2014
Received: 7 February 2014
Accepted: 3 April 2014
Published: 11 April 2014
Indium tin oxide (ITO) and titanium dioxide (TiO2) anti-reflective coatings (ARCs) were deposited on a (100) P-type monocrystalline Si substrate by a radio-frequency (RF) magnetron sputtering. Polycrystalline ITO and anatase TiO2 films were obtained at room temperature (RT). The thickness of ITO (60 to 64 nm) and TiO2 (55 to 60 nm) films was optimized, considering the optical response in the 400- to 1,000-nm wavelength range. The deposited films were characterized by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), and atomic force microscopy (AFM). The XRD analysis showed preferential orientation along (211) and (222) for ITO and (200) and (211) for TiO2 films. The XRD analysis showed that crystalline ITO/TiO2 films could be formed at RT. The crystallite strain measurements showed compressive strain for ITO and TiO2 films. The measured average optical reflectance was about 12% and 10% for the ITO and TiO2 ARCs, respectively.
To deposit titanium dioxide (TiO2) and indium tin oxide (ITO) films, several techniques have been used, including radio-frequency (RF) sputtering, chemical vapor deposition , sol–gel , spray deposition , and electron-beam evaporation . Low-deposition temperatures are required because high temperatures can degrade a substrate material for solar cells and plastic devices . RF sputtering is a sophisticated process with high deposition rate and good reproducibility . Most of these techniques require a type of heat treatment (250°C to 650°C) for the substrates during or after the deposition [1, 2, 4], due to insufficient crystallization at RT. This phenomenon leads to poor optical and structural properties . RT deposition is important for photovoltaic devices as the thermal treatments may change the intended compositional distribution and also introduce defects that act as recombination centers for charge carriers in the solar cell device. Many attempts have been made to deposit ITO and TiO2 thin films on silicon substrates by RF sputtering technique at RT [8, 9]. The ITO film exhibits excellent conductivity and it can be used as an ohmic contact on a p-type c-Si. De Cesare, et al. achieved good electrical properties with ITO/c-Si contact at RT . ITO has also become the attractive material for its anti-reflection (AR) properties and enhanced relative spectral response in the blue-visible region. Optical device performance depends greatly on the surface morphology and crystalline quality of the semiconductor layer .
Another material, TiO2, is well known in silicon processing technology and has wide applications in optics and optoelectronics [12, 13]. TiO2 films can be distinguished into three major polymorphs: anatase, rutile, and brookite. Each phase exhibits a different crystal configuration with unique electrical, optical, and physical properties. Anatase is the most photoactive but thermally instable and it converts into rutile phase above 600°C [14, 15]. In this paper, RF sputtering of ITO/TiO2 is used to eliminate the standard high-temperature deposition process required for the formation of AR films. This also guarantees that the critical surface layer of the monocrystalline Si is not damaged. Present work reports the crystal structure, optical reflectance, and microstructure of the ITO/TiO2 AR films, RF sputter deposited on monocrystalline Si p-type (100) at RT.
The growth parameters and results of the ITO and TiO 2 film deposition on the Si substrate
Distance from substrate
2.68 × 10-5 mbar
2.97 × 10-5 mbar
Vacuum (plasma) pressure
7.41 × 10-3 mbar
6.75 × 10-3 mbar
RF sputtering power
2.1 Å · s-1
0.5 Å · s-1
60 to 64 nm
55 to 60 nm
n (λ = 500 nm)
Results and discussion
where D is the average crystallite size, λ is the X-ray radiation wavelength (0.15406 nm), β is the full width at half maximum (FWHM) value, and θ is the diffraction Bragg angle.
The work presents the structural and optical characteristics of ITO and TiO2 ARCs deposited on a (100) P-type monocrystalline Si substrate by a RF magnetron sputtering at RT. X-ray diffraction proved the anatase TiO2 and polycrystalline ITO films structure. Residual compressive strain was confirmed from the Raman analysis of the ITO and TiO2 films which exhibited blue shifts in peaks at 518.81 and 519.52 cm-1 excitation wavelengths, respectively.
FESEM micrographs showed that the granules of various scales are uniformly distributed in both ITO and TiO2 films. Reflectance measurements of ITO and TiO2 films showed 25% and 23% improvement in the absorbance of incident light as compared to the as-grown Si. Low reflectivity value of 10% in the ITO film as compared to 12% of the TiO2 film is attributed to the high rms value. Our results reveal that the highly absorbent polycrystalline ITO and photoactive anatase TiO2 can be obtained by RF magnetron sputtering at room temperature. Both ITO and TiO2 films can be used as ARCs in the fabrication of silicon solar cells.
The authors acknowledge the Short Term Research Grant Scheme (1001/PFIZIK/845015) and Universiti Sains Malaysia (USM) for the Fellowship to Khuram Ali.
- Guo D, Ito A, Goto T, Tu R, Wang C, Shen Q, Zhang L: Effect of laser power on orientation and microstructure of TiO2 films prepared by laser chemical vapor deposition method. Mater Lett 2013, 93: 179–182.View ArticleGoogle Scholar
- Sasani Ghamsari M, Bahramian AR: High transparent sol–gel derived nanostructured TiO2 thin film. Mater Lett 2008, 62: 361–364. 10.1016/j.matlet.2007.05.053View ArticleGoogle Scholar
- Nguyen-Phan T-D, Pham VH, Cuong TV, Hahn SH, Kim EJ, Chung JS, Hur SH, Shin EW: Fabrication of TiO2 nanostructured films by spray deposition with high photocatalytic activity of methylene blue. Mater Lett 2010, 64: 1387–1390. 10.1016/j.matlet.2010.03.033View ArticleGoogle Scholar
- Senthilkumar V, Vickraman P, Jayachandran M, Sanjeeviraja C: Structural and optical properties of indium tin oxide (ITO) thin films with different compositions prepared by electron beam evaporation. Vacuum 2010, 84: 864–869. 10.1016/j.vacuum.2009.11.017View ArticleGoogle Scholar
- Kurdesau F, Khripunov G, da Cunha AF, Kaelin M, Tiwari AN: Comparative study of ITO layers deposited by DC and RF magnetron sputtering at room temperature. J Non-Cryst Solids 2006, 352: 1466–1470. 10.1016/j.jnoncrysol.2005.11.088View ArticleGoogle Scholar
- Lee H-C, Seo J-Y, Choi Y-W, Lee D-W: The growth of indium-tin-oxide thin films on glass substrates using DC reactive magnetron sputtering. Vacuum 2003, 72: 269–276. 10.1016/j.vacuum.2003.08.001View ArticleGoogle Scholar
- Quaas M, Steffen H, Hippler R, Wulff H: Investigation of diffusion and crystallization processes in thin ITO films by temperature and time resolved grazing incidence X-ray diffractometry. Surf Sci 2003, 540: 337–342. 10.1016/S0039-6028(03)00850-1View ArticleGoogle Scholar
- Park J-O, Lee J-H, Kim J-J, Cho S-H, Cho YK: Crystallization of indium tin oxide thin films prepared by RF-magnetron sputtering without external heating. Thin Solid Films 2005, 474: 127–132. 10.1016/j.tsf.2004.08.172View ArticleGoogle Scholar
- Guillén C, Herrero J: Comparison study of ITO thin films deposited by sputtering at room temperature onto polymer and glass substrates. Thin Solid Films 2005, 480–481: 129–132.View ArticleGoogle Scholar
- De Cesare G, Caputo D, Tucci M: Electrical properties of ITO/crystalline-silicon contact at different deposition temperatures. IEEE Electron Device Let 2012, 33: 327–329.View ArticleGoogle Scholar
- Raoufi D, Kiasatpour A, Fallah HR, Rozatian ASH: Surface characterization and microstructure of ITO thin films at different annealing temperatures. Appl Surf Sci 2007, 253: 9085–9090. 10.1016/j.apsusc.2007.05.032View ArticleGoogle Scholar
- Vallejo B, Gonzalez-Mañas M, Martínez-López J, Morales F, Caballero MA: Characterization of TiO2 deposited on textured silicon wafers by atmospheric pressure chemical vapour deposition. Sol Energ Mat Sol C 2005, 86: 299–308. 10.1016/j.solmat.2004.07.011View ArticleGoogle Scholar
- Ali K, Khan SA, Mat Jafri MZ: Enhancement of silicon solar cell efficiency by using back surface field in comparison of different antireflective coatings. Sol Ener 2014, 101: 1–7.View ArticleGoogle Scholar
- Libardi J, Grigorov KG, Guerino M, da Silva Sobrinho AS, Maciel HS, Soares JP, Massi M: High quality TiO2 deposited by reactive sputtering. Structural and electrical peculiarities influenced by the specific experimental conditions. In Microelectronics Technology and Devices (SBMicro), 2013 Symposium on; 2–6 Sept 2013, 1: 2013.Google Scholar
- Zhang J-Y, Boyd IW, O'Sullivan BJ, Hurley PK, Kelly PV, Sénateur JP: Nanocrystalline TiO2 films studied by optical, XRD and FTIR spectroscopy. J Non-Cryst Solids 2002, 303: 134–138. 10.1016/S0022-3093(02)00973-0View ArticleGoogle Scholar
- Kim H, Horwitz JS, Kushto G, Pique A, Kafafi ZH, Gilmore CM, Chrisey DB: Effect of film thickness on the properties of indium tin oxide thin films. J Appl Phys 2000, 88: 6021–6025. 10.1063/1.1318368View ArticleGoogle Scholar
- Ishida T, Kobayashi H, Nakato Y: Structures and properties of electron‒beam‒evaporated indium tin oxide films as studied by X‒ray photoelectron spectroscopy and work‒function measurements. J Appl Phys 1993, 73: 4344–4350. 10.1063/1.352818View ArticleGoogle Scholar
- Lien S-Y: Characterization and optimization of ITO thin films for application in heterojunction silicon solar cells. Thin Solid Films 2010, 518: S10-S13. 10.1016/j.tsf.2010.03.023View ArticleGoogle Scholar
- Dai S, Wu Y, Sakai T, Du Z, Sakai H, Abe M: Preparation of highly crystalline TiO2 nanostructures by acid-assisted hydrothermal treatment of hexagonal-structured nanocrystalline titania/cetyltrimethyammonium bromide nanoskeleton. Nanoscale Res Lett 2010, 5: 1829–1835. 10.1007/s11671-010-9720-0View ArticleGoogle Scholar
- Cullity BD: Element of X-ray Diffraction. 3rd edition. USA: Wesley Publishing Company; 1967.Google Scholar
- Yang Y, Zhang Q, Zhang B, Mi WB, Chen L, Li L, Zhao C, Diallo EM, Zhang XX: The influence of metal interlayers on the structural and optical properties of nano-crystalline TiO2 films. Appl Surf Sci 2012, 258: 4532–4537. 10.1016/j.apsusc.2012.01.020View ArticleGoogle Scholar
- Alhomoudi IA, Newaz G: Residual stresses and Raman shift relation in anatase TiO2 thin film. Thin Solid Films 2009, 517: 4372–4378. 10.1016/j.tsf.2009.02.141View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.