- Nano Express
- Open Access
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.
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