Comparison of anti-reflective properties of single layer anatase and rutile TiO 2 on GaAs based solar cells
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Comparison of anti-reflective properties of single layer anatase and rutile TiO2 on GaAs based solar cells R. Vasan, Y. F. Makableh, and M. O. Manasreh Department of Electrical Engineering, 3217 Bell Engineering Center, University of Arkansas, Fayetteville, AR 72701 ABSTRACT Anatase and rutile titanium dioxide thin films grown by a low temperature process are investigated for their use as a single layer antireflection coating for GaAs solar cells. The thin films are obtained by spin coating a layer from the TiO2 sol-gel and subsequently annealing at 150 oC. The sol-gel is synthesized by the hydrolysis of titanium isopropoxide in the presence of an acid or a base. By controlling the pH of the sol-gel during growth, pure anatase and rutile phases are obtained. A pH of around 3.0 yields anatase phase while a pH of 9.0 yields pure rutile phase TiO2. The two different phases of TiO2 are characterized by measuring the Raman scattering spectra. The optical constants, thickness and reflectance of the thin films on GaAs are obtained using a spectroscopic ellipsometer. The sol-gel is spin coated on GaAs based solar cells and annealed at 150 oC to form the anti-reflective layer. The performance of the solar cells is evaluated before and after coating with the TiO2 films. The anatase TiO2 anti-reflective films performed better than the rutile with a maximum power conversion efficiency enhancement of 50%. Quantum efficiency enhancement of 58% and 25% are obtained with anatase and rutile phase films respectively. The performance enhancement of the solar cells using these thin films can be attributed to the destructive interference of light associated with a single layer coating on the solar cell surface. INTRODUCTION Development of high efficiency photovoltaic devices was intensively investigated in recent years. Several attempts were made to improve on the performance of GaAs based devices using various surface modification techniques [1-12]. A polished GaAs surface reflects more than 35% of the incident light, which accounts for part of efficiency loss [14]. There are reports of planar [1-7], nanostructured and multilayer nanostructured antireflection coating [8-12] (ARC) being used on GaAs based solar cells to reduce the reflection. Planar ARC on GaAs was implemented by ZnO [1], aluminum doped ZnO [2], polyllysine [3], as single layer and SiO2/TiO2 [4], MgF2/ZnS [5], ZrO3/Al2O3 [6], ZnS [7], as bilayer coatings. These planar coatings reduce reflection by destructive interference of light reflected from two different medium. On the other hand nanostructured ARC enables better light trapping and confinement by tailoring the size, shape, and distribution of the nanostructures, which cannot be implemented using planar coatings. Yu et al have reported an efficiency enhancement of 30% using indium tin oxide nanocolumns on GaAs solar cells [8], while Lam et al reported a 12.3% enhancement using hexagonal surface texturing [9]. Better performance enhancements on GaAs solar cells were reported by using multilayer nanostructured ARC lik
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