Optimization of ZnO Films for Amorphous Silicon Solar Cells

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Abstract
ZnO films prepared by the metalorganic chemical vapor deposition (MOCVD) method were applied to amorphous silicon (a-Si) solar cells as a transparent conductive oxide (TCO). The B2H6 flow rate and the thickness of the film were optimized at 0.5 µmol/min and 2.0 µm, respectively. We also proposed a preannealing technique of the ZnO film before fabrication of a-Si solar cells, and it was shown that the open circuit voltage was increased with use of this technique. Furthermore, we fabricated a novel p-i-n a-Si solar cell structure, which had a 0.1-µm-thick ZnO layer between a-Si and the metal electrode. Theoretical calculation and experimental results proved that the reflectance of the ZnO/Ag back reflector was higher than that of the Ag electrode at wavelengths between 600 nm and 800 nm. As a result, a marked enhancement of collection efficiencies in the long-wavelength region was achieved, resulting in the increase of the short-circuit current of about 1 mA/cm2. Using both the preannealing technique of ZnO film and a ZnO/Ag/Al back reflector, a conversion efficiency of 11.9% (V oc=0.893 V, I sc=18.6 mA/cm2, F F=0.715) was obtained.