Design and Fabrication of Advanced TCO and ARC with Enhanced Light Trapping for Silicon Thin Film Solar Cells

Abstract

Solar cells have received a lot of attention as generating electric energy technology among various renewable energy sources. Silicon (Si) thin film solar cells are promising due to their potentials of low cost and large area manufacture. Light confinement into photoactive Si layers is one of the essential techniques to increase conversion efficiency with decreasing Si thickness. In this thesis, various approaches about transparent conductive oxide (TCO) and anti-reflection coating (ARC) was introduced in order to enhance the light trapping in the Si thin film solar cells. As for the TCO, important factors to determine texture-etching profiles of Al-doped ZnO (AZO) was investigated by controlling substrate temperature and sputter pressure. It was thought that crystallinity and compactness of AZO films significantly affect the surface morphology of textured AZO films. In order to obtain highly light scattering capability of AZO, I have focused on an improvement of the crystallinity of AZO films. Oxygen controlled seed layer and ITO buffer were employed before the bulk AZO deposition to enhance the polycrystalline growth of AZO films. The improved crystallinity by those techniques allowed for better optoelectronic properties of AZO films. Reliability of TCOs resistivity is one of the critical issues for maintaining solar cell performance. In this work, Ga- and Al- codoped ZnO (GAZO) and B- and Al- codoped ZnO (BAZO) films were suggested and they exhibited pronounced stable resistivity compared to the widely used AZO film under thermal annealing in atmospheric air and H2O vapor ambient. In a field of the ARC, optoelectronic properties of TiO2 ARC was systematically investigated and optimized. Also, novel GaN ARC was proposed at the TCO/Si interface and the promising results for replacing the TiO2/ZnO bilayer ARC by the GaN was demonstrated. In addition to those ARC, I proposed Al2O3 and MgO ARC at glass/TCO interface to further improve light trapping at front interfaces. Microcrystalline Si (µc-Si:H) solar cells employing those ARC exhibited decreased cell reflectance and improved quantum efficiency. The light trapping was maximized by employing glass texture in combination with the proposed TCO and ARC. The light trapping capability was enhanced through texture-etching of air/glass interface, which demonstrated the improvement of light absorption without sacrificing Voc and FF. Those approaches above indicates that light trapping at front interfaces can provide the promising results for improving the absorption in Si thin film solar cells.