High-Efficiency Triple-Junction Amorphous Silicon Alloy Photovoltaic Technology; Annual Technical Progress Report, 6 March 1998--5 March 1999

Abstract

This report describes the research performed during Phase I of this three-phase, three-year program. The research program is intended to expand, enhance, and accelerate knowledge and capabilities for developing high-performance, two-terminal multijunction amorphous silicon (a-Si) alloy cells and modules with low manufacturing cost and high reliability. To improve efficiency, United Solar uses a spectral-splitting, triple-junction cell structure. In this configuration, the top cell uses an amorphous silicon alloy of {approx}1.8 eV bandgap to absorb the blue photons. The middle cell uses an amorphous silicon germanium alloy ({approx}20% germanium) of {approx}1.6 eV bandgap to capture the green photons. The bottom cell has {approx}40% germanium to reduce the bandgap to {approx}1.4 eV to capture the red photons. The cells are deposited on stainless steel with a predeposited silver/zinc oxide back reflector to facilitate light trapping. A thin layer of antireflection coating is applied to the top of the cell to reduce reflection loss. During this year, research activities were carried out in the following four areas: (1) fundamental studies to improve our understanding of materials and devices, (2) small-area cell research to obtain the highest cell efficiency, (3) deposition of small-area cells using a modified very high frequency (MVHF) technique to obtain higher deposition rates, and (4) large-area cell research to obtain the highest module efficiency.