Research on improvement of the efficiency and stability of glass-based amorphous silicon (a-Si) photovoltaic submodules is described. Materials aspects of this work include (1) the formation of graded-index, antireflecting glass surfaces, (2) the deposition of high-quality, 5 Ω/□ polycrystalline films of zinc oxide, (3) the use of carbon-graded a-Si1-xCx:H p-layers in devices, and (4) the use of TiOx layers between the stacks of tandem devices to increase their fill factor. A 9.3% efficient a-Si/a-Si tandem cell is reported. Stability tests indicate that a 10% advantage in power output accrues to tandem cells relative to single-junction cells. Submodule results include a laser-scribed 1 ft sq (900 cm2) tandem junction panel with an active area efficiency of 7.7%. The elimination of localized defects proved crucial to this achievement. Two new low-loss module designs are described; analysis of a type involving dot contacts indicates that the area plus I2R losses of conventional modules can be reduced by a factor of 2-3. Finally, a brief description is given of a 10 MWp yr-1 a-Si:H processing line now undergoing trials.
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