Abstract
We review the field of thin-film silicon solar cells with an active layer thickness of a few micrometers. These technologies can potentially lead to low cost through lower material costs than conventional modules, but do not suffer from some critical drawbacks of other thin-film technologies, such as limited supply of basic materials or toxicity of the components. Amorphous Si technology is the oldest and best established thin-film silicon technology. Amorphous silicon is deposited at low temperature with plasma-enhanced chemical vapor deposition (PECVD). In spite of the fundamental limitation of this material due to its disorder and metastability, the technology is now gaining industrial momentum thanks to the entry of equipment manufacturers with experience with large-area PECVD. Microcrystalline Si (also called nanocrystalline Si) is a material with crystallites in the nanometer range in an amorphous matrix, and which contains less defects than amorphous silicon. Its lower bandgap makes it particularly appropriate as active material for the bottom cell in tandem and triple junction devices. The combination of an amorphous silicon top cell and a microcrystalline bottom cell has yielded promising results, but much work is needed to implement it on large-area and to limit light-induced degradation. Finally thin-film polysilicon solar cells, with grain size in the micrometer range, has recently emerged as an alternative photovoltaic technology. The layers have a grain size ranging from 1 μm to several tens of microns, and are formed at a temperature ranging from 600 to more than 1000∘C. Solid Phase Crystallization has yielded the best results so far but there has recently been fast progress with seed layer approaches, particularly those using the aluminum-induced crystallization technique.
Highlights
Faced with the threat of global warming caused by fossil fuel consumption, humankind is looking for alternative forms of power production that lead to minimal CO2 emission
The dominating photovoltaic technology is still based on the concept that led to the first practical solar cell in 1954, that is, the “bulk” or wafer-based crystalline silicon technology
Most types of amorphous silicon solar cells are in superstrate configuration, which means that the light enters the solar cell through the supporting substrate
Summary
Faced with the threat of global warming caused by fossil fuel consumption, humankind is looking for alternative forms of power production that lead to minimal CO2 emission. Solar insolation is quite well distributed around the globe, and it is plentiful ( quite dilute) while photovoltaics converts solar irradiation directly into a high quality energy form with an efficiency ranging from 5 to 20% This is much higher than most of the other energy technologies that convert (often very indirectly) solar energy into useful work. Even if wire sawing is improved drastically, it is unlikely that an Si consumption per wafer (wafer thickness + kerf loss), much less than 250 μm, can be obtained (today, this value is typically 400 μm) This is a relatively large amount of a costly material prepared with a very high energy input, which is, not necessary for high efficiency. The term “thin-film crystalline Si” is quite broad It covers a wide range of technologies, from amorphous Si to monocrystalline lift-off Si solar cells. Another review paper with a similar scope to the present article but even shorter can be found in a recent issue of the MRS bulletin [4]
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