The growing phenomenon of global warming is causing the world adapting strict control on carbon dioxide emissions. Together with fossil fuel depletion, these factors have contributed to the emphasis on alternative energy and demand, also stimulate the booming of solar industry. Semiconductor solar cells, which can directly convert sunlight into electricity, has been one of the alternative energy sources to meet our expectation[1].
1-1 The advantage of solar cell
The solar cell is a promising new type of power supply with three major advantages:
permanent, clean, and flexible. Solar power can be available as long as there is sun, so the solar cells can be an investment in long-term use; and compared with thermal power, nuclear power, solar cells will not cause environmental pollution. Different solar cells can be adapted into different formats, which greatly increase its flexibility in the applications.
1-2 Types of solar cells
Today, there are many different types of solar cells. Therefore, we will give a brief introduction about their characteristics of the different types solar cells.
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Fig. 1-1 Types of solar cells.
http://en.wikipedia.org/wiki/File:PVeff(rev100414).png Silicon solar cell
There are three major categories of silicon solar cells: single-crystal silicon,
polycrystalline silicon and amorphous silicon. At present, most of the applications are using single-crystal and polycrystalline silicon because of the following reasons: A.
single-crystal silicon has the highest efficiency. B. the polysilicon technology is mature, and the price is cheaper. Most important of all, its efficiency can be competitive with the single-crystal silicon. Meanwhile, amorphous silicon has lowest efficiency, and it can only be used in low-end products [2]. Single-crystal silicon solar cell boasts the highest efficiency in the silicon device family. The current available devices ranging from 11% to 27.6% depend on their sizes and designs. The multiple
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crystalline device can be as high as 20.4%, and the amorphous device can be 12.5%
when stacking different cells together. Even though amorphous cell is not very popular at this moment, we have to remember that its manufaturing is currently the lowest cost. If the lifetime and efficiency issues can be solved, the flexibility of amorphous silicon will be the key for its wide application.
Fig. 1-2 Monocrystalline solar cell (source: http://www.archiexpo.com) Group II-VI material related solar cell
Group II-VI materials (like CdTe) with special characteristics such as good p-type conductivity and right energy band gap remain a strong contender for the advanced solar cell technology. Emission layer with n-type CdS or ZnSe, and other third group elements are usually needed to adjust the energy gap for the optimized design. In order to reduce the cost of CdTe thin films, often polycrystalline structure mainly available a variety of methods for the preparation.
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Many designs of n-CdS/p-CdTe solar cells have been published and the highest record of power conversion efficiency can achieve 17.3%. Large area CdTe solar cell module has recently been in the mass production stage, and the energy conversion efficiency of is more than 10%, with the lifetime up to 25 years. The CdTe solar cell unit with one US dollar per watt, is the lowest-cost thin film solar cells. One of the great concerns about this technology, however, is the Cd element, which is environmentally unfriendly. In Europe, due to the environmental protection protocol, most of the hazardous material are banned from use, such as the electronic parts. The solar cell currently is not in the list. But in the long run, more stringent environmental requirements will be executed, and it is not good for CdTe solar cells.
Fig. 1-3 Cadmium telluride thin film solar cell
(source:http://mrsec.wisc.edu/Edetc/SlideShow/slides/pn_junction/CdTe.html)
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Copper-Indium Gallium Selenide
Copper Indium Gallium Selenide (CIGS) is a direct band gap semiconductor. With different combinations with ZnO (3.30eV), CdS (2.42eV), and CIS (1.02eV), high performance solar cells can be achieved. Energy conversion of CIS-based absorption layer solar cells is up to 15.4%[4], and can be as high as 20.3% in the research lab
sample.
The efficiency of CIGS solar cell module is almost comparable to silicon family.
Although CIGS production technology is not mature, a small amount of production lines have already come out.
Fig. 1-4 CIGS device
(source:http://www.solarserver.com/solar-magazine/solar-energy-system-of-the-mo nth/photovoltaic-production-in-the-cigsfab-integrated-factories-provide-competitive-s
olar-electricity.html) Gallium arsenide multijunction
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Multi-junction photovoltaic cells are composed of multiple epitaxial layers. By using different alloys of III - V semiconductors, the band gap of each layer may be adjusted to absorb the specific ranges of electromagnetic radiation from sun, but each layer must be lattice matched and each cell needs to be current-matched. These matching criteria dominate the design and performance of the current multi-junction solar cell design.
Each layer of III - V solar cells has optical absorption in series which has the highest band gap material at the top. It receives the entire spectrum in the first junction. Then top layer with a higher band gap material will first absorb the shorter wavelength (i.e. higher energy) photons, and bottom layer will absorb photons which are transmitted through the top cell.
GaAs-based devices are the most efficient multi-junction solar cells so far. By October 2010, the triple junction metamorphic cell reached a record high of 42.3%[5]
[6].
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Fig. 1-5 Gallium arsenide multijunction solar cell
(source:http://spie.org/Images/Graphics/Newsroom/Imported-2010/003124/003124 _10_fig1.jpg)