How Solar Works
Thin Film Technology.
Like computer chips, PV devices are semi conductors. Accordingly, many of the lessons learned developing computer technologies have been applied to improving PV. One of the scientific discoveries of the computer semiconductor industry that has shown great potential for the PV industry is thin-film technology.
Rather than growing, slicing, and treating a crystalline ingot, as with crystalline silicon, a PV material can be created by sequentially depositing thin layers of the different materials into a very thin structure. The resulting thin-film devices require very little semiconductor material and have the added advantage of being easy to manufacture.
Several different deposition techniques are available, and all of them are potentially cheaper than the ingot-growth techniques required for crystalline silicon. Best of all, these deposition processes can be scaled up easily so that the same technique used to make a 2-inch x 2-inch laboratory cell can be used to make a 2-foot x 5-foot module (in a sense, a huge cell!).
Thin-Film Forms:
The three principal thin-film technologies are Amorphous Silicon (a-Si), Cadmium Telluride (CdTe) and Copper Indium Gallium diSelenide (CIGS).
Amorphous Silicon (a-Si)
Amorphous solids, like common glass, are materials in which the atoms are not arranged in any particular order. They do not form crystalline structures at all, and they contain large numbers of structural and bonding defects. In the '70s, researchers began to realize that amorphous silicon could be used in PV devices by properly controlling the conditions under which it was deposited and by carefully modifying its composition. Similar to other thin-film PVs, amorphous silicon absorbs solar radiation 40 times more efficiently than single-crystal silicon, so a film only at 1 micron (one one-hundredth of a centimeter) thick can absorb 90 percent of the usable solar energy. Today, amorphous silicon is the predominant form of thin-film PV and is commonly used for solar-powered consumer devices that have low power requirements (e.g. wristwatches and calculators).
Limitations
Cell efficiency is an important measure of the performance of a PV product. It defines how much energy in sunlight is actually converted into electricity. A major drawback of amorphous silicon modules is that they have lower efficiency than other PV materials. Moreover, long-term, their cell efficiency degrades progressively with use. This efficiency degradation is caused, ironically, by exposure to light. In the first three months, cells can lose up to 30% of their efficiency, depending on the technology. To combat this phenomenon, manufacturers found that by making the layers even thinner, degradation was not as serious. Unfortunately, making the layers thinner also lowers the product's overall cell efficiency.
Multi-junction
Thin-film PV is created by sequentially depositing thin layers of the different materials into a very thin "sandwich-like" structure. One way to improve cell efficiency that has been employed by amorphous silicon manufacturers is to stack two of these PV "sandwiches" on top of each other. The top sandwich will absorb some of the light energy (photons) and create electricity. Any photons that pass through the first sandwich can be absorbed by the second sandwich to create additional electricity. Each of these sandwiches creates a single electrical interface known as a "junction". Logically, a stack of these junctions is referred to as a "multi-junction" cell. Multi-junction devices can achieve higher total conversion efficiency because they can convert more of the energy spectrum of light into electricity.
Limitations
There is, of course, a downside to multi-junction PV devices. To make these devices work, each sandwich has to be "tuned" to respond to sunlight energy from a unique range of the solar spectrum (its unique "band-gap"). In this way, the top cell captures the high-energy photons and passes the rest of the photons on to be absorbed by the bottom cell (or cells). The bottom cells also have to be tuned to respond to lower band-gap energies. These multi-gap, multi-junction designs have proven very costly to manufacture.
Other serious problems persist with amorphous silicon technology. The best demonstrated laboratory module efficiency for single junction amorphous silicon is much less than that of the CIGS technology. Furthermore, triple-junction amorphous silicon designs still have lower cell efficiency than a single-junction CIGS. Moreover, incremental increases in cell efficiency have not occurred in the last several years. Recent independent studies have also suggested that the efficiency degradation of amorphous silicon is much more serious than previously believed. (TISO Centre, and independent testing laboratory funded by the Swiss Federal Office of Energy, has published testing results at http://leee.dct.supsi.ch/pv/tiso_tests_results.htm).
Nonetheless, many companies continue to emphasize manufacturing amorphous silicon because the technology is relatively well explored and many of the patents have expired. Thus, most production techniques are in the public domain.
Cadmium Telluride (CdTe)
Cadmium Telluride, another thin-film technology, has high cell efficiencies (over 16% in the laboratory). Manufactured module efficiencies have been achieved and may increase to over 10% over time. Limitations
CdTe exhibits certain limitations that may keep CdTe from full market acceptance.
First, the perception has historically been that CdTe devices will be unstable in the outdoor environment due to an inherent nature of the material to "self-compensate", thereby causing degradation of initially high-performance electronic contacts and reducing power output over time.
Second, CdTe deposition and crystal formation requires high processing temperatures. As a consequence of this and other issues, CdTe is only manufactured in a "superstrate" configuration; that is, sunlight must pass through the substrate to get to the PV material. Glass is the only material that can withstand the temperature and still be adequately transparent. Due to its fragile nature, the glass used must necessarily be thick and heavy to endure the stresses found during product life in the field. High processing conditions can build stress into the glass, leading to fracturing after deployment.
A third limitation of CdTe is that the toxicity of Cadmium is of concern to health officials and policy makers (Cadmium is a heavy metal). This is expected to limit access to many high-volume consumer applications.
Copper Indium Gallium diSelenide (CIGS)
Copper Indium diSelenide (CuInSe2) has an extremely high absorption that allows 99 percent of available light to be absorbed in the first micron of the material. This makes it an optimal, effective PV material. Adding small amounts of Gallium to the CuInSe2 boosts its light-absorbing band gap, which makes it more closely match the solar spectrum, thereby improving the voltage and the efficiency of the PV cell. CIGS cells have reached efficiencies of more than 19 percent - much higher than other thin-film PV. CIGS also has a demonstrated ability to pass appropriate environmental certification and waste-handling requirements. (See Technology - Global Solar)
Technology Comparison Summary:
As the table below summarizes, c-CIGS (crystalline-Copper Indium Gallium diSelenide)
compares favorably against the industry's dominant technology, crystalline silicon, as well as other thin-film technologies. This is especially true when evaluating module performance, since it is modules, not cells that ultimately are used in the marketplace.
