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Photovoltaics | Crystalline Silicon | Thin-film

Global Solar has developed a proprietary process for manufacturing thin-film Copper Indium Gallium diSelenide (CIGS) photovoltaic (PV) modules. Unlike traditional solar panels that are rigid, heavy and fragile, Global Solar's thin-film solar modules are lightweight, flexible and durable. While other companies produce CIGS on glass, Global Solar is the only company with CIGS on flexible materials.

CIGS create more electricity from the same amount of sunlight than does other thin-film PV and therefore has a higher "conversion efficiency". CIGS conversion efficiency is also very stable over time, meaning its performance continues unabated for many years. The performance of many other PV materials can rapidly decline with use. Customers are well aware that Global Solar's CIGS does not suffer the degradation in cell efficiency associated with other thin-film PV. (See conclusions of Israel's Weizmann Institute as published in Renewable Energy World, September 1999, "CIGS Cells are Self-Repairing, Say Researchers").

No other company comes close to matching our approach to PV manufacturing and its resultant PV products.
This is because:

Global Solar uses thin-film PV which is less expensive to manufacture due to reduced labor, lower material, energy, handling and capital costs.
Global Solar uses Copper Indium Gallium diSelenide (CIGS) which is the best of the thin-film PV.
Global Solar uses "roll-roll" manufacturing which is significantly more cost-effective than traditional in-line manufacturing approaches.
Global Solar's PV can be shipped in very compact, lightweight packages to the most remote parts of the earth. This significantly reduces the cost of the complete installation.
Global Solar's PV being so compact provides better storage and when coupled with high damage tolerance is a significant advantage for many military, space, and commercial applications.

Global Solar's technology and products offer great opportunity to the future of renewable energy.

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PHOTOVOLTAICS

How it works

Photovoltaics (PV) systems produce electricity when exposed to sunlight. Sunlight is composed of particles of energy called photons. When sunlight strikes a PV material, photons will either pass through, be reflected, or be absorbed. If the photon is absorbed, its energy will be transferred to an electron in an atom of the PV material. With its new found energy, the electron is able to escape from its normal position in orbit around that atom. In this way, the electron can become part of, and augment, the current in an electrical circuit. This "photovoltaic effect" is the basic physical process through which sunlight is converted into electricity.

The primary building block of a PV system is the PV cell. A typical PV cell is about 3 X 3-in. and very thin. By itself, a single PV cell produces only a small amount of electricity. Fortunately, we can easily increase the total power in a PV system by connecting several cells to form larger units called modules. Modules, in turn, can be connected to form even larger units know as arrays, which can be interconnected to produce more power, and so on. In this way, we can build a PV system to meet almost any power need, no matter how small or great.

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CRYSTALLINE SILICON

All commercially viable PV products are made using one of two groups of technologies; Crystalline Silicon or Thin-film materials.

Crystalline Silicon - Overview

Traditional crystalline silicon is by far the most common solar cell material for commercial applications because:

It has been in use for more than 50 years, and its manufacturing processes are well known. Those processes are now largely in the public domain.
The raw material used, silicon, is very abundant (it's the second most abundant element in the Earth's crust - second only to oxygen)

Although raw silicon is readily available, the silicon used in solar cells must be refined to an extremely high purity (99.9999 percent) - far more refined than most prescription medicines. Refining to this degree makes the silicon quite expensive. In the past, silicon PV producers economized by reclaiming silicon waste from Integrated Circuit (IC) manufacturers (IC chips require even higher silicon purity). This source is rapidly becoming less available as IC producers (1) improve their manufacturing yields and thereby reduce waste; (2) reclaim silicon waste for their own uses; and (3) develop chip designs that can employ lower grade silicon. As a result, PV grade silicon is becoming even more expensive.

Forms of Crystalline Silicon PV

There are two basic forms of crystalline silicon PV:

Single-crystalline silicon, which is more efficient at creating electricity from sunlight but is more expensive to manufacture
Poly-crystalline silicon, which is less efficient at creating electricity from sunlight, but is less expensive to manufacture.

As their name suggests, single-crystalline silicon cells are prepared from slices of a large single crystal ingot. This crystalline material is structurally uniform with very few disturbances in the orderly arrangement of atoms. As a result, single-crystalline silicon is more efficient at converting sunlight power into electricity. In contrast, poly-crystalline silicon is composed of many crystals or "grains". At the interfaces of the grains, the atomic order is disrupted. These interfaces make poly-crystalline silicon less efficient at converting sunlight power into electricity.

Crystalline silicon PV is made in several ways, all of which are capital and labor intensive and require the costly melting of high-purity silicon. The most widely used technique for making single-crystalline silicon lowers a "seed" of single-crystalline silicon into the top of a vat of molten silicon. As the seed is slowly raised from the vat, atoms of the molten silicon solidify around the seed, creating a long cylindrical ingot of silicon. All the crystals within this ingot will have the same crystalline structure as the seed. In contrast, semi-crystalline PV is typically made through a much simpler process of casting, in which molten silicon is poured directly into a mold and allowed to solidify into an ingot.

Either way, once the crystalline ingots are produced, they must be sliced into very thin, fragile wafers. After several additional manufacturing steps, wafers will work as PV cells; however, because the cells are fragile, they usually must be encapsulated between two thick sheets of glass. The glass allows sunlight to enter the PV material while helping prevent damage from light impacts. Unfortunately, the result is a very heavy, cumbersome product that cannot survive serious impact, requires excessive protection during shipping, and is costly to ship and handle.

Some PV manufacturers are pursuing new techniques to manufacture crystalline silicon. In one such technique called Silicon-Film approach, the silicon layer is grown directly on a ceramic substrate, resulting in a silicon wafer that is reportedly one-half the thickness of a traditional cell. In another technique, two parallel strings are pulled through molten silicon which spans, then solidifies, between the strings. Both processes eliminate the inherent cost and waste of sawing an ingot of silicon into wafers. Nonetheless, each approach still requires several additional manufacturing treatments before these fragile wafers will work as PV cells. Moreover, the resulting PV product still has most of the inherent limitations of other silicon PV such as fragile cells and heavy packaging.
The characterization of either of these products as thin-film PV is not supportable. Although the resultant silicon wafers may be one-half the thickness of traditional silicon, they are, nonetheless, 50 times as thick as true thin-film PV.

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THIN-FILM

Thin-Film - Overview

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. 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)

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Technology Comparison Summary

As the table below summarizes, CIGS 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.

Photovoltaic Technology	Photovoltaic Efficiency, %			Flexi-bility	Remarks
Photovoltaic Technology	*Record Cell	*Record Module	Typical Module	Flexi-bility	Remarks
Crystalline Silicon
Single Crystalline Silicon	24.7	22.7	12-14	Rigid	Fully Mature Technology Further Reduction in Price or Increase in Performance Difficult
Polycrystalline Silicon	20.3	15.3	10-12	Rigid	Similar to Single Crystalline Silicon
String Ribbon Silicon	16.6	8.2		Rigid	Similar to Single Crystalline Silicon
Thin-Film
Amorphous Silicon	12.7	N/A	5-7	Rigid/ Some Flexible	Requires Hazardous Gases Low Performance No Clear Pathway to Increase Efficiency Exhibits Instability
Multi-Junction Amorphous Silicon	12.4	10.4	6-8	Rigid	Similar to Amorphous Silicon
Cadmium Telluride	16.5	10.7	7-8	Rigid	Toxicity of CdTe is an Issue No Clear Pathway to Increase Efficiency Requires Superstrate Configuration
Copper Indium Gallium Diselenide	19.2	13.4	9-11	Global Solar is Flexible	Economical Material and Processes Highest Performance Thin Film Clear Pathways to Improve Performance Compatible with Flexible Substrates and Economical Roll-to-Roll Processing

*Green MA, Emery K, Ling DL, Igari S, and Warta W. Solar Cell Efficiency Tables (Version 24). Progress in Photovoltaics: Research and Applications. 2004; 12: 365-372.