With so many types of solar technologies on the market, it’s easy to be confused about how one differs from another. US Master Electrician David Herres finds out more about thin-film solar cells.

There seems to be no downside to solar as it steadily approaches grid parity across the globe. Utility-scale operations are in place throughout the world, not to mention the large amount of interactive, gridconnected equipment and, for remote regions, stand-alone versions.

Only in recent times has the cost of solar power declined to an extent where it may be considered a viable alternative on a utility level. At present it is attracting a great deal of interest and the future looks good.

Solar power, to operate, depends upon a semiconductor that absorbs light and converts it to electron-hole pairs. All of the radiant energy that strikes a solar cell is reflected, absorbed or transmitted. Some of the absorbed radiation is lost as heat; the remainder is converted into electricity.

The way this works is at the semiconductor junction. Light-generated carriers, holes and electrons are separated, creating a usable voltage differential between top and bottom outer surfaces of the solar cell. When light strikes any absorbing substance, atomic motion results, but in a semiconductor the holes and electrons are separated and sent in opposite directions, making for usable electrical current.

Obviously, there must be conducting material bonded to the two surfaces, so that wires may be attached in order to connect a load and harness the resulting current flow. The bottom contact can be a flat, opaque, metallic plate but the topside contact is a bit more of a challenge since materials that conduct electricity (like copper) do not ordinarily transmit light. The solution has been to create a grid that blocks an acceptably small amount of light and yet is substantial enough to conduct the desired electrical current across the upper surface of the solar cell to the outside where it can be connected to the output of additional cells and eventually become the electrical product.

The topside pickup wiring must be of sufficient ampacity that is large enough to transmit the electrical energy without too much heat loss, while small enough so as not to block too much incoming light. One solution has been to make these conductors rectangular in cross section, high but not wide.

This is the basic structure of a solar cell. It may take one of two forms, crystalline silicon (c-Si) or thin-film.

C-Si is the more prevalent material in most solar cells manufactured at present. But it is an imperfect light-absorbing medium and as a result, the solar cell must be relatively thick. Despite this drawback, c-Si is widely used because of comparatively good efficiencies, approaching 20% and beyond.

Silicon is very abundant at the earth’s surface, but for solar cell fabrication, great purity is required, making for a costly end product. Nevertheless, due to market inertia and other factors, c-Si continues to comprise close to 80% market share.

It may take either of two forms. Monocrystalline silicon is made by slicing wafers from a single silicon crystal that has been grown in a highlycontrolled environment. Most of the requisite technology was already in place (thanks to microprocessing knowhow) prior to its use in solar cell fabrication. Multicrystalline silicon is produced by sawing a previously cast block into bars, then wafers. Obviously, in both of these techniques, there is a certain amount of waste in the form of sawdust. Also, these processes are fairly labour-intensive.

In both forms, a minute amount of phosphorous is diffused into the top layer of the silicon wafer. This is key to the whole process since it enables the semiconductor action to occur. Screenprint technology is used to create the contacts.

Since c-Si cells generate only 0.5V, it is necessary to connect them in series-parallel configuration to achieve higher power levels. The final step is to seal the cells under very high-quality glass to make a weather-resistant assembly that will last for years under all environmental conditions.

Thin-film technology

In the continuing effort to find less expensive solutions, the industry for years has looked at another approach, known as thin-film technology. This solution has a strange mix of advantages and disadvantages. They need to be analysed in order to improve the product if it is to finally prevail.

At present, thin-film solar power has captured about 20% of the market. No one knows for certain what the future may bring, but it is possible that thin-film solar power will surge ahead, primarily due to the fact that it is the more suitable of the dominant forms of solar power to be used in conjunction with Building Integrated Photovoltaics (BIPV), which is considered by many to be the way of the future. We shall see why this is so.

Thin-film solar cells are, as the name implies, much thinner than c-Si products. Some versions are flexible, so that they may be packed in rolls and affixed to curved surfaces.

They are made of amorphous (noncrystalline) silicon, or certain crystalline materials – cadmium telluride (CdTe), copper indium (gallium) or diselenide (CIS or CIGS). These materials lend themselves to a variety of manufacturing processes whereby they are deposited over suitable materials in wide sheets, allowing them to be mass-produced.

We are witnessing great progress in amorphous silicon. Thin layers are very stable, but due to decreased light absorption, their efficiency is less than c-Si. This only means, however, that they have to be deployed in slightly larger arrays and the cost advantage is not completely lost.

In thin-film technology, the solar cell is created by depositing a very thin layer of amorphous silicon onto an obliging substrate. Thus, little of the costly material is required.

Amorphous silicon may be deposited on a light, flexible plastic substrate, making it ideal for use in BIPV. Early in the development of solar power, thin-film technology was used for handheld calculators and small consumer electronics items.

Despite somewhat less efficiency and the possibility that it may not be as durable as c-Si, thin-film is coming on strong due to its cost advantage and suitability for BIPV usage.

In BIPV, the PV array does not merely sit upon the roof, supported by additional hardware, but it becomes the roof, or building facade, or window material. Since it is actually integrated into the construction of the building, it replaces, in terms of materials and labour, conventional building elements. Accordingly, these may be subtracted from the finished price of the PV installation to calculate the installed cost.

BIPV elements may take the form of roof shingles, tiles or flat panels, or facade or window materials, and they make for a highly effective final product with a pleasing visual aspect. Of course, designers and installers must be knowledgeable and accomplished workers if a quality product is to result.

Those electricians who are willing to expend the effort to acquire BIPV knowledge and expertise will benefit by having access to abundant work. The demand for BIPV installations is bound to accelerate as thin-film, with greater manufacturing efficiency, continues to drop in price and conventional energy costs spiral ever upward. At present, BIPV represents only 2% of the total PV scenario, so there is ample room for expansion.

There are numerous modes of entry into this lucrative niche market. To begin, it is necessary to acquire a complete knowledge of applicable codes and regulations, which vary widely from place to place. Any required licensing should be obtained. Let the unlicensed individuals who choose to work under the table have the bottom segment of the market. Trunk slammers will always be around, but they will never prosper.

The examination process should be viewed as a valuable learning opportunity as opposed to an ordeal to be endured.

A full knowledge of all applicable feedin tariffs, tax incentives and available grants is useful so that the tradesperson may communicate this information to the client. This is a great buyer incentive and helps the customer afford the project.

Where to begin? Consider a BIPV installation for your own home or work facility. Besides displaying your expertise and ability to successfully complete such an undertaking, you will enjoy the same economic and aesthetic benefits that you advocate for your customers.