Photovoltic Cell Technologies

In making comparisons between alternative power technologies, the most important measure is the energy cost per kWh delivered. In pv power, this cost primarily depends on two parameters, the photovoltaic energy conversion efficiency, and the capital cost per watt capacity.

The continuing development efforts to produce more efficient low cost cells have resulted in various types of pv technologies available in the market today, in terms of the conversion efficiency and the module cost. The major types are discussed in the following sections:

Crystalline solar cells

Most solar cells are made of a single crystal or multi-crystalline silicon material. Silicon ingots are made by the process of crystal growth, or by casting in specially designed furnaces. The ingots are then sliced into thin wafers. Single crystal wafers are usually of 125 × 125 mm or larger sizes with ‘pseudo-square’ shape; multi-crystalline wafers are typically square-shaped with a dimension of 100 × 100 mm or larger. Using high temperature diffusion furnaces, ‘impurities’ like boron or phosphorous are introduced into the silicon wafers to form a p–n junction. The silicon wafers are thus converted into solar cells. When exposed to sunlight, a current is generated in each cell. Contacts are attached to the top and bottom of each solar cell to enable inter-connections and drawing of the current.

Single-Crystalline Silicon

The single crystal silicon is the widely available cell material. In the most common method of producing this material, the silicon raw material is first melted and purified in a crucible.
A seed crystal is then placed in the liquid silicon and drawn at a slow constant rate. This results in a solid, single-crystal cylindrical ingot. The manufacturing process is slow and energy intensive, resulting in high raw material cost. The ingot is sliced using a diamond saw into 200 to 400 μm (0.005 to 0.010 inch) thick wafers. The wafers are further cut into rectangular cells to maximize the number of cells that can be mounted together on a rectangular panel. Unfortunately, almost half of the expensive silicon ingot is wasted in slicing ingot and forming square cells.
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Polycrystalline and Semicrystalline

This is relatively a fast and low cost process to manufacture thick crystalline cells. Instead of drawing single crystals using seeds, the molten silicon is cast into ingots. In the process, it forms multiple crystals. The conversion efficiency is lower, but the cost is much lower, giving a net reduction in cost per watt of power.

Thin-film solar cells

Thin-film solar cells are made from amorphous silicon (a-Si), copper indium selenide/cadmium sulphide (CuInSe2/CdS) or cadmium telluride/cadmium sulphide (CdTe/CdS), by using thin-film deposition techniques.  These are new types of photovoltaics entering the market. Copper Indium Diselenide, Cadmium Telluride, and Gallium Arsenide are all thin film materials, typically a few μm or less in thickness, directly deposited on glass, stainless steel, ceramic or other compatible substrate materials. This technology uses much less material per square area of the cell, hence, is less expensive per watt of power generated.

Amorphous Silicon

In this technology, amorphous silicon vapor is deposited on a couple of μm thick amorphous (glassy) films on stainless steel rolls, typically 2,000-feet long and 13-inches wide. Compared to the crystalline silicon, this technology uses only 1 percent of the material. Its efficiency is about one-half of the crystalline silicon at present, but the cost per watt generated is projected to be significantly lower.

Spheral

This is yet another technology that is being explored in the laboratories. The raw material is low-grade silicon crystalline beads. The beads are applied on typically 4-inch squares of thin perforated aluminum foil. In the process, the impurities are pushed to the surface, from where they are etched away. Since each sphere works independently, the individual sphere failure has negligible impact on the average performance of the bulk surface.

Concentrated Cells

In an attempt to improve the conversion efficiency, the sunlight is concentrated into tens or hundreds of times the normal sun intensity by focusing on a small area using low cost lenses. The primary advantage is that such cells require a small fraction of area compared to the standard cells, thus significantly reducing the pv material requirement. However, the total module area remains the same to collect the required sun power. Besides increasing the power and reducing the size or number of cells, such cells have additional advantage that the cell efficiency increases under concentrated light up to a point. Another advantage is that they can use small area cells. It is easier to produce high efficiency cells of small areas than to produce large area cells with comparable efficiency. On the other hand, the major disadvantage of the concentrator cells is that they require focusing optics adding into the cost.