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Last updated: 4 April 2002

There are only a handful of materials, especially treated semi-conductors, that are known to display the PV effect with reasonable energy conversion efficiency. At present the vast majority of photovoltaic cells are made from silicon. In general, cells are classified as either crystalline (sliced from ingots or castings or grown ribbons) or thin film (deposited in thin layers on a low cost backing).

Crystalline silicon

Example of multi-crystalline silicon cell Single crystal silicon cells are usually manufactured from a single crystal ingot, most commonly grown by the Czochralski method. PV cells made from multicrystalline silicon have now become popular as they are less expensive to produce, although slightly less efficient. Multicrystalline cell manufacture usually begins with a thermal process in which silicon is melted and solidified such a way that crystals are oriented in a predetermined direction. This produces a rectangular ingot of multicrystalline silicon that is then cut into blocks or bricks which are finally sliced into thin wafers that are used to make the cells. One way of eliminating the sawing step is to grow ribbons of multicrystalline silicon that are already wafer thin and the correct width for use as PV cells.

The maximum recorded cell efficiency for crystalline silicon is 24,7%. Cell efficiencies greater than 25% have been recorded for cells made from III-V semiconductor material (for example gallium arsenide concentrator systems and for space applications because of their high cost.
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Thin film

Thin film modules are constructed by depositing extremely thin layers of photovoltaic materials on a low cost backing such as glass, stainless steel or plastic. Individual 'cells' are formed by then scribing through the layers with a laser. Thin film cells offer the potential for cost reductions. Firstly, material costs are lower because much less semiconductor material is required and, secondly, labour costs are reduced because the films are produced as large, complete modules and not as individual cells that have to be mounted in frames and wired together.

The most fully developed thin film technology is hydrogenated amorphous silicon. This is the material normally used in consumer applications, although it is used, but less frequently, in power modules. The efficiency of commercial amorphous silicon modules has improved from around 3,5 % in the early 1980's to over 7% currently. The most efficient modules are made with multiple layers of photovoltaic material, for instance three layer amorphous silicon modules with germanium added to two of the layers (a-Si/a-SiGe/a-SiGe) which have a record cell efficiency of 13,5 %. Other types of thin films can be made using polycrystalline silicon, cadmium telluride (CdTe), and copper indium gallium diselenide (CIGS).

Typical and maximum module and cell conversion efficiencies (at Standard Test Conditions, i.e., 1 000 Wm-2, 25 degrees Celsius, solar spectrum AM1.5) are given in the table below for some of the commercially available PV technologies.

Typical and maximum module and cell conversion efficiencies at Standard Test Conditions
Type Typical module efficiency
[%]		 Maximum recorded module efficiency
[%]		 Maximum recorded laboratory efficiency
[%]
Single crystalline silicon 12-15 22,7 24,7
Multicrystalline silicon 11-14 15,3 19,8
Amorphous silicon 5-7 - 12,7
Cadmium telluride - 10.5 16.0
CIGS - 12,1 18,2

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