Short description of PV technology

Single crystalline silicon solar cells and modules

Its base material is a single crystalline silicon wafer. The crystal is grown by the Czochralski method. The cell consists of a pn-junction whereby the substrate is a p-type silicon and the n-type emitter is formed by a diffusion process in a diffusion oven at a relatively high temperature. The front side which is illuminated by the sun is contacted via a conductive grid consisting of bus bars and fingers. This grid is created by screen printing technique. The front surface is textured and an antireflection coating is also formed to decrease reflection from the front side. Then the cells are connected series and are laminated between a glass sheet and backside foil using EVA foil in between. Aluminium frame gives a higher mechanical stability and makes the mounting of the solar module easier. The degradation of this type of solar cell is minimal and contributed mainly to the degradation of the EVA foil. The cell and module efficiency is relatively high. However, because of the high thermal budget of the manufacturing process and the need of large amount of raw material, these modules are relatively expensive.

Multicrystalline silicon solar cells and modules

Its technology and structure is very similar to that of the single crystalline silicon solar cell. However, its base material is a multicrystalline silicon wafer. The raw silicon is molten in a shape and is let cool slowly forming an ingot. The ingot then is sawn into wafers. This technology has a lower thermal budget than the single crystalline silicon solar cell technology but the substrate is of a lower quality. Therefore the price as well as the efficiency of multicrystalline silicon solar cells is lower than in case of single crystalline silicon.

Thin film silicon solar cells and modules

This type of cell consists of thin, submicron p-, i- (intrinsic) and n-type silicon layers deposited on a TCO (transparent conductive oxide) coated glass superstrate or on a steel foil substrate. It is possible to manufacture flexible solar modules of this type. These layers are amorphous or microcrystalline (in this case the thickness can be upto 2 microns) depending on the technology. Since this material is of low quality, it needs hydrogen passivation and an i-layer has to be inserted between the p- and n-layers as opposed to crystalline silicon solar cells. The deposition technique is PECVD (plasma enhanced chemical vapour deposition). The absorption co-efficient of this material is high enough to use very thin layers as absorbers and it is possible to make two or three cells on top of each other which is then called double or triple junction solar cells. The first cell of the tandem cell is amorphous silicon while the others can be either amorphous or microcrystalline. The creation of microcrystalline layers requires more energy and time and results in a material of superior quality. Module production takes place simultaneously with cell production. Unfortunately, amorphous silicon degrades to a certain level. Thus, amorphous technology is the cheapest of all and has the poorest quality and micromorph can be positioned somewhere between amorphous and microcrystalline technology regarding both price and quality.

Other, non-silicon based technologies

Solar cells can be made from semiconductor compounds or organic materials. By varying the ratio of the components one can tune the bandgap of the semiconductor which is a key property of the solar cell base material. It is also possible to manufacture double and triple junction solar cells using materials with various bandgaps thus achieving excellent effeciency but at a higher cost. As single crystalline solar cells are very efficient but very expensive at the same time, too. Highly efficient but expensive solar cell types are used together with concentrators requiring solar tracker which makes the system more expensive and on the other hand allowing to extend the time period of solar energy generation. There are thin film semiconductor compound solar cells as well, like CdTe, CIS and CIGS for which PVD (Physical Vapour Deposition) is the deposition method. They have quite good cost-performance ratio. However, these technologies use rare and, in some cases, toxic materials. Organic solar cells are rather under development, although they are already available in the market. Their cost as well as their efficiency is very low and their lifetime is short yet. The most efficient third generation solar cell type is the dye-sensitized solar cell which have stability issues, too.