What are solar photovoltaics?
Solar photovoltaics enable sunlight to be transformed directly into electrical power. This transformation is made by the photovoltaic effect, or interaction between radiating sunlight and the semiconductor material of the solar cell. This generates electrical charges which are conducted away by metal contacts. The direct current produced can be transformed into alternating current suitable for the grid by connecting a AC/DC inverter. The most important element of a photovoltaic generator is the solar or photovoltaic cell. Several solar cells are combined in series or parallel into an electrical unit, the solar module.
How does a solar cell function?
A solar cell consists of a very thin layer of semiconductor material (usually silicon). This is doped with impurities (other elements) on both sides. As a result one side acquires a negative charge (a surplus of electrons) and the other a positive charge (electron deficiency). When sunlight falls on the material which has been changed in this way, electrons are forced from one side to the other by its radiative energy. This produces electrical voltage, and thus direct current at the terminals.
What is a solar cell made of?
The basic material of the solar cell is silicon (Si), which is extracted from silica sand. Silicon cells can be divided into two main groups depending on how they have been processed:
1. Crystalline silicon cells. These are formed as single or multicrystal cells, and offer a high efficiency: fourteen to eighteen per cent. Crystalline silicon cells are used in cladding materials, roofs, skylights etc.
2. Amorphous silicon cells. Amorphous silicon solar cells are relatively inexpensive, and used in consumer applications such as watches and pocket calculators as well as buildings. Efficiency is five to eight percent.
# Current status of photovoltaic technology
Single crystal silicon cells currently have the greatest market share. These cells have been on the market for some years and the technology is fully developed.
Companies in Germany, the USA and Japan are now working on developing the material and on new casting and crystallizing processes which would lower production costs. Work is also going on worldwide to achieve higher efficiency rates.
Ribbon silicon: the biggest cost factor in the production of silicon wafers is the slicing. One option is to grow silicon in ribbons. However, only one company is currently working on this technology.
Thin film silicon: besides the ribbon technique, which produces independent silicon ribbons, another possibility is the diffusion of silicon onto a substrate. The diffusion may be from a solution or gas.
The production of all solar cells will have some environmental costs, all of which are many times lower than existing power production. However, Greenpeace cannot encourage the use of solar cells which incorporate heavy metals, and we would encourage consumers to use silicon cells.
Amorphous silicon (a-Si) solar cells: there is a stable market for amorphous silicon cells in such things as pocket calculators and watches, minimal energy consumers. In supplying power in the kW range, amorphous silicon is still not often used on account of its low efficiency (5-7%) and its degradation (its efficiency falls in the first few years of operation). However, its use as a cladding component for large areas is being explored particularly in applications where its semi-transparent nature is useful.