Solar radiation received on earth in just one hour is more than what the whole world’s population consumes in one year, so solar cells play a vital role in developing the world’s future energy needs. The 3,850,000 exajoules (EJ) amount of solar energy received by the Earth’s surface per year. In 2011, worldwide primary energy consumption was 550 EJ. 7 hours of Solar Energy at Deserts can meet the annual global energy requirement.
Note: 1 EJ = 10 18 J
(i.e. in the order, Joule (J), Kilo Joule, Mega Joule, Giga Joule, Tera Joule, Peta Joule, Exa Joule, Zeeta Joule, Yotta Joule, and so on.)
What is Photovoltaics (“PV”)?
- Photovoltaic (phōs) = light (@ Greek )and “voltaic” = electric (from Volta, Italian physicist)
- Conversion of sunlight directly into electricity through semiconductor materials
- Electricity for 2 billion people around the world
What are photovoltaic (PV) systems?
A PV system consists of:
- Photovoltaic cells are connected into modules and encapsulated modules are grouped into panels.
- Panels are grouped into arrays,
- A power conditioning unit,
History of Photovoltaic Cells:
In 1839, a young French physicist named Edmund Becquerel discovered the photovoltaic effect. He noted that while working with two metal electrodes in an electricity-conducting solution, the apparatus generates a voltage when exposed to light.
In 1904, Albert Einstein published a paper on the photoelectric effect, and the general scientific community stopped looking at photovoltaics as some type of scientific hoax.
1839 Becquerel: Photogalvanic Efficacy
1873 Smith: Photoconductivity of Se
1883 Fritts: Se films solar cells
1946 Russel Ohl Modern Solar cell Patent
1954 Bell Lab: 6% Si solar cells
1959 Hoffman: 2% Si cells (industrial)
1966 NASA: 1 kW array
1974 Sunshine: Alternatives to Si
1980 Thin-film 10% Cu2S/CdS
1985 High Efficiency 20%; Conc. 25% (Si)
1999 1GW installed (cumulative)
2007 ~3 GW
2008 Sunpower 18.5% PV panel
2009 Low cost (<1$/wp) module by First solar
Note: Se – Selenide, Si – Silicon, CdS- Cadmium Sulfides, Cu2S- Copper Sulfides.
What is Solar Cell?
The basic component of a PV system is a solar cell.
Two fundamental functions:
- Photo-generation of charge carriers (electrons and holes) in a light-absorbing material
- Separation of the charge carriers to a conductive contact to transmit electricity
If light with adequate energy falls onto silicon arranged to form a p-n junction and penetrates to a point near the junction, then, because of the photo-electric effect, it will create free electrons near the junction. These electrons immediately move under the influence of the p-n junction’s electric field. The electrons continue to move through the cell to the surface of the cell.
Some of the electrons may be re-absorbed by the silicon atoms on the way towards the surface of the cell, but many electrons still reach the surface of the cell. These electrons can be collected by a metallic grid and an electric current will flow if the grid is connected to the metal contact on the other side of the cell by an external circuit.
- Crystalline silicon (c-Si)
- Amorphous silicon (s-si)
- Cadmium Telluride (CdTe)
- Copper Indium Diselenide (CIS)
- III-V Family (Gallium arsenide)
- Dye-sensitized Solar Cells (DSSC)
- Organic Photovoltaics (OPV)