semiconductor Archives

How do solar cells work?

By Pratik Barve | October 31, 2017 | Comments 0 Comment

The solar cell is an important candidate for an alternative terrestrial energy source because it can convert sunlight directly to electricity with good conversion efficiency, can provide nearly permanent power at low operating cost, and is virtually non-polluting. Solar cell also called as photovoltaic cell and are building blocks of solar panels. You must have seen these panels (large collection of solar cells) in green energy campaigns and also in developed cities in large arrays. It is used as a primary source of energy in space applications. ISS has large solar panels! Let’s understand the basics of solar cells.

The most commonly known solar cell is configured as a large-area p-n junction made from silicon. p-n junctions of silicon solar cells are made by diffusing an n-type dopant into one side of a p-type wafer (or vice versa). To get an idea about p-n junction click here. Now when light or photon hits the p-n junction it dislodges an electron and creates a hole in its place. Now the dislodged electron and hole are free to move in silicon crystal. Due to the electric field present, the electron moves to the n-type region and hole moves to the p-type region. The mobile electrons created in n-type are collected by thin metal fingers on the top of n-type region. Photons having energy equal to the band gap of silicon crystals are the only ones contributing in cells electrical output. Energy greater than band gap is lost as heat. We might also lose energy if electrons and holes recombine as soon as they are formed. So basically a solar cell works by knocking off electron of same energy as band gap of crystal material used and essentially converting light energy into electrical energy.

Schematic representation of silicon p-n junction Solar cell.

The radiative energy output from the sun derives from a nuclear fusion reaction. In every second, about 6 x 10^11 kg hydrogen is converted to helium, with a net mass loss of about 4 x 10^3 kg. We get a lot of energy from sun and if there is a way to harness this energy what’s stopping us from being completely reliant on solar power? There are various factors at play.

Terrestrially the sunlight is attenuated by clouds and by atmospheric scattering and absorption. Also we don’t receive sunlight during night time and during bad weather conditions. This is something we can’t do anything about. As mentioned above, only the photon having energy equal to band gap contributes in electrical output. Photons having energies other than band gap of semiconductor crystals either reflects back or goes through or the energy is just lost as heat energy. Antireflective coating is done to avoid reflection of photons. To deal with photons of energies other than band gap of that semiconductor crystal there is a different approach.

Spectrum Splitting :

Spectrum splitting is a very good way of increasing efficiency of solar cells by splitting sunlight into narrow wavelength bands and directing each band to a cell that has a band gap optimally chosen to convert just this wavelength band of light. There is one more way, by simply stacking cell on top of one another with the highest band gap cell at the top which automatically achieves an identical spectral-splitting effect, making this “tandem” cell approach a reasonably practical way of increasing cell efficiency.

This is an area of active research where scientists all over the world are trying to increase the efficiency of solar cells. The most efficient solar cell yet still converts only 46% of solar energy into electricity. And most commercial systems convert only 15-20% of solar energy. Solar energy is a source of green and sustainable energy and probably a candidate for future source of energy.

What is p-n junction?

By Pratik Barve | October 30, 2017 | Comments 0 Comment

The p-n junctions are elementary building blocks of semiconductor electronic devices. As the name suggest it is a junction between two types of semiconductor materials the p-type and the n-type. The p-type is the positive side of junction and n-type is the negative side. The p-type consists of excess holes (missing electrons) and n-type consists of excess electrons. The junction is the boundary or interface between these 2 types inside a single crystal of semiconductor. When a p-n junction is formed i.e when it is fabricated there are excess electrons on n side and holes on p sides so electrons and holes combine at the junction. Departure of electrons from n side to p side leaves with a positive donor ion on n side and likewise hole leaves negative acceptor ion on p side. The uncompensated ions are positive on n side and negative on p side which creates an electric field which provides a force opposing the continued exchange of charge carriers.

When the electric field is sufficient to stop further transfer of charge carriers the depletion layers reaches an equilibrium. When a positive voltage is applied to the p-side with respect to the n-side, a large current will flow through the junction. However, when a negative voltage is applied, virtually no current flows. This “rectifying” behaviour is the most important characteristic of p-n junction. This is a very short description of p-n junction and how it works.

The p-n junction was invented by Russell Ohl an American physicist of Bell Laboratories in 1939. The p-n junction is created by doping (adding impurities) by various methods on any one type of base semiconductor material. Various methods like ion implantation, diffusion, etc. Semiconductors for instance silicon, is doped with boron, phosphorus, or arsenic by ion implantation method. In diffusion method the impurity moves into the semiconductor crystal by solid-state diffusion. In the ion-implantation method, the intended impurity is introduced into the semiconductor by accelerating the impurity ions to a high-energy level and then implanting the ions in the semiconductor.

The p-n junction can be used in electronics circuits in two different configurations, forward bias and reverse bias.

In forward bias, the p-type is connected with the positive terminal and the n-type is connected with the negative terminal. With a battery connected this way, the holes in the p-type region and the electrons in the n-type region are pushed toward the junction and start to neutralize the depletion zone, reducing its width. And thus current flows through the junction.Connecting p-type region to the negative terminal of the battery and the n-type region to the positive terminal corresponds to reverse bias. Very little current will flow until the diode breaks down in reverse bias configuration.

The forward-bias and the reverse-bias properties of the p–n junction imply that it can be used as a diode. A p–n junction diode allows electric charges to flow in one direction, but not in the opposite direction; negative charges (electrons) can easily flow through the junction from n to p but not from p to n, and the reverse is true for holes. When the p–n junction is forward-biased, electric charge flows freely due to reduced resistance of the p–n junction. When the p–n junction is reverse-biased, however, the junction barrier (and therefore resistance) becomes greater and charge flow is minimal. p-n junction is used in many semiconductor devices such as diodes, transistors, solar cells, LED’s, integrated circuits, etc.