Photovoltaic (PV) Cells

PVs or solar cells (SC) convert sunlight directly into electricity. When photons strike certain semiconductor materials, such as silicon, they dislodge electrons which causes a potential difference to form between the specially treated front surface of the SCs and the back surface. In order to increase the voltage, individual cells are combined in a panel form. The most advanced photon utilization technology is the SC to which the PV effects of semiconductors are applied. SCs are the stan­dard bearer of the new energy technologies because of their great potential. Their successful development is dependent on cost reduction of the power-generating sys­tems that include SCs. They must either be used together with storage devices or as supplements to conventional facilities. Due to their high cost they are still not prac­tical for large-scale power generation. The few central solar generation facilities in operation are experimental and need large areas of land. With current technol­ogy about 10 m2 of PV panels are required to generate 1 kW of electricity in bright sunlight. It would take hundreds of square kilometers of solar panels to replace an average nuclear power plant. For instance, 220,000 km2 would be needed to supply the world with power. Some scientists suggest that the size of the solar power foot­print could be reduced by as much as 75% by placing satellites in space to collect sunlight, convert it into electricity, and then beam the power to the earth’s surface in the form of microwaves. Currently, the problems for researchers in SC technology are making SCs more reasonable in price and more efficient. Unfortunately, high efficiency and low cost tend to be mutually exclusive.

Photovoltaic cells consist of a junction between two thin layers (positive, p, and negative, n) of dissimilar semiconducting materials. When a photon of light is ab­sorbed by a valance electron of an atom, the energy of the electron is increased by the amount of energy of the photon. If the energy of the photon is equal to or more than the band gap of the semiconductor, the electron with the excess energy will jump into the conduction band where it can move freely. However, if the electron does not have sufficient energy to jump into the conduction band, the excess energy of the electron is converted to excess kinetic energy of the electron, which mani­fests as an increase in temperature. If the absorbed photon has more energy than the band gap, the excess energy over the band gap simply increases the kinetic energy of the electron. One photon can free up only one electron even if the photon energy is greater than the gap band. Figure 7.11 indicates schematically a PV device.

As free electrons are generated in the n layer by the photon action they can ei­ther pass through an external circuit or recombine with positive holes in the lateral direction, or move toward the p-type semiconductor. However, the negative charges in the p-type semiconductor at the p-n junction restrict their movement in that direc­tion. If the n-type semiconductor is made extremely thin, the movement of electrons and therefore the probability of recombination within the n-type semiconductor are greatly reduced unless the external circuit is open. In this case the electrons recom­bine with the holes and an increase in the temperature of the device is observed. The energy of a photon is already expressed by Eq. 3.3 and by considering the light

speed from Eq. 3.1 the photon energy can be obtained as

Photovoltaic cells are usually manufactured from silicon although other materials can also be used. n-type semiconductors are made of crystalline silicon that has been “doped” with tiny quantities of an impurity (usually phosphorous) in such a way that the doped material possesses a surplus of free electrons. On the other hand, p-type semiconductors are also made from crystalline silicon, but they are doped with very small amounts of a different impurity (usually boron) which causes the material to have a deficit of free electrons. Combination of these two dissimilar semiconductors produces an n-p junction, which sets up an electric field in the region of the junction (Fig. 7.11). Such a set up will cause negatively (positively) charged particles to move in one direction (in the opposite direction).

Light is composed of a steam of tiny energy particles called photons, and if pho­tons of a suitable wavelength fall within the p-n junction, then they can transfer their energy to some of the electrons in the material so prompting them to a higher en­ergy level. When the p-n junction is formed, some of the electrons in the immediate vicinity of the junction are attracted from the n-type layer to combine with holes on the nearby p-type layer. Similarly, holes on the p-type layer near the junction are attracted to combine with electrons on the nearby n-type layer. Hence, the net effect is to set up around the junction a layer on the n-type semiconductor that has more positive charges than it would otherwise have.

In recent years, power generation from renewable resources has been counted upon to bridge the gap between global demand and supply of power. The direct conversion technology based on solar PV devices has several positive attributes and

seems to be most promising. Extensive research activities over the past 25 years have led to significant cost reduction and efficiency amelioration (De Meo and Steitz 1990; Kaushika 1999).

Generation of electricity from sunlight started in the 1950s when the first PV cell was invented, which converted solar radiation directly into electric current via a complex photo-electric process. PV technology has advanced during the last five decades, making it possible to convert a larger share of sunlight into electricity; it has reached as much as 14% in the most advanced prototype systems. Although the cost of PV devices has fallen drastically during recent decades, it is still four to six times the cost of power generation from fossil fuels. PV devices are already the most economical way of delivering power to homes far from utility lines. It is expected that this technology will become an economic way of providing supple­mentary utility power in rural areas, where the distance from plants tends to cause a voltage reduction that is otherwise costly to remedy. As they become more versa­tile and compact, PV panels could be used as roofing material on individual homes, bringing about the ultimate decentralization of power generation. For instance, the desert areas are the most attractive and rich regions of the world for the solar radi­ation conversion into electric power. One day in the future the world’s deserts may become very large solar power plants, which may centralize power in the same way as do today’s coal and nuclear power plants. PV panels are much more effective in hazy or partly cloudy conditions and they can be installed even on very small scale residential rooftops.

Photovoltaic solar cells are semiconductor diodes that are designed to absorb sunlight and convert it into electricity. The absorption of sunlight creates free mi­nority carriers, which determine the solar cell current. These carriers are collected and separated by the junction of the diode, which determines the voltage. PV SCs have been the power supply of choice for satellites since 1958. Light drives the PV process and provides the energy that is converted into electricity. PV cells use pri­marily visible radiation. The distribution of color within light is important because a PV cell produces different amounts of current depending on the various colors shining on it. Infrared radiation contributes to the production of electricity from crystalline silicon and some other materials. In most cases infrared radiation is not as important as the visible portion of the solar spectrum (Chap. 3).

Terrestrial applications of PV devices developed rather slowly. Some of the main advantages of their use as an electric power source can be given as follows (Deniz 2006, unpublished):

1. Direct conversion of solar radiation into electricity

2. No mechanical moving parts and no noise

3. No high temperatures

4. No pollution

5. PV modules are very robust and have a long life

6. The energy source (sun) is free and inexhaustible

7. PV energy is a very flexible source; its power ranges from microwatts to megawatts

Updated: June 30, 2015 — 9:20 pm