Photovoltaics is the direct conversion of light into electricity at the atomic level. Some material exhibit a property known as the photoelectric effect that causes them to absorb photons of light and release electrons. When these free electrons are captured, an electric current results that can be used as electricity.
The photoelectric effect was first noted by a French physicist, Edmund Becquerel, in 1839, who found that certain materials would produce small amounts of electric current when exposed to light. In 1905, Albert Einstein described the nature of light and the photoelectric effect on which photovoltaic technology is based, for which he later won a Noble prize in physics. The first photovoltaic module was built by Bell Laboratories in 1954. It was billed as a solar battery and was mostly just a curiosity as it was too expensive to gain widespread use. In1960s, the space industry began to make the first serious use of the technology to provide power abroad spacecraft. Through the space program, the technology advanced, its reliability was established, and the cost began to decline. During the energy crisis in the 1970s photovoltaic technology gained recognition as a source of power for non-space applications.
11.8.1 Photovoltaic Process
The height of the potential barrier is an open circuit, dark (non-illuminated) P-N junction always adjusts itself in such a way that the resultant current is zero and the electric field at the junction is in such a direction that it repels the majority carriers.
When light falls on diode surface, minority carriers get injected and hence the minority current increases. Since the diode is open circuited, the resultant current must remain zero. Therefore, majority current must increases by the same amount as the minority carrier current. This increases in majority current is possible only when retarding electric field at the junction is reduced resulting in the lowering of the barrier height. Therefore, across the diode terminals, a voltage appears, which is equal to the decrease in the barrier potential. This constitutes the photovoltaic e.m.f. and is of the order of 0.1 V for Ge cell and 0.5 V for Si cell.
Expression for Photovoltaic e.m.f.: The photovoltaic e.m.f. Vp appears across the diode when the net current I in the diode is zero. Substituting I=0 in the volt-ampere characteristics f a photodiode given by
The above equation shows that the photovoltaic e.m.f. Vp increases logarithmically with Is and thus illumination. Figure 11.24 shows graph for Vp versus light intensity.
11.8.2 Photovoltaic (PV) Cell/Solar Cells
Figure 11.25 illustrates the operation of a basic photovoltaic cell, also called a solar cell. Solar cells are made of the same kinds of the semiconductor materials such as silicon, used in the microelectronics industry. For solar cells, a thin semiconductor wafer is specially treated to form an electric field, positive on one side and negative on the other. When light energy strikes the solar cells, electrons are knocked loose from the atoms in the semiconductor material. If electrical conductors are attached to the positive and negative sides, forming an electrical circuit, the electrons can be captured in the form of an electric current i.e., electricity. This electricity can then be used to power to power to load, such as a light or a tool.
A number of solar cells electrically connected to each other and mounted in a support structure or frame is called photovoltaic module. Modules are designed to supply electricity at a certain voltage, such as a common 12V system. The current produced is directly dependent on how much light strikes the module.
Multiple modules are be wired together to form an array (Figure 11.26). In general, the larger the area of a module or array, the more electricity that will be produced. Photovoltaic modules and arrays produce direct current (dc) electricity. They can be connected in both series and parallel electrical arrangements to produce any required voltage and current combination. Today’s most common PV devices use a single junction, or interface, to create an electric field within a semiconductor such as a PV cell. In a single junction PV cell, only photons, whose energy is equal to or greater than the band gap of the cell material can free an electron for an electric circuit. In other words, the photovoltaic response of single junction cells is limited to the portion of the Sun’s spectrum, whose energy is above the band gap of the absorbing material, and lower-energy photons are not used.
One way to get around this limitation is to use two or different cells, with more than one band gap and more than one junction, to generate a voltage. These are referred to as “multi junction” cells (also called “cascade” or “tandem” cells). Multi junction devices can achieve a higher total conversion efficiency because they can convert more of the energy spectrum of light to electricity.
As shown in Figure 11.27, a multi junction device is a stack of individual single-junction cells in descending order of band gap (Eg). The top cell captures the high energy photons and passes the rest of the photons on to be absorbed by lower band gap cells. Must of today’s research in multi junction cells focuses on gallium arsenide as one (or all) of the component cells. Such cells have reached efficiencies of around 35% under concentrated sunlight. Other materials studied for multi junction devices have been amorphous silicon and copper indium diselenide.
As an example (Figure 11.28), the multi junction device used to top cell of gallium indium phosphide, “a tunnel junction” to aid the flow of electrons between the cells and a bottom cell of GaAs.