of ~11% in case of industrial grade cells [85,86]. GaAs is next grade semiconductor material finding many applications in high-speed electronics, space applications and optoelectronics industry. Alloy-based solar materials include copper-indium-selenide (CIS), copper-indium-gallium-selenide (CIGS) and copper-indium-gallium-selenide-sulphur (CIGSS) [87]. The two types of PV plants are stand-alone plants and grid-based plants. Standalone plants are not connected to the grid and the core structure consists of PV panels and an energy storage system which ensures electric power supply when sunlight is not available or poorly available. A PV generator produces direct electric current (DC) power supply [88-90]. In case, an alternate current (AC) power supply is required to meet the requirements at the demand end, an inverter for conversion of DC to AC power supply is required [88-90]. The grid-connected plants fetch electric power from the grid in the case of PV generators not being efficient enough to produce enough power for meeting the clients’ requirements. On production of surplus electric power, the excess can be stored in the grids and can be used for further utilization. This is not a centralized power technology rather it is a distributed energy production technology. This offers the advantages of reduced transmission losses in addition to decreased expenses on electrical transport systems.
1.3 Energy Production by Equivalent Cell Circuitry
The circuit of a solar cell as a current generator can be represented as shown in Figure 1.19. If the current generated by absorption of photons is IS, diode current is ID and leakage current is IL, then, the current I obtained at the output terminals is given as follows [88-90],
Figure 1.19 Schematic shows the electrical circuit of solar cell with current contribution due to PV effect (IS), diode current (ID), leakage current (IL), open-circuit voltage (VOC) when no current flows across the load, l eakage conductance (G) and output current (I). It also shows a diode through which ID flows, and series RS offered due to resistance offered to movement of charge carriers.
In the circuit diagram shown below, a series resistance (RS) has been shown which depends on the thickness of the junction resisting the flow of current across the barrier, types of impurities and contact resistance. The leakage conductance (G) directly relates to the leakage current in series under normal conditions of operations. The conversion efficiency of a solar cell is least affected in a variation of value of G, while a small variation of RS has a pronounced effect on it. In case of open circuit when no current flows across the load, the voltage is given as follows,
The diode current ID is expressed by formula for the DC flow and is given as,
where, q is the charge on an electron, T is temperature, k is Boltzmann constant, A is the identity factor of diode depending on recombination effects inside the diode and ISS is the saturation current of diode. As a result, current generated across the load is given as follows from equations (1.29) & (1.30) in equation (1.28),
(1.31)
The typical current-voltage characteristics of a solar cell module are shown in Figure 1.20. The generated current is the highest (ISC) under short-circuit conditions, whereas it is the least in case of open-circuit and voltage is the maximum (VOC). The produced electric power in both these conditions is zero. Under all the conditions other than these two conditions, the produced power increases as a function of the voltage. Hence, different parameters involved in the characterization of a solar cell module are the short-circuit current (ISC), open-circuit voltage (VOC), current and voltages (Im and Vm) produced at the maximum power. The filling factor (FF) is the ratio of the maximum power (Pm) to the product of the open-circuit voltage (VOC) multiplied by the short-circuit current (ISC).
Figure 1.20 Schematic shows the electrical circuit of solar cell with current contribution due to PV effect (IS), diode current (ID), leakage current (IL), open-circuit voltage (VOC) when no current flows across the load, leakage conductance (G) and output current (I). It also shows a diode through which ID flows, and series RS offered due to resistance offered to movement of charge carriers.
Solar panels will display the maximum efficiency when angle of incidence of sunlight is always perpendicular in a direction to the surface of the panel. The panels must be oriented in a direction specified by azimuthal angle (γ) which is the deviation in reference to the optimal direction to the north in the Southern Hemisphere or with respect to optimum direction in south in the Southern Hemisphere [88-90]. Azimuth angle can be defined as the angular distance determined from north to east along the horizontal line which is the point of intersection of sphere with the axis.
1.4 Conclusion
Fabrication of solar cells is based on semiconductor fabrication technology through which crystalline silicon solar cells are produced at the industrial scale. The conversion efficiency of industrial-grade silicon solar cells is still around 15%, which is a matter of concern. This is a costly technology and constant efforts are directed towards developing a cheaper method with maximized conversion efficiency of the solar cell. Intense research has been going on worldwide to develop organic solar cells, which is a low-cost process. Semiconductor manufacturing and subsequent device fabrication is a technology which requires certain well-defined procedures and protocols, such as, requirement of a clean room. A clean room is a room with special types of filters attached, for example, HEPA or high energy particulate filter with purifier) for extraction of smaller particles of 0.3 μm or larger in size. In certain cases, ultra-low particulate air (ULPA) filters are required when there is need to extract particles smaller than the aforementioned particles. There are well-defined protocols to work in these clean room fabrication laboratories or manufacturing facilities. A special clean room suite or gown is required before entering the laboratory or manufacturing facility and it is a requirement to pass through the chamber which sucks all smaller particles before entering
