Green Energy. Группа авторов

The back-contacted solar cell is the most efficient configuration of commercial solar cell and its manufacturing requires multiple complicated steps. Figure 1.16 shows the various steps involved in the fabrication of a monofacial solar cell. Similarly, bifacial and back-contacted solar cells can be fabricated utilizing various growth, patterning and deposition steps as discussed in general fabrication methodology in Figure 1.14 schematically.

Figure 1.15 (a) Shows the schematic of (a) monofacial solar cell, (b) bifacial solar cell and (c) back-contacted solar cell configurations.

Figure 1.16 Fabrication steps involved in the preparation of a monofacial solar cell.

The generation of electricity by impinging light on a semiconductor material requires production of electrons and holes such that electrons in the valence band become free and jump to the conduction band by absorbing energy [72-74]. Thus, jumping of highly energetic electrons to different material generates an electromotive force (EMF) converting light energy into electrical signals. This is known as the photovoltaic (PV) effect. The first PV cell was fabricated by Charles Fritts in 1883 by depositing a thin layer of gold (Au) over the semiconductor material selenium (Se) to form junctions [72-74]. This first fabricated solar cell was only 1% efficient. A solar cell or PV cell is basically a p-n junction exhibiting nonlinear current-voltage (I-V) characteristics. Bell Laboratories, USA, developed the first practical solar cell in 1954 by fabrication of a diffused Si p-n junction with 6% efficiency. Si is widely used in PV cell technology since it is cheaper, abundant and Si-fabrication technology is highly developed [72-74]. First of all, polished Si wafers cut from highly pure industrial grade Si boules are prepared which can be single-crystalline, polycrystalline or even amorphous. After wafer procurement/fabrication, Si is doped selectively to make p-n junctions and is processed to furnish a solar cell. The various methods of fabrication of solar cells are listed as follows [72-74],

      1 (i) Screen printed fabrication technology

      2 (ii) Buried contact fabrication technology

A process flow chart for fabrication of solar cell panels has been shown in Figure 1.17. The PV technology is based on the photoelectric effect in which a doped semiconductor produces electricity as a result of electron-hole generation on illumination of solar radiation [4,5,9,72-74]. The main merits of PV technology include reduced dependency on orthodox sources of energy of fossil fuels, pollution-free energy technology with zero emission, significantly reduced operational and maintenance costs, long life of solar panels of over twenty years with robust & reliability features. Moreover, system modularity provides the flexibility of enhancement of power production by simply increasing the number of solar panels. A solar energy production plant consists of generators or the solar panels and a frame or hard-casing for mounting the panels in a particular orientation or angle. This is supported by an electrical power control system and an energy storage system frequently nested in industries, houses, factories, big farmhouses, universities, colleges, offices and buildings. The basic component of a PV generator is the solar cell which converts solar radiation into electrical power.

Figure 1.17 Shows the process flow for fabrication of solar cells to manufacture photovoltaic (PV) array. Various steps involved in the fabrication process have been demonstrated pictorially.

On illumination of light on the solar cell, E-H pairs arise in both the p- and n-type regions [4,5,9,72-74]. As discussed previously, an internal electric field is developed due to the charge carriers in the semiconductor junction. It pushes the extra electrons to segregate from electronic charge pairing due to the absorption of optical energy and makes them move in a direction opposite to the movement of holes. This electric field is directed from p-type region to n-type region, and as a result, electric force prevents them from transportation in the reverse direction after crossing the electric barrier. For electrical transport, the junction is interconnected by an external conductor making a closed circuit as shown in Figure 1.18. In this circuit on illumination with light, current flows from n-type block or layer with higher potential to another n-type block or layer at a lower potential. The saga of electrical conduction lies in the fact that an electron in the valence band absorbs sufficiently energetic photon to get excited to the conduction band, a case typical for semiconductor materials with bandgap energies slightly higher than the metals [4,5,9,72-74].

Figure 1.18 Schematic shows the electrical contacting of n-type layers with current flowing from high potential to low layer on illumination of light. The inset shows the electron jump from valence band (VB) to conduction band (CB) on absorption of optical energy in form of the quanta of light.

      An important fact is that the larger the surface area of the solar panel, the larger is the current produced. The genesis is that the region surrounding the p-n junction works as a factory of charges, while holes and electrons generating in far-flung areas, means away from the junction, recombine because there is no force supplied by an electric field to drive them off. This is the main reason for non-conversion of most of the solar energy into electrical energy. Therefore, the solar energy is responsible for segregation of charge carriers, recombination of hole-electron charge carriers leading to annihilation, transmission of solar energy, reflection from the front contacts on surface of the panel in addition to the shadow effects.

      Thousands and millions of solar cells fabricated on a solar module are assembled on a single surface called as the panel. Many panels are assembled or connected in series to form an array. Several of such arrays are connected in parallel to form the PV generator for obtaining desired output of electrical power. As a matter of fact, due to subtle manufacturing defects, the solar cells in the modules are not identical and hence no two blocks connected in parallel can have the same magnitude of voltage. This results in generation of a current flowing from cells at higher potential to cell blocks having lower voltages. This creates mismatch losses due to the fact that a portion of converted power is lost inside the module itself. Similar problems arise in case of arrays connected in parallel because of dissimilarity of modules, shadow effects, defects and possessing different irradiance issues. To avoid these problems, by-pass diodes can be additional electrical components in the non-linear circuitry to short-circuit the shadowed or maligned part of the module. Mismatch losses due to dissimilar electrical features of the cells can also be measured by solar irradiances on shadowed or damaged solar cells. These cells block the current produced by other solar cells, are subject to the voltage of other cells and cause local overheating and damages.

      Nowadays, crystalline Si solar cells are used with efficiency of 14%-17%, which can be single crystalline and polycrystalline silicon solar cells [4,5,9,72-74]. These are derived by chopping-off slices of wafers from the cylindrical ingot. Microgrooves