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Solar Photovoltaic Technology

Solar cells are essentially made of p-n junction diodes. Electrical connections are taken by using metal. A detailed description of the fabrication technology is out of the scope of the present chapter. Interested readers may refer to the book (Roy and Bose, 2018) co-authored by the author of this chapter. c-Si solar cell is typically made using a 156 mm x 156 mm p-type silicon wafer, by converting the top portion of it to n-type using doping. This forms the n-p junction. Metal contacts are taken from top (n-type) and bottom (p-type) for electrical connections. Front and back view of a typical c-Si solar cell is shown in Figure 9. The metal connection is made up of fingers appearing as thin horizontal lines and bus bars appearing as thick vertical lines. The fingers collect the photo-generated carriers (electrons) from the n-side of the diode. The bus bars accumulate these carriers so that these can be taken out from the cells (see Figure 9). The finger configuration is required, as the complete front surface cannot be covered by the metal. As the metal is opaque, no light will then reach the silicon. The back surface has no such restrictions. Therefore, the entire back surface is covered by metal, which also includes bus bars for external connections.

Several advancements in c-Si technology are made to improve the efficiency. Some of the important advancements are (a) double side printing (Roy and Bose, 2018), (b) selective emitter (Roder et al., 2010), (c) Back Surface Field (BSF) (Roy and Bose, 2018), (d) buried contact (Antoniadis et al., 2010), (e) Light Induced Plating (LIP) (Roy and Bose, 2018), (f) Metal Wrap Through (MWT) (Magnone et al., 2014), (g) Emitter Wrap Through (Kiefer et al., 2011), (h) Passivated Emitter and Rear Contact (PERC) (Wang and Green, 1994), (i) Passivated Emitter Rear Locally Diffused (PERL) (Wang et al., 1995),

(a) Front and (b) Back view of a c-Si solar cell

Figure 9. (a) Front and (b) Back view of a c-Si solar cell.

(j) Passivated Emitter Rear Totally Diffused (PERT) (Zaho et al., 1999) and (k) Inter-digitated Back Contact (IBC) (Zanuecoli et al., 2015).

High efficiency III-V cell technology (Roy and Bose, 2018) uses thin films of high-quality single crystal III-V compound semiconductor materials. Stacks of n-type and p-type thin films are typically deposited on Ge substrate to form solar cells. Multi-Junction (MJ) solar cells are made by using stacks of p-n junctions using ternary and quaternary III-V compound semiconductor materials. The band gaps of such materials are adjusted by varying the composition of the constituent elements. Photolithography steps are also used to define various regions. The cost of such solar cells is very high due to costly materials (in gas form) as well as processes, such as deposition and photolithography. These cells are used in space applications, such as satellites, where the overall cost implication of the solar cell is not significant. These cells are also used in Concentrated Photovoltaic (CPV) systems where the cost implication of using solar cells is significantly reduced as the area covered by the cells is much smaller.

a-Si, CdTe and CIGS solar cell technologies also use thin film deposition techniques (Roy and Bose, 2018). However, the materials (in gas form) and processes used in these technologies are not that costly. Amorphous or micro-crystalline thin film layers of p and n are deposited on a TCO (Transparent Conducting Oxide) coated glass substrate to form the solar cells. Instead of photolithography, LASER are used to define various regions and reduce the processing cost. Although these solar cells are cheaper, the efficiencies are lower due to lower mobilities of the deposited materials.

The fabrication of Dye Sensitized Solar Cell (DSSC) and organic solar cell is even simpler. Light sensitive dye can be coated on cheap plastic or ceramic substrates to form the solar cells. Some problems, such as stability, are yet to be solved before they can be commercially used.

A representative specification of a commercially available c-Si solar cell is given in Table 2. It may be noted that the parameters are measured at STC. The power of the individual c-Si solar cell is very small and many cells need to be electrically connected in order to enhance the available power. This is done by making modules (Roy and Bose, 2018), sometime called panels, generally consisting of series connected cells. A 60 cells module, which is common, has 60 cells connected in series, generally consisting of a 10 x 6 matrix. Modules with cells other than 60, e.g., 12, 24, 36, 48, 72, or 96, are also available. The top bus bars of the first cell are connected to the second cell’s bottom bus bars in order to connect the two cells in series. One string consisting of 10 cells are made using this process. Six such strings are made for a 60 cells module. The strings are made by an automated process called Tabbing and Stringing (T&S). These strings are then connected in series by a process called bussing. In this, the strings are electrically connected using copper strips. The entire cell matrix is then laminated using an arrangement called “layup”, consisting of layers of glass, Ethyl Vinyl Acetate (EVA)-I, cell matrix, Ethyl Vinyl Acetate (EVA)-II and tedlar sheet from top to bottom in this sequence.

The lamination is done to protect the solar cells from the external environment after installation. Galvanized aluminum frames are then attached on all four sides to provide more support and make it compatible for easy installation. A Junction Box (JB) consisting of two external cables is attached at the back side of the module. Electrical connections from output of the cell matrix to the cables are provided by the JB. The electrical connection between modules are established using these cables. Both series and

Table 2. Representative specifications of a commercially available c-Si solar cell.

Parameter

Value

Voc

0.625 Y

V»

0.508 Y

Isc

9.08 A

к

8.71 A

F.F.

0.78

P

m

4.425 Wp*

Size

156 mm x 156 nun

Efficiency

18.2%

* Subscript P in Wp indicate power extracted at peak (MPP) of the P-Y curve.

(a) Front and (b) Back view of a c-Si solar module (panel)

Figure 10. (a) Front and (b) Back view of a c-Si solar module (panel).

Table 3. Representative specifications of commercially available 60 cells c-Si solar module.

Parameter

Value

Voc

37.45 Y

v.

30.46 Y

Isc

9.08 A

!m

8.71 A

F.F.

0.78

p

m

265 Wp*

Size

1639 mm x 982 mm

Efficiency

16.46%

* Subscript P in Wp indicate power extracted at peak (MPP) of the P-Y curve.

parallel connections of modules are possible. Figure 10 shows the front and back side views of a 60 cells c-Si module.

A representative specifications of a commercially available 60 cells c-Si module measured at STC are given in Table 3. III-V compound semiconductor based solar cells technology also has similar approach. The cells, typically 100 mm in diameter, are made first and then several of them are assembled to make the solar panels for the satellite. There are no standard sizes of the panels as it is tailor made as per the requirement of a particular satellite. a-Si, CdTe and CIGS thin film solar cell are made using a large TCO coated glasses. Therefore there are no separate cell manufacturing and module assembly steps; the modules are made directly.

 
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