An individual solar cell is fragile and can only generate limited output power. For real-world applications, photovoltaic modules are fabricated by electrically connecting typically 36 to 72 solar cells together in a so-called PV module. A PV module (or panel) is an assembly of solar cells in a sealed, weather-proof packaging and is the fundamental building block of photovoltaic (PV) systems.
All finished solar cells are tested on electrical and optical parameters for quality control and are sorted on the basis of current or power output. Solar cells which are similar in terms of electrical performance and optical aesthetics are used for the fabrication of a PV module. A schematic of a PV module with series connected solar cells is shown in Figure 1. The negative contact of one solar cell is connected to the positive contact of the next cell. Most industrial solar cells have the negative contact on the front and the positive contact at the rear of the solar cell.
The schematic process flow for the fabrication of a PV module is shown in Fig. 2. In the interconnection step, solar cells in one column of the PV module are soldered either manually or by a tabber and stringer machine. These strings are typically inspected by electroluminescence imaging to identify defects early on in the production process. Subsequently, these strings are placed on top of a glass sheet with a layer of EVA (Ethylene Vinyl Acetate) and the strings are interconnected as well. Subsequently, another layer of EVA is put on top of the interconnected strings and a backsheet (typically tedlar) is placed on top of the EVA and the whole stack is placed in a laminator where it is heated at ~200 oC for 10-15 minutes. Then the junction box and the frame are added to the stack to complete the module. In between every step, quality checks can be done to ensure a high-quality product. A schematic cross-section of a PV module is shown in Fig. 3. The output voltage of a module depends on the number of interconnected cells and if the solar cells are connected in series of (partly) in parallel. The voltage output of a typical solar cell at maximum power point is about 0.5 V at 25 ºC, and consequently, the output voltage of a 72-cell module is 36 V (or higher if the individual cells have higher voltage) when connected in series while the current is identical to the lowest maximum power current of the solar cells in the module.
The next section gives some more detail about the requirements and functionality of the various components of the PV module.
- Front glass: Tempered glass, typically with an antireflection coating, is used at the front of a solar module. The front surface of the module has to be stable against water and oxygen and also ultraviolet radiation as all of these factors can result in degradation of the individual solar cells. In order to ensure a high transmission of the glass, it is important that the iron content is very low as this limits the light transmission to the solar cells.
- Encapsulant: The encapsulant acts as a binding material between multiple layers of a PV module and provides resistance towards vibrational failure. Ethylene Vinyl Acetate (EVA) is the industry standard encapsulant for silicon PV modules. Cells are laminated between translucent EVA sheets. The structure is then heated up to 150 °C, at which EVA melts and provide adhesion to the cells. Important characteristics that make EVA a great commercial choice are:
- It has high electrical resistivity and thus serves as an excellent insulator.
- EVA films are a good barrier against moisture and other contaminants.
- They can potentially absorb the influence of shocks and vibrations and protects the module.
- It is stable under high UV exposure.
- It has a low thermal resistance and high optical transmission.
- Its refractive index is similar to glass which provides efficient coupling of light.
- It is relatively cheap.
There has been significant interest in exploring alternative encapsulant materials for silicon-based modules to either reduce cost or to further improve performance or reliability. Some investigated encapsulants include silicones, PVB (polyvinyl butyral) and ionomers. Silicones possess high transparency to UV-visible light, almost perfectly matching refractive index with glass, and excellent hydrophobic properties. PVB was used in the initial days of PV industry and recently, their improved counterparts have outperformed EVA under UV exposure and extreme heat in some 70-day stability tests, however, their use can be restricted by their low resistance to moisture. Ionomer can potentially provide good adhesion to glass and protection from corrosion, with the added advantage of easy handling and enhanced durability.
- Backsheet: Modules typically incorporate a backsheet at the rear side. Tedlaris typically used at the back of the modules. A backsheet electrically isolates the connections from the rear of the cells. It provides physical strength to the module from mechanical shocks and moisture. It is conventionally white and has a high reflection in the visible part of the solar cell spectrum. This means that a significant part of the light that hits the white backsheet is reflected and can, due to internal reflection at the module glass-air interface, be reabsorbed by the solar cells. This can result in a cell-to-module gain. Other suitable candidates are polyester, Tedlar/Polyester/Tedlar trilaminate (TPT), Tedlar/Polyester/EVA (TPE), Tedlar/polyester/Aluminum foil/Tedlar (TPAT) and glass.
- Junction box: A junction box is connected at the back of the module and contains the electrical strings coming out for subsequent wiring into standard connectors used in PV systems (fig 5). It contains bypass diodes that prevent output power from feeding back into the panel in the dark (e.g., due to partial shading). They are strongly sealed (sometimes permanently) to protect the cells and module components from moisture.
- Frame: The frame is usually made of aluminium to ensure robustness of the assembly under extreme weather conditions (e.g., wind). Its primary function is to hold the structure firm and strong. It also comes with clamps to assist in the mounting of the module.