Photovoltaic Electrical Contacts and Cell Coatings
The outermost layers of photovoltaic (PV) cell, or solar cell, are the electrical contacts and anti-reflective coating. These layers provide essential functions to the cell's operation.
Electrical contacts are essential to PV cells because they bridge the connection between the semiconductor material and the external electrical load, such as a light bulb.
The back contact of a cell—the side away from the incoming sunlight—is relatively simple. It usually consists of a layer of aluminum or molybdenum metal.
But the front contact—the side facing the sun—is more complicated. When sunlight shines on a PV cell, a current of electrons flows over the surface. To collect the most current, contacts must be placed across the surface of the cell. This is normally done with a grid of metal strips or "fingers." However, placing a large grid, which is opaque, on top of the cell shades active parts of the cell from the sun and reduces the cell's conversion efficiency. To improve conversion efficiency, shading effects must be minimized.
Another challenge in cell design is to minimize the electrical resistance losses when applying grid contacts to the solar cell material. These losses are related to the solar cell material's property of opposing the flow of an electric current, which results in heating the material. Therefore, shading effects must be balanced against electrical resistance losses. The usual approach is to design grids with many thin, conductive fingers that spread to every part of the cell's surface. The fingers of the grid must be thick enough to conduct well (with low resistance) but thin enough not to block too much incoming light.
Grids can be expensive to make and can affect the cell's reliability. To make top-surface grids, metallic vapors are deposited on a cell through a mask or painted on via a screen-printing method. An alternative to metallic grid contacts is a transparent conducting oxide (TCO) layer made of, for example, tin oxide (SnO2). The advantages o TCOs are that they are nearly invisible to incoming light and they form a good bridge from the semiconductor material to the external electrical circuit. TCOs are very useful in manufacturing processes involving a glass superstrate, which is the covering on the sun-facing side of a PV module. In this process, the TCO is generally deposited as a thin film on the glass superstrate before the semiconducting layers are deposited. The semiconducting layers are then followed by a metallic contact that is actually the bottom of the cell. The cell is therefore constructed "upside down," from the top to the bottom.
The sheet resistance of the semiconductor is also an important consideration in grid design. In crystalline silicon, for example, the semiconductor carries electrons well enough to reach a finger of a metallic grid. Because the metal conducts electricity better than a TCO, shading losses are less than losses associated with a TCO. Other semiconductors, such as amorphous silicon, conduct very poorly in the horizontal direction. Therefore, they benefit from having a TCO over the entire surface.
Silicon is a shiny gray material that can act as a mirror by reflecting more than 30% of the light that shines on it. To improve the conversion efficiency of a solar cell, the amount of light reflected must be minimized.
Two techniques are commonly used to reduce reflection. The first technique is to coat the top surface with a thin layer of silicon monoxide (SiO). A single layer reduces surface reflection to about 10%, and a second layer can lower the reflection to less than 4%. The second technique is to texture the top surface. Chemical etching creates a pattern of cones and pyramids, which captures light rays that might otherwise be deflected away from the cell. Reflected light is redirected into the cell, where it has another chance to be absorbed.