Skip Navigation to main content U.S. Department of Energy U.S. Department of Energy Energy Efficiency and Renewable Energy
Bringing you a prosperous future where energy is clean, abundant, reliable, and affordable EERE Home
Solar Energy Technologies Program
 
About the ProgramProgram AreasInformation ResourcesFinancial OpportunitiesTechnologiesDeploymentHome
Concentrating Solar Power
Photovoltaics Why PV is Important PV Basics PV Physics PV Devices PV Systems PV in Use Research and Development For Builders For Consumers For Students, Educators and Trainers
Solar Heating
Solar Lighting
Solar FAQ
Solar Glossary
Solar Timeline

Electrical Contacts

Illustration of grid contacts installed on a PV cell.

Grid contacts on the top surface of a typical cell are designed to have many thin, conductive fingers spreading to every part of the cell's surface.

Electrical contacts are essential to a photovoltaic (PV) cell 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 — on 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 — on the side facing the sun — is more complicated. When sunlight shines on the PV cell, a current of electrons flows all over its surface. If we attach contacts only at the edges of the cell, it will not work well because of the great electrical resistance of the top semiconductor layer. Only a small number of electrons would make it to the contact.

To collect the most current, we must place contacts across the entire surface of a PV cell. This is normally done with a "grid" of metal strips or "fingers." However, placing a large grid, which is opaque, on the top of the cell shades active parts of the cell from the sun. The cell's conversion efficiency is thus significantly reduced. To improve the conversion efficiency, we must minimize these shading effects.

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, in designing grid contacts, we must balance shading effects against electrical resistance losses. The usual approach is to design grids with many thin, conductive fingers spreading 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 much of the incoming light. This kind of grid keeps resistance losses low while shading only about 3% to 5% of the cell's surface.

Grids can be expensive to make and can affect the cell's reliability. To make top-surface grids, we can either deposit metallic vapors on a cell through a mask or paint them on via a screen-printing method. Photolithography is the preferred method for the highest quality, but has the greatest cost. This process involves transferring an image via photography, as in modern printing.

An alternative to metallic grid contacts is a transparent conducting oxide (TCO) layer such as tin oxide (SnO2). The advantage of TCOs is 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. Some thin-film PV cells, such as amorphous silicon and cadmium telluride, use superstrates. 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 will actually be the bottom of the cell. As you can see, the cell is actually constructed "upside down," from the top to the bottom.

But the construction technique isn't the only thing that determines whether a metallic grid or TCO is best for a certain cell design. The sheet resistance of the semiconductor is also an important consideration. In crystalline silicon, for example, the semiconductor carries electrons well enough to reach a finger of the metallic grid. Because the metal conducts electricity better than a TCO, shading losses are less than losses associated with using a TCO. Amorphous silicon, on the other hand, conducts very poorly in the horizontal direction. Therefore, it benefits from having a TCO over its entire surface.

See these pages for more information on crystalline silicon solar cells:

To learn more about PV physics, see: