Types of Solar Cells

The performance of a solar or photovoltaic (PV) cell is measured in terms of its efficiency at converting sunlight into electricity. There are a variety of solar cell materials available, which vary in conversion efficiency.

Semiconductor Materials

A solar cell consists of semiconductor materials. Silicon remains the most popular material for solar cells, including these types:

• Monocrystalline or single crystal silicon
• Multicrystalline silicon
• Polycrystalline silicon
• Amorphous silicon

The absorption coefficient of a material indicates how far light with a specific wavelength (or energy) can penetrate the material before being absorbed. A small absorption coefficient means that light is not readily absorbed by the material. Again, the absorption coefficient of a solar cell depends on two factors: the material making up the cell, and the wavelength or energy of the light being absorbed.

The bandgap of a semiconductor material is an amount of energy. Specifically, the bandgap is the minimum energy needed to move an electron from its bound state within an atom to a free state. This free state is where the electron can be involved in conduction. The lower energy level of a semiconductor is called the "valence band." The higher energy level where an electron is free to roam is called the "conduction band." The bandgap (often symbolized by Eg) is the energy difference between the conduction band and valence band.

Solar cell material has an abrupt edge in its absorption coefficient; because light with energy below the material's bandgap cannot free an electron, it isn't absorbed.

Thin Film

Thin film solar cells use layers of semiconductor materials only a few micrometers thick. Thin film technology has made it possible for solar cells to now double as these materials:

• Rooftop or solar shingles
• Roof tiles
• Building facades
• Glazing for skylights or atria.

Thin-film rooftop or solar shingles, made with various non-crystalline materials, are just now starting to enter the residential market. The following are benefits of these solar shingles:

• Attractive integration into homes
• Dual purpose—serves as both roofing material and pollution-free electricity producer
• Durability.

Current issues with commercially-available solar shingles include their lower efficiencies and greater expense compared with the standard small solar electric system.