Although crystalline silicon cells are the most common type, photovoltaic (PV), or solar cells, can be made of many semiconductor materials. Each material has unique strengths and characteristics that influence its suitability for specific applications. For example, PV cell materials may differ based on their crystallinity, bandgap, absorbtion, and manufacturing complexity.
Learn more about each of these characteristics below or learn about these solar cell materials:
- Silicon (Si)—including single-crystalline Si, multicrystalline Si, and amorphous Si
- Polycrystalline Thin Films—including copper indium diselenide (CIS), cadmium telluride (CdTe), and thin-film silicon
- Single-Crystalline Thin Films—including high-efficiency material such as gallium arsenide (GaAs).
The crystallinity of a material indicates how perfectly ordered the atoms are in the crystal structure. Silicon, as well as other solar cell semiconductor materials, comes in various forms, including single-crystalline, multicrystalline, polycrystalline, and amorphous. In a single-crystal material, the atoms that make up the framework of the crystal are repeated in a very regular, orderly manner from layer to layer. In contrast, in a material composed of numerous smaller crystals, the orderly arrangement is disrupted moving from one crystal to another.
The bandgap of a semiconductor material 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, and 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 and valence bands.
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.
The absorption coefficient of a solar cell depends on two factors: the material of the cell and the wavelength or energy of the light being absorbed. Solar cell material has an abrupt edge in its absorption coefficient because light with energy below the material's bandgap cannot free an electron.
The most important parts of a solar cell are the semiconductor layers because this is where electrons are freed and electric current is created. Several semiconductor materials can be used to make the layers in solar cells, and each material has its benefits and drawbacks.
The cost and complexity of manufacturing varies across materials and device structures based on many factors, including deposition in a vacuum environment, amount and type of material used, number of steps involved, and the need to move cells into different deposition chambers.