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Solar electric devices, commonly referred to as photovoltaics, are able to generate electricity directly from sunlight. Within the semiconductor material present on solar panels, photons released from the sun cause outer electrons to break free of their atomic bonds1. As electrons are forced in one direction, they create a flow of electrical current capable of powering electrical devices or sending electricity to the grid.
Originally theorized in 1839 by French physicist Alexandre-Edmond Bequerel, photovoltaic technology remains a subject of scientific inquiry. Today, the photovoltaic industry continues to grow, as major programs in the United States, Japan, and Europe are rapidly accelerating their own implementation of solar energy systems.
The Photovoltaic Module
While the materials within photovoltaic systems vary, all modules contain a number of layers originating from the outer light-facing side to the back. Typically made from glass, light penetrates a protection layer, followed by a transparent front contact layer allowing light to enter the cell. At the center of the module is adsorption material, and this layer absorbs and converts light into the electric current2. This integral layer of the photovoltaic system contains semiconductor material which can vary, depending on the system.
Following the adsorption material is a conduction completing the electric circuitry referred to as a metal back contact. In order to ensure that the structure is water-proof and insulated from heat, the laminate film is placed behind the metal back contact. As an extended layer of protection, a layer of either glass, aluminum, or plastic is placed on the back side of the module, which is attached to the connectors and cables allowing the module to be wired2.
Semiconductor materials within a photovoltaic system can vary from silicon, polycrystalline thin films, or single-crystalline thin films. The silicon materials include single-crystalline silicon, multi-crystalline silicon, and amorphous silicon. Single-crystalline silicon has a regular structure, allowing a better response rate compared to that seen in the multi-crystalline structure.
Amorphous silicon is comprised of atoms that are randomly placed together, exhibiting a lower response rate than that observed in the single-crystalline structure. Capable of capturing more light than the crystalline silicone, alloying amorphous silicon with germanium or carbon can intensify this property2.
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Copper indium diselenide (CIS), cadmium telluride (CdTe), and thin-film silicon are certain polycrystalline thin film materials often used, whereas high-efficiency material such as gallium arsenide (GaAs) often comprise single-crystalline thin film materials3. Each of these materials possesses unique strengths and characteristics that influence its suitability for the desired application. Of these traits include the material’s crystallinity, band-gap, absorption, and manufacturing complexity.
External Influences on the Semiconductor
Determined based on order arrangement of atoms within the crystal structure, the crystallinity of a semiconductor can affect the charge transport, current density, and power conversion efficiency within a solar cell4. The bandgap of a semiconductor material refers to the minimum energy required to move an electron from its bound state to a free state, allowing the conduction of the electron to occur3. The valence band of a semiconductor is the lower energy level, and the conduction band is the higher energy level.
Often symbolized by Eg, the band-gap describes the energy difference between the valence and conduction bands. Indicated by how far light with a specific wavelength can penetrate the material, the absorption coefficient determines the ability of light to be absorbed by the material. This coefficient depends on the material of the cell and the wavelength of the light being absorbed3.
The cost and manufacturing complexity of the wide variety of materials and devices for semiconductor materials is based on many factors, including the amount and type of material used, the time required for production, and the movement of cells into different deposition chambers within the module. Each of these characteristics plays a significant role in determining the capability of a photovoltaic system to efficiently produce energy for a given application.
References and Further Reading
- Photovoltaic (Solar Energy)
- The Principle of Photovoltaics
- Photovoltaic Cell Material Basics
- Improved Photovoltaic Performance by Enhanced Crystallinity of Poly(3-hexyl)thiophene