Solar Photovoltaic Cells

Data/Images/figure_1_no2.jpgThe photovoltaic effect is the basis of photovoltaic cell technology. The key to this effect is a class of materials called semiconductors, the same materials used in the transistors and diodes that have replaced vacuum tubes and switches in consumer electronics.

A typical crystalline photovoltaic cell is made of a semiconductor material such as silicon, into which impurities, such as boron and phosphorus, are intentionally added to create two distinct "layers" within the photovoltaic cell. One layer will have a negative charge, the other a positive charge. The electric field, or voltage, created between these charged layers sets the stage for the photovoltaic effect.

Light energy, in the form of photons excites the electrons in the photovoltaic cell, knocking them free of the crystal structure of the semiconductor. The electric field in the photovoltaic cell moves the excited electrons across the cell where they are collected by metal contacts placed on the front and back surface of the cell. When the photovoltaic cell is connected in a circuit, an electric current flows.

Figure on the right shows the illustration of а crystalline silicon cell, the typical solar cell in use today. The electrical current generated in the semiconductor is extracted by contacts to the front and rear of the cell. The top contact structure, which must allow light to pass through, is made in the form of widely-spaced thin metal strips (usually called fingers) that supply current to а larger bus bar. The cell is also covered with а thin layer of dielectric material - the antireflection coating or ARC - to minimize light reflection from the top surface.

A photovoltaic cell produces electricity with no moving parts and no consumption of material. Figure at the bottom shows examples of the three most common silicon photovoltaic technologies. Silicon is the basic material for most of commercially available photovoltaic cells. To produce a high efficiency photovoltaic cell, silicon is refined to 99.9999% purity before it is fabricated into a photovoltaic cell. Single crystal silicon is grown as a solid ingot. Precision wire saws cut the ingot into many wafers, each about the thickness of a human hair. Additional processing steps such as diffusion of impurities and screen printing of contacts are needed to turn the wafer into a photovoltaic cell. Crystalline silicon photovoltaic cells are predominant in the industry although there are many other materials and manufacturing processes that are currently undergoing theoretical and applied research, especially thin-film photovoltaic devices.

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