This diagram shows a typical crystalline silicon solar cell. 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 a larger bus bar. The cell is covered with a thin layer of dielectric material - the anti-reflection coating, ARC - to minimise light reflection from the top surface.
This animation comprises five images, which change every 6 seconds.
Solar cells are essentially semiconductor junctions under illumination. Light generates electron-hole pairs on both sides of the junction, in the n-type emitter and in the p-type base. The generated electrons (from the base) and holes (from the emitter) then diffuse to the junction and are swept away by the electric field, thus producing electric current across the device. Note how the electric currents of the electrons and holes reinforce each other since these particles carry opposite charges. The p-n junction therefore separates the carriers with opposite charge, and transforms the generation current between the bands into an electric current across the p-n junction.
A more detailed consideration makes it possible to draw an equivalent circuit of a solar cell in terms of a current generator and a diode. This equivalent circuit has a current-voltage relationship.
In solar cell applications this characteristic is usually drawn inverted about the voltage axis, as shown below. The cell generates no power in short-circuit (when current Isc is produced) or open-circuit (when cell generates voltage Voc). The cell delivers maximum power Pmax when operating at a point on the characteristic where the product IV is maximum. This is shown graphically below where the position of the maximum power point represents the largest area of the rectangle shown.
The efficiency (n) of a solar cell is defined as the power Pmax supplied by the cell at the maximum power point under standard test conditions, divided by the power of the radiation incident upon it. Most frequent conditions are: irradiance 100 mW/cm2 , standard reference spectrum, and temperature 25 0 C. The use of this standard irradiance value is particularly convenient since the cell efficiency in percent is then numerically equal to the power output from the cell in mW/cm2.
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