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# Effect of Light Intensity

The change in the incident intensity on a solar cell causes change in all solar cell parameters, which include short circuit current, open circuit voltage, the Fill Factor, efficiency and the series and shunt resistances effect. Light intensity on a solar cell is named the number of suns, where 1 sun is equal to standard illumination at AM1.5, or 1 kW/m2. As an example a system which has 10 kW/m2 incident on the solar cell would be said to operate at 10 suns, or at 10X. A PV module which is designed to operate under 1 sun conditions is called a flat plate module while the others using concentrated sunlight are named concentrators.

#### Concentrators

A concentrator is a solar cell which is designed to operate under an illumination that is greater than 1 sun. The incident sunlight is guided or focused by optical elements so that a high intensity light beam strikes a small solar cell. Concentrators have multiple possible advantages, which include the higher efficiency potential than the 1 sun solar cell and the lower cost possibility. The short circuit current from a solar cell linearly depends on light intensity, so that the device operating under 10 suns would be 10 times the value of short circuit current of a similar device under one sun conditions. Yet, this effect will not provide an increase in efficiency, because the incident power increases linearly as well with the concentration. However, the benefits of efficiency is based on the logarithmic dependence of the open circuit voltage on short circuit. Thus, with concentration, Voc logarithmically increases with light intensity, as given in the equation below: where X: sunlight concentration The price of a concentrating PV system might be less than a similar flat plate PV system because a smaller area of solar cells is required. The efficiency benefits of concentration might be reduced by the increased losses from the series resistance as short circuit current increases and as well by the increased temperature of the solar cell operation. As losses from short circuit current is dependent on the square value of the current, power loss from series resistance increases with the concentration square.

#### Low Light Intensity

Solar cells experience daily light intensity variations, with the sun’s incident power ranging between 0 and 1 kW/m2. At lower light levels, the shunt resistance impact becomes increasingly essential. With reduction in light intensity, the bias point and current through solar cell decrease as well, and the solar cell’s equivalent resistance can start to approach the shunt resistance. If these two resistances are equal, the total current fraction flowing through shunt resistance increases, therefore increasing the fractional power loss from shunt resistance. Thus, in cloudy conditions, solar cells with higher shunt resistance retains a bigger portion of the original power than  solar cells with lower shunt resistance.
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