Submitted by elsbeth on Tue, 2009-10-27 18:25

Why isn't the efficiency 100%?

 

Sunlight consists of various colours, which are absorbed by solar cells to a greater or lesser degree. The rate of absorption is determined by the semiconductor material used to produce the solar cell. These materials are not equally sensitive to all colours of light. Solar panels use visible light (approximately 45% of sunlight): light that we can observe with our eyes. Light is made up of photons (= the light particles from solar radiation = packets of energy) which determine the colour of the light.

The photon requires a minimum amount of energy to release electrons in the semiconductor material. If the energy is higher than the required minimum, this excess energy is emitted in the form of heat. Approximately 55% of the energy in light is lost in this process. Furthermore, electrons have the property of falling back to their original state. Therefore, conversion efficiency rates greater than 20% - 30% are not theoretically attainable.

At this time, the best small solar cells with optimum colour-sensitivity in laboratories have not exceeded 25% efficiency. Commercially available solar cells currently have efficiency rates of between 6% and 16%.