Solar Spectrum in the Electromegnetic Spectrum: Solar Cells

Nearly all energy in one form or another can be traced back to solar energy, and it is harness in many different ways. Solar cells are the most direct way to convert incident solar radiation into electricity, this process is also known as photovoltaics. Solar cells produce no pollution, noise, and do not contain any moving parts, allowing them to be tough, reliable, and long lasting. The natural light that we enjoy from the sun and see every day is only a fraction of the total energy emitted by the sun incident on the earth. The visible portion of sunlight is from just a small portion of the electromagnetic spectrum. [1]

Fig. 1. The solar spectrum in the electromagnetic spectrum, as a function of wavelength, frequency, and energy. [2]

Due to the nature of solar energy, the two components required to have a functional solar energy generator are a collector and a storage unit. The job of the collector is just that, to collect the radiation that falls on it and converts a fraction of it to other forms of energy, usually electricity. The storage unit is necessary due to the inconsistent nature of solar energy; weather and time of day effect the amount of radiation that can be received. The storage unit holds excess energy produced during the periods of maximum productivity, and releases it as needed during times of low energy production. The most common type of collector is the flat plate collector, an array of solar panels arranged in a plane. The output of these collectors is directly related to the size, cleanliness and amount of solar radiation they are exposed to. [3] The photovoltaics available on the market today are only about 15% efficient [4] nut advances in technology have recently produced a solar cell with a staggering...

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...ower can be decreased by increasing the series resistances or decreasing the shunt resistances.

Fig. 9. Photovoltaic I-V characteristics affected by non-ideal resistances.

The resistances used to produce Figure 9 have not changed the values of VOC or ISC because as RSH decreases, the value of VOC also decreases and ISC is left unaffected. An excessive increase in RS can result in a drop of ISC, with VOC remaining constant. The more drastic the deviations from the ideal form, the more dramatic the changes in ISC and VOC become.

The internal resistances can be approximated from the absolute value of the V/I slopes of the current voltage curve at VOC and ISC, as shown in Figure 10. The slope of the curve at VOC is proportional to the series resistance RS at best, but it is generally larger than RS. The slope of the V/I curve at ISC is a much better estimate for RSH.