# Investigate the effects of two different variables on a solar cell output.

Solar Cell Investigation

Planning

Aim:

To investigate the effects of two different variables on a solar cell output.

Solar cell information:

The solar cells that are on calculators and satellites are photovoltaic (PV) cells or modules, which are basically a group of cells, which are electrically connected and packaged in one frame. Photovoltaics, are made up of two words: photo = light, voltaic = electricity, and therefore you can deduce that they convert light into electricity.

These PV cells are made of special materials called semiconductors such as silicon. When the light strikes the cell, a certain amount of it is absorbed within the  material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely and therefore producing a current. PV cells also all have one or more electric fields that act to force electrons freed by light absorption, to flow in a certain direction. By placing metal contacts on the top and bottom of the PV cell, you can draw that current off to use externally. E.g. the current can power a calculator. This current, together with the cell's voltage defines the power that the solar cell can produce. (P=IV)

When light, in the form of , hits our solar cell, its energy frees electron-hole pairs. Each photon with enough energy will normally free exactly one electron, and result in a free hole as well. If this happens close enough to the electric field, or if free electron and free hole happen to wander into its range of influence, the field will send the electron to the N side and the hole to the P side. This causes further disruption of electrical neutrality, and if we provide an external current path, electrons will flow through the path to their original side (the P side) to unite with holes that the electric field sent there, doing work for us along the way. The electron flow provides the current, and the cell's electric field causes a voltage. With both current and voltage, we have power, which is the product of the two.

(a=n-type silicon and b=p type silicon)

The most that the cell could absorb is around 25 percent, and more likely is 15 percent or less.

This is because light can be separated into different wavelengths, and we can see them in the form of a rainbow. Since the light that hits our cell has photons of a wide range of energies, it turns out that some of them won't have enough energy to form an electron-hole pair. They'll simply pass through the cell as if it were transparent. Still other photons have too much energy. Only a certain amount of energy is required to knock an electron loose. This is called the band gap energy of a material. If a photon has more energy than the required amount, then the extra energy is lost. These two effects alone account for the loss of around 70 percent of the radiation energy incident on our cell.

Structure of the solar cell:

Variables:

When investigating a solar cell, there are several variables we could investigate. Below, I have analysed all the variables that could be investigated, and evaluated which one I will investigate.

## Light

Light can have different colours, and different coloured lights are known to have different frequencies. This in turn would cause the different coloured lights to emit different levels of power. We know that this is the case because when combining the two below formulae, we can see that energy and frequency are related.

Wavelength x Frequency= Wave Speed

Planck’s Constant x frequency= Energy

The second formula states that frequency is directly proportional to energy.

When rearranging the first formula to display frequency as the subject of the formula, and then substituting the value for frequency given (wave speed/wavelength) into the second formula, we get:

Planck’s Constant x wavespeed/wavelength= energy

Using this formula, we can find out what kinds of light give out the most energy. As all light travels at the same speed (300,000,000 m/s), we know that the wavelength of the light will determine how much energy is given out from the light. The wavelength and frequency are directly related in light, because both multiplied must give a product of 300,000,000 m/s.

We can gather by the formula that light which has a smaller wavelength will give out more energy, because when a smaller number is divided by the ...