Reverse Bias in Solar Cells

Solar Cell Basics

A photo cell (solar cell) is a p-n junction. A photo cell convert’s light energy into electrical energy and the photons is the current source.

When photons incident the silicon, it either travels through the material if its energy is lower than the band gap energy of the silicon semiconductor (transmission), or is absorbed by the silicon if its energy is higher than the band gap energy , or reflected. If the energy is high enough then an electron-hole pair is produced, and the electron and hole are separated by depletion region of the p-n junction and a current is generated through a connected circuit , since the positive carriers prefer the p-type material and the negative carriers prefer the n-type material, this increases the conductivity of the material.

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Diagram showing electron-hole pairs being affected by incident photons.

When no light or photons are incident on the solar cell, then there is a balance between drift and diffusion and the number of electrons and holes are equal between the two regions, and no net current flow is present.

The light detector or solar cell is in fact a photodiode where is will readily conduct current in one direction and hardly at all in the other. With photocells, we need to apply a reverse bias in order to increase the effect of an internal electric field in the junction, thus causing an imbalance of drift and diffusion across the depletion region.

For the photocell, the holes tend to enjoy staying in the p region and the electrons in the n region, reverse bias enhances this tendency. When a stream of photons are absorbed into the silicon and are within the transition region, then electron-hole pairs are formed, then photon generated charge carriers will prefer their n/p type material respectively.

This will result in an additional reverse current through the junction.

In the above diagram, you will notice electron-hole pairs outside of the depletion region, still being able to make the transition, this will be due if they live long enough to slip into the depletion region.

In practise it is preferred to have one of the p-type or n-type materials to be much less than the other to ensure the depletion region extends further on one side only, this ensures. Assuming the p-type is much thinner, then the absorption length of the silicon which is the reciprocal of the absorption co-efficient will need to be tuned with some light doping to ensure the transition region is wider which will increase the probability that incident photons will occur around the transition region where electron-hole pairs are close to the depletion region, else the pairs will recombine due to lower lifetime constants. Therefore the doping requirements will be that one of the material types will have a weaker doping than the other. Also the p-Type material is in contact with the substrate and the initial layers close to the substrate where the electrical contacts are located will have a high doping population P+ the p layer closer to the junction will have a lower doping population p.

Diffusion is responsible for the leakage of charge across the p-n junction when it is experiencing reverse bias due to thermal energy, charge carriers move from high concentration to low concentration. Diffusion pushes minority charge towards the edge of the junction within the depletion region where it is swept across to the other side and becomes a majority charge carrier. See the diagram above, where charge carries are swept across (charge separation). Diffusion is negligible in reverse bias

Drift is where the positive charge will drift in the direction of the positive electric field (negative end of battery) and negative charge will go the opposite way. The total current flowing through the depletion region under reverse biasing is made up of mostly of minority carrier drift.

There are various manufacture processes that can create drift or diffusion dominant diodes depending on the dopant used (Science Direct, 2009)

We will look at the materials used in the next section, that and a battery, where the battery will be used to store the current generated by the solar cell.

The materials required to fabricate a p-n junction are:

· Metal or similar material that is electrically conducting

· Insulator to isolate the voltage applied

· A Semiconductor where the conductivity can be altered by doping and bias voltages

The device to build is in fact a complementary metal oxide semiconductor field effect transistor (CMOSFET). This is where n-type and p-type devices are situated side by side. We will focus on a n-channel silicon MOSFET.

The junction is the region where the n-type and the p-type silicon are in contact with each other.

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Diagram representing a p-n junction

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As we can see from the above, when in equilibrium the electrons “diffuse” across and combine with the holes; this area is known as the depletion region.

When a current is applied by making the p-type more positive and the n-type more negative, it will flow readily in one direction (Forward Bias) but not in the other (Reverse Bias).

To form a pn junction different conductive regions must be adjacent to each other and a rapid spatial change in the dopant species from donor to acceptor must be created.

The reverse bias enhances the large field potential in the device and thus charge carriers are rapidly accelerated to the respective anode and cathode electrodes, thus contributing to the current.

What about forward bias?

In forward bias, the internal field would essentially be destroyed and the charge carriers would move very slowly and hence your solar cell would be less effective.

Furthermore applying a forward bias would temporarily reduce or destroy the junction as it would induce carriers to move in the same direction as the diffusive flow, thus smoothing out the carrier differences.

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