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## What is a p-n junction?

1. A pn junction can be used as a solar cell. Describe how this is possible. Describe the limitations to the conversion efficiency of solar radiation to electric power in a silicon pn junction solar cell. Explain how you would improve the pn-junction conversion efficiency

A p-n junction can be used as a solar cell. Describe how this is possible

P–N junctions are the boundary or interface between the several forms of semiconductor that make up a single crystal of semiconductor. There are several ways to achieve this, such as ion implantation or diffusion of dopants or epitaxy. There will be a grain boundary between the semiconductors if they are made of two distinct materials, which will drastically reduce their value by spreading electrons and holes over the material's surface.

Electrical energy can only be created when light-generated carriers are accumulated. Electricity can only be generated when there is a voltage and current present. Photovoltaic effect" creates the voltage in a solar cell. Carriers created by light enter the junction and move to the other side of the p-n junction, where electrons are collected and holes are dispersed. As light-generated energy takes over when a circuit is shorted, no charge is left in the gadget. (Gharghi et al., 2006).

Describe the limitations to the conversion efficiency of solar radiation to electric power in a silicon p-n junction solar cell

There is an increase in electrons and holes on the n-type side of the p-n junction as a result of the accumulation of light-generated carriers. An opposing electric field is created at the connection as a result of this charge separation. As the electric field functions as a barrier, it is important to reduce the electric field in order to enhance the forward’s bias diffusion current (Shockley et al., 1961). A new equilibrium is established when a voltage from across p-n junction is applied. The voltage output of the solar cell is the difference between the forward current and the IL current. Forward bias grows under open-circuit circumstances until the light-generated current is entirely matched by the junction's forward bias diffusion current, at which time the net flow is zero. To equalize these two currents, "open-circuit voltage" is the phrase used. (Gharghi et al., 2006).

Explain how you would improve the pn-junction conversion efficiency.

1. Minimizing the amount of light being reflected away from the cell surface
2. Using anti-reflective coatings
3. Using and promoting light scattering visible spectrum
4. Using light trapping photonic structures to increase the cell’s conversion efficiencies.
5. Use of optimum transparent conductors
6. Improving charge carrier collection

2.One way to improve power conversion efficiency is to design a p-i-n device. Explain how this improves the efficiency and what are the limitations of this method

To activate the PIN photodiode when the reverse bias voltage is applied, the space charge area must entirely cover the intrinsic region. In the space charge area, photon absorption forms electron-hole pairs. Due to its limited lifespan, its switching speed is inversely proportional to it.

A short minority carrier lifespan contributes to the acceleration of the switching process. To maximize switch speed in light detector applications where response time is crucial, the depletion area width should be as large as possible with a very short minority carrier lifetime (William et al., 1999). PIN photodiodes may do this by introducing an intrinsic region that increases the breadth of the space charge. The image below shows a standard PIN photodiode:

How this improves the efficiency:

1. Has a low bias current
2. Has low dark current
3. Has the high-speed response
4. Has a low junction capacitance
5. Has a large depletion region
6. Can tolerate high reverse bias voltages. (William et al., 1999).

What are the limitations of this method?

1. It is less sensitive
2. Has a high reverse recovery time that makes it lose a lot of power
3. Does not have an internal gain
4. Has a smaller area. (William et al., 1999).

3. Another way to improve efficiency is to add quantum wells to the i-region of the p-i-n

device. Explain how this improves the efficiency of conversion and what are the limitations

of this method. (30 marks)

A solar cell

When photons strike the p-n junction in the presence of light, electrons form pairs and an electric current flows through the depletion zone, where electrons from n-type Silicone have spread into holes in the p-type Silicone. When photons are absorbed by atoms, the free one electron from the atom and cause a hole to form in the atom's structure. Those that have enough energy to make it out of the depletion zone without depleting all of their resources are considered to be survivors (Barnham et al., 1993).

When a wire is connected from the cathode (N) to the anode (O), it is feasible for electrons to flow through it (p). The introduction of an external load causes electrons to be pulled to P and holes to be drawn to N, resulting in the passage of current across the circuit (Barnham et al., 1993).

Limitations of this method.

1. It has a high maintenance cost
2. Bandgap selects only at a few frequencies to shift electrons
3. As sunlight is not monochromatic and energy is spread over a spectrum

References

Barnham, K., Barnes, J., Haarpaintner, G., Nelson, J., Paxman, M., Foxon, T., & Roberts, J. (1993). Quantum-well solar cells. MRS Bulletin, 18(10), 51-55. Retrieved from: https://www.cambridge.org/core/journals/mrs-bulletin/article/quantumwell-solar-cells/79E0B02E8AA2E2231E90775F0E420F07

Gharghi, M., Bai, H., Stevens, G., & Sivoththaman, S. (2006). Three-dimensional modeling and simulation of pn junction spherical silicon solar cells. IEEE transactions on electron devices, 53(6), 1355-1363. References: https://ieeexplore.ieee.org/abstract/document/1637631/

Shockley, W., & Queisser, H. J. (1961). Detailed balance limit of efficiency of p?n junction solar cells. Journal of applied physics, 32(3), 510-519. Retrieved from: https://aip.scitation.org/doi/abs/10.1063/1.1736034

Williams, K. J., & Esman, R. D. (1999). Design considerations for high-current photodetectors. Journal of Lightwave Technology, 17(8), 1443. Retrieved from: https://www.osapublishing.org/abstract.cfm?uri=JLT-17-8-1443

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