SOLAR CELL – Photovoltaic Cell

In this lecture, we are going to learn about the solar cell, how solar cell works, what is the principle of the solar cell, construction, application, and advantage of solar cells also will be discussed in this lecture.

What is Solar Cell?

A Solar cell is a device that converts solar energy into electrical energy. 

• The solar cell or solar energy converter is essentially a large photodiode to operate as a photovoltaic device and to give as much output power as possible.

• Semiconductor materials like silicon, gallium arsenide (GaAs), indium arsenide (InAs), and cadmium arsenide (CdAs) are used for manufacturing solar cells.

• Silicon and selenium are the most widely USD materials for solar cells.

• The solar cell can convert energy directly into electrical energy with high conversion efficiency and can provide nearly permanent power at a low operating cost and also free of pollution.

• The maximum theoretical efficiency of a solar cell depends on the bandgap of the semiconductor material.

Principle of Solar Cell

Solar cells operate on the principle of photovoltaic action. The photovoltaic action refers to their voltage-generating capability. Since these cells generate a voltage proportional to sunlight intensity, the solar cells are also called photovoltaic cells.

Construction of Solar Cell

• The solar cells consist of a single semiconductor crystal that has been doped with both p- and n-type impurities, thereby forming a p-n junction.

• The basic construction of a p-n junction solar cell and its circuit symbol is shown in the figure below.

• The thickness of p-type material is made extremely thin so that light can penetrate into the junction. A nickel-plated being around the p-type material is the positive output terminal. The thickness of an n-region is also kept small to allow holes generated near the surface to diffuse to the junction before they recombine. A metal plating at the bottom of the n-material is the negative output terminal. The p-n diode is enclosed in a can with a glass window on top so that light may fall upon p- and n-type materials.

Working of Solar Cell

• The net current id an open circuit p-n junction is zero. This is because the current due minority carriers are balanced by the current due to the majority carriers.

• However, when the p-n junction is illuminated, the incident light photon at the junction may collide with a valance electron and impart sufficient energy to make a transition to the conduction band.

• As a result, an electron-hole pair is formed. The newly formed minority carriers in the p-region and n-region get injected across the junction, thereby increasing the current due to minority carriers.

• Since the junction is open-circuited, the net current must still remain zero. Therefore the current due to majority carriers must increase an equal amount.

• The rise in carrier current is possible only if the field at the junction is reduced. Thus, the barrier height is lowered. This leads to the accumulation of majority carriers on both sides of the junction.

• This gives rise to a photovoltaic voltage ( also called open-circuit voltage V_oc) across the junction in the open circuit condition. This voltage is equal to the decrease in the barrier potential.

• When the p-n junction photovoltaic cell is used as a solar cell, it is important to use an optimum load resistance so as to extract maximum power.

• The conversion efficiency of solar cells is given by,

\eta = \frac{Output\;Power}{Input\;Power}=\frac{I_mV_m}{P_{in}}

Where V_m and I_m are the voltage and current at maximum power point and Pin is the power density of the sunlight.

• The maximum efficiency of silicon solar cells is about 14% even though the theoretical efficiency is about 25% when using sunlight as the light source.

• Apart from the bandgap energy, the current-voltage characteristics of a solar cell also influence its efficiency.

• Typical V-I characteristics of a solar cell corresponding to different levels of illumination are sown in the figure below:

• The maximum power output is obtained when the cell is operated at the knee of the curve.

• The available output current depends upon the light intensity, cell efficiency, and the size of the active area of the cell face. The conversion efficiency depends upon the spectral content and the illumination.

Advantages of Solar Cell

1. They can be operated satisfactorily over a wide range of temperatures.
2. They have the ability to generate voltage without any bias.
3. They have extremely fast responses.

Applications of Solar Cell

1. Solar cells are extensively used as a source of power in satellites and solar vehicles, to supply power to electronic and other equipment or to charge the battery.
2. Solar cells are used to generate power in calculators and watches.
3. Solar cells are used in photodetection(visible and invisible), demodulation, logic circuits, switching, and so on.

Frequently Asked Questions Related to Solar Cells