Fabrication of PV Cell

Fabrication of PV Cell

A PV cell means photovoltaic cell or solar cell which works on photovoltaic effect. It generates voltage proportional to the light incident on it.

The Fig. 1.16.1 (a) shows the construction of a photovoltaic cell while the Fig. 4.5.1 (b) shows the symbl of a photovoltaic cell.

The surface layer of p-type material is extremely thin so that light can penetrate to the junction. The nickel plated ring around p-type is the positive output terminal. The nickel plated ring around n-type material acts as the negative output terminal.

When the light strikes the cell, the photon light energy is absorbed by the semiconductor material and number of electron-hole pairs are generated. This happens on both the sides of the junction.

The holes are directed towards the p-region while the electrons are directed towards the n-region due to the electric field present. Due to this movement the minority current is set up across the junction due to which the voltage is developed across the p and n regions. This voltage is taken out as the output voltage.

Most practical photovoltaic cells use silicon as it is easily available and cheap compared to other materials.

1. Steps of Fabrication of PV Cell

The various steps of fabrication of PV cell are,

Refining silicon : The available mineral in earth is silicon dioxide (SiO₂). This is used to obtain silicon by removing oxygen. For this, it is heated to 1500 - 2000 °C in an electrode arc furnace with coal and charcoal i.e., reaction with carbon. The silicon obtained is 98 % pure which contains the impurities like iron, aluminium etc. To remove these impurities, further purification is necessary. For this purpose, powdered silicon is reacted with anhydrous HC1 to obtain SiHCl₃. Along with the halides of iron and aluminium (FeCl₃, AlCl₃). The S1HCI3 is reacted finally with hydrogen at 1100 °C to produce pure silicon.

Once the pure silicon is available than various methods can be used for the fabrication of PV cell.

a. Screen Printed PV Cell Fabrication

This is the simplest process of fabrication of a PV cell.

The first step in this process is to obtain silicon wafer of 10 × 10 sq.cm and 0.5 mm thick. It is then p-type doped so as to obtain p-type wafer.

The wafer is then heated in a furnace at 800 - 1000 °C with a phosphorous. This causes small amounts of phosphorous to be deposited on the outer layers of silicon. The phosphorous is required below the screen printed contact to maintain low contact resistance.

The wafers cut are then textured by etching pyramids on the wafer surface with a chemical solution. The texturing ensures better light travel inside the wafer by reducing reflection.

The cells are stacked one on top of the other for the removal of the junction at the edge of the wafer. The edges of the cells are etched by a highly reactive plasma gas to remove the junction.

Then the wafer is placed upside down and a screen is lowered onto the rear of the cell along with the metal paste. A full aluminium layer printed on the rear on the cell, with subsequent alloying through firing, produces back surface field.

This is done by placing the cell in a second furnace at a very high temperature. This firing process destroys the rear n-layer so that the aluminium makes contact with the p-type bulk.

The second print of aluminium is required for solderable contact.

The cell is then flipped over for the printing on the front. The front contact is printed in a similar manner as the rear contact.

The finished cell is ready for encapsulation into a module. An extra metal contact strip is soldered to the busbar during the encapsulation to lower the cell series resistance. The square shape of a multi crystalline substrate simplifies the packing of cells into a module.

Review Question

1. Explain the fabrication of a PV cell.