Understanding Solar Energy 101 : The Photovoltaic Cell

To understand photovoltaic cells, we must first venture into the world of semiconductors. In the realm of science and technology, semiconductors enable modern marvels to come to life. These materials neither perfect conductors nor insulators, possess a remarkable ability to control the flow of electricity. One can imagine semiconductors as a versatile switchboard allowing the controlled movement of electrons within. In the world of solar energy, semiconductors particularly silicon, play an important role in capturing and converting the sun’s energy into usable electricity.

Solar Energy Through Time

Solar energy has long been an inspiration for humankind. But it was not until the 19th century that the photovoltaic effect was discovered by Alexandre-Edmond Becquerel, a French physicist who observed that certain materials produced electricity when exposed to light. Fast-forward to the 20th century, scientists have built upon Becquerel’s work and created photovoltaic cells. By the 1950s, Bell Laboratories realized that semiconducting materials such as silicon were more efficient than other materials and managed to build the first practical device for converting solar energy into electricity.

The choice of silicon as the primary material for solar cells was driven by a combination of availability, and technological feasibility. Silicon, the second most abundant element on Earth’s crust, has a unique structure that allows it to conduct electricity under certain conditions. While other materials have been explored for solar cell technology, silicon’s combination of abundant availability, suitable energy properties, and manufacturing processes made it the material of choice for many early and current solar cell technologies. It has paved the way for the widespread adoption of solar energy as a viable and sustainable power source.

Cross section of a typical solar cell

Photovoltaic Cell Structure

Imagine a photovoltaic cell like a sandwich made of special materials

1. Top Layer : The top layer is a small thin layer of silicon that is good at catching sunlight. This is known as the emitter layer or the N-Type layer. A large fraction of light is absorbed close to this surface. In this N-Type layer there are lots of free electrons hanging around. The top layer thickness is less than 1 µm (micrometer).
2. Bottom Layer : The bottom layer is the substrate layer which is also made up of silicon. It is also known as the P-Type layer or the base layer. The bottom P-Type layer is a bit different, here there is a shortage of electrons instead here we have an abundance of something called holes which are like empty spaces where electrons could be but they’re not. The bottom layer thickness ranges from 100 –500 µm (micrometer).
3. Middle Layer : This is the boundary layer between the top and the bottom layer where the photovoltaic process happens. This is known as the P-N junction layer. By combining the P-Type and N-Type layer we get the P-N junction layer.
4. Metal Contacts: Metal contacts are attached to the N-type and P-type layers, providing a pathway for the electric current generated by the photovoltaic process to flow out of the cell.
5. Front and Back Surface : The top surface of the photovoltaic cell is typically coated with an anti-reflective material to minimize the reflection of sunlight and the back surface of the cell is often textured to improve the efficiency of the cell.

In a nutshell , we can think of the cell structure as zones where there are abundance of electrons in one side and and missing electrons in the other . By cleverly joining these zones together, we create a situation that allows us to control the movement of electrons, leading to electricity generation. A variety of materials can satisfy the cell structure but in practice a typical P-N junction semiconductor material is used in the photovoltaic cell structure.