Photovoltaic solar cells, often referred to as solar panels, are at the forefront of renewable energy technologies. Harnessing the energy from the sun, these devices have the potential to revolutionise our energy landscape and reduce our dependency on fossil fuels. There is a lot to understand about the intricate workings of photovoltaic solar cells, exploring their components, mechanisms, and the underlying scientific principles that enable them to convert sunlight into usable electricity.
By understanding the science behind solar cells, we can appreciate their remarkable efficiency and discuss their potential to drive sustainable energy production on a global scale.
How Were Photovoltaic Cells Developed?
The development of photovoltaic cells can be traced back to the early 19th century when scientists began experimenting with the properties of light and electricity. In 1839, Alexandre Edmond Becquerel discovered the photovoltaic effect, which demonstrated that light falling on certain materials could generate an electric current. This discovery laid the foundation for understanding the conversion of light energy into electricity.
In the late 19th century, inventors such as Charles Fritts and William Grylls Adams and Richard Evans Day worked on developing solid-state devices that directly converted sunlight into electricity. Fritts created the first known solar cell in 1883, which consisted of selenium coated with a thin layer of gold. However, early solar cells had low efficiencies and were primarily used for scientific experiments.
The modern era of photovoltaics began in the 1950s with the development of silicon-based solar cells. Bell Laboratories researchers Daryl Chapin, Calvin Fuller, and Gerald Pearson created the first practical silicon solar cell in 1954, achieving an efficiency of around 6% and demonstrating its commercial potential.
Over the following decades, researchers focused on improving efficiency and performance, experimenting with different materials, cell structures, and manufacturing techniques. Silicon solar cells surpassed 10% efficiency in the late 1950s, and thin-film solar cells utilising materials such as amorphous silicon, cadmium telluride, and copper indium gallium selenide emerged as alternative technologies in the 1970s and 1980s.
Multi-junction solar cells with multiple semiconductor layers came into prominence in the 1990s, offering higher efficiencies by capturing a broader spectrum of sunlight. Continued research and development efforts have led to significant advancements, with silicon solar cells surpassing 25% efficiency and thin-film technologies also making progress.
What Is the Photovoltaic Effect?
At the heart of photovoltaic solar cells lies the photovoltaic effect, a phenomenon that enables the conversion of sunlight into electricity. This effect is based on the properties of semiconductors, materials with unique electrical conductive characteristics.
When photons, which are tiny particles that comprise waves of electromagnetic radiation, from the sun strike the solar cell, they transfer their energy to the electrons within the semiconductor material, exciting them to a higher energy state. This creates electron-hole pairs, where an electron breaks free from its atom and leaves behind a positively charged hole.
To facilitate the separation of these electron-hole pairs, solar cells are typically designed with a p-n junction, created by doping the semiconductor material with impurities. The n-type region is doped with atoms that provide extra electrons, while the p-type region is doped with atoms that create electron deficiencies or “holes.” This creates an electric field at the junction that helps to separate the charges and prevents them from recombining immediately.
What Are Photovoltaic Solar Cells Made Of?
Photovoltaic solar cells are complex structures composed of multiple layers of semiconductor materials, each serving a specific function in the energy conversion process. The most common semiconductor material used is silicon, due to its abundance and favorable electrical properties. Silicon atoms have four valence electrons, forming a crystalline lattice structure, which initially makes it a rather poor electrical conductor.
However, by introducing impurities, known as dopants, into the silicon crystal, we can modify its electrical conductivity.
How Do Photovoltaic Cells Create Electricity?
In a typical silicon solar cell, the top layer consists of an anti-reflective coating to reduce the loss of sunlight through reflection. Beneath this layer is a thin grid-like pattern of metal conductors, called busbars, which collect the generated electricity.
The next layer is the p-type silicon, which has an excess of holes. Adjacent to the p-type layer is the n-type silicon, which has an excess of electrons. This p-n junction facilitates the separation of the electron-hole pairs generated by the incident photons.
Within the p-n junction, there exists an electric field created by the difference in charge between the p-type and n-type regions. When photons strike the solar cell, they transfer their energy to electrons in the valence band of the semiconductor material, allowing them to break free and move to the conduction band. The electric field within the p-n junction then causes these free electrons to move toward the n-type side and the holes toward the p-type side.
As the electrons and holes move through the different regions of the solar cell, they encounter the grid-like conductors that collect and direct the flow of electricity. This collected electrical current can then be utilised as a direct current (DC) source or converted to alternating current (AC) using inverters for use in our electrical grid systems.
