All light contains photons that carry energy proportional to the wavelength of light. When a wavelength of light hits a surface, some of that light energy is absorbed. If there is enough energy hitting the surface, the electrons in the material can escape; this results in electricity.
Every material is photoelectric— if there is enough energy, you can excite an electron in the material. However, certain materials are easier to work with, the most common being silicon.
Materials that use this phenomenon to create and harness electricity are called photovoltaics. All solar cells are fundamentally similar. Solar cells consist of an active material with semi-conductive properties that create electrons and other layers that help move the electrons out of the material into a circuit where the electricity can be used.
Typically, solar cells are made of solar grade silicon. Additional coatings can be applied to the surface glass to help reduce the amount of light reflected, increase the amount of light absorbed, or help protect the cell from degradation, including humidity, oxygen, and mechanical stresses.
The solar technology industry has approximately 60 years of cumulative research, and still, 90% of commercial photovoltaics are crystalline silicon (c-Si). Within crystalline silicon, there is single-crystal, also known as monocrystalline (sc-Si), and multicrystalline (mc-Si). Monocrystalline has higher efficiency and is approximately 20-30% more expensive than multicrystalline. Monocrystalline silicon is made using crystal growth technology, specifically through a method known as the Czochralski process that grows single crystals of silicon into large ingots. In comparison, mc-Si requires silicon to be melted into a cast, producing many randomly oriented crystals. A downside of crystalline silicon is its low ability to absorb light. To compensate, a thick, brittle wafer of silicon is used in solar cells to increase the module’s efficiency.
Solar grade silicon also requires ultra-high purity. Electronic grade silicon is 99.99% pure, while solar grade silicon is 99.9999999% pure. This process requires multiple refinement and purification methods at high temperatures of 1000°C – 2000°C. Despite this, crystalline silicon will likely remain the leading technology due to the many years invested in research and the abundance of silicon (It is made from sand!)
Some companies have worked to pioneer other technologies. The progress of solar technology has mainly improved solar modules’ efficiency by exploring various other materials. It should be noted that for some of the emerging technologies, such as perovskites, GaAs or hybrid, only small modules have been created. These technologies face challenges with the scale of the material costs and methods of production. In most cases, it is difficult to increase production scale while maintaining the quality of the modules.