Bench Talk

(Source: jaochainoi - stock.adobe.com)

Solar cells, otherwise known as photovoltaics (PVs), are a rapidly growing class of renewable energy devices that change solar rays (photons) into electricity. Solar cells come in many forms, especially when it comes to the active material that changes sunlight into electricity. The most traditional solar cells rely on inorganic materials as the active material, while the newest solar cells use everything from organic materials to printed inks. Here, we’re focusing on the traditional inorganic solar cell because even though the newer classes bring different benefits (printable, flexible), the inorganic solar cells are still the most widely implemented and efficient solar cells in use today.

Solar cells rely upon the principle of the photoelectric effect, which is when a material exhibits a chemical and physical phenomenon that generates voltage and current when exposed to light. Inorganic solar cells have traditionally used silicon, that is doped to create a semiconducting junction. In many cases, designers use silicon on both sides of the junction, where one side is doped with an atom with one less electron than silicon (p-type), and another side is doped with an atom that has an extra electron than silicon (n-type). In most cases, the silicon is doped with boron or gallium and phosphorous to produce p- and n-type semiconducting regions, respectively. In recent years, there has been a shift from using silicon to various nanomaterials, as the nanomaterials often yield greater device efficiencies. While these efficiencies are greater than silicon, silicon-based solar cells are still the most common inorganic solar cells in use.

Regardless of the material used—whether it is two differently doped silicon materials or nanomaterials—a semiconducting junction is formed where the two regions contain holes and electrons separated by a depleted region. How the electrons and holes are formed and aligned in the semiconducting junction is largely due to the chemical properties of the material and the level of chemical doping. When the silicon is doped to become a p-type region, the doped atom can only form three covalent chemical bonds compared to silicon’s four. This leaves a space where a chemical bond should be. This lack of a bond is known as a hole, which is positively charged. On the other hand, when the silicon is doped with an atom to form an n-type region, the extra electron is left over in the lattice because the lattice geometry that silicon adopts can only accommodate four bonds (the same goes for many nanomaterials, as it is often carbon nanomaterials that are used, and they can still only accommodate four covalent bonds if the material is to be stable). Hence, the extra electron becomes delocalized in the lattice. These doped regions then become hole-rich and electron-rich, respectively..

Before the solar cell is exposed to any light, these electrons and holes are separated by the depletion region, which is the interface of the n-region and the p-region and is often coined a p-n junction. This interfacial area is a region where some of the electrons and holes have come together, causing the other positively and negatively charged carriers to be separated by this ‘neutral zone’. As well as being a neutral zone, it also generates an internal electric field that prevents all the holes and electrons in the two regions from combining.

While the charge carriers are initially separated, this changes when sunlight hits the solar cell. Photons of light possess energy, and when the photons strike the solar cell, this energy then transfers to the holes and electrons that are free on either side of the depletion zone. This extra energy also enables the free charge carriers to move into the depletion zone. As this happens, the width of the depletion zone reduces. Eventually, the internal electrical field can’t counteract the energetic motion of the charge carriers, and the electrons move to the opposite side of the junction, where they recombine with the holes. This generates a constant current. Once the charge carriers start to recombine, the depletion zone increases again, but it never returns to its rest state so long as there is energetic input—i.e., light on the solar cell. So, as long as there is sunlight, a continuous current will flow that can be harvested and stored. Once the sunlight is no longer present, the depletion zone will return to its original thickness, and the device is essentially reset until more sunlight hits the cell.

The inorganic solar cell relies on chemistry, chemical principles, and the effects of chemical reactions to efficiently convert sunlight into electricity through a semiconducting p-n junction. The doping of two regions, be it silicon or another inorganic material, creates a region that enables oppositely charged particles to combine and produce an electrical current under solar irradiation. Inorganic cells have much higher efficiencies than other types of solar cells, but the materials used are highly crystalline in nature and have a regular solid-state lattice. So, this means they are often less flexible than other types of solar cells. However, they are still the most common type, and until flexible and printable solar cells can reach similar efficiency levels, it is likely that inorganic solar cells will be the frontrunner for many years.