Another record was broken: Silicon-perovskite photovoltaics reached an efficiency of 34%

Silicon solar panels are already hitting physical efficiency limits, so the best chance to increase it further is to make better use of the sunlight spectrum. This will be made possible by the combination of silicon and perovskite.

Perovskite crystals can be layered on silicon, creating a panel with two materials that absorb different regions of the spectrum. Moreover, perovskites can be made from relatively cheap raw materials. Unfortunately, until now it has been difficult to produce perovskites that are highly efficient and at the same time have a lifetime of tens of years like silicon. However, developers are working hard to improve the technology, and the latest progress has produced perovskite/silicon cells with conversion efficiencies of up to 34 percent.

Perovskite is a mineral composed of calcium titanate (chemical formula CaTiO3) and forms a crystal structure based on pseudohexahedrons or pseudooctahedrons. However, it can also be minerals composed of other elements that form the same crystalline structure. Perovskite-based photovoltaics are typically created through solution processing, where all the raw materials are dissolved in a liquid that is then layered onto the future panel. This will allow the formation of perovskite crystals on its entire surface. The downside is that this process tends to create crystals with different orientations on a single surface, reducing performance.

Another disadvantage is the lower stability and thus the lifetime of perovskites. They are usually made of a combination of positively and negatively charged ions and must be in the right ratio to form a perovskite. However, some of these individual ions diffuse over time, which disrupts the crystal structure. In solar cells, the material absorbs a lot of energy, which heats it up and the rate of diffusion increases.

However, silicon itself has a theoretical limit of light-to-electricity conversion efficiency of about 27%, so currently silicon photovoltaics are approaching the limit of maximum efficiency. Tandem cells with perovskite could push this limit by up to a quarter.

ARS Technica magazine reported on two scientific researches that try to solve the problems of perovskite photovoltaics. In the first research, scientists from Saudi Arabia and Turkey, using a technique called density functional theory, created a molecule called tetrahydrotriazinium, which has a six-membered ring composed of alternating carbon and nitrogen atoms.

Tetrahydrotriazinium has a neutral charge when only two of the nitrogens have hydrogen bonds. However, it captures the charged hydrogen (proton) from the solution and gives it a net positive charge. This leaves each of its three nitrogens bonded to a hydrogen and allows a positive charge to spread between them. This makes this interaction incredibly strong, which also stabilizes the crystal structure and should make perovskites much more stable.

However, tetrahydrotriazinium reacts with a number of other chemicals, so it is not very suitable as a raw material for a perovskite-forming solution. However, the researchers created the tetrahydrotriazinium directly in the perovskite-forming solution so that it was immediately incorporated into the perovskite crystal and had no chance to react with anything else.

After layering on silicon, the cell achieved an efficiency of 33 to 34 percent, which is the current record. Researchers have several ideas that should increase the efficiency up to 35%.

The crystals were reasonably stable in the light, but the combination of light and heat caused a significant drop in performance. The researchers claim that the devices will retain over 90% of their initial performance "for up to 1,000 hours". However, degradation of up to 10% in 6 weeks makes the commercial deployment of such cells impassable.

The second research focused on improving the crystalline process in the formation of perovskite. To control this process, the researchers focused on using an "anti-solvent" that reduces the solubility of other chemicals in solution. They used a long hydrocarbon chain linked to an ammonium ion and a bromine atom, both of which are typically components of perovskites. Addition to the solution could control the formation of perovskite crystals with a wide range of compositions. The result was a more robust crystal with fewer defects that affect performance and stability.

In tandem with the silicon layer, these cells achieved an efficiency of 30 to 33%. Even in this case, resistance at elevated temperatures is a problem. At room temperature, the material had more than 98 percent of its original efficiency after 100 days, but in practical use, we can forget about room temperatures and the degradation will be much faster.

Addressing the photothermal stability of perovskite–silicon tandem solar cells is a multifaceted challenge that requires addressing multiple complications in the areas of interfaces, contacts, electrodes, and encapsulation. So we still have to wait for commercially viable articles of this type.