Scientists from Kaunas University of Technology and Vilnius University in Lithuania and University of Colorado in the U.S have proposed a method for increasing the stability and performance of perovskite solar cells. The team synthesized a new class of carbazole-based cross-linkable materials, which are resistant to various environmental effects, including strong solvents used in the production of solar cells.
When applied as hole transporting layers, the new materials helped achieve the 16.9% efficiency of the inverted-architecture perovskite cells at the first attempt. It is expected to reach higher efficiency upon optimization.
Perovskite-based solar cells usually have one of two architectures—regular (n-i-p) and inverted (p-i-n) structures. In the latter, the hole transporting materials are deposited under the perovskite absorber layer.
"Although p-i-n cells have numerous advantages when compared to the perovskite solar cells of regular architecture, they have serious shortcomings. For instance, the hole transporting compounds should be able to withstand the strong polar solvents used to form light-absorbing perovskite layer, which is placed above," explains Professor Vytautas Getautis, chief researcher at KTU Faculty of Chemical Technology.
To solve this problem, in p-i-n architectures polymers are often used as hole transporting materials. However, due to solubility issues, a polymer layer is not easy to form; moreover, it is difficult to control the recurrence of reactions and synthesize the same structure. Aiming to address this issue, the researchers made a hole transporting layer of carbazole-based molecules, which then was thermally polymerized in situ to reach cross-linking effect.
"The cross-linked polymer has a three-dimensional structure. It is very resistant to various effects, including the strong solvents used while forming a light-absorbing perovskite layer. We used several groups of molecules and developed materials, which, while used as a hole transporting layer, can improve the efficiency of an inverted perovskite solar cell to almost 17 percent," says a Ph.D. student Šarūnė Daškevičiūtė-Gegužienė, who synthesized these compounds.
The research group, headed by Prof Getautis, has developed numerous cutting-edge inventions, aimed at improving the efficiency of solar cells. Among them are synthesized compounds, which self-assemble into a molecule-thin layer that acts as a hole transporting material. The silicon-perovskite tandem solar produced using the said materials reached an efficiency of over 29 percent. According to Prof Getautis, the latter tandem combination will soon become the commercially available alternative to silicon-based solar cells—more efficient and cheaper.
"Our field of research aims to improve the existing technologies for perovskite solar elements and in this field, we have achieved the best results with the self-assembling-monolayer technology. However, science is often developed in multiple directions, as we need to explore ways to use solar energy the best we can," says Prof Getautis.