Researchers from HZB, University of Naples Federico II, Kaunas University of Technology, Humboldt University of Berlin and the Italian Institute of Technology (IIT) recently succeeded in reducing losses in the lower contact layer of tin perovskite solar cells. The team identified chemical compounds that self-assemble into a molecular layer that fits very well with the lattice structure of tin perovskites. On this monolayer, tin perovskite with excellent optoelectronic quality can be grown, which increases the performance of the solar cell.
A self assembled monolayer of Phenothiazine enables the formation of perovskite films with good optoelectronic quality and minimizes recombination losses. Image credit: HZB
Tin perovskite solar cells are lead-free and potentially more stable than their lead-containing counterparts. Thanks to their special electro-optical properties, they are particularly well suited to tandem and triple solar cells. Nevertheless, tin perovskite solar cells still have to catch up with the high efficiencies of lead-based perovskites.
In current tin perovskite solar cells, the lowest contact layer is produced using PEDOT:PSS. This is not only a cumbersome process, but it also results in losses. However, in lead perovskites, the PEDOT:PSS layer can be replaced with a more elegant solution: self-organized monolayers (SAMs) which have even led to new record efficiencies.
Until now, experiments with SAMs based on the MeO-2PACz compound in tin perovskites have yielded poorer results than with PEDOT:PSS. Nevertheless, principal investigator Dr. Artem Musiienko was convinced that SAMs can also offer advantages in tin perovskites. With the assistance of his partners, the team analyzed potential issues with using MeO-2PACz as a contact layer for tin perovskite. Density functional theory calculations revealed that the resulting interface did not align well with the adjacent perovskite lattice, resulting in substantial losses.
The team therefore sought alternative self-assembled monolayer (SAM) molecules that would allow a better fit. They discovered phenothiazine, a sulphur-containing functional group abbreviated as Th-2EPT. Dr. Tadas Malinauskas and Mantas Marčinskas from Kaunas University of Technology in Lithuania synthesized the new compound. Compared to PEDOT, Th-2EPT enables the formation of perovskite films with comparable crystallinity, albeit with smaller grains. Tin perovskite solar cells with a SAM made of Th-2EPT outperform control cells made with either PEDOT or MeO-2PACz. Th-2EPT results in an exceptionally good interface that minimizes recombination losses.
"We have demonstrated that the performance of tin perovskite photovoltaics can be significantly enhanced through targeted and rational molecular design", says Dr. Musiienko.
The new tin perovskite solar cells with Th-2EPT achieve an efficiency of 8.2%. These results lay the groundwork for further improvements to tin perovskite interfaces, paving the way for the development of pure tin perovskite tandem solar cells. "We prove that the higher performance stems from the excellent optoelectronic quality of perovskite grown on the novel SAM", says Valerio Stacchini, one of the first authors of the paper.
