In January 2020, Germany's Helmholtz Zentrum Berlin (HZB) researchers claimed a record efficiency level of 29.15% for a perovskite and silicon tandem solar cell hailed as the highest ever efficiency level achieved for these cells (see Perovskite Tandem Cell 29.15% Efficiency Record).
Buoyed by the success, the team is confident of exceeding 30% efficiency after it analyzed the two sub-cells individually in the HySPRINT laboratory and at the PVcomB at the HZB and calculated a maximum possible efficiency of 32.4%, which can be achieved with exactly this structure.
Back then they did not reveal much, but now in their research work published in the scientific journal Science, the team has discussed at length how it was able to achieve the 'current world record' of 29.15% in a tandem solar cell. It is now at the top of the list of emerging PV category in the National Renewable Energy Laboratory (NREL) chart, HZB claims. HZB claimed the top spot from Oxford PV, which published news about a 28% perovskite and silicon tandem solar cell end of 2018 (see 28% Efficiency For Oxford PV Perovskite Cell).
Headed by Prof. Steve Albrecht, HZB researchers used silicon and perovskite in the tandem cell arrangement bringing together a complex perovskite composition with a 1.68 eV band gap and focused on optimizing the substrate interface. They deployed an intermediate layer of organic molecules that arrange themselves to form a self-assembled monolayer or SAM. This SAM is applied to the electrode to improve the flow of charge carriers. The team also used what it calls a novel carbazole-based molecule with methyl groups (Me-4PACz).
"We first set up the perfect bed, so to speak, on which the perovskite layer lies," said lead author of the study, Amran Al-Ashouri.
Once all was in, the team analyzed the different processes at the interfaces between perovskite, SAM and the electrode. "We have optimized the so-called fill factor in particular, which is influenced by how many charge carriers make their way out of the perovskite sub-cell get lost," said Al-Ashouri. "While the electrons in the direction of sunlight through the C 60Layer, the 'holes' must move in the opposite direction and flow through the SAM layer into the electrode. However, we saw that holes are extracted much more slowly than electrons, which limited the fill factor."
They found the SAM layer helps considerably with removal while contributing to better stability.
The team comprised other partners including teams from Kaunas University of Technology, Lithuania, the University of Ljublana, Slovenia, the University of Sheffield, UK, the University of Potsdam and the Physikalisch-Technische Bundesanstalt (PTB), the Berlin University of Applied Sciences and Technical University of Berlin.
The detailed research work titled Over 29% – efficient Monolithic Perovskite / Silicon Tandem Solar Cell Enabled by Enhanced Hole Extraction can be read online on Science website.