Scientists led by Saudi Arabia’s King Abdullah University of Science and Technology (KAUST) have achieved a power conversion efficiency of 28.2% for a tandem solar cell with an area of around 1 cm2, based on an n-i-p perovskite stacked on top of a silicon heterojunction. The remarkable performance was achieved through a novel strategy to suppress halide segregation and reduce nonradiative recombination loss, which consists of using carbazole, a nitrogen-containing heterocyclic molecule, as an additive for the cell perovskite precursor.
“The cell could be used for solar panels for large-scale applications, space application, and also other applications that now mainstream silicon can cover,” researcher Jiang Liu told pv magazine. “The cell technology could be easily transferable to commercial production, as Oxford PV and other Chinese manufacturers have shown.”
The researchers claim the molecule is able to passivate deep charge traps and suppress phase segregation in wide-band-gap perovskites, such as that used for their tandem cell with a bandgap of 1.68 eV.
“We find that this organic nitrogen-containing heterocyclic molecule interacts with halide anions through hydrogen bonds, immobilizes halide species, and passivates trap states,” they said. “Specifically, this happens at the grain boundaries because the carbazole molecules, with a relatively large molecular radius, cannot be incorporated into the perovskite lattice.”
The additive was found to prevent the migration of ions in the perovskite film even under humid conditions – one of the main causes for halide segregation.
“Our results show that the illumination intensity, the environmental atmosphere, and trap passivation all alter the dynamics of halide segregation,” the academics said. “In the case of superposition of multiple stressors, halide segregation occurs faster, but the addition of carbazole minimizes this behavior.”
The single-junction perovskite sub-cell was built on a substrate made of glass and an indium tin oxide (ITO) layer, a hole transport layer (HTL) made of the 2PACz carbazole additive, the perovskite layer, lithium fluoride (LiF), a bathocuproine (BCP) buffer layer, and a silver (Ag) metal contact.
The champion solar cell developed by the Saudi group achieved an efficiency of 28.6% for a device with a 1cm2 surface and 27.1% for an area over 3.8 cm.
“We verified our in-house measurements with 28.2% stabilized power output certified by an accredited laboratory, the Japan Electrical Safety & Environment Technology Laboratories,” the scientists said. “Our tandem device is commensurate with the state-of-the-art tandem devices in the literature and represents the record-certified PCE for double-textured perovskite/Si tandem devices.”
The KAUST researchers also conducted a stability test on the cell. They found it could retain around 93% of its original performance over 43 days in hot and humid outdoor environments. The team published all of the technical details in a recent report that was published in Joule. In July, the team also demonstrated a perovskite-silicon tandem cell that reached a 27% conversion efficiency rating.
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