An international research team led by the Gwangju Institute of Science and Technology (GIST) in South Korea developed a passivation process for formamidinium lead iodide (FAPbI3) perovskite films, which it demonstrated in solar cells with certified 23.69% power conversion efficiency, and modules with 21.44% certified efficiency.
As for stability, the hetero-polytypic passivated devices with an additional low-dimensional perovskite (LDP) layer reportedly maintained 92% of their initial efficiency after 1,000 h compared to 63% in an untreated reference device. And the 6H polytype perovskite devices without the LDP maintained 81%.
Their work was described in “Shallow-level defect passivation by 6H perovskite polytype for highly efficient and stable perovskite solar cells,” published by nature communications.
“A typical approach so far has been to introduce an external chemical reagent to deal with the defect problem. However, bringing in external reagents could directly impact the crystalline quality of the perovskite during crystal growth, so our work does not rely on such stabilizers,” corresponding author Hobeom Kim of Gwangju Institute of Science and Technology (GIST), South Korea, said in a statement. “Instead, we employ a chemically identical polytype of perovskite, hexagonal polytype perovskite (6H) containing a corner-sharing component that effectively suppresses the formation of defects in perovskite.”
To create the component, the team used an excess of lead iodide and methylammonium chloride that intervened with the dominant defect site. Specifically, they inserted 6H into the cubic polytype (3C) of polycrystalline in FAPbI3. The process reportedly improved the structural integrity and carrier dynamics of FAPbI3, resulting in an “ultralong” carrier lifetime of greater than 18 microseconds.
The defect passivation involved the deposition of a surface passivation layer of octylammonium iodide on top of the 3 C/6H hetero-polytypic perovskite film to induce the formation of a low-dimensional perovskite (LDP) passivation layer.
The fabricated cells were made with an n-i-p structure. They had a fluorine-doped tin oxide substrate, a titanium dioxide electron transport layer, a mesoporous titanium dioxide/potassium chloride layer, a perovskite absorber, a spiro-OMeTAD hole transport layer, and a gold metal contact.
The control device had an efficiency of 20.32% while the 6H bridged hetero-polytypic polycrystalline film enabled a device with 22.35%. Further efficiency improvement was achieved via an LDP passivation layer on top of the 6H bridged perovskite film, resulting in a cell with 24.13% efficiency, an open-circuit voltage of 1.156 V, short-circuit current density of 25.58 mA cm−2, and fill factor of 81.60%.
The best cell certified by the Korea Institute of Energy Research had an efficiency of 23.69%.
The team also fabricated 6.5 cm × 7.0 cm modules in a low-temperature process with eight LDP passivated 3 C/6H hetero-polytypic perovskite cells with P1, P2, and P3 interconnection. The best-performing module was certified by Newport Photovoltaic Testing and Calibration Laboratory with a 21.44% efficiency.
Besides the team from S. Korea-based GIST, the study included researchers from Korea Basic Science Institute (KBSI) and Korean Research Institute of Chemical Technology (KRICT), along with Japan’s Toin University of Yokohama, Lomonosov Moscow State University in Russia, École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland, and King Abdulaziz University, Saudi Arabia.
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