Inverted perovskite solar cell based on synergistic bimolecular interlayer achieves 25.53% efficiency

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A group of scientists led by the Fudan University in China has developed an inverted perovskite solar cell that utilizes a synergistic bimolecular interlayer (SBI) and reportedly achieves the smallest nonradiative recombination induced open-circuit voltage loss reported to date

Inverted perovskite cells have a device structure known as “p-i-n”, in which hole-selective contact p is at the bottom of intrinsic perovskite layer i with electron transport layer n at the top. Conventional halide perovskite cells have the same structure but reversed – a “n-i-p” layout. In n-i-p architecture, the solar cell is illuminated through the electron-transport layer (ETL) side; in the p-i-n structure, it is illuminated through the hole‐transport layer (HTL) surface.

The researchers' SBI strategy consisted of depositing 4-methoxyphenylphosphonic acid (MPA) and 2-phenylethylammonium iodide (PEAI) as modulators to functionalize the perovskite surface.

“MPA induces an in-situ chemical reaction at the perovskite surface via forming strong P-O-Pb covalent bonds that diminish the surface defect density and upshift the surface Fermi level,” they explained. “PEAI further creates an additional negative surface dipole so that a more n-type perovskite surface is constructed, which enhances electron extraction at the top interface.”

They also stressed that the proposed strategy does not affect the perovskite surface morphology, crystallinity, or optical absorption properties, while also contributing to a more effective passivation of defects.

The scientists built the cell with a substrate made of fluorine-doped tin oxide (FTO), a hole transport layer (HTL) made of phosphonic acid called methyl-substituted carbazole (Me-4PACz), a perovskite absorber, the SBI layer, a buckminsterfullerene (C60) electron transport layer (ETL) relying on phenyl-C61-butyric acid methyl ester (PCBM), a bathocuproine (BCP) buffer layer, and a silver (Ag) metal contact.

Using ultraviolet photoelectron spectroscopy (UPS), the team also found that the perovskite surface can pin to the negative polaron transport state of PCBM ETL, thus further promoting electron transfer across the perovskite/ETL interface.

“Moreover, the SBI-modified perovskite film displays a smaller surface potential distribution difference and lower surface roughness,” said the team. “A smoother perovskite surface with a more uniform surface potential distribution is beneficial for forming an efficient contact with the adjacent ETL that prevents nonradiative recombination.”

Tested under standard illumination conditions, the solar cell achieved an efficiency of up to 25.53% and a short-circuit current density of 24.31 mA cm2. It also obtained one of the smallest nonradiative recombination-induced open-circuit voltage losses of “only 59 mV” and a certified efficiency of 25.05%. “In addition, the target device also features good stability, retaining 95% of its initial efficiency for aging over 1000 h,” the academics added.

They described the new cell concept in the study “Reducing nonradiative recombination for highly efficient inverted perovskite solar cells via a synergistic bimolecular interface,” which was recently published in nature communications. “These results well demonstrate the significant roles of our SBI strategy on perovskite surface properties, which show significant effectiveness in minimizing trap density and constructing beneficial perovskite surface energetics, and pave ways for the further improvement of perovskite solar cells,” the scientists concluded.

 

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