Perovskite-HJT tandem solar cell based on phosphonic acid, self-assembled monolayer achieves 30.22% efficiency
Scientists from the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have fabricated a tandem solar cell based on a perovskite top cell and a heterojunction (HJT) bottom device with a thickness of 100–150 μm and double-sided micro-textured surfaces.
“The concept of double-sided textured bottom cells is a key adaptation in the design of the current champion devices, such as those from the King Abdullah University of Science and Technology (KAUST), which achieve a power conversion efficiency of 33.7%, and Longi, which holds the world record with an outstanding efficiency of 33.9% and recently 34.6% on Cz-silicon (Cz-Si) bottom cells,” they stated.
The research team explained that the HJT cell was fabricated by wet-etching random pyramids, with their height being adjusted by alkaline texturing with no current loss in the cell.
“To achieve high-performance shunt-free perovskite/silicon tandem solar cells with solution-processed with high yield top cells, the perovskite absorber thickness must be thicker than the pyramid texture height,” it stressed. “Since high-quality wide band gap perovskite absorbers are typically 600–800 nm thick and do not usually conformally cover the surface when processed from solution, the pyramid height must be adjusted accordingly.”
The scientists built the top cell with a hole transport layer (HTL) with a self-assembled monolayer (SAM) based on [2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz), a perovskite absorber, an electron transport layer (ETL) made of tin oxide (SnO2) and buckminsterfullerene (C60), a transparent back contact made of indium zinc oxide (IZO), and a silver (Ag) metal contact.
The bottom cell was fabricated with an a-Si:H(i)/nc-SiOx:H(n) layer stack on the front side and an a-Si:H(i)/a-Si:H(p) layer stack on the rear side deposited using plasma-enhanced chemical vapor deposition (PECVD). “On the rear a layer stack of transparent conducting oxide was sputtered by DC sputtering and 20 nm on the front, defining the cell area of around 1 cm2 by aligned sputter masks on both sides.
Tested under standard illumination conditions, the tandem cell achieved an efficiency of 30.22%, an open-circuit voltage of 1.954 V, a short-circuit current density of 18.92 mA/cm2, and a fill factor of 81.74%.
“These results demonstrate the potential of submicron-sized textured Cz-silicon bottom cells for industry-compatible tandem cells with solution-processed perovskite top cells to achieve high efficiencies,” the scientists noted. “Furthermore, our work demonstrates a simple, implementable solution to circumvent the film formation issue associated with Me-4PACz while maintaining a high charge extraction rate and good passivation properties.”
The new cell concept was described in “Perovskite/Silicon Tandem Solar Cells Above 30% Conversion Efficiency on Submicron-Sized Textured Czochralski-Silicon Bottom Cells with Improved Hole-Transport Layers,” published in ACS Applied Materials & Interfaces.
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