A group of researchers led by the Indian Institute of Technology Roorkee has proposed to use hybrid heterojunction solar cells (HHSCs) as bottom devices in four-terminal (4T) perovskite-silicon tandem PV cells.
The scientists explained that HHSCs have a simple structure and low-temperature fabrication processes, while also showing “promising” efficiencies. “These HHSCs aim to eliminate up to 35 % of energy consumption during solar cell production and exploit the advantages of both functional layer and Si materials in a cost-effective approach,” they stated, noting that these cells are based on carrier-selective functional layers relying on either molybdenum oxide (MoOx), graphene, carbon nanotube (CNT), PEDOT:PSS, or poly(3-hexylthiophene) (P3HT).
For their new HHSC architecture, the research team chose PEDOT:PSS, which is a polymer known for its low cost and easy preparation properties. It was fabricated with a silver (Ag) metal contact, a textured n-type silicon (n-Si) absorber coated with PEDOT:PSS, and an indium gallium (In:Ga) interface. Its power conversion efficiency reached 10.92% and the fill factor was 66.04%.
The textured surface of the n-Si absorber is claimed to reduce surface reflectance while providing saw and damage-free smooth surface and increased junction area, which is crucial for high quality and efficient PEDOT:PSS/n-Si junction formation. “The addition of ethylene glycol in the PEDOT:PSS enhances its electrical conductivity and induces a strong inversion layer at PEDOT:PSS/n-Si interface near the n-Si surface,” the group said, noting that the cell was prepared using a solution processing technique.
The top perovskite device was constructed with a transparent fluorine-doped tin oxide (FTO) substrate, a hole transport layer (HTL) made of nickel(II) oxide (NiOx), a perovskite absorber, an electron transport layer (ETL) based on phenyl-C61-butyric acid methyl ester (PCBM) and molybdenum trioxide (MoO3), an Rh101 buffer layer, a silver (Ag) metal contact.
The academics explained that it gauged the energy bandgap and transparency of the top perovskite cell to increase light absorption and power conversion efficiency. The wide bandgap light absorber layer of the top cell was based on a perovskite material known as Cs0.1FA0.9PbI2Br. “The top perovskite sub-cell absorbs the short wavelength photons incident on the device and the remaining photons transmit through it and fall on the silicon bottom sub cell thereby maximizing the light absorption,” they added.
A detailed report about the new cell concept is available in the paper “Solution processable perovskite-hybrid heterojunction silicon 4T tandem solar cells,” which was recently published in materialstoday ADVANCES. The research team included academics from India’s CSIR-National Physical Laboratory.
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