New passivation strategy paves the way for lead-free perovskite-silicon tandem solar cells

Researchers at the University of Electro-Communication in Japan have developed a tin perovskite solar cell with a bandgap wider than 1.6 eV that may be used as a top layer in lead-free perovskite-silicon tandem solar cells.

The scientists acknowledged that replacing lead (Pb) with tin (Sn) in perovskite solar cells implies a considerable reduction in power conversion efficiency and explained that this depends on the presence of tin(IV)-sulfide (Sn⁴⁺), which is formed by the oxidation of Sn during the perovskite ink preparation, keeping perovskite inks, or baking the perovskite film.

In order to reduce the presence of Sn⁴⁺ concentrated on the surface of the perovskite film, the research group adopted a new passivation strategy based on the use of phenylsilane (PhSiH3) as a reducing agent.

“As far as we know, there is no report on the surface passivation with organic solutions consisting of reducing agents,” it explained, noting that using PhSiH3 results in the formation of a siloxane layer, which creates better contact between the hydrophilic tin perovskite absorber and hydrophobic buckminsterfullerene (C₆₀) used for the electron transport layer (ETL). “After the surface passivation, the contact angle became larger, demonstrating that the perovskite surface became more hydrophobic.”

The academics built the cell with a substrate made of glass and a fluorine-doped tin oxide (FTO), a hole transport layer (HTL) made with PEDOT:PSS, a polymer known for its low cost and easy preparation properties, an absorber made of a lead-free perovskite material known as ASnI₂Br, an ETL based on C₆₀, a bathocuproine (BCP) buffer layer, and a silver (Ag) metal contact.

The scientists compared the performance of the solar cell with that of a reference device that did not go through the novel passivation treatment. They found the latter achieved an efficiency of 3.65%, while the treated device reached 5.50%, which they said proves the effectiveness of the PhSiH₃ treatment.

The results are explained by faster photoluminescence decay after the surface treatment, lower series resistance and higher charge recombination resistance after the surface treatment, they noted. “It was proved that PhSiH₃ is effective for decreasing Sn⁴⁺ accumulated on the perovskite films and for achieving better contact of the perovskite layer with C60 layer,” they concluded.

The solar cell design and the passivation techniques are described in the paper “Efficiency-enhancement of lead-free ASnI₂Br perovskite solar cells by phenyltrihydrosilane passivation effective for Sn⁴⁺ reduction and hydrophobization,” published in Next Materials.