The poor stability of tin-based perovskites under ambient conditions has limited the progress of this promising type of solar cell. Now, a study by researchers at Imperial College London and the University of Bath has shed light on the degradation mechanisms that affect these devices – oxygen and moisture induce the formation of iodine, an aggressive species that further poisons the perovskites. In addition, the researchers identified materials that prevent oxidation and could inspire new solar cell designs that are more environmentally friendly and stable.1
Solar cells based on lead perovskites have exceeded expectations in terms of efficiency. However, chemists move away from this metal because of its toxicity. The element that lives on lead in the periodic table – tin – has proven to be a promising alternative that has the highest power conversion efficiency of the lead-free alternatives. In addition, studies show that while tin is toxic, it has a lower bioavailability. In the event of a leak, tin is likely to oxidize to insoluble products and the biological effects would be minimal.2
“This tendency towards slight oxidation is, however, a double-edged sword,” explains first author Luis Lanzetta from Imperial College London. “Stability is the main problem with tin-based perovskites, largely due to the oxidation reactions that take place when the materials are exposed to air and moisture.” The mechanism of these degradation processes was only partially known – now researchers have combined X-ray diffraction, spectroscopy and computational calculations to fill the gaps.
They soon found a suspect – iodine. “This element normally occurs as a counterion in tin perovskites,” explains Lanzetta. Oxygen and water break down tin (II) to inactive tin (IV) salts, but are also responsible for the formation of undesirable iodine compounds such as hydrogen iodide and molecular iodine. “We discovered that this species further triggers the oxidation of tin (II) to tin (IV) salts and feeds a vicious circle that further breaks down the perovskite,” he says.
“The stabilization of tin (II) in perovskites remains a challenge due to the high stability of the tin (IV) species,” explains Maria Antonietta Loi, an expert on tin-based perovskites at the University of Groningen, Netherlands. “This paper clarifies the role of iodine in this process and offers a very clear way of explaining the degradation reactions – this information is very valuable for future designs.”
In addition to uncovering the dark side of iodine, the researchers also realized the importance of choosing the right material to support the perovskite. “Perovskite solar cells have a sandwich-like structure,” explains Saif Haque from Imperial College London, who led the study. “We have conductive glass, a hole transporter, the perovskite, an electron transporter and then a metallic contact.” The team discovered that the hole transporter – a material that carries positive charges – is key to preventing degradation.
They compared several commonly used hole transporters – nickel oxide, copper thiocyanate and PEDOT: PPS, a polymer mixture – and found that the polymers provide excellent protection for tin against oxidation. “This suggests that further investigation into the effects of some of the ingredients in the sandwich could result in more efficient and stable tin-perovskite solar cells,” added Haque. “Everyone uses PEDOT: PPS as a hole transport material, it works very well with tin solar cells,” explains Loi. “This research underscores the importance of the interface layers, and perhaps others will look for better materials for both hole and electron transport,” she adds.
Loi believes the real breakthrough is in unraveling the harmful properties of iodine. “When cells are made, the perovskite is usually heated, which releases halogens into the environment, which can potentially contaminate the device,” she says. The team discovered that as little as 350 ppm iodine triggered the breakdown.
“Tin perovskites are becoming increasingly popular,” says Haque. “We hope that these new discoveries will gain even more recognition and that more people will adopt them as a green, efficient alternative to lead.”