University of Groningen scientists are investigating in situ how lead-free perovskite crystals form and how the crystal structure affects the functioning of the solar cells, as part of their quest to find alternatives to lead-based perovskites.

The best results in solar cells have been obtained using perovskites with lead as the central cation. As this metal is toxic, tin-based alternatives have been developed, for example, formamidinium tin iodide (FASnI₃). This is a promising material; however, it lacks the stability of some of the lead-based materials. Attempts have been made to mix the 3D FASnI₃ crystals with layered materials, containing the organic cation phenylethylammonium (PEA). "My colleague, Professor Maria Loi, and her research team showed that adding a small amount of this PEA produces a more stable and efficient material," says Assistant Professor Giuseppe Portale.

Researchers at the Pohang University of Science & Technology (POSTECH) have developed eco-friendly-solvent processable hole transport polymers by using peppermint oil and walnut aroma food additives and the polymer can prevent lead leakage.

The POSTECH research team consisted of Prof. Taiho Park and Junwoo Lee, that developed Alkoxy-PTEG - hole transport polymers that could be dissolved in peppermint oil, by applying ethylene glycol side chains when producing perovskite solar cells. Also, the team confirmed that this polymer captured leaking lead in aging perovskite solar cells.

Researchers at Northern Illinois University and the U.S. Department of Energy's (DOE) National Renewable Energy Laboratory (NREL) in Colorado have reported on a potential breakthrough in the development of hybrid perovskite solar cells.

Led by Tao Xu of NIU and Kai Zhu of NREL, the scientists have developed a technique to sequester the lead used to make perovskite solar cells and minimize potential toxic leakage by applying lead-absorbing films to the front and back of the solar cell.

Researchers led by Prof. Antonio Abate at the Helmholtz-Zentrum Berlin have designed a study to investigate lead hazards relating to perovskite soar cells. They cooperated with plant scientists from the Fujian Agriculture and Forestry University, China, where the experiments were carried out, and with a group from the university of Naples, Italy.

Mint plants grown on control soil (left) and perovskite-contaminated soil (right). Credit: Nature

The plant experts prepared contaminated soil samples with different concentrations of lead from either perovskite solar cells or other lead sources and cultivated different plants. After a growth period, they analyzed the lead content in leaves and other parts of the plant. They found that lead from perovskite solar cells is 10 times more bioavailable than lead from other industrial sources.

Chinese manufacturer GCL recently indicated that it is nearing commercialization of perovskite solar technology. 'Once the conversion rate [of] perovskite is close to what monocrystalline product does, which is coming soon, the only obstacle for perovskite to take [the] place of mono is the limitation of its production capacity,' GCL Nano Technology general manager Fan Bin said at a recent industry conference which considered the potential of perovskite.

Discussing GCL's work with perovskites, Fan said his company's lab has achieved a conversion efficiency of 16% on a large panel and he is confident 18% could be achieved by the end of the year. With a theoretical conversion limit of around 33% thought to apply to perovskite cell efficiency ' and possibly up to 47% for a tandem device ' the manager voiced confidence perovskites would soon surpass the 18% threshold.

Researchers from the University of Surrey's Advanced Technology Institute (ATI) have produced a perovskite solar cell which contains 50% less lead, replaced with the more innocuous tin. By fine-tuning their tin solar cell, the researchers were able to create a product that is able to absorb infrared light in a similar manner as silicon cells. They also found that by stacking lead-only cells with the ones mixed with tin can lead to power conversion results that outperform those of silicon-only power cells.

Indrachapa Bandara, lead author of the study and PhD student at ATI, said: 'We are starting to see that many countries are treating the threat of climate change with the seriousness it deserves. If we are to get a handle on the problem and put the health of our planet on the right track, we need high-performing renewable energy solutions.... Our study has shown that tin based perovskite solar cells have an incredible amount of potential and could help countries such as the United Kingdom reach its target of becoming carbon neutral by 2050'.

Researchers from the Okinawa Institute of Science and Technology Graduate University (OIST) have found that a protective layer of epoxy resin helps prevent the leakage of pollutants from perovskite solar cells (PSCs). Adding a 'self-healing' polymer to the top of a PSC can drastically reduce how much lead it discharges into the environment. This may give a boost to prospects for commercializing the technology.

'Although PSCs are efficient at converting sunlight into electricity at an affordable cost, the fact that they contain lead raises considerable environmental concern,' explains Professor Yabing Qi, head of the Energy Materials and Surface Sciences Unit, who led the study. "While so-called 'lead-free' technology is worth exploring, it has not yet achieved efficiency and stability comparable to lead-based approaches. Finding ways of using lead in PSCs while keeping it from leaking into the environment, therefore, is a crucial step for commercialization.'

Researchers at Huazhong University of Science and Technology (HUST) in China, University of Toledo in the U.S, Monash University in Australia, Jilin University and Tsinghua University in China, the Dalian Institute in China and the University of Toronto in Canada have examined a lead-free double perovskite that exhibited stable and efficient white light emission. In its mechanism of action, the material produced self-trapped excitons (STEs) due to Jahn-Teller distortion of the AgCl₆ octahedron in the excited state of the complex, observed when investigating exciton-phonon coupling in the crystal lattice.

The research team stated that a fifth of global electricity consumption is based on lighting, and efficient and stable white-light emission with single materials is ideal for such applications. Photon emission that covers the entire visible spectrum is, however, difficult to attain with a single material. Metal halide perovskites, for instance, have outstanding emission properties but contain lead, and so yield unsatisfactory stability. The perovskite in this study is, therefore, lead-free.

A KAIST research team has proposed a perovskite material, Cs2Au2I6 that serves as a potential active material for highly efficient lead-free thin-film photovoltaic devices. This material is expected to lay the foundation to overcome previously known limitations of perovskite including its stability and toxicity issues.

The joint team led by Professor Hyungjun Kim from the KAIST Department of Chemistry and Professor Min Seok Jang from the School of Electrical Engineering analyzed a previously discovered perovskite material, Cs2Au2I6, consisting of only inorganic substances and investigated its suitability for application in thin-film photovoltaic devices. Theoretical investigations suggests that this new perovskite material is not only as efficient but also more stable and environment friendly compared to the conventional perovskite materials.

A research team at the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences has fabricated a sensitive photodetector based on lead-free perovskite single crystals.

"We have developed a high performance photodetector based on MA3Sb2I9 microsingle crystals (MSCs)," said Prof. HAN. Scientists found that MA3Sb2I9 single crystals exhibited a low trap-state density of ~1010 cm-3, high carrier mobility of 12.8 cm2 V-1 s-1 and long carrier diffusion length reaching 3.0 μm.