Researchers from the University of Cambridge (UK) have shown how bismuth could present a non-toxic alternative to lead in next-generation, low-cost solar cells.
Only a few years into the exploration of lead-based hybrid halide perovskites in laboratories around the world, the remarkable properties of these materials have researchers abuzz with hope for a new generation of cheap and easy-to-make, high-performance solar cells. But the controversial lead content could pose a barrier to their global commercial deployment. “It is scientifically and technologically important to find alternatives to the perovskites based on non-toxic elements,” thus notes Dr Robert Hoye of Cambridge's Cavendish Laboratory and Department of Materials Science & Metallurgy. Hoye is also the lead author of the paper “Strongly Enhanced Photovoltaic Performance and Defect Physics of Air-Stable Bismuth Oxyiodide (BiOI),” published in Advanced Materials.
One approach has been to identify and better understand why these perovskites enable such promising device performances and to aim to replicate those properties in lead-free materials. “A key property that scientists have recognized as important is the ability of perovskites to tolerate defects,” Hoye points out. “That is, you can grow perovskites by cheap methods that lead to a defective material, but these defects do not prevent the material from achieving high efficiencies.” The expert notes that a recent computational search through 27000 different materials has identified compounds that could replicate the electronic structure of the perovskites that may lead to defect tolerance.
Among these compounds is bismuth oxyiodide.
Bismuth oxyiodide had previously been dismissed as a photovoltaic candidate because it proved unstable in certain electrolytes and initial efficiencies were low. Computations, though, predicted that bismuth oxyiodide should have been defect tolerant. So Hoye and his colleagues revisited the compound through a rigorous set of experiments and calculations. They found that not only was this material stable in air for at least 197 daysbut that it was also defect tolerant.
The solid-state photovoltaic device the team made by sandwiching the bismuth oxyiodide light absorber between two oxide electrodes achieved double the conversion efficiency previously reported. “We analyzed the losses in our devices using optical and electrical methods as well as modeling and found that improvements in efficiency can readily be obtained,” Hoye reports. “Our work shows that bismuth oxyiodide is a promising material for photovoltaics.”
Another research group calculated that based on the optical properties the theoretical limit for this material is 22%, which is comparable to silicon and perovskite solar cells. “There is a lot more we can get out from this material,” Hoye adds.
Designing a new generation of solar cells
While scientists searching for lead-free alternatives to the perovskites have identified, grown and characterized many new compounds, photovoltaic efficiencies have been low. “This has raised the question of whether the new design criteria based on defect tolerance are flawed, or whether there has not been enough time to optimize efficiencies,” Hoye says. “Our work shows the design criteria do work, and we also show that one of the materials identified can be an efficient device.”
The discovery, the expert hopes, will provide scientists with a non-toxic alternative to lead-based materials and motivate them to press on in the search for other efficient, non-toxic materials.
“Bismuth is an element next to lead on the periodic table,” Hoye says. “It has displayed very little evidence of toxicity and is commonly found in medicines, solders, fireworks and cosmetics.” A recent article in Nature Chemistry is titled "Green bismuth."
Hoye emphasizes that bismuth, though, exhibits many of the electronic properties of lead responsible for the defect-tolerant electronic structure of the hybrid lead halide perovskites. Furthermore, bismuth remains stable in the desired oxidation state for making defect tolerant compounds. Many of the new bismuth-based materials have been found to be air-stable.
Performance of solar cells based on bismuth vs lead
“Our bismuth oxyiodide solar cells convert photons of certain wavelengths to electrical charge with an efficiency of up to 80%,” reports Hoye, referring to the external quantum efficiency. “An external quantum efficiency of 80% exceeds previous reports of photovoltaics made using bismuth-based materials and is catching up to the performance of solar cells based on lead.”
Impact on the future of perovskite solar cells
The expert says there has been a growing momentum in the perovskites community to find lead-free alternatives that are stable and efficient. “Our work shows that bismuth oxyiodide is a viable alternative that is stable in air, defect tolerant and efficient, with a lot of potential for future improvements,” he says. “We hope that our work will inspire perovskites groups to look beyond chemical substitutions into the perovskite crystal structure and look towards materials that do not necessarily have the perovskite crystal structure but rather can replicate its electronic properties.”
Challenge with bismuth-based solar cell
Hoye shares that achieving a continuous inorganic device stack was particularly challenging but is important to avoid device performance losses due to shunting. The Cambridge team grows bismuth oxyiodide by chemical vapour transport inside a tube furnace. They report they screened through several substrates before finding a material with a favorable surface energy to achieve continuous films.
Results of the team´s loss analysis have shown that improving the collection of carriers could improve the efficiency. “To do this, we will engineer the oxide electrodes to tune the energy level alignment in our device stack,” Hoye projects. “We will also engineer the device structure to achieve an optimal distribution of light in our solar cells.”
Written by Sandra Henderson, Research Editor, Solar Novus Today