The low cost, reduced weight, and non-toxicity of the cells make them ideal for integration into vehicles or mobile devices, and the new, streamlined way they are created paves the way for their large-scale production.
Conventional silicon-based solar cells are very efficient at generating electricity from sunlight. However, their manufacture is an expensive and energy-intensive process, and the resulting devices are heavy and extremely bulky. Thin-film solar cells are, in some ways, a great alternative – but they often contain toxic elements such as lead, cadmium, or scarce and expensive elements like indium or tellurium.
In the mid-2010s, another alternative emerged when researchers at the Institute of Photonic Sciences (ICFO) in Spain developed a low-cost, non-toxic cell based on AgBiS 2. These nanocrystals can be fabricated into a solar cell as thin as 35 nm using a layer-by-layer deposition process. Their efficiency is about 6% compared to 25% silicon, the material of which was found to be commercially uncompetitive.
Cation disorder engineering
To increase the optical absorption of cell-based AgBiS 2 , researchers at ICFO, together with collaborators from University College and Imperial College in the UK, investigated the effect of disordered positive ions (cations) on the material’s optoelectronic properties. After finding evidence of inhomogeneities due to regions rich in Ag or Bi that form inside the nanocrystals, the researchers used density functional theory calculations to determine the effects of these inhomogeneities. Based on these calculations, they came to the somewhat controversial conclusion that careful placement of defects in the crystal lattice—a technique they call “cation disorder engineering”—results in a more uniform distribution of cations, which promotes ion migration. They then used a process called low temperature annealing to produce AgBiS2 samples with the specified characteristics.
When the researchers placed cells made from the optimized material under artificial sunlight, they recorded energy conversion efficiency in excess of 9%, a record for this type of ultrathin solar cell. They also observed absorption in a wide spectral range, from ultraviolet (400 nm) to infrared (1000 nm). Their device, which they fabricated on glass/indium tin oxide and coated with a polytriarylamine solution, is no more than 100 nm thick, making it 10 to 50 times thinner than current thin-film photovoltaic (PV) technologies. and 1000 times thinner than PV silicon.
ICFO physicist Gerasimos Constantatos, who led the study and co-authored the paper in Nature Photonics, said the team’s work demonstrates for the first time how changing the order of atoms in a material affects its optoelectronic properties. This type of materials science may also prove useful in other areas such as catalysis. Constantatos noted that the team’s method meets many of the requirements of the photovoltaic industry, including low cost, scalability, and the use of non-toxic cells.