In the photovoltaic solar power industry, crystalline silicon remains the dominant technology in the creation of solar cells. Silicon is considerably cheaper than the alternative, thin-film technology, and still achieves superior efficiencies, though the manufacturing process is more time-consuming and complex than the thin-film method. But while thin film is faster to make, it requires more expensive materials and still achieves considerably lower efficiencies than silicon.
In recent years, research has pursued a third method based on perovskite, a class calcium titanium oxide minerals. Perovskite solar cells are generally made from a hybrid organic-inorganic lead or tin halide-based material as the light-harvesting active layer. Perovskite is cheap and abundant, and solar cells created from it have the potential to achieve higher efficiencies even than silicon. When experiments first began with perovskite solar cells in 2009, efficiencies of 3.8 percent were achieved. Today, that figure stands at about 22.1 percent, and researchers believe the material could ultimately meet or exceed silicon’s high water mark of 30 percent efficiency. To date, the goal of perovskite solar cell research has been to find ways to improve production techniques, boost efficiency even further and solve perovskite’s biggest drawback, which is a faster decay rate than either crystalline silicon or thin-film.
Perovskite technology for solar cell applications is attractive because it has good tunability of bandgap, strong light absorption, and high defect tolerance. The rapid efficiency improvements in perovskite solar cells over the past several years have been achieved using the lab-scale spin coating technique in which a coating material is applied to the center of the substrate and then “spun off” using rotation. This method, however, isn’t scalable, so it’s unlikely to lead to mass production.
One of the perovskite method’s most interesting features, however, is that it can be prepared using layer-by-layer solution processing, which is scalable and requires lower capital expenditures for production equipment than spin coating. There is potential that ultimately, perovskites can be subjected to a scalable deposition method by processing it into something resembling “ink,” which could then be simply rolled onto panels to create high-performance and low-cost PV technology in high volume.
There’s still a wide gap in device performance between spin coating and scalable deposition methods, however, which has made it difficult for the latter process to reach suitability for mass production. The discrepancy between these techniques lies in the markedly different results achieved from their casting, drying, crystallization, conversion mechanisms. To date, the ink chemistry of perovskite for scalable deposition for solar PV cells has been underexplored.
Researchers at the National Renewable Energy Lab (NREL) recently achieved significant progress with scalable deposition of perovskite material using blade coating onto a substrate. The team used a general chlorine-containing methylammonium lead iodide precursor perovskite formulation, along with solvent tuning, to produce a new solution-ink chemistry.
|Solar cells grown using perovskite ink. Photo courtesy of NREL.|
With previous work using this method, researchers were able to forms photovoltaic crystals easily, but long drying times under high temperatures made it impractical for large-scale production. The new research by