Using halide perovskites for photovoltaic (PV) solar-energy cells could dramatically reduce production costs compared to silicon-based PV cells, because perovskites are solution-processible at relatively low temperatures. However, long-term stability of these materials has been a significant challenge. Now, a research team at Rice University (Houston; www.rice.edu), has developed a method for synthesizing the perovskite formamidinium lead iodide (FAPbI3) that imparts structural stability while also retaining high efficiency in converting solar energy to electricity.
As crystals of FAPbI3 form from solution, the material can adopt multiple crystal structures: one, the so-called black phase, is highly photoactive, but not thermodynamically favored; another (yellow phase) has more structural stability, but is photoactively undesirable.
The Rice researchers have been working on ways to improve the quality and durability of the perovskite materials to move them closer to market readiness. In this project, they “seasoned” the precursor FAPbI3 solution with specially designed two-dimensional perovskites (photo) that served as a stable structural template for the crystal lattice of the bulk material. The 2-D template kinetically “traps” the photoactive black phase as the material crystallizes, creating phase-pure films of FAPbI 3. Also, the 2-D “seed” template imparts compressive strain to the black phase, increasing its stability. The resulting FAPbI3 films showed high (24.1%) power conversion efficiency, while also exhibiting what the researchers called “exceptional durability” — maintaining 97% of that efficiency for 1,000 hours at 85°C.
“Right now, we think that this is state of the art in terms of stability,” says Rice engineering professor Aditya Mohite. “FAPbI3 films templated with 2-D crystals were of higher quality, showing less internal disorder and exhibiting a stronger response to illumination, which translated as higher efficiency,” explains Isaac Metcalf, a Rice graduate student and one of the first authors on the study, which was published in a recent issue of the journal Science.