While solar researchers had been adopting lead-halide perovskites into solar cell designs in recent years for the low cost, processability and power conversion efficiencies above 20%, scientist Javier Vela at Ames Lab has been focusing on organolead mixed-halide perovskites. We spoke with him about his recent findings.
“Through the use of 207Pb ssNMR, we discovered the presence of persistent dopants and phase segregation in organolead mixed-halide perovskites that are in agreement with calculated miscibility gaps and spontaneous spinodal decomposition,” Vela says.
In his work, Vela has aimed to better understand mixed-halide perovskites because he says these materials are promising for a variety of reasons, including increased moisture stability, visible range tunability, and their potential for tandem solar cells, colored solar cells, and light emitting diodes (LED’s).
Vela, who is also an associate professor of chemistry at Iowa State University (ISU), worked with scientists with expertise in solid-state nuclear magnetic resonance (NMR) at both Ames Laboratory and ISU — and gained tremendous new knowledge from their interdisciplinary inquiry. An analytical chemistry technique, NMR provided the team with insights about how the chemistry, composition and structure of these materials can affect their behavior. “The observation of dopants and phases segregation was only possible through collaboration with Professor Aaron Rossini, Michael Hanrahan, and Sarah Cady, who are experts in ssNMR, because these phases are not observed by widely used optical measurements and x-ray diffraction,” Vela reports.
The experiments also revealed that solid state synthesis is far superior to solution-phase synthesis in making mixed-halide perovskites. “Iodide-based organolead halide perovskites are more desirable for photovoltaics because of their ideal band gap,” says Vela, whose work revealed that mixed-halide perovskites that contain both iodide and bromide are highly dependent on the specific synthesis used. “We found that solution phase synthesis results in semicrystalline phases that are not observed by solid phase synthesis. Thus, solid phase synthesis results in a more homogeneous structure that will be better for photovoltaics.”
The disruptive research coming out of Ames Lab could lead to advances in the design of the next generation of solar cells and LEDs. “Previously, organolead mixed-halide perovskites were believed to be perfect alloys, but we now know these structures contain multiple phases that lead to an inhomogenous film, which should directly impact the film’s electronic properties and, by extension, device performance,” Vela says, further nothing that this is especially important because perovskite solar cells are transitioning towards hybrid structures composed of multiple cations and halides that allow researchers to fine-tune film properties, such as Csx([CH3NH3]0.17[HC(NH2)2]0.83)1-xPb(I0.83Br0.17)3. “Therefore, further increase in the efficiency of perovskite solar cells will depend on studying how, and to what extent, these additional phases impact the electronic properties of the film,” he concludes.
What is next for Vela and his team? “First, we plan to further probe the presence of dopants and phase segregation in organolead mixed-halide perovskites with additional advanced spectroscopic techniques through our Ames Lab Department of Energy funded collaboration with ISU Professors Emily Smith and Jacob Petrich,” he shares. “In addition, we plan to develop more advanced ssNMR methods based on dynamic nuclear polarization (DNP) ssNMR through collaboration with Professor Aaron Rossini that will allow faster data acquisition and the ability to study perovskite films by ssNMR.”
Written by Sandra Henderson, Research Editor, Novus Light Technologies Today