Solar cell designers know that their creations have to cope with a wide range of temperatures and all sorts of weather conditions – conditions that can affect their efficiency and lifespan.
Lea Nienhaus, Assistant Professor of Chemistry and Biochemistry at Florida State University, and former FSU Postdoctoral Fellow Sarah Wieghold help understand the basic processes in a material known as perovskite. This work could lead to more efficient solar cells that are also more resistant to degradation. They found that small changes to the chemical composition of materials and the strength of the electrical field to which they are exposed can have a major impact on the overall stability of the material.
Her latest work is published in two studies in the Journal of Materials Chemistry C and the Journal of Applied Physics.
Her research focuses on improving the potential of perovskites, a material with a crystal structure based on positively charged lead ions known as cations and negatively charged halide anions. In a cubic perovskite crystal structure, the octahedra formed by the lead and halide ions are surrounded by additional positively charged cations.
The first perovskite solar cells, developed in 2006, had solar energy conversion efficiency of about 3 percent, but the cells developed in 2020 have energy conversion efficiency of more than 25 percent. This rapid increase in efficiency makes them promising material for further research, but they have drawbacks to economic viability such as a tendency to deteriorate quickly.
“How can we make perovskites more stable in the real-life conditions in which they are used?” Said Nienhaus. “What is causing the degradation? We are trying to understand. Perovskites that do not decompose quickly could be a valuable tool for extracting more energy from solar cells.”
Despite the ionic bonds of the crystal lattice that make up their structure, perovskites are a so-called “soft material”. The halides or cations in the material can move through this lattice, which can increase their rate of degradation, leading to a lack of long-term stability.
In the work of the Journal of Materials Chemistry C, the researchers examined the combined influence of light and elevated temperature on the performance of mixed-cation-mixed halide perovskites.
They found that adding a small amount of the element cesium to the perovskite film increased the stability of the material under light and elevated temperatures. Adding rubidium, on the other hand, resulted in poorer performance.
“We found that, depending on the choice of cation, two degradation pathways can be observed in these materials, which we then correlated with a decrease in performance,” said Wieghold, now a research fellow at the Center for Nanoscale Materials and the Advanced Photon Source at Argonne National Laboratory. “We have also shown that adding cesium increases film stability under our test conditions, which are very promising results.”
They also found that a decrease in film performance for the less stable perovskite mixtures correlated with the formation of the lead bromide / iodide compound and an increase in electron-phonon interactions. The formation of lead bromide / iodide is based on the undesired degradation mechanism that must be avoided in order to achieve long-term stability and performance of these perovskite solar cells.
In the Journal of Applied Physics, they investigated the relationship between voltage and performance of perovskite materials. This showed that the movement of ions in the material changes the underlying electrical response, which will be a critical factor in photovoltaic performance.
“Perovskites are a great opportunity for the future of solar cells, and it’s exciting to be advancing this science,” said Nienhaus.
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Materials provided by Florida State University. Originally written by Bill Wellock. Note: The content can be edited by style and length.