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NREL, SLAC Determine Ideal Conditions for Making Perovskite Photovoltaic Absorbers

February 16, 2017

Researchers at the Energy Department’s National Renewable Energy Laboratory (NREL) and the SLAC National Accelerator Laboratory have mapped the processing phase space where ideal conditions exist to create perovskite photovoltaic absorbers.

Using high-speed X-ray diffraction (XRD), the scientists were able to see the formation of formamidinium lead iodide (FAPbI3) from precursor into perovskite crystals. A transparent substrate was coated with the FAPbI3 precursor and annealed using unfiltered light from halogen lamps to provide controlled heating. Observing the X-ray diffraction patterns during the annealing process allowed researchers to capture over time the phase transitions from the formation of the perovskite to its degradation into lead iodide (PbI2).

Perovskites emerged in the last several years as a promising material for solar cells because of their ability to efficiently convert sunlight into electricity.

A new paper published in Nature Communications, “Thermal engineering of FAPbI3 perovskite material via radiative thermal annealing and in situ XRD,” details the time and temperature ranges NREL and SLAC researchers determined can be used to produce high quality perovskite films. A series of experiments revealed a much broader range than the standard annealing time for FAPbI3 of 10 minutes at 170° Celsius (338° Fahrenheit).

Instead, any temperature between 150°C and 230°C (446°F) can be used, and the annealing time can be less than 40 seconds. From the traditional starting time of 10 minutes, the researchers were able to complete the process 93 percent faster.

The research confirmed the hypothesis that once the perovskite is formed, its efficiency is fixed. In other words, heating the material longer doesn’t produce a perovskite absorber with better initial solar cell efficiency.

“You can basically take any time and temperature that’s within this zone and make the material,” said Maikel van Hest, a senior scientist at NREL. “The realization that you can process this in that short a time is ultimately a good thing for enabling commercialization.”

Van Hest is co-author of the paper along with NREL colleagues Benjia Dou and Talysa R. Klein. Other co-authors are Vanessa L. Pool, Douglas G. Van Campen, Md. I. Ahmad, and Michael F. Toney from the Energy Department’s SLAC National Accelerator Laboratory, and Frank S. Barnes and Sean E. Shaheen from the University of Colorado Boulder.

Van Hest and Toney directed the research project.

So far, most of the annealing of perovskites in a research environment has been performed on hot plates. The efficiency of a FAPbI3 cell made through radiative thermal annealing was almost identical to a cell annealed using a hot plate. That discovery clears the way for a more cost-effective way for large-scale roll-to-roll manufacturing of the perovskite cells. At a production speed of 1 meter per second a hot plate used to anneal the FAPbI3 onto a substrate over 10 minutes would need to be 600 meters long, which the researchers dismissed as impractical. The demonstrated shorter annealing times would reduce a processing oven or hot plate to an acceptable length.

For this research effort, NREL designed the experiments, participated in the measurements at SLAC, and jointly analyzed the X-ray data with the California laboratory housed at Stanford University.

“The collaboration is great,” van Hest said. “We are experts on perovskites. They are experts on XRD. So by bundling these two together, you get way more than one plus one.”

The research was funded by the Bridging Research Interactions through collaborative Development Grants in Energy (BRIDGE) program from the Energy Department’s SunShot Initiative.

The U.S. Department of Energy SunShot Initiative is a national effort to drive down the cost of solar electricity and support solar adoption. SunShot aims to make solar energy a low cost electricity source for all Americans through research and development efforts in collaboration with public and private partners. Learn more at energy.gov/sunshot.