Redefining perovskite

Redefining perovskite
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Laura Schelhas explains why perovskites are causing a buzz in solar.

If you follow the field of photovoltaics, it’s likely that you’ve heard the term ‘perovskite’ sneak into conversations more and more frequently. So, what is a perovskite and who should be listening?

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Technically, perovskite is a specific arrangement of atoms into a crystal or structure with a composition of ABX3, where A, B, and X describe specific types of atoms. Even more specifically, perovskite traditionally refers to calcium titanate (CaTiO3). Perovskite-structured materials have long been a topic of research with their oxide formulations having applications in magnetic and piezoelectric materials.

However, more recently, perovskite has become the common term for a class of halide-containing materials. They often contain Pb as the B-site atom and a whole slew of atoms on the A-site. Perovskite has even come to describe materials that are inspired by the ‘original’ halide perovskite, Methylammonium Lead Iodide, or MAPI for short, that do not have the perovskite structure. These halide perovskites, MAPI specifically, originally emerged as a potential dye in dye-sensitized PV cells.

Before long, researchers realised they functioned remarkably well as solar absorbers on their own and the field of perovskite PV was born. Since then, over the last decade, researchers have explored a number of different formulations pushing to higher photoconversion efficiencies. As researchers approach the fundamental efficiency limits of this technology, they are exploring applications in tandem structures which would allow for even greater gains.

Another positive for this technology is the ease of fabrication. It is amenable to a variety of deposition techniques ” such as solution phase, vacuum deposition ” and substrates (e.g. rigid, flexible) providing a lot of avenues for manufacturing and deployment.

The US-MAP Consortium, led out of the National Renewable Energy Laboratory (NREL), a national laboratory of the US Department of Energy, is poised to create a bridge between this emerging industry and the partner laboratories (the Washington Clean Energy Testbeds at the University of Washington, the University of North Carolina at Chapel Hill, and the University of Toledo) to accelerate the commercialisation of this technology.

While highly promising, there’s still work to be done before you will be seeing perovskite solar modules in mass deployment. Large scale field testing of perovskite PV modules is still lacking.

Proven field performance is a critical step in assessing the bankability of the technology.

While reports of fielded perovskites are starting to reach the literature, more work to create market confidence and enable deployment is on the horizon. Proven field performance is a critical step in assessing the bankability of the technology and one of the remaining hurdles before widespread commercialisation of this technology can move forward.

In addition to US-MAP, recent funding opportunities from the US Department of Energy focused on exactly these outstanding research questions; therefore, we anticipate that the next few years will see rapid advances regarding these and other outstanding questions.

However, the excitement for perovskites shouldn’t stop at traditional solar modules. Recent work out of NREL has demonstrated their use as smart windows. The tailoring of the thermochromic properties can enable changes from transparent to a variety of colours when exposed to warming sunlight (see image below). Additionally, the darkened window is a fully functioning solar cell in the dark state, creating a two-for-one gain in using these windows.

Perovskites tunability is not limited to its composition. Beyond solar applications, perovskites have also shown promise in a number of different application spaces. For example, they are being explored for use as radiation detectors.

NREL researchers Bryan Rosales and Lance Wheeler hold a perovskite window prototype. Image: NREL

Their tunable bandgap, large light absorption coefficient, large mobility, and long carrier recombination lifetime, while good properties for PV applications, also make them useful in both imaging and spectroscopy applications across a wide energy range. Again, the solution processability of these materials and ease of manufacturing have also excited the detector community.

Other potential applications for halide perovskites are as emitters; for example as LEDs in solid state lighting, and various display applications. This shouldn’t be a surprise to anyone familiar with Professor Eli Yablonovich’s mantra: “A great solar cell has to be a great LED.”

The research teams at NREL have recognised the promise of these technologies and have active research programmes to help see these technologies succeed. Through broad industry and academic collaboration, the groups work across these technology spaces to address some of the outstanding research questions.

A recent example of these partnerships is highlighted by the recent R&D 100 award for the Aplex Flex PV technology for flexible all-perovskite tandem devices.

The last ten years of research into halide perovskites has been exciting and fast paced. The next decade is likely to show the emergence of even more new application spaces for this technology. As we race to commercialisation, perovskite has been redefined to mean so much more than its original crystal. The remaining question now is which of these applications will win the race to market?

ABOUT THE AUTHOR

Laura Schelhas is a research scientist and group manager at the National Renewable Energy Laboratory, Colorado, USA. Her current research interests are focused on the intersection between photovoltaic reliability, emerging new technologies, and materials characterization.

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