Currently, silicon is the dominant technology for photovoltaic solar power. There are a handful of competing thin-film technologies, which are easier to manufacture but rely on more expensive raw materials and don't reach the same efficiencies as silicon. The trade offs between these two technologies have helped drive research into a third: perovskite solar cells, which rely on cheap and abundant raw materials but have the potential for much higher efficiencies.

Still, perovskites have two significant issues. One is that the ability to integrate them into mass production techniques hasn't been demonstrated. The second is that they tend to decay pretty rapidly in the real world. There's been some progress on issue two recently, and now a team at the National Renewable Energy Lab (NREL)* has figured out a way to make a perovskite "ink" that should allow mass-manufacturing of the material.

Perovskites aren't actually a single material; instead, they're a class of materials that all share the same general crystal structure. Many of them involve a small organic chemical and metals like lead, along with some other simple chemicals. The best perovskites are closing in on silicon, with efficiencies well over 20 percent (meaning over 20 percent of the incoming sunlight is converted to electricity). Critically, crystals of perovskites are easy to form from a water-based solution, meaning that it should be possible to coat all sorts of materials with a photovoltaic material using manufacturing tech that's been in use for decades.

Photovoltaic ink

The new work started with an extremely simple perovskite formed from iodine, lead, and methylammonium. Under normal conditions, the material forms photovoltaic crystals easily, but it takes a while at elevated temperatures to dry the material out afterwards. This would slow down manufacturing, which would in turn add significantly to the cost.

So, the team tried an extensive search for conditions that would speed up crystal formation. These included substituting various amounts of chlorine in for the iodine and adding what they term a "negative solvent"—something that would cause the perovskite to settle out of solution quickly.

The end result is what they refer to as a perovskite "ink," which only takes one minute at 100°C to dry onto a surface. That's quick enough to be incorporated into what's called roll-to-roll manufacturing, where a sheet of material is spun off one roll, coated, and then rolled back on to a new one. The team demonstrated the ink also worked with what's called blade coating, where an excess of ink is applied and the unused portion is simply pushed off by passing the surface being coated under a blade.

Individual cells made with this process had efficiencies of over 17 percent in some cases. And by adding an additional coating of fullerenes, the efficiency could be boosted to over 19 percent. That's not up there with silicon, but it's a lot easier to make.

That's the good news. The bad news is that one of the problems with perovskites—long term stability—was still there. When used for over 3,000 hours, about 20 percent of the material degraded.

While the new ink doesn't solve all of the issues facing perovskites, this is the sort of problem that can often be solved in parallel. Some labs can work on stability, others efficiency, and still others ease of manufacture. As issues get solved, researchers can start to look for a single material that has properties identified by the earlier work. Ideally, by combining features, we can finally get to something that can be developed into a useful product.

Nature Energy, 2017. DOI: 10.1038/nenergy.2017.38 (About DOIs).

*NREL is part of the Department of Energy. Under the proposed budget, the DOE would end all renewable energy research, including work such as this.