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The Fall and Rise of Spray-On Solar.

A Silicon Valley proverb says that we overpromise what we can deliver in three years and underestimate what we can do in ten. Nowhere has that been borne out more vividly than in the field of spray-on solar, which has seen repeated waves of excitement over the past decade, followed by loss of interest when early promises failed to pan out.

The first reports of a solar "paint" that could be sprayed onto windows or walls to turn them into photovoltaic (PV) solar panels came nearly a decade ago from the University of Leicester and Norwegian company EnSol, which patented a power-generating spray-on film in 2010. EnSol has since shelved the project, but at the time, it generated a fair amount of attention. "We had articles and website hits from more countries in the world than I even knew existed!" said Phil Denby, technical manager for EnSol, in an e-mail. The story receded then, until 2014, when Oak Ridge National Laboratories in Tennessee and the University of Toronto, among others, announced advances in spray-on PV coatings.

In 2010, the excitement was over metal nanoparticles called quantum dots. At the time, quantum dot-based PV cells were reaching about 10 percent efficiency in the lab, half of what commercial solar panels produce. Part of the interest in the 2014 developments, said Sjoerd Hoogland of the University of Toronto's Sargent Group, was generated by the fact that the Toronto team made its first quantum dot solar cell using an airbrush bought from a local art store.

Another company formed to commercialize spray-on solar technologies, Oxford PV (it's a spinoff of the university), also began developing solar windows about the time EnSol made its breakthrough. But like EnSol, Oxford PV dropped those plans in 2014, according to the company's Chief Technology Officer, Dr. Chris Case, because it would have taken five to ten years or more to bring to market.

Another exotic material for spraying virtually anything with a solar-electric layer also made headlines in 2014. Perovskite, named for a mineral discovered in the Ural Mountains in the 1830s, refers to any of a number of compounds that share a particular crystal structure. Like quantum dots, perovskites can be dissolved and sprayed onto a wide range of surfaces and cure at fairly low temperatures. Perovskites had been largely ignored until Tsutomu Miyasaka, a professor at Toin University of Yokohama, first sparked interest in them with a 2009 research paper. Fewer than a dozen papers a year followed until 2013, when the material broke the 10 percent efficiency barrier.

Michael Gratzel, director of the Laboratory of Photonics and Interfaces at the Swiss Federal Institute of Technology in Lausanne (EPFL), announced in 2013 that his research team had boosted the efficiency of perovskite-based PV elements to 15.5 percent. By spring 2017, small perovskite solar cells in labs were recording power conversion efficiencies above 22 percent, closing in on the current record for silicon-based solar cells of 26.7 percent efficiency and topping the typical commercial solar cell's 20 percent rating. Perovskite-based solar cells are fast becoming as efficient as monocrystalline silicon--and the research world is watching. More than 3,000 papers on perovskites were published in 2017.

Persistent problems have held the technology back. Chief among them has been stability. The first perovskite found to be good at carrying photons, methylammonium lead tri-iodide, degrades in air or moisture. It also undergoes phase changes at around 50 degrees C, in the range of operating temperatures for a solar panel. While phase changes are reversable, the behavior is not desirable.

But in the last few years, Oxford PV's Case said, researchers "looked at that composition and said, 'What can we do to enhance the chemical stability, its moisture resistance and its phase stability?' It turned out the answer is, there was a lot." A team from the Imperial College London proposed fixing it by adding an extra dose of iodide in the manufacturing process. Other combinations of metals such as tin, and halides like bromine or chlorine in place of the iodine, have also produced more air-stable perovskites. "The compositions that people use today are much more elaborate than the methylammonium lead iodide," said Case. "This material hasn't been out in the field at all, ... so you don't have 10-year field data on this. What you do have is the same tests the silicon panels have gone through. All indications are they passed those reliability tests."

Meanwhile, a group at Stanford University took an architectural approach, fashioning a compound solar cell modeled after a bee's eye, with a honeycomb-shaped epoxy scaffolding to hold the perovskite in position without affecting its electro-optical properties.

The other problem is perovskites' lead content. Concerns about environmental contamination have led researchers to try tin, titanium, and other metals, with disappointing results. Case said lead's special properties make it hard to substitute. "You look at the band structure ... and how it twists the orbitals of these organic materials in such a way that they really want to transfer their charge, and that's one of the things that makes this material so effective." Still, he said, the lead can be reduced by using a lead-tin alloy.

Furthermore, Case and others argue, the amount of lead in perovskites is trivial, because of how little perovskite is involved. Typically, the perovskite layer in a solar cell is about 300 nm, compared to hundreds of microns in silicon solar cells (a human hair is about 100 microns wide). As a result, the perovskite layer contains far less lead than a silicon solar cell, even before the lead solder used in constructing silicon solar panels is considered.

Coming up on 10 years since the first research papers on the unusual electro-optical properties of perovskites, there have again been significant advances. Last year, EPFL said its scientists achieved an efficiency of 21.6 percent by adding rubidium to improve stability and, this June, produced a perovskite-silicon tandem cell with a record efficiency of 25.2 percent.

Once again, analysts and researchers are predicting that the disruptive new technology will hit the market in the next two to three years. Just don't go looking for thin-film perovskite solar cells to replace the familiar black silicon rectangles. Companies are taking an if-you-can't-beat-them-join-them approach. Since perovskites, quantum dots, and silicon all absorb light in different parts of the spectrum, they are making tandem solar cells by spraying a perovskite or quantum dot layer on top of a standard silicon solar cell, boosting overall efficiency by 25 percent.

"One of the problems of trying to introduce a new technology into the marketplace, if you try to disrupt the incumbent, they resist," said Oxford PV's Case. "Silicon is 92 percent of the market. By incorporating the strategy of putting it on the silicon, that 92 percent of the market didn't become a defender, it became a potential customer." His company is currently running a demonstration plant in Germany and partnering with an unnamed silicon PV manufacturer with the goal of bringing tandem solar panels out by the end of 2019.

Hoogland said the Toronto researchers are even considering making a "triple junction cell," sandwiching a silicon solar cell between perovskite and quantum dot layers. In theory, such a cell could reach better than 43 percent efficiency, far exceeding the theoretical limit of either silicon or perovskites alone.

It may still be a decade or more before skyscrapers are powered by their windows. Long before that, a sprayed on nanolayer of perovskite or other materials could make solar, already cost-competitive with oil and natural gas, twice as efficient.

"The world," said Case, "needs all the solar it can get." By combining new spray-on nanomaterials with tried-andtrue silicon technology, he hopes to build a disruptive new technology on top of the existing solar industry.

Manny Frishberg, Contributing Editor

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Title Annotation:News and Analysis of the Global Innovation Scene
Author:Frishberg, Manny
Publication:Research-Technology Management
Geographic Code:1USA
Date:Sep 1, 2018
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