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The new perovskite solar cells have achieved an efficiency of 20.1 per cent and can be manufactured at low temperatures, which reduces the cost and expands the number of possible applications. Credit: Kevin Soobrian/ the University of Toronto

A new innovation could make printing solar cells both simple and inexpensive, as University of Toronto engineers have cleared a critical hurdle in the development of a relatively new class of solar devices.

The alternative solar technology, known as perovskite solar cells, could lead to low-cost, printable solar panels capable of turning nearly any surface into a power generator.

“Economies of scale have greatly reduced the cost of silicon manufacturing,” professor Ted Sargent, an expert in solar technologies and the Canada Research chair in Nanotechnology, said in a statement. “Perovskite solar cells can enable us to use techniques already established in the printing industry to produce solar cells at very low cost.”

“Potentially, perovskites and silicon cells can be married to improve efficiency further but only with advances in low-temperature processes,” he added.

Currently most commercial solar cells are made from thin slices of crystalline silicon, which must be processed to a very high purity. The energy-intensive process requires temperatures higher than 1,000 degrees Celsius and large amounts of hazardous solvents.

However, perovskite solar cells depend on a layer of tiny crystals—each about 1,000 times smaller than the width of a human hair—made of low-cost, light-sensitive materials.

The perovskite raw materials can be mixed into a liquid to form a kind of ‘solar ink,’ that could be printed onto glass, plastic or other materials using a simple inkjet printing process.

 Until now in order to generate electricity, electrons excited by solar energy must be extracted from the crystals so they can flow through a circuit. That extraction occurs in a special layer called the electron selection layer (ESL), which has been difficult to manufacture in the past.

“The most effective materials for making ESL’s start as a powder and have to be baked at high temperatures, above 500 degrees Celsius,” Hairen Tan, Ph.D., said in a statement. “You can't put that on top of a sheet of flexible plastic or on a fully fabricated silicon cell—it will just melt.”

The research team has developed a new chemical reaction that enables them to grown an ESL made of nanoparticles in solution, directly on top of the electrode. While heat is still required, the process stays below 150 degrees Celsius.

The new nanoparticles are coated with a layer of chlorine atoms, which helps them bind to the perovskite layer on top.

This strong binding allows for efficient extraction of electrons and the efficiency of solar cells are made using the new method at 20.1 percent, which is the best ever reported for low-temperature processing techniques.

The new process is also more stable than the previous methods. Many perovskite solar cells experience a severe drop in performance after only a few hours, but the new cells retained more than 90 percent of their efficiency even after 500 hours of use.

“I think our new technique paves the way toward solving this problem,” Tan said.

By lowering the temperature during the manufacturing process, perovskite solar cells can produce a number of applications, from smartphone covers that provide charging capabilities to solar-active tinted windows that offset building energy use.

In the near future the new technology could be used in tandem with conventional solar cells.

“With our low-temperature process, we could coat our perovskite cells directly on top of silicon without damaging the underlying material,” Tan said. “If a hybrid perovskite-silicon cell can push the efficiency up to 30 percent or higher, it makes solar power a much better economic proposition.”

The study was published in Science.  

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