University of Toronto researchers move closer to low-cost, printable perovskite solar cells

University of Toronto researchers more closer to low-cost, printable perovskite solar cells The researchers have developed a new chemical reaction than enables them to grow an ESL made of nanoparticles in solution, directly on top of the electrode

University of Toronto researcher Dr. Hairen Tan and his team have overcome a "critical" manufacturing hurdle in the development of perovskite solar cells. This alternative solar technology could lead to low-cost, printable solar panels capable of turning nearly any surface into a power generator.

According to the researchers today, virtually all commercial solar cells are made from crystalline silicon which must be processed to a very high purity. It's an energy-intensive process, requiring temperatures higher than 1,000 degrees Celsius and large amounts of hazardous solvents.

In contrast, 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. Because the perovskite raw materials can be mixed into a liquid to form a kind of 'solar ink', they could be printed onto glass, plastic or other materials using a simple inkjet printing process.

However, the researchers said there has been a catch: 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 happens in a special layer called the electron selective layer - ESL. The difficulty of manufacturing a good ESL has been one of the key challenges holding back the development of perovskite solar cell devices.

Tan and his colleagues developed a new chemical reaction than enables them to grow an ESL made of nanoparticles in solution, directly on top of the electrode. While heat is still required, the process always stays below 150 degrees C, much lower than the melting point of many plastics.

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. In a paper recently published in Science, Tan and his colleagues report the efficiency of solar cells made using the new method at 20.1 per cent.

Another advantage is stability. Many perovskite solar cells experience a severe drop in performance after only a few hours, but Tan's cells retained more than 90 per cent of their efficiency even after 500 hours of use. "I think our new technique paves the way toward solving this problem," said Tan, who undertook this work as part of a Rubicon Fellowship.

In the nearer term, Tan's technology could be used in tandem with conventional solar cells.

Professor Ted Sargent, an expert in emerging solar technologies and the Canada Research Chair in Nanotechnology, said, "Economies of scale have greatly reduced the cost of silicon manufacturing." Ted, added, "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."

University of Toronto researcher Dr. Hairen Tan, said, "The most effective materials for making ESLs start as a powder and have to be baked at high temperatures, above 500 degrees Celsius." Hairen, added, "You can't put that on top of a sheet of flexible plastic or on a fully fabricated silicon cell -- it will just melt."

Hairen continued, "This is the best ever reported for low-temperature processing techniques. Perovskite solar cells using the older, high-temperature method are only marginally better at 22.1 per cent, and even the best silicon solar cells can only reach 26.3 per cent."

Professor Alan Aspuru-Guzik, an expert on computational materials science in the Department of Chemistry and Chemical Biology at Harvard University, who was not involved in the work, said, "The Toronto team's computational studies beautifully explain the role of the newly developed electron-selective layer. The work illustrates the rapidly-advancing contribution that computational materials science is making towards rational, next-generation energy devices."

,blockquote>Professor Luping Yu of the University of Chicago's Department of Chemistry - an expert on solution-processed solar cells and was not involved in the work, said, "To augment the best silicon solar cells, next-generation thin-film technologies need to be process-compatible with a finished cell. This entails modest processing temperatures such as those in the Toronto group's advance reported in Science."

Hairen, concluded by saying , "With our low-temperature process, we could coat our perovskite cells directly on top of silicon without damaging the underlying material. If a hybrid perovskite-silicon cell can push the efficiency up to 30 per cent or higher, it makes solar power a much better economic proposition."