Dr. Hairen Tan and his colleagues from the University of Toronto have overcome a crucial manufacturing challenge in developing a comparatively new type of solar devices known as perovskite solar cells. This alternative solar technology can allow easy and low-cost building of printing cells, similar to printing of a newspaper, which in turn leads to inexpensive and printable solar panels with the ability to turn almost any surface into a power generator.
Economies of scale have greatly reduced the cost of silicon manufacturing. 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.
Ted Sargent (ECE) Professor, University of Toronto
Sargent is the senior author of the paper and the Canada Research Chair in Nanotechnology, who is also a specialist in new-generation solar technologies.
Currently, nearly all commercial solar cells are developed using thin slices of crystalline silicon, necessitating the process of ensuring higher purity, which is an energy-intensive process carried out at temperatures of over 1000ºC and using greater quantities of harmful solvents.
Conversely, perovskite solar cells depend on a layer of small crystals made of light-sensitive and low-cost materials. The crystals are 1000 times tinier than the width of a strand of human hair. The ability of the perovskite raw materials to be mixed into a liquid to form a type of ‘solar ink’ enables them to be printed onto plastic, glass, or other materials by means of an easy inkjet process.
However, to generate electricity, electrons excited with solar energy have to be extracted from the crystals to enable them to flow in a circuit. The extraction process occurs in a unique layer known as the electron-selective layer (ESL). The challenges encountered in the production of a high-quality ESL are the main hurdles in developing perovskite solar cell devices.
The most effective materials for making ESLs start as a powder and have to be baked at high temperatures, above 500 degrees Celsius. You can’t put that on top of a sheet of flexible plastic or on a fully fabricated silicon cell—it will just melt.
Dr Hairen Tan, University of Toronto
Tan and his research team formulated an innovative chemical reaction for growing an ESL formed of nanoparticles in solution, right on top of the electrode. Although heat is still mandatory, the temperature of the process is always less than 150ºC, which is considerably lower than the melting point of different plastics.
A layer of chlorine atoms is coated onto the new nanoparticles, assisting in binding of the particles to the perovskite layer on top. Such a strong binding ensures effective extraction of electrons. Tan and his collaborators have reported the outcomes of their research in a recently published paper in the journal Science. They describe that the efficiency of solar cells fabricated by means of the new method is 20.1%.
“This is the best ever reported for low-temperature processing techniques,” stated Tan, who further added that perovskite solar cells fabricated using the conventional, high-temperature technique are only slightly better with an efficiency of 22.1% per cent. He also stated that even the most efficient silicon solar cells could only attribute 26.3% efficiency.
An additional benefit of the new perovskite solar cells is their stability. While conventional perovskite solar cells undergo an acute reduction in performance after just a few hours, Tan’s cells were able to retain over 90% of their efficiency even after using them for 500 hours. “I think our new technique paves the way toward solving this problem,” stated Tan, who carried out the research under a Rubicon Fellowship.
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.
Professor Alán Aspuru-Guzik, from the Department of Chemistry and Chemical Biology at Harvard University
Professor Aspuru-Guzik is a computational materials science specialist and was not involved in the study.
“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,” stated Professor Luping Yu, a specialist in solution-processed solar cells from the University of Chicago’s Department of Chemistry, who also was not involved in the study.
Maintaining lower temperature during the manufacturing procedure enables the perovskite solar cells to be used in a number of applications such as solar-active tinted windows that counterbalance energy use in a building, smartphone covers with charging capabilities, and so on. Tan anticipates that in the near future, his technology can be used alongside traditional solar cells.
“With our low-temperature process, we could coat our perovskite cells directly on top of silicon without damaging the underlying material,” stated Tan. “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.”