| 11 June 2012
Interdisciplinary discourse mixed with a dose of serendipity — that was the recipe for research success at Northwestern University in Evanston, Illinois (US), where a nanoparticles expert and a chemist have created the first all-solid-state dye-sensitized solar cell (DSC) system that promises to exceed the performance of the Grätzel cell.
Replacing the liquid electrolyte in Grätzel cells with a solid has been a major ongoing field of research. Grätzel cells are environmentally friendly and cheap to make. But they have a problem researchers have been unable to solve for two decades — the organic liquid can freeze in cold temperatures or expand and leak out as the system warms up in higher temperatures (which is hardly avoidable with exposure to solar radiation). Among those researchers is Robert P. H. Chang, a professor of materials science and engineering at Northwestern’s McCormick School of Engineering and Applied Science. “He was working on the Grätzel cells for some time. I listened to him complain about the liquid problem enough times that it eventually dawned on me that we might have such a material lying around in our lab,” recalls Mercouri Kanatzidis, the Charles E. and Emma H. Morrison Professor of Chemistry in Northwestern’s Weinberg College of Arts and Sciences.
Prof. Kanatzidis is talking about CsSnI3, a thin-film compound made up of cesium, tin and iodine. “We have a new material that is soluble, and that is key here.” The new solar cell the Kanatzidis group and the Chang group created collaboratively as a result of that momentous conversation between colleagues uses both n-type and p-type semiconductors. Spherical titanium dioxide nanoparticles (the n-type semiconductor) are coated with Kanatzidis’ soluble CsSnI3 thin-film material (the p-type semiconductor). When the solvent evaporates, a solid mass results. While the Northwestern team initially thought they were merely searching for a material to replace the leaky liquid in the Grätzel cell, what they ended up creating may not even be a Grätzel cell. “Our material itself absorbs light at the same time, whereas the old liquid didn’t absorb light,” Prof. Kanatzidis explains. “So now we have two things that absorb light, the dye and our material. And our material also accepts a hole and carries it to the other side. So it is a new type of cell essentially.”
Could the Northwestern cell be the breakthrough commercializable DSC cell researchers had been waiting for? “That is what we hope,” Prof. Kanatzidis says. “Our cell is new and essentially solves the electrolyte problem people have been complaining about.” He goes on to explain that in a conventional Grätzel cell, the liquid leeches off the dye. Without the dye, the solar device does not create electrons and holes. “Now this cannot happen in the solid state, so in that regard, then our cell is expected to have a greater lifetime,” the Northwestern chemist says. He goes on to say, “20 years have not passed for us to know whether our cells are stable or not, obviously. However, it raises new hope as most all-solid-state devices are more ruggedized and more stable than a liquid-state device.”
At approximately 10%, the Northwestern cell comes close to the highest reported performance for a Grätzel cell, which is between 11% and 12%. “Our efficiency is impressively high given that our cell is so new,” says Kanatzidis, who still remembers his disappointment after the Northwestern cell achieved a very low efficiency — only half a percent — in the first tests. “My colleague, Professor Chang, was very excited by the 0.5%. He told me the fact that it works at all using this material is remarkable.” So Kanatzidis and Chang challenged their postdoctoral fellows with improving the cell, telling them “it would be nice if we could get 1% a month improvement,” Kanatzidis says. “They went to their lab and sure enough, it went up to one-and-a-half, two, three percent. And finally we hit about close to 10% several months later,” the professor says. “It was a matter of actually learning how to make this type of cell; and then the efficiency was really up.”
The next step for the interdisciplinary team is to conduct more stability tests. “And we want to work with private sector in trying to push this forward and commercialize it. And also, of course, increase the efficiency from here,” Kanatzidis says. His ultimate performance goal for the new cell is 15%.
Kanatzidis and Chang are the senior authors of the paper “All-Solid-State Dye-Sensitized Solar Cells With High Efficiency” published in the journal Nature.
Written by Sandra Henderson, Research Editor, Solar Novus Today
