Solar sandwich

André Taylor of New York University and his colleagues there and at Yale University, Northwestern University, and the University of Electronic Science and Technology of China, have developed a new way to make "organic" solar cells more efficient by side-stepping the use of fullerenes and making a solar sandwich instead. [Y Zheng et al Mater Today (2018); 10.1016/j.mattod.2017.10.003]

The conventional organic solar cell structure is a sandwich with an active layer comprising electron donors and acceptors as the filling. It is the filling that absorbs the light energy that drives the processes that allow an electric current to be drawn by the positive and negative electrodes, the two slices of "bread" of the sandwich. Taylor and his colleagues wanted to extend the spectral range of absorption by the filling but without compromising how well the sandwich fits together. "My group works on key parts of the sandwich, such as the electron and hole transporting layers of the bread, while other groups may work only on the interlayer materials. The question is: How do you get them to play together? The right blend of these disparate materials is extremely difficult to achieve."

The team has turned to a squaraine molecule as a crystallizing agent that is also an electron donor to enhance the absorption of the active layer. "By adding this small molecule, it facilitates the orientation of the donor-acceptor polymer (called PBDB-T) with the non-fullerene acceptor, ITIC, in a favorable arrangement," explains Taylor. PBDB-T is poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione))]). ITIC is 3,9-bis(2-methylene(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b'] dithiophene).

With this architecture in hand, the team also added another design feature of their own, FRET, or Förster resonance energy transfer. FRET was first observed in photosynthesis so it is apt that science is using the mechanism in solar cells. The team has now achieved 10% efficiency, which was considered an unreachable level just a few years ago. "There are now newer polymer non-fullerene systems that can perform above 13 percent, so we view our contribution as a viable strategy for improving these systems," Taylor says. Lead author on the study and a former student of Taylor, Yifan Zheng suggests that, "We expect that this crystallizing-agent method will attract attention from chemists and materials scientists affiliated with organic electronics. The next step will be to optimise the approach still further. 

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".