Polymers: One stone, two birds

Polymers: One stone, two birds

11:57 PM, 17th November 2011
Polymers: One stone, two birds
A transmission electron micrograph of a thin film made from a mixture of POD2T-DTBT polymer and PC71BM ester. © 2011 Wiley-VCH.


CENTROS, SINGAPORE: Polymers are the material of choice for making thin-film transistors and solar cells. They are also potentially suitable for manufacture using economical, high-throughput techniques, such as roll-to-roll and inkjet printing processes. However, transistors and solar cells have traditionally used different kinds of polymers and this can severely complicate the fabrication process. Zhi-Kuan Chen at the A STAR Institute of Materials Research and Engineering and co-workers have now developed a versatile polymer that is suitable for both kinds of devices.

Polymers with high-charge mobilities are ideal to use in the manufacture of transistors. However, these materials are susceptible to having large energy bandgaps, which prevent them from absorbing portions of the solar spectrum. Such materials could severely hamper the energy conversion efficiency if made into solar cells.

The researchers focused on a class of polymers called polythiophenes, derivatives of which have been measured to have high hole (or positive charge) mobilities. However, polythiophenes also have a large energy bandgap, which prevents them from absorbing light with red-orange wavelengths longer than 650 nm, thus reducing solar cell performance.

Previous work by other researchers has shown that this bandgap can be lowered by making modifications to the backbone of polythiophene chain with groups of atoms that are able to accept charge. Even so, the power conversion efficiency of resulting solar cells was below 2.3 per cent, less than half of the best-performing polymer cells.

Chen and co-workers followed in steps of their predecessors by modifying a polythiophene polymer. The result was a novel polymer called POD2T-DTBT that was measured to have a relatively low bandgap which resulted in an optical absorption range that extended to red-orange wavelengths of 780 nm, thus taking in more of the solar spectrum. At the same time, the hole mobility of the polymer was measured to be 0.20 cm2 per volt per second, comparable to unmodified polythiophene. This allowed for fabrication of high-performance transistors and solar cells. In particular, by combining POD2T-DTBT with the ester PC71BM, the research team constructed a solar cell with a power conversion efficiency of 6.26 per cent, comparable to efficiency of the best polymer cells to date.

This strong performance was drawn in part from morphology of thin films that resulted from POD2T-DTBT / PC71BM mixture. Electron microscopy of films showed that two components were intimately mixed together: long white fibre, which are 20-25 nm in width, correspond to the polymer and the darker domains correspond to the ester. The high-charge mobility of POD2T-DTBT polymer itself also boosted performance.

The article, “A versatile low bandgap polymer for air-stable, high-mobility field-effect transistors and efficient polymer solar cells,” was first published online on 21 January 2011. It was also available in the journal Advanced Materials. The authors of the article are Kok-Haw Ong, Siew-Lay Lim, Huei-Shuan Tan, Hoi-Ka Wong, Jun Li, Zhun Ma, Lionel C H Moh, Suo-Hon Lim, John C de Mello, Zhi-Kuan Chen.

(C) Agency for Science, Technology and Research News




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