ENGINEERING & NANOTECHNOLOGY
Polymers One stone, two birds
A single polymer can be used to fabricate both thin-film transistors and solar cells
Polymers are the material of choice for making thin-film transis- tors and solar cells. They are also potentially suitable for manu- facture 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 Engi- neering and co-workers1
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 suscep- tible 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 polythio-
phenes, 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 nanometers (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 the polythiophene chain with groups of atoms that are able to accept charge. Even so, the power conversion efficiency of the resulting solar cells was below 2.3%, less than half of the best-performing polymer cells. Chen and co-workers followed in the steps of their predeces-
sors 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 PC71
BM, the research team constructed a solar cell with a power conversion 72 200 nm
A transmission electron micrograph of a thin film made from a mix- ture of POD2T-DTBT polymer and PC71
BM ester.
efficiency of 6.26%, comparable to the efficiency of the best polymer cells to date. This strong performance was drawn in part from the morphol-
ogy of the thin films that resulted from the POD2T-DTBT / PC71
BM mixture. Electron microscopy of the films showed
that the two components were intimately mixed together: the long white fibers, which are 20–25 nm in width, correspond to the polymer, and the darker domains correspond to the ester (see image). The high-charge mobility of the POD2T-DTBT polymer itself also boosted performance.
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1. Ong, K.-H. et al. A versatile low bandgap polymer for air-stable, high-mobility field-effect transistors and efficient polymer solar cells. Advanced Materials 23, 1409–1413 (2011).
A*STAR RESEARCH OCTOBER 2011– MARCH 2012
© 2011 Wiley-VCH
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