Organic electronics: Two birds, one polymer

(Nanowerk News) Although most electronics are made in billion-dollar fabrication facilities, it may also soon be possible to print electronic devices using a continuous printing press—at much lower cost. This is one of the motivations for research into the use of organic semiconductors, like polymers, for electronics. Thin-film transistors and photovoltaic cells based on polymers have been studied for over a decade, and different polymers have been optimized for each kind of device. Zhi-Kuan Chen and colleagues at the A*STAR Institute of Materials Research and Engineering have now demonstrated a polymer that works well in both device types ("A Versatile Low Bandgap Polymer for Air-Stable, High-Mobility Field-Effect Transistors and Efficient Polymer Solar Cells").
A transmission electron microscopy image of a POD2T-DTBT film, showing a fibrous polymeric phase (light) and a non-polymeric phase (dark)
A transmission electron microscopy image of a POD2T-DTBT film, showing a fibrous polymeric phase (light) and a non-polymeric phase (dark).
Their polymer combines two classes of materials: polythiophenes and benzothiadiazoles. Polythiophenes have high mobilities for positive charge, which makes them attractive for use in both transistors and solar cells. But they also have a large bandgap, which means they cannot absorb light in the deep red or infrared part of the spectrum, making them impractical for use in solar cells. Benzothiadiazoles, on the other hand, are electron acceptors that when bonded to polythiophenes reduces the bandgap, leading to increased absorption in the important region of the solar spectrum.
Chen and his colleagues followed this general approach to make a polymer called POD2T-DTBT, which incorporates a polythiophene and a benzothiadiazole, as well as long, branched alkyl groups that ensure the polymer has high solubility to facilitate fabrication. It has a lower bandgap than pure polythiophene, and transmission electron microscopy images of thin films of the polymer revealed the presence of two phases—a polymer phase consisting of long fibers and a non-polymeric phase (see image). This morphology enables efficient charge separation and transport, which are essential for solar cell performance.
Whereas previous solar cells that combine polythiophenes and benzothiadiazoles have been demonstrated to give power conversion efficiencies of up to 2.2%, Chen and his colleagues were able to fabricate solar cells using POD2T-DTBT with efficiencies as high as 6.3%. They also used their novel polymer to make thin-film transistors with good hole mobilities and large on/off ratios. The ability to make both types of devices with a single polymer will help the integration of these technologies into single devices and bring printing-press fabrication a step closer to reality.
Source: Tokyo Institute of Technology