|Posted: Nov 06, 2015|
Light-absorbing polymers stand to attention(Nanowerk News) Semiconducting polymers that absorb or emit light are used in solar cells and light-emitting diodes. But their optical properties can be difficult to predict, because they depend on how the polymer molecules are stacked together in thin films.
|The flat, linear polymer molecules typically lie lengthways along a substrate, like a raft of logs. Making them stand vertically, as a ‘forest’ of polymer strands, is much more challenging but could dramatically affect the way they interact with light.|
|Now, Keisuke Tajima at the RIKEN Center for Emergent Matter Science and colleagues have developed a reliable technique for producing these vertically aligned polymer films, allowing them to study their behavior ("Optical Anisotropy and Strong H-Aggregation of Poly(3-Alkylthiophene) in a Surface Monolayer").|
|Figure 1: A fluorinated cap causes strands of the polymer poly(3-butylthiophene) to pull away from a polystyrene film (left). On crystallizing, the strands line up vertically (right). (© Wiley)|
The polymers possess clouds of
|Tajima’s team created a polymer called P3BT-F17, composed of poly(3-butylthiophene) capped at one end with a chemical group loaded by fluorine atoms. When this polymer was mixed with polystyrene to form a thin film, the fluorinated groups separated from the polystyrene, pulling the poly(3-butylthiophene) sections into vertical alignment.|
|Optical measurements indicated that poly(3-butylthiophene) units next to the polystyrene base were laid flat and tended to absorb longer wavelengths, whereas those further away stood vertically and absorbed shorter wavelengths.|
|Heating the film rearranged the strands into a more ordered, crystalline structure, in which all the poly(3-butylthiophene) units were vertical (Fig. 1). This shifted their absorption to shorter wavelengths—probably due to strong interactions between the side-by-side chains. “We were very surprised,” says Tajima. “Such a large blue-shift after crystallization is the opposite of what we expected.”|
|The team then took two films of P3BT-F17 on polystyrene and sandwiched them together so that the fluorinated groups on the surface of each film were touching. This red-shifted the absorption by increasing the head-to-tail interactions of the poly(3-butylthiophene) units.|
“We have shown that a vertical orientation of
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