Electroluminescence from a single polymer chain

(Nanowerk News) Polyfluorenes are a class of semiconducting π-conjugated polymers with excellent optical, electrical and mechanical properties that make them suitable for applications in optoelectronic devices, such as organic light-emitting diodes (OLED) and related display and lightning technology.
However, the blue-light emission of polyfluorenes often shows an undesirable feature, an additional emission in the green spectral region which degrades the color purity and decreases the device efficiency and stability. Over the past decade, great effort has been spent to clarify this phenomenon but so far there has been no clear consensus on its physical origin.
Now, the groups of Martin Vacha from the Department of Organic and Polymeric Materials, and Tomokazu Iyoda from the Chemical Resources Laboratory succeeded in detection and spectral characterization of electroluminescence (EL) emission from single polyfluorene chains. This first report on detection of EL from one chain of a conjugated polymer was made possible by isolating individual polyfluorene chains in vertical cylinders of a phase-separated block copolymer ("Single-molecule electroluminescence and photoluminescence of polyfluorene unveils the photophysics behind the green emission band").
Single-molecule electroluminescence device
(A) Scheme of the single-molecule EL device; (B) Example of time evolution of spectrally resolved EL intensity from a single BDOH-PF chain; (C) EL spectra at selected times reconstructed from (B); (D) Chemical structure of the polyfluorene BDOH-PF; (E) Histogram of peak positions of EL spectra from 202 molecules (bars), together with a bulk EL spectrum of a control device prepared with a neat film instead of the BC (solid line). (click on image to enlarge)
The researchers observed different types of EL spectra within the region of the green band, as well as strong spectral dynamics, that is, large and frequent changes of the spectra with time. They explained some of the observed phenomena in terms of oxidation of the chains. Further, based on quantum chemical calculations performed by the group of Susumu Kawauchi from the Department of Organic and Polymeric Materials they assigned the novel observations to EL emission from aggregates of the polyfluorene chains which are formed and stabilized due to the presence of the charges in the OLED device.
These results will have broad impact in the field of photophysics of conjugated polymers, and will contribute to the material design of polyfluorene materials. Also, as the smallest possible optoelectronic device, EL emitting single polymer chains could pave the way towards the field of nanotechnology applications in optoelectronics.
Source: Tokyo Institute of Technology
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