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Significance Statement
Thermal annealing is a widely used method for tailoring properties of organic electronic devices. In our work published in Solid-State Electronics 103 (2015) p. 184–189, we showed that it can also be effectively utilized to modify spectra of organic light-emitting diodes made of undoped and doped polymers. Importantly, it was demonstrated that thermal annealing can be applied to reduce the undesired green emission caused by keto-defects that is generally occurring in blue light emitting polyfluorene organic light-emitting diodes. The organic light-emitting diodes in the study were made of poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) and the temperature of 190 °C was shown to have beneficial impact besides of light emission color also on luminance values. Considering the doped organic light-emitting diodes made of blend of PFO and poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol- 4,8-diyl)] (F8BT), thermal annealing had a negative impact on luminance, while it caused a blue emission peak to appear in the spectrum of the devices originally indicating only green light emission taking place via F8BT. This was attributed to raised PFO emission caused by impaired Förster energy transfer followed from phase separation of the polymers. It was however concluded that the phenomenon could be used advantageously. This was done later on, when we exploited thermal annealing on inducing color conversion of PFO:F8BT organic light-emitting diodes from green to white and proposed applications in display technology in creation of multicolor light emitting devices. This work “Light emission color conversion of polyfluorene-blend organic light-emitting diodes induced by thermal annealing” was published in IEEE Transactions on Electron devices 62 (2015) p. 2238 – 2243.
Journal Reference
Solid-State Electronics, Volume 103, 2015, Pages 184-189.
Jokinen, A. Bykov, R. Sliz, K. Remes, T. Fabritius, R. Myllylä
Optoelectronics and Measurement Techniques Laboratory, P.O. Box 4500, 90014 University of Oulu, Finland.
Abstract
The effect of thermal annealing on electroluminescent (EL) properties of polyfluorene-based organic light-emitting diodes (OLEDs) was studied. Two types of light-emitting layers were investigated: undoped layers made of poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) and doped layers consisting of 95 wt% of PFO and 5 wt% of poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT). Thermal annealing at 190 °C and 290 °C was performed prior to top contact thermal deposition. The experiments indicated that the annealing treatment results in significant changes of EL properties in both types of the considered organic light-emitting diodes. However, the annealing affects them differently. For the undoped organic light-emitting diodes, the devices annealed at 190 °C presented the highest luminance and current density. In the case of doped organic light-emitting diodes, the highest luminance was observed in the non-annealed devices and the highest current density in the devices annealed at 190 °C. Remarkably, the annealing was noticed to affect the EL spectra of both types of organic light-emitting diodes. In the undoped organic light-emitting diodes the purest blue emission was observed from the organic light-emitting diodes annealed at 190 °C, whereas the organic light-emitting diodes annealed at 290 °C and the non-annealed organic light-emitting diodes in particular had pronounced undesired green emission, a common problem for the polyfluorene-based blue light-emitting polymers. Green emission originating from F8BT was observed for the doped organic light-emitting diodes in the case of non-annealed devices and the devices annealed at 290 °C, whereas the spectrum of the similar devices annealed at 190 °C had an additional blue component corresponding to PFO emission
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