Effects of Postannealing on the Characteristics and Reliability of Polyfluorene Organic Light-Emitting Diodes

We studied the effects of postannealing on the performance of polyfluorene organic light-emitting diodes (OLEDs) with a pure poly(9, $9'$ -dioctylfluorene-co-benzothiadiazole) (F8BT) emissive layer. After annealing at 150 °C–200 °C, the OLED exhibited the lowest voltage and highest luminance. This correlated well with the highest crystallinity of the F8BT film, as revealed by X-ray diffraction. As the temperature was raised up to 300 °C, the voltage at 50 mA/cm2 increased by 2.7 V, and the luminance decreased by 44%, whereas the device lifetime was markedly extended by as much as 13 times. The loss of crystallinity at high temperatures suggested that structural disorders were created, resulting in degraded charge transport and radiative process. Our study demonstrates the necessity of posttreatment above the glass transition temperature to obtain desirable efficiency and lifetime of solution-processed polyfluorene OLEDs.