Light Emission Color Conversion of Polyfluorene-Blend OLEDs Induced by Thermal Annealing

This paper investigated the applicability of thermal annealing to be used for the color conversion of organic light-emitting diodes (OLEDs) made of a blend of light-emitting polyfluorenes. Blue-light-emitting polyfluorene poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) was used as the host material and green-light-emitting polyfluorene poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT) as the dopant material. Annealing treatment was conducted at the temperatures of 150 °C and 270 °C, before top contact deposition. Both nonannealed OLEDs and those annealed at 270 °C showed green color emission, implying that emission was taking place via the dopant material F8BT. Instead, the light emission color of the OLEDs annealed at 150 °C was shifted to white upon a rise of a blue spectral component. The origin of the blue light emission was attributed to PFO and indicated considerable phase separation between PFO and F8BT. This paper showed that thermal annealing can be applied to modify the light emission color of polyfluorene-blend OLEDs.