The interplay between the optical and electronic properties of light-emitting-diode applicable conjugated polymer blends and their phase-separated morphology

We have investigated a series of different blends of the device applicable conjugated polymers poly(9,9-dioctylfluorene) [F8] and poly(9,9-dioctylfluorene-alt-benzothiadiazole) [F8BT]. Scanning force microscopy indicates that blend films of F8 and F8BT spin-cast from toluene have more coarsely structured phase-separated domains, whereas films cast from chloroform are much more finely structured. This conclusion is confirmed using photoluminescence (PL) measurements, where we show that the degree of energy transfer from F8 to F8BT can be used as a measure of de-mixing in the blend. We have investigated different F8:F8BT blend films as the active semiconductor in light emitting diodes (LEDs). We find that the electroluminescence (EL) efficiency of the LED is a strong function of the relative composition of the blend, with highest efficiency occurring in blends of 95:5 F8:F8BT. In this particular polymer system, the morphological structure of the blend appears less important in determining LED efficiency than does the blend composition.