Charge-Carrier Balance and Color Purity in Polyfluorene Polymer Blends for Blue Light-Emitting Diodes

A study of an efficient blue light-emitting diode based on a fluorescent aryl polyfluorene (aryl-F8) homopolymer in an inverted device architecture is presented, with ZnO and MoO3 as electron- and hole-injecting electrodes, respectively. Charge-carrier balance and color purity in these structures are achieved by incorporating poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)-diphenylamine (TFB) into aryl-F8. TFB is known to be a hole-transporting material but it is found to act as a hole trap on mixing with aryl-F8. Luminance efficiency of approximate to 6 cd A(-1) and external quantum efficiency (EQE) of 3.1% are obtained by adding a small amount (0.5% by weight) of TFB into aryl-F8. Study of charge injection and transport in the single-carrier devices shows that the addition of a small fraction of hole traps is necessary for charge-carrier balance. Optical studies using UVvis and fluorescence spectroscopic measurements, photoluminescence quantum yield, and fluorescence decay time measurements indicate that TFB does not affect the optical properties of the aryl-F8, which is the emitting material in these devices. Luminance efficiency of up to approximate to 11 cd A(-1) and EQE values of 5.7% are achieved in these structures with the aid of improved out-coupling using index-matched hemispheres.