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

• Published in: Advanced functional materials Vol. 22; no. 1; pp. 144 - 150
• Main Authors: Lu, Li-Ping, Kabra, Dinesh, Johnson, Kerr, Friend, Richard H.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 11.01.2012; WILEY‐VCH Verlag
• Subjects: charge-carrier balance; color purity; hole traps; PLEDs; polyfluorenes
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.201101892

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 ≈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 UV–vis 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 ≈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. Charge‐carrier balance and color purity is achieved in an arylated polyfluorene‐based blue‐emitting light‐emitting diode (LED) by use of a macromolecular additive (TFB). This additive provides excellent control over current density while keeping the optical properties of emissive materials intact. Luminance efficiency of up to 5.9 cd A−1 along with a pure blue emission is achieved in the blend instead of 1.2 cd A−1 in the homopolymer‐based LED.