Donal Bradley

Abstract:

We report a novel approach to achieve deep-blue, high-efficiency, and long-lived solution-processed polymer light-emitting diodes (PLEDs) via a simple molecular level conformation change of an emissive conjugated polymer. We introduce rigid β-phase segments into a 95% fluorene-5% arylamine copolymer emissive layer. The arylamine moieties at low density act as efficient exciton formation sites in PLEDs, whereas the conformational change alters the nature of the dominant luminescence from a broad, charge transfer like emission to a significantly blue-shifted and highly vibronically structured excitonic emission. As a consequence, we observe a significant improvement in the Commission International de L'Eclairage ( x, y) coordinates from (0.149, 0.175) to (0.145, 0.123) while maintaining high efficiency and improved stability. We achieve a peak luminous efficiency, η = 3.60 cd/A, and a luminous power efficiency, ηw = 2.44 lm/W, values that represent state-of-the-art performance for single copolymer deep-blue PLEDs. These values are 5-fold better than for otherwise-equivalent, β-phase poly(9,9-dioctylfluorene) PLEDs (0.70 cd/A and 0.38 lm/W). This report represents the first demonstration of the use of molecular conformation as a simple but effective method to control the optoelectronic properties of a fluorene copolymer; previous examples have been confined to homopolymers.

Abstract:

We report state-of-the-art fluorene-based yellow-green conjugated polymer blend gain media using Förster resonant-energy-transfer (FRET) from novel blue-emitting hosts to yield low threshold (≤ 7 kW cm-2) lasers operating between 540 and 590 nm. For poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) (15 wt.%) blended with our newly synthesised 3,6-bis(2,7-di([1,1'-biphenyl]-4-yl)-9-phenyl-9H-fluoren-9-yl)-9-octyl-9H–carbazole (DBPhFCz) a highly desirable more than four-times increase (relative to F8BT) in net optical gain to 90 cm-1 and thirty four-times reduction in amplified spontaneous emission threshold to 3 µJ cm-2 is achieved. Detailed transient absorption studies confirm effective exciton confinement with consequent diffusion-limited polaron-pair generation for DBPhFCz. This delays formation of host photoinduced absorption long enough to enable build-up of the spectrally overlapped, guest optical gain and resolves a longstanding issue for conjugated polymer photonics. Our comprehensive study further establishes that limiting host conjugation length is a key factor therein, with 9,9-dialkylfluorene trimers also suitable hosts for F8BT but not pentamers, heptamers or polymers. We additionally demonstrate that the host highest occupied and lowest unoccupied molecular orbitals can be tuned independently from the guest gain properties. This provides the tantalizing prospect of enhanced electron and hole injection and transport without endangering efficient optical gain; a scenario of great interest for electrically pumped amplifiers and lasers.