No abstract

Benzobisoxazoles (BBOs) are known to increase the electron affinities and improve the electron transporting properties of materials containing them. However, BBO copolymers generally do not perform well as emissive guests in guest−host PLEDs due to inefficient Forster resonance energy transfer (FRET) between host and guest. The incomplete FRET results in a large amount of host emission and limits the potential efficiencies of the devices. In all previously reported BBO copolymers, the conjugation pathway was through the oxazole rings. Herein we report six new BBO copolymers with backbone connectivity directly on the central benzene ring, resulting in a conjugation pathway for the polymers that is perpendicular to the previously reported pathway. Guest−host PLEDs made using these polymers show that the new conjugation pathway improves FRET between the poly(N-vinylcarbazole) host and the BBO-containing polymer guest. Because of highly efficient FRET, no host emission is observed even at lower guest concentrations. The improved energy transfer results in devices with luminous efficiencies up to 3.1 Cd/A, a 3-fold improvement over previously reported BBO-based PLEDs. These results indicate that the conjugation pathway plays a critical role in designing emissive materials for guest−host PLEDs.

Solution-processed OLEDs with polymer hosts and polymer or small-molecule guests have been studied extensively. More recently, efficient solution-processed OLEDs with small molecule hosts and small molecule guests were also reported. However, small molecule hosts of polymer guests in solutionprocessed fluorescent OLEDs have not been investigated. In this work guest:host systems consisting of the small molecule 4,4 0 -bis(9-carbazolyl)-biphenyl (CBP) as host to polymer guests such as novel benzobisoxazole (BBO)-containing copolymers and well-known poly(2-methoxy-5-(2 0 -ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) are compared to those with poly(N-vinyl carbazole) (PVK) host, which previously yielded highly efficient phosphorescent OLEDs. In the case of MEH-PPV, guest:host OLEDs are also compared to those with a neat MEH-PPV emitting layer. It is found that replacing the polymer host PVK with the small molecule host CBP improves efficiencies by up to 100%. A blue emissive BBO-polymer:CBP device reaches a luminous efficiency (h L,max ) of 3.4 cd A À1 (external quantum efficiency h ext ¼ 2.4%), while the PVK-based device exhibits h L,max ¼ 1.7 cd A À1 (h ext ¼ 1.2%). A green emissive BBO:CBP OLED exhibits h L,max ¼ 5.7 cd A À1 (h ext ¼ 2.1%), while that in the PVK host is 3.1 cd A À1 (h ext ¼ 1.1%). For MEH-PPV:CBP these values are 3.7 cd A À1 (h ext ¼ 1.4%), compared to 2.9 cd A À1 (h ext ¼ 1.0%) for MEH-PPV:PVK and 0.7 cd A À1 (h ext ¼ 0.4%) for the neat MEH-PPV device. Possible origins of the improvement are discussed, including increased charge mobility, smoother film morphology, and the potential effect of multiple non-coiling host small molecules (in contrast to the likely coiled PVK) surrounding a polymer guest.