Fluorescent polymer guest:small molecule host solution-processed OLEDs

Abstract

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 solution-processed fluorescent OLEDs have not been investigated. In this work guest:host systems consisting of the small molecule 4,4′-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′-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 (η_L,max) of 3.4 cd A⁻¹ (external quantum efficiency η_ext = 2.4%), while the PVK-based device exhibits η_L,max = 1.7 cd A⁻¹ (η_ext = 1.2%). A green emissive BBO:CBP OLED exhibits η_L,max = 5.7 cd A⁻¹ (η_ext = 2.1%), while that in the PVK host is 3.1 cd A⁻¹ (η_ext = 1.1%). For MEH-PPV:CBP these values are 3.7 cd A⁻¹ (η_ext = 1.4%), compared to 2.9 cd A⁻¹ (η_ext = 1.0%) for MEH-PPV:PVK and 0.7 cd A⁻¹ (η_ext = 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.