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 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.

Synthesis, characterization, and electrogenerated chemiluminescence of deep blue emitting eumelanin‐inspired poly(indoylenearylene)s for polymer light emitting diodes

“…In comparison, devices fabricated with neat PIF showed a very poor performance. This may be due to the strong concentration quenching of the polymer in its solid state . The highest brightness (∼200 Cd/m 2 ) was achieved for a device with 5.0 wt % of PIF in mCP (Supporting Information Figure S7).…”

Carbazole-based polymers as hosts for solution-processed organic light-emitting diodes: Simplicity, efficacy

“…Using tris(2-phenylpyidine) Ir(ppy) 3 as the dopant, a brightness of 7000 cd/m 2 , a CE of 38 cd/A, a PE of 13 lm/W and an EQE of 11% were obtained for unoptimized devices. Recently, a comparison between the small molecule 4,4 0 -bis(9-carbazolyl)biphenyl (CBP) and PVK as hosts for a light-emitting polymer was established [104]. It has to be noticed that this approach is unusual for solution-processed devices and was reported for the first time by these authors.…”

Deep blue/ultraviolet microcavity OLEDs based on solution-processed PVK:CBP blends

“…AFM scans of the EML on [ITO or Ag 40 nm]/MoO x [5 or 20 nm] show that the PVK:CBP layer is smooth (R RMS 6 1 nm), slightly more so with Ag than with ITO. Previous studies have shown that smooth AFM images correspond to films that are homogeneous [38][39][40]. We therefore conclude that the PVK:CBP is likely not phase separated or aggregated.…”

“…However, as there is still obvious PVK emission in the long optical length devices, some excitons form on or transfer to the slightly lower gap sites on PVK molecules. We also suspect that as the HOMO level of PVK is shallower than that of CBP, it likely traps holes in coiled sites [40]. We note that while the absorption edge of PVK is at shorter wavelength than that of CBP, the source emission of CBP is at shorter wavelength, i.e., the Stokes shift of PVK is considerably larger than that of CBP.…”