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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.

Benzobisoxazole polymers possessing a conjugation pathway directly through the central benzene ring possess reduced optical band gaps and more efficient electroluminescence in polymer light-emitting diodes.

The spin-1/2 single modulation (SM) and double modulation (DM) photoluminescence (PL) detected magnetic resonance (PLDMR) in poly(2-methoxy-5-(2'-ethyl)-hexoxy-1,4-phenylene vinylene) (MEH-PPV) films and poly(3-hexylthiophene) (P3HT) films is described, analyzed, and discussed. In particular, the models based on spin-dependent recombination of charge pairs (SDR) and triplet-polaron quenching (TPQ) are evaluated. By analyzing the dependence of the resonance amplitude on the microwave chopping (modulation) frequency using rate equations, it is demonstrated that the TPQ model can well explain the observed resonance behavior, while SDR model cannot reproduce the results of the observed DM-PLDMR. Thus the observed spin-1/2 PLDMR is assigned to TPQ rather than SDR, even though the latter may also be present.

Polymer light-emitting diodes based on poly(N-vinylcarbazole) (PVK) with molecular weights MW of 1.1 × 106 and ∼7.5 × 104 are compared. For devices without an electron transport layer (ETL), the high MW PVK yields higher external quantum efficiency (0.67% vs 0.18%), but for devices with an ETL, the low MW PVK yields higher efficiency (1.13% vs 0.83%). This intriguing difference is believed to result from higher energetic disorder in the high MW polymer and different recombination zone-quenching metal electrode distances, in agreement with Konezny et al. [Appl. Phys. Lett. 97, 143305 (2010)].

Keywords:UV-to-blue OLED arrays UV-to-blue microcavity OLED arrays PVK:CBP OLED arrays Polymer/small molecule mixed emission layer Ab initio simulations of OLED EL spectra a b s t r a c t There is an increasing need to develop stable, high-intensity, efficient OLEDs in the deep blue and UV. Applications include blue pixels for displays and tunable narrow solid-state UV sources for sensing, diagnostics, and development of a wide band spectrometer-on-a-chip. With the aim of developing such OLEDs we demonstrate an array of deep blue to near UV tunable microcavity (lc) OLEDs (k $373-469 nm) using, in a unique approach, a mixed emitting layer (EML) of poly(N-vinyl carbazole) (PVK) and 4,4 0 -bis(9-carbazolyl)-biphenyl (CBP), whose ITO-based devices show a broad electroluminescence (EL) in the wavelength range of interest. This 373-469 nm band expands the 493-640 nm range previously attained with lcOLEDs into the desired deep blue-to-near UV range. Moreover, the current work highlights interesting characteristics of the complexity of mixed EML emission in combinatorial 2-d lcOLED arrays of the structure 40 nm Ag/x nm MoO x /$30 nm PVK:CBP (3:1 weight ratio)/y nm 4,7-diphenyl-1,10-phenanthroline (BPhen)/1 nm LiF/100 nm Al, where x = 5, 10, 15, and 20 nm and y = 10, 15, 20, and 30 nm. In the short wavelength lc devices, only CBP emission was observed, while in the long wavelength lc devices the emission from both PVK and CBP was evident. To understand this behavior simulations based on the scattering matrix method, were performed. The source profile of the EML was extracted from the measured EL of ITO-based devices. The calculated lc spectra indeed indicated that in the thinner, short wavelength devices the emission is primarily from CBP; in the thicker devices both CBP and PVK contribute to the EL. This situation is due to the effect of the optical cavity length on the relative contributions of PVK and CBP EL through a change in the wavelength-dependent emission rate, which was not suggested previously. Structural analysis of the EML and the preceding MoO x layer complemented the data analysis.

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