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

Abstract

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.

Characterization of higher harmonic modes in Fabry–Pérot microcavity organic light emitting diodes

“…Compared to the broadband emission of conventional OLEDs 2 , the emission of microcavity OLEDs offer potential advantages for spectroscopy, optical communications and displays 6 , 9 – 12 . The thickness of the transport layers can be varied to tune the emission through the visible 13 and into the deep blue and ultraviolet 14 , a color range that has proven challenging for conventional OLEDs.…”

“…The goal of this work is to address a range of these modes in the microcavity OLED, which requires that the molecular emitter has some electroluminescence intensity at the resonant wavelength. Thin-film OLEDs are ideally suited to address the fundamental resonant mode ( ) with emission peaks in the visible range between 400 and 700 nm, since the thickness ( d ) of an OLED is roughly 100 nm, and the index of refraction ( n ) of the organic semiconductors is generally between 1.5 and 2 3 , 4 , 13 , 14 . The optical path length within such an OLED microcavity is therefore around 200 nm, giving a fundamental, resonant peak at 400 nm.…”

Realization of Flexible Ultraviolet Organic Light‐Emitting Diodes: Key Design Issues

“…The seminal work on UV OLEDs was reported a Berggren et al, who combined the polymeric organic polymer poly(3‐(4‐octyl‐phenyl)‐2,2′‐bithiophene) (PTOPT) with 2‐(4‐Biphenylyl)‐5‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazole (PBD), a small‐molecular organic material, realizing UV OLEDs with a peak wavelength of 394 nm and 0.1% external quantum efficiency (EQE) at 15 V. [ 8 ] Various kinds of organic UV emitters were subsequently reported such as polysilane materials, [ 9,10 ] spirobifluorene materials, [ 11,12 ] heavy metal complexes, [ 13,14 ] thermally activated delayed fluorescence (TADF), [ 15 ] etc. [ 16–21 ] Recently, a few UV OLED works using thermal evaporation of a common wide‐gap electron transport layer (ETL) called 3‐(Biphenyl‐4‐yl)‐5‐(4‐tert‐butylphenyl)‐4‐phenyl‐4H‐1,2,4‐triazole (TAZ) as a UV emitter have been reported. [ 22–30 ] Efforts were focused mainly on identifying UV emitters and on improving the injection of holes into the deep highest occupied molecular orbital (HOMO) level of UV emitters (see the summary of the performance of previous UV OLEDs in Figure S1 and Table S1 in Supporting Information).…”

Highly Efficient Microcavity Organic Light-Emitting Devices with Narrow-Band Pure UV Emission

“…Therefore, the restriction of non-UV emission of organic materials to achieve narrow-band pure UV emission is still a significant challenge to obtain superior performance in the case of UVOLEDs. The vast majority of the presented device structures are focused on ITO-based devices with a broadband UV emission, and a limited number of reports have integrated metal microcavity (μC) into symmetric UVOLEDs to realize a narrow-band UV emission. , Meanwhile, the spectra of the reported μC UVOLEDs have an obviously visible-light component. Over all, the stability and color purity of UVOLEDs are still required to be further improved to meet the increasing need for portable compact near-UV sources with narrow-band pure UV emission.…”