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

• Deep blue to near UV microcavity OLEDs with mixed PVK:CBP emitting layer.
• Microcavity OLED pixel array with peak emissions ranging from ∼370 to 470 nm.
• Addressing interest in UV OLEDs excitation sources for analytical applications.
• Ab initio simulations in good agreement with experimental spectra.
• Interesting characteristics of mixed polymer/small molecule emission layers.

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 (μc) OLEDs (λ ∼373–469 nm) using, in a unique approach, a mixed emitting layer (EML) of poly(N-vinyl carbazole) (PVK) and 4,4′-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 μcOLEDs 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 μcOLED arrays of the structure 40 nm Ag/x  nm MoOx/∼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 μc devices, only CBP emission was observed, while in the long wavelength μc 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 μc 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 MoOx layer complemented the data analysis.

(a) Top: source profile EL spectra calculated from the experimental data with Gaussian fits. Bottom: the measured and calculated (based on these Gaussians) EL of microcavity (μc) PVK:CBP OLEDs. (b) EL spectra of a μc OLED array with a mixed PVK:CBP EML. (c) The molecular packing in the PVK (large spheres):CBP (small spheres) films.Figure optionsDownload as PowerPoint slide