Three new solution processable small molecular host materials based on bis-[3,5-di(9H-carbazol-9-yl)phenyl] structural moiety have been developed for blue phosphorescence (FIrpic dopant) organic light-emitting diodes. Whereas N,N-bis-[3,5-di(9H-carbazol-9-yl)phenyl]methylamine (CzPAMe) has the highest solid state triplet energy gap (ET) of 2.73 eV, tetrakis-[3,3',5,5'-(9H-carbazol-9-yl)]triphenylphosphine oxide (CzPPO) and N,N-bis-[3,5-di(9H-carbazol-9-yl)phenyl]pyrimidin-2-amine (CzPAPm) are two host materials potentially being bipolar for charge transport due to the electron deficient unit of phenylphosphine oxide and pyrimidine, respectively. Due to the insufficient ET (2.56 eV) of CzPAPm, CzPPO or CzPAMe devices are significantly better than CzPAPm devices with or without 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD-7) co-host. Particularly, having no OXD-7 co-host and no vacuum-thermal-deposited extra electron transporting layer, single-layer devices of CzPPO surpassing CzPAMe devices reach current efficiency as high as 9.32 cd/A (or power efficiency of 4.97 lm/W), which is one of the highest of the kind. Corresponding single-layer white phosphorescence OLEDs are also fabricated with the small molecular host material demonstrated herein.
In this paper, we demonstrate simulation results of top-emission organic light-emitting devices (TOLEDs) with red, green and blue (RGB) colors. We take the RGB spectral peaks at 460, 520, and 600 nm with full width at half maximum of 100 nm. Device structures are thick silver (Ag) anode /hole-transport layer 60 nm/ emitting layer (EML) /semi-transparent Ag cathode 20 nm/ dielectric layer. The dielectric material capped upon the cathode is zinc selenium with a refractive index of 2.6 for providing high output intensity and narrow FWHM. When monitoring the peak wavelengths of RGB device and varying the EML and dielectric thicknesses, we found the optimized value of the EML are 71, 47 and 31 nm for the red, green and blue devices, respectively. The optimized dielectric thicknesses are 80, 70 and 50 nm with periods of 117, 98 and 90 nm, respectively, for the RGB devices. Due to the limitations of the experiments, the EML thicknesses can be different and the dielectric thickness must be the same of the RGB devices. For optimizing the BGB devices simultaneously, the thickness of dielectric layer of the OLED is 667 nm. The RGB peak intensities are 0.96, 0.99 and 0.83, normalized to their optimized value. Typically, in a TOLED, green device exhibits higher efficiency than red and blue ones. That means the intensity of the green TOLED can be lower. When the dielectric layer thickness is 314 nm, the normalized RGB peak intensities are 0.99, 0.26 and 0.97.