Search

Solution-based processing was applied to fabricate green phosphorescent organic light-emitting diodes (OLEDs). EPH31 was used as a phosphorescent host, doped with guest dopant green phosphorescent Ir(ppy)3, and dissolved in chlorobenzene solvent to form the emitting layer. Device structural parameters were controlled by changing the spin coating speed of the emitting layer and hole injection layer [poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate), PDOT:PSS] to adjust the thickness of the electron transport layer [tris(8-hydroxyquinolinato)aluminum, Alq3]. In addition, the differences in using CsF and LiF materials as the electron injection layer were investigated. A maximum current efficiency of 13.6 cd·A-1 was obtained at a high emitting layer spin coating speed. Despite the close resemblance in both the luminance intensity and current efficiency when using CsF and LiF as the electron injection layer, CsF devices had a low driving voltage. Smooth and stable films resulting from the spin coated hole injection layer, along with the control of the thickness of the electron transport layer (Alq3) and electron injection layer (CsF), effectively improved the performance of green OLEDs. The emitting layer host material (CBP) and three guest dopants [Firpic, Ir(ppy)3, and Ir(piq)2] were dissolved in toluene solvent during solution preparation to fabricate white OLEDs. The properties of the resulting solution-processed white PHOLEDs are a current efficiency of 2.4 cd·A-1 at 20 mA·cm-2 and CIE coordinates of (0.33, 0.33) at 9 V. Results of these experiments demonstrate that solution processing can be used as an alternative to and in conjunction with thermal evaporation.

This study is carried out to discuss how to reduce the driving voltage of blue phosphorescent organic light-emitting diodes (PHOLEDs) by using a thin double-emission layer. A hole transport-type host (TCTA) is inserted between the hole transport layer (TAPC) and the emitting layer (EML), constituting a buffer layer between them with the aim of improving charge carrier balance. Furthermore, in this study, we also utilize the interface between double light-emitting layers of devices by codoping them with a red phosphorescent dopant [Os(bpftz)2(PPh2Me)2]. An Os complex with a high-lying highest occupied molecular orbital (HOMO) energy level (trapping holes) is codoped at the interface between emitting layers and an exciton-formation zone is expanded to obtain a white PHOLED with high efficiency. From the results, the optimal structure of the white device exhibits a yield of 35 cd A-1, a power efficiency of 22 lm W-1, and CIE coordinates of (0.33,0.38) at a luminance of 1000 cd m-2. Furthermore, the power efficiency can be improved to 30 lm W-1 by attaching the outcoupling enhancement film.