Material design of hole transport materials capable of thick-film formation in organic light emitting diodes

Abstract

In this study, the authors show an empirical guideline for designing hole transport materials (HTMs) that suppress rises in driving voltage even with a few hundred nanometer thick film in the organic light emitting diodes (OLEDs). In a device structure of indium tin oxide (110nm)/hole transport layer (HTL) (Xnm)∕4,4′-N,N′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (10nm)/tris-(8-hydroxyquinoline)aluminum (Alq3) (50nm)∕MgAg (100nm)∕Ag (10nm), the authors compared electroluminescence characteristics of the OLEDs having a thin-film HTL (X=50nm) and a thick-film HTL (X=300nm) using 13 kinds of HTMs. They observed a closed correlation between suppression of the driving voltage and the HTMs’ thermal characteristics. Highly thermally stable HTMs resulted in a small increase in the driving voltage.

Simple model-free estimation of orientation order parameters of vacuum-deposited and spin-coated amorphous films used in organic light-emitting diodes

“…Figure 1 shows the chemical structures of the main OLED materials used in this study including -triphenylbiphenyl-4,4A-diamine) (TPT1), and 2,7-bis(9,9-spirobifluoren-2-yl)-9,9-spirobifluorene (TSBF), which have glass transition temperatures (T g ) of 60, 96, 110, 144, and 231°C, respectively. [9][10][11][12] For each material, we fabricated vacuum-deposited films with thicknesses of 20, 50, and 100 nm on fused silica and Si(100) substrates at a deposition rate of 2 Å=s under a vacuum of <1 × 10 −3 Pa. Spin-coated films were also fabricated on the same types of substrates using solutions of each material in chloroform (15 mg=mL) at spin speeds of 1000, 3000, 5000, and 7000 rpm, followed by mild baking under nitrogen atmosphere for 30 min on a hot plate at a temperature lower than T g : 50°C for TPD and 80°C for the other materials. We measured the absorption spectra of all the sample films on fused silica substrates using a spectrophotometer (Shimadzu UV2450).…”

Small Organic Molecules

“…Here S will be −0.5 if the molecules are completely parallel to the surface, 0 for random orientation and 1 if they are perpendicular to the surface [42]. A correlation between S and the OLEDs driving voltage was found [42,45], indicating again that horizontally oriented molecules provide better electrical characteristics. In ref.…”

A Dithienylbenzothiadiazole Pure Red Molecular Emitter with Electron Transport and Exciton Self‐Confinement for Nondoped Organic Red‐Light‐Emitting Diodes

“…A thicker HTL is often preferable to prevent short circuits and dielectric breakdown in ''thin film'' electroluminescent devices, as long as the driving voltage is not significantly affected. [8,9] On the other hand, however, any time the layer thickness is changed, the device efficiency may be affected due to the cavity effect. Generally, when hole transport is predominant in the emissive layer, the addition of a hole-blocking layer contributes to confining exciton formation at the EML/HBL interface, thus increasing external quantum efficiency.…”