Investigation of solution-processed ultrathin electron injection layers for organic light-emitting diodes.
We study two types of water/alcohol-soluble aliphatic amines, polyethylenimine (PEI) and polyethylenimine-ethoxylated (PEIE), for their suitability as electron injection layers in solution-processed blue fluorescent organic light-emitting diodes (OLEDs). X-ray photoelectron spectroscopy is used to determine the nominal thickness of the polymer layers while ultraviolet photoelectron spectroscopy is carried out to determine the induced work-function change of the silver cathode. The determined work-function shifts are as high as 1.5 eV for PEI and 1.3 eV for PEIE. Furthermore, atomic force microscopy images reveal that homogeneous PEI and PEIE layers are present at nominal thicknesses of about 11 nm. Finally, we solution prepare blue emitting polymer-based OLEDs using PEI/PEIE in combination with Ag as cathode layers. Luminous efficiency reaches 3 and 2.2 cd A(-1), whereas maximum luminance values are as high as 8000 and 3000 cd m(-2) for PEI and PEIE injection layers, respectively. The prepared devices show a comparable performance to Ca/Ag OLEDs and an improved shelf lifetime.
Light Technology Institute, Karlsruhe Institute of Technology , Engesserstrasse 13, 76131 Karlsruhe, Germany.
Sebastian
S
Stolz
Michael
M
Scherer
Eric
E
Mankel
Robert
R
Lovrinčić
Janusz
J
Schinke
Wolfgang
W
Kowalsky
Wolfram
W
Jaegermann
Uli
U
Lemmer
Norman
N
Mechau
Gerardo
G
Hernandez-Sosa