Heterolayer light-emitting diodes based on poly-phenylene vinylene

Abstract

In order to enhance the quantum efficiency of poly‐p‐phenylene vinylene (PPV) light‐emitting diodes (LEDs), we have fabricated metal/insulator/polymer (MIP) LEDs and heterolayer LEDs based on PPV and oxadiazole polymers. The current–voltage (I–V) characteristics and electroluminescence (EL) intensity of the MIP structures display a pronounced dependence of the insulator thickness and we detect an increase in the quantum efficiency of more than a factor of 30 at an AlOx layer thickness of 3–6 nm. The device characteristics are qualitatively understood within inorganic metal insulator semi‐conductor (MIS) theory and can be explained by a voltage‐dependent barrier for minority carrier injection in connection with a hole‐blocking barrier at the PPV/insulator interface. Our oxadiazole polymers used in the heterolayer polymeric devices are characterized by a high thermal stability and excellent film‐forming properties. These materials act as efficient hole‐blocking, electron transport and injection layers in PPV‐based LEDs and we measure a significantly improved device performance with external quantum efficiencies of more than 0.5%. Temperature‐dependent investigations point to a relatively balanced charge carrier injection and reveal the influence of space charge limited currents on the device performance at low temperature. © 1997 John Wiley & Sons, Ltd.

Synthesis and photophysical properties of soluble low‐bandgap thienothiophene polymers with various alkyl side‐chain lengths

“…The HOMO level variation of PTtx‐O according to alkyl side‐chain lengths roughly agrees with the results from electrochemical measurement. As it has been common practice to estimate LUMO level by subtraction of the optical bandgap from HOMO level determined either electrochemically or via optical absorption spectra,40–42 here we show the energy band diagram derived from UPS and UV–vis measurement [Fig. 7(a)] and band diagram derived from electrochemical measurement [Fig.…”

Enhancement of electroluminescence efficiency for organic light-emitting-diodes due to the introduction of a co-evaporated layer

“…In recent papers, it was reported that the introduction of ionic materials such as LiF [7] and CaO [8] at the interface between a metal cathode and an organic layer enhances electron injection from cathode, resulting in improved EL efficiency in the organic LED. It was also reported that the introduction of good inorganic insulating materials such as AlOx [9,10] and SiO [11] at the interface enhances the EL efficiency. It is interesting and significant to clarify the carrier transfer behaviour at the inorganic-organic and organic-organic interfaces.…”