Enhanced quantum efficiency in polymer electroluminescence devices by inserting a tunneling barrier formed by Langmuir–Blodgett films

Abstract

Quantum efficiency in a polymer electroluminescence device is significantly improved by inserting a thin insulating layer with the thickness of tunneling range. Four times higher quantum efficiency was obtained without the increase of the threshold voltage. Poly(methyl methacrylate) Langmuir–Blodgett films were used as the thin tunneling barrier. The enhancement may result from the lowering of the effective barrier height for electron injection while increasing the effective barrier for hole injection. The effects improve the balanced injection of electrons and holes into the light-emitting devices.

Energy Level Alignment and Interfacial Electronic Structures at Organic/Metal and Organic/Organic Interfaces

“…By inserting a LiF layer, a lowering of the injection barrier by about 0.3 eV was observed, [52] suggesting that a similar lowering in the deposition of Al/LiF/Alq 3 system is a possible origin of enhanced efficiency, although other factors are also proposed. [153] Similar reduction of electron injection barrier was observed by Tokito and collaborators, [88] while Shaheen et al [156] reported still larger work function lowering of Al by depositing LiF, although they did not examine the deposition of Alq 3 on these modified substrates.…”

Energy‐Level Alignment at Organic/Metal and Organic/Organic Interfaces

“…The different possibilities for chemical-bonding sites can cause local variations in both dipole energy and molecular-orbital modification, creating a distribution of injection barriers and trap energies at the electrode in a device. These effects in part explain the utility of the thin insulating so-called spacer layers [101][102][103][104][105][106][107] inserted between the metal cathode and the p-conjugated organic film in many devices. The spacer layers are typically thin enough to allow for tunneling and for decoupling the organic film from the metal cathode, bringing it into the physisorption regime covered by the ICT model.…”

Performance Improvement of Organic Light Emitting Diodes Using Poly(N‐vinylcarbazole) (PVK) as a Blocking Layer

“…[2][3][4][5][6][7][8][9] Among them, an inserting of thin layer of conducting, semiconducting or insulating materials such as polyaniline, poly(phenylenevinylene) and Teflon [10][11][12] between the electrode and the emitting layer was used as simple methods to increase efficiency. In these studies, it seems that the excess holes from an anode were controlled by the inserted thin layers, which may help improve the balance of hole and electron.…”