The Li3PO4/Al bilayer: An efficient cathode for organic light emitting devices

Abstract

In this contribution an efficient cathode material for organic light emitting diodes (OLEDs) is introduced consisting of a thin layer of the metal salt lithium phosphate (Li3PO4) deposited between the organic semiconductor and an Al cathode. The bilayer cathode Li3PO4/Al enables a device performance of small molecule based OLEDs competitive to the benchmark cathode LiF/Al. While current densities and luminances of both systems are alike, the use of Li3PO4 substantially increases the device lifetime. It will be shown that the improved device characteristics can be ascribed to a stably enhanced electron injection. We demonstrate that neither a field enhancement across the Li3PO4 layer due to accumulated holes nor a possible charge transfer doping by the Li3PO4 is the reason for the improved electron injection. Investigation of the Li3PO4/Al interface by Kelvin probe techniques disclosed a work function lowering of the cathode that facilitates electron injection and finally explains the improvement in performance of respective organic light emitting diodes.

The Challenge of Producing Fiber-Based Organic Electronic Devices

“…Figure 3 shows the current density j as a function of the electric field F of an OLED on a glass stick (1 mm diameter) with 1 mm length, which was measured in dry nitrogen atmosphere using a HP 4155A semiconductor parameter analyzer and a photodiode (BPW 34, Osram, Regensburg, Germany). It has to be noted that the electric field corresponds to voltages of up to 21 V and is thus higher than typically applied to flat OLEDs using the same organic semiconductors [ 17 ]. This mainly results from the voltage drop along the cathode which on the one hand has to be kept thin to out-couple the generated light (see Figure 4 ) but resulting on the other hand in a high resistivity.…”

“…In the second step, the handling of the substrates during device preparation had to be elaborated and the layer formation was investigated separately. Finally, top emitting OLEDs were produced using a prototypical material stack [ 17 ] with process parameters adapted from flat geometry devices. In the course of these experiments critical parameters and material-related issues could be identified which will be discussed in detail.…”

Recombination Zone Inside Transistor Channel in an Alq3/Pentacene Organic Light-Emitting Transistor

“…The device also has asymmetric electrodes. The source electrode is made of gold (Au) (work function of 5.1 eV), while the drain electrode material is Lithium Fluoride/aluminum (LiF/Al) (work function of 2.9 eV), leading to an easy electron injection in the Lowest unoccupied molecular orbital (LUMO) of Alq3 [17]. Both of them are 1 µm long and 50 nm thick.…”

Systematic study of photoluminescence, lyoluminescence and mechanoluminescence in Ce3+‐ and Eu3+‐activated Li3PO4 phosphors

“…Baginskiy and co-workers discovered the application of LiZnPO 4 :Mn for an LED (1). A bilayer cathode consisting of a thin Li 3 PO 4 layer with an optimal thickness of 7 Å capped with Al may provide efficient electron injection in Organic light emitting diodes (OLEDs) (2). The combined effect of thermal and electric (dc and ac, V = 0.15-1) fields on the crystallization behaviour of Li 3 PO 4 + Li 4 GeO 4 causes melting and the composition of the crystallization products has been described by Ksenofontov et al (3).…”