Role of LiF in polymer light-emitting diodes with LiF-modified cathodes

Organic light-emitting diodes (OLEDs) are promising for full-color, full-motion, flat panel display applications [1][2][3] because they offer several advantageous features, for example, ease of fabrication, low costs, light weight, bright self-emission, a wide viewing angle, and the possibility of flexible displays. The basic OLED structure consists of a number of organic semiconductor layers sandwiched between a cathode and an anode. For efficient electron injection into the organic layers, low-work-function materials are required for the cathode. A very thin LiF layer with a thick Al capping is widely used for this purpose.[4] For the anode, indium tin oxide (ITO)is the predominant choice because it offers transparency in the visible range of the electromagnetic spectrum as well as electrical conductivity.[ [5][6][7] However, several aspects of ITO are far from optimal for high-performance OLEDs. It is known that the migration of indium and oxygen from ITO into organic semiconductors during OLED operation causes device degradation. [8,9] The electrical properties of ITO greatly depend on the film preparation. [10,11] The rough surface of the deposited ITO film and the work function of ITO, ca. 4.7 eV, limit the efficiency of the hole injection. [12] The typical sheet resistance of a 100 nm thick ITO layer, 20-80 X/&, is still high, which causes a voltage drop along the addressing line, thus limiting the operation of a large-area passive matrix OLED array. [13] Moreover, the cost of ITO has escalated in recent years because of the jump in price of the element indium. Several alternative materials, for example, TiN, [14] Al-doped ZnO, [15] and fluorine tin oxide, [16] have been investigated as anode materials instead of ITO; however, none are optimal as anode in OLEDs because they have either a lower work function or a lower conductivity than ITO. Other transparent conducting oxides, such as, and Zn-In-O (ZIO), that have a higher work function and a similar electrical conductivity when compared to ITO have also been examined as OLED anode materials.[17] However, they are potentially problematic because they also contain the element indium that i) may diffuse into the organic layer in the OLED; and ii) has a high price, making these electrodes expensive. Besides these materials several metals with a high work function, such as Au, [12] Ni, [18] and Pt, [19] have been investigated as anodes for OLEDs. In these cases the metal was used to modify the surface of the ITO electrode, or as an anode for top-emitting devices. A surface-modified thin Ag film [20,21] has been used as a semitransparent electrode instead of ITO, but its transparency was low. Recently, carbon nanotube films have been investigated as transparent, conductive electrodes, [22][23][24][25] but they have a high sheet resistance that may limit the device performance.In this Communication, we report semitransparent metal electrodes fabricated by nanoimprint lithography (NIL), and evaluate their potential as OLED anodes. NIL, an emerging lithographic te...

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“…A very thin LiF layer with a thick Al capping is widely used for this purpose. [4] For the anode, indium tin oxide (ITO)…”

mentioning

confidence: 99%

Nanoimprinted Semitransparent Metal Electrodes and Their Application in Organic Light‐Emitting Diodes

2007

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“…The thickness of the lithium phosphate was chosen to be 7 Å according to commonly reported optimal thicknesses of LiF interlayers in OLEDs. 8,12,15 The devices were finished with a 100 nm thick Al capping layer.…”

Section: Methodsmentioning

confidence: 99%

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

Gassmann

Melzer

Seggern

2009

Journal of Applied Physics

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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.

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“…Jin et al 35 applied the LiF/Al cathode in polymer light-emitting diodes (PLEDs), and found the evaporation of Al onto LiF produced space charges close to the cathode/polymer interface due to the doping of polymer with Li + ions instead of Li atoms. This space charge caused modest ndo ping and a shift of the built-in potential, thus decreasing the barrier for electron injection.…”

Section: Inorganic Compoundsmentioning

confidence: 99%

Review Paper: Progress on efficient cathodes for organic light‐emitting diodes

2011

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Abstract— Balanced charge injection is essential for highly efficient and stable OLEDs. Various cathode materials, such as elemental metals, metal alloys, and metal compounds, have been adopted to facilitate electron injection. Currently, composite cathodes utilizing an electron‐injection layer and an air‐stable metal, such as LiF/Al, is the most common choice. This article will review the progress of efficient vacuum‐evaporated cathodes for OLEDs and their mechanisms.

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Role of LiF in polymer light-emitting diodes with LiF-modified cathodes

Cited by 30 publications

References 51 publications

Nanoimprinted Semitransparent Metal Electrodes and Their Application in Organic Light‐Emitting Diodes

Nanoimprinted Semitransparent Metal Electrodes and Their Application in Organic Light‐Emitting Diodes

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

Review Paper: Progress on efficient cathodes for organic light‐emitting diodes

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