Mechanisms of injection enhancement in organic light-emitting diodes through an Al/LiF electrode

Heil, Holger; Steiger, J.; Karg, Siegfried; Gastel, M.; Ortner, H.; Seggern, Heinz von; Stößel, M.

doi:10.1063/1.1331651

Cited by 195 publications

(130 citation statements)

References 11 publications

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“…15 Others claimed the same mechanism for LiF on Alq 3 . 7,11 Al/CsF on poly͑9,9-dioctylfluorene͒ ͑PFO͒ follows the same reaction scheme according to Greczynski et al 13 However, these authors did not observe this for Al/LiF on PFO, which suggests that the proposed reaction scheme is not generally applicable. Also, Yang et al noted that significant photoluminescence quenching should be expected if alkali metal ions diffuse into the active organic layer.…”

Section: Introductionmentioning

confidence: 66%

The interfaces of poly(p-phenylene vinylene) and fullerene derivatives with Al, LiF, and Al/LiF studied by secondary ion mass spectroscopy and x-ray photoelectron spectroscopy: Formation of AlF3 disproved

Gennip

Duren

Thüne

et al. 2002

The Journal of Chemical Physics

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Two mutually exclusive mechanisms have been proposed to explain the improved electron injection by the insertion of a LiF layer between the metal cathode and the active organic layer of organic photoelectronic devices: the dipole and the doping mechanism. The possibility of the doping mechanism was studied by investigating the interface of poly͓2-methoxy-5-͑3Ј,7Ј-dimethyl-octyloxyl ͒ -1, 4-phenylenevinylene ͔ ͑MDMO-PPV ͒ or 1-͑ 3-͑ methoxycarbonyl ͒ propyl͒-1-phenyl͓6,6͔C 61 ͑PCBM͒ with Al, LiF, or Al/LiF. In this mechanism, Li dopes the organic layer, after liberation via the reaction Alϩ3LiF→AlF 3 ϩ3Li. If this reaction takes place, AlF 3 should be detectable at the surface. However, SIMS measurements showed that AlF 3 is not present at the Al/LiF/MDMO-PPV and Al/LiF/PCBM interfaces. This is evidence that the proposed reaction does not occur. Other evidence that the doping mechanism cannot be the general mechanism to explain the enhanced electron injection comes from the presence of LiF on both organic surfaces. XPS measurements indicate that there is a reaction of Al with the carboxylic oxygen of PCBM, and that a LiF layer between PCBM and Al prevents this reaction.

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Section: Introductionmentioning

confidence: 66%

The interfaces of poly(p-phenylene vinylene) and fullerene derivatives with Al, LiF, and Al/LiF studied by secondary ion mass spectroscopy and x-ray photoelectron spectroscopy: Formation of AlF3 disproved

Gennip

Duren

Thüne

et al. 2002

The Journal of Chemical Physics

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“…Presently, the challenge is to further improve their performances (lifetime, stability, etc.) by understanding the phenomena that occur at the interfaces inside their multilayer structure (interlayer diffusion or degradation mechanisms depending on illumination, temperature variations, and atmospheric conditions [11,12]). Despite their excellent performance on organic samples, cluster ions are rather inefficient to depth profile inorganic samples [13], so that it remains a challenge to use them efficiently on hybrid samples [14].…”

Section: Introductionmentioning

confidence: 99%

Hybrid Organic/Inorganic Materials Depth Profiling Using Low Energy Cesium Ions

Noël

Houssiau

2016

J. Am. Soc. Mass Spectrom.

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Abstract. The structures developed in organic electronics, such as organic light emitting diodes (OLEDs) or organic photovoltaics (OPVs) devices always involve hybrid interfaces, joining metal or oxide layers with organic layers. No satisfactory method to probe these hybrid interfaces physical chemistry currently exists. One promising way to analyze such interfaces is to use in situ ion beam etching, but this requires ion beams able to depth profile both inorganic and organic layers. Mono-or diatomic ion beams commonly used to depth profile inorganic materials usually perform badly on organics, while cluster ion beams perform excellently on organics but yield poor results when organics and inorganics are mixed. Conversely, low energy Cs + beams (<500 eV) allow organic and inorganic materials depth profiling with comparable erosion rates. This paper shows a successful depth profiling of a model hybrid system made of metallic (Au, Cr) and organic (tyrosine) layers, sputtered with 500 eV Cs + ions. Tyrosine layers capped with metallic overlayers are depth profiled easily, with high intensities for the characteristic molecular ions and other specific fragments. Metallic Au or Cr atoms are recoiled into the organic layer where they cause some damage near the hybrid interface as well as changes in the erosion rate. However, these recoil implanted metallic atoms do not appear to severely degrade the depth profile overall quality. This first successful hybrid depth profiling report opens new possibilities for the study of OLEDs, organic solar cells, or other hybrid devices.

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“…The use of this material combination for inverted OLEDs, however, is complicated by the crucial nature of the deposition LiF/Al sequence: When deposition of LiF is followed by the Al layer, the resulting bilayer works well as EIL. However, bilayers fabricated with the inverted sequence (Al →LiF) function poorly (14,15). This striking difference comes primarily from the reactive nature of the Al atoms created by thermal evaporation.…”

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confidence: 99%

“…This striking difference comes primarily from the reactive nature of the Al atoms created by thermal evaporation. Their reaction with the earlier deposited LiF layer leads to the liberation of Li metal, forming reaction layers with a low work function (13,14). Thus, the LiF/Al combination does not provide a viable route to OLEDs with an inverted structure.…”

mentioning

confidence: 99%

Transparent amorphous oxide semiconductors for organic electronics: Application to inverted OLEDs

Hosono

Kim

Yoshitake

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

119

112

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Efficient electron transfer between a cathode and an active organic layer is one key to realizing high-performance organic devices, which require electron injection/transport materials with very low work functions. We developed two wide-bandgap amorphous (a-) oxide semiconductors, a-calcium aluminate electride (a-C12A7:e) and a-zinc silicate (a-ZSO). A-ZSO exhibits a low work function of 3.5 eV and high electron mobility of 1 cm 2 /(V · s); furthermore, it also forms an ohmic contact with not only conventional cathode materials but also anode materials. A-C12A7:e has an exceptionally low work function of 3.0 eV and is used to enhance the electron injection property from a-ZSO to an emission layer. The inverted electron-only and organic light-emitting diode (OLED) devices fabricated with these two materials exhibit excellent performance compared with the normal type with LiF/Al. This approach provides a solution to the problem of fabricating oxide thin-film transistor-driven OLEDs with both large size and high stability.inverted OLEDs | electron injection | electron transport | amorphous oxide semiconductor | low work function material E lectronic and photonic devices based on organic semiconductors have attracted much attention due to their intrinsic characteristics that are difficult to achieve in inorganic semiconductors, such as flexibility and the capability of precise molecular design of active organic layers (1-3). However, organic devices suffer from the material properties that organic semiconductors in general have: rather high lowest unoccupied molecular orbital (LUMO) levels and low electron mobilities, such as 10 −6 to 10 −3 cm 2 /(V·s). This leads to inefficiency in electron injection/transport between an electrode and an organic active layer and a large energy loss. In other words, the creation of materials suitable for efficient electron injection layers (EILs) and electron transport layers (ETLs), with very low work functions, reasonable chemical stability, and high electron mobility, should lead to organic devices with improved performance.Organic light-emitting diodes (OLEDs) are regarded as nextgeneration flat-panel displays (4). Although small high-resolution OLED displays are used widely for smart phones/tablets, and large televisions are being commercialized, several issues still remain, such as lifetime, image sticking, power consumption, and production cost (5). The recent high-resolution OLED pixels (6) with reduced aperture ratios [areal ratio of thin-film transistor (TFT) to pixel] raise the importance of the top emissiontype structure for practical use. For large OLEDs, it is unrealistic to use low-temperature polycrystalline silicon (LTPS) TFTs for backplanes. Only oxide TFTs, as represented by a-In-Ga-Zn-O (a-IGZO) (7), are practical candidates for the backplane (8, 9). The current small OLEDs use normal-type structures combined with p-channel LTPS TFTs. However, only n-channel TFTs are possible for oxide TFTs in actual applications. Simple substitution of the p-channel LTPS TFTs wi...

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Mechanisms of injection enhancement in organic light-emitting diodes through an Al/LiF electrode

Cited by 195 publications

References 11 publications

The interfaces of poly(p-phenylene vinylene) and fullerene derivatives with Al, LiF, and Al/LiF studied by secondary ion mass spectroscopy and x-ray photoelectron spectroscopy: Formation of AlF3 disproved

The interfaces of poly(p-phenylene vinylene) and fullerene derivatives with Al, LiF, and Al/LiF studied by secondary ion mass spectroscopy and x-ray photoelectron spectroscopy: Formation of AlF3 disproved

Hybrid Organic/Inorganic Materials Depth Profiling Using Low Energy Cesium Ions

Transparent amorphous oxide semiconductors for organic electronics: Application to inverted OLEDs

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