Interface formation between poly(2,5-diheptyl-p-phenylenevinylene) and calcium: implications for light-emitting diodes

Dannetun, Per; Fahlman, Mats; Fauquet, C.; Kaerijama, K.; Sonoda, Yoriko; Lazzaroni, Roberto; Brédas, Jean‐Luc; Salaneck, W. R.

doi:10.1016/0379-6779(94)90026-4

Cited by 69 publications

(49 citation statements)

References 11 publications

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“…From XPS measurements it has been shown that interaction between PPV and calcium occurs. 5,6,8,38 The influence of interaction on the diffusion can be determined by comparing diffusion coefficients from interacting and noninteracting species in PPV, which are comparable in size. The diffusion coefficient of (noninteracting) oxygen in OC 1 C 10 PPV is many orders of magnitude higher ͑D =10 −9 m 2 /s at T = 298 K͒ 39 than the diffusion coefficient for calcium and barium into PPV that we measured.…”

Section: Comparison Between Calcium and Barium Diffusionmentioning

confidence: 99%

“…However, it was suggested 7 that the interface stability does play an important role in PLED degradation. Diffusion of metals into PPV derivatives and PPV model systems during deposition has been observed, [8][9][10][11][12] but the dynamics of the diffusion process was not investigated. Furthermore, it has been shown that PLEDs with barium cathodes have better efficiencies and longer lifetimes than devices with calcium cathodes.…”

Section: Introductionmentioning

confidence: 99%

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Interface formation between metal and poly-dialkoxy-p-phenylene vinylene

et al. 2004

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In this work we address the dynamics and stability of calcium/PPV and barium/PPV interfaces during and after deposition of the metal. Diffusion of calcium and barium into OC 1 C 10 PPV is studied with low energy ion scattering (LEIS) and x-ray photoelectron spectroscopy (XPS). During metal deposition the diffusivity is found to be orders of magnitude higher than after deposition and the diffusion coefficient was found to be dependent on the metal concentration in the PPV. Furthermore, the amount of metal inside the polymer films was found to depend on the deposition rate. These observations were explained in a two-stage diffusion model. In the first stage atoms land on the surface and diffuse fast into the polymer and in the second stage metal ionizes and is trapped and diffusion is strongly decreased. The diffusion coefficient of barium into PPV at T = 298 K is found to be almost an order of magnitude lower than the diffusion coefficient of calcium into PPV [͑0.35± 0.05͒ ϫ 10 −23 m 2 / s and ͑2.7± 0.4͒ ϫ 10 −23 m 2 / s, respectively]. Furthermore, the activation energy of the diffusion process of barium into PPV ͑0.75± 0.07eV͒ is significantly higher than the activation energy of the diffusion process of calcium into PPV ͑0.62± 0.05 eV͒. The difference in diffusion coefficient and activation energy between calcium and barium are discussed in terms of an Arrhenius law of diffusion. Finally, polymer LED performance was studied as a function of the amount of metal diffused into the polymer layer. It was observed that the light output and the efficiency decreased as the amount of metal in the PPV increased. This indicates that the metal ions form charge carrier traps and exciton quenching sites in the PPV.

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Section: Comparison Between Calcium and Barium Diffusionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Interface formation between metal and poly-dialkoxy-p-phenylene vinylene

et al. 2004

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“…Observed clearly first for DP7 [32], and subsequently for DHPPV [33], calcium diffuses into the near-surface region of a PPV film, donates electrons to the p-system, and forms Ca 2 -ions. The interfacial region between the Ca±metal contact and the polymer has an approximate scale in the range of 20±40 A Ê (similar to the case of A1 atoms, where the range is about 20±30 A Ê ).…”

Section: Calcium On Ppvmentioning

confidence: 99%

“…5 where the ideal interface model is shown at top, and the two extremes are shown below: doped bipolaron states in the near-surface region for the case of clean calcium on clean PPVs [32,33]; and the calcium ªoxideº interfacial layer, in the case of calcium on PPV with a high density of oxygen-containing species on the surface [34].…”

Section: Calcium On Ppvmentioning

confidence: 99%

Conjugated polymer surfaces and interfaces in polymer-based light-emitting diodes

Salaneck

Lögdlund

1998

Polym. Adv. Technol.

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Since the discovery of high electrical conductivity in doped polyacetylene in 1977, π‐conjugated polymers have emerged as viable semiconducting electronic materials for numerous applications. In the context of polymer electronic devices, it is of critical importance to understand the nature of the electronic structure of the polymer surface and the interface with metals. It has been shown that, for conjugated polymers, photoelectron spectroscopy, especially in connection with quantum chemical modeling, provides a maximum amount of both chemical and electronic structural information in one type of measurement. There is no such thing as the ideal metal‐on‐polymer contact; there is always some chemistry that occurs at the interface. © 1998 John Wiley & Sons, Ltd.

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“…4 -6 In by far most cases the metallic cathode is deposited by thermal evaporation or sputter deposition on the organic material directly onto the emissive organic layer, or onto an organic charge transport and electron-injection layer. The deposition is a crucial step in the fabrication process since the metals diffuse into, [7][8][9] and interact with, the organic material. [10][11][12][13][14][15] Such indiffusion is a general phenomenon in polymer metallization.…”

Section: Introductionmentioning

confidence: 99%

Liquid metals as electrodes in polymer light emitting diodes

Andersson

Gommans

Gon

et al. 2003

Journal of Applied Physics

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We demonstrate that liquid metals can be used as cathodes in light emitting diodes ͑pLEDs͒. The main difference between the use of liquid cathodes and evaporated cathodes is the sharpness of the metal-polymer interface. Liquid metal cathodes result in significantly sharper metal-organic interfaces than vapor deposited cathodes, due to the high surface energy of the metals. The sharper interface in pLEDs with liquid metal cathodes is observed by neutral impact collision ion scattering spectroscopy and low energy ion scattering spectroscopy measurements. The influence of interface sharpness on device performance was studied by comparing current-voltage-light characteristics of devices with OC 1 C 10 paraphenylenevinylene ͑PPV͒ as electroluminescent polymer and indium tin oxide ͑ITO͒ as hole injection electrode, and different cathodes. Comparison of devices using a liquid Ga cathode and an evaporated Al cathode showed that light emission for the liquid Ga cathode is two orders of magnitude larger than for the evaporated Al cathode, and that the external light efficiency is increased by an order of magnitude. Since the work function of Ga and Al is nearly the same, the poor performance for evaporated Al LEDs is attributed to the formation of an interfacial layer where Al has diffused into, and reacted with, the PPV. This interfacial layer has poor electrical conduction compared to pure PPV, and contains quenching sites which reduce light emission. Low work function liquid metal cathodes were studied by using liquid Ca and Ba amalgams. The improved performance of liquid amalgam pLEDs is attributed to the different structure of the metal-polymer interface. The enormous increase in light and current through the amalgam devices compared to those using pure Hg demonstrate that less than 1 ML of a metal with a low work function at the polymer-cathode interface can have a dramatic effect on the performance of the devices. Devices with a liquid Ca amalgam cathode showed an increase of the current ͑by 50%͒ and brightness ͑80%͒ compared to devices with an evaporated Ca cathode, which is ascribed to reduced diffusion of Ca into the emissive PPV layer.

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Interface formation between poly(2,5-diheptyl-p-phenylenevinylene) and calcium: implications for light-emitting diodes

Cited by 69 publications

References 11 publications

Interface formation between metal and poly-dialkoxy-p-phenylene vinylene

Interface formation between metal and poly-dialkoxy-p-phenylene vinylene

Conjugated polymer surfaces and interfaces in polymer-based light-emitting diodes

Liquid metals as electrodes in polymer light emitting diodes

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