Liquid metals as electrodes in polymer light emitting diodes

Abstract

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.

Thermodynamics and structure of liquid surfaces investigated directly with surface analytical tools**

“…No non surface‐active impurities were found in reference . Such impurities could have been be detected at a concentration greater than about 0.1 atom% based on reference where chlorine could clearly be identified as an impurity within the polymer poly(p‐phenylenvinylen) (PPV) at a concentration of about 0.1 atom%.…”

Organic memory device based on 3,3′-bis-(3,5-di-tert-butyl-4- methoxyphenyl)-2,2′-bithiophene with high endurance and robustness to ambient air operation

“…10 With respect to the Al-DPBT-ITO cell case, measurements were performed at a more moderate vacuum of 40 mbars to avoid Hg evaporation. Also in this case the cell shows a voltage driven conductance switching behavior, as shown in Fig.…”

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

“…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.…”