PT Unknown
AU Felbier, P
TI All-inorganic heterostructure light-emitting devices based on ZnO nanoparticles
PD 07
PY 2015
LA en
DE Nanoparticles; Quantum Dots; LED; ZnO
AB All-inorganic ZnO nanoparticle LEDs provide an interesting alternative to traditional OLEDs and epitaxial LEDs. Inorganic materials are typically more stable than organic materials, ZnO shows tunable luminescence from the near UV to the low energetic end of the visible spectral range and a nanoparticle material allows the application of advanced concepts, e.g. LEDs from ink-jet printing and flexible LEDs. Nevertheless, all-inorganic ZnO LED concepts are suffering from low external quantum efficiencies. To discuss the reason behind this, the external quantum efficiency can be divided into three major components: the out-coupling efficiency, the injection efficiency and the luminescent quantum yield of the active material. The out-coupling efficiency describes the share of generated photons inside the device that actually leave it to light the surrounding. Because concepts to enhance the out-coupling efficiency can be transferred from other LED types, the focus of this work is set on the injection efficiency and the luminescent quantum yield, which bear ZnO specific challenges.

The injection efficiency describes the share of charge carriers that form an electron-hole pair from those that flow through the device in total. While electrons are easily transferred into ZnO, e.g. from an aluminum electrode, injecting holes is assumed to be much more difficult. The reason behind this is the very low valence band level of ZnO at -7.5 eV, which has even led to application of ZnO as hole blocking layer in solar cell concepts. Different hole injection layers, NiO, WO3 and GaN, are studied to reduce the injection barrier holes are facing at the anode interface of the ZnO layer. Apart from the valence band level, the studied hole injection layers differed in significant other properties like whether being n- or p-conducting and providing an electron barrier or not, giving an indication of their influence. It is demonstrated, that the injection efficiency could be enhanced from 0.01% for a bare ITO substrate up to 31% for a p-GaN:Mg injection layer.

The luminescent quantum yield of the active materials describes the share of electron-hole pairs inside the active material that recombine radiatively. While band gap luminescence of ZnO nanoparticles is typically very inefficient (< 1%), efficient blue, green and yellow emitting ZnO nanoparticles have been presented. These particles, however, have been made by wet-processing sol-gel methods and have to be capped with insulating ligands for stabilization, and are therefore disadvantageous for use in electrical devices. Here, a method to produce highly luminescent, green emitting ZnO quantum dots from the gas phase and without ligands from a nonthermal plasma is presented. Exceptional high quantum yields of up to 60% have been recorded for the smallest ZnO quantum dots after exposure to ambient air for one day, which is the highest reported value for any compound semiconductor nanoparticle material from the gas phase. Systematic studies reveal the influence of the surrounding atmosphere on the luminescence properties and are used to understand the mechanism of the efficient green emission.
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