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All-inorganic heterostructure light-emitting devices based on ZnO nanoparticles
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 ...
All-inorganic heterostructure light-emitting devices based on ZnO nanoparticles
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 ...
All-inorganic heterostructure light-emitting devices based on ZnO nanoparticles
Felbier, Patrick (author) / Bacher, Gerd
2015-07-15
Theses
Electronic Resource
English
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