Transparent conducting oxides (TCOs) are widely investigated in purview of potential applications as charge collectors in light-emitting devices and thin-film photovoltaics where optical transparency in the visible photon energy range are required. TCOs that have been investigated as charge collectors in optoelectronic devices include indium-doped and fluorine-doped tin oxides, zinc oxides, nickel oxide (NiO), and their alloys. The strong interest in NiO stems from the fact that no dopants are needed to turn on its electrical conductivity while keeping its transparency at acceptable levels, differently from tin oxide, where In or F doping is required, and zinc oxide where conductivity is enhanced by Al doping. Our work will present our defect engineering efforts to optimize NiO and (MgNi)O thin films for their applications as TCOs. In the final part of our talk, we will present our recent work where NiOx thin films are used as hole transport layers (HTL) in green organic photovoltaics, with water-soluble poly[2-(3-thienyl)-ethoxy-4-butylsulfonate]-sodium (PTEBS) and fullerenes as active layer, showing improved external quantum efficiency for UV light.
Transparent conducting oxides (TCOs) are extensively investigated because of their applications as transparent electrodes in solar cells and light-emitting devices. TCOs of interest include indium-tin oxide, aluminum-doped zinc oxide, nickel oxide (NiO), and their combinations. There is strong interest in NiO because no heteroatoms are required to “dope” it at high transparency levels. It has been speculated that paramagnetic defects due to Ni3+ centers and O interstitials are responsible for the electrical conductivity of otherwise insulating and antiferromagnetic NiO, but direct investigation of such defects has been limited. Here, the electrical conductivity in nanostructured NiO thin films is investigated and correlated to the paramagnetic defect density extracted from electron spin resonance (ESR). Two types of ESR-active centers are identified. Our work points at defect engineering as a necessary step to optimize NiO thin films for their applications as TCOs.
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