Photoactive ZnO-Organic Nanostructures: Development and Characterization
|Kustantaja||Tampere University of Technology|
|Tila||Julkaistu - 27 toukokuuta 2016|
|Nimi||Tampere University of Technology. Publication|
Photoinduced processes in hybrid semiconductor-organic nanostructures were studied in this thesis. The work was divided into three tasks: (1) preparation of ZnO thin films and nanorod arrays in a controlled and cost-effective way, (2) functionalization of the ZnO surfaces with organic, photoactive layers and (3) study of photoinduced reactions on the surfaces by both steady state and time-resolved methods.
Aluminum doped zinc oxide (AZO) electrodes were tested as an alternative for the traditionally used indium tin oxide semitransparent electrodes in organic solar cell devices.The electrodes were prepared by atomic layer deposition method. Devices with AZO electrodes showed performance comparable to that of the reference device but were more stable in open air showing no degradation during 40 days time interval.
ZnO nanorod arrays were prepared and used as model substrates to study electronic interactions at semiconductor-organic interface. The growth was optimized to achieve well-aligned nanorods with high specific surface area. To control the semiconductor electronic properties, while keeping the morphology unchanged, the nanorods were further modified with thin layers of Al2O3 or TiO2 prepared by atomic layer deposition.
Self-assembled monolayers (SAM) of three different porphyrin derivatives and one phthalocyanine derivative were formed on the ZnO nanorods using carboxylic acid or siloxane as anchor groups. The fastest electron transfer from zinc porphyrin (ZnP) to the semiconductor was observed for the ZnO nanorods modified with a 5 nm layer of TiO2 (<0.2 ps). On the contrary, the charge recombination was not any faster compared to that of ZnP on the unmodified nanorods. This indicates that the charge recombination depends mainly on the semiconductor bulk properties, whereas the charge separation is determined by the surface properties of the semiconductor.
The charge generation mechanisms in the hybrid systems consisting of zinc phthalocyanine (ZnPc) SAM on ZnO nanorods covered by a spin coated layer of hole transporting materials, P3HT or Spiro-OMeTAD, were studied with time-resolved absorption spectroscopy. After selective excitation of ZnPc the primary electron transfer step was controlled by the hole transporting material. In the system with P3HT the first reaction step is a fast (1.8 ps) electron transfer to ZnO, whereas in the Spiro-OMeTAD system a fast (0.5 ps) hole transfer from the excited ZnPc to Spiro-OMeTAD is the dominant primary electron transfer step. However, in both cases long-lived (> 5 ns) chargeseparated states are formed. In these states electrons are localized in ZnO and the holes in the organic donor layer, while ZnPc is in the ground state.