(0001)-Oriented Single-Crystal-Like Porous ZnO on ITO Substrates via Quasi-Topotactic Transformation from (001)-Oriented Zinc Hydroxychloride Crystals

Novel ZnO nanostructures composed of ⟨0001⟩-oriented porous ZnO were prepared on indium tin oxide (ITO) substrates from ⟨001⟩-oriented zinc hydroxychloride (ZHC) crystals. The ZHC crystals were obtained by simple electrochemical deposition on layer-by-layer (LbL)-coated ITO in aqueous solution and transformed into porous ZnO via calcination in air. X-ray diffraction and scanning electron microscopy were used to characterize the ZHC crystals and porous ZnO. The well-faceted ZHC crystals obtained on the LbL-coated ITO had hexagonal-plate shapes with diameters of 3–7 μm and exhibited strongly ⟨001⟩-oriented simonkolleite crystal structures. The calcination of ZHC crystals at 450–550 °C for 1 h in air led to the formation of ⟨0001⟩-oriented porous ZnO comprising nanocrystals with diameters of ∼50 nm while maintaining the original hexagonal-plate shapes. Electron backscatter diffraction analyses and epitaxial growth of ZnO nanorod array revealed that the porous ZnO hexagonal plates exhibited features of single crystals. Finally, a plausible mechanism for the topotactic-like transformation of ZHC crystals to porous ZnO is discussed on the basis of similarities in their Zn–O frameworks

Hybrid ZnO/Phthalocyanine Photovoltaic Device with Highly Resistive ZnO Intermediate Layer

We report a hybrid photovoltaic device composed of a 3.3 eV bandgap zinc oxide (ZnO) semiconductor and metal-free phthalocyanine layers and the effects of the insertion of the highly resistive ZnO buffer layer on the electrical characteristics of the rectification feature and photovoltaic performance. The hybrid photovoltaic devices have been constructed by electrodeposition of the 300 nm thick ZnO layer in a simple zinc nitrate aqueous solution followed by vacuum evaporation of 50–400 nm thick-phthalocyanine layers. The ZnO layers with the resistivity of 1.8 × 10³ and 1 × 10⁸ Ω cm were prepared by adjusting the cathodic current density and were installed into the hybrid photovoltaic devices as the n-type and buffer layer, respectively. The phthalocyanine layers with the characteristic monoclinic lattice showed a characteristic optical absorption feature regardless of the thickness, but the preferred orientation changed depending on the thickness. The ZnO buffer-free hybrid 50 nm thick phthalocyanine/n-ZnO photovoltaic device showed a rectification feature but possessed a poor photovoltaic performance with a conversion efficiency of 7.5 × 10^–7 %, open circuit voltage of 0.041 V, and short circuit current density of 8.0 × 10^–5 mA cm^–2. The insertion of the ZnO buffer layer between the n-ZnO and phthalocyanine layers induced improvements in both the rectification feature and photovoltaic performance. The excellent rectification feature with a rectification ratio of 3188 and ideally factor of 1.29 was obtained for the hybrid 200 nm thick phthalocyanine/ZnO buffer/n-ZnO photovoltaic device, and the hybrid photovoltaic device possessed an improved photovoltaic performance with the conversion efficiency of 0.0016%, open circuit voltage of 0.31 V, and short circuit current density of 0.015 mA cm^–2