Y. Y. Xi

ZnO nanorods for solar cells: Hydrothermal growth versus vapor deposition

Performance of dye-sensitized solar cells (DSSCs) based on ZnO nanorods prepared by hydrothermal and vapor-deposition methods has been investigated. In spite of their inferior optical properties, DSSCs based on hydrothermally grown rods exhibit higher power conversion efficiency, which can be attributed to the higher dye adsorption. Hydrothermally grown and vapor deposited nanorods also exhibit different dependence of photovoltaic performance on the annealing conditions of the rods, indicating significant effect of the native defects on the achievable photocurrent and power conversion efficiency. Efficiency of 0.22% is obtained for both as grown hydrothermally grown nanorods and vapor deposited nanorods annealed in oxygen at 200°C.

NiO∕ZnO light emitting diodes by solution-based growth

Heterojunction NiO∕ZnO light emitting diodes have been fabricated using low temperature solution-based growth methods. While negligible light emission has been obtained for the as-grown NiO film, devices with annealed NiO film exhibit room-temperature electroluminescence (EL), which was attributed to the detrimental effects of nickel oxide hydroxide in as-grown NiO layers. The device performance can be further modified by insertion of the organic layers between NiO and ZnO and the EL spectra exhibited dependence on the bias voltage. For higher bias voltages, strong UV-violet emission peak can be obtained in spite of the dominance of defect emission in the photoluminescence spectra.

Dye-sensitized solar cells using ZnO tetrapods

One dimensional (1D) ZnO nanostructures are of interest for applications in dye-sensitized solar cells (DSSCs) since they exhibit significantly improved electron transport compared to that in porous films. However, 1D nanostructures also have a significantly lower surface area than the porous films. Thus, the achieved solar cell efficiencies are typically much lower in spite of the improved charge transport. In this work, we investigated DSSCs based on ZnO tetrapods to achieve an increased surface area compared to that of 1D nanostructures. The cell performance as a function of the tetrapod film thickness and the dye used was studied. To further increase the surface area, mixed morphologies (tetrapods with nanoparticles) were also investigated. Under optimal conditions, an AM 1.5 power conversion efficiency of ∼1.2% was achieved.

GaN/ZnO nanorod light emitting diodes with different emission spectra

Light emitting diodes (LEDs) consisting of p-GaN epitaxial films and n-ZnO nanorods have been fabricated and characterized. The rectifying behavior and emission spectra were strongly dependent on the electronic properties of both GaN film and ZnO nanorods. Light emission under both forward and reverse bias was obtained in all cases, and emission spectra could be changed by annealing the ZnO nanorods. The emission spectra could be further tuned by using a GaN LED epiwafer as a substrate. Both forward and backward diode behavior has been observed and the emission spectra were significantly affected by both the properties of the GaN substrate and the annealing conditions for the ZnO nanorods.

Enhancement of spontaneous emission rate and reduction in amplified spontaneous emission threshold in electrodeposited three-dimensional ZnO photonic crystal

ZnO photonic crystal (PC) with face-center-cube type structure is fabricated by electrodeposition using holographic lithographically made organic (SU-8) template. Photonic band gap effect (reflection peak and transmission dip in infrared spectral region) is clearly seen. Observation of strong enhancement and blueshift of the emission peak (from 383.8 to 378.8 nm), shortening of the exciton photoluminescence lifetime (from 88 to 34 ps), and reduction in amplified spontaneous emission threshold of ZnO PC compared to that of the reference nonstructured electrodeposited ZnO showed clear evidence of PC structure affecting the ZnO exciton emission.

Effect of annealing on the performance of CrO3/ZnO light emitting diodes

Heterojunction CrO3/ZnO light emitting diodes have been fabricated. Their performance was investigated for different annealing temperature for ZnO nanorods. Annealing in oxygen atmosphere had significant influence on carrier concentration in the nanorods, as well as on the emission spectra of the nanorods. Surprisingly, annealing conditions, which yield the lowest band edge-to-defect emission ratio in the photoluminescence spectra, result in the highest band edge-to-defect emission ratio in the electroluminescence spectra. The influence of the native defects on ZnO light emitting diode performance is discussed.

Influence of Annealing on ZnO Nanorod/NiO LEDs

Zinc oxide (ZnO) is a promising candidate for short‐wavelength optoelectronic devices. Due to the lack of stability and reproducibility of p‐type ZnO, ZnO heterojunction devices are of considerable interest. In this work, the performance of ZnO nanorod/NiO light emitting diodes (LEDs) was investigated. Both materials were fabricated using inexpensive solution based process. Due to the low fabrication temperature, ZnO nanorods contained considerable concentration of point defects. Post‐fabrication treatments could significantly change ZnO nanorod properties. The effect of post‐fabrication annealing on ZnO properties and device performance were discussed.

Core‐shell ZnO solar cell application

In this study, the ZnO nanorod arrays coated with different oxide shells (TiOx, MgO, SnO2, and Al2O3) were fabricated. We compared the effect of various kinds of oxide layer coating (TiOx, MgO, SnO2, and Al2O3) on the performance of ZnO based solar cells. The thickness of the oxide shell layer is determined by transmission electron microscopy. Different kinds of oxide layers had a significant effect on the photovoltaic performance. The effect of different shells on the properties and performance of ZnO based core/shell nanorod array photoanode is discussed.

ZnO nanorod heterojunctions and LEDs

Zinc oxide (ZnO) nanorods are of great interest due to their potential use in optoelectronic devices. Hydrothermal synthesis and electrodeposition are two very promising techniques for ZnO growth as they are simple and inexpensive and also involve low fabrication temperature, favoring the use of large area and/or flexible substrates. In this work, we investigated the influence of the seed layer on the morphology and optical properties of ZnO nanorods, as well as the influence of post-fabrication treatments, such as annealing under different conditions. To further demonstrate the potential use of ZnO nanorods in optoelectronic devices, light emitting diodes (LEDs) were also studied in this paper. We have focused on heterojunction devices with ZnO as an n-type layer, since heterojunction ZnO LEDs typically exhibit better performance compared to homojunctions. We have investigated devices using GaN and NiO as a p-type material, and in addition hybrid organic/inorganic devices were also prepared.

Hydrothermal growth of periodic ZnO nanorod arrays using polystyrene sphere templates

Zinc Oxide (ZnO) photonic crystals (PCs) are of great interest in recent years due to its potential applications in optoelectronic devices. However, most of the growth temperatures in fabricating ZnO nanostructures in periodic structures reported up to date are very high (>500°C), which is not favorable for low cost manufacture. In this work, a method involving low growth temperature (90°C), hydrothermal synthesis, was used to fabricate ZnO nanostructures. Self-assembly of polystyrene (PS) spheres was used to fabricate periodic arrays. PS spheres were self-assembled on the silicon (Si) or fluorine-doped tin oxide (FTO) substrate with ZnO seed layer derived from zinc acetate. After crosslinking the PS sphere layer, hydrothermal growth was performed. The influence of growth conditions (concentration, addition of PEI, etc.) on fabricated ZnO nanorod array has been studied. Then, PS spheres were removed by annealing in air, and fabricated ZnO nanorod arrays were studied by scanning electron microscopy (SEM) and photoluminescence (PL) spectroscopy. Periodicity of the array was strongly substrate dependent (better on Si than on FTO). Light emitting diodes were also fabricated in order to demonstrate the potential use of this ZnO periodic array.