Chun-Pei Cho

Growth of AlQ3 nanowires directly from amorphous thin film and nanoparticles

Amorphous AlQ3 thin film and nanoparticles can directly grow into crystalline nanowires by a one-step heat treatment. With appropriate Ar pressure and heating time at 190 °C, growth of α-phase nanowires from the film occurs. Similar growth from amorphous nanoparticles is also demonstrated by heat treatment at 150 °C. The growth is dictated by the anisotropic crystallographic nature of α-AlQ3. The growth mechanism can be illustrated in terms of nucleation and molecular migration within the film or nanoparticles. Complete structural transformation to nanowires leads to a spectral blue shift and an enhanced intensity of photoluminescence.

Molecular modification on dye-sensitized solar cells by phosphonate self-assembled monolayers

Molecular modification by phosphonic acids forms phosphonate dipole layers on TiO2 and influences photoelectrochemical characteristics of DSSCs. Voc is dipole-related and η is dominated by Voc. A phosphonic acid with a negative dipole moment shifts the TiO2ECB closer to the vacuum level and causes a larger Voc. A higher ECB increases the EF gradient and results in a larger Jsc. APPA has the largest negative dipole moment, so the APPA-modified DSSC shows the largest Voc and Jsc, and the highest η. The dark currents of modified DSSCs are not just dipole-related. They are dominated mainly by the electron tunneling effect. A shorter phosphonate dipole layer offers a smaller tunneling barrier and makes reverse electron injection easier. So the CEPA- and APPA-modified DSSCs show smaller charge transfer impedances at the TiO2/dye/electrolyte interfaces and larger dark currents than the other four modified DSSCs. Although the TFBPA dipole layer is not the longest, the TFBPA-modified DSSC exhibits the largest impedance and smallest dark current because the charge recombinations at the interface and reverse injection of electrons are suppressed by the strong electron-withdrawing property.

Crystalline Gaq 3 Nanostructures: Preparation, Thermal Property and Spectroscopy Characterization

Crystalline Gaq31-D nanostructures and nanospheres could be fabricated by thermal evaporation under cold trap. The influences of the key process parameters on formation of the nanostructures were also investigated. It has been demonstrated that the morphology and dimension of the nanostructures were mainly controlled by working temperature and working pressure. One-dimensional nanostructures were fabricated at a lower working temperature, whereas nanospheres were formed at a higher working temperature. Larger nanospheres could be obtained when a higher working pressure was applied. The XRD, FTIR, and NMR analyses evidenced that the nanostructures mainly consisted of δ-phase Gaq3. Their DSC trace revealed two small exothermic peaks in addition to the melting endotherm. The one in lower temperature region was ascribed to a transition from δ to β phase, while another in higher temperature region could be identified as a transition from β to δ phase. All the crystalline nanostructures show similar PL spectra due to absence of quantum confinement effect. They also exhibited a spectral blue shift because of a looser interligand spacing and reduced orbital overlap in their δ-phase molecular structures.

Field emission of Alq3 nanoprotrusions

Quasi-amorphous Alq3 (tris-(8-hydroxyquinoline) aluminium) thin films with nanoprotrusions on the surface were fabricated by vapour condensation under different conditions. The films fabricated under a liquid nitrogen cold trap have a smoother surface with smaller nanoprotrusions and exhibit a turn-on field of 6–9 V µm−1. The films fabricated without a cold trap, with a rougher surface and larger nanoprotrusions, exhibit a turn-on field of 2–12 V µm−1. The electrons are emitted from an effective area on the tip of the nanoprotrusions. Both larger thickness and smaller effective protrusion radius lead to a larger field enhancement. A linear relationship between the effective and measured radii of the nanoprotrusions demonstrates that surface roughness indeed has a significant influence on the field emission efficiency. The nanostructured thin films exhibit a relatively lower emission threshold field than Alq3 nanowires and most other film emitters.

Decreased phase transition temperatures of Alq3 nanoparticles

Spherical amorphous Alq3 nanoparticles of various diameters could be obtained under a cold trap by adjusting the He pressure in a vapour condensation system. The growth of nanoparticles is related to the collisions between the sublimed Alq3 molecules and gaseous He atoms. Both the diameter and size distribution of nanoparticles decrease with a decreased He pressure. Smaller nanoparticles have no spectral blue shift but lead to stronger photoluminescence because of the increased optical absorption that results from the larger specific surface area. With a larger surface-to-volume ratio, smaller nanoparticles exhibit higher surface energy and require less enthalpy for phase transition, resulting in decreased temperatures for α–δ phase and melting transitions.

Modified photoanodes by amino-containing phosphonate self-assembled monolayers to improve the efficiency of dye-sensitized solar cells

Surface modification of TiO2 electrodes by selected molecules could lower the energy barrier of electron transfer and improve DSSC performance. The category of the terminal group and molecular length of a self-assembled monolayer influence the surface and interfacial properties of a TiO2 electrode, and the photovoltaic parameters of a DSSC could thereby be adjusted. By electrochemical approaches, it was discovered that the redox current and photocurrent increased when a lower work function and thus a reduced impedance at the TiO2/dye/electrolyte interface were achieved by using an amino-containing phosphonic acid. However, a smaller photocurrent would be caused when the path for electron transport and charge recombination probability were increased by employing a longer molecule. Both the photocurrent and dark current of a DSSC were suppressed when a larger impedance restrained electron transport through the interface. The TiO2 electrode modified by 2-aminoethylphosphonic acid showed the largest redox current. The corresponding DSSC exhibited the smallest impedance, largest photocurrent and highest efficiency of 6.67%. This study has demonstrated that a monolayer formed on the TiO2 surface by an amino-containing phosphonic acid enhanced DSSC performance efficiently.

Performance improvement of dye-sensitized solar cells by surface patterning of FTO electrodes

Patterned FTO electrodes for DSSCs were fabricated by a facile wet etching method. Pattern depth could be controlled by etching time. Most DSSCs with patterned FTO electrodes exhibited both larger open-circuit voltage and photocurrent density. Energy conversion efficiency gradually increased with longer etching time and achieved a highest value when etching time is 240s. An optimum pattern depth was required to acquire best DSSC performance. The improved device performance could be mainly attributed to more dye adsorption, enhanced light harvesting and scattering due to larger amount of TiO2 nanoparticles filled in the pattern. More contact between TiO2 nanoparticles and patterned FTO with larger surface area was also an advantage. It was revealed from Nyquist plots that the charge transfer impedance at the TiO2/dye/electrolyte interface apparently influenced the magnitude of photocurrent density and device performance. Electron transfer became easier and higher performance was thereby obtained when a DSSC had a smaller interfacial impedance. This study has demonstrated obvious improvement in DSSC performance by surface patterning of FTO electrodes. With appropriate etching condition, the highest energy conversion efficiency of 7.71% was achieved, which was around 16 % higher than that of the DSSC with unpatterned FTO electrode.