Dye-sensitized solar cells (DSSCs) based on nanocrystalline TiO2 photoelectrodes on indium tin oxide (ITO) coated
polymer substrates have drawn great attention due to its lightweight, flexibility and advantages in commercial
applications. However, the thermal instability of polymer substrates limits the process temperature to below 150 °C. In
order to assure high and firm interparticle connection between TiO2 nanocrystals (TiO2-NC) and polymer substrates, the
post-treatment of flexible TiO2 photoelectrodes (F-TiO2-PE) by mechanical compression was employed. In this work,
Degussa P25 TiO2-NC was mixed with tert-butyl alcohol and DI-water to form TiO2 paste. F-TiO2-PE was then prepared
by coating the TiO2 paste onto ITO coated polyethylene terephthalate (PET) substrate using doctor blade followed by
low temperature sintering at 120 °C for 2 hours. To study the effect of mechanical compression, we applied 50 and 100
kg/cm2 pressure on TiO2/PET to complete the fabrication of F-TiO2-PE. The surface morphology of F-TiO2-PE was
characterized using scanning electron microscopy. The resultant F-TiO2-PE sample exhibited a smooth, crack-free
structure indicating the great improvement in the interparticle connection of TiO2-NC. Increase of compression pressure
could lead to the increase of DSSC photoconversion efficiency. The best photoconversion efficiency of 4.19 % (open
circuit voltage (Voc) = 0.79 V, short-circuit photocurrent density (Jsc) = 7.75 mA/cm2, fill factor (FF) = 0.68) was
obtained for the F-TiO2-PE device, which showed great enhancement compared with the F-TiO2-PE cell without
compression treatment. The effect of compression in DSSC performance was vindicated by the electrochemical
impedance spectroscopy measurement.
A columnar Ta2O5 thin film was fabricated with oblique angle deposition and applied in a prism coupling system as a polarization converter. The transmission, reflection and propagation for the wave reflected in one-round trip from the biaxial film were analyzed. Constructive interference occurred for polarization coupling to achieve polarization conversion. Birefringence of the biaxial thin film provided a sophisticated phase compensation as the incident angle varied over a large range to have a wide-angle polarization conversion. For broadband illumination applications, high polarization conversion efficiency was achieved over visible wavelengths.
We propose a design method of an achromatic waveplate that comprises anisotropic thin films as a multi-cell layered system. Each cell is a symmetrical three-layered ABA structure with equivalent refractive indices and phase thicknesses. A merit function defined as the rate of change of the phase retardation with respect to wavelength is introduced to control the variation of phase retardation by selecting proper birefringence and thickness for each film. The allowed thickness of each birefringent film is investigated for different requirement about the uniformity of phase retardation. An achromatic waveplate with a smaller number of cells is demonstrated by arranging the two composed films made of different materials with high contrast of refractive index between them. The arrangement of a matching layer, which is associated with the performance of the achromatic waveplate, is also discussed.
In this work, the birefringence of bideposited symmetric nanorod array is investigated. The Ta2O5 nanorod arrays
composed of several subdeposits are fabricated by serial bideposition (SBD) technique. Each nanorod consists of several
identical units and each unit consists of symmetrical sections ABA. From the lateral view of the structure, the nanorod
array is a symmetrical multilayered. The deposition planes for layer A and layer B are perpendicular to each other. For
normal incident ray, the polarization-dependent refractive indices and phase thicknesses of the film are presented as
functions of wavelength and optical constants of each layer. The transmittance spectra of symmetrical sections have a
pass band property as the equivalent refractive indices are real. The principal indices of the Ta2O5 nanorod arrays with
each subideposition thickness of 3 nm associated with the two orthogonal polarizations are measured by ellipsometer
when the deposition angle is changed from 70° to 80°. According to principal indices database, a uniform phase
retardation between the two orthogonal polarization directions can be designed for a specific wavelength range.
This work presents a wide angle phase retarder by using a single anisotropic Ta2O5 columnar thin film. The single
anisotropic Ta2O5 columnar thin film can provide phase retardation between two tangential eigenvectors to modulate
the polarization state of light reflected from the prism-coupling system (BK7 prism/anisotropic thin film/air). In
experiment, glancing angle deposition technique is used to prepare single layer film of Ta2O5 tilted nanorod array with
thickness 270nm. In this analysis, we use wave tracing based on the Berreman calculus to calculate the variations of
phases of eigen-waves in the anisotropic thin film as the electromagnetic wave is incident to the prism-coupling system.
The uniform phase retardation can be observed in a wide angle range. A linearly polarized incident ray can be reflected
as a specific elliptical polarized ray uniformly over the range. Similarly, the wide angle and broadband polarization
conversion reflectance with high efficiency also exists in the single anisotropic Ta2O5 columnar thin film. The single
anisotropic Ta2O5 columnar thin film can be useful for the further application in optical components design.
To develop diverse anisotropic thin films, asymmetric bideposition technique is introduced to
fabricate tilt columnar Ta2O5 films with biaxial optical property. The asymmetric bideposition is
achieved using two different opposite deposition angles (a+,a-) and two different thicknesses of
opposite deposited subdeposits. The two sets of Ta2O5 columnar thin films associated with deposited
subdeposits (d+,d-)=(5.2,2.8) are prepared at the opposite deposition angles (a+,a-)=(70,-40), (75,-40),
(80,-40) and at the opposite deposition angles (a+,a-)=(70,-50), (75,-50), (80,-50). Columnar thin films
with various column angle and biaxial properties are measured their planar birefringence and three
principal indexes. The larger column angle leads to higher principal indices. It is demonstrated that the
asymmetric bideposition can enhance the birefringence of a tilted columnar thin film.ntut.edu.tw
Semi-continuous silver films (SSFs) with different filling fractions are papered on BK7 substrate in electron beam
evaporation system. The microstructural and optical properties are studied near the percolation threshold. From
morphological evolution, the transition from nanograins, through nanoclusters to near uniform film can be observed in
SEM images. As the filling fraction increasing, the transmittance variation over the wavelength range from 800 nm to
2400 nm is changed from an increasing behavior to a decreasing behavior. The spectral transmittance and reflectance of
the SSFs are essentially constant near the percolation threshold in the infrared. The thickness of growing MgF2 film is
measured at the wavelength of 532 nm by optical monitoring and stopped coating at the crest of the curve in monitor
chart. Equivalent admittance of a semi-continuous silver film at the MgF2/SSF can be retrieved using characteristic
matrix. With different filling fraction, the variations of equivalent admittance of a semi-continuous silver film at the
MgF2/SSF are discussed.
The glancing angle deposition (GLAD) technique is applied to fabricate a three-layered dielectric-metal-dielectric
anisotropic thin film. The silver nanorod array is between two SiO2 nanorod arrays (NRA). The three-layered thin film is
arranged in the Kretschmann configuration and measured the reflectance of the system. The polarization conversion
reflectance is measured from this system as the plane of incidence is not coincident with the deposition plane. From the
angular spectra of reflectance for RPP, RSS, RSP and RPS (Rij, i denotes the polarization of the incident beam and j denotes
the polarization of the reflected light) measured at angles larger than critical angle, the absorption for s-polarization and
p-polarization incident light can be calculated. The resonant peaks varied with the orientation of the deposition plane are
also measured. The configuration combines polarization conversion, transverse surface plasmon resonance and
longitudinal surface plasmon resonance of the rods. Those resonances are associated with the relationship between the
rod directions and the oscillating electric field vector. The variations of the absorption peaks affected by those resonant
modes are discussed in this work.
The absorbances of two samples of a bilayer comprising a metal film and a periodically nonhomogeneous sculptured nematic thin film (SNTF) were measured in the Kretschmann configuration, the incident plane waves being linearly polarized with their wave vectors lying wholly in the morphologically significant plane of the SNTF. Comparison of experimental data with theoretical predictions confirmed the possibility of independently launching multiple surface-plasmon-polariton waves of the same color (frequency) but different phase speeds and polarization states. Exciting prospects are in store for plasmonics-based sensing, imaging, and communication.
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