Porous TiO2 films are a crucial part of mesostructured solar cells (MSCs), both dye-sensitized solar cells (DSSCs) and perovskite solar cells (PSCs). However, the literature does not provide a clear description of the optical properties especially of the refractive index and scattering for those films relevant to MSCs. In DSSCs, two different porous TiO2 layers are included, the mesoporous active layer and the blocking layer. While the first is essential for the charge separation, electron collection and ion conduction, the second is intended for suppressing the loss of generated electrons to the electrolyte. Both layers consist of the same chemical compound, TiO2, but they have different porosities. For PSCs, the perovskite is deposited on a mesoporous TiO2 structure for enhancing the I–V characteristics
This paper investigates TiO2 films really used in fabricated MSCs. We utilize a technique allowing the determination of the effective refractive index and the film porosity for two different film kinds fabricated using sol-gel methods, discussed in our previous work, to determine the thickness of TiO2 films typically used in fabricating MSCs.
Thin film solar cells (TFSCs) where first introduced as a low cost alternative to conventional thick ones. TFSCs show low conversion efficiencies due to the used poor quality materials having weak absorption capabilities and to thin absorption layers. In order to increase light absorption within the active layer, specially near its absorption edge, photon management techniques were proposed. These techniques could be implemented on the top of the active layer to enhance the absorption capabilities and/or to act as anti-reflecting coating structures. When used at the back side, their purpose is to prevent the unabsorbed photons from escaping through the back of the cell.
In this paper, we coupled the finite difference time-domain (FDTD) algorithm for simulating light interaction within the cell with the commercial simulator Comsol Multiphysics 4.3b for describing carrier transports. In order to model the dispersive and absorption properties of various used materials, their complex refractive indices were estimated using the Lorentzian-Drude (LD) coefficients. We have calculated the absorption profile in the different layers of the cell, the external quantum efficiency and the power conversion efficiency achieved by adding dielectric nanospheres on the top of the active layer. Besides that, the enhancement observed after the addition of dielectric nanospheres at the back side of the active layer was computed. The obtained results are finally compared with the effects of using textured surface and nanowires on the top in plus of cascaded 1D and 2D photonic crystals on the back.
The advent of nanolithographic techniques has enabled electrically-driven liquid crystal devices to be addressed
using nanoscale electrodes. Tiny periodic phase grating structures, which can now be realized, may find applications
in optical communications and displays devices. Introducing optical discontinuities in the nematic liquid
crystal (NLC) material will enable these structures to act as photonic band gap devices. This paper addresses the
properties of band gap structures formed by NLC discontinuities using a 10micron thick homeotropically aligned
NLC. It demonstrates how the position of the optical discontinuities may be altered by a symmetric voltage pattern
and thus tune the photonic band gap.
Nanolithographic fabrication techniques may soon enable electrically-driven LCoS
devices to be manipulated using ultra-nanoscale CMOS transistors. However, questions
as to the switching properties of such LCoS devices arise due to the diminishing dimensions
of their transistors. Thus, experimental investigations into the response times and
the onset-threshold voltages for LCoS devices were embarked upon. Such measurements
were obtained for various electrode dimensions and cell gaps. Furthermore, an interdigitated
(IDT) electrode pattern was used to drive the homeotropically-aligned NLC material
in a direction parallel to the bounding planes of the cell. Experimental findings
revealed that faster response times were achieved when the electrode spacings were decreased.
Such results have shown that a 10μm-thick device with an electrode pitch of 2μm
can achieve a switch-on time of < 5ms. In addition, decreasing the electrode spacing results
in the threshold voltage to drop. The results therefore indicate that improvements in
a LCoS device's switching properties can be realised by using smaller electrode dimensions.
Liquid crystal on silicon (LCoS) devices have been exploiting the ever-diminishing CMOS silicon process, which is pushing towards 45nm dimensions. Consequently, such fine metallic structures are bound to influence the alignment of the liquid crystal material. To illustrate this, a number of 1D metal patterns with differing mark-to-space ratio were fabricated using an Electron-Beam exposure technique. The results confirmed Dwight Berreman's topological alignment theory regarding the pitch of the surface topography and how this influences the quality of the planar alignment. Patterns with a metal to spacing ratio of 1:1 were shown to yield higher contrast ratios and hence better planar alignment. Such findings could be useful for developing non-intrusive alignment methods for nanoscale LCoS devices.
The effective demultiplexing of WDM signals requires a tuneable filter, which is able to arbitrarily select and filter more than one channel to a particular output at any one time. Thus, such optical filters need to posses a large dynamic tuning range, narrow bandwidth and high sidelobe-suppression capabilities. Important applications of these tuneable filters include Add/Drop multiplexers and wavelength-selectors for tuneable lasers. The proposed architecture consists of cascading a Polarisation Independent Acousto-Optic Filter (PIAOTF) with a Fabry Perot Interferometer (FPI). The frequency domain characteristics of this architecture outperforms those currently offered by tuneable technologies, including the Fibre-Bragg Grating, the Mach-Zehnder Interferometer, the Fabry-Perot Filter and the Double-Stage PIAOTF. Consequently, our architecture is very favourable for WDM purposes since the filter boasts a sidelobe-level attenuation of <-25dB when simultaneously switching four channels, whereas the leakage at the through-port is only -16dB. In addition, the filter proves to demonstrate a convincingly flat passband with a 3dB bandwidth of ~0.35 nm. Consequently, a simple model is built to simulate the WDM (demultiplexing) environment in which the novel tuneable filter may be deployed. In this case, the tuneable filter's architecture enables a light stream consisting of multiple wavelength-channels, each with a spectral width of 0.1nm, to be split into two output ports via its bandpass and notch spectra. Hence, there is both a filtered and non-filtered light stream.