D-shaped optical fibres are coated with silica nanoparticles by horizontal dip and slide method. Visual determination of film coverage is difficult. Thick coatings are discernible through thin film interference coloring but thinner coatings require SEM imaging. Here, we show that fluorescence imaging, using Rhodmaine B in this example, can provide some qualitative assessment of coverage.
The porous properties of self-assembled waveguides made up of nanoparticles are characterised. Atomic force microscopy (AFM) reveals predominantly hcp or fcc packing suggesting a remarkably well ordered and distributed porous structure. N2 adsorption studies estimate a surface area SA ~ 101 m2/g, a total interstitial volume Vi ~ 1.7 mL/g and a pore size distribution of r ~ (2 - 6) nm. This distribution is in excellent agreement with the idealised values for identically sized particles obtained for the octahedral and tetrahedral pores of the hcp and fcc lattices, estimated to lie within and rtet ~ (2.2 – 3.3) nm and roct ~ (4.2 – 6.2) nm for particles varying in size over 20 to 30 nm. Optical transmission based percolation studies reveal rapid penetration of Rhodamine dye (< 5 s) with very little percolation of larger molecules such as ZnTPP observed under similar loading conditions. In the latter case, laser ablation was used to determine the transport of hydrated Zn2+ to be D ~ 3 x 10-4 nm2s-1. By comparison, ZnTPP was not able to percolate into the wire over the time of exposure, t = 10 mins, effectively demonstrating the self-assembled structure acting as a molecular sieve. We discuss the potential of such structures more broadly and conclude that the controllable distribution of such nano-chambers offers the possibility of amplifying, or up-scaling, an otherwise local interaction or nanoreactions to make detection and diagnostics much simpler; it also opens up a new approach to material engineering making new composites with periodic nanoscale variability. These and other unique aspects of these structures are embodied in an overall concept of lab-in-wire, or similar self-assembled structures, extending our previous concept of lab-in-fibre from the micro domain into the nano domain.
Triplet-triplet annihilation photon upconversion (TTA-UC) is a promising candidate for mitigating sub-band gap absorption losses in solar cells. In TTA-UC, sensitiser dyes absorb sub-band gap photons, cross to a triplet state, and transfer triplet excitons to emitter dyes. Two triplet-excited emitters can undergo TTA, raising one emitter to a higher-energy bright singlet state. The quadratic efficiency of TTA-UC at device-relevant light intensities motivates a push towards the higher chromophore densities achievable in the solid phase. We have begun this process by tethering tetrakisquinoxalino palladium porphyrin to 20nm silica nanoparticles using peptide chemistry techniques, achieving a total-volume concentration of 1.5mM. The phosphorescence kinetics of the tethered porphyrins was measured to quantify quenching by rubrene emitter. Upconverter performance was measured in a solar cell enhancement experiment.
The room temperature deposition of self-assembling silica nanoparticles onto D-shaped optical fibres (“D-fibre”), drawn from milled preforms fabricated by modified chemical vapor deposition, is studied and preliminary results reported here. Of various techniques explored, an automated “dip-and-withdraw” approach is found to give the most reproducible layers. Vertical dip-and-withdraw produces tapered layers with one end thicker (surface coverage < 0.85) than the other whilst horizontal dip-and-withdraw produces much more uniform layers over the core region. The problem of induced fracturing is shown to originate from the sides of the D-fibre flat, attributed to an extended, linear “coffee stain effect”, and is greatest for horizontal dip-and-withdraw. Under optimal preparation conditions they can be minimised and prevented from extending over the core region. Alternatively, these structures can be made periodic potentially enabling some unique structures to be fabricated since post-deposition of functional species will be highest in these cracks.
KEYWORDS: Thin film solar cells, Laser induced plasma spectroscopy, Solar energy, Chemistry, Thin films, Nanotechnology, Photovoltaics, Silicon solar cells
This article [J. Photon. Energy. 3, , 034598 (2013)] was originally published on 6 February 2013 with an error in the denominator of Eq. (8). The value k 1 has been added as follows:
KEYWORDS: Solar cells, External quantum efficiency, Reflectors, Photons, Absorption, Molecules, Solar energy, Thin films, Ray tracing, Thin film solar cells
All presently available types of solar cells transmit light with energies below their band gaps, foregoing energy. An elegant way toward overcoming these subbandgap losses and using a larger fraction of the incident light is the re‐shaping of the solar spectrum by upconversion (UC) of photons. Recently, first results on solar cells augmented by either lanthanide-based UC or triplet-triplet-annihilation UC in organic chromophores were presented. Both of these UC strategies are characterized by a nonlinear response on the illumination density under conditions relevant to solar energy conversion, opening a route for increasing the UC yield by concentrating the light. While operation of the whole cell under concentrated sunlight is in most cases undesirable, application of micro-optical focusing of the transmitted light in the upconverting layer is a promising strategy. In the present work, a more than two-fold enhancement of the current gain by UC behind an amorphous silicon solar cell through optimization of the upconverter optical design is demonstrated, including employing a focusing microstructured back reflector. The experimental data is rationalized using a simple ray tracing modeling approach, highlighting a further enhancement potential of a microstructured UC unit.
We report on recent advances made in fabricating photonic microwires from silica nanoparticles using evaporative self-assembly. Silica microwires up to 7 cm in length and widths as small as 10 µm were fabricated with rectangular cross sections. A model of fabrication is put forward and confirmed by experimental observation. A crude dependence on concentration for an idealised spherical droplet is explored.
Free-base porphyrins are bound to titania sol-gel layers deposited on glass slides. The porphyrin-containing titania layers
show the UV-VIS spectra of the porphyrin and are found to be uniformly and evenly distributed. By addition of a metal
salt to the titania layer, it was possible to metallate the free-base porphyrin within and change the UV-VIS absorbance of
the porphyrin. The metalloporphyrins based on Cu and Zn ions could be detected by laser ablation inductive coupled
plasma mass spectroscopy (LA-ICP-MS). Aggregation of metals is observed indicating that metal ions are also attaching
directly to the titania. In samples where already metalized porphyrins are used little or no aggregation is observed,
indicating that the titania sol gel is non-uniform in its affinity for metal ions.
Yuen Yap Cheng, Burkhard Fückel, Tim Schulze, Rowan MacQueen, Murad Tayebjee, Andrew Danos, Tony Khoury, Raphaël G. Clady, N. Ekins-Daukes, Maxwell Crossley, Bernd Stannowski, Klaus Lips, Timothy Schmidt
KEYWORDS: Solar cells, Solar energy, Upconversion, Photovoltaics, Amorphous silicon, External quantum efficiency, Absorption, Molecules, Energy efficiency, Silicon films
Photovoltaics (PV) offer a solution for the development of sustainable energy sources, relying on the sheer
abundance of sunlight: More sunlight falls on the Earth’s surface in one hour than is required by its inhabitants in a
year. However, it is imperative to manage the wide distribution of photon energies available in order to generate
more cost efficient PV devices because single threshold PV devices are fundamentally limited to a maximum
conversion efficiency, the Shockley-Queisser (SQ) limit. Recent progress has enabled the production of c-Si cells
with efficiencies as high as 25%,1 close to the limiting efficiency of ∼30%. But these cells are rather expensive, and ultimately the cost of energy is determined by the ratio of system cost and efficiency of the PV device. A strategy to radically decrease this ratio is to circumvent the SQ limit in cheaper, second generation PV devices. One promising approach is the use of hydrogenated amorphous silicon (a-Si:H), where film thicknesses on the order of several 100nm are sufficient. Unfortunately, the optical threshold of a-Si:H is rather high (1.7-1.8 eV) and the material
suffers from light-induced degradation. Thinner absorber layers in a-Si:H devices are generally more stable than
thicker films due to the better charge carrier extraction, but at the expense of reduced conversion efficiencies,
especially in the red part of the solar spectrum (absorption losses). Hence for higher bandgap materials, which
includes a-Si as well as organic and dye-sensitized cells, the major loss mechanism is the inability to harvest low
energy photons.
A simple demonstration utilising the optical light source of an Android tablet to obtain the absorption spectra of
Rhodamine dye stained self-assembled silica microwires is demonstrated. The spectrum is collected using a portable
Spectrometer. This highlights the potential of tablet technology as portable optical hardware in its own right and we
discuss how to potentially achieve complete integration of spectrometer onto the tablet.
The structure and physical properties of a thin titania sol-gel layer, prepared on silicon and silica surfaces by cold
processing and spin-coating techniques, were examined. A series of spectroscopic (FTIR, UV-VIS spectroscopy and
ellipsometry) and microscopic (light microscopy, SEM and EDS) techniques were used to examine the chemical and
physical uniformities of the sol-gel layers. Conditions were established to generate uniform layers reproducibly. The
high refractive index, selective binding to organic functional groups and the light and gas transmission properties of the
titania layers can be successfully made use of for new optical sensor applications.
A simple method of fabricating low loss microwire waveguides from silica nanoparticles via evaporative self-assembly is
demonstrated. Light guidance within the microwires is characterised. The photonic microwires assemble into rectangular
slab waveguides with a typical cross-sectional dimension of (20×10) μm and are up to 15 mm in length. Rhodamine B
was incorporated into the structures and characterized with fluorescent microscopy, absorption spectroscopy and
scanning electron microscopy (SEM). We discuss the relevance of these structures to sensing.
The self-assembly of TiO2 nanoparticles is used to create a high index layer within a
structured optical fibre. We show both experimentally (using a novel porphyrin probe) and theoretically
that this approach leads to more than order of magnitude enhanced localisation of the optical field at
the layer-air interface of the hole, both through edge localisation and through novel resonance
localisation as a result of a ring resonator whispering gallery modes.
We review recent work on evaluating the performance of a simple porphyrin-based acid sensor using structured fibre
technology. Specifically, the same sensor in a multimode liquid core is compared to that in a sol-gel coated structured
optical fibre. General implications for fibre chemical sensing are discussed.
We propose enhancing the absorption of a species inside the channels of a structured optical fibre by depositing a high
index layer within the air holes. This layer draws out the optical field within the mode increasing the overlap interaction.
Simulations support the general idea and an experimental demonstration is reported using a novel approach to film
formation with TiO2 nanoparticles. For the sake of demonstration, we use porphyrin with carboxylic groups that attach to
the TiO2. The deposition of particles well coated with porphyrin is compared to those not fully coated prior to deposition
in the holes. The latter case is found to give the best results since scattering loss is reduced when the porphyrins are not
initially attached to the TiO2 particle. This is expected if film formation through intermolecular forces has occurred.rs.
A porphyrin containing sol-gel layer has been deposited within the interior of the channels of a silica structured optical
fibre. Gaseous HCl detection based on protonation of the porphyrin and observed as a change in the spectrum is
demonstrated. This system is compared to previous work based on an acid sensor within a liquid-core fibre. The signal-to-
noise of this type of fibre system shows a higher level of sensitivity than the liquid-core and has a forty-fold acid
diffusion rate increase due to the different medium for acidification.
We have investigated a photochemical up-conversion system comprising a molecular mixture of a palladium
porphyrin to harvest light, and a polycyclic aromatic hydrocarbon to emit light. The energy of harvested
photons is stored as molecular triplet states which then annihilate to bring about up-converted fluorescence.
The limiting efficiency of such triplet-triplet annihilation up-conversion has been believed to be 11% for some
time. However, by rigorously investigating the kinetics of delayed fluorescence following pulsed excitation, we
demonstrate instantaneous annihilation efficiencies exceeding 40%, and limiting efficiencies for the current system
of ≈60%. We attribute the high efficiencies obtained to the electronic structure of the emitting molecule, which
exhibits an exceptionally high T2 molecular state. We utilize the kinetic data obtained to model an up-converting
layer irradiated with broadband sunlight, finding that ≈3% efficiencies can be obtained with the current system,
with this improving dramatically upon optimization of various parameters.
Spectroscopic characterisation of water soluble porphyrins using a structured optical fibre are presented and discussed.
Porphyrin thin-films were also fabricated inside the holes of structured fibres. The thin-films self-assemble inside the fibres leading
to energy coupling between the molecules. These are the first steps towards future chemically tailored optical fibre sensors for
molecular detection.
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