Modelling the performance of fluorescent solar collectors

The theoretical power conversion efficiency of a silicon solar cell with a fluorescent solar collector is believed to reach 90% of the maximum efficiency of an ideal silicon solar given by the Shockley-Queisser detailed balance limit, but the practical efficiencies are significantly lower due to several loss mechanisms. This work presents an analytic model which take the non-ideal coupling between the collector and the solar cell mounted at the edge into consideration and it is shown in good agreement with experimental results

Designing experiments for an application in laser and surface Chemistry

We consider the design used to collect data for a Second Harmonic Generation (SHG) experiment, where the behaviour of interfaces between two phases, for example the surface of a liquid, is investigated. These studies have implications in surfactants, catalysis, membranes and electrochemistry. Ongoing work will be described in designing experiments to investigate nonlinear models used to represent the data, relating the intensity of the SHG signal to the polarisation angles of the polarised light beam. The choice of design points and their effect on parameter estimates is investigated. Various designs and the current practice of using equal-spaced levels are investigated, and their relative merits compared on the basis of the overall aim of the chemical study

Photon tunneling into a single-mode planar silicon waveguide

We demonstrate the direct excitation of a single TE mode in 25 nm thick planar crystalline silicon waveguide by photon tunneling from a layer of fluorescent dye molecules deposited by the Langmuir-Blodgett technique. The observed photon tunneling rate as a function of the dye- silicon separation is well fitted by a theoretical tunneling rate, which is obtained via a novel approach within the framework of quantum mechanics. We suggest that future ultrathin crystalline silicon solar cells can be made efficient by simple light trapping structures consisting of molecules on silicon

Hot photons and open-circuit voltage in molecular absorbers

Hot carrier solar cells have attracted interest for many years. Although no working exemplars exist today, the challenges to overcome have become clearer and a substantial research effort has been underway with a focus on inorganic semiconductors, including quantum wells. In this paper we propose a novel strategy to potentially exploit hot photons, based on organic absorbers. Our approach, when combined with photon management structures similar to photonic fluorescent collectors, can potentially enhance the efficiency of complete photovoltaic devices. We present a characterisation method of fluorescent collectors by evaluating the chemical potential and temperature of the emitted fluorescence photon flux. We report on observation of temperatures of the emitted photon flux well above the ambient temperature, indicating the presence of hot photons. We propose a theoretical background to describe how excess thermal energy carried by hot photons can be exploited to increase the chemical potential of the photon flux which is closely related to the open-circuit voltage of the solar cell

Light harvesting in silicon(111) surfaces using covalently attached protoporphyrin IX dyes

We report the photosensitization of crystalline silicon via energy transfer using covalently attached protoporphyrin IX (PpIX) derivative molecules at different distances via changing the diol linker to the surface. The diol linker molecule chain length was varied from 2 carbon to 10 carbon lengths in order to change the distance of PpIX to the Si(111) surface between 6 A and 18 A. Fluorescence quenching as a function of the PpIX-Si surface distance showed a decrease in the fluorescence lifetime by almost two orders of magnitude at the closest separation. The experimental fluorescence lifetimes are explained theoretically by a classical Chance-Prock-Silbey model. At a separation below 2 nm, we observe for the first time, a Forster like dipole-dipole energy transfer with a characteristic distance of R-o = 2.7 nm

Observation of energy transfer at optical frequency to an ultrathin silicon waveguide

Energy transfer from a submonolayer of rhodamine 6G molecules to a 130 nm thick crystalline silicon (Si) waveguide is investigated. The dependence of the fluorescence lifetime of rhodamine on its distance to the Si waveguide is characterized and modeled successfully by a classical dipole model. The energy transfer process could be regarded as photon tunneling into the Si waveguide via the evanescent waves. The experimentally observed tunneling rate is well described by an analytical expression obtained via a complex variable analysis in the complex wavenumber plane

Silicon photosensitisation using molecular layers

Silicon photosensitisation via energy transfer from molecular dye layers is a promising area of research for excitonic silicon photovoltaics. We present the synthesis and photophysical characterisation of vinyl and allyl terminated Si(111) surfaces decorated with perylene molecules. The functionalised silicon surfaces together with Langmuir-Blodgett (LB) films based on perylene derivatives were studied using a wide range of steady-state and time resolved spectroscopic techniques. Fluorescence lifetime quenching experiments performed on the perylene modified monolayers revealed energy transfer efficiencies to silicon of up to 90 per cent. We present a simple model to account for the near field interaction of a dipole emitter with the silicon surface and distinguish between the 'true' FRET region (<5 nm) and a different process, photon tunnelling, occurring for distances between 10-50 nm. The requirements for a future ultra-thin crystalline solar cell paradigm include efficient surface passivation and keeping a close distance between the emitter dipole and the surface. These are discussed in the context of existing limitations and questions raised about the finer details of the emitter-silicon interaction

Vacancy-Ordered Double Perovskite Cs2TeI6 Thin Films for Optoelectronics

Alternatives to lead- and tin-based perovskites for photovoltaics and optoelectronics are sought that do not suffer from the disadvantages of toxicity and low device efficiency of present-day materials. Here we report a study of the double perovskite Cs2TeI6, which we have synthesized in thin film form for the first time. Exhaustive trials concluded that spin coating CsI and TeI4 using an anti-solvent method produced uniform films, confirmed as Cs2TeI6 by XRD with Rietveld analysis. They were stable up to 250°C, had an optical band gap of ~1.5 eV, absorption coefficients of ~6 x 104 cm-1, carrier lifetimes of ~2.6 ns (unpassivated 200 nm film), a work function of 4.95 eV and had p-type surface conductivity. Vibrational modes probed by Raman and FTIR spectroscopy showed resonances qualitatively consistent with DFT Phonopy-calculated spectra, offering another route for phase confirmation. It was concluded that the material is a candidate for further study as a potential optoelectronic or photovoltaic material

Non-linear spectroscopic studies at interfaces : experiment and theory

The structure and properties of the neat air/toluene interface were studied with SHG using a fundamental wavelength of 532 nm.  The second harmonic wavelength (266 nm) was resonantly enhanced with the first electronic transition and a polarisation dependence study showed the toluene molecule to be oriented by 36° to the surface normal.  Extensive theoretical calculations were carried out using Hartree-Fock (HF), Møller-Plesset (MP2) and Density Functional theory (DFT) methods on the geometries, harmonic frequencies, excitation energies and frequency dependent (hyper) polarisabilities on the toluene molecule. The non-linear optical properties of the aromatic amino acid L-phenylalanine (Phe) were investigated at the air/water interface.  Polarisation and concentration dependence studies were performed on the air/phenylalanine(aq) interface for concentrations up to 0.8 M.  The phenylalanine molecules were found to complete a monolayer for concentrations above 0.2 M and the adsorption could be described by a Langmuir isotherm.  The orientation of the molecule at the interface didn’t change significantly with increasing bulk concentration and the molecule could be approximated to being weakly aligned at the interface.  The surfactant C20-Phe was studied as a Langmuir-Blodgett (LB) film at the air/water interface.  The orientation of the Phe chromophore was studied by SHG from the gas phase of the film monolayer and it was found to be shifted 6° away from the surface normal with respect to the orientation of the free Phe molecule at the air/water interface.  A frequency dependent calculation on the hyperpolarisability of the Phe molecule with different Phe conformations revealed the sensitivity of the hyperpolarisability values on the conformation of the molecule.</p