Lefteris Danos

Large surface photovoltages observed at methyl-terminated silicon surfaces synthesised through a two-step chlorination-alkylation method

Methylation of the silicon surface through a chlorination-alkylation method has been used to improve the electronic properties of silicon. Upon alkylation of the surface, an increase in the minority carrier recombination lifetime and the surface photovoltage is observed, in line with an increase in surface charge. A likely explanation of this unusually large band bending is charge accumulation during the removal of chlorine from the surface.

Photon frequency management for trapping & concentration of sunlight

This paper considers a range of techniques which – within the realm of classical optics – can be used to enhance light capture as a first step in photovoltaic energy conversion. Examples include a simple case of downshifting, fluorescent collectors which reduce the size of a light beam, and a novel form of light trapping to increase the path length of light within the solar cell. The results are discussed using a thermodynamic framework where the energy exchange with an absorbing/fluorescent medium allows the entropy of the captured photon gas to be lowered, reducing the etendue of the emitted beam. We show that frequency management represents a powerful tool, allowing enhancement in light trapping above the Yablononovitch limit, leading to potentially highly efficient but very thin crystalline silicon solar cells.

Photon reabsorption in fluorescent solar collectors

Understanding photon transport losses in fluorescence solar collectors is very important for increasing optical efficiencies. We present an analytical expression to characterize photon reabsorption in fluorescent solar collectors, which represent a major source of photon loss. A particularly useful universal form of this expression is found in the limit of high reabsorption, which gives the photon reabsorption probability in a simple form as a function of the absorption coefficient and the optical etendue of the emitted photon beam. Our mathematical model predicts fluorescence spectra emitted from the collector edge, which are in excellent agreement with experiment and provide an effective characterization tool for photon transport in light absorbing media.

GaSb quantum rings in GaAs/AlxGa1−xAs quantum wells

We report the results of continuous and time-resolved photoluminescence measurements on type-II GaSb quantum rings embedded within GaAs/AlxGa1−xAs quantum wells. A range of samples were grown with different well widths, compensation-doping concentrations within the wells, and number of quantum-ring layers. We find that each of these variants have no discernible effect on the radiative recombination, except for the very narrowest (5 nm) quantum well. In contrast, single-particle numerical simulations of the sample predict changes in photoluminescence energy of up to 200 meV. This remarkable difference is explained by the strong Coulomb binding of electrons to rings that are multiply charged with holes. The resilience of the emission to compensation doping indicates that multiple hole occupancy of the quantum rings is required for efficient carrier recombination, regardless of whether these holes come from doping or excitation.

Silicon sensitisation using light harvesting layers

Abstract Langmuir–Blodgett monolayers of a cyanine dye mixed with stearic acid were deposited on glass and silicon substrates with spacer layers of pure stearic acid monolayers or silicon dioxide films deposited by PECVD. By using the time correlated single photon counting technique, time resolved emission spectra (TRES) and decay curves were measured to characterise the dependence of energy transfer rate on the separation between the dye monolayer and the silicon surface and also for the dye concentrations in the monolayers. We observe interlayer energy transfer between monomers, dimers and higher aggregates present in the monolayer deposited on glass but also competing directly with energy transfer to silicon at close distances. We find that the fluorescence lifetime of the dye monolayer is significantly shortened when deposited close to the silicon surface signifying efficient energy transfer. The dissipation of the excitation energy near silicon is explained using the classical theory developed for metals and a deviation is observed for monolayers deposited at distances close to the silicon surface.

Kelvin probe studies of alkyl monolayers on silicon (111) for surface passivation

A chlorination–alkylation procedure has been investigated with a view to improving the surface passivation properties on silicon. We have found that increasing surface coverage of the alkyl monolayer raises the measured SPV and recombination lifetime. A logarithmic trend is observed between the SPV and recombination, showing that monolayer preparation is an important factor in determining the lifetimes observed. The high SPVs measured by Kelvin probe suggest the presence of a silicon surface with a lower concentration of electrons, reducing surface recombination.

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.

Emission Wavelength Tuning in Rare Earth Fluoride Upconverting Nanoparticles Decorated with Dye-Coated Titanate Nanotubes

The radiative energy transfer from rare earth fluoride upconverting (UC) Na(x)Li(y)YF(4):Yb(3+),Er(3+) nanoparticles to rhodamine dyes has been systematically studied in colloidal solutions at room temperature. The UC emission bands at 520 and 550 nm have been shifted to the longer-wavelength (ca. 600 nm) region suitable for biomedical applications. To decrease the optical length between the upconverting emitter and the fluorophore, the UC nanoparticles were decorated with titanate nanotubes coated with a dense layer of dye molecules providing possible resonance-energy transfer between them. The fabricated nanostructured composite shows efficient harvesting of UC emission within the proximity of the nanoparticles, allowing the local generation of light suitable for photodynamic therapy applications.

Efficient Light Harvesting with LB Films for Application in Crystalline Silicon Solar Cells

We have carried out fluorescence lifetime measurements using time correlated single photon counting (TCSPC) for a cyanine dye near the silicon surface. The measurements have been carried out for both (100) and (111) crystal orientations of the silicon surface, showing the dependence of energy transfer rate as a function of the separation between the dye monolayer and the silicon surface. Langmuir Blodgett fatty acid layers were used to create a multistep structure and a monolayer of a cyanine dye was deposited on top of the stepped structure. Spectroscopic ellipsometry has been used to measure the thickness of the fatty acid steps and provide an accurate estimate of the distance of the dye monolayer to the silicon surface. Time resolved emission spectra and fluorescence decay curves were measured with a single photon picosecond time correlated system. We find that the fluorescence lifetime of the dye monolayer is significantly shortened when present close to the silicon surface signifying efficient energy transfer. The dissipation of the excitation energy near silicon is explained using the classical theory developed for metals and a deviation is observed for distances close to the silicon surface (d<5nm). The model can be reconciled with the observed data by modifying the value of the silicon extinction coefficient which can provide an insight into the energy transfer process in the near field dye-silicon interaction.

Enhancing light capture by silicon - with the help of molecules

Efficient capture of sunlight remains one of the great challenges to photovoltaics today. This is particularly so for the dominant photovoltaic material - crystalline silicon - which, as an indirect gap semiconductor, needs several hundred micrometers thickness for efficient operation. This paper gives an overview of the principal concepts that are currently being considered to enhance light capture by the solar cell. We shall, in particular, compare and contrast two main ideas of thought that underpin the current status of the field. The first, based on thermodynamics, makes use of light trapping where photon path within a structure is extended by virtue of a stochastic photon distribution inside a dielectric / weakly absorbing semiconductor. The second approach rests on the use of sub-wavelength or nano-scale structures which allow the possibility of electromagnetic energy injection into very thin semiconductor layers, by direct interaction with the trapped modes or via the near field of an intermediate dipole absorber or scatterer. We review a range of techniques which are available to reducing the thickness of crystalline silicon solar cells to below 1μm with the use of molecular layers deposited on thin crystalline silicon layers by spin coating, as Langmuir-Blodgett films, or directly anchored to silicon by covalent bonding.