Raphaël G. C. R. Clady

Improving the light-harvesting of amorphous silicon solar cells with photochemical upconversion

Single-threshold solar cells are fundamentally limited by their ability to harvest only those photons above a certain energy. Harvesting below-threshold photons and re-radiating this energy at a shorter wavelength would thus boost the efficiency of such devices. We report an increase in light harvesting efficiency of a hydrogenated amorphous silicon (a-Si:H) thin-film solar cell due to a rear upconvertor based on sensitized triplet–triplet-annihilation in organic molecules. Low energy light in the range 600–750 nm is converted to 550–600 nm light due to the incoherent photochemical process. A peak efficiency enhancement of (1.0 ± 0.2)% at 720 nm is measured under irradiation equivalent to (48 ± 3) suns (AM1.5). We discuss the pathways to be explored in adapting photochemical UC for application in various single threshold devices.

Photochemical Upconversion Enhanced Solar Cells: Effect of a Back Reflector

Photochemical upconversion is applied to a hydrogenated amorphous silicon solar cell in the presence of a back-scattering layer. A custom-synthesized porphyrin was utilized as the sensitizer species, with rubrene as the emitter. Under a bias of 24 suns, a peak external quantum efficiency (EQE) enhancement of ~2 % was observed at a wavelength of 720 nm. Without the scattering layer, the EQE enhancement was half this value, indicating that the effect of the back-scatterer is to double the efficacy of the upconverting device. The results represent an upconversion figure of merit of 3.5 × 10–4 mA cm–2 sun–2, which is the highest reported to date.

High-efficiency, broad band, high-damage threshold high-index gratings for femtosecond pulse compression.

High efficiency, broad-band TE-polarization diffraction over a wavelength range centered at 800 nm is obtained by high index gratings placed on a non-corrugated mirror. More than 96% efficiency wide band top-hat diffraction efficiency spectra, as well as more than 1 J/cm(2) damage threshold under 50 fs pulses are demonstrated experimentally. This opens the way to high-efficiency Chirped Pulse Amplification for high average power laser machining by means of all-dielectric structures as well as for ultra-short high energy pulses by means of metal-dielectric structures.

Entropically driven photochemical upconversion.

Conventional photochemical upconversion (UC) through homo-geneous triplet-triplet annihilation (TTA) is subject to several enthalpic losses that limit the UC margin. Here, we address one of these losses: the triplet energy transfer (TET) from the sensitizer to the emitter molecules. Usually, the triplet energy level of the emitter is set below that of the sensitizer. In our system, the triplet energy level of the emitter exceeds that of the sensitizer by ∼600 cm(-1). Choosing suitable concentrations for the sensitizer and emitter molecules, we can exploit entropy as a driving force for the migration of triplet excitation from the sensitizer to the emitter manifolds. Thereby we obtain a new record for the peak-to-peak TTA-UC energy margin of 0.94 eV. A modified Stern-Volmer analysis yields a TET rate constant of 2.0 × 10(7) M(-1) s(-1). Despite being relatively inefficient, the upconverted fluorescence is easily visible to the naked eye with irradiation intensities as low as 2 W cm(-2).

Towards an aligned luminophore solar concentrator

Luminescent solar concentrators promise to reduce the cost of solar energy, but are hindered by a number of losses. Escape of luminescence through the large waveguide-air interfaces can be attenuated through alignment of the optical transition dipole of the luminophore along the waveguide surface normal, directing the maximum possible proportion of luminescence into waveguide modes. We demonstrate such alignment using a guest-host dye-doped liquid crystal sandwiched between conductive glass slides. Application of a potential while illuminating through a narrow edge caused a drop in the intensity of luminescence escaping the large surfaces, and an increase in the intensity of light escaping the narrow edges of the system. This is explained in terms of alignment of the transition dipoles of the dye. We discuss implementation in a luminescent solar concentrator.

InGaAs/GaAsP quantum wells for hot carrier solar cells

Hot carrier solar cells have a fundamental efficiency limit well in excess of single junction devices. Developing a hot carrier absorber material, which exhibits sufficiently slow carrier cooling to maintain a hot carrier population under realistic levels of solar concentration is a key challenge in developing real-world hot carrier devices. We propose strain-balanced In0.25GaAs/GaAsP0.33 quantum wells as a suitable absorber material and present continuous-wave photoluminescence spectroscopy of this structure. Samples were optimised with deep wells and the GaAs surface buffer layer was reduced in thickness to maximise photon absorption in the well region. The effect of well thickness on carrier distribution temperature was also investigated. An enhanced hot carrier effect was observed in the optimised structures and a hot carrier distribution temperature was measured in the thick well (14 nm) sample under photon flux density equivalent to 1000 Suns concentration.

Efficient up-conversion by triplet-triplet annihilation

Following experimental determination of kinetic parameters governing the up-conversion of light by triplet-triplet annihilation (TTA-UC), we develop a kinetic model to determine the conditions required for efficient up-conversion. We discuss the assumptions underpinning statistical arguments for an upper limit to TTA-UC and argue that no such limit exists.

Hot carrier dynamics in InGaAs/GaAsP quantum well solar cells

A hot carrier solar cell is a device with a steady-state carrier population which is described by a higher temperature than the surrounding lattice. Thermalisation loss is reduced in such a device, offering the potential for substantial efficiency advantages over single junction solar cells. Despite clear efficiency benefits no real world device has ever been developed, partly because of the difficulty of developing a suitable absorber material with sufficiently limited interaction between excited carriers and lattice phonons. This study evaluates the suitability of strain balanced InGaAs/GaAsP quantum well structures as hot carrier absorbers. Ultrafast time resolved photoluminescence (TRPL) spectroscopy measurements are presented which demonstrate hot carrier populations beyond 2ns after excitation in a deep well sample. Continuous wave photoluminescence (CWPL) spectroscopy was used to compare steady-state carrier populations in deep and shallow well samples. In both cases hot distributions were observed under photon flux density greater than 10,000 Suns equivalent. Increasing incident photon flux density was shown to increase carrier distribution temperature, suggesting that the hot carrier effect might be enhanced in a multiple QW structure with better well region absorption. It was also found that the deep well sample achieved significantly higher carrier distribution temperatures than the shallow well sample, demonstrating that increasing quantum confinement further inhibits thermalisation pathways. This study provides a guide to the development of hot carrier solar cells as it indicates deep multiple quantum well samples might exhibit an enhanced hot carrier effect. Strain Balanced InGaAs/GaAsP is a particularly suitable material system for growing this type of structure, making it an exciting prospect for the development of a hot carrier absorber.

The structure and luminescence properties of europium(III) triflate doped self-assembled pyromellitamide gels

Addition or doping of anion-sensitive self-assembled gels formed from tetrahexyl pyromellitamide 1 in cyclohexane with a few equivalents of europium(III) triflates results in materials with red luminescence arising from the europium(III) ions. Gelation and spectroscopic studies suggest that the europium(III) ions bind to the amides in 1 with an apparent association constant of 770 M−1. Luminescence lifetime studies show a 273 ± 8 μs lifetime for the 5D0 → 7F2 transition upon excitation with laser light at 300 nm. Interestingly, the red europium emission can also be observed using a multi-photon microscope and a near infrared (NIR) excitation source of 780 nm but this is almost certainly a quadratic electro-optic effect (Kerr effect) due to the intense laser beam generating a white light continuum excitation into the ultraviolet range. These results do demonstrate that self-assembled gels can be doped with europium salts generating smart soft material for applications in time-gated spectroscopy and microscopy.

Effects of non-ideal energy selective contacts and experimental carrier cooling rate on the performance of an indium nitride based hot carrier solar cell

The performance of an InN based hot carrier solar cell with a bulk InN absorber has been evaluated using an innovative approach that takes into account absorber energy-momentum dispersion relations, energy conservation, Auger recombination and impact ionization mechanisms simultaneously. The non ideality of the energy selective filters has also been included in the model. In order to obtain practical achievable values of conversion efficiency, the actual thermalisation velocity of hot carriers in InN has been measured using time resolved photoluminescence. Results of the computations shown limiting efficiencies of 24% for 1000 suns and 36.2% for maximal concentration.