Louise C. Hirst

Hot Carriers in Quantum Wells for Photovoltaic Efficiency Enhancement

In a hot carrier solar cell, the steady-state carrier population is hot relative to the surrounding lattice. This requires an absorber material which restricts carrier-phonon interaction and, therefore, reduces entropic loss during thermalization. The limiting efficiency of these devices approaches 85%: the Carnot limit for a solar energy collector. A spectroscopic analysis of GaAsP/InGaAs quantum well structures shows that carrier cooling in single quantum well samples is dominated by the rate of radiative recombination, leading to unprecedented carrier cooling lifetime (τ = 5.8 ±0.1 ns). This exceptional lifetime arises due to state saturation, frustrating the carrier scattering processes. A steady-state carrier population temperature >100 K above the lattice temperature is measured under illumination equivalent to 10 000 Suns. We calculate the projected efficiency >40% for a device with these characteristics, amounting to a 3% efficiency enhancement over equivalent single-junction devices.

Experimental demonstration of hot-carrier photo-current in an InGaAs quantum well solar cell

An unambiguous observation of hot-carrier photocurrent from an InGaAs single quantum well solar cell is reported. Simultaneous photo-current and photoluminescence measurements were performed for incident power density 0.04–3 kW cm−2, lattice temperature 10 K, and forward bias 1.2 V. An order of magnitude photocurrent increase was observed for non-equilibrium hot-carrier temperatures >35 K. This photocurrent activation temperature is consistent with that of equilibrium carriers in a lattice at elevated temperature. The observed hot-carrier photo-current is extracted from the well over an energy selective GaAs barrier, thus integrating two essential components of a hot-carrier solar cell: a hot-carrier absorber and an energy selective contact.

Spatially indirect radiative recombination in InAlAsSb grown lattice-matched to InP by molecular beam epitaxy

A photoluminescence (PL) spectroscopy study of the bulk quaternary alloy InAlAsSb is presented. Samples were grown lattice-matched to InP by molecular beam epitaxy and two different growth temperatures of 450 °C and 325 °C were compared. Interpolated bandgap energies suggest that the development of this alloy would extend the range of available direct bandgaps attainable in materials lattice-matched to InP to energies as high as 1.81 eV. However, the peak energy of the observed PL emission is anomalously low for samples grown at both temperatures, with the 450 °C sample showing larger deviation from the expected bandgap. A fit of the integrated PL intensity (I) to an I∝Pk dependence, where P is the incident power density, yields characteristic coefficients k = 1.05 and 1.18 for the 450 °C and 325 °C samples, respectively. This indicates that the PL from both samples is dominated by excitonic recombination. A blue-shift in the peak emission energy as a function of P, along with an S-shaped temperature depe...

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.

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 efficiency limit of solar cells with molecular absorbers: A master equation approach

A master equation approach to the limiting efficiencies of solar cells with molecular absorbers is presented. A number of model absorbers are analyzed that possess identical absorption spectra, but with differing numbers of electronic excited states. The Shockley–Queisser limit is reproduced for a molecule resembling a semiconductor, with an infinity of electronic levels in the excited state manifold. However, when only a few electronic states contribute to the absorption spectrum, the limiting efficiency is reduced. In the extreme case, where only a single electronic excited state participates in the absorption spectrum, the efficiency limit is 28.9%. At high energy thresholds, built-in thermal up-conversion results in solar cells with efficiencies exceeding the Shockley–Queisser curve. The analysis is applicable to any single threshold photovoltaic device, including those based on semiconductor, polymer, and small molecule absorbers.

Enhanced Hot-Carrier Effects in InAlAs/InGaAs Quantum Wells

Hot-carrier solar cells require absorber materials with restricted carrier thermalization pathways, in order to slow the rate of heat energy dissipation from the carrier population to the lattice, relative to the rate of carrier extraction. Absorber suitability can be characterized in terms of carrier thermalization coefficient (Q). Materials with lower Q generate steady-state hot-carrier populations at lower levels of incident solar power and, therefore, are better able to perform as hot-carrier absorbers. In this study, we evaluate Q = 2.5±0.2 W·K-1 · cm-2 for a In0.52 AlAs/In0.53 GaAs single-quantum-well(QW) heterostructure using photoluminescence spectroscopy. This is the lowest experimentally determined Q value for any material system studied to date. Hot-carrier solar cell simulations, using this material as an absorber yield efficiency ~39% at 2000X, which corresponds to a >5% enhancement over an equivalent single-junction thermal equilibrium device.

Controlling radiative loss in quantum well solar cells

The inclusion of quantum well layers in a solar cell provides a means for extending the absorption and therefore increasing the photocurrent of the cell. In 2009, a single-junction GaAsP/InGaAs quantum well solar cell attained a peak efficiency of 28.3% under solar concentration. Since then InGaP/MQW/Ge quantum well devices have attained efficiencies in excess of 40% under concentration and over 30% under AM0. The principle motivation for incorporating a quantum well stack into a multi-junction solar cell is to increase the photocurrent delivered by the middle junction over the conventional In0.01GaAs bulk junction. This enables additional current to flow through the top and middle cells, resulting in a sharp rise in efficiency. However, quantum wells also provide some freedom to manipulate the radiative recombination in the quantum well solar cell. We show that under radiatively dominated, anisotropic emission, strong radiative coupling between sub-cells takes place, resulting in a multi-junction solar cell that is tolerant to daily and seasonal changes to the solar spectrum.

Intrinsic radiation tolerance of ultra-thin GaAs solar cells

Radiation tolerance is a critical performance criterion of photovoltaic devices for space power applications. In this paper we demonstrate the intrinsic radiation tolerance of an ultra-thin solar cell geometry. Device characteristics of GaAs solar cells with absorber layer thicknesses 80 nm and 800 nm were compared before and after 3 MeV proton irradiation. Both cells showed a similar degradation in Voc with increasing fluence; however, the 80 nm cell showed no degradation in Isc for fluences up to 1014 p+ cm−2. For the same exposure, the Isc of the 800 nm cell had severely degraded leaving a remaining factor of 0.26.

Modeling, design and experimental results for high efficiency multi-junction solar cells lattice matched to InP

The high conversion efficiencies demonstrated by multi-junction solar cells over the past three decades have made them indispensable for use in space and are very attractive for terrestrial concentrator applications. The multi-junction technology consistently displays efficiency values in excess of 30%, with record highs of 37.8% under 1 sun conditions and over 44% under concentration. However, as material quality in current III-V multi-junction technology reaches practical limits, more sophisticated structures will be required to further improve on these efficiency values. In a collaborative effort amongst several institutions we have developed a novel multi-junction solar cell design that has the potential to reach the 50% conversion efficiency value. Our design consists of a three junction cell grown on InP substrates which achieves the optimal bandgaps for solar energy conversion using lattice matched materials. In this work, we present the progress in the different subcells comprising this multi-junction structure. For the top cell, InAlAsSb quaternary material is studied. For the middle, InGaAlAs and InGaAsP materials and devices are considered and for the bottom, a multi-quantum well structure lattice matched to InP for fine bandgap tunability for placement in an InGaAs cell is demonstrated.