Ian Ballard

Quantum well solar cells

In this paper we discuss the potential advantages of quantum wells (QWs) for enhancing solar cell efficiency. We present recent experimental results which show that the open-circuit voltage (Voc) of the quantum well solar cell (QWSC) is enhanced over that of comparable conventional cells formed from the well material, by more than the change in the absorption edge. We also report on theoretical and experimental studies which seek to determine the quasi-Fermi level separation in quantum wells inside a p-i-n system in order to understand the voltage behaviour of QWSCs and to be able to estimate the efficiency enhancements which may be achieved in the radiative limit. We discuss QWSCs in the InPInGaAs lattice matched system and present results which show that QWSCs in this deep well system have a better variation of efficiency with temperature than conventional cells made from either the well or barrier material. This is important for applications involving concentrated sunlight. We also consider the advantages of quantum well cells in the area of thermophotovoltaics (TPV).

Photoconductivity and charge trapping in porous nanocrystalline titanium dioxide

We have measured the photoconductivity of symmetrically contacted nanocrystalline anatase TiO2 films in different chemical environments. The photocurrent can be attributed to the conduction band electron density and can be described quantitatively by a rate equation model which incorporates trapping, recombination and scavenging. In ambient air and in an acetonitrile solvent, the photocurrent is controlled by a competition between two electron loss processes, probably electron–hole recombination and electron scavenging by surface adsorbed species. In vacuum, electron scavenging is suppressed, leading to photocurrents which are increased by a factor of 10 5 –10 6 compared to ambient air. This is attributed to the removal of molecular oxygen. Addition of methanol to acetonitrile similarly appears to extend the electron lifetime and leads to much larger photocurrents. This is attributed to the hole scavenging effect of methanol. At low light intensities, in conditions where electron loss processes are suppressed, trap filling effects may be observed. If the photogeneration rate is known, the density of trap states may be deduced. For films in vacuum, the density of effective electron traps is estimated to be less than 10 20 cm −3 . The method is applicable to dye sensitised solar cells, for instance in the evaluation of different hole transporting media, and to photocatalysis.

Charge transport in porous nanocrystalline titanium dioxide

Abstract The dark conductivity and photoconductivity of porous, anatase titanium dioxide films have been studied in different ambient conditions. The films are nanocrystalline with a particle size of 5– 15 nm and porosity of around 50%. Films are resistive (10 4 – 10 6 Ω m ) in the dark in ambient air, and exhibit space charge limited current–voltage behaviour, modified by the presence of traps. Vacuum reduces the dark conductivity by a factor of 10 2 –10 3 . This effect is tentatively attributed to the removal of water, which is known to adsorb dissociatively on TiO 2 surfaces and may dope the material by proton insertion and Ti 3+ formation. The photoconductivity in vacuum is 10 6 larger than that in air at maximum photocurrent and increases with decreasing pressure. In this case the effect is attributed to the loss of surface adsorbed oxygen, a known electron scavenger, in vacuum. Removal of oxygen extends the electron lifetime and results in a much larger saturation photocurrent. In vacuum, a point of inflexion is observed in the transient rise and the shapes of the curves are intensity dependent. Both these observations are consistent with the presence of traps. No correlation was observed between the photoconductivity decays and temperature, which suggests that the decay occurs by band-to-band recombination and not thermionic emission. On the basis of these observations, a model based on competition between photogeneration, trapping and scavenging has been developed. By varying the trapping and recombination rates we can simulate the effects of air and vacuum. The intensity dependent results can be simulated by changing the generation rate alone which allows us to estimate a trap density of less than 10 20 cm −3 . We propose that photoconductivity may be used as a direct probe of the electron lifetime and can serve to evaluate different chemical environments for dye sensitised solar cells, and to study photocatalytic function.

Observation of photon recycling in strain-balanced quantum well solar cells

Photon recycling in strain-balanced quantum well solar cells grown on distributed Bragg reflectors has been observed as a suppression of the dark current and a change in electroluminescence spectra. Comparing devices grown with and without distributed Bragg reflectors we have demonstrated up to a 33% reduction in the ideality n=1 reverse saturation current. Furthermore, to validate the observations we demonstrate how both the measured dark currents and electroluminescence spectra fit very well to a photon recycling model. Verifying our observations with the model then allows us to calculate optimized device designs.

Effect of quantum well location on single quantum well p-i-n photodiode dark currents

The photocurrent available from a p-i-n solar cell can be increased by the addition of quantum wells (QWs) to the undoped region. At the same time the QWs reduce the open-circuit voltage by introducing areas of lower band gap where recombination is enhanced. This increase in recombination should be as small as possible for the most favorable effect on the photovoltaic efficiency of the device. Theoretical considerations indicate that nonradiative recombination, which is the dominant loss mechanism in AlxGa1−xAs/GaAs QW structures, may be reduced by positioning the QWs away from the point where the electron-hole product is a maximum. For p-i-n diodes, where recombination is greatest at or near the center of the space charge region, this means locating the QWs closer to the doped regions. Spectral response should not be affected so long as the QWs are still located within the field bearing region. Thus, improved photovoltaic performance may be expected through strategic location of the QWs. We report on mea...

Photovoltaic efficiency enhancement through thermal up-conversion

A scheme is presented whereby ambient thermal energy is coupled to sub-band-gap photons through the creation and recombination of electron hole pairs, thereby increasing the short-circuit current and the efficiency of a solar cell. The necessary photon-phonon coupling is demonstrated experimentally, through the observation of anti-Stokes luminescence from QW p-i-n devices. A modest efficiency enhancement is possible for a constrained PV device, but the maximum efficiency does not exceed that of the Shockley-Queisser limit. Only when a temperature difference between the PV device and QW thermal up-convertor is established, can the system exceed the Shockley-Queisser limit.

Simulating multiple quantum well solar cells

The quantum well solar cell (QWSC) has been proposed as a route to higher efficiency than that attainable by homojunction devices. Previous studies have established that carriers escape the quantum wells with high efficiency in forward bias and contribute to the photocurrent. Progress in resolving the efficiency limits of these cells has been dogged by the lack of a theoretical model reproducing both the enhanced carrier generation and enhanced recombination due to the quantum wells. Here we present a model which calculates the incremental generation and recombination due to the QWs and is verified by modelling the experimental light and dark current-voltage characteristics of a range of III-V quantum well structures. We find that predicted dark currents are significantly greater than experiment if we use lifetimes derived from homostructure devices. Successful simulation of light and dark currents can be obtained only by introducing a parameter which represents a reduction in the quasi-Fermi level separation.

The application of quantum well solar cells to thermophotovoltaics

Abstract We discuss the advantages of quantum well solar cells (QWSCs) for thermophotovoltaic (TPV) applications and illustrate them with InP/InGaAs and GaInAsP/InGaAs QWSCs which were designed for other applications and have not been optimised for TPV. It is shown that an InP p-i-n solar cell with 15 lattice matched InGaAs quantum wells (QWs) in the i region has an increase in open circuit voltage ( V oc ) of (1.7 ± 0.1) times that of a control cell of InP with InGaAs in the i-region under an illuminating spectrum close to that expected from an ideal ytterbia emitter. Also, using an InGaAsP quaternary cell of band gap wavelength of 1.1 Am with 60 InGaAs QWs under the same illuminating spectrum the current density is increased by a factor of (2.4 ± 0.1) over that of the InP QWSC. The quaternary cell also absorbs longer wavelengths without any significant loss in V OC . Better temperature coefficients for the former quantum well solar cell than the control cell are observed in a spectrum approximating a black body at 3000 K. Further advantages of QWs for narrow band and broad band illuminating spectra are discussed.

Quantum Well Solar Cells and Quantum Dot Concentrators

Publisher Summary This chapter focusses on quantum well solar cells and quantum dot concentrators. This chapter reviews the development over the past half a decade of the quantum well solar cell (QWSC) and the quantum dot concentrator (QDC). The study of nanostructures such as quantum wells (QWs) and quantum dots (QDs) has dominated opto-electronic research and development for the past two decades. The chapter also reviews recent advances since then, concentrating in particular on studies of the strain-balanced quantum well solar cell (SB-QWSC) as a concentrator cell and the thermodynamic modelling of the QDC. The SB-QWSC offers a way to extend the spectral range of the highest efficiency single-junction cell, the GaAs cell. It discusses the way this can in principle lead to higher efficiency in both single-junction and multi-junction cells and offers particular advantages in high-concentration systems. The high efficiency, wide spectral range, and small cell size make these systems particularly attractive for high concentration, building-integrated applications using direct sunlight. The chapter also demonstrates a 3D thermodynamic model capable of describing the performance of dye-doped and QD-doped slabs of luminescent concentrators. The model is a powerful tool for analyzing the performance of the luminescent concentrators. The fits show that concentrator performance is currently limited by the quantum efficiency (QE) of the QDs dispersed in the plastics.

InP-based lattice-matched InGaAsP and strain-compensated InGaAs∕InGaAs quantum well cells for thermophotovoltaic applications

Quantum well cells (QWCs) for thermophotovoltaic (TPV) applications are demonstrated in the InGaAsP material system lattice matched to the InP substrate and strain-compensated InGaAs∕InGaAs QWCs also on InP substrates. We show that lattice-matched InGaAsP QWCs are very well suited for TPV applications such as with erbia selective emitters. QWCs with the same effective band gap as a bulk control cell show a better voltage performance in both wide and erbialike emission. We demonstrate a QWC with enhanced efficiency in a narrow-band spectrum compared to a bulk heterostructure control cell with the same absorption edge. A major advantage of QWCs is that the band gap can be engineered by changing the well thickness and varying the composition to the illuminating spectrum. This is relatively straightforward in the lattice-matched InGaAsP system. This approach can be extended to longer wavelengths by using strain-compensation techniques, achieving band gaps as low as 0.62eV that cannot be achieved with lattice-...