Mazzer, M.; Barnham, K. W. J.; Ballard, I. M.; Bessiere, A.; Ioannides, A.; Johnson, D. C.; Lynch, M. C.; Tibbits, T. N. D.; Roberts, J. S.; Hill, G.; Calder, C.
Thin Solid Films, Vol. 511
A quantum well solar cell is a special multiple-band gap device with intermediate properties between heterojunction cells (sum of the currents generated in the different materials but voltage controlled by the lowest of the two band gaps) and tandem cells (sum of the voltages but current determined by the worst of the two sub-cells).
Strain-balanced GaAsP/InGaAs multi-quantum wells move the absorption edge of GaAs solar cells closer to the optimum value for single junction cells with no need for any partially relaxed buffer layer to accommodate lattice mismatch between the absorbing layers and the substrate. Covering a large spectral range in a single-junction cell has the benefit that the cell remains close to optimal efficiency in the varying spectral conditions of a …
Johnson, D. C.; Ballard, I. M.; Barnham, K. W. J.; Connolly, J. P.; Mazzer, M.; Bessiere, A.; Calder, C.; Hill, G.; Roberts, J. S.
Applied Physics Letters, Vol. 90, (Issue No. 21)
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.
Johnson, D. C.; Ballard, I.; Barnham, K. W. J.; Bishnell, D. B.; Connolly, J. P.; Lynch, M. C.; Tibbits, T. N. D.; Ekins-Daukes, N. J.; Mazzer, M.; Airey, R.; Hill, G.; Roberts, J. S.
Solar Energy Materials and Solar Cells, Vol. 87, (Issue No. 1-4)
Strain-balanced quantum well solar cells (SB-QWSC) extend the photon absorption edge beyond that of bulk GaAs by incorporation of quantum wells in the i-region of a p-i-n device. The addition of a distributed Bragg reflector (DBR) can substantially increase the photocurrent with little or no detriment to the dark-current. Experimental results are presented that show improvements of DBR cell efficiencies over SB-QWSC's without DBR's. In addition, at high dark-current levels appropriate to high concentration, we observe that the dark-currents of the SB-QWSC's exhibit ideal diode behaviour. We present evidence that the ideality n = 1 dark-current is reduced in the DBR cells and discuss the possible efficiency improvements if the dark-current is radiatively dominant.
Rohr, C.; Abbott, P.; Ballard, I.; Connolly, J. P.; Barnham, K. W. J.; Mazzer, M.; Button, C.; Nasi, L.; Hill, G.; Roberts, J. S.; Clarke, G.; Ginige, R.
Journal of Applied Physics, Vol. 100, (Issue No. 11)
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 …
Barnham, K. W. J.; Mazzer, M.; Clive, B.
Nature Materials, Vol. 5, (Issue No. 3)
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Bushnell, D. B.; Tibbits, T. N. D.; Barnham, K. W. J.; Connolly, J. P.; Mazzer, M.; Ekins-Daukes, N. J.; Roberts, J. S.; Hill, G.; Airey, R.
Journal of Applied Physics, Vol. 97, (Issue No. 12)
The effect of increasing the number of quantum wells in a strain-compensated, multiquantum-well solar cell is investigated. It is found that as the well number is increased, dark current level close to the operating point rises linearly. Short-circuit current in the AM0 spectrum also rises linearly with the inclusion of more quantum wells. This allows the cell to maintain a constant open-circuit voltage irrespective of the number of wells grown. This is anticipated to have advantages when the cell is used as a replacement for the GaAs junction in the existing generation of tandem and triple-junction cells since current levels can be matched to the upper junction without detriment to the voltage performance. This result allows us to predict a tandem cell AM0 efficiency of …
Johnson, D. C.; Ballard, I.; Barnham, K. W. J.; Bishnell, D. B.; Connolly, J. P.; Lynch, M. C.; Tibbits, T. N. D.; Ekins-Daukes, N. J.; Mazzer, M.; Airey, R.; Hill, G.; Roberts, J. S.
Energy Materials and Solar Cells, Vol. 87, (Issue No. 1-4)
Strain-balanced quantum well solar cells (SB-QWSC) extend the photon absorption edge beyond that of bulk GaAs by incorporation of quantum wells in the i-region of a p-i-n device. The addition of a distributed Bragg reflector (DBR) can substantially increase the photocurrent with little or no detriment to the dark-current. Experimental results are presented that show improvements of DBR cell efficiencies over SB-QWSC's without DBR's. In addition, at high dark-current levels appropriate to high concentration, we observe that the dark-currents of the SB-QWSC's exhibit ideal diode behaviour. We present evidence that the ideality n = 1 dark-current is reduced in the DBR cells and discuss the possible efficiency improvements if the dark-current is radiatively dominant.
Lynch, M. C.; Ballard, I. M.; Bushnell, D. B.; Connolly, J. P.; Johnson, D. C.; Tibbits, T. N. D.; Barnham, K. W. J.; Ekins-Daukes, N. J.; Roberts, J. S.; Hill, G.; Airey, R.; Mazzer, M.
Journal of Materials Science, Vol. 40, (Issue No. 6)
The inclusion of quantum wells in p-i-n solar cells leads to both an increase in photocurrent and a reduction in open circuit voltage. It has been shown that up to 50 shallow strain-balanced GaAsP/InGaAs quantum wells can be inserted into a GaAs cell resulting in an increase in photocurrent that is greater than the reduction in V-oc, leading to higher cell efficiency [1]. We present an investigation into the effects of a further increase in well number. The spectral response and IV characteristics of a 65 well quantum well solar cell and an otherwise identical 50 well cell are presented and compared. In addition, the ideality n = 1 dark currents obtained from these samples at concentrator current levels are studied. The predicted and measured …
Nasi, L.; Ferrari, C.; Lanzi, A.; Lazzarini, L.; Balboni, R.; Clarke, G.; Mazzer, M.; Rohr, C.; Abbott, P.; Barnham, K. W. J.
Journal of Crystal Growth, Vol. 274, (Issue No. 1-2)
Different strain-balanced InGaAs/InGaAs multi-quantum wells (MQWs) were grown on (0 0 1) Inp to be used as active layers of thermophotovoltaic devices. Transmission electron microscopy (TEM) and high-resolution X-My diffraction (HRXRD) were performed to correlate the evolution of the layer interfaces from planar to wavy and the consequent nucleation of extended defects with the well and barrier compositions and thicknesses and the growth temperature.
The existence of a critical elastic energy density for the wavy growth onset has been experimentally confirmed by changing both the well and barrier misfit and the multi-quantum well layer thickness. A decrease of the gowth temperature shifts the critical energy to higher values. An empirical model to predict the maximum number of layers that can be grown …
Torsello, G.; Lomascolo, M.; Licciulli, A.; Diso, D.; Tundo, S.; Mazzer, M.
Nature Materials, Vol. 3, (Issue No. 9)
Rare-earth oxide materials emit thermal radiation in a narrow spectral region, and can be used for a variety of different high-temperature applications, such as the generation of electricity by thermophotovoltaic conversion of thermal radiation. However, because a detailed understanding of the mechanism of selective emission from rare-earth atoms has so far been missing, attempts to engineer selective emitters have relied mainly on empirical approaches. In this work, we present a new quantum thermodynamic model to describe the mechanisms of thermal pumping and radiative de-excitation in rare-earth oxide materials. By evaluating the effects of the local crystal-field symmetry around a rare-earth ion, this model clearly explains how and why only some of the room-temperature absorption peaks give rise to highly efficient emission bands at high temperature …
Chatten, A. J.; Barnham, K. W. J.; Buxton, B. F.; Ekins-Daukes, N. J.; Malik, M. A.
Semiconductors, Vol. 38, (Issue No. 8)
The luminescent properties of core-shell quantum dots are being exploited in an unconventional solar concentrator, which promises to reduce the cost of photovoltaic electricity. Luminescent solar collectors have advantages over geometric concentrators in that tracking is unnecessary and both direct and diffuse radiation can be collected. However, development has been limited by the performance of luminescent dyes. We present experimental and theoretical results with a novel concentrator in which the dyes are replaced by quantum dots. We have developed a self-consistent thermodynamic model for planar concentrators and find that this three-dimensional flux model shows excellent agreement with experiment
Peng, R. W.; Mazzer, M.; Barnham, K. W. J.
Applied Physics Letters, Vol. 83, (Issue No. 4)
We present an efficiency analysis of ideal photovoltaic solar cells based on multi-intermediate band structures. It is shown that the difference between the thermodynamic limit of photovoltaic conversion and the limit of efficiency of traditional bulk semiconductor solar cells can be gradually bridged if an optimum energy band structure is achieved. Efficiency enhancement originates from photonic excitations among multiple energy bands. Several possible ways to design the optimum energy band structures are proposed.
Ekins-Daukes, N. J.; Ballard, I.; Calder, C. D. J.; Barnham, K. W. J.; Hill, G.; Roberts, J. S.
Applied Physics Letters, Vol. 82, (Issue No. 12)
Anti-Stokes emission is observed experimentally from a series of quantum well p-i-n structures and is attributed to a quasithermal equilibrium being established between the quantum well and barrier material. A device is described whereby the anti-Stokes emission from a quantum well p-i-n structure can be used to increase the short-circuit current of a second conventional solar cell, essentially coupling sub-band-gap sunlight to the ambient thermal reservoir. For a GaAs p/n cell at 300 K, this effect is calculated to raise the one-sun power conversion efficiency from 30.0% to 31.3%. Greater efficiency increases are possible if a thermal gradient is established across the structure.
Chatten, A. J.; Barnham, K. W. J.; Buxton, B. F.; Ekins-Daukes, N. J.; Malik, M. A.
Solar Energy Materials and Solar Cells, Vol. 75, (Issue No. 3-4)
Luminescent collectors have advantages over geometric concentrators in that tracking is unnecessary and both direct and diffuse radiation can be collected. However, development has been limited by the performance of luminescent dyes. We have recently proposed a novel concentrator in which the dyes are replaced by quantum dots (QDs). Advantages over dyes include that the absorption threshold can be tuned by choice of dot diameter, and that the red shift between absorption and luminescence is related to the spread of dot sizes. In this paper we discuss how we have developed a self-consistent thermodynamic model for planar concentrators which allows for re-absorption by the QDs.
Bushnell, D. B.; Ekins-Daukes, N. J.; Barnham, K. W. J.; Connolly, J. P.; Roberts, J. S.; Hill, G.; Airey, R.; Mazzer, M.
Solar Energy Materials and Solar Cells, Vol. 75, (Issue No. 1-2)
GaInP/GaAs tandem cells are limited by the current generated in the bottom GaAs junction. Strain-balanced multi-quantum well (MQW) solar cells offer a way of achieving a lower band gap for the lower junction, whilst retaining the lattice parameter of GaAs, and avoiding non-radiative recombination through dislocations. Further, the addition of a distributed Bragg reflector (DBR) allows the possibility of light not absorbed by the wells being reflected back into the structure, whilst allowing sub-well band-gap light through to a third Ge junction. Experimental results are presented from MQW cells grown with and without DBRs. These show a higher internal quantum efficiency in the 880 nm - 1 mum region without detriment to the bulk response, when compared to MQW cells without DBRs.
Barnham, K. W. J.; Ballard, I.; Connolly, J. P.; Ekins-Daukes, N. J.; Kluftinger, B. G.; Nelson, J.; Rohr, C.
Physica E-Low-Dimensional Systems & Nanostructures, Vol. 14, (Issue No. 1-2)
This paper reviews the experimental and theoretical studies of quantum well solar cells with an aim of providing the background to the more detailed papers on this subject in these proceedings. It discusses the way quantum wells enhance efficiency in real, lattice matched material systems and fundamental studies of radiative recombination relevant to the question of whether such enhancements are possible in ideal cells. A number of theoretical models for quantum well solar cells (QWSCs) are briefly reviewed and more detail is given of our own group's model of the dark-currents. The temperature and field dependence of QWSCs are all briefly reviewed.
Ekins-Daukes, N. J.; Bushnell, D. B.; Connolly, J. P.; Barnham, K. W. J.; Mazzer, M.; Roberts, J. S.; Hill, G.; Airey, R.
Physica E-Low-Dimensional Systems & Nanostructures, Vol. 14, (Issue No. 1-2)
The exceptional power conversion efficiency of monolithic multi-junction solar cells requires careful current matching in the individual junctions. It is shown that the bulk GaInP/GaAs tandem configuration is not optimal for solar conversion. A means for achieving optimum tandem power conversion is demonstrated through the incorporation of thin strain-balanced layers, resulting in a multi-quantum-well (MQW) structure. Experimental results are presented and an efficiency of 27% AM0 projected for a GaInP/MQW tandem cell.
Rohr, C.; Connolly, J. P.; Ekins-Daukes, N.; Abbott, P.; Ballard, I.; Barnham, K. W. J.; Mazzer, M.; Button, C.
Physica E-Low-Dimensional Systems & Nanostructures, Vol. 14, (Issue No. 1-2)
Strain-compensated layers in photovoltaic devices can yield unique advantages as the absorption threshold can be extended towards longer wavelengths beyond that of the lattice-matched material, which is particularly important for thermophotovoltaic (TPV) applications. In such a nanostructure, where InGaAs barriers and InGaAs quantum wells of appropriate compositions are strain compensated on an InP substrate, the absorption of a quantum well cell (QWC) can be extended to similar to2 mum. Due to the higher band-gap barriers, the dark current remains at a low level more appropriate to lattice-matched InGaAs. Great care has to be taken in design and growth to achieve a situation that is close to strain balance with zero stress. Results are presented on a strain-compensated QWC that absorbs out to 1.77 mum. Predictions …
Salviati, G.; Ferrari, C.; Lazzarini, L.; Nasi, L.; Drigo, A. V.; Berti, M.; De Salvador, D.; Natali, M.; Mazzer, M.
Applied Surface Science, Vol. 188, (Issue No. 1-2)
A systematic study of strain relaxation mechanisms by TEM XRD, RBS-channelling, SEM-CL and AFM m InxGa1=xAs/InP heterostructures grown by MOCVD under tensile and compressive initial misfit is reported. It is found that the layers under compression (0.61 < x < 0.74) relax nearly symmetrically along the two <110> directions. The residual strain vs. the layer thickness follows the same law experimentally determined for compressive MBE-grown InxGa1-xAs/GaAs (x < 0.2) specimens showing no composition, growth technique or temperature effect.
The tensile layers (0.2 < x <0.36) relax asymmetrically with a critical thickness larger than for the compressive case, The relaxation is faster along the [110]-direction with the asymmetry that increases by increasing the tensile misfit, Cracks, formed after the growth, are found to …
Ekins-Daukes, N. J.; Barnes, J. M.; Barnham, K. W. J.; Connolly, J. P.; Mazzer, M.; Clark, J. C.; Grey, R.; Hill, G.; Pate, M. A.; Roberts, J. S.
Solar Energy Materials and Solar Cells, Vol. 68, (Issue No. 1)
The state of GaAs/InGaAs quantum well solar cell research is reviewed. The effect of strain upon the GaAs/InGaAs cells is discussed and the limits to a strained GaAs/InGaAs cell established. The strain-balance approach is suggested as a means of overcoming the limits inherent to the strained approach and the principle is demonstrated in two differing device configurations. The strain-balance devices show enhanced efficiencies over their strained counterparts and in one case, comparable efficiency to a good GaAs control cell. The application of these cells to tandem structures is discussed, indicating the potential for a substantial efficiency enhancement.
Kluftinger, B.; Barnham, K.; Nelson, J.; Foxon, T.; Cheng, T.
Solar Energy Materials and Solar Cells, Vol. 66, (Issue No. 1-4)
lQuantum well solar cells (QWSC) have been proposed as a way to achieve a higher efficiency than conventional homogeneous cells. A key quantity for understanding the potential of the QWSC is the spatial variation of the quasi-Fermi level separation (Delta phi (F)) which is directly linked to the radiative losses. For the first time we have successfully measured Delta phi (F) in asymmetric double QW (DQW) p-i-n diodes in the AlGaAs/GaAs system at temperatures between 200 and 295 K. We have made several significant observations: (1) radiative losses through the QW decrease with increasing temperature and are clearly below the detailed balance-based predictions at room temperature. (2) Delta phi (F) is not the same in both wells which contradicts the assumption made by some researchers. …
Barnham, K. W. J.; Nelson, J.; Stevens, R. A.
Nature, Vol. 407, (Issue No. 6806)
Did civil reactors supply plutonium for weapons?
Barnham, K.; Marques, J. L.; Hassard, J.; O'Brien, P.
Applied Physics Letters, Vol. 76, (Issue No. 9)
The use of quantum dots can turn the old concept of a luminescent solar collector into a practical concentrator. The quantum efficiency, tunability of absorption threshold, and size of the redshift make quantum dots an ideal replacement for the organic dyes whose performance limited this inexpensive technology. Progress in photovoltaic cells, in particular, the ability of quantum-well cells to tune the band gap, also suggests high efficiency is possible in solar and thermophotovoltaic applications. A thermodynamic model is used to show quantitatively how the separation of absorption and luminescent peaks under global illumination is related to the spread of quantum-dot sizes. Hence, the redshift can be determined during the growth process. The model can be used to optimize concentrator performance and to study the effect …
Mazzer, M.; Grunbaum, E.; Barnham, K. W. J.; Barnes, J.; Griffin, P. R.; Holt, D. B.; Hutchison, J. L.; Norman, A. G.; David, J. P. R.; Roberts, J. S.; Grey, R.
Materials Science and Engineering B-Solid State Materials for Advanced Technology, Vol. 42, (Issue No. 1-3)
The location, density and nature of misfit dislocations (MDs) in lattice-strained multi-quantum well (MQW) structures were investigated by depth-resolved electron-beam-induced current (EBIC) and cathodoluminescence (CL) modes in a scanning electron microscope. A planar network of dark recombination lines due to MDs was observed at the lower and upper interfaces of the MQW stack. Their density was correlated with the MQW average strain before relaxation, giving information on the equilibrium and catastrophic strain relaxation processes which take place at the two MQW stack interfaces. High-resolution transmission electron microscopy (HRTEM) showed the location and nature of the MDs at an atomic level; they are mostly close to the lower MQW stack interface, on a {111} plane constituting glissile-60 degrees dislocations, composed of two partials including a stacking …
Barnham, K. W. J.; Braun, B.; Nelson, J.; Paxman, M.; Button, C.; Roberts, J. S.; Foxon, C. T.
Applied Physics Letters, Vol. 59, (Issue No. 1)
We have studied the forward bias behavior of AlGaAs/GaAs p-i-n multiquantum well (MQW) photodiodes. In samples with low background impurity levels in the intrinsic region the high quantum efficiency observed in reverse bias is maintained into forward bias even for carriers photoexcited in the wells. We compare our MQW devices with structures which are identical apart from having AlGaAs intrinsic regions without quantum wells. The short-circuit currents in the MQW structures are much higher than in the control samples though the open-circuit voltages are somewhat smaller. In one case the energy conversion efficiency of the MQW device in white light is 110% higher than the control. We discuss the implications of our results for the development of low-dimensional structure solar cells.
Barnham, K. W. J.; Duggan, G.
Journal of Applied Physics, Vol. 67, (Issue No. 7)
A New Approach to High-Efficiency Multi-Band-Gap Solar-Cells