Carsten Rohr

Quantum well solar cells

Abstract 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 groups model of the dark-currents. The temperature and field dependence of QWSCs are all briefly reviewed.

EFFECT OF STRAIN RELAXATION ON FORWARD BIAS DARK CURRENTS IN GAAS/INGAAS MULTIQUANTUM WELL P-I-N DIODES

The effect of the dislocation line density produced by the relaxation of strain in GaAs/InxGa1−xAs multiquantum wells where x=0.155–0.23 has been studied. There is a strong correlation between the dark line density, observed by cathodoluminescence, before processing of the wafers into photodiode devices, and the subsequent low forward bias (<1.5 V) dark current densities of the devices. A comparison is made of the correlation between the reverse bias current density and dark line density and it is found that, in this range of strain, the forward bias current density varies more. Two growth methods, molecular beam epitaxy and metal organic vapor phase epitaxy, have been used to produce the wafers and no difference between the growth methods has been found in dark line or current density variations with strain.

Recent results on quantum well solar cells

The Quantum Photovoltaic group at Imperial College has pioneered the use of quantum wells in photovoltaic applications, using material supplied by our collaborators in the EPSRC III-V Facility, University of Sheffield, University of Nottingham and the Center for Electronic Materials and Devices. We have shown, in a number of lattice-matched multi-quantum well systems, that quantum wells enhance the output current and efficiency compared to homogeneous cells made from the barrier material and output voltage compared with conventional cells made from the material in the well. This paper discusses recent results in three areas of our work: (1) fundamental studies relevant to the question of whether similar efficiency enhancements will be possible in an ideal cell where radiative recombination dominates, (2) the materials problems we have had to solve in order to enhance the highest efficiency GaAs solar cells with InGaAs wells and (3) the application of quantum well cells in the area of thermophotovoltaics.

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-...

InGaAs=InGaAs strain-compensated quantum well cells for thermophotovoltaic applications

Abstract 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 ∼2 μm . 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 μm . Predictions show that strain-compensated InGaAs/InGaAs QWCs have superior performance when compared with bulk InGaAs on InP monolithic interconnected modules and GaSb TPV cells.

Electron-beam-induced current and cathodoluminescence characterization of InGaAs strain-balanced multiquantum well photovoltaic cells

InxGa1−xAs/InyGa1−yAs strain-balanced quantum well cells (QWCs) have been shown to be beneficial for photovoltaic applications in particular to extend the light absorption edge of a single-junction cell toward the near infrared with a lower reduction of the open-circuit voltage compared to a single band-gap cell. The strain-balancing condition ensures that the multi-quantum well as a whole does not relax. However, if the mismatch between wells and barriers exceeds a critical limit, the structure becomes vulnerable to morphological or compositional fluctuations, which can lead to a local structural breakdown with the generation of extended defects of a completely different nature from misfit dislocations. In this work, we investigated a series of strain-balanced InGaAs QWCs grown on InP for thermophotovoltaic applications by means of electron-beam-induced current (EBIC) and cathodoluminescence (CL) measurements. Despite being electrically active, these defects appear to have a minor impact on the dark curr...

A comparative study of bulk InGaAs and InGaAs/InGaAs strain-compensated quantum well cells for thermophotovoltaic applications

One of the main requirements for thermophotovoltaic (TPV) systems powered by fuel combustion is a low level of pollution. To achieve this, low combustion temperatures are needed. The most efficient narrow band emitters emit at long wavelengths, necessitating low band gap cells. Erbium oxide emits around 1500 nm and we report an InGaAs p-n cell which is well matched to this spectrum. Two more suitable emitters are thulium oxide and holmium oxide, which emit around 1700nm and 1950nm respectively, beyond the band gap of lattice matched InGaAs. To absorb this emission, lattice mismatched materials must be used. The technique of strain compensation can prevent the creation of dislocations within the structure. We present results of a strain-compensated InGaAs/InGaAs Quantum Well Cell (QWC) which demonstrates the success of this structure in allowing wavelength response to be extended whilst displaying a lower dark current.

Extended defects in InGaAs/InGaAs strain-balanced multiple quantum wells for photovoltaic applications

Different strain-balanced InGaAs/InGaAs multiple quantum wells (MQWs) were grown on (001) InP changing the In composition in the wells/barriers in order to extend the absorption edge beyond 2 μm for thermophotovoltaic applications. The strain increase in the structures results in the formation of isolated highly defected regions taking their origin from lateral layer thickness modulations. Experimental results are consistent with the existence of a critical elastic energy density for the development of MQW waviness. An empirical model for predicting the maximum number of layers that can be grown without modulations as a function of the strain energy stored in the MQW period is presented.

Characterisation Of Strain‐Compensated InGaAs/InGaAs Quantum Well Cells For TPV Applications

Thermophotovoltaic (TPV) generators can reduce pollution by lowering their operating temperature, but the choice of semiconductor materials for this purpose is limited. We present results on an InGaAs p‐n cell lattice‐matched to InP which is optimised for the Erbia emission spectrum peak at a wavelength of 1.5μm. However, for lower temperature TPV applications at longer wavelengths one is constrained by the lack of lattice‐matched materials. In order to extend the absorption towards 1900 nm for a selective emitter based on Thulium strain‐compensated InGaAs/InGaAs quantum well cells (QWCs) on InP have been designed and characterised. We present data showing that strain‐compensated QWCs extend the spectral response (SR) to longer wavelengths and can show a lower dark current density than the bulk InGaAs p‐n cell despite the QWC having a lower band‐gap. We have developed a model for the SR of strained multi‐quantum well (MQW) systems in InGaAsP, including quantum effects as well as strain‐induced changes. SR...

Strain-balanced In/sub 0.62/Ga/sub 0.38/As/In/sub 0.47/Ga/sub 0.53/As(InP) quantum well cell for thermophotovoltaics

For thermophotovoltaic (TPV) applications, there is considerable interest at present in extending the absorption to longer wavelengths for higher overall system efficiencies with lower temperature sources. With strain-balanced In/sub 1-x/Ga/sub x/As/In/sub 1-y/Ga/sub y/As (InP) quantum well cells (QWCs) the absorption can be extended, while retaining a low dark current. We present a strain-balanced In/sub 0.62/Ga/sub 0.38/As/In/sub 0.47/Ga/sub 0.53/As QWC, which extends the absorption edge beyond that of lattice-matched bulk InGaAs to about 1.8 /spl mu/m, which is similar to that of GaSb, while the dark current remains at a lower level. We can model the spectral response of InP-based-including strain-balanced-QWCs. Efficiencies for solar (AM1.5G), black-body spectra of 1500-3200 K and selective emitters are presented. Lattice-matched InGaAsP and strain-balanced InGaAs (InP) QWCs show superior performance when compared with bulk InGaAs monolithic interconnected modules and bulk GaSb TPV cells.