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 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 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.
The effect of the dislocation line density produced by the relaxation of strain in GaAs/In x Ga 1Ϫx As multiquantum wells where xϭ0.155-0.23 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 ͑Ͻ1.5 V͒ 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.
For thermophotovoltaic (TPV) applications, there is considerable interest a t present in extending the absorption t o longer wavelengths for higher overall system efficiencies with lower temperature sources. With strainbalanced In1 --+ Ga, As/ln 1 -, Ga, As (In P) Quantum Well Cells (QWCs)the absorption can be extended, while retaining a low dark current. We present a strain-balanced In0.6~Gao.3~As/lno.47Gao.53As QWC, which extends the absorption edge beyond that of lattice-matched bulk InGaAs to about 1.8 pm, which is similar t o 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. Latticematched InGaAsP and strain-balanced InGaAs (InP) QWCs show superior performance when compared with bulk InGaAs monolithic interconnected modules and bulk GaSb TPV cells.
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