III-V Dilute Nitride multi-quantum well structures are currently promising candidates to achieve 1 sun efficiencies of >40% with multi-junction design (InGaP/ GaAs/ GaAsN/ Ge). In other works, we have discussed the design having III-V Dilute Nitride GaAsN multi-quantum well (MQW) structures with resonant tunneling setup in the intrinsic region, in order to improve the response potentially yielding 1 sun efficiencies greater than 40%. Earlier efforts in this direction had yielded samples with considerable incorporation of N at the QW/barrier interface, leading to the formation of nitridation and reducing the overall quantum efficiency. In this work we discuss the results of the growth of MQW solar cells in MBE, with a modified run-vent system for the RF N-plasma setup aimed at increasing the sharpness of the well-barrier transition, and the change in quality of the quantum wells grown.
Reduction of defects by use of thick sophisticated graded metamorphic buffers in inverted metamorphic solar cells has been a requirement to obtain high efficiency devices. With increase in number of metamorphic junctions to obtain higher efficiencies, these graded buffers constitute a significant part of growth time and cost for manufacturer of the solar cells. It's been shown that ultrathin 3 and 4 junction IMM devices perform better in presence of dislocations or/and radiation harsh environment compared to conventional thick IMM devices. Thickness optimization of the device would result in better defect and radiation tolerant behavior of 0.7ev and 1.0ev InGaAs sub-cells which would in turn require thinner buffers with higher efficiencies, hence reducing the total device thickness. It is also shown that for 3 and 4 junc. IMM, with an equivalent 1015 cm-2 1 MeV electron fluence radiation, very high EOL efficiencies can be afforded with substantially higher dislocation densities (<2×107 cm-2) than those commonly perceived as acceptable for IMM devices with remaining power factor as high as 0.85. The irregular radiation degradation behavior in 4-junc IMM is also explained by back photon reflection from gold contacts and reduced by using thickness optimization of 0.7ev and 1.0ev InGaAs sub-cells.
Using drift-diffusion model and considering experimental III-V material parameters, AM0 efficiencies of lattice-matched
multijunction solar cells have been calculated and the effects of dislocations and radiation damage have been analyzed.
Ultrathin multi-junction devices perform better in presence of dislocations or/and radiation harsh environment compared
to conventional thick multijunction devices. Our results show that device design optimization of Ga0.51In0.49P/GaAs multijunction devices leads to an improvement in EOL efficiency from 4.8%, for the conventional thick device design, to
12.7%, for the EOL optimized thin devices. In addition, an optimized defect free lattice matched Ga0.51In0.49P/GaAs solar
cell under 1016cm-2 1Mev equivalent electron fluence is shown to give an EOL efficiency of 12.7%; while a
Ga0.51In0.49P/GaAs solar cell with 108 cm-2 dislocation density under 1016cm-2 electron fluence gives an EOL efficiency of
12.3%. The results suggest that by optimizing the device design, we can obtain nearly the same EOL efficiencies for high
dislocation metamorphic solar cells and defect filtered metamorphic multijunction solar cells. The findings relax the need
for thick or graded buffer used for defect filtering in metamorphic devices. It is found that device design optimization
allows highly dislocated devices to be nearly as efficient as defect free devices for space applications.
In our simulation of reflection losses for 1D and 2D subwavelength dielectric grating, surface texturing was done
while comparing reflection losses with various incident angles for photovoltaic materials like Si and III-Vs GaAs.
Transfer matrix formalism is modeled by treating each grating's effective refractive index as being composed of
several layers of varying refractive indexes. Discrete parameterization on intervals with different profiles such as 1D
rectangles and triangle, as well as 2D pyramids and hemispheres are used to minimize power reflected for black
body radiation. This simulation treats each layer to be uniform, which requires the texturing to be in the
subwavelength region. We compared the reflection loss and incident angle dependence for dielectric layers, dielectric
gratings, and the combination of both dielectric layers and gratings, and found that with gratings, reflection losses
are less dependent on incident angle. By optimizing the texturing and design parameters, we can obtain reflection
losses around 1% for spectral range of solar cell with a very small increase in incidence angle.
Crystalline defects (e.g. dislocations or grain boundaries) as well as electron and proton induced defects cause reduction
of minority carrier diffusion length which in turn results in degradation of efficiency of solar cells. Hetro-epitaxial or
metamorphic III-V devices with low dislocation density have high BOL efficiencies but electron-proton radiation causes
degradation in EOL efficiencies. By optimizing the device design (emitter-base thickness, doping) we can obtain highly
dislocated metamorphic devices that are radiation resistant. Here we have modeled III-V single and multi junction solar
cells using drift and diffusion equations considering experimental III-V material parameters, dislocation density, 1 Mev
equivalent electron radiation doses, thicknesses and doping concentration. Thinner device thickness leads to increment in
EOL efficiency of high dislocation density solar cells. By optimizing device design we can obtain nearly same EOL
efficiencies from high dislocation solar cells than from defect free III-V multijunction solar cells. As example defect free
GaAs solar cell after optimization gives 11.2% EOL efficiency (under typical 5x1015cm-2 1 MeV electron fluence) while
a GaAs solar cell with high dislocation density (108 cm-2) after optimization gives 10.6% EOL efficiency. The approach
provides an additional degree of freedom in the design of high efficiency space cells and could in turn be used to relax
the need for thick defect filtering buffer in metamorphic devices.