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). Previously under the assumption of complete carrier collection from wells, we have shown that III-V Dilute Nitride GaAsN multi-quantum well (MQW) structures included in the intrinsic region of the third cell in a 4 junction configuration could yield 1 sun efficiencies greater than 40%. However for a conventional deep well design the characteristic carrier escape times could exceed that of radiative recombination hence limiting the current output of the cell, as has been indicated by prior experiments. In order to increase the current extraction here we evaluate the performance of a cascaded quantum well design whereby a thermally assisted resonant tunneling process is used to accelerate the carrier escape process (<30ps lifetime) and hence improve the photo generated carrier collection efficiency. The quantum efficiency of a p-i-n subcell where a periodic sequence of quantum wells with well and barrier thicknesses adjusted for the sequential extraction operation is calculated using a 2D drift diffusion model and taking into account absorption properties of resulting MQWs. The calculation also accounts for the E-field induced modifications of absorption properties and quantization in quantum wells. The results are then accounted for to calculate efficiencies for the proposed 4 junction design, and indicate potential for reaching efficiencies in excess of this structure is above 42% (1 sun) and above 50% (500 sun) AM1.5.
KEYWORDS: Solar cells, Quantum wells, Gallium arsenide, Solar energy, Quantum efficiency, Photovoltaics, Picosecond phenomena, Nanostructures, Absorption, Electron transport
Using material systems displaying a band offset only on the conduction (GaAs/(In)GaAsN) or valence
(GaAs/GaAsSb(N)) band, we offer device designs that rely on intra-subband thermal transitions accompanied by
resonant tunneling to adjacent wells, which greatly accelerates the carrier escape process. Typically, photo-excited
carriers in the well regions need about several nanoseconds to make their way out of the well, but a proper design of
energy states in successive quantum wells can reduce this escape time to few picoseconds, leading to reduced
recombination and higher carrier collection. Using a solar cell modeling program based on the drift-diffusion framework,
we show that quantum well solar cells displaying such thermo-tunneling carrier escape process can substantially surpass
the efficiency limit of their bulk counterpart.
KEYWORDS: Solar cells, Gallium arsenide, Indium gallium phosphide, Diffusion, Solar radiation, Multijunction solar cells, Doping, Photons, Ions, Solar radiation models
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.
KEYWORDS: Solar cells, Quantum wells, Solar energy, Electrons, Nanostructuring, Gallium arsenide, Photovoltaics, Nanostructures, Group III-V semiconductors, Sun
Nanostructured solar cells are touted to lead to super high photo-conversion efficiencies. Nevertheless the inclusion of
potential energy fluctuations associated with those structures hinders the smooth vertical transport of photo-generated
carriers. We present an innovative energy level engineering design that significantly facilitates the collection of all
photo-generated carriers. Using dilute nitride III-V semiconductor quantum wells embedded in a conventional III-V
GaAs host, we demonstrate the possibility of achieving a quasi-flat valence band that will ease the smooth transport of
holes. The conduction band confinement energies are designed in a way that promotes thermo-tunneling electrons from
their potential wells to the conduction band continuum. Energy levels were calculated by including strain and spin-orbit
interaction. The calculation of confinement energies was also undertaken. Once confinement energies and potential
barrier heights were determined we complemented the theoretical evaluation by calculating carrier escape times via
thermionic and tunneling routes at 300 K. Here we demonstrate that an optimized resonant thermo-tunneling design
leads to ultra rapid escape. The suggested approach is thus expected to circumvent recombination losses and lead to a
substantial carrier collection and efficiency improvements.
The current high-efficiency triple junction (Al)InGaP (1.9eV)/GaAs(1.42eV)/ Ge(0.66eV) design for a solar cell can be
improved upon by the use dilute nitrides to include a sub-cell in the 1eV range. Addition of a small percentage of
nitrogen to III-V semiconductor alloys (such as GaAsN) enables us to achieve the required bandgap, however these bulk
dilute nitride structures suffer from a reduced minority carrier lifetime, decreasing the overall current output. The route
suggested herein is to include dilute nitride multi-quantum wells (with thicknesses much lesser than the minority carrier
diffusion length) within the intrinsic region of a GaAs subcell. Modeling has been done for this structure to obtain the
confined energies of the electrons and holes, as well as the absorption coefficient and thereby the spectral response of the
4-junction cell. The results show that it is possible to achieve with the appropriate current matching, a conversion
efficiency of ~40% under AM0 (1 sun) with up to ~18 mAcm-2 short circuit current.
KEYWORDS: Solar cells, Gallium arsenide, Diffusion, Doping, Solar radiation, Multijunction solar cells, Crystals, Solar radiation models, Radiation effects, Absorption
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.
KEYWORDS: Gallium arsenide, Solar cells, Photovoltaics, Absorption, Solar energy, Nanostructuring, Heterojunctions, Superlattices, Electron transport, Photons
Successions of dilute nitride-based III-V semiconductor staircase like superlattice structures are incorporated in the
intrinsic region of common III-V p-i-n solar cells. The choices of material system and energy band design are tuned
towards facilitating the collection of all photo-generated carriers while minimizing recombination losses. Band structure
calculations including strain effects, band anti-crossing models and transfer matrix methods are used to theoretically
demonstrate optimum conditions for enhanced vertical transport. High electron quantum tunneling escape probability,
together with a free movement of quasi-3 D holes, is predicted here to result in enhanced PV device performance.
Furthermore, the increase in electron effective mass due to the incorporation of N translates in enhanced absorptive
properties, ideal for PV application.
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