A. Mehrotra

Investigation of dilute-nitride alloys of GaAsNx (0.01 < x < 0.04) grown by MBE on GaAs (001) substrates for photovoltaic solar cell devices

Dilute-nitride GaAsNx epilayers were grown on GaAs (001) substrates at temperatures of ∼450 °C using a radio-frequency plasma-assisted molecular/chemical beam exitaxy system. The concentration of nitrogen incorporated into the films was varied in the range between 0.01 and 0.04. High-resolution electron microscopy was used to determine the cross-sectional morphology of the epilayers, and Z-contrast imaging showed that the incorporated nitrogen was primarily interstitial. {110}-oriented microcracks, which resulted in strain relaxation, were observed in the sample with the highest N concentration ([N] ∼ 3.7%). Additionally, Z-contrast imaging indicated the formation of a thin, high-N quantum-well-like layer associated with initial ignition of the N-plasma. Significant N contamination of the GaAs barrier layers was observed in all samples, and could severely affect the carrier extraction and transport properties in future targeted devices. Dilute-nitride quantum-well-based photovoltaic solar cells were fabri...

Modeling and optimal designs for dislocation and radiation tolerant single and multijunction solar cells

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.

Single crystalline gallium arsenide photovoltaics on flexible metal substrates

Combining the unsurpassed performance of GaAs based multi-junction technologies with a conventional roll to roll processing standards of thin film industry could lead to paradigm-shifting reduction of the cost of solar electricity and increase of specific efficiencies. But thus far, attempts toward the direct deposition of GaAs and related compounds on metal foils have yielded to poorly performing polycrystalline films and devices Here we report on the fabrication of single crystalline GaAs-based epilayers and solar cells on thin (50 microns) flexible polycrystalline Ni-based metallic substrates. A rapid (1m/hour) roll to roll ion beam assisted deposition is used to deposit oxide based adaptation buffers on the metal substrates followed by a growth of highly textured thin Ge films and the subsequent growth of GaAs epilayers by molecular beam epitaxy. RHEED, X-ray diffraction and transmission electron microscopy analysis confirm the (001) orientation and the single crystalline nature of the GaAs films. The fabricated samples exhibit strong photoluminescence response attesting the optoelectronic quality of the fabricated films and analysis of near band edge excitons confirms minimal (or no) thermoelastic/lattice mismatch strain in GaAs epilayer.

Modeling of defect tolerance of IMM multijunction photovoltaics for space application

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. Its 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.

Modeling of defect-tolerant thin multi-junction solar cells for space application

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.

Optimized device design for radiation resistant and high dislocation solar cells for space

Electron and proton-induced point defects as well as extended crystalline defects (e.g. dislocations or grain boundaries) cause reduction of minority carrier diffusion length which in turn results in solar cell efficiency degradation. For hetero-epitaxial or metamorphic III–V devices dislocation densities have to be below 106 cm−2 to obtain highest BOL efficiencies, yet EOL efficiency optimization, which favors thinner cell designs, synergistically yield to more defect tolerant devices. In this work and within the framework of the GaAs-archetype solar cell material system we have computed the efficiencies as function of the combined effect of dislocation densities, radiation doses (1 MeV equivalent electrons) and device emitter and base design (doping-thickness) and show that by implementing an appropriate design EOL efficiencies for highly dislocated devices can be significantly improved over that of a conventionally designed state of the art defect free device.

Superlattice intermediate band solar cell with resonant upper-conduction-band assisted photo-absorption and carrier extraction

In this work we propose and theoretically evaluate a superlattice intermediate band solar cell design, wherein a superlattice comprising lattice matched layers of electronically mismatched alloys and thin barriers are inserted within the intrinsic region of a wide bandgap p-i-n diode. The shallow valence band offsets in the design favor the minority hole extraction, and the intermediate levels is build through superlattice minibands formed by coupling lower band gap wells (lower E-conduction branch of mismatched alloys like GaAsN) and higher bandgap Kane-like semiconductors. In the proposed design the upper conduction band E+ of the mismatched alloys is maintained in resonance with the barrier and bandgap of host material to promote an efficient extraction of electrons and preserve the 3D nature of the upper band, thus favoring a strong intermediate to band second photon absorption. In this design carriers can be promoted either directly to the conduction band or via the intermediate band, permitting the absorption of low energy photons whilst maintaining a high cell voltage. Also the characteristic lengths of the wells are substantially smaller than typical diffusion lengths of the electronically mismatched alloy which should overcome the minority carrier losses observed in bulk like devices fabricated with these alloys. To attain the necessary combination of high and low bandgaps and low dislocation density, we use materials that are lightly strained or lattice-matched to an GaAs (or Ge) substrate. GaAsN (Sb) quantum well layers are incorporated into direct bandgap low Al content (x<;30%)) AlGaAs host material, to attain high bandgaps of 1.7-1.9 eV, and low-energy bandgaps of 1.1-1.3 eV. A preliminary detailed balance evaluation of the proposed device that incorporates calculation of the absorption properties of the SL region and the host AlGaAs crystal suggest potential for exceeding 1sun and 1000 sun efficiencies of 39% and 55% respectively.

Minimizing solar cell reflection loss through surface texturing and implementation of 1D and 2D subwavelength dielectric gratings

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 gratings 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.

Increased radiation resistance of thin 4J-IMM solar cells by recycling transparency photon losses

In this work we have evaluated thickness dependent efficiency of 4J-IMM solar cells as a function of radiation doses and dislocations. Its been shown that bottom 0.7ev InGaAs sub-cells radiation resistance and/or dislocation tolerance can be improved by use of back gold reflection. Photon confinement results in fabrication of thinner sub-cells thus increasing the radiation hardness of the sub-cells. Its been shown that for moderate to high doses of radiation, very high EOL efficiencies can be afforded with substantially higher dislocation densities than those commonly perceived as acceptable for IMM devices i.e. even in the presence of dislocation densities in both sub-cells as large as 107 cm-2, for typical 1015 cm-2 1MeV electron fluence, a remaining power factor >85% (ηEOL~32%). These finding could in turn be used to simplify manufacturing (thinner graded buffers) or/and increase yield for IMM space cells and also explain the irregular radiation behavior seen in 4J-IMMs.

Modification of MBE for growth of dilute nitride quantum well photovoltaics

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.