Mitchell F. Bennett

Effect of quantum dot position and background doping on the performance of quantum dot enhanced GaAs solar cells

The effect of the position of InAs quantum dots (QD) within the intrinsic region of pin-GaAs solar cells is reported. Simulations suggest placing the QDs in regions of reduced recombination enables a recovery of open-circuit voltage (VOC). Devices with the QDs placed in the center and near the doped regions of a pin-GaAs solar cell were experimentally investigated. While the VOC of the emitter-shifted device was degraded, the center and base-shifted devices exhibited VOC comparable to the baseline structure. This asymmetry is attributed to background doping which modifies the recombination profile and must be considered when optimizing QD placement.

Intrinsic radiation tolerance of ultra-thin GaAs solar cells

Radiation tolerance is a critical performance criterion of photovoltaic devices for space power applications. In this paper we demonstrate the intrinsic radiation tolerance of an ultra-thin solar cell geometry. Device characteristics of GaAs solar cells with absorber layer thicknesses 80 nm and 800 nm were compared before and after 3 MeV proton irradiation. Both cells showed a similar degradation in Voc with increasing fluence; however, the 80 nm cell showed no degradation in Isc for fluences up to 1014 p+ cm−2. For the same exposure, the Isc of the 800 nm cell had severely degraded leaving a remaining factor of 0.26.

Epitaxial lift-off of quantum dot enhanced GaAs single junction solar cells

InAs/GaAs strain-balanced quantum dot (QD) n-i-p solar cells were fabricated by epitaxial lift-off (ELO), creating thin and flexible devices that exhibit an enhanced sub-GaAs bandgap current collection extending into the near infrared. Materials and optical analysis indicates that QD quality after ELO processing is preserved, which is supported by transmission electron microscopy images of the QD superlattice post-ELO. Spectral responsivity measurements depict a broadband resonant cavity enhancement past the GaAs bandedge, which is due to the thinning of the device. Integrated external quantum efficiency shows a QD contribution to the short circuit current density of 0.23 mA/cm2.

Rapid thermal annealing of InAlAsSb lattice-matched to InP for top cell applications

The effect of rapid thermal annealing on the optical properties of In_xAl_1-xAs _ySb _1-y was analyzed and compared to that for In_0.52 Al_0.48As. Initial ellipsometry and photoluminescence experiments performed before the annealing indicate the presence of a low energy Urbach tail in the absorption spectrum. Rapid thermal annealing produces a blue-shift in the PL emission when annealed at 650°C for 60s and a decrease in the full-width-half-maximum, which originates from a reduction of the emission from the longer wavelength states. For the In_0.52 Al_0.48As, the emission energy and the full-width-half-maximum remain constant during the annealing study. The elimination of sub-bandgap states in In_0.52 Al_0.48As is critical for achieving a realistic path towards high efficiency multijunction cells lattice-matched to InP.

Analysis of gaas photovoltaic device losses at high MOCVD growth rates

Gallium arsenide material has been deposited via metal organic chemical vapor deposition (MOCVD) at growth rates varying between 14 μm/hr and 56 μm/hr. Photovoltaic device results indicate a 6-7% relative decrease in efficiency between 14 and 56 μm/hr GaAs solar cells, due to a reduction in short-circuit current and open-circuit voltage. By simulating the experimental characterization data, it is established that performance losses are associated with rear surface recombination velocity and Shockley-Read-Hall lifetime. The relative impact of these loss mechanisms will be quantified and conclude with discussions on their mitigation.

Investigation of the design parameters of quantum dot enhanced III-V solar cells

The incorporation of nanostructures, such as quantum dots (QD), into the intrinsic region of III-V solar cells has been proposed as a potential route towards boosting conversion efficiencies with immediate applications in concentrator photovoltaic and space power systems. Necessary to the optimization process of this particular class of solar cells is the ability to correlate nanoscale properties with macroscopic device characteristics. To this purpose, the physics-based software Crosslight APSYS has been developed to investigate the design parameters of QD enhanced solar cells with particular focus on the InAs/GaAs system. This methodology is used to study how nanoscale variables, including size, shape and material compositions, influence photovoltaic performance. In addition, device-level engineering of the nanostructures is explored in optimizing the overall device response. Specifically, the effect of the position of the QDs within the intrinsic regions is investigated. Preliminary simulations suggest strategically placing the QDs off-center reduces non-radiative recombination and thereby the dark saturation current, contributing to a marked increase in opencircuit voltage and fill factor. The short-circuit current remains unchanged in the high field region resulting in an increase in overall conversion efficiency. To further explore this finding, a series of three samples with the QDs placed in the center and near the doped regions of a pin-GaAs solar cell have been grown using MOCVD, fabricated and fully characterized. Contrary to predictions, the emitter-shifted devices exhibit a marked decrease in open-circuit voltage and fill factor. This behavior is attributed to non-negligible n-type background doping in the intrinsic region which shifts the region of maximum recombination towards the p-type emitter.

Development of recessed contacts for mechanical stacking of GaSb solar cells

Transfer printing is a formidable technique to stack multijunction solar cells while avoiding existing epitaxial limitations. Thus, a larger portion of the solar spectrum can be harvested through careful design optimization. In this process, metal contacts are embedded within a highly conductive lateral conduction layer (LCL) on the lower subcells. A readily available low bandgap candidate for a multijunction stack includes a GaSb solar cell with a 1.0 eV Al0.2Ga0.8Sb LCL. Processing of a bare AlGaSb layer is difficult, as standard photolithography processes and wet etch chemistries leave the surface excessively roughened and inhibit good ohmic contact. Several dry etch recipes are investigated to etch the AlGaSb LCL while leaving a smooth surface. A recipe based on a gas mixture of BCl3/Ar was determined to be a viable candidate for dry etching of AlGaSb and used to fabricate a GaSb solar cell with recessed contacts in an AlGaSb LCL.

Effect of carrier confinement and doping in quantum well tunnel junctions

Quantum well tunnel junctions have attractive properties for the design of multi-junction solar cells and represent a good alternative to homo-junction tunnel diodes. Tunnel junctions based on InAlAs quantum wells were grown strain balanced on InP with varying well and barrier thicknesses. We characterized their current voltage characteristics and found that devices with thicker well and thinner tunnel barrier performed best provided the doping profile across the structure is accurately controlled. We explain the effects of barrier and well thicknesses due to quantum confinement as well as doping using a Poisson-Schrodinger model coupled to a non-local tunneling model that were implemented in a numerical drift-diffusion solver.

Evaluation of strained InAlAs as a window layer for wide bandgap materials lattice matched to InP

In order to be considered a viable option as a window layer, a number of requirements must be met. These include high transparency, small valence band offset, high conduction band offset, low surface recombination velocity, ability to be passivated in air, and availability of a selective etchant with the contact layer. In this paper, we demonstrate that strained In0.3Al0.7As meets all of these requirements for widebandgap, lattice-matched InAlAs grown on InP substrates. We expect the results to be applicable to the InAlAsSb system which could enable triple junction photovoltaic devices with record efficiencies.

Alpha radiation effects on n-i-p quantum dot epitaxial lift-off solar cells

Embedded nanostructures such as quantum dots (QDs) have been studied for many applications including enhanced mini-band absorption in intermediate-band solar cells and current matching in multi junction cells. Furthermore, solar cells with QDs have shown a radiation hardness and temperature tolerance that has been improved by adding nanostructures. InAs/GaAs QD cells were grown by MOVPE, fabricated and processed by epitaxial lift off, creating thin and flexible devices that exhibit enhanced sub-GaAs bandgap current collection. Due to the thinning of these devices, the sub-GaAs bandgap eternal quantum efficiency curves are more pronounced than those for a thicker cell, indicating the presence of a cavity mode effect. Champion devices incorporating QDs have short circuit currents exceeding those of baseline samples with no QDs by an absolute value of 0.12 mA/cm2 under 1-sun AM0 illumination. In addition to optical, materials, and electrical characterization, devices were exposed to alpha radiation to gauge the effects of a harmful environment on cell performance. In this area QD cells also outperformed baseline devices, with a relative end of life remaining maximum power factor increase of 10%.