Kenneth J. Schmieder

Effect of Growth Temperature on GaAs Solar Cells at High MOCVD Growth Rates

Increasing epitaxial growth rate is an important path toward III-V solar cell cost reductions; however, photovoltaic device performance has been shown to degrade with increasing growth rate. In this study, gallium arsenide (GaAs) material has been deposited via metal-organic chemical vapor deposition (MOCVD) at growth rates varying between 14 and 60 μm/h. Deep-level transient spectroscopy is utilized to elucidate an exponential rise in EL2 trap density as a function of growth rate when all other growth conditions are held constant. Evidence is provided that this EL2 defect is responsible for limiting the Shockley-Read-Hall (SRH) lifetime of very high growth rate solar cells. The effect of growth temperature on devices at high growth rate is subsequently investigated as a strategy to reduce trap density and improve solar cell performance. From this investigation, EL2 trap density is suppressed, and single-junction on-substrate GaAs solar cells grown at 60 μm/h are reported with 1.01 V 1-sun open-circuit voltage and 23.8% AM1.5G efficiency.

Modeling, design and experimental results for high efficiency multi-junction solar cells lattice matched to InP

The high conversion efficiencies demonstrated by multi-junction solar cells over the past three decades have made them indispensable for use in space and are very attractive for terrestrial concentrator applications. The multi-junction technology consistently displays efficiency values in excess of 30%, with record highs of 37.8% under 1 sun conditions and over 44% under concentration. However, as material quality in current III-V multi-junction technology reaches practical limits, more sophisticated structures will be required to further improve on these efficiency values. In a collaborative effort amongst several institutions we have developed a novel multi-junction solar cell design that has the potential to reach the 50% conversion efficiency value. Our design consists of a three junction cell grown on InP substrates which achieves the optimal bandgaps for solar energy conversion using lattice matched materials. In this work, we present the progress in the different subcells comprising this multi-junction structure. For the top cell, InAlAsSb quaternary material is studied. For the middle, InGaAlAs and InGaAsP materials and devices are considered and for the bottom, a multi-quantum well structure lattice matched to InP for fine bandgap tunability for placement in an InGaAs cell is demonstrated.

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.

Analysis of GaAs solar cells at High MOCVD growth rates

Single junction GaAs solar cells grown by MOCVD are fabricated over a range of growth rates targeting up to 56 μm/hr in order to evaluate the effect on photovoltaic device performance. MOCVD recipe conditions are provided. Dopant incorporation efficiency is found to increase at high growth rates, potentially due to reduced Zn desorption as the time required to deposit a monolayer of GaAs is reduced. Device results are characterized by light and dark-IV as well as external quantum efficiency and verified against bulk minority carrier lifetime data from time-resolved photoluminescence. High growth rate solar cells degrade less than 4% relative to baseline devices with Voc and Jsc losses of 1% and 3%, respectively. The comparison suggests that both bulk Shockley Read Hall (SRH) lifetime and surface recombination velocity (SRV) are affected by growth rate and contribute to a reduction in performance.

Simulated Potential for Enhanced Performance of Mechanically Stacked Hybrid III-V/Si Tandem Photovoltaic Modules Using DC-DC Converters

Abstract. This work examines a tandem module design with GaInP2 mechanically stacked on top of crystalline Si, using a detailed photovoltaic (PV) system model to simulate four-terminal (4T) unconstrained and two-terminal voltage-matched (2T VM) parallel architectures. Module-level power electronics is proposed for the 2T VM module design to enhance its performance over the breadth of temperatures experienced by a typical PV installation. Annual, hourly simulations of various scenarios indicate that this design can reduce annual energy losses to ∼0.5% relative to the 4T module configuration. Consideration is given to both performance and practical design for building or ground mount installations, emphasizing compatibility with existing standard Si modules.

Defect characterization of proton irradiated GaAs pn-junction diodes with layers of InAs quantum dots

In order to expand the technology of III-V semiconductor devices with quantum structures to both terrestrial and space use, radiation induced defects as well as native defects generated in the quantum structures should be clarified. Electrically active defects in GaAs p+n diodes with embedded ten layers of InAs quantum dots (QDs) are investigated using Deep Level Transient Fourier Spectroscopy. Both majority carrier (electron) and minority carrier (hole) traps are characterized. In the devices of this study, GaP layers are embedded in between the QD layers to offset the compressive stress introduced during growth of InAs QDs. Devices are irradiated with high energy protons for three different fluences at room temperature in order to characterize radiation induced defects. Seven majority electron traps and one minority hole trap are found after proton irradiation. It is shown that four electron traps induced by proton irradiation increase in proportion to the fluence, whereas the EL2 trap, which appears be...

Characterization, modeling and analysis of InAlAsSb Schottky barrier solar cells grown on InP

In this paper we present the first photovoltaic devices made from the promising quaternary InAlAsSb, grown lattice matched to InP by molecular beam epitaxy. Schottky barrier solar cells using semi-transparent contacts have been fabricated, characterized and simulated using a drift-diffusion model to extract information about the barrier height, minority carrier diffusion length and optical performance of devices fabricated from this material. We have compared the performance to analogous InAlAs devices, and present a wide range of optical and electrical characterization for the materials.

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.

Wide bandgap, strain-balanced quantum well tunnel junctions on InP substrates

In this work, the electrical performance of strain-balanced quantum well tunnel junctions with varying designs is presented. Strain-balanced quantum well tunnel junctions comprising compressively strained InAlAs wells and tensile-strained InAlAs barriers were grown on InP substrates using solid-source molecular beam epitaxy. The use of InAlAs enables InP-based tunnel junction devices to be produced using wide bandgap layers, enabling high electrical performance with low absorption. The impact of well and barrier thickness on the electrical performance was investigated, in addition to the impact of Si and Be doping concentration. Finally, the impact of an InGaAs quantum well at the junction interface is presented, enabling a peak tunnel current density of 47.6 A/cm2 to be realized.