Eric B. Clark

High efficiency GaAs-on-Si solar cells with high V/sub oc/ using graded GeSi buffers

Single junction AlGaAs/GaAs and InGaP/GaAs solar cells and test structures have been grown by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD), respectively, on Si wafers coated with compositionally-graded GeSi buffers. The combination of controlled strain relaxation within the GeSi buffer and monolayer-scale control of the Ill-V layer nucleation is shown to reproducibly generate minority carrier lifetimes exceeding 10 nanoseconds within GaAs overlayers. The III-V layers are free of long-range antiphase domain disorder, with threading dislocation densities in the high-10/sup 5/ cm/sup -2/ range, consistent with the low residual dislocation density in the Ge cap of the graded buffer structure. Single junction GaAs cells grown by both MBE and MOCVD on the Ge/GeSi/Si substrates demonstrated high V/sub oc/ values for GaAs cells grown on Si. Record V/sub oc/ values for MOCVD-grown single junction InGaP/GaAs cells exceeded 980 mV (AMO) with fill factors of 0.79. Additionally, external quantum efficiency data indicates no degradation in carrier collection from GaAs homoepitaxial cells for current single-junction cell designs grown by MBE. Based on these results, cell efficiencies in excess of 18.5% under AM0 conditions should be attainable with cell designs demonstrating state of the art J/sub sc/ values. Such cell performance demonstrates the potential and viability of graded GeSi buffers for the development of Ill-V cells on Si wafers.

Impact of threading dislocations on both n/p and p/n single junction GaAs cells grown on Ge/SiGe/Si substrates

Single junction GaAs solar cells having an n/p polarity were grown on p-type Ge/SiGe/Si substrates for the first time. The cell performance and material properties of these n/p cells were compared with p/n cells grown on n-type Ge/SiGe/Si substrates for which record high minority carrier hole lifetimes of 10 ns and open circuit voltages (V/sub oc/) greater than 980 mV (AM0) were achieved. The initial n/p experimental results and correlations with theoretical predictions have indicated that for comparable threading dislocation densities (TDD), n/p cells have longer minority carrier diffusion lengths, but reduced minority carrier lifetimes for electrons in the p-type GaAs base layers. This suggests that a lower TDD tolerance exists for n/p cells compared to p/n cells, which has implications for the optimization of n/p high efficiency cell designs using alternative substrates.

Non-solar photovoltaics for small space missions

NASA has missions planned to targets in the solar system ranging from the permanently shadowed craters of Mercury to the icy reaches of the Kuiper belt and beyond. In 2011, the NASA Office of the Chief Technologist (OCT) requested the NASA Ames and Glenn Research Centers to assess the potential of small power supplies based on direct conversion of energy from radioisotope sources for future NASA missions; and in particular to assess whether alphavoltaic and betavoltaic power sources could be of potential benefit in small missions, as well as examining the use of miniaturized thermophotovoltaic power supplies. This paper summarizes the results of that assessment.

Preferentially etched epitaxial liftoff of indium phosphide

The removal of an indium phosphide (InP) epitaxial layer from an InP substrate has been demonstrated by a preferentially etched epitaxial liftoff (PEEL) technique. The release layer is lattice matched InGaAs which is selectively etched. Results of several combinations of etchants are included. Two etchants yielded the desired peeling effect: HF: H/sub 2/O/sub 2/: H/sub 2/O (1:1:10) and citric acid: water: peroxide (1:1:8). Apiezon black wax is used to provide compression to the InP film to facilitate peeling of the film. The film and substrate remain in good condition after the PEEL process. Application of this technique to thin InP solar cells is discussed.<<ETX>>

Improved radiation resistance of epitaxial lift-off inverted metamorphic solar cells

The inverted metamorphic (IMM) solar cell has a high specific power compared to traditional germanium-based multi-junction solar cells, which may prove beneficial for space applications where costs are weight-driven. In addition, the mechanical flexibility of the IMM cell may be beneficial for new technologies, such as high-power, flexible, deployable arrays currently under development. However, IMM solar cells have not yet demonstrated radiation resistance equal to that of traditional Ge-based multi-junction cells, largely due to degradation in the InGaAs bottom subcell. A structure and process have been developed to incorporate a back surface reflector on the epitaxial lift-off (ELO) IMM solar cell, permitting the InGaAs subcell to be thinned whilst maintaining high optical absorption. The thinner subcell can better tolerate degraded base diffusion lengths following irradiation. In this manner, a significant improvement in the end of life efficiency of ELO IMM solar cells is demonstrated following irradiation with 1 MeV electrons at a fluence of 1×1015 cm-2.

Solar cells for NASA RAINBOW concentrator

The RAINBOW concentrator system is based on a concept of splitting the solar spectrum and focussing each portion on a solar cell having a bandgap matching the input spectral portion. Efficiencies over 40% are predicted for systems using a four-bandgap cell assembly under 20/spl times/ concentration. Reported here are the results of materials growth, processing, and testing of four different solar cells designed to populate the RAINBOW testbed, under development at JPL. The cells are 0.74-eV InGaAs on lattice-matched InP, 1.1-eV InGaAs on lattice-mismatched GaAs, 1.43-eV GaAs on GaAs, and 1.85-eV InGaP on lattice-matched GaAs. Quantum efficiencies between 0.8 and 1.0 have been realized for the spectral region from 0.5 to /spl sim/1.5 /spl mu/m.

Integrated solar cell array antenna for satellite and terrestrial communications

This paper describes a feasibility study of integrating photovoltaic solar cells in planar antenna structure. The approach for the solar array development is based on monolithic interconnected module (MIM) and GaAs technologies, which is structurally more compact and has higher efficiency compared to Si solar array based on hybrid approaches. To facilitate integration and provide maximum real estate for the solar array, miniaturized patch antennas with coplanar waveguide (CPW)/slotline feed were employed. In the paper, the design of a 2times2 array, the main feature of the proposed approach and experimental results would be presented and discussed

Lattice mismatched dual junction tandem cells

High performance solar cells with capabilities covering a broad range of mission parameters are of great interest to the space photovoltaic community. Current areas of interest include improving efficiency of multi-junction cells by adjusting bandgaps to more optimum values, adding junctions to existing structures and investigating the effects of various substrate materials. The goal is to merge the highest efficiency multijunction solar cell with a low cost, lightweight substrate. This paper focuses on developing a multijunction solar cell with optimum bandgaps by relaxing the constraint for lattice matching between the substrate and the epitaxial cell structure. A III-V lattice mismatched dual junction solar cell composed of a 1.6 eV InGaP top cell and a 1.1 eV InGaAs bottom cell has been grown with an Air Mass Zero (AM0) efficiency of 16.4% without an antireflective coating (ARC). An AM0 efficiency of 23% is anticipated when a dual layer antireflective coating is applied. Both sub-cells are lattice matched to each other but mismatched to the GaAs substrate. Accommodation of the lattice strain was accomplished via an InGaAs buffer structure. Extension of the lattice mismatched approach to three junction devices holds the promise to demonstrate AM0 efficiencies in excess of 30%.

One-Step Synthesis of Dithiocarbamates from Metal Powders

Neutral metal dithiocarbamate complexes (M(NR2CS2)X) are well-known precursors to metal sulfides, a class of materials with numerous technological applications. We are involved in a research effort to prepare new precursors to metal sulfides using simple, reproducible synthetic procedures. We describe the results of our synthetic and characterization studies for M = Fe, Co, Ni, Cu. and In. For example, treatment of metallic indium with tetramethylthiuram disulfide (tmtd) in 4-methylpyridine (4-Mepy) at 25 deg C produces a new homoleptic indium (III) dithiocarbamate, In(N(CH3)2CS2)3(I), in yields of over 60 percent. The indium (III) dithiocarbamate was characterized by X-ray crystallography; (I) exists in the solid state as discrete distorted-octahedral molecules. Compound (I) crystallizes in the P1bar (No. 2) space group with lattice parameters: a = 9.282(1) A, b = 10.081(1) A, c = 12.502 A, alpha = 73.91(1) deg, beta = 70.21(1) deg, gamma = 85.8(1)deg, and Z = 2. X-ray diffraction and mass spectral data were used to characterize the products of the analogous reactions with Fe, Co, Ni, and Cu. We discuss both use of dithiocarbamates as precursors and our approach to their preparation.

Progress Towards III-V Photovoltaics on Flexible Substrates

Presented here is the recent progress of the NASA Glenn Research Center OMVPE group’s efforts in the development of high efficiency thin-film polycrystalline III-V photovoltaics on optimum substrates. By using bulk polycrystalline germanium (Ge) films, devices of high efficiency and low mass will be developed and incorporated onto low-cost flexible substrates. Our progress towards the integration of high efficiency polycrystalline III-V devices and recrystallized Ge films on thin metal foils is discussed.