Jeremiah S. McNatt

The future of space photovoltaics

A review of state-of-the-art solar cell efficiencies and their application to future space array utilization is discussed. New emerging technologies are presented and the impact on cost and efficiency is projected for space use.

Thin film poly III–V space solar cells

Results on the development of polycrystalline III–V based devices grown by OMVPE on thin metallic foil substrates are presented. It has previously been demonstrated that device quality polycrystalline Ge suitable for OMVPE growth can be produced on metallic foils using a recrystallization process. This work reports on the development of textured metal foil substrates with low misfit grain boundary orientations designed to improve the semiconducting device parameters of the “epitaxially” deposited Ge films, the use of innovative device structures, and grain boundary passivation approaches for the polycrystalline GaAs films that are all designed to address performance issues associated with these types of solar cells. The Ge which serves as the III-V growth template could be activated and serve as the bottom junction of a conventional triple junction III-V cell design using this approach. The crystallographic, morphological, and electro-optical properties associated with these substrates and related epitaxial films will be presented. In addition, the thermal and radiation behavior, that is critical for the potential use of these devices in space, was investigated. The potential for these devices for future space development and exploration will be discussed.

Ultra-lightweight space power from hybrid thin-film solar cells

The development of hybrid inorganic/organic thin-film solar cells on flexible, lightweight, space-qualified, durable substrates provides an attractive solution for space power generation with high mass specific power (W/kg). The high-volume, low-cost fabrication potential of organic cells will allow for square miles of solar cell production at one-tenth the cost of conventional inorganic materials. Plastic solar cells take a minimum of storage space and can be inflated or unrolled for deployment. We explore a cross-section of NASA in-house and sponsored research efforts that aim to provide new hybrid technologies that include both inorganic and polymer materials as active and substrate materials. For NASA applications, any solar cell or array technology must not only meet weight and AMO efficiency goals, but also must be durable enough to survive launch and space environments. Also, balance of system technologies must be developed to take advantage of ultra-lightweight solar arrays in power generation systems.

Design of photovoltaic power system for a precursor mission for human exploration of mars

This project analyzed the viability of a photovoltaic power source for technology demonstration mission to demonstrate Mars in-situ resource utilization (ISRU) to produce propellant for a future human mission, based on technology available within the next ten years. For this assessment, we performed a power-system design study for a scaled ISRU demonstrator lander on the Mars surface based on existing solar array technologies.

Thin film III–V solar cells on Mo foil

Space bound photovoltaics utilize crystalline III–V material systems to achieve extremely high conversion efficiencies. The measure of these devices is not ultimate conversion efficiency but specific power density, which can be limited by the bulky non-contributing substrates they must be epitaxially grown on. By replacing these substrates with thin metal foil, the overall weight of the device can be significantly reduced. This can be achieved by deposition of amorphous Ge on a metal foil and recrystallizing it through a thermal process to create a large-grain polycrystalline surface allowing for epitaxial growth. In this paper, the changes in device characteristics in the transition from single-crystal GaAs to polycrystalline Ge, as well as adhesion layers and annealing conditions for the recrystallization of Ge on Mo foil, was studied. It was shown that cells grown on poly-Ge substrates exhibited an increased JSC compared to those grown on crystalline Ge substrates. A process window for recrystallization was observed between 700°C and 850°C. Delamination of the Ge was observed using both W/Ti and Cr as a barrier layer to Mo, but Cr exhibited promising recrystallization results in need of further study.

Ultra-Lightweight Hybrid Thin-Film Solar Cells: A Survey of Enabling Technologies for Space Power Applications

The development of hybrid inorganic/organic thin-film solar cells on flexible, lightweight, space-qualified, durable substrates provides an attractive solution for fabricating solar arrays with high mass specific power (W/kg). Next generation thin-film technologies may well involve a revolutionary change in materials to organic-based devices. The high-volume, low-cost fabrication potential of organic cells will allow for square miles of solar cell production at one-tenth the cost of conventional inorganic materials. Plastic solar cells take a minimum of storage space and can be inflated or unrolled for deployment. We will explore a cross-section of in-house and sponsored research efforts that aim to provide new hybrid technologies that include both inorganic and polymer materials as active and substrate materials. Research at University of Texas at Arlington focuses on the fabrication and use of poly(isothianaphthene-3,6-diyl) in solar cells. We describe efforts at Norfolk State University to design, synthesize and characterize block copolymers. A collaborative team between EIC Laboratories, Inc. and the University of Florida is investigating multijunction polymer solar cells to more effectively utilize solar radiation. The National Aeronautics and Space Administration (NASA)/Ohio Aerospace Institute (OAI) group has undertaken a thermal analysis of potential metallized substrates as well as production of nanoparticles of CuInS2 and CuInSe2 in good yield at moderate temperatures via decomposition of single-source precursors. Finally, preliminary work at the Rochester Institute of Technology (R.I.T.) to assess the impact on performance of solar cells of temperature and carbon nanotubes is reported. Technologies that must be developed to enable ultra-lightweight solar arrays include: monolithic interconnects, lightweight array structures, and new ultra-light support and deployment mechanisms. For NASA applications, any solar cell or array technology must not only meet weight and AMO efficiency goals, but also must be durable enough to survive launch conditions and space environments.

Aerosol-Assisted Chemical Vapor Deposited Thin Films for Space Photovoltaics

Abstract Copper indium disulfide thin films were deposited via aerosol-assisted chemical vapor deposition using single source precursors. Processing and post-processing parameters were varied in order to modify morphology, stoichiometry, crystallography, electrical properties, and optical properties in order to optimize device-quality material. Growth at atmospheric pressure in a horizontal hot-wall reactor at 395 °C yielded best device films. Placing the susceptor closer to the evaporation zone and flowing a more precursor-rich carrier gas through the reactor yielded shinier, smoother, denser-looking films. Growth of (112)-oriented films yielded more Cu-rich films with fewer secondary phases than growth of (204)/(220)-oriented films. Post-deposition sulfur-vapor annealing enhanced stoichiometry and crystallinity of the films. Photoluminescence studies revealed four major emission bands (1.45, 1.43, 1.37, and 1.32 eV) and a broad band associated with deep defects. The highest device efficiency for an aerosol-assisted chemical vapor deposited cell was 1.03 percent.

Epitaxial regrowth contacts for the nipi photovoltaic device

The simulation and fabrication of a multi-period GaAs n-type / intrinsic / p-type / intrinsic (nipi) doping superlattice solar cell has been demonstrated. A fabrication procedure has been developed using regrown contacts in wet etched V-grooves. Devices have been fabricated and characterized. Current-Voltage measurements in the dark and under one sun AM0 illumination were taken both experimentally and in simulation. Devices with epitaxial regrown contacts having a shunt resistance of 3.17 kΩ, demonstrates an improvement over prior work. Simulations show the potential for high current collection, with non anti-reflection coated AM0 results achieving 24.02 mA/cm2 short circuit current, due to a drift dominated current collection mechanism.

Transparent Conducting Oxide Thin-Films and Junctions for Semi-Transparent Photovoltaic Devices

The NASA Glenn Research Center (GRC) is collaborating with Alpha Micron, Inc. (AMI) to develop a transparent solar cell for smart window applications. This window uses liquid crystals powered transparent solar cells to vary the tinting of the window throughout the day. A p/n junction is formed between transparent conducting oxide (TCO) thin films to produce a solar cell with conversion efficiency high enough to meet the power requirements of the window. The TCO films are deposited by radio frequency (RF) sputtering. The deposited films are characterized for optimal optical and electronic properties. This paper presents the optimization of the experimental variables producing n-type aluminum-doped zinc oxide (ZnO:Al), p-type copper oxide (Cu2O), and indium tin oxide (ITO) thin-films along with first attempts at producing a working photovoltaic device. The analysis of this data will be used produce the framework for the transparent solar cell.

Recrystallization of Ge for III-V photovoltaic substrates

Amorphous germanium (Ge) is RF sputtered onto 25 µm thick molybdenum (Mo) foils and recrystallized at 675 °C under an AsH3 environment. After annealing, the Ge is polycrystalline with grain boundary regions that range from 1 um2 to 0.5 cm2. The polycrystalline surface behaves electrically much like that of commercial single crystal Ge. This polycrystalline Ge on thin Mo foil will serve as a substrate for a high efficient, high mass-specific-power, photovoltaic device.