David M. Wilt

InGaAs PV device development for TPV power systems

Indium gallium arsenide (InGaAs) photovoltaic devices have been fabricated with bandgaps ranging from 0.75 eV to 0.60 eV on Indium Phosphide (InP) substrates. Reported efficiencies have been as high as 11.2 percent (AMO) for the lattice matched 0.75 eV devices. The 0.75 eV cell demonstrated 14.8 percent efficiency under a 1500 K blackbody with a projected efficiency of 29.3 percent. The lattice mismatched devices (0.66 and 0.60 eV) demonstrated measured efficiencies of 8 percent and 6 percent respectively under similar conditions. Low long wavelength response and high dark currents are responsible for the poor performance of the mismatched devices. Temperature coefficients have been measured and are presented for all of the bandgaps tested.

Testing and modeling of a solar thermophotovoltaic power system

A solar thermophotovoltaic (STPV) power system has attractive attributes for both space and terrestrial applications. This paper presents the results of testing by McDonnell Douglas Aerospace (MDA) over the last year with components furnished by the NASA Lewis Research Center (LeRC) and the National Renewable Energy Lab (NREL). The testing has included a large scale solar TPV testbed system and small scale laboratory STPV simulator using a small furnace. The testing apparatus, instrumentation, and operation are discussed, including a description of the emitters and photovoltaic devices that have been tested. Over 50 on‐sun tests have been conducted with the testbed system. It has accumulated over 300 hours of on‐sun time, and 1.5 MWh of thermal energy incident on the receiver material while temperatures and I‐V measurements were taken. A summary of the resulting test data is presented that shows the measured performance at temperatures up to 1220u2009°C. The receiver materials and PV cells have endured the hi...

Electrical and Optical Performance Characteristics of 0.74-eV p/n InGaAs Monolithic Interconnected Modules

There has been a traditional trade-off in thermophotovoltaic (TPV) energy conversion development between system efficiency and power density. This trade-off originates from the use of front surface spectral controls such as selective emitters and various types of filters. A monolithic interconnected module (MIM) structure has been developed which allows for both high power densities and high system efficiencies. The MIM device consists of many individual indium gallium arsenide (InGaAs) cells series-connected on a single semi-insulating indium phosphide (InP) substrate. The MIM is exposed to the entire emitter output, thereby maximizing output power density. An infrared (IR) reflector placed on the rear surface of the substrate returns the unused portion of the emitter output spectrum back to the emitter for recycling, thereby providing for high system efficiencies. Initial MIM development has focused on a 1u2009cm2 device consisting of eight series interconnected cells. MIM devices, produced from 0.74 eV InG...

High-performance, lattice-mismatched InGaAs/InP monolithic interconnected modules (MIMs)

High performance, lattice-mismatched p/n InGaAs/InP monolithic interconnected module (MIM) structures were developed for thermophotovoltaic (TPV) applications. A MIM device consists of several individual InGaAs photovoltaic (PV) cells series-connected on a single semi-insulating (S.I.) InP substrate. Both interdigitated and conventional (i.e., non-interdigitated) MIMs were fabricated. The energy bandgap (Eg) for these devices was 0.60 eV. A compositionally step-graded InPAs buffer was used to accommodate a lattice mismatch of 1.1% between the active InGaAs cell structure and the InP substrate. 1×1-cm, 15-cell, 0.60-eV MIMs demonstrated an open-circuit voltage (Voc) of 5.2 V (347 mV per cell) and a fill factor of 68.6% at a short-circuit current density (Jsc) of 2.0 A/cm2, under flashlamp testing. The reverse saturation current density (Jo) was 1.6×10−6u200aA/cm2. Jo values as low as 4.1×10−7u200aA/cm2 were also observed with a conventional planar cell geometry.

Materials and process development for the monolithic interconnected module (MIM) InGaAs/InP TPV devices

The selection, development, and testing of materials and processes for MIM fabrication are described. Topics covered include isolation trenches, contact and interconnect metallization, dielectric isolation barriers, back surface reflectors, and antireflection coatings. (AIP)

Development and testing of coatings for orbital space radiation environments

Specific coating processes and materials were investigated in the quest to develop multilayer coatings with greater tolerance to space radiation exposure. Ultraviolet reflection (UVR) and wide-band antireflection (AR) multilayer coatings were deposited on solar cell covers and test substrates and subsequently exposed to simulated space environments and also flown on the Materials International Space Station Experiment-7 (MISSE-7) to determine their space environment stability. Functional solar cells integrated with these coatings underwent simulated UV and MISSE-7 low earth orbit flight exposure. The effects of UV, proton, and atomic oxygen exposure on coatings and on assembled solar cells as related to the implemented deposition processes and material compositions were small. The UVR/AR coatings protected flexible polymer substrate materials that are intended for future flexible multijunction cell arrays to be deployed from rolls. Progress was made toward developing stable and protective coatings for extended space-mission applications. Test results are presented.

Novel flexible solar cell coverglass for space photovoltaic devices

A flexible space solar cell coverglass replacement called Pseudomorphic Glass (PMG) has been under investigation in hopes of providing a robust, high transmissivity replacement for conventional coverglass. PMG is composed of ceria doped borosilicate or fused silica beads incorporated in a variety of polymer matrices. The glass beads provide the primary radiation protection and the polymer matrix provides the mechanical integrity. PMG development has recently focused on optimization of optical performance (transmissivity and scattering), bead material and development of optical coatings.

Technology opportunities to enable high mass specific power

Inverted Metamorphic Multijunction (IMM) technology has demonstrated excellent energy conversion efficiency, 32% AM0. In addition to high conversion efficiency, this technology also offers the potential for ultra-high mass specific power at the blanket level. Because the substrate is removed, the thin, flexible epitaxial cell can be incorporated in a variety of novel blanket structures. Several novel array technologies have been proposed which would take advantage of the flexible nature of the IMM by incorporating rolled stowage for launch. The flexibility of the IMM may lead one to assume that the IMM is a much higher efficiency drop-in replacement for conventional thin-film photovoltaics (ex. amorphous silicon, copper indium gallium diselenide). An important differentiation between these technologies is the radiation hardness of the different technologies to the space environment. This paper presents a study to examine the photovoltaic blanket specific mass achievable with IMM technology depending upon the orbit of interest and the end-of-life performance requirement. The impact of radiation shielding, both front and back, is assessed.

Lattice-matched and strained InGaAs solar cells for thermophotovoltaic use

Lattice‐matched and strained indium gallium arsenide solar cells can be used effectively and efficiently for thermophotovoltaic applications. A 0.75 eV bandgap InGaAs solar cell is well matched to a 2000 K blackbody source with a emission peak around 1.5 μm. A 0.60 eV bandgap InGaAs cell is well suited to a Ho‐YAG selective emitter and a blackbody at 1500 K which have emission peak around 2.0 μm. Modeling results predict that the cell efficiencies in excess of 30% are possible for the 1500 K Ho‐YAG selective emitter (with strained InGaAs) and for the 2000 K blackbody (with lattice‐matched InGaAs) sources.

n/p/n tunnel junction InGaAs Monolithic Interconnected Module (MIM)

The Monolithic Interconnected Module (MIM), originally introduced at the First NREL thermophotovoltaic (TPV) conference, consists of low-bandgap indium gallium arsenide (InGaAs) photovoltaic devices, series interconnected on a common semi-insulating indium phosphide (InP) substrate. An infrared reflector is deposited on the back surface of the substrate to reflect photons, which were not absorbed in the first pass through the structure. The single largest optical loss in the current device occurs in the heavily doped p-type emitter. A new MIM design (pat pend.) has been developed which flips the polarity of the conventional MIM cell (i.e., n/p rather than p/n), eliminating the need for the high conductivity p-type emitter. The p-type base of the cell is connected to the n-type lateral conduction layer through a thin InGaAs tunnel junction. 0.58 eV and 0.74 eV InGaAs devices have demonstrated reflectances above 90% for wavelengths beyond the bandgap (>95% for unprocessed structures). Electrical measurement...