David J. Brinker

High efficiency indium gallium arsenide photovoltaic devices for thermophotovoltaic power systems

The development of indium gallium arsenide (Eg=0.75 eV) photovoltaic devices for thermophotovoltaic power generation is described. A device designed for broadband response had an air mass zero efficiency of 11.2 % and an internal quantum yield of over 90% in the range of 800 to 1500 nm. Devices designed for narrow‐band response have also been developed. Both structures are based on a n/p junction which also makes them applicable for integration into indium phosphide based, monolithic, tandem solar cells for solar photovoltaic applications.

Amorphous diamond-like carbon films—a hard anti-reflecting coating for silicon solar cells

Abstract Amorphous diamond-line carbon (a:DLC) films are suitable for use as a protective layer and/or anti-reflecting coating for silicon solar cells. Microhardness tests show a high hardness of about 4700 kg mm−2. Optical measurements in the visible light on a silicon solar cell without anti-reflecting coating where a:DLC was deposited as an anti-reflecting (AR) coating, show a significant reduction in the reflection of the light of about 25–45% as compared with the reflection from the solar cell before deposition of the a:DLC film. The current-voltage (I–V) characteristics of solar cells without an anti-reflecting coating and with a:DLC as the AR coating layer show an improvement in the short circuit current (from 107 mA to 116 mA) and in the efficiency of the cell by 1% (from 8% to 9%). I–V characteristics of a solar cell with AR coating and deposited a:DLC (400Athickness) over that film as a protective coating, show a slight reduction of the short circuit current (from 168 mA to 145 mA), and the efficiency of the solar cell decreases (16% to 14%). However, the microhardness was significantly increased.

InGaAs monolithic interconnected modules (MIMs)

A monolithic interconnected module (MIM) structure has been developed for thermophotovoltaic (TPV) applications. The MIM device consists of many individual InGaAs cells series-connected on a single semi-insulating InP substrate. An infrared (IR) back surface reflector (BSR), placed on the rear surface of the substrate, returns the unused portion of the TPV radiator output spectrum back to the radiator for recuperation, thereby providing for high system efficiencies. Also, the use of a BSR reduces the requirements imposed on a front surface interference filter and may lead to using only an anti-reflection coating. As a result, MIMs are exposed to the entire radiator output, and with increasing output power density. MIMs were fabricated with an active area of 0.9/spl times/1 cm, and with 15 cells monolithically connected in series. Both lattice-matched and lattice-mismatched InGaAs/InP devices were fabricated, with bandgaps of 0.74 and 0.55 eV, respectively. The 0.74 eV MIMs demonstrated an open-circuit voltage (Voc) of 6.16 V and a fill factor of 74.2% at a short-circuit current (Jsc) of 0.84 A/cm/sup 2/, under flashlamp testing. The 0.55 eV modules demonstrated a Voc of 4.85 V and a fill factor of 57.8% at a Jsc of 3.87 A/cm/sup 2/. The near IR reflectance (2-4 /spl mu/m) for both lattice-matched and lattice-mismatched structures was measured to be in the range of 80-85%. Latest electrical and optical performance results for these MIMs is presented.

High altitude current-voltage measurement of GaAs/Ge solar cells

Measurements of high-voltage (V/sub oc/ of 1.2 V) gallium arsenide on germanium tandem junction solar cells at air mass 0.22 showed that the insolation in the red portion of the solar spectrum is insufficient to obtain high fill factor. On the basis of measurements in the LeRC X-25L solar simulator, these cells were believed to be as efficient as 21.68% AM0. Solar simulator spectrum errors in the red end allowed the fill factor to be as high as 78.7%. When a similar cells current-voltage characteristic was measured at high altitude in the NASA Lear Jet Facility, a loss of 15 percentage points in fill factor was observed. This decrease was caused by insufficient current in the germanium bottom cell of the tandem stack.<<ETX>>

Iced Aircraft Flight Data for Flight Simulator Validation

NASA is developing and validating technology to incorporate aircraft icing effects into a flight training device concept demonstrator. Flight simulation models of a DHC-6 Twin Otter were developed from wind tunnel data using a subscale, complete aircraft model with and without simulated ice, and from previously acquired flight data. The validation of the simulation models required additional aircraft response time histories of the airplane configured with simulated ice similar to the subscale model testing. Therefore, a flight test was conducted using the NASA Twin Otter Icing Research Aircraft. Over 500 maneuvers of various types were conducted in this flight test. The validation data consisted of aircraft state parameters, pilot inputs, propulsion, weight, center of gravity, and moments of inertia with the airplane configured with different amounts of simulated ice. Emphasis was made to acquire data at wing stall and tailplane stall since these events are of primary interest to model accurately in the flight training device. Analyses of several datasets are described regarding wing and tailplane stall. Key findings from these analyses are that the simulated wing ice shapes significantly reduced the C , max, while the simulated tail ice caused elevator control force anomalies and tailplane stall when flaps were deflected 30 deg or greater. This effectively reduced the safe operating margins between iced wing and iced tail stall as flap deflection and thrust were increased. This flight test demonstrated that the critical aspects to be modeled in the icing effects flight training device include: iced wing and tail stall speeds, flap and thrust effects, control forces, and control effectiveness.

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 1.6×10−6 A/cm2. Jo values as low as 4.1×10−7 A/cm2 were also observed with a conventional planar cell geometry.

Uncertainty analysis of high altitude aircraft air mass zero solar cell calibration

Recently, the ISO standards organization has requested the PV community to establish AM0 calibration methodologies for space power solar cells. The PV community responded by organizing a series of workshops to review and recommend AM0 calibration techniques. One of the activities of the workshop is to review the various calibration methodologies and conduct a comprehensive uncertainty analysis of each method. This paper outlines NASAs methodology of AM0 calibration using the high-altitude aircraft method.

Peeled film GaAs solar cell development

Thin-film, single-crystal gallium arsenide (GaAs) solar cells could exhibit a specific power approaching 700 W/kg including coverglass. A simple process has been described whereby epitaxial GaAs layers are peeled from a reusable substrate. This process takes advantage of the extreme selectivity (>10/sup 6/) of the etching rate of aluminum arsenide (AlAs) over GaAs in dilute hydrofluoric acid (HF). The feasibility of using the peeled film technique to fabricate high-efficiency, low-mass GaAs solar cells is presently demonstrated. A peeled film GaAs solar cell was successfully produced. The device, although fractured and missing the aluminum gallium arsenide (Al/sub x/Ga/sub 1-x/As) window and antireflective (AR) coating, has a V/sub oc/ of 874 mV and a fill factor of 68% under AM0 illumination.<<ETX>>

The performance of advanced solar cells for interplanetary missions

Recent advances in space solar cell technology have produced substantial increases in Air Mass Zero (AM0) efficiency. Since these cells have been developed primarily for Earth orbiting missions, little is known of their behavior at distances far from the Sun. In order to better define the photovoltaic performance of arrays for deep space missions, JPL has completed initial measurements on a number of advanced cells under a variety of LILT (low intensity, low temperature) conditions. These include high efficiency silicon, and multi-junction III-V devices. The test results show that multi-junction cells suffer from LILT degradation and that at 5AU (approximately the solar distance of Jupiter), efficiency advantages over high efficiency silicon are minimal. Silicon cells optimized for 3-6 AU operation not only equal the efficiency available from 2 and 3 junction cells, but also tend to be more uniform.

Low intensity low temperature (LILT) measurements and coefficients on new photovoltaic structures

As RTGs have become increasingly less desirable for outer-planetary missions, solar cell performance data at very low temperatures and intensities are required. The authors have measured low intensity, low temperature (LILT) I-V data on single junction and multi-junction high efficiency solar cells, representing the state-of-the-art in photovoltaic technology. Using this LILT data to calculate I/sub sc/, V/sub oc/, and FF as a function of temperature and intensity, an accurate prediction of cell performance under the AMO spectrum can be determined. When combined with quantum efficiency at low temperature (QULT) data, one can further enhance the prediction by the addition of spectral variation. This paper presents an overview of LILT measurements taken at temperatures as low as -180/spl deg/C and intensities ranging from 1 Sun to 0.02 Suns. The temperature dependency coefficients presented in this paper are experimental results intended to provide a guideline for array design.