David A. Scheiman

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.

High-bandgap solar cells for near-sun missions

High bandgap solar cells are to be preferred for near-Sun, high operating-temperature environments, such as will be encountered by a Mercury orbiter or the Solar Probe mission. A GaInP solar cell is well suited for elevated temperature performance because it is available and has a bandgap high enough to produce reasonable performance at temperatures above 400 °C. The cell is currently commercially available as the top cell of a multi-junction solar cell. A cell contact metallization needs to be developed that can operate without degradation at high temperature.

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-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.

Extended Temperature Solar Cell Technology Development

Future NASA missions will require solar cells to operate both in regimes closer to the sun, and farther from the sun, where the operating temperatures will be higher and lower than standard operational conditions. NASA Glenn is engaged in testing solar cells under extended temperature ranges, developing theoretical models of cell operation as a function of temperature, and in developing technology for improving the performance of solar cells for both high and low temperature operation.

Mars Solar Power

NASA missions to Mars, both robotic and human, rely on solar arrays for the primary power system. Mars presents a number of challenges for solar power system operation, including a dusty atmosphere which modifies the spectrum and intensity of the incident solar illumination as a function of time of day, degradation of the array performance by dust deposition, and low temperature operation. The environmental challenges to Mars solar array operation will be discussed and test results of solar cell technology operating under Mars conditions will be presented, along with modeling of solar cell performance under Mars conditions. The design implications for advanced solar arrays for future Mars missions is discussed, and an example case, a Martian polar rover, are analyzed.

Temperature coefficient of multijunction space solar cells as a function of concentration

Measurements of multijunction space solar cells were taken as a function of temperature for low intensity operation (down to 0.02 suns), and also at high intensities (up to 60 suns). Cells from two vendors were tested. The effect of intensity on the measured temperature coefficient was analyzed. The temperature coefficient of the short circuit current was directly proportional to the intensity, while the temperature coefficient of the open circuit current decreased proportionally to the open circuit voltage. Commercial triple-junction space cells were tested to temperatures as high as 400°C, showing a slight deviation from linear performance but no catastrophic degradation.

Characterization and modeling of InGaAs/InAsP thermophotovoltaic converters under high illumination intensities

Thermophotovoltaic converters based on In0.69Ga0.31As/InAs0.34P0.66 have been fabricated, characterized experimentally, and modeled. Good device performance has been achieved with an open-circuit voltage of 1.46 V, short-circuit current density of 1.06 A/cm2, and a fill factor of 71.3% for a four-junction cell under an optical power density of 3.4 W/cm2. Key material parameters have been extracted from measured device characteristics, providing a detailed quantitative understanding of the dependence of device performance on the electro-optical properties of the InGaAs/InAsP material system. Extracted minority carrier lifetimes of 106 ns in the p-type base and 0.3 ns in the n-type emitter regions were obtained, limited by radiative and Auger recombination, respectively. The recombination velocity for the InGaAs/InAsP interface is found to be below 2000 cm/s. The parameter analysis provides guidance for the design of a high-efficiency monolithically integrated module for use under high illumination intensities.