A three-terminal, monolithically integrated InP photovoltaic (PV) device with an InGaAs/InAlAs electroabsorption modulator for simultaneous power generation and data modulation has been designed and experimentally validated. A discrete modulator with a data rate approaching 1 Mbps with an on / off ratio of 1.20 was demonstrated. The InP power generating junction of the monolithic device attained an AM0 efficiency of 14.1% for a 0.25 cm 2 area, with the efficiency loss primarily coming from the expected loss in the lateral conduction layer that was designed to minimize fractional power loss in the PV device and to minimize insertion loss at 1.55 $\mu$ m. The monolithically integrated device demonstrated the potential for data modulation up to 0.5 Mbps at 1.55 $\mu$ m. Simultaneous power generation and data modulation were observed in this monolithic device, and the device has application in areas such as low size, weight, power (SWaP) satellites, debris tagging, medical applications, and high altitude, long endurance unmanned aerial systems.

Photovoltaics (PV) provide an enabling platform for integrating data transmission capabilities. A four-terminal, mechanically stacked hybrid PV device/electroabsorption modulator (EAM) is presented, combining both solar power generation and data transmission via free space optical at 1.55 μm in one package. A 23 % AM0 efficient dual junction, InGaP/GaAs solar cell was bonded to an InGaAs/InAlAs segmented EAM with a designed cutoff frequency approaching 1 MHz. Simultaneous power generation and data transmission will be demonstrated.

A three-terminal, monolithically integrated device is presented combining both power generation and optical communication at 1.55 µm, A discrete InP cell without an anti-reflection coating with an AMO efficiency of 12.8 % and an InGaAs/lnAlAs electroabsorption modulator with a peak ON/OFF ratio of 3.0 have been developed. A monolithic device has been grown and fabricated, and is undergoing simultaneous PV collection/free space optical communication testing.

We report a low power ocean salinity sensor based on capacitance measurements. Our design compares two carefully tuned astable multivibrator oscillators to measure small frequency shifts in the sensing oscillator induced by changes in salt concentration. Comparing the difference in the number of oscillator cycles during a measurement time interval enables the calculation of a frequency shift from which a corresponding salinity change is fitted. We demonstrate a design by which this oscillator cycle-count comparison can be conducted without the need for a microcontroller which enables very low power operation. Our fabricated sensor is sensitive to a frequency shift as small as 0.024% allowing sensitivity to changes in salinity of less than 0.1 parts-per-thousand (ppt). Our sensor’s sensitivity to changes in salinity is amplified by a conductive hydrogel occupying the space between the parallel plates and whose permittivity varies with the salinity of the water absorbed.

The current standard for communications system on satellites are high size, weight, and power (SWaP) RF transceivers, which is contrasted with the low SWaP solar arrays that implement III-V semiconductors for optimal solar collection. An alternative, low SWaP solution is to implement a hybrid photovoltaic (PV)/electroabsorptive modulator (EAM) coupled with a retroreflector and use free space optical communication over 1.55 μm rather than RF communication. A design which minimizes parasitic losses and optimizes contrast ratio and cutoff frequency in a 1 cm2 device is discussed.

The concept of a hybrid solar cell with an integrated multi-quantum well modulating retroreflector is presented. This device can be fabricated by monolithic growth of III-V compounds or via mechanical stacking. The DataCell device allows for simultaneous electrical energy harvesting (photovoltaic effect) AND high-speed optical data communication (quantum-confined Stark effect) for free-space optical (FSO) links in a single integrated device. The fabrication and successful operation of first prototype devices is discussed.

The Naval Research Laboratory has developed a low-power, real-time displacement damage dosimeter (RT3D). RT3D consists of two components; one being the measurement electronics and the second being the actual dosimeter (GaAs diode) exposed to the radiation source. RT3D accumulates displacement damage passively (without power) and the displacement damage dose (DDD) is measured at will with measurement electronics by measuring the GaAs diode dark current at one or more forward bias voltages. The DDD is subsequently determined in real-time from a calibration curve derived from ground-based testing. The dosimeter is capable of measuring approximately three orders of magnitude in DDD. The dosage measurement range of the sensor can be modified by placing an absorber material over the sensor. Proton testing performed in situ yielded an error estimate determination to within 10% for simulated space use.

Flight endurance and power are limiting factors affecting UAV applications today due to battery weight and capacity. Solar arrays integrated into the wing surface can provide additional power dependent on the sun, weight, wing area, and efficiency, and have demonstrated more than doubled flight times. With most wing surfaces having some degree of curvature and flexure during flight, stresses can be induced on the solar arrays. Photoluminescence is used to assess wing stresses by optically identifying crack propagation in the cells. NRL has built a variety of wings for UAVs from solar cell technologies. This paper intends to provide a demonstration of using this technique to study solar cell cracking through the array assembly process from wing integration to flight.

The frequency of high altitude balloons flights for the purpose of AM0 calibration of solar cells has diminished substantially in the last decade. There is increasing interest in routine calibration flights to characterize the next generation of space solar cells, which have substantially different spectral response than the incumbent GaInP/InGaAs/Ge cells. This paper describes several ongoing efforts to restore high-altitude balloon solar cell calibration capability.

The remote military installation at Diego Garcia is used as an economic and strategic example of the many possible future naval or commercial sites and scenarios that could be utilized for the production of sustainable low carbon fuel from non-fossil electrical energy. In addition to photovoltaic (PV) and wind electrical sources, this analysis includes light water reactors (LWRs) on mobile platforms as the main electrical energy source. Using published capital cost estimates and a range of solar and wind renewable electrical energy scenarios, costs ranging between $6.40 and $12.57 per gallon of standard fuel are estimated for a 129,000 gallons/day fuel process. The cost estimate for a floating nuclear electrical scenario is between $4.66 and $9.47 per gallon. Larger area potential sites of strategic naval and/or commercial importance such as Guam and Djibouti provide additional operational scenarios. Most importantly these military examples provide a framework for how such a process could be used commercially to supply stored energy as a sustainable fuel source for remote countries poor in fossil fuel resources. This information may also be used by policy analysts to support alternative energy implementation strategies and greatly expand the naval and commercial role in nuclear and renewable energy aimed at sustainable production of low carbon fuel. Published by Elsevier Ltd.

Modern silicon photovoltaic (PV) cells have high external quantum efficiencies (>70%) from 900nm-1070nm, and are ideally suited as laser power receivers to match the wavelength of high power lasers available today. Silicon PV cells are ~300X less expensive than TTT-V photovoltaic cells making them economical alternatives for large area receivers. A large receiver benefits the laser side of a wireless power system by reducing the requirement for maintaining a small beam at a great distance and eases the wireless receiver design by allowing waste heat to be spread over a larger surface area. Finally, a silicon PV array can efficiently combine solar energy harvesting during the day, and laser energy power transfer at night with a single low-cost array. In this paper we study commercially available silicon solar cells, evaluate their suitability for a laser power converter, and discuss some of the system related aspects of fielding a laser power converter.

Unmanned Aerial Vehicles (UAVs) are expanding in both military and commercial markets. Most UAVs are limited in flight duration and due in part to the weight of energy storage. Solar cells integrated into the wing surface can provide additional power dependent on the sun, weight, wing area, and efficiency. NRL has built a variety of wings for UAVs from solar cell technologies which include Si, thin flexible GaAs, triple junction InGaP/GaAs/Ge, and Inverted Metamorphic Multi-Junction (IMM) for comparison. NRL has flown these solar technologies demonstrating flights in excess of 10 hrs with only 4 hrs of onboard energy storage. This paper intends to provide a side-by-side comparison of these technologies for design and cost considerations on mission feasibility.

Experimental results of solar cells tested under deep space conditions are presented. The impact of low intensity and low temperature (LILT) conditions on the performance characteristics, in particular the fill factor, is discussed. The paper closes with a review of the elimination and mitigation of LILT effects by dedicated solar cell device design.

Metal matrix composites (MMCs) have been fabricated as potential advanced solar cell electrodes. Test structures have been developed for evaluating electrical performance of the MMC electrodes upon substrate fracture and under subsequent tensile stress to simulate stress fractures and mechanical fatigue of solar cell electrodes. Electrical analysis reveals that MMCs utilizing single-wall carbon nanotubes (SWCNTs) provide electrical continuity for gaps of ~6 μm or less; however, incorporation of longer multi-walled CNTs (MWCNTs) into the MMCs enables bridging of gaps approaching 30 μm. The results indicate that CNT-MMCs may provide a more robust solar cell electrode, particularly for fragile IMM solar cells.

Unmanned Aerial Vehicles (UAVs) are rapidly growing in both military and commercial markets. One shortfall of UAVs is the amount of time they can fly, being limited by the energy storage. Solar cells can be integrated into the wing surface to provide additional power, dependent on the sun, weight, wing area, and efficiency. NRL is building a UAV with wings from a variety of solar cell technologies, which includes high efficiency Si, thin flexible GaAs, triple junction InGaP/GaAs/Ge, and Inverted Metamorphic Multi-Junction (IMM) for direct comparison and to improve flight endurance. The UAV also incorporates the necessary power management system required to maximize the solar power available. In addition to solar cells, this plane can also utilize thermal updrafts to soar. Flight data is provided which includes all electrical parameters for comparison studies.

This study investigates maximizing net power input from a solar-photovoltaic array and/or a thermal updraft while performing constant bank angle, circular flight-path turns (that is, an orbit). The process inputs are the aircraft sink polar (or, equivalently, the power required curve), the date, the time, the location, and the flight altitude. A solar insolation model is combined with a best-turn performance calculation to determine the bank angle that maximizes power input from a solar-photovoltaic array at varying sun elevation angles. In general, for low sun elevation angles, the maximum net power gain from solar input and drag output is found at higher bank angles and shows 15% (absolute) gain over the limiting case of wings-level orbits in the same conditions. For high sun elevation angles, the maximum net power is found at low bank angles. The break point between high and low sun elevation angles varies with aircraft parameters and is approximately 25deg for the example aircraft in this paper. The relative power losses of off-optimum orbits at a given bank angle are quantified. Finally, a thermal-updraft model is included in the net power calculations to determine an optimal bank angle that maximizes the average power input from a combination of solar photovoltaics and thermal updrafts. This paper shows that soaring and solar photovoltaics can be mutually beneficial, and it provides a method to calculate losses at off-optimum conditions.

Applications and uses for UAVs are rapidly expanding as the constituent technologies mature. One major drawback of UAVs is the length of time they can fly, which is limited by the energy storage. Solar cells can be added to the wing surface and provide additional power dependent on the sun, weight, wing surface area, and efficiency. Inverted Metamorphic Multi-junction (IMM) solar cells offer advantages over conventional cell technology because they are lighter, more efficient, and more mechanically flexible than crystalline Si allowing for coverage of the curved surface of the wing. NRL is building an UAV with IMM solar cells to improve flight endurance. The aircraft also incorporates the necessary power management system required to maximize the solar power available. In addition to solar cells, this plane will also utilize thermal updrafts to soar.

Laboratory measurement of the IV parameters of multi-junction (MJ) solar cells is performed using solar simulators designed to match the desired illumination environment as closely as possible. This paper examines the impact of non-idealities in solar simulator spectrum on the measurement of MJ solar cell IV parameter through the use of a multi-zone solar simulator capable of varying light output on each MJ subcell independently. It is shown through offsets in solar simulator spectrum that standard characterization methodology may result in significant error in IV parameter measurement resulting in over or underestimation of cell performance, largely as a result of luminescent coupling.

We have developed an InGaP solar cell structure capable of operating at 450°C under N 2 ambient. This structure has been annealed for over 70h without degradation in room temperature performance. This type of structure has applications in hybrid solar energy plants which combine photovoltaic and thermal collection systems to maximize overall conversion efficiency. We anticipate that this device will be able to achieve up to 17% efficiency at 400°C and 500x concentration based on simulations with incorporated optical and electrical high-temperature semiconductor parameters.

A high-altitude AM0 solar cell calibration platform capable of attaining conditions close to those of space, i.e. 30km (100,000ft) or more is presented. The platform leverages the latest advances in the fields of miniaturized flight control electronics and sun pointing systems. This provides the ability to build a calibration platform of comparable or superior performance to previous airplane and balloon based systems but at two to three orders of magnitude less weight and size which results in launches with lower operating cost and short notice. Further advantages are the capability to measure temperature coefficients and a guided retrieval system. Test and high-altitude flight data are presented.