Clay Mayberry

Multijunction solar cell iso-junction dark current study

Single-crystal multijunction solar cells show great promise for achieving 30-40% conversion efficiency under air mass zero (AM0) conditions, and have been identified as an enabling technology for next-generation government and commercial satellites. In this note we report on an approach to better understand the dark current-voltage (I-V) behavior in multijunction solar cells and its effect on conversion efficiency. This technique is based on determining the impact of dark-current behavior within individual p-n junctions on monolithic triple-junction GaInP/sub 2//GaAs/Ge solar cell performance. The GaInP/sub 2//GaAs/Ge tandem solar cells used in this study were developed, in part, under the US Air Forces Manufacturing Technology (ManTech) Program and exhibited measured efficiencies of 24-25% (AM0).

A Thermal Model for Carbon Nanotube Interconnects

In this work, we have studied Joule heating in carbon nanotube based very large scale integration (VLSI) interconnects and incorporated Joule heating influenced scattering in our previously developed current transport model. The theoretical model explains breakdown in carbon nanotube resistance which limits the current density. We have also studied scattering parameters of carbon nanotube (CNT) interconnects and compared with the earlier work. For 1 µm length single-wall carbon nanotube, 3 dB frequency in S12 parameter reduces to ~120 GHz from 1 THz considering Joule heating. It has been found that bias voltage has little effect on scattering parameters, while length has very strong effect on scattering parameters.

Analytical Current Transport Modeling of Graphene Nanoribbon Tunnel Field-Effect Transistors for Digital Circuit Design

A semi-classical and a semi-quantum current transport model for p-i-n n-type armchair graphene nanoribbon tunnel field effect transistor (TFET) are studied analytically. The results are compared with the numerical quantum transport simulation method using an atomistic Schrodinger-Poisson solver within the non-equilibrium Green function (NEGF) formalism. The channel length and width are 20 and 4.9 nm and a-GNR band gap is 0.289 eV. Current ratio ION/IOFF at 0.1 V supply voltage is calculated as follows: 122, 16.3 and 116 with a subthreshold slopes 26, 69 and 27.4 mV/decade from semi-classical, semi-quantum and NEGF simulation, respectively. Performance of a-GNR TFET is also studied analytically and numerically considering a-GNR width variation. Voltage transfer characteristics of a-GNR TFET inverter are computed for 0.1 V and 0.2 supply voltages using three current transport models which are in close agreement.

Experimental investigation of multi-AMTEC cell ground demonstration converter systems based on PX-3 and PX-5 series AMTEC cells

Two first generation power conversion systems which utilize multiple AMTEC cells (based on PX-3 or PX-5 series cell designs) have been studied. The experimental setup and results are discussed. The experiments demonstrated the feasibility of AMTEC cells operated in a series connected circuit, while surfacing several key issues which must be addressed for successful future power systems. The 8-cell converter demonstrated an efficiency of 11.12% and a maximum power of 28.4 We at a hot end temperature of 1123 K and condenser temperature of 553 K. The 4-cell converter demonstrated an efficiency of 10.5% and a maximum power of 12.54 We at a heater input of 120 We and a condenser temperature of 535 K.

Effects of dimensional nanoscaling on the optical and electrical properties of crystalline Si thin films

Thin film Si structures between 10 and 200nm in thickness and configured into two terminal metal-semiconductor-metal structures have been characterized for optical and electrical properties. Dark currents, spectral response, dc quantum efficiency, and ultrafast time response up to 400nm femtosecond laser illuminations at low fields have been studied. Dark currents and dc photocurrent measurements showed an increase in the film conductivity between 75 and 35nm, suggesting an increase in the carrier effective velocities due to confinement. An increase in the carrier effective velocity below 75nm was also confirmed through the transient response analysis. The measured spectral responses are in good agreement with Fresnel’s theoretical model for thin film coupling. The electron-limited transient signal has a full width at half maximum (FWHM) approximately 40ps for the 10nm Si film as compared to 490ps for a 200nm structure. For a hole-limited transit time signal the FWHM was about 82ps for the 10nm film as co...

Monolithic crystalline multijunction solar cell development and analysis at the US Air Force research laboratory

As satellite payload electrical power system requirements continue to grow, satellite systems employing flat panel arrays have reached limits set by either on-orbit dynamics that limit the size and shape of the deployed array, mass constraints set by the launch vehicle, or by the limits set by the volume constraints of the launch shroud. This has caused several satellite programs to approach power margin limits early in the design cycle, and to either compromise on satellite capabilities or perform costly redesigns. A very leveraging parameter for raising satellite power levels and reducing costs is the efficiency of the solar cells employed by satellite systems. State of the art efficiencies have reached 26.5% efficiency at load, and 30.1% for prototype cells, and solar arrays using GaAs based multijunction solar cells have achieved deployed solar array power densities of 70 W/kg and stowed volume power densities of 8 kW/m3. A simplified approach to the unwieldy dark current electrical analysis of multijunction solar cells has been developed, correlated with the performance of dual and triple junction solar cells, and explains ideality factors and reverse saturation currents that appear large. It was found that introducing a fourth junction with modest performance could raise the efficiency of multijunction solar cells to 31.5% efficiency at load, raise total power levels to 22 kW, raise the power densities to 100 W/kg and 9 kW/m3 with no impact to the configuration or operation of satellite solar arrays.

Modeling of the X-irradiation Response of the Carrier Relaxation Time in P3HT:PCBM Organic-Based Photocells

Initial experimental work has demonstrated that x-ray bombardment of organic-based photocells (specifically P3HT:PCBM-based) leads to a reduction in the open-circuit voltage (Voc) without apparent change in the carrier relaxation time. The variation of Voc was suggested to be due to the injection and trapping of holes near the anode, which resulted in a decrease in the built-in potential. We have extended the experimental measurements to higher total dose (~1300 krad(SiO2)). Using standard inorganic modeling tools, a device model of the organic cell has been developed and predictions made. These predictions have been compared to the results of the previous and new experimental measurements and they demonstrate reasonable agreement between the two, thereby supporting the initial charge buildup hypothesis. Questions about the origin and behavior of the photo-carrier relaxation arise.

Super-alloy, AMTEC cells for the pluto/express mission

A number of design changes for improving the performance of super-alloy AMTEC cells, and options of integrating them to General Purpose Heat Source (GPHS) modules in the configuration proposed by Advanced Modular Power Systems were investigated, for satisfying the electric power requirements for the Pluto/Express mission. A power system consisting of 6 fresh-fuel GPHS modules and 48, 7-tube super-alloy cells connected electrically in 4 parallel strings would weight 28.75 kg and provide 174 We at EOM, at an efficiency of 13.2%. The cells’ BASE brazes and evaporator temperatures would be below 1059 K and 971 K, respectively. A power system consisting of 5 fresh-fuel GPHS modules and 4 parallel strings of 8, 9-tube cells each would deliver 156 We at EOM and weight only 24.53 kg. In this system, the AMTEC cells would also operate at low BASE brazes and evaporator temperatures (1071 K and 995 K, respectively).

Directions in US Air Force space power technology for global virtual presence

Recent trends in the development of high efficiency, light-weight, compact, reliable and cost-effective space power technologies needed to support the development of next-generation military and commercial satellites will be discussed. Development of new light-weight and reduced volume electrical power system (EPS) technologies are required to enable the design of future “smallsats” with power requirements less than 1500W, to “monstersats” having projected power levels ranging from 10–50kW for commercial communication and military space based radar type satellites. In support of these projected requirements a complement of power generation, power management and distribution, and energy storage technologies are under development at the Air Force Research Laboratory’s Space Vehicles Directorate. The technologies presented in this paper include high efficiency multijunction solar cells, alkali metal thermal electric converters (AMTEC), high-voltage (70–130V)/high-efficiency/high-density power management and ...

Alkali metal thermal-to-electric converter development

Initial impedance measurements of multi-BASE tube AMTEC cells have been accomplished. These measurements were based on theoretical analyses performed by the Jet Propulsion Laboratory. These factors are derived from a plot of the apparent charge transfer resistance, Ract, as a function of the electrical potential across the beta alumina solid electrolyte. These include the morphology factor, G, the exchange current J00, and α, the transfer coefficient. The factor G was given a value of 50 as a result of the inability of present equipment to achieve 1.6 V across a single electrolyte element. Average values for the temperature independent charge exchange factor, B, are also reported. These results were taken from cells with as few as 3300 h to as many as 12,000 h of operation. The results show that if the value for G is somewhat close to actual values, then J00 and α are much lower than those reported in previous literature, resulting in an increase in the charge transfer resistance.