David Scheiman

Mobile Solar Power

The militarys need to reduce both fuel and battery resupply is a real-time requirement for increasing combat effectiveness and decreasing vulnerability. Mobile photovoltaics (PV) is a technology that can address these needs by leveraging emerging, flexible space PV technology. In this project, the development and production of a semirigid, lightweight, efficient solar blanket with the ability to mount on, or stow in, a backpack and recharge a high-capacity rechargeable lithium-ion battery was undertaken. The 19% efficient blanket consists of a 10 × 3 solar array of 20 cm2 and single-junction epitaxial lift-off solar cells, which have an efficiency of ∼22% under AM1.5G illumination. A power-conditioning module was also developed to interface the solar panel to the battery. Thirteen systems were outfitted during a Limited Objective Experiment-1 in February 2012, and based on the results, a second version of the system is in development.

Concentrator photovoltaic module architectures with capabilities for capture and conversion of full global solar radiation

Significance Concentrator photovoltaic (CPV) systems, wherein light focuses onto multijunction solar cells, offer the highest efficiencies in converting sunlight to electricity. The performance is intrinsically limited, however, by an inability to capture diffuse illumination, due to narrow acceptance angles of the concentrator optics. Here we demonstrate concepts where flat-plate solar cells mount onto the backplanes of the most sophisticated CPV modules to yield an additive contribution to the overall output. Outdoor testing results with two different hybrid module designs demonstrate absolute gains in average daily efficiencies of between 1.02% and 8.45% depending on weather conditions. The findings suggest pathways to significant improvements in the efficiencies, with economics that could potentially expand their deployment to a wide range of geographic locations. Emerging classes of concentrator photovoltaic (CPV) modules reach efficiencies that are far greater than those of even the highest performance flat-plate PV technologies, with architectures that have the potential to provide the lowest cost of energy in locations with high direct normal irradiance (DNI). A disadvantage is their inability to effectively use diffuse sunlight, thereby constraining widespread geographic deployment and limiting performance even under the most favorable DNI conditions. This study introduces a module design that integrates capabilities in flat-plate PV directly with the most sophisticated CPV technologies, for capture of both direct and diffuse sunlight, thereby achieving efficiency in PV conversion of the global solar radiation. Specific examples of this scheme exploit commodity silicon (Si) cells integrated with two different CPV module designs, where they capture light that is not efficiently directed by the concentrator optics onto large-scale arrays of miniature multijunction (MJ) solar cells that use advanced III–V semiconductor technologies. In this CPV+ scheme (“+” denotes the addition of diffuse collector), the Si and MJ cells operate independently on indirect and direct solar radiation, respectively. On-sun experimental studies of CPV+ modules at latitudes of 35.9886° N (Durham, NC), 40.1125° N (Bondville, IL), and 38.9072° N (Washington, DC) show improvements in absolute module efficiencies of between 1.02% and 8.45% over values obtained using otherwise similar CPV modules, depending on weather conditions. These concepts have the potential to expand the geographic reach and improve the cost-effectiveness of the highest efficiency forms of PV power generation.

Quantum-Well Solar Cells for Space: The Impact of Carrier Removal on End-of-Life Device Performance

In this paper, a detailed analysis on the radiation response of solar cells with multi quantum wells (MQW) included in the quasi-intrinsic region between the emitter and the base layer is presented. While the primary source of radiation damage of photovoltaic devices is minority carrier lifetime reduction, we found that in the case of MQW devices, carrier removal (CR) effects are also observed. Experimental measurements and numerical simulations reveal that with increasing radiation dose, CR can cause the initially quasi-intrinsic background doping of the MQW region to become specifically n- or p-type. This can result in a significant narrowing and even the collapse of the electric field between the emitter and the base where the MQWs are located. The implications of the CR-induced modification of the electric field on the current-voltage characteristics and on the collection efficiency of carriers generated within the emitter, the MQW region, and the base are discussed for different radiation dose conditions. This paper concludes with a discussion of improved radiation hard MQW device designs.

Characterization of high fluence irradiations on advanced triple junction solar cells

Reported is the characterization of irradiated InGaP2/GaAs/Ge multijunction (MJ) solar cells using the cathodoluminescence (CL) imaging/spectroscopy and electron beam induced current (EBIC) modes of scanning electron microscopy (SEM). These techniques were applied to verify the influence of radiation damage on the optoelectronic properties of each subcell in the monolithic triple junction structure and correlate them with the illuminated (AM0, 1 sun, 25°C) current-voltage (IV) and quantum efficiency (QE) characteristics.

Modeling and analysis of high-performance, multicolored anti-reflection coatings for solar cells

In this work solar cell anti-reflection coatings tuned to give a specific hue under solar illumination are investigated. We demonstrate that it is possible to form patterned coatings with large color contrast and high transmittance. We use colorimetric and thin film optics models to explore the relationship between the color and performance of bilayer anti-reflection coatings on Si, and predict the photocurrent generation from an example Si solar cell. The colorimetric predictions were verified by measuring a series of coatings deposited on Si substrates. Finally, a patterned Si sample was produced using a simple, low-cost photolithography procedure to selectively etch only the top layer of a bilayer coating to demonstrate a high-performance anti-reflection coating with strong color contrast.

Analysis of radiation hardness and subcell I-V characteristics of GaInP/GaAs/Ge solar cells using electroluminescence measurements

The voltage degradation of GaInP/GaAs/Ge triple-junction solar cells after exposure to proton irradiation is analyzed using electroluminescence (EL) measurements. It is shown that EL measurements in combination with the reciprocity relationship allow accurate determination of the degradation of the open-circuit voltage (Voc) of each individual subcell. The impact of different proton energies on the voltage degradation of each subcell is analyzed. For solar cells exposed to extremely high radiation levels, a correlation between the degradation of the quantum efficiency of the Ge subcell and its EL properties is presented.

High efficiency flexible triple junction solar panels

The Marines have increasing battery needs as fighting technology puts higher demands on the power they use. In an effort to offset this demand, the marines are investigating alternative energy sources, one being solar power. Mobile photovoltaics (PV) are a technology that can address these needs by leveraging flexible high efficiency III-V photovoltaic technology. The development of a lightweight, high efficiency solar panel to mount on, or stow in, a backpack and used to recharge a warfighters battery was demonstrated. The panel consists of a 10 × 3 solar array of 20 cm2 epitaxial lift-off (ELO) Inverted Metamorphic (IMM) triple junction solar cells. In the first two phases of the project, single-junction GaAs cells with an efficiency of ~ 21% under AM1.5 illumination were used. Several of these systems were outfitted during Limited Objective Experiments (LOE) in February 2012 and August 2012. In the third and most current phase of this project, panels of triple-junction cells with an expected efficiency of 28-30% under AM1.5 illumination. Data from these LOEs are presented here. Although the panels are expensive, they have been demonstrated as a viable technology.

Transparent conducting oxide-based, passivated contacts for high efficiency crystalline Si solar cells

In this work, we investigate a transparent conducting oxide (TCO)-based, passivated contact for the potential use as a passivated tunnel contact to p-type Si. As a surface passivation layer, the Al₂O₃ films with varying the thickness are deposited using plasma-enhanced atomic layer deposition (PEALD) at 200 °C, followed by post-deposition annealing. For a ~15 nm thick Al₂O₃ layer, a high level of surface passivation is achieved, characterized by the effective surface recombination velocity (S_eff,max) of <;30 cm/s. The samples with ultrathin Al₂O₃ layer <;3 nm, however, shows degradation in passivation quality, reaching the S_eff,max<;500 cm/s. When Al-doped zinc oxide (ZnO:Al) as TCO contact is directly deposited onto a ~10.6 nm thick Al₂O₃ coated p-Si via RF magnetron sputtering, the final passivation quality (p-Si/Al₂O₃/ZnO:Al) is characterized by the saturation current density at contact (J_0,contact) of 92.1 fA/cm² with the implied open-circuit voltage (iVoc) of 653 mV, showing the passivation quality is not severely degraded after sputtering without thermal treatment. Further process optimization of PEALD is in progress to produce an improved quality of surface passivation with the S_eff,max<;10 cm/s for ultrathin passivation layers less than 2 nm, enabling a passivated tunneling contact.

Detailed Characterization of the Radiation Response of Multijunction Solar Cells Using Electroluminescence Measurements

The response of triple-junction solar cells to proton and electron irradiation is analyzed using electroluminescence (EL) measurements. This analysis allows the dark current of each individual subjunction to be determined providing insight into the radiation response mechanisms.

High fluence irradiations on triple junction solar cells

Triple junction GaInP/InGaAs/Ge solar cells were irradiated with 3 MeV protons to extreme fluences as high as 1015 p+/cm2. In this paper, IV and QE results will be presented and a model will be developed to explain the radiation behavior. A carrier removal damage mechanism is evident which actually aids in the radiation hardness of these devices.