Justin R. Lorentzen

Correlation of Electron Radiation Induced-Damage in GaAs Solar Cells

GaAs solar cells with different structures and polarities were irradiated with 1 and 5 MeV electrons. The energy dependence of the electron damage coefficients for the photocurrent, photovoltage, and maximum power were found to vary approximately linearly with NIEL in contrast to what has been found for other GaAs cells.

W-structured type-II superlattice-based long- and very long wavelength infrared photodiodes

W-structured type-II superlattices (W-SLs) were initially developed to increase the gain of mid-wave infrared (MWIR) lasers. The design addressed the reduced optical transition matrix elements due to the spatial displacement between valence and conduction band wavefunctions in the type-II superlattice (T2SL), and further improved the differential optical gain by providing a mostly two-dimensional density of states. As a result, W-SL and W interband cascade lasers have lower thresholds and higher pulsed and cw operating temperatures than any other III-V interband MWIR lasers. These same features give W-SLs desirable properties for IR detectors, and here we report for the first time on characteristics of W-SLs used for long-wave and very long-wave IR photodiodes. IR transmission measurements of W and conventional T2SL photodiodes revealed absorption characteristics that are well described by theory, including line shape and peak absorption coefficient values which are about a factor of 2 greater in the W-SLs. Similarly, the low temperature photoluminescence shows much higher and sharper emission intensity in the W-SLs. While the W-SLs have demonstrated superior optical properties, as predicted, additional work is needed to achieve higher detector quantum efficiency. Results suggest that the excess carrier collection in the W-structures is reduced with respect to similar T2SL structures, especially for the lowest energy state. Possible mechanisms of excess carrier loss, as well as new designs to improve charge collection, in the W-SL, will be discussed.

In Situ Irradiation and Measurement of Triple Junction Solar Cells at Low Intensity, Low Temperature (LILT) Conditions

The performance of triple junction InGaP/(In)GaAs/Ge space solar cells was studied following high energy electron irradiation at low temperature. Cell characterization was carried out in situ at the irradiation temperature while using low intensity illumination, and, as such, these conditions reflect those found for deep space, solar powered missions that are far from the sun. Cell characterization consisted of I-V measurements and quantum efficiency measurements. The low temperature irradiations caused substantial degradation that differs in some ways from that seen after room temperature irradiations. The short circuit current degrades more at low temperature while the open circuit voltage degrades more at room temperature. A room temperature anneal after the low temperature irradiation produced a substantial recovery in the degradation. Following irradiation at both temperatures and an extended room temperature anneal, quantum efficiency measurement suggests that the bulk of the remaining damage is in the (In)GaAs sub-cell.

Thin-film photovoltaic radiation testing and modeling for a MEO orbit

A radiation test plan for thin-film photovoltaic technologies focused on a MEO flight experiment is outlined. The radiation response of thin film, triple junction amorphous Si solar cells, with and without a space coating, is presented. The degradation of the photovoltaic output under 2 MeV proton irradiation is measured and analyzed. Irradiations performed both at room temperature, in the dark, and at open circuit and at elevated temperature, under illumination, and under load were performed. The experimental data are presented and analyzed. These data will form the basis for an on-orbit prediction model as applied to a high-radiation MEO orbit.

Deep level defects in proton radiated GaAs grown on metamorphic SiGe/Si substrates

The effect of 2MeV proton radiation on the introduction of deep levels in GaAs grown on compositionally graded SiGe∕Si substrates was investigated using deep level transient spectroscopy (DLTS). Systematic comparisons were made with identical layers grown on both GaAs and Ge substrates to directly assess the influence of threading dislocations on radiation-related deep levels for both n-type and p-type GaAs. DLTS revealed that for p+n structures, proton irradiation generates electron traps at Ec−0.14eV, Ec−0.25eV, Ec−0.54eV, and Ec−0.72eV in the n‐GaAs base, and, for n+p structures, radiation-induced hole traps appear at Ev+0.18eV, Ev+0.23eV, Ev+0.27eV, and Ev+0.77eV in the p-type GaAs base, irrespective of substrate choice for both polarities. The primary influence of substituting SiGe∕Si substrates for conventional GaAs and Ge substrates is on the introduction rates of the individual traps as a function of proton radiation fluence. Substantially reduced concentrations are found for each radiation-induce...

Radiation response mechanisms in multijunction III-V space solar cells

The radiation response mechanisms operative in multijunction (MJ) III-V space solar cells are described. The effects of electron and proton radiation-induced defects on the cell performance are identified, and a detailed description of the MJ solar cell radiation response is presented for several different stoichiometries. Special attention is paid to the case of low energy proton irradiation for both unidirectional, monoenergetic laboratory experiments and omni-directional, spectrum irradiation experienced in a space environment.

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.

Initial results from the second Forward Technology Solar Cell Experiment

The success of the first Forward Technology Solar Cell Experiment (FTSCE) which flew as part of the 5th Materials on the International Space Station Experiments (MISSE-5) [1,2,3], led to a follow on experiment, FTSCE II. This experiment is currently operating on the International Space Station (ISS) as part of MISSE7. Solar cells are characterized with I–V curves. The test cells include current and next generation triple-junction production cells from Emcore and Spectrolab as well as advanced inverted metamorphic (IMM) and other thin film III–V cells from Emcore, Spectrolab and MicroLink Devices, Inc. Other thin film technologies include, advanced amorphous silicon concepts from United Solar Ovonic and a Copper Indium Selenide based two-cell string from Dutch Space. In this paper, The FTSCE II experiment is described and current status presented.

Measurement and characterization of triple junction solar cells using a close matched multizone solar simulator

The photovoltaic characterization of triple-junction InGaP2/GaAs/Ge solar cells is presented. Measurements made using a single light source solar simulator are compared with other measurements made using a multi-light source solar simulator that provides a close match to the air mass zero (AM0) solar spectrum. The output spectrum of the solar simulators has been measured, and two methods for calibrating the simulator output intensity haven been employed. The spectral response of the solar cells has been characterized through quantum efficiency measurements. These data are analyzed to determine the effect of the simulator spectrum on the measured photovoltaic response, and in particular, areas where spectral mismatch between the simulator and AM0 can lead to inaccurate performance predictions are highlighted. In addition, the effects of the different calibration techniques on the measured data are studied. Exploiting the capabilities of the multi-source, close matched simulator, the response of each of the three sub-junctions are studied individually, and the interplay between the spectral response of the sub-junctions and the incident spectrum is investigated.