Christel Nömayr’s research while affiliated with Airbus Defence and Space and other places

The solar arrays of spacecrafts are subjected to a severe radiative environment. Associated degradation rates are dependent on operational orbit configuration. The current approach used in models and tools to determine power degradation shall thus be validated to ensure good prediction accuracy. This paper reports on extensive irradiation test campaigns realized on triple‐junction solar cells to compare degradation induced by different configurations of irradiation reproducing nominally the same displacement damage dose (DDD) in the active layer. These test campaigns data are used to support the validation of a new tool developed to optimize prediction accuracy of solar cell degradation. The DDD equivalence between front/rear irradiation on solar cell assemblies and on bare solar cells, between different electron and proton energies, as well as the verification of the superposition principle are addressed. Results are conclusive regarding the equivalence of degradation obtained with electrons, protons and/or combined electrons and protons, accounting for local shielding.

The Bepi Colombo spacecraft was successfully launched on 20th October 2018 and is now on its way to Mercury. The materials and coatings on the Mercury Planetary Orbiter (MPO) and Mercury Transfer Module (MTM) solar arrays will need to withstand a prolonged extreme environment of high-intensity UV radiation at high-temperature (HIHT). The solar arrays must be gradually off-pointed at close sun distances in order to avoid over-heating, up to an angle of over 70° at 0.3 AU for the MPO. This means that some of the materials and coatings, as well as the solar cells, will always be operating reasonably close to their temperature limits. Extensive on-ground testing was performed to qualify the solar arrays for the BepiColombo mission environment and during this testing some materials and coatings were pushed beyond acceptable performance limits. In this paper we give an overview of selected test programmes related to the solar array materials, and we present results of experimental investigations which provide some scientific insight into the likely causes of the materials degradation observed, and the lessons learnt in the cases where over-testing had occurred. This included testing on bare and bonded optical solar reflectors, solar cell cover glass and adhesive, and the solar array edge shields.

GaAs single junction cells, representative of the middle cell in triple junction Ga 0.5In 0.5P/GaAs/Ge cells, were irradiated with various fluences of 1- and 3-MeV electrons as well as 1-MeV protons. The light I-V curves measured at room temperature exhibit a voltage-dependent photocurrent. The photocurrent is modeled taking into account the voltage-dependent width of the space charge region in combination with a strongly decreased minority carrier diffusion length. By extracting the width of the space charge region from capacitance measurements and the base layer diffusion length from the external quantum efficiency of the cell, the experimental behavior is reproduced accurately.

A method of making charge dissipative both surfaces of Flexible Cable Connectors (FCCs), used for Solar Arrays in GEO, by specially developed ion beam surface treatment is described in this study. FCC’s made of dielectric space polymer Kapton100HN with shaped/grooved surfaces and back surfaces containing embedded inorganic particles have been made charge dissipative, with required values of surface resistivity and in a wide space-defined temperature range. The method comprises controllably carbonizing the surface of the polymer-based material in a vacuum environment with simultaneous surface renewal, by bombarding the surface with an ion beam from a linear gaseous high-current technological ion beam source of back gas with added required amount of a carbonaceous gas for simultaneous carbonized surface renewal in a dynamic way. Surface resistivity values of ~10 MΩ/square at room temperature were achieved. The treated FCCs kept the surface charge dissipation properties at least in the space-related temperature range of ±150 °C, have been shown to be resistant to thermal cycling, being durable, radiation resistant and surface charge dissipative in imitated GEO environment for at least 15 years of GEO space equivalent, with the influence on the final thermal optical properties almost the same as on original FCCs surfaces.

Characteristic degradation curves for proton and electron induced degradation of triple junction (3J) and isotype Ga0.5In0.5P/GaAs/Ge solar cells were obtained. The displacement damage dose methodology in combination with a varying effective threshold energy for atomic displacement Td,eff was used to analyze 3G28 and 3G30 3J cell data. The nonionizing energy loss (NIEL) was calculated analytically, and Td,eff was explicitly introduced as a fit parameter. Using the GaAs NIEL in fitting the 3J degradation data, a Td,eff of 21 eV was determined, whereas a Td,eff of 36 eV was found using the Ga0.5In0.5P NIEL. In GaAs and Ga0.5In0.5P single junction cells, the effective threshold energies for atomic displacement of 22 and 34 eV were determined. Copyright

BepiColombo is the joint mission of the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA) to explore the planet mercury. The European contributions, namely the mercury transfer module (MTM) and the mercury planetary orbiter (MPO), are both powered by deployable solar arrays. Many materials and technologies are at their limit under the harsh high-intensity, high-temperature (HIHT) conditions of the mission. Synergistic effects like photo fixation and photo enhanced contamination by ultra violet and vacuum ultra violet radiation (UV/VUV) on sunlit surfaces are considered to play an important role in the HIHT environment of the BepiColombo mission. A design verification test under UV/VUV conditions of sun exposed materials and technologies on component level is presented which forms part of the overall verification and qualification of the solar array design of the MTM and MPO. The test concentrates on the self-contamination aspects and the resulting performance losses of the solar array under high intensity and elevated temperature environment representative for the photovoltaic assembly (PVA).

Results of making surfaces of flexible cable conductors, used for solar arrays in geostationary orbit, charge dissipative by a specially developed method of ion-beam surface treatment with simultaneous surface renewal are presented. All flexible cable conductor surfaces, including those with rough morphology and foreign inclusions, achieved surface resistivity of ∼10–20 MΩ/sq. at room temperature and kept charge dissipation in a wide ±150 °C space-related temperature range. A testing space qualification program confirmed all treated flexible cable conductors being resistant to thermal cycling and humidity, mechanically durable, and radiation resistant in geostationary orbit. All flexible cable conductors successfully survived 15 years space-equivalent geostationary orbit imitating testing. A comparison of beginning- and end-of-life characteristics of original and treated flexible cable conductors after this testing confirmed additional decrease of surface resistivity and almost equal final thermal-optical characteristics. Electrostatic discharge testing in a geostationary orbit charging testing facility demonstrated the long-lasting charge-dissipation properties of the treated flexible cable conductors, fully preventing them from building up critical charges in geostationary orbit environment, when a reliable grounding path is ensured for the surfaces.

Scanning transmission ion microscopy (STIM) has been applied to measure sputter yields of thin Kovar foil. The results have been found in very good agreement with values determined by the weight loss method, demonstrating STIM as a feasible alternative measurement technique for sputter yield estimation of thin material samples. Measurements have been carried out under normal xenon ion incidence for ion energies in the range between 100 eV and 1000 eV. In addition, sputter yields of Kovar bulk samples are reported. The data might be interesting for ion beam applications such as solar electric propulsion, in which materials with low sputter yields are preferred to ensure a long operational lifetime of the system components.

The harsh environment at mercury during the BepiColombo mission results in temperature ranges for the photovoltaic assembly (PVA) up to 200°C in combination with 5 to 11 times the air mass zero (AM0) solar spectrum. Standard solar cell assembly (SCA) technologies representative of the PVA materials and design could not met the required performance in tests simulating the high intensity, high temperature (HIHT) environment. Independently of the injected state of the art triple junction technologies maximum power losses of up to 20% were observed after only several hundreds of test hours. Systematic tests revealed that the degradation was linked to the interaction of the solar cell assembly materials. The formation of a shunt in the top cell was identified as a root cause for the degradation of the electrical performance. It was possible to prevent this effect by means of a modified cell design.

The behavior of standard space photovoltaic assemblies in a high intensity, high temperature environment (HIHT) is addressed. Experimentally, an HIHT environment, typical for missions to the inner planets of the solar system such as Mercury, characterized by temperatures of 500K and 11 solar constant irradiance in the ultraviolet region below 400 nm, was simulated in a vacuum. Independently of the triple junction cell technology used, module degradation up to 20% in power was observed during several hundred hours of test. Electroluminescence analysis identified discrete top cell shunts close to the cell edge, in particular around the frontside contact pads. Cross-sectional transmission electron microscopy performed on several degraded cells revealed an etched contact pad metallization/cap layer interface and more importantly, several 100-nm large, oriented Cu3P inclusions at the shunted locations. A chemical degradation mechanism is proposed. Short wavelength ultraviolet light interacting with polysiloxanes used as module encapsulant produces hydrogen and methyl radicals. With these building blocks, an organic acid can be formed on external reaction surfaces such as the Ag busbars that simultaneously serve as a source of oxygen. Cu traces present in the Ag segregate to the surface and are transported by this acid to the contact pad of the cell in the liquid phase. An adapted cell design was developed to prevent this degradation mechanism believed to be of relevance for all HIHT space environments. A several hundred micrometer-wide rim composed of the outermost cell area is electrically separated from the inner cell area and provides a barrier against environmental attack. None of the photovoltaic assemblies featuring this mesa cell design showed any fill factor-induced power degradation any more. Copyright © 2011 John Wiley & Sons, Ltd.