Low-energy proton irradiation effects on GaAs/Ge solar cells

This paper reports the low-energy proton irradiation effects on GaAs/Ge solar cells for space use. The proton irradiation experiments were performed with a fluence of 1.2×10¹³ cm⁻², energies ranging from 0.1 to 3.0 MeV. The results obtained demonstrate that the irradiation with a proton energy of 0.3 MeV gives rise to the most degradation rates of I_sc, V_oc and P_max of the solar cells with no coverglass, which is related to the proton irradiation-induced vacancies near the pn junction in GaAs/Ge cells. The degradation rates of I_sc, V_oc and P_max of the solar cells with coverglass increase as the proton energy increases due to the cascade ions induced by collision processes. It is found that the coverglass has an obvious protection effect against the irradiation with the proton energy below 0.5 MeV.     

Measurement of silicon and GaAs/Ge solar cells ac parameters

The ac parameters (cell capacitance and cell resistance) of Silicon (Si) and Gallium Arsenide (GaAs/Ge) solar cells are measured at different temperatures using time domain technique. The cell capacitance is calculated from the Open circuit voltage decay (OCVD) and the cell resistance from solar cell I–V characteristics measured under dark condition. It is observed that the solar cell capacitance increases whereas the cell resistance decreases with increase in temperature.     

GaAs/Ge solar cell AC parameters under illumination

The ac parameters of GaAs/Ge solar cell were measured under illumination at different cell temperatures using impedance spectroscopy technique. They are compared with the dark measurements. It is found that the cell capacitance is higher and cell resistance is lower under illumination than in dark for all cell terminal voltages. The cell capacitances at the corresponding maximum power point voltage (terminal) do not vary with temperature where as the cell resistance decreases. The cell capacitance under illumination is estimated from the dark cell capacitance and it is in good agreement with the measured illumination data.     

Measurement of silicon and GaAs/Ge solar cell device parameters

The device parameters (carrier lifetime, ideality factor), and physical parameters (built-in voltage, doping concentration) of silicon (Si) and gallium arsenide (GaAs/Ge) solar cells are measured at different temperatures using time domain technique. Carrier lifetime is calculated from open circuit voltage decay (OCVD). Built-in voltage and doping concentration are derived from the cell capacitance measured at different bias voltages. Ideality factor is derived from the I–V characteristics of solar cell. Carrier lifetime increases while built-in voltage decreases with increase in temperature. Ideality factor of the solar cell decreases with temperature.     

Characteristics of GaAs solar cells on Ge substrate with a preliminary grown thin layer of AlGaAs

Characteristics of GaAs solar cell on Ge substrate with a new buffer layer structure is reported. The buffer layer structure, which consisted of a preliminarily grown thin layer of A1_xGa_1−xAs and a 1 μm thick GaAs layer, was designed to obtain a high quality GaAs layer on Ge substrate by metalorganic chemical vapor deposition (MOCVD). Performance of a GaAs solar cell fabricated on Ge substrate with the buffer layer structure was compared with that fabricated on Ge substrate with a conventional GaAs buffer layer and also that fabricated on GaAs substrate. A conversion efficiency of 23.18% (AM1.5G) was successfully obtained for the cell fabricated on Ge substrate with the new buffer layer structure, while it was 20.92% for the cell fabricated on Ge substrate with the conventional GaAs buffer layer. Values of V_oc and J_sc, for the cell fabricated on Ge substrate with the new buffer layer structure were approximately comparable to those of a 25.39% efficiency GaAs solar cell fabricated on GaAs substrate.     

GaAs/Ge solar cell AC parameters at different temperatures

The AC parameters of Gallium Arsenide (GaAs/Ge) solar cell were measured at different cell temperatures (198–348 K) by varying the cell bias voltage (forward and reverse) under dark condition using impedance spectroscopy technique. It was found that the cell capacitance increases with the cell temperature where as the cell resistance decreases, at any bias voltage. The measured cell parameters were used to calculate the intrinsic concentration of electron–hole pair, cell material relative permittivity and its band gap energy. The diode factor and the cell dynamic resistance at the corresponding maximum power point decrease with the cell temperature.     

Electron irradiation and thermal annealing effect on GaAs solar cells

This paper describes the effect of electron irradiation and thermal annealing on LPE AlGaAs/GaAs heterojunction solar cells with various p/n junction depths. The electron irradiation experiments were performed with energy of 3 MeV, fluences ranging from 1×10¹⁴ to 5×10¹⁵ e/cm². The results obtained demonstrate that the irradiation-induced degradation of performances of the cells is mainly in the short circuit current and could be mostly recovered by annealing at 260°C for 30 min. Four electron traps, E_c−0.24 eV, E_c−0.41 eV, E_c−0.51 eV, E_c−0.59 eV, were found by DLTS analysis, only two shallow levels of which could be removed by the annealing.     

Catalytic hydrogenation for improvement of GaAs/Ge solar cell efficiency

Hydrogen passivation on MOCVD grown p-GaAs epilayers on Ge substrate have been studied by plasma and catalytic hydrogenation and the results were compared. The conversion efficiency of the GaAs/Ge solar cells was found to increase by 10% after catalytic hydrogenation at AM1.5. This increase in efficiency is probably due to passivation of surface dangling bonds.     

Degradation behaviors of electrical properties of GaInP/GaAs/Ge solar cells under <200 keV proton irradiation

The degradation effects of the GaInP/GaAs/Ge triple-junction solar cells irradiated by <200 keV protons are investigated on the basis of the spectral response analysis and measurements of electric property. The experimental results show that with increasing proton fluence I_sc, V_oc and P_max decrease obviously. The proton energy exhibits an important influence on the degradation effects of the triple-junction cells dependent on the proton penetration range in the cells. As the proton energy is lower than 100 keV, irradiation-induced damage occurs in the top cell, while the irradiation with proton energy higher than 100 keV causes damage mainly in the middle sub-cells. Comparing the changes in the electrical properties of the triple-junction cells, a conclusion can be made that the GaAs middle sub-cell plays a major role in leading to more severe degradation. In this case, the 170 keV protons are suggested to be used to evaluate the performance of the GaAs triple-junction solar cells, for they can produce more severe degradation effects.     

1 MeV electron irradiation influence on GaAs solar cell performance

The results of experimental study of radiation resistance of GaAs-based solar cells are presented. The solar cells were irradiated by 1 MeV electrons at room temperature with the fluence up to . The radiation influence on the dark current and short-circuit current under illumination was investigated both experimentally and theoretically. It is shown that the radiation-produced electron traps E5 and hole traps H1 are responsible for irradiation-induced degradation of such solar cells. The radiation tolerance of the basic parameters (the short-circuit current, the output power) of GaAs solar cells is primarily determined by the radiation damage in p-regions.     

Radiation-resistant solar cells for space use

This paper reviews the present status of radiation-resistant solar cells made with Si, GaAs, InP and InGaP/GaAs for space use. At first, properties of radiation-induced defects in semiconductor materials and solar cells are described based on an anomalous degradation of Si space solar cells under high-energy, high-fluence electron and proton irradiations. Advantages of direct bandgap materials as radiation-resistant space cells are presented. Unique properties of InP as radiation-resistant cells have also been found. A world-record efficiency of 26.9% (AM0) has been obtained for an InGaP/GaAs tandem solar cell. Radiation-resistance of the InGaP/GaAs tandem cells is described.     

On the effects of tilted light in a global prediction of AlGaAs/GaAs solar cell performance

A global optimisation of AlGaAs/GaAs solar cells under light with different angles of incidence is carried out in this work. Not only are the optimum internal and external device structures calculated, but also the thicknesses of the anti-reflecting coatings layers. For this reason, a study of the whole structure is carried out from a global point of view taking state-of-the-art technology into account. In fact, this kind of work has not been carried out up till now. The best structure, working at 1000 suns (100 W/cm²), reaches an efficiency of 28.8% for normal incidence of light. This efficiency is nearly constant up to an incident angle of around 60° and it then sharply decreases for higher angles. Variations of parameters that can be optimised (thicknesses and doping levels) as a function of angle of incidence ranging from 0° to 90° are presented. Deviations from the optimum structure are also analysed with the purpose of determining their influence in the solar cell manufacturing process.     

Characterization of antiphase domains on GaAs grown on Ge substrates by conductive atomic force microscopy for photovoltaic applications

The role of antiphase domains formed on GaAs grown on Ge is analyzed by means of conductive atomic force microscopy. The correlation of the derivative topography scans with the conductive scans shows a constant current value in most of the surface under study; although at certain locations high current leaks occur causing an inhomogeneous conductivity through the GaAs layer as the density of antiphase domains increases. This result implies that the existence of antiphase domains decreases the parallel resistance of solar cells, helping to understand the impact of these defects on the electrical behavior of these devices     

Optimal growth procedure of GaInP/GaAs heterostructure for high-efficiency solar cells

We have developed an optimal growth procedure for gas-source MBE production of a GaInP/GaAs heterointerface. The interface quality is crucial to obtaining high-performance GaAs solar cells with a GaInP barrier layer because minority carrier lifetime depends strongly on the interface structure. In situ Reflective High-Energy Electron Diffraction (RHEED) observation during the growth across the GaInP/GaAs heterointerface revealed that the phosphorus atoms are replaced by arsenic atoms in the near-interface region of the GaInP layer, and a transient layer acting as a carrier trap is formed. Introduction of a GaP layer into the interface was found to be effective in suppressing carrier loss. From Composition Analysis by Thickness Fringe-Transmission Electron Microscopy (CAT-TEM) images, it was also found that the optimum thickness of inserted GaP to avoid the generation of misfit dislocations is 1 nm.     

High-voltage (1.8 V) tandem solar cell system using a GaAs/AlXGa(1−X)As graded solar cell and dye-sensitised solar cells with organic dyes having different absorption spectra

Stacked multijunction (tandem) solar cells have been prepared by mechanically stacking dye-sensitised solar cells (DSCs) and a GaAs/Al_XGa_(1−X)As graded solar cell (GGC) as the top and bottom cells, respectively. Three organic dyes with different absorption spectra (D131, D102 and D205) were used in the DSCs, in order to match the photocurrent density between the DSC and the GGC. Tuning the absorption range of the DSC by choosing an appropriate dye, increased the overall photovoltaic conversion efficiency due to the optimal utilisation of the solar spectrum in the individual cells. The open circuit photovoltages (V_OC) of the GGC and the DSC with D131 were 1.11 V and 0.76 V, respectively, resulting in a V_OC of 1.85 V and a photovoltaic conversion efficiency of 7.63% for the tandem cell. Although the overall conversion efficiency has not exceeded that of the GGC (7.66%), these tandem cells provide adequate V_OC values for water splitting applications.     

High efficiency AlxGa1−xAs/GaAs solar cells: Fabrication, irradiation and annealing effect

High efficiency Al_xGa_1−xAs/GaAs heteroface solar cells have been fabricated by an improved multi-wafer squeezing graphite boat liquid phase epitaxy (LPE) technique, which enables simultaneous growth of twenty 2.3 × 2.3cm² epilayers in one run. A total area conversion efficiency of 17.33% is exhibited (1sun, AMO, 2.0 × 2.0cm²). The shallow junction cell shows more resistance to 1 MeV electron radiation than the deep one. After isochronal or isothermal annealing the density and the number of deep level traps induced by irradiation are reduced effectively for the solar cells with deep junction and bombardment under high electron fluences.     

A detailed study of H2 plasma passivation effects on GaAs/Si solar cell

The hydrogen plasma passivation effects of MOCVD-grown GaAs solar cell on Si substrate have been studied in detail. To get a more reproducible increase of conversion efficiency and test the thermal stability of the plasma-exposed GaAs/Si solar cell, both the plasma exposure and post-passivation annealing conditions were optimized. Annealing the H₂ plasma passivated GaAs/Si solar cell at 450°C in AsH₃/H₂ ambient seems a very essential parameter to restore the carrier concentration, especially, without losing the beneficial effects of H incorporation into GaAs on Si. For the H₂ plasma passivated GaAs/Si solar cell, a highest conversion efficiency of 18.3% was obtained compared with that of the as-grown cell (16.6%) due to the H passivation effects on nonradiative recombination centers, which increased the minority carrier lifetime.     

Mechanism for the anomalous degradation of Si solar cells induced by high-energy proton irradiation

High-energy and high-fluence proton irradiation of Si space solar cells has provoked an anomalous increase in short-circuit current, followed by its abrupt decrease and cell failure. A model is proposed which explains the phenomena by expressing a reduction in the carrier concentration of the base region, in addition to a decrease of minority-carrier diffusion length. The reduction in carrier concentration due to majority-carrier trapping by radiation-induced defects has the effect of (1) broadening the depletion region width and (2) increasing the resistivity of the base layer. The anomalous change in the quantum efficiency of the cells under high-fluence ( 10¹⁴cm⁻²) irradiation is also explained by considering the generation of a donor-type defect level with the irradiation.     

Computational modelling of the reflectivity of AlGaAs/GaAs and SiGe/Si quantum well solar cells

In this paper the modelling of the reflectivity of two quantum well solar cells (QWSC) are theoretically developed and computationally analysed. The new reflectivity model is based on the Modified Single Effective Oscillator model combined with Fresnel's equation. The model takes into consideration the effects of the design parameters including concentration levels, structural properties of the device (well length, etc.), operating temperature and electric field effects. The results generated are for a bare AlGaAs/GaAs cell and the same cell with a ZnS antireflection coating (ARC). Further investigations include a bare SiGe/Si cell and the same cell with a Ta₂O₅ ARC. The results generated are accurate and match with experimental data for similar cells. The analysis is performed for AM 1.5 spectrum. The model is intended to be an aid to QWSC designers.     

Thin film silicon solar cells for space applications: Study of proton irradiation and thermal annealing effects on the characteristics of solar cells and individual layers

The paper reports on the effects of a proton irradiation campaign on a series of thin-film silicon solar cells (single- and double-junction). The effect of subsequent thermal annealing on solar cells degraded by proton irradiation is investigated. A low-temperature annealing behaviour can be observed (at temperatures around 100 to 160°C) for microcrystalline silicon solar cells. To further explore this effect, a second proton irradiation campaign has been carried out, but this time on microcrystalline silicon layers. The effect of proton irradiation and subsequent thermal annealing on the optical and electronic properties of microcrystalline silicon is, thus, thoroughly investigated.