Chiharu Morioka

Change in the electrical performance of InGaAs quantum dot solar cells due to irradiation

PiN structure GaAs solar cells with In<inf>0.4</inf>Ga<inf>0.6</inf>As quantum dot layers are irradiated with electrons at 1 MeV in fluencies up to 3×10¹⁵ /cm³. The change in the electrical performance under AM0 and the quantum efficiency are investigated. The decrease in the open circuit voltage for the solar cells with quantum dot layers is smaller than that for GaAs PiN solar cells with no quantum dot layer, although no significant difference in the degradation of the short circuit current is observed between solar cells with and without quantum dot layers. As a result of quantum efficiency measurements, it is revealed that the currents generated by In<inf>0.4</inf>Ga<inf>0.6</inf>As dot layers still remain by 60 % of the initial value, even after 1 MeV electron irradiation at 3×10¹⁵ /cm².

Effect of Base Doping Concentration on Radiation-Resistance for GaAs Sub-Cells in InGaP/GaAs/Ge

National Institute for Astronomy and Geophysics Research, Helwan, Cairo 11421, EgyptReceived July 30, 2010; revised September 13, 2010; accepted October 1, 2010; published online December 20, 2010GaAs solar cells with the lower base carrier concentration under low energy proton irradiations had shown experimentally the better radiation-resistance. Analytical model based on fundamental approach for radiative and non-radiative recombination has been proposed for radiationdamage in GaAs sub-cells. The radiation resistance of GaAs sub-cells as a function of base carrier concentration has been analyzed by usingradiative recombination lifetime and damage coefficient for minority carrier lifetime. Numerical analysis shows good agreement with experimentalresults. The effect of carrier concentration upon the change of damage constant and carrier removal rate have been studied.# 2010 The Japan Society of Applied PhysicsDOI: 10.1143/JJAP.49.121202

Radiation degradation and damage coefficients of InGaP/GaAs/Ge triple-junction solar cell by low-energy electrons

Low-energy electrons were irradiated to InGaP single-junction and InGaP/GaAs/Ge triple-junction (3J) solar cells. The electron energy values were selected to approach the threshold energy of gallium and indium atoms recoiling in the InGaP lattice system (∼300 keV). Simultaneous electron irradiation and current-voltage characteristics measurement of the cells revealed the fact that the short-circuit currents (Isc) of InGaP cells and consequently the 3J cells does not degrade when the cells are irradiated with electrons with energies of less than 300 keV, while the open-circuit voltage (Voc) considerably degrades for both types of cells, independent of electron energy. This result implies that the effects of radiation-induced defects originating from the recoil of phosphorus are insufficient to degrade the minority-carrier lifetime in InGaP. In addition, the degradation of the Voc is considered attributable not to an increase of reverse saturation current but to the increased surface recombination for irradiations with < 300 keV electrons. The relative damage coefficients (RDCs) of the 3J cell were derived using the degradation trend obtained. The RDCs for Isc approximately followed the extrapolation line from high-energy electron irradiation results, but those for Voc were lower than the extrapolation.

Theoretical Optimization of Base Doping Concentration for Radiation Resistance of InGaP Subcells of InGaP/GaAs/Ge Based on Minority-Carrier Lifetime

One of the fundamental objectives for research and development of space solar cells is to improve their radiation resistance. InGaP solar cells with low base carrier concentrations under low-energy proton irradiations have shown high radiation resistances. In this study, an analytical model for low-energy proton radiation damage to InGaP subcells based on a fundamental approach for radiative and nonradiative recombinations has been proposed. The radiation resistance of InGaP subcells as a function of base carrier concentration has been analyzed by using the radiative recombination lifetime and damage coefficient K for the minority-carrier lifetime of InGaP. Numerical analysis shows that an InGaP solar cell with a lower base carrier concentration is more radiation-resistant. Satisfactory agreements between analytical and experimental results have been obtained, and these results show the validity of the analytical procedure. The damage coefficients for minority-carrier diffusion length and carrier removal rate with low-energy proton irradiations have been observed to be dependent on carrier concentration through this study. As physical mechanisms behind the difference observed between the radiation-resistant properties of various base doping concentrations, two mechanisms, namely, the effect of a depletion layer as a carrier collection layer and generation of the impurity-related complex defects due to low-energy protons stopping within the active region, have been proposed.

Radiation-resistance analysis of GaAs and InGaP sub cells for InGaP/GaAs/Ge 3-junction space solar cells

Recently, InGaP/GaAs/Ge 3-junction solar cells are widely used for space because of their higher conversion efficiency and better radiation-resistance compared to GaAs and Si solar cells. In this study, effects of base carrier concentration in GaAs and InGaP sub-cells upon their radiation resistance are analyzed by using radiative recombination lifetime and damage constant K for minority-carrier lifetime of GaAs and InGaP. In addition, analytical results are also compared with the experimental results of InGaP solar cells irradiated with 1-MeV electrons, 30-keV and 200-keV protons. In low irradiation fluence, n-on-p structure cells are found to be more radiation resistant than p-on-n structure cells. Better radiation-resistance of sub-cells can be realized by optimal design based on fundamental approach for radiative and non-radiative recombination properties of InGaP and radiation-resistance of InGaP/GaAs/Ge 3-junction cells will also be improved by optimal design of sub cells.

Study the effects of proton irradiation on GaAs/Ge solar cells

Proton energy dependence of radiation damage to GaAs/Ge solar cells irradiated by protons with various energies (50 keV, 200 keV, 1 MeV and 9.5 MeV) were analyzed by using PC1D simulation together with SRIM simulations to investigate their electrical properties. The degradation of the open-circuit voltage is highest for 50 keV irradiation and lowest for 9.5 MeV irradiation. According to SRIM simulations the above changes in electrical properties are mainly related to damage in different regions of the solar cells.

Niel analysis of radiation degradation parameters derived from quantum efficiency of triple-junction space solar cell

Degradation modeling of InGaP/GaAs/Ge triple-junction (3J) solar cells due to proton/electron irradiation is performed with the use of a one-dimensional optical device simulator; PC1D. By fitting external quantum efficiencies of the 3J solar cells degraded by proton/electron irradiation, the short-circuit currents (ISC) and open-circuit voltages (VOC) are simulated. The validity of this model is confirmed by comparing the results of both ISC and VOC to the experimental data. Then, the degradation level in each sub-cell is evaluated. The carrier removal rate of base layer (RC) and the damage coefficient of minority carrier diffusion length (KL) in each sub-cell are also estimated. In addition, NIEL (Non-Ionizing Energy Loss) analysis for both radiation degradation parameters RC and KL is discussed.

Current Injection Effects on the Electrical Performance of 3J Solar Cells Irradiated with Low and High Energy Protons

To compare high and low energy proton irradiation effects on the recovery behavior of triple-junction (3J) solar cells, 3J solar cells designed for space applications were irradiated with protons at 50 keV and 10 MeV, and their electrical performance was measured in situ under AM0 illumination. The electrical performance of the solar cells decreases with increasing proton fluence. For 50 keV-proton irradiation, the remaining factors of short circuit current (Jsc), open circuit voltage and maximum power become 81, 81 and 56 % respectively at a fluence of 1.2times1012/cm2. For 10 MeV-proton irradiation, these values are 83, 69 and 47 % at a fluence of 3times10 13/cm2. After proton irradiation, current injection into solar cells was performed at current densities between 0.03 and 0.25 A/cm2. The values of Jsc increase with increasing injected charge, and no significant difference in the increase behavior of Jsc is observed between 3J solar cells irradiated with 50 keV- and those irradiated with 10 MeV-protons. The obtained result suggests that the origin of defects annealed by current injection in 3J solar cells irradiated with protons at 50 keV is the same as that in 3J solar cells irradiated with protons at 10 MeV

Study on Optimum Structure of AlInGaP Top Cell for Triple-Junction Space Solar Cell

The purpose of this study is to improve the performance of triple-junction solar cells. The AlInGaP single-junction (SJ) cell was studied because it has greater radiation resistance than the conventional InGaP top cell. AlInGaP SJ cells with varied layer thicknesses and carrier concentrations of the base layer were prepared, and structural dependencies were investigated in order to determine an optimum structure for the AlInGaP top cell. The preferred cell for the 10-year mission on geostationary Earth orbit would have a base layer thickness of 1250 nm and a base layer carrier concentration of 3.0times1016 cm-3 or lower. This paper also compared radiation resistance of the AlInGaP SJ cell with that of the InGaP SJ cell. Even though InGaP is known to have excellent radiation resistance, the AlInGaP SJ cell exhibited better resistance than the InGaP SJ cell. It was demonstrated that AlInGaP is a superior radiation-resistant material for advanced 3J top cell