An investigation of the physical mechanisms governing the response of III-V based solar cells to particle irradiation is presented. The effect of particle irradiation on single and multijunction solar cells is studied through current vs. voltage, spectral response, and deep level transient spectroscopy measurements. The basic radiation response mechanisms are identified, and their effects on the solar cell electrical performance are described. In particular, a detailed analysis of multijunction InxGa1-xP/InyGa1-yAs/Ge devices is presented. The MJ cell response is found to be more strongly affected by the internal cell structure than by the In content.

Solar cells made of multiple absorbers are a commonly used approach for improving efficiency due to their extended range of spectral sensitivity. Indeed, efficiencies nearing the theoretical maximum have been achieved with a triple-junction device made of In0.51Ga0.49P (InGaP2), GaAs, and Ge solar cells connected in series. For extraterrestrial applications, there is the added requirement of radiation tolerance. The main challenge for space power-generation is therefore the development of highly efficient and radiation-tolerant devices. We have investigated several aspects of the radiation response of solar cells made of multiple absorbers, such as multijunction devices and quantum-well solar cells. Novel possibilities such as quantumdot solar cells and ordered-disordered heterostructures are proposed.

In this paper, the photovoltaic response of p + n and n + p GaAs solar cells is monitored as a function of proton fluence at different proton energies. The energy dependence of displacement damage coefficients (DCs) describing the photovoltaic degradation for these devices are compared with calculations of nonionizing energy loss (NIEL). The short circuit current DCs for both device types follows the same energy dependence. In contrast, the open circuit voltage DCs follows a different energy dependence at higher proton energies (E ≫ 10 MeV).

This paper presents a method for using the SPENVIS on-line computational suite to implement the displacement damage dose (D(sub d)) methodology for calculating end-of-life (EOL) solar cell performance for a specific space mission. This paper builds on our previous work that has validated the D(sub d) methodology against both measured space data [1,2] and calculations performed using the equivalent fluence methodology developed by NASA JPL [3]. For several years, the space solar community has considered general implementation of the D(sub d) method, but no computer program exists to enable this implementation. In a collaborative effort, NRL, NASA and OAI have produced the Solar Array Verification and Analysis Tool (SAVANT) under NASA funding, but this program has not progressed beyond the beta-stage [4]. The SPENVIS suite with the Multi Layered Shielding Simulation Software (MULASSIS) contains all of the necessary components to implement the Dd methodology in a format complementary to that of SAVANT [5]. NRL is currently working with ESA and BIRA to include the Dd method of solar cell EOL calculations as an integral part of SPENVIS. This paper describes how this can be accomplished.

We characterize radiation induced defects in n-type GaAs following electron, proton, and silicon ion irradiations using deep level transient spectroscopy (DLTS) and electron beam induced current (EBIC) measurements. EBIC micrographs show the existence of radiation induced recombination centers following high energy proton (E MeV) or 22 MeV silicon ion irradiations, which were not observed following 1 MeV electron or 2 MeV proton irradiations. The evolution of the U-band defect as determined by DLTS seems to occur when active recombination centers are observed in the EBIC images and therefore, appears to be produced by high energy recoils probably creating defect clusters.

An analysis of the effects of low energy proton irradiation on the electrical performance of triple junction (3J) InGaP 2 /GaAs/Ge solar cells is presented. The Monte Carlo ion transport code SRIM is used to simulate the damage profile induced in a 3J solar cell under the conditions of typical ground testing and that of the omnidirectional space environment. The results are used to present a quantitative analysis of the defect, and hence damage, distribution induced in the cell active region by the different radiation conditions. The modeling results show that, in the space environment where the incident radiation is omnidirectional, the solar cell will experience a uniform damage distribution through the active region of the cell. The cases of directional spectrum irradiation and omnidirectional irradiation through very thin shielding are also considered. Through an application of the displacement damage dose analysis methodology, the implications of this result on mission performance predictions are investigated

Current space solar cell degradation modeling relies on proton to electron correlation at a prescribed solar cell degradation level to determine the end-of-life (EOL) characteristics. This topic is reviewed and an alternate, degradation-dependent method is proposed.

A quantitative analysis of low energy proton effects on space solar cell materials is presented. The results show that the space environment produces a uniform damage distribution, so any effects caused by the non-uniform damage profile induced by monoenergetic ground testing will not be seen on orbit. The solar cell space radiation response is shown to be best represented by ground testing with protons of high enough energy to fully penetrate the solar cell active region (on the order of 1 MeV or greater). Calculations are also performed for the case of an unshielded solar cell representative of a thin-film solar cell. A non-uniform damage profile is observed in an unshielded cell, but the profile is seen to decrease with depth with no Bragg peak forming. Therefore, ground testing at extremely low energies where the proton stops within the cell active region does not accurately model the space environment effects. Addition of a coating as thin as 10 mum is seen to render the damage profile essentially uniform

We discuss the applicability of the displacement damage dose method and the non-ionizing energy loss (NIEL) concept to the effect of protons on GaAs devices. Comparison of the relative damage coefficients (RDCs) for different GaAs devices with the energy dependence of the proton NIEL shows differences for proton energies >10 MeV. However, since actual RDCs are always bound between the total and Coulombic NIELs, calculations were performed for the total displacement damage dose using both NIEL curves to determine the range of dose expected for devices shielded with various thickness of fused silica coverglass. These calculations were performed for two different incident proton spectra. The overall effect of using the total or Coulombic NIEL for calculating the total displacement dose deposited is minimal until extreme thicknesses of shielding are used

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.

An index that would reliably rank the relative radiation hardness of semiconductors would be very helpful for selecting materials for many applications. The concept of "concentration of primary defects" (CPD) has been proposed as such an index. Here we show that for the GaAs-InP family of semiconductors used in solar cells, CPD yields results that do not agree with experiment. The long-established concept of damage constants for various semiconductor properties is a more reliable index. For solar cells the relevant damage constant is that for the degradation of minority carrier lifetime. Although the damage constants reported for different semiconductors are specific to a particular type and energy of radiation, we show they can be extended beyond the type and particle energy for which they were originally determined by using nonionizing energy loss (NIEL), thereby greatly extending their possible application.

Damage correlations resulting from neutron and proton irradiation are described using the nonionizing energy loss (NIEL) approach. The method is applied on data generated on single junction GaAs/Ge solar cells.

It is shown how the widely available computer code SRIM can be used to calculate proton relative damage coefficients (RDCs) for solar cells. This approach is important in cases when the incident proton energy is reduced significantly in traversing the active volume of the cell and, consequently, when analytic calculations of the nonionizing energy loss (NIEL) are not satisfactory for calculating the energy dependence of the RDCs. In the SRIM approach it is assumed that the RDCs are proportional to the total number of vacancies formed. The calculation involves integrating the 'vacancy.txt' files produced by SRIM over the active depth of the cells and normalizing the results to the number of total vacancies produced by 10 MeV protons. The RDCs calculated from SRIM are found to agree wen with experimentally measured values for both Si and multijunction (MJ) solar cells. Copyright (C) 2005 John Wiley Sons, Ltd.

An analysis of the effects of low energy proton irradiation on the electrical performance of triple junction (3J) InGaP2/GaAs/Ge solar cells is presented. The Monte Carlo ion transport code (SRIM) is used to simulate the damage profile induced in a 3J solar cell under the conditions of typical ground testing and that of the space environment. The results are used to present a quantitative analysis of the defect, and hence damage, distribution induced in the cell active region by the different radiation conditions. The modelling results show that, in the space environment, the solar cell will experience a uniform damage distribution through the active region of the cell. Through an application of the displacement damage dose analysis methodology, the implications of this result on mission performance predictions are investigated.

The development of a photovoltaically (PV) powered, laser communication system that constitutes a miniature, highly energy-efficient wireless communication technology is described. The technology is based on the direct integration of a multi-quantum well (MQW) modulating retroreflector (MRR) optical communication node and a monolithically integrated module (MIM) PV power source. The MQW MRR optical data link exploits the shift in the MQW absorption peak under an applied reverse bias to modulate incident laser light enabling binary encoding of data for transfer. A MIM consists of many individual solar cells monolithically integrated on a single substrate and offers the design versatility necessary to allow efficient electrical conversion of both incident sunlight and the system laser-light and the ability to match the voltage output to the MRR requirements. A description of the development of the MRR and MIM components of the system along with the power management and distribution circuitry is given. Results of bench-top demonstrations of the operational system are presented.

An analysis of solar array data from the Microelectronic and Photonic Testbed (MPTB) space experiment is presented. The data are analyzed using the displacement damage dose (Dd) methodology developed by the US Naval Research Laboratory (NRL) as implemented in the Solar Array Verification and Analysis Tool (SAVANT). SAVANT is a WindowsTM‐based computer code that predicts the on‐orbit performance of a solar cell in a specified Earth orbit. The predicted solar cell performance produced by the SAVANT code are compared with the measured on‐orbit data. In addition, the calculated data are compared with onboard dosimeter measurements. The results allow both a validation of the SAVANT code and a comparison of the space environment models with measured on‐orbit data. The results show the models to match the measured data within a factor of 2. Published in 2005 by John Wiley & Sons, Ltd.

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

We present results of displacement damage correlation between 2 MeV protons and 22 MeV silicon ion irradiation damage in p/sup +/n GaAs solar cells. The radiation induced degradation of the photovoltaic response correlates well in terms of displacement damage dose.

The effect of incident particle range, straggling, primary recoil atom range and target volume on the expected applicability of nonionizing energy loss (NIEL) as a basis for correlating displacement damage effects are quantified. Examples are given for protons and alpha particles incident on both silicon and gallium arsenide.

Gallium nitride (GaN)-based blue-emitting diodes (CREE Model C430-DH85) were irradiated at room temperature with protons in the energy range 2 to 115 MeV at fluences varying from 1/spl times/10/sup 11/ to 1/spl times/10/sup 15/ cm/sup -2/. Light output degradation curves were obtained for each energy and the damage constant (A) associated with these curves was determined according to the theory of Rose and Barnes. For proton energies less than 10 MeV, A varies inversely with the proton energy (E). At higher energies, A is consistently above the 1/E relationship. The change in nature of the energy dependence is attributed to nuclear interactions. Nonionizing energy loss calculations for the case of protons on GaN are presented. Good agreement between theory and experiment is obtained.