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Voltage-dependent photocurrent in irradiated GaAs solar cells
Progress in Photovoltaics: Research and Applications
Abstract
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.
... Even though the PV-cells in a space environment are degraded due to different reasons, the degradation due to the exposure to strong particle radiation is one of the major concerns of PV manufacturers and space research societies considering the severe damages that can be caused by it. Near the Earth, this represents a big challenge to satellites [6,7,8,9,10]. The spectrum near the Earth is considered omnidirectional, except for solar flare times when the particles direction will be ruled by the geomagnetic lines. ...
... [15] The current-voltage (IV)-characteristics slope (from zero to the maximum power point (MPP) voltage) becomes steeper. [8,23] Decrease of the external quantum efficiency (EQE). The longer the wavelength region, the more severe the damage. ...
... [37,38,39] Although a great deal of effort has been put into radiation-induced degradation analysis of PV-cells over the last decades, still there exist many issues to be overcome for the efficient and vast deployment of PV system technologies The radiation-induced degradation is mostly due to atomic displacements (such as vacancies, interstitials, or anti-sites). (b) The defects create levels in the otherwise forbidden bandgap, which might act like minority-carrier traps, majority-carrier traps, recombination centers, generation centers, or temporary trapping centers [14,15,8]. (c) Profile of energy absorbed by recoils due to different streams of mono-energetic and unidirectional (normally incident) protons using SRIM [11]. ...
The growing interest in space exploration demands exploring new energy resources as well as improvement of the existing sources of energy used in space environments in terms of robustness, reliability, resiliency, and efficiency. This especially applies to the photovoltaic (PV) systems that are required to work efficiently in very hostile environments of radiation under extreme temperatures and vacuum conditions to name a few. In this respect, many efforts have been made to enhance III-V PV-cells technologies towards lighter and more efficient cells. Besides, especial interest has been expressed in understanding and modeling the degradation mechanisms of PV-cells due to the radiation of particles, such as electrons and protons, aiming to improve their radiation resistance. Therefore, an in-depth analysis of the conducted experiments and developed mathematical approximations with updated information is highly useful to guide the research efforts towards the current challenges in the field. In this regard, this paper aims to provide a chronological review of papers published between the 1990s up to the present discussing their main outcome and providing useful information about the experiments and simulation analysis carried out by such studies. The goal is to contribute to understanding the degradation mechanisms of III-V PV-cells caused by the radiation of nuclei particles, as well as to identify the remaining challenges that should be dealt with to improve the current III-V PV technologies for future deep space explorations.
... Currently, most of the space solar power applications are directed towards communication satellites operating in Geostationary Earth Orbits (GEO) or in satellite constellations in Low Earth Orbits (LEO) [10]. However, new advances in satellite technology, such as elliptical orbits that partially operate in the vicinity of the Van Allen radiation belt, require the understanding of solar cell degradation upon higher levels of radiation exposure [11]. Electron irradiation is the most commonly studied characterization method to replicate cell performance under irradiation damage in the space environment [12][13][14][15][16][17][18]. ...
... The equivalent fluence usually associated to GEO missions is achieved with 1 × 10 15 1-MeV electrons/cm 2 , while 5 to 10 times lower doses are required for the simulation of LEO missions. The solar cells degradation constants for performance prediction available in literature, however, differ significantly [11,19,20]. Commonly, degradation is described in terms of experimentally determined parameter remaining factors which are valid for one particular cell architecture rather than for the applied absorber material in general [21]. ...
... The direct conversion of the previously reported electron irradiation degradation constants, based on the particle equivalent displacement damage dose (DDD), resulted in a good representation of the experimentally obtained dark and illuminated J − V and external quantum efficiency (EQE) curves. Furthermore, an earlier reported voltage-dependency of the photocurrent [11] is seen in the irradiated cells. ...
In this study a recently developed physics-based model to describe the performance degradation of GaAs solar cells upon electron irradiation is applied to analyze the effects of proton irradiation. For this purpose GaAs solar cells with significantly different architectures are subjected to a range of proton irradiation fluences up to 5×10¹² H⁺/cm². The resulting J−V and EQE characteristics of the cells are measured and compared with the simulations from the model. The model requires individual degradation constants for the SRH lifetimes and the surface recombination velocities as an input. In this study these constants were obtained from the recently determined associated constants for electron irradiation using the particles non-ionizing energy loss (NIEL) values for conversion. The good fit between the simulated and experimentally obtained results demonstrate that this is a valid approach. Moreover, it suggests that the physics based model allows for a good prediction of GaAs cell performance under particle irradiation of any kind independent of the particular cell architecture as long as the layer thicknesses and doping levels are known. In addition the applied proton irradiation levels in this study were not found to induce additional Cu-related degradation in the investigated thin-film cells, indicating that the use of copper foil as a convenient carrier and rear contact does not require reconsideration for thin-film cells intended for space applications.
... Based on the hypothesis that the permanent displacement damage produced by the incidence of charged particles is the main aspect that degrades the device performance in space, the mission equivalent damage from electrons, protons, ions, and neutrons of different energies can be averaged by a certain electron fluence. [30][31][32][33] Geostationary orbit missions (GEO) usually last for 15 years, and the damage created by the irradiation environment is equivalent to that obtained by a fluence of 1 × 10 15 1-MeV electrons/cm 2 . For low earth orbit (LEO) missions, which last for approximate 10 years at a lower altitude, the equivalent fluences are five to 10 times lower. ...
... Solar cells with different structures were subjected to a total electron fluence of 1 × 10 15 e − /cm 2 of 1-MeV radiation, which corresponds to a 15-year GEO mission. 32 The irradiation was performed at the Reactor Institute Delft (RID) of the Delft University of Technology, using a van der Graaf accelerator with an electron flux of 5 × 10 11 e − /cm 2 s. The available structures for this study consisted of two substrate-based (SB) and three thin-film (TF) GaAs solar cells, with either a thin n-doped emitter and a thick p-doped base, here called a shallow junction (SJ) geometry, or a thick n-doped emitter and a thin p-doped base, here called a deep junction (DJ) geometry, as depicted in Figure 1. ...
... Following this approach specifically clarifies that the V oc of the thin-film cells turns out to be limited by the surface recombination velocity at the base-BSF interface (S n ). Upon exposure to radiation, the minority carriers lifetime reduces (most likely due to radiation displacement defects [30][31][32][33][34] ). Furthermore, in all solar cell structures a degradation of the emitter-window (GaAs/AlInP) and base-BSF (GaAs/InGaP) hetero-interfaces quality is identified, characterized by an increase in the surface recombination velocity. ...
The effects of electron irradiation on the performance of GaAs solar cells with a range of architectures is studied. Solar cells with shallow and deep junction designs processed on the native wafer as well as into a thin‐film were irradiated by 1‐MeV electrons with fluence up to 1×10¹⁵ e⁻/cm². The degradation of the cell performance due to irradiation was studied experimentally and theoretically using model simulations, and a coherent set of minority carriers' lifetime damage constants was derived. The solar cell performance degradation primarily depends on the junction depth and the thickness of the active layers, whereas the material damage shows to be insensitive to the cell architecture and fabrication steps. The modeling study has pointed out that besides the reduction of carriers lifetime, the electron irradiation strongly affects the quality of hetero‐interfaces, an effect scarcely addressed in the literature. It is demonstrated that linear increase with the electron fluence of the surface recombination velocity at the front and rear hetero‐interfaces of the solar cell accurately describes the degradation of the spectral response and of the dark current characteristic upon irradiation. A shallow junction solar cell processed into a thin‐film device has the lowest sensitivity to electron radiation, showing an efficiency at the end of life equivalent to 82% of the beginning‐of‐life efficiency.
... The resultant trap and recombination centers impact critically on the electrical parameters of solar cells. 30 Moreover, the interaction between the incident irradiation particles and lattice atoms affects the crystal quality and increase the hetero-junction interface surface recombination velocity, and, subsequently, reduce the minority carrier lifetime. 15 Using Equation (1) and (2), the minority carrier lifetimes of each subcell under different electron irradiation fluences were calculated. ...
The degradation characteristics of lattice‐matched (LM) GaInP/InGaAs/Ge solar cell under 1 MeV electron irradiation are numerically simulated using APSYS finite element analysis software and Essential Macleod Program. Main output parameters of solar cells are simulated based on the material energy band, minority carrier lifetime, and interface surface recombination velocity. The results show that the adverse effects of the displacement damages in solar cell materials induced by irradiation are the primary reasons for overall cell performance degradation. The minority carrier lifetime degraded mostly in InGaAs middle subcell after irradiation, and it became to the current‐limiting unit in the end‐of‐life condition. The simulation results of the current‐voltage curves and the external quantum efficiency spectra are well agreed with the irradiation experimental data under similar conditions. Furthermore, an approach to radiation hardening of LM GaInP/InGaAs/Ge solar cell by adjusting the base and emitter layer thickness in the InGaAs middle subcell is proposed based on this model.
... The diffusion length L n in the base was determined by fitting the quantum efficiency data of the cells. 11,13 The DDD analysis is performed on L n , J 01 , and J 02 as illustrated in Figure 2. The degradation curves for L n −2 , J 01 , and J 02 follow a linear law, and the fitting is performed with the generalized equation ...
GaAs component cells, representative of the middle cell in Ga0.5In0.5P/GaAs/Ge triple junction solar cells, were irradiated with protons and electrons of various energies and fluences. The local ideality factor, calculated from the measured dark J‐V curves, exhibits a characteristic signature of the irradiating particle. With the help of an analytical model based on Shockley‐Read‐Hall statistics, the recombination current in the space‐charge region is calculated, and the local diode ideality factor is reproduced accurately. The inclusion of defect levels away from the intrinsic Fermi level in the bandgap is found to be essential, since a classical two‐diode model fails to describe the experimental data. On the basis of literature data of known defect levels in irradiated GaAs, the associated lifetimes and defect introduction rates are derived.
The recent surge of interest in electrostatic actuators, particularly for soft robotic applications, has placed increasing demands on high voltage control technology. In this respect, optoelectronic bidirectional switching and analogue regulation of high voltages is becoming increasingly important. One common problem is the leakage current due to dark resistance of the material or device used. Another is the physical size of such elements. However, their ability to provide galvanic separation makes them a very attractive alternative to conventional (wired) semiconductor elements. This paper gives an overview of available methods and devices before introducing a concept based on the combination of photoresistive and magnetoresistive effects in Gallium Arsenide that are potentially applicable to other semiconductor materials.
The degradation of solar cells under irradiation by high energy particles (electrons, protons) is the consequence of the introduction of defects trapping minority carriers, which are then not collected by the junction. However, at low temperature, defects located in the space charge region can also induce a tunneling current that results in an apparent decreases of the maximum power. The degradation produced by this tunneling current can depend on temperature, since the concentration of defects created by an irradiation is usually temperature dependent, and can be larger than the degradation associated with carrier recombination. For instance, as we shall see below, an irradiation with 1 MeV electrons at 120 K with a fluence of 3.0 × 1015 /cm2 induces a decrease of less than 10 % in the short-circuit current (Isc) and open-circuit voltage (Voc) of triple junction (TJ) cells, but a decrease of about 40 % in the maximum power (Pmax), which implies that more than half of the total degradation of Pmax should be assigned to another loss mechanism, tunneling in this case. In this work, we demonstrate that this additional degradation must indeed be ascribed to a tunneling process and we investigate the variation of the tunneling current versus fluence induced by electron irradiation in TJ cells, in order to tentatively ascribe the tunneling components to specific sub-cells.
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
The response of triple-junction solar cells to proton and electron irradiation is analyzed using electroluminescence (EL) measurements. This analysis allows the dark current of each individual subjunction to be determined providing insight into the radiation response mechanisms.
Low-thrust geostationary transfer orbits (LT2GEO) are found to offer significant low cost options thus making them very attractive for GEO missions. However, LT2GEOs increase the transfer orbit time from days to months, thereby causing a significant increase in the time that satellites traverse the most intense trapped particle radiation belts. This paper describes the LT2GEO radiation environment and provides some practical implications using new solar cell technologies and materials. New solar cell and coverglass testing protocols are established using Monte Carlo transport modeling and experimental results are given for Qioptiq CMG borosilicate coverglass.
Highly elliptical orbits can be used to provide targeted satellite coverage of locations at high latitudes. We review the history of use of these orbits for communication. How elliptical orbits can be used for broadband communication is outlined. We propose an addition of known elliptical orbits to the new equatorial O3b satellite constellation, extending O3b to cover high latitudes and the Earth's poles. We simulate the O3b constellation and compare this to recent measurement of the first real Internet traffic across the newly deployed O3b network.
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.
The investigation of solar cells degradation and the prediction of its
end-of-life performance is of primary importance in the preparation of a space
mission. In the present work, we investigate the reduction of solar-cells'
maximum power resulting from irradiations with electrons and protons. Both GaAs
single junction and GaInP/GaAs/Ge triple junction solar cells were studied. The
results obtained indicate how i) the dominant radiation damaging mechanism is
due to atomic displacements, ii) the relative maximum power degradation is
almost independent of the type of incoming particle, i.e., iii) to a first
approximation, the fitted semi-empirical function expressing the decrease of
maximum power depends only on the absorbed NIEL dose, and iv) the actual
displacement threshold energy value (Ed=21 eV) accounts for annealing
treatments, mostly due to self-annealing induced effects. Thus, for a given
type of solar cell, a unique maximum power degradation curve can be determined
as a function of the absorbed NIEL dose. The latter expression allows one to
predict the performance of those solar cells in space radiation environment.
The paper reports about the possibility of predicting the degradation of III-V multi-junction solar cells due to particle irradiation in space solely based on the radiation response of the respective sub cells. State-of-the art triple-junction solar cells of the 3G28 class manufactured by AZUR Space GmbH are used as an example to demonstrate how to model the degradation behavior of multi-junction cell from the degradation behavior of the respective component cells, as investigated in this study. The advantages of component cells for understanding and modeling the behavior of multi-junction cells under different conditions are discussed.
Fraunhofer ISE and RWE SSP have developed a lattice-matched GaInP/GaInAs/Ge triple-junction space solar cell with a begin-of-life efficiency of 28.0 % (AM0, 1367 W/m 2 , T=28°C) and excellent remaining factors of 90.5% after 5x10 14 and 86.6% after 1x10 15 1 MeV electron irradiation per cm 2 . This was accomplished by systematic optimisation of the middle cell design for maximum end-of-life performance. This new triple cell, the RWE3G-28%, constitutes the second generation of fully European triple-junction space solar cells which is now going into qualification. Future concepts such as light weight solar cells, five and six-junction cells and the use of metamorphic materials are under investigation.
The current–voltage (J(V)) data measured in light and dark from a-Si-based solar cells have been analyzed to yield six parameters that completely specify the illuminated J(V) curve from reverse bias to beyond open-circuit voltage (Voc). A simple photocurrent collection model is used, which assumes drift collection in a uniform field. The method has been applied to J(V) data from over 20 single-junction a-Si or a-SiGe devices from five laboratories measured under standard simulated sunlight. Very good agreement is obtained between measured and calculated J(V) performance with only one adjustable parameter, the ratio of collection length to thickness Lc/D. Some of these devices have also been analyzed after extended light soaking or under filtered illumination. The effect of the voltage-dependent photocurrent collection on the fill factor and Voc is considered in detail. Results under 1 sun illumination for both a-Si and a-SiGe devices are consistent with hole-limited collection. Photocurrent collection in very thin devices (D ∼ 0.1 μm), or thicker devices under blue light, may be strongly influenced by interface recombination or back diffusion. The flatband voltage (Vfb) is dependent on the intensity and spectrum of illumination, and hence is not a fundamental device property and is not equivalent to the built-in potential. The Voc is limited by Vfb, not junction recombination current Jo, in typical devices. The illuminated solar cell performance is nearly independent of the forward diode current for low values of Lc/D, as occurs after light soaking or with a-SiGe. The model is also useful to investigate the intensity dependence of the fill factor and to predict the influence of Lc/D and Vfb on solar cell performance. © 1997 John Wiley & Sons, Ltd.
The voltage dependence of the photocurrent JL(V) of CdTe/CdS solar cells has been characterized by separating the forward current from the photocurrent at several illumination intensities. JL(V) reduces the fill factor (FF) of typical cells by 10–15 points, the open circuit voltage (VOC) by 20–50 mV, and the efficiency by 2–4 points. Eliminating the effect of JL(V) establishes superposition between light and dark J(V) curves for some cells. Two models for voltage dependent collection give reasonable fits to the data: (1) a single carrier Hecht model developed for drift collection in p-i-n solar cells in which fitting yields a parameter consistent with lifetimes of 10−9 s as measured by others; or (2) the standard depletion region and bulk diffusion length model fits almost as well. The simple Hecht-like drift collection model for photocurrent gives very good agreement to J(V) curves measured under AM1·5 light on CdTe/CdS solar cells with FF from 53% to 70%, CdTe thickness from 1·8 to 7·0 µm, in initial and stressed states. Accelerated thermal and bias stressing increases JL(V) losses as does insufficient Cu. This method provides a new metric for tracking device performance, characterizes transport in the high field depletion region, and quantifies a significant FF loss in CdTe solar cells. Copyright © 2007 John Wiley & Sons, Ltd.
Internal collection efficiency of solar cells is calculated as a function of depletion width, minority‐carrier diffusion length, solar spectrum, and absorption coefficient. Particular attention is given to the variation of depletion width with voltage, which may reduce collection efficiency by a few percent at operating voltages and may be misinterpreted as a shunting effect. Calculations of collection‐efficiency magnitude and voltage dependence, plus comparisons with experiment, are presented for several types of solar cells. Primary emphasis is placed on polycrystalline thin‐film cells with relatively short diffusion lengths and hence potentially more pronounced collection efficiency effects.
Isochronal and isothermal annealing experiments on defects produced by 1‐MeV electron irradiation at room temperature have been performed on n‐type GaAs (vapor phase epitaxy layers). In addition to the previously reported irradiation‐induced defects E2 to E5, three new traps have been observed (P1 to P3) and their thermal behavior has been studied together with the thermal behavior of the traps E2–E5. The trap E2 is shown to exhibit an annealing kinetics which can be decomposed into the sum of two first‐order kinetics, the first one having the same annealing rate as the annealing kinetics of the traps E3 and E5. These observations lead to an interpretation of the annealing mechanism: annihilation of vacancy‐interstitial pairs or vacancy‐antisite defect pairs, and E2 is tentatively identified as a vacancy.
Chopped beam measurements on CuInSe 2 /Cd(Zn)S solar cells have revealed a failure of the superposition principle, with the light‐generated current showing a strong voltage dependence. The light I‐V curve can be fully explained with no change in the diode current. The voltage‐dependent light‐generated current dominates the light I‐V curve below V oc , requiring that the analysis of all the traditional solar cell characterization measurements be modified. We have derived the general expressions needed to obtain the diode factor from the light I‐V and V oc versus intensity curves. We have also shown how the temperature variation of V oc is affected.
Degradation modeling of InGaP/GaAs/Ge triple-junction (3J) solar cells subjected to proton irradiation is performed with the use of a one-dimensional optical device simulator, PC1D. By fitting the external quantum efficiencies of 3J solar cells degraded by 30 keV, 150 keV, 3 MeV, or 10 MeV protons, the short-circuit currents (ISC) and open-circuit voltages (VOC) are simulated. The damage coefficients of minority carrier diffusion length (KL) and the carrier removal rate of base carrier concentration (RC) of each sub-cell are also estimated. The values of ISC and VOC obtained from the calculations show good agreement with experimental values at an accuracy of 5%. These results confirm that the degradation modeling method developed in this study is effective for the lifetime prediction of 3J solar cells.
Space charge Shockley-Read-Hall recombination currents in the presence of discrete or continuous distributions of recombination centres are analysed. For a single level trap, depending on its position inside the forbidden band, asymptotic values both for the ideality factor of the current-voltage characteristic and for the activation energy of the saturation current are obtained. The analysis is extended to continuous trap distributions and the current-voltage characteristics obtained are explained in terms of the simple theory developed for single level traps.
- Abou-Ras
Solar cells utilizing small molecular weight organic semiconductors
- Hegedus
Proton irradiation of 3J solar cells at low temperature
- Parks Bourgoinjc
- Cavanio
Dose Dependence for Solar Cells Irradiated with Electrons and Protons
- C Baur
- M Gervasi
- P Nieminen
- Niel
Solar cell degradation analysis applying the displacement damage dose approach using appropriate Niel values
- P Baur C Gervasi M Nieminen
- P G Rancoita
- Tacconi
Solar cell degradation analysis applying the displacement damage dose approach using appropriate Niel values
- Baurc Gervasim Nieminenp Rancoitapg Tacconim
Development status of european multi-junction space solar cells with high radiation hardness
- Meuselm Baurc Guterw