A Yoshikawa

A. Amine

DJ Friedman

HJ Hovel

J Nelson

JC Bourgoin

K Nakayama

K. Zazi

M Idali Oumhand

M. Bouabdellaoui

M. Zazoui

N Angelis de

RJ Walters

S Makham

Y. Mir

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Opt Quant Electron (2013) 45:1189–1197DOI 10.1007/s11082-013-9735-8The window layers effect on the hardness improvementof space solar cells exposed to the 1 MeV electronirradiationsY. Mir · A. Amine · M. Bouabdellaoui · K. Zazi ·M. ZazouiReceived: 4 March 2013 / Accepted: 3 July 2013 / Published online: 13 July 2013© The Author(s) 2013. This article is published with open access at Springerlink.comAbstract Because of their state of art technology, GaAs solar cells are generally preferred forspatial applications. Exposure to proton and electron irradiations, solar cells suffer significantdegradation in their performance such as short circuit current and open circuit voltage. Addinga window layer helps in effectively reducing the surface recombination at the emitter surfaceof the solar cell without absorbing the useful light required for the device. It remains tostudy the physics of the window-emitter hetero-interface in order to understand how thewindow layer presence increases the minority carrier lifetime of the solar cell exposed toparticles irradiation. In this work Numerical simulation has been used to study the AlxGa1-xAs window composition effect on the current–voltage characteristics of a GaAs solar cellunder AM0 illumination and exposed to 1 MeV electron irradiation. To predict the effectof window layers on solar cells degradation, the current voltage characteristic are evaluatedfor different electron irradiation fluences. The findings are supported by experimental data.They lead us to get to know how the window layer improves resistance to electron irradiationthrough its own parameters.Keywords Carrier lifetime · Window layer · Effective recombination velocity ·Degradation · Electrons irradiation1 IntroductionWindow layers help in effectively reducing the surface recombination at the emitter surfaceof the solar cells, without absorbing the useful light required for the device, resulting insignificant improvement in energy conversion efficiency (Jain 2003). Various window layermaterials have been investigated for III–V compound semiconductor based solar cells. ToY. Mir · A. Amine · M. Bouabdellaoui · K. Zazi · M. Zazoui (B)Laboratory of Condensed Matter and Renewable Energy, Faculty of Sciences and Techniques,University of Hassan II Mohammedia, Boulevard Hassan II, B. P. 146, 28800 Mohammedia, Moroccoe-mail: zazouimimoun@yahoo.fr1231190 Y. Mir et al.systematically reach this end, it will be important to understand the physics of the window-emitter hetero-interface as well as growth related issues (Lammasniemi et al. 1997).In the case of Ga As solar cell, it has been demonstrated with efficiency near to 25 % atone sun illumination and high efficiency Ga As solar cells all use a wide band-gap windowlayer on the surface to reduce front surface recombination losses (Vernon et al. 1990). Oneof the more critical design areas in high efficiency Ga As solar cell is AlGa As window layerand in order to minimize its optical absorption, high aluminium content alloys are favouredones (Gee and Drummond 1990). In previous work, new approach, justified theoreticallyand experimentally is used, allowing to deduce the relationship between electron and holeminority-carrier lifetime then we have shown, by using this method, the effect of the deviceparameters, i.e. the thickness X p and Xn respectively of emitter and base, and the leveldoping NA and ND on each side of the junction of the emitter and the base and the frontsurface recombination velocity, on how is the variation of characteristics and the electronirradiation resistance of solar cells (Idali Oumhand et al. 2008, 2009). In addition, the validityand accuracy of this approach has been justified by the fact that the degradation parameters,τn0, τp0, Kσn and Kσp which fit the experimental data are the same for both the shortcircuit current JSC and the open circuit voltage, VOC in the case of p+/n − I nGa P andn+/p − I nGa P solar cell (Idali Oumhand et al. 2008, 2009).This paper is a suite of the study and prediction of solar cell device parameters. We rewritethe calculation principle of new approach by taking into account the physics of the window-emitter hetero- interface and we apply this approach to p− AlGa As p+/n−Ga As solar cellunder AM0 illumination and exposed to 1 MeV electron irradiation and then a comparisonbetween both calculated and measured values would be studied (Friedman and Olson 2001).Moreover, we will show how the effect of window layer p − AlGa As on p+/n − Ga Asthrough three parameters, absorption coefficient αw(λ) at a given wavelength λ and effectivehetero-interface recombination velocity Snef f and its thickness variation Xw on variation ofJSC and VOC versus the fluence of the irradiation, φ (Yoshikawa and Kasai 1981; Ueda etal. 1998 ; Nakayama et al. 2008). The comparison between p − AlGa As and p − I nGa Pwindow-layers effects on degradation of JSC and VOC are investigated.2 Experimental results and principle of degradation parameter calculationThe p − AlGa As p+/n − Ga As structure studied is MOCVD epitaxially grown and it hasbeen irradiated with increasing fluences, in the range 1014–1017 electrons cm−2, at energyof 1 MeV. The irradiation was performed at room temperature with a scanned beam in orderto ensure the uniformity of the fluence over the area of the cell (1–4 cm2). The intensity ofthe beam (2 µA) was chosen to keep negligible the temperature increase. The cell is placedin a Faraday cup allowing accurate measurement of the fluence with a current integrator.Current–voltage measurements were performed in situ in the accelerator under a 1 AM0illumination provided by a Xe arc lamp (Bourgoin and Zazoui 2002). Defects induced byelectron irradiation in n- and p-type materials have been extensively studied (for a review seePons and Bourgoin 1985). These studies demonstrated that the defects introduced by roomtemperature irradiation are primary defects, resulting from the displacements of As in the Assub-lattice (Stievenard et al. 1986).For simplicity it is assumed that the series resistance RS and shunt resistance RSh havenegligible effects and that there is no shadowing effect on the illuminated surface (Walterset al. 1998).The calculated effective hetero-interface recombination velocity Sne f f assumes that thewindow layer effectively passive the solar cell front surface by reducing surface recombi-123The window layers effect on the hardness 1191Fig. 1 Band structure diagram of p-AlGaAs-p/n-GaAsTable 1 Physical parameter values of p − AlGaAs − p+/n − GaAs cell (Makham et al. 2005)p − AlGa Asp+/n − Ga AsWindow layerthicknessXw (10−6m)EmitterthicknessX p (10−6m)BasethicknessXn (10−6m)EmitterdopingNA (cm−3)Base dopingND (cm−3)Solar cellparameters0.035 0.44 3.1 1.2 × 1018 6.7 × 1016nation at the emitter surface of the solar cells without absorbing the useful light requiredfor the device and that the large band discontinuities at the hetero-interface provide a greatpotential barrier for the minority carriers, as it is illustrate in Fig. 1, and then well improvingthe performance of the solar cell (Jain and Landis 1991; Nelson 2003). When calculating thecontribution of each side of the junction and the depletion region to current in presence ofwindow layer, the incident flux, ϕ(λ), and the electron recombination velocity, Sn , shouldbe modified respectively to ϕ(λ)e−αw(λ)Xw and to Sne f f which is expressed as (Nelson 2003;Hovel 1979),Snef f = Swn e(Eg−Egw/K B T ) Dwn /Swn Lwn tanh (Xw/Lwn ) + 1Swn Lwn /Dwn tanh (Xw/Lwn ) + 1Eg and Ewg are the energy gap in GaAs and in window respectively .3 Mechanism of degradation and parameters effects3.1 Mechanism of degradationThe calculation of degradation parameters τn0, τp0, Kσn and Kσp of p− AlGa As p+/n−Ga As solar cell will be possible, when the thickness of window layer and doping respectivelyof the emitter and base are given further information in (Idali Oumhand et al. 2009). Theparameters of the studied cell are listed in Table 1 below (Makham et al. 2005).The knowledge of J expSC0 and VexpOC0 under given illumination, prior irradiation is alsonecessary in order to derive the minority carrier lifetimes τp0 and τn0.1231192 Y. Mir et al.Table 2 Calculated degradation parameters of p-AlGaAs-p+/n-GaAs cell (Makham et al. 2005)p − AlGa As p+/n − Ga As τn0 (s) τp0 (s) Kσn (cm) Kσp (cm)Degradation parameters 1.23 × 10−11 1.37 × 10−8 1.12 × 10−12 7.57 × 10−14Fig. 2 Calculated degradation (solid line), induced by 1 MeV electron irradiation, of the short circuit current(a), and open circuit voltage (b) in the p-AlGaAs-p+/n-GaAs cell under 1 AM0 illumination and comparisonwith measured data (open square) cited in Makham et al. (2005)Once the initial values of the minority carrier lifetimes are determined, we inject theminto calculation (Idali Oumhand et al. 2009).The knowledge of J expSCφ and VexpOCφ under given illumination and amount of irradiationφ allow it to deduce the values of τnφ and τpφ and hence Kσn, Kσp , for more detailedinformation see (Idali Oumhand et al. 2009).Then from the parameters τn0 and Kσn in emitter, and τp0 and Kσp in base, see Table 2,we derive the calculated values of JSCφ and VOCφ as shown in curve Fig. 2a , b respectively.123The window layers effect on the hardness 1193Fig. 3 Variation of the short circuit current Jsc (a) and the open circuit voltage Voc (b) under 1 AM0illumination, versus the fluence of 1 MeV electron irradiation, calculated for different values Xw such as:(solid line), Xw = 10−6cm, (dashed line), Xw = 3.5×10−6cm and (dotted line) Xw = 7 × 10−6cm ofp-AlGaAs window-layer thicknessWe add that these values extracted from our analysis are different from the values deter-mined by other authors (Bourgoin and Angelis 2001; Angelis et al. 2001) because of therelationship between two carriers lifetime τp0 and τn0 (Idali Oumhand et al. 2009), and morethe validity and accuracy of this approach was justified by the fact that the degradation para-meters, τn0, τp0, Kσn and Kσp which fit the experimental data, J expSC () and VexpOC (), arethe same for both the short circuit current, J calSC (solid line), and the open circuit voltage, V calOC(solid line) in case of p − AlGa As p+/n − Ga As.3.2 The window-layer parameters effectNow the degradation parameters,τn0, τp0, Kσn and Kσp of p − AlGa As − p+/n − Ga Asare calculated with the help of our calculation method, and we proceed to study the effectsof parameters1231194 Y. Mir et al.Fig. 4 Variation of the short circuit current Jsc (a) and the open circuit voltage Voc (b) under 1 AM0illumination, versus the fluence of 1 MeV electron irradiation, calculated for both values of Sne f f , 2.5×106cm/s(dashed line) and 5 × 106cm/s (solid line) in p-AlGaAs window-layerFor instance, thickness Xw and effective hetero-interface recombination velocity Snef f onthe variation of JSC and VOC versus the fluence of the irradiation φ, then a comparisonbetween p − AlGa As − p+/n − Ga As and p − I nGa P − p+/n − Ga As is investigatedby using variation of absorption coefficient αw(λ).The variations of the short-circuit currents and open circuit voltages versus fluence of thiscell are given in Fig. 3a, b, under 1AM0 illumination, versus the fluence of 1 MeV electronirradiation, calculated for different values of the thickness xw . When we vary the thicknessxw from 10 to 35 and to 70 nm the curve of the short-circuit current JSC versus the fluenceof the irradiation φ leads to significant reduction in JSC . Indeed, when the window layerthickness xw will be large, the attenuation becomes pronounced which causes a reductionof the spectral response and therefore a diminution of the value of JSC . At high fluence ofirradiation, we note a fast decrease of the curves representing the cells of which the thicknessxw is large. On the contrary, the VOC curve exhibits no significant reduction at the time ofthe increase of xw.123The window layers effect on the hardness 1195Fig. 5 Normalized degradation parameters as: τn0 (open square), τp0 (asterisk), JSC (solid line) and VOC(dotted line) under 1 AM0 illumination, versus fluence of 1 MeV electron irradiationTable 3 Variation of initial minority carrier lifetime versus effective hetero-interface recombination velocitySne f fJsc (mA cm−2) Voc (V) Sef fn (cm/s) τn0 (s) τp0 (s) Kσn (cm) Kσp (cm)22.33 0.91 7.5 × 106 2.9 × 10−11 5.28 × 10−9 1.195 × 10−12 1.378 × 10−1322.33 0.91 5.1 × 106 1.23 × 10−11 1.37 × 10−8 1.12 × 10−12 7.57 × 10−1422.33 0.91 1.1 × 106 5.99 × 10−12 3.27 × 10−8 1.1 × 10−12 7.57 × 10−1422.33 0.91 5.1 × 105 5.53 × 10−12 3.626 × 10−8 1.11 × 10−12 7.2 × 10−1422.33 0.91 1.1 × 105 5.188 × 10−12 3.93 × 10−8 1.11 × 10−12 6.9 × 10−1422.33 0.91 5.1 × 104 5.147 × 10−12 3.97 × 10−8 1.11 × 10−12 6.87 × 10−14The Fig. 4a shows an important effect of variation of the effective hetero-interface recom-bination velocity on JSC while, this effect remains a little weak on the variation of the Vocparameter as is exhibited in Fig. 4b.By comparing the degradation parameters of p/n- GaAs junction, as τp, τn, JSC and VOCwe find that the short circuit current JSC is sensitive to degradation. Moreover, the minoritycarrier lifetime τp in base is degraded faster than τn , in emitter, as shown in Fig. 5.Whereas, the degradation of VOC is less pronounced relatively to that of minority carrierlifetime. In addition, the study degradation of a series of solar cells technologically similarbut differs in their effective front surface velocity was achieved. It can be seen from Table 3,that the degradation parameters τn0, τp0, Kσn and Kσp of solar are sensitive to variation ofeffective hetero-interface recombination velocity Sne f f , when the value of this later is greaterthan a limit value of 5 × 106cm/s.While these degradation parameters become non sensitive to this variation, if the valuesof Sne f f are smaller than a limit value. This explains why the optimum value of Sne f f must besmaller than a limit value of 5 × 106cm/s.1231196 Y. Mir et al.Fig. 6 variation of the short circuit current Jsc, (a) and the open circuit voltage Voc (b) under 1 AM0illumination, versus the fluence of 1 MeV electron irradiation, calculated in case of p-AlGaAs (solid line) andp-InGaP (dashed line) window-layerFinally, the Fig. 6a, b exhibit the comparison between p − AlGa As − p+/n −Ga As andp − I nGa P − p+/n − Ga As solar cells, by using the absorption coefficient αw(λ) and theeffective hetero-interface recombination velocity Sne f f . We note for both curves, Fig. 6a, b,that the variations of Jsc and Voc are clearly important in the case of p-AlGaAs than that ofp InGaP and consequently AlGa As window layer structure is good candidate for solar cell.4 ConclusionIn this paper we have investigated the effect of the window layer through three parameters,absorption coefficient αw(λ), effective hetero-interface recombination velocity Sne f f and itsthickness variation xw , on the variation of JSC and VOC versus the fluence of the irradiation,φ. We have demonstrated that the short circuit current JSC is sensitive to variations of thethickness of the window-layer, and the effective surface recombination velocity but the openvoltage VOC remain little sensitive to these variations.123The window layers effect on the hardness 1197Open Access This article is distributed under the terms of the Creative Commons Attribution License whichpermits any use, distribution, and reproduction in any medium, provided the original author(s) and the sourceare credited.ReferencesBourgoin, J.C., Angelis, N.: Radiation-induced defects in solar cell materials. Sol. Energy Mater. Sol. Cell 66,467–477 (2001)Bourgoin, J.C., Zazoui, M.: Irradiation-induced degradation in solar cell: characterization of recombinationcentres. Semicond. Sci. Technol. 17, 453–460 (2002)de Angelis, N., Bourgoin, J.C., Takamoto, T., Khan, A., Yamaguchi, M.: Solar cell degradation by electronirradiation. Comparison between Si, GaAs and GaInP cells. Sol. Energy Mater. Sol. 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Optical and Quantum Electronics

The window layers effect on the hardness improvement of space solar cells exposed to the 1 MeV electron irradiations

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The window layers effect on the hardness improvement of space solar cells exposed to the 1 MeV electron irradiations

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