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7IMPROVEMENT OF IRRADIATION RESISTANCE OF SOLAR CELLS BY VARIATION OF THE DEVICE PARAMETERS: APPLICATION
TO N+/P InGaP
M. Idali Oumhand*, Y. Mir**, M. Khalis*, M. Zazoui*
Laboratoire de Physique de la Matière Condensée, Université Hassan II Mohammedia BP 146, Bd Hassan II, F.S.T. Mohammedia, Maroc Telephone/fax: 023315353 E-mail: [email protected]
Ecole Nationale des Sciences Appliquées de Safi, Université CADI AYYAD BP 63 route Sidi Bouzid, Safi principale
Received: 8 Oct. 2007; accepted: 14 Nov. 2007
The degradation of solar cell materials in space has already been studied and is a consequence of the defects induced by electron and proton irradiation. The nature, characteristics and introduction rates of these defects are typical of a specific material. We propose here a method allowing to study the variation of the short circuit current Jsc and open circuit voltage Voc versus the fluence of the irradiation using a new approach taking account of the dependence of two minority carriers lifetime T0n and T0p before irradiation. Then we show the effect of the device parameters, i.e. the variation of different values of parameters respectively the emitter thickness, base thickness, emitter doping, base doping and the front surface recombination velocity.
Organization(s): professor and thesis L.P.M.C. Laboratoire de Physique de la Matière Condensée. Education: Université Hassan II. Mohammedia, Faculté des sciences et
techniques F.S.T. Mohammedia. Maroc (2004-2007).
Keywords: photovoltaic effect in semiconductor structures; modelisation, degradation
Organization: Professor. Education: Université Hassan II Mohammedia (1995-2007). Main range of scientific interests: solar cells, semiconductor, condensed matter. Publications: in Phys Rev, Appl. Phys. Letter, J. Appl. Phys, Semicond. Scien. Tech...
Mimoun Zazoui
Mohammed Idali Oumhand
Introduction
Previously we have proposed a new method, justified theoretically and experimentally, allowing to deduce T0n versus T0p, the minority carriers lifetime respectively in the base and in the emitter of solar cell, before irradiation and j versus Tjp, the minority carriers lifetime respectively in the base and in the emitter, after irradiation, and versus qj, the fluence irradiation. The validity of this method was illustrated in the case of the degradation of the n+/p InGaP cells [1]. In this paper, we recall briefly the calculation principle of new approach and we will show the effect of the device parameters, i.e. the emitter thickness and doping (Xn, Nd) and base thickness and doping (Xp, Na) and the front surface recombination velocity, Sp, on the variation of characteristics and consequently on the resistance to electron irradiation of solar cells.
Principle of calculation of minority carrier lifetime
The electron or proton irradiation introduces recombination centres which tend to affect solar cell performance by reducing the minority carrier lifetimes Tjn and Tjp through equation [2],
1 1 , 11,
— =—+k°nv nqj, —=—+pv pqj,
Tjn T0n Tjp T0 p
where the subscripts 0n, 0p and jn, jp indicate values before and after irradiation in the n and p region. The lifetimes T0np associated with the recombination of minority carriers (electrons and holes in the emitter and (or) base before irradiation) depend on the nature and concentration of the native recombination centres, i.e. on the mode of growth of the material and on the process treatments which are applied to realize the cell.
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In the previous work, we have shown that the minority carrier lifetime can be written as:
T jn =
jp + bJ
/Tjp - aJrp
The complete expressions for a,j, equation were derived from [1].
bJ, rn
Once the degradation of n+/p InGaP is calculated, we proceed to study the parameters effects on the degradation of Voc and Jsc [6].
and rp of last
Mecanism of degradation and parameters effects
Mecanism of degradation It is possible to calculate, and hence predict, the degradation of n+/p InGaP solar cell, when the thickness and doping respectively of the emitter and base are given. The parameters of the studied cell are listed in Table 1 below.
Table 1
Calculated parameters of n+/p InGaP cells
22 20 18
"Ê 16 u
< 14
JE
"o 12
U) 12 -3
10 8 6
Xp=33.10 m Xn=8,2.10-6m Xn=2,2.10-6m Xn=1.10-6m
1E9 1E10 1E11 1E12 1E13 1E14 1E15 1E16 1E17 Log(fluence) (cm-2)
Cell To„(s) Top(s) &a„(cm-1) Äoi,(cm"1)
InGaP (n+/p) 3.44-10"8 7.2-10"12 1.2-10"14 8.6-10"13
The knowledge of Jsc0 and Voc0 under given illumination, before irradiation is also necessary in order to derive the minority carrier lifetimes T0n, T0p in the base and emitter. Once the initial values of the minority carrier lifetimes are determined, one inject them into calculation. The knowledge of Jscj and Vocj under given illumination and amount of irradiation ty allow it to deduce the values of Tjn, TjP and hence kan, kap.
Often some authors fits experimental data to the values normalized prior irradiation [3, 4]. In our case we calculate the absolute theoretical data as shown in Fig. 1, a and b where the absolute experimental [5] (with yellow star (*) symbol) and the theoretical data of Voc, Jsc are represented.
From the parameters T0n, kan (in emitter region), and T0p, kap (in base region) (see Table 2) we derive the calculated values of Voc and Jsc. We add that these values extracted from our analysis are different of the values determined by other authors [3, 4] because of the relationship between two carriers lifetime T0n and T0p.
Parameters of ]
Log(fluence) (cm2)
Fig. 1. Variation (a) of the short circuit current, Jsc, and (b) of the open circuit voltage, Voc, under 1AM0 illumination, versus the fluence of 1 MeVelectron irradiation, calculated for different values, Xn, of emitter thickness
Table 2
solar cells
InGaP-n> Emitter thickness Xn (1-10-6 m) Base thickness Xp (110-5 m) Emitter doping Nd (1-1018 cm-3) Base doping Na (11017 cm-3) Recombination velocity Sp (1-105 cm-s-1)
Solar cell 8.2 2.2 1.0 3.3 2 1 4.5 1.5 0.85 4 5 1 0.5
parameters 2 1
In the present work, we interest to the effects of parameters as the emitter thickness and doping (Xn, Nd) and base thickness and doping (Xp, Na) and the front surface recombination velocity, Sp. We show in Fig. 1-5 respectively the variation of the short circuit current, Jsc, and open circuit voltage, Voc, under 1 AM0 illumination,
(the AM0 spectrum is the relevant one for satellite and space-vehicle applications), versus the fluence of 1 MeV electron irradiation, calculated for different values of the emitter and base thickness and doping level and of the front surface recombination velocities.
International Scientific Journal for Alternative Energy and Ecology № 6 (62) 2008
© Scientific Technical Centre «TATA», 2008
M. Idali Oumhand, Y. Mir, M. Khalis, M. Zazoui. Improvement of irradiation resistance of solar cells.
The effect of the variation of the thickness For the studied cell InGaP-n+/p, we vary only each of the emitter thickness, Xn, and the base thickness, Xp, the other parameters being fixed at the values of Table 2. The curves of Jsc, short-circuit current and the open circuit voltage, Vtcc, obtained are represented in Fig. 1, 2. We observe an important reduction in Jsc with the increase values of the emitter thickness, Xn. On the contrary Jsc increases but slightly with increasing values of the bases thickness, Xp.
Indeed, when the emitter thickness, Xn, will be big, enough carriers don't reach the base region, what causes a reduction of the spectral response and therefore an attenuation of the value of Jsc.
At high fluence of irradiation (Fig. 2, a), we note a fast decrease of the curves representing the cells of which the thickness Xp is large.
Not indeed of surprise for the Voc parameter, it doesn't vary practically at the time of the variation of the Xn and Xp (in the Fig. 1, b and Fig. 2, b; the three representative curves are nearly confounded).
и ic4
Xn=8,2.10m Xp=33.10-6m Xp=20.10-6m Xp=10.10-6m
The effect of the variation of the level doping The effect of the variation of the level doping of two regions, emitter and base, is illustrated in Fig. 3, 4. The Fig. 3, b and Fig. 4, b show that the effect of the doping remains weak on the variation of the Voc parameter.
The contribution of the two regions to the short circuit current, Jsc, is shown in Fig. 3, a and Fig. 4, a. This contribution takes place in opposite sense, i.e., the Jsc increases rapidly when the base is well doped by the quantity Na whereas the emitter is doped weakly by the quantity Nd.
Moreover, at high fluence of irradiation, to see Fig. 3, b and Fig. 4, a, Jsc resists to the irradiations when the cell (n+/p) is doped weakly by the acceptors Na, on the other hand Voc resists well to the irradiations when the junction (n+/p) is doped weakly by the donors Nd.
1E13 1E14 1E15 1E16 Log(fluence) (cm-2)
1,40 —i
1,35 -1,30-
1,25- \
1,20- \
1,15 - Xn=8,2.10-6m
1,10- ■ Xp=3,3.10-5m
1,05- * Xp=2.10-5m
* Xp=1.10-5m
1,00- .....'1 ......"I ........ 1 1 ......1 ' ' и mu i i и inri i i и Mili i i шип i i inn. i
1E17
1E9 1E101E111E121E131E141E151E161E17 log(fluence) (cm-2)
Fig. 2. Variation (a) of the short circuit current, Jsc, and (b) of the open circuit voltage, Voc, under 1AM0 illumination, versus the fluence of 1MeV electron irradiation, calculated for different values, Xp, of base thickness
о и —i
Fig. 3. Variation (a) of the short circuit current, Jsc, and (b) of the open circuit voltage, Voc, under 1AM0 illumination, versus the fluence of 1 MeV electron irradiation, calculated for different values Nd of the emitter doping level
b
18
a
16-
14
12
8
6
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¥
\ \
—-^
Nd=1,5.1017cm-3 \
■ Na=4.1017cm-3 \
Na=2.1017cm-3 \
» Na=1.1017cm-3 N
1E13 1E14 1E15 1E16 Log(fluence) (cm-2)
1E17
1,35 1,30 1,25
>
? 1,20 >
1,15 1,10 1,05
b \
---
Nd=1,5.1017cm-3 \
■ Na=4.1017cm-3
Na=2.1017cm-3
17 -3 Na=1.10 cm
..........................^
1E11 1E12 1E13 1E14 1E15 1E16 1E17 log(fluence) (cm2)
Fig. 4. Variation (a) of the short circuit current, Jsc, and (b) of the open circuit voltage, Voc, under 1AM0 illumination, versus the fluence of 1 MeV electron irradiation, calculated for different values, Na, of the base doping level
The effect of the variation of the front surface recombination velocities
The Fig. 5 shows a not very significant effect of the front surface recombination velocities, Sp, on the characteristics Jsc and Voc especially at low fluence of
irradiation but at high fluence of irradiation, we can note a very significant effect of the Sp on the irradiation resistance of the Jsc and Voc parameters, i.e., more the Sp decreases and more the Jsc and Voc increase.
¥
......
\
■ Sp=5.105cm.s-1
4 Sp=1.105cm.s-1
t Sp=5.104cm.s-1
1E14 1E15 1E16
log(fluence) (cm-2)
1E17
1,40 1,35 1,30 1,25 ~ 1,20 £ 1,15 1,10 1,05 1,00 0,95
1E13 1E14 1E15
Log(fluence) (cm2)
1E16
1E17
Fig. 5. Variation (a) of the short circuit current, Jsc, and (b) of the open circuit voltage, Voc, under 1AM0 illumination, versus the fluence of 1MeV electron irradiation, calculated for different values, Sp, offront surface recombination velocities
Conclusion
References
In this study, we have shown that the short circuit current, Jsc, is sensitive to variations of he emitter thickness, base thickness, emitter doping, base doping and the front surface recombination velocities. At high irradiation of the n+/p InGaP solar cell and when the level of doping of the donors, Nd, is large then that of the acceptors, Na, is low, we note a resistance of the two characteristics Jsc and Voc to the electron irradiations. Finally, the front surface recombination velocities, Sp, has a profound effect on the short circuit current, Jsc, especially at high fluence irradiation.
rx~>
- TATA — l_XJ
1. Idali Oumhand M., Zazoui M. M. // J. Condensed Matter.
2. Sze S.M. // Physics of Semiconductors Devices. Murray Hill New-Jersey. 1991.
3. Makham S. et al. // Semicond. Sci. Technol. 20
(2005). P. 699-704.
4. Bourgoin J.C., Angelis N. // Solar Energy Material & Solar Cell. 66 (2001). P. 467-477.
5. Yamaguchi M., Takamoto T., Araki K., Ekins-Daukes N. // Solar Energy. 79 (2005). P. 78-85.
6. Hadrami M. // Solar Energy Material & Solar Cell. 90
(2006). P. 1486-1497.
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