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Characterization of monolithic III–V multi-junction solar cells—challenges and application
ARTICLE IN PRESS
Solar Energy Materials & Solar Cells 90 (2006) 3268–3275 www.elsevier.com/locate/solmat
Characterization of monolithic III–V multi-junction solar cells—challenges and application M. Meusela,b,, C. Baura, G. Siefera, F. Dimrotha, A.W. Betta, W. Wartaa a
Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstr. 2, D-79110 Freiburg, Germany b Freiburger Materialforschungszentrum FMF, Stefan-Meier-Str. 21, 79104 Freiburg, Germany Received 24 May 2005; accepted 19 September 2005 Available online 28 August 2006
Abstract The characterization of monolithic III–V multi-junction solar cells is still a challenging task. In this paper we show that quantum efficiency measurements have to be performed under optimized lightand voltage-bias conditions to minimize measurement artifacts. They appear, if the subcell to be measured has a low shunt resistance or a low reverse breakdown voltage. Moreover, cells with increasing number of junctions, such as our five-junction cell, pose new challenges to this characterization technique. Accurate I–V measurements of multi-junction cells under correct spectral conditions are usually very time consuming. However, ISE CalLab has developed a faster procedure for spectral correction. It is used in the methodology of spectrometric characterization. This characterization technique can be used to determine the current matching of the subcells. Applying spectrometric characterization in degradation experiments yields the degradation of the individual subcells additionally to the degradation of the total multi-junction device. Furthermore, it can be used to give a realistic estimation of the annual power output of terrestrial concentrator systems. r 2006 Published by Elsevier B.V.
1. Introduction Monolithic III–V multi-junction (MJ) solar cells are widely applied in space owing to their high efficiency of upto 30% (AM0, BOL) [1] and their high radiation hardness. Corresponding author. Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstr. 2, D-79110 Freiburg, Germany. Tel.: +49 761 4588 5223; fax: +49 761 4588 9250. E-mail address: [email protected] (M. Meusel).
0927-0248/$ - see front matter r 2006 Published by Elsevier B.V. doi:10.1016/j.solmat.2006.06.025
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Further on, they have a high potential to reduce terrestrial PV-electricity costs, if used in concentrator systems. Efficiencies of 35.2%(low AOD AM1.5 direct, C ¼ 66) [2] and 36%(AM1.5 G, 100–500 suns) [3], respectively, have already been reached by MJ concentrator cells and prototype concentrator modules and systems are being built by several groups. The I–V parameters of monolithic MJ cells are very dependent on the incident spectrum due to the internal series connection of the subcells. This has to be carefully considered for the calibration of such devices. Further on, these effects have to be taken into account for the application of these cells in space and in terrestrial concentrator systems. The presented paper discusses issues and challenges for analyzing monolithic MJ cells. 2. EQE measurements and measurement artifacts The basic procedure for measuring the external quantum efficiency (EQE) of MJ cells was established several years ago [4]. Recently, the ISE CalLab has worked on refining this procedure in order to minimize a certain measurement artifact often encountered when measuring the Ge bottom cell of a GaInP/GaInAs/Ge triple junction (3J) cell (see Fig. 1). The origin of this measurement artifact lies in the changing current mismatch of the subcells during measurement caused by the chopped monochromatic light. Consequently, the operating voltages of the subcells are shifting. This leads to artifact signals if the subcell to be measured has sloped characteristics in the voltage range close to short circuit, as for example caused by a low shunt resistance or a low reverse breakdown voltage. A detailed discussion of this effect as well as a guideline for how to adjust bias light and voltage in order to minimize this artifact can be found in Ref. [5]. In general, the measurement artifact can easily be detected in the case of GaInP/ GaInAs/Ge 3J cells (see Fig. 1). However, in the case of the recently developed 5J cell [6], the subcells have similar bandgaps and consequently overlapping response bands (Fig. 2). In this case a measurement artifact might be difficult to detect. 100
2.0
80
1.6
60
1.2
40
0.8
20
0.4
measured AC current [mA]
EQE [%]
measurement with artifacts measurement without artifacts
0.0
0 400
600
800 1000 1200 1400 1600 1800 wavelength [nm]
Fig. 1. EQE measurements of the Ge-bottom cell of a GaInP/GaInAs/Ge 3J cell. The measurement with artifacts shows response in the range of the top cell, a too-low response of the bottom cell and a distorted shape of the curve correlated to the signal height [5].
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3. I–V measurements and calibration For accurate I–V measurements of MJ cells, the spectrum of the solar simulator has to be adjusted such that each subcell generates the same photocurrent as under the reference spectrum. This can be achieved by using spectrally adjustable or multi-source solar simulators. The mathematical procedure usually used is a generalization of the procedure for single-junction cells (see e.g. Ref. [7]). This procedure involves the calculation of the socalled spectral mismatch factor for each junction. Following this approach leads to an iterative procedure, because after each adjustment of the simulator the spectrum has to be remeasured and the spectral mismatch factors recalculated. At ISE CalLab an alternative procedure was developed. We use a three-source simulator with independent light sources. The intensity of each source can be adjusted without changing its spectral distribution. The total simulator spectrum can therefore be calculated as the additive mixture of the individual sources. Thus, the iterative procedure can be avoided and the time for the measurement is significantly reduced. A detailed discussion of this procedure can be found in Ref. [8]. This mathematical approach can be well used for the calibration of 5J cells. However, the practical problems exists, that no five-source simulators are yet available. Therefore, it is still a challenge to measure a calibrated I–V curve of our 5J cell.
4. Spectrometric characterization Spectrometric characterization describes the behavior of the MJ cell under different spectral conditions. The I–V curve is recorded while the multi-source simulator is adjusted to a set of different spectra by the mathematical approach mentioned above. A detailed discussion in of 2J cells can be found in Ref. [8]. Figs. 3 and 4 show the spectrometric characterization of a 3J cell. In principle, spectrometric characterization of a 3J cell can be performed in analogy to 2J cells (Fig. 3), because the Ge-bottom cell usually has a large excess photocurrent and therefore acts only as a ‘‘voltage booster’’. 100
EQE [%]
80
60 AlGaInP
40
GaInP AlGaInAs GaInAs
20
Ge 0
400
600
800
1000
1200
1400
1600
1800
wavelength [nm]
Fig. 2. EQE of a five-junction cell fabricated at Fraunhofer ISE and measured at ISE CalLab [6].
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However, an additional measurement can be performed to analyze the Ge bottom cell as shown in Fig. 4. From these measurements it can be observed, that the Ge-bottom cell becomes current limiting at the maximum-power point (MPP), if its photocurrent is reduced to less than 65% of its value under AM0. At short circuit this cannot be observed because of the low reverse breakthrough voltage of the Ge-bottom cell.
5. Characterization of space MJ cells Considering the application of MJ cells in space, it is known, that the spectral response of the individual subcells degrades differently under the high-energy radiation in space. Therefore, the goal is to achieve maximum end-of-life (EOL) performance by current matching the subcells for EOL conditions. Consequently, a precise determination of the current matching is very important, which can be performed by spectrometric characterization. Fig. 5 shows the spectrometric characterization of a 2J cell at begin-oflife (BOL) and after irradiation with 3 MeV electrons at a fluence of 1 1015 cm2. It can be observed, that the measured cell is current matched at BOL but strongly mismatched Jphoto 1.08
MID
(E(λ)) / Jphoto
1.04
1.00
MID
(AM0)
0.96
0.92
FF [%]
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86 AM0 1367 W/m2
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32
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17
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34 current
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14
V [V]
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2.48
V [V]
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VMPP
0.92
0.96 TOP
Jphoto (E(λ))
1.00 / Jphoto
1.04
1.08
TOP
(AM0)
Fig. 3. Spectrometric characterization of a 3J cell. Starting from the reference spectrum (here AM0), the spectra E(l) are varied to change the photocurrent of the top and middle cell, while the photocurrent of the bottom cell is kept constant. Spectra are blue rich on the right of AM0 (x-value ¼ 1) and red rich on the left. Since the sum of top- and middle-cell photocurrent is kept constant, the JSC is maximum when top and middle cell are currentmatched. On the right of this maximum the middle cell is limiting the current of the 3J cell, on the left the top cell is current limiting.
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FF [%]
Jphoto
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/ Jphoto
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80
V [V]
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JMPP
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2.24 2.22 2.20 0.5
0.6
0.7
0.8
Jphoto BOT(E(λ)) / Jphoto
0.9
1.0
BOT
(AM0)
Fig. 4. Spectrometric characterization of a 3J cell. The spectra E(l) are varied to change the photocurrent of the bottom cell, while the photocurrents of the top and middle cell are kept constant.
after irradiation. Additionally, the model of a series connection of two two-diode models, including parasitic resistances was fitted to the measurements (see Fig. 5). Therefore the I–V curves of the individual subcells can be calculated from the fitted parameters. Thus, spectrometric characterization applied in degradation experiments yields the degradation of the individual subcells additionally to the degradation of the total multi-junction device.
6. Characterization of terrestrial concentrator MJ cells and modules Procedures for calibrating MJ concentrator cells are described in Ref. [7]. However, it is important to point out that the one-sun calibration of MJ concentrator cells does not eliminate the need to use a simulator with correctly adjusted spectrum for the measurement under high light intensity. For illustration, Fig. 6 shows the measurement of a 2J cell under high light intensity and two different simulator spectra. The subcells are current matched for the AM1.5 direct reference spectrum. Therefore, only the measurement under a simulator spectrum spectrally matched to AM1.5 direct ensures that both subcells are irradiated by the same concentration calculated as C ¼ I SC =I SC ð1 sunÞ. Only this measurement yields the correct efficiency, in this case 31.1%. The measurement under the red-rich spectrum irradiates the bottom cell at a higher concentration than calculated. Although this does not affect the ISC of the 2J cell, it still causes the FF to increase (see Fig. 6). In general, current mismatch between the subcells increases the fill factor of the MJ cell, which is a well known effect and can well be observed in spectrometric
ARTICLE IN PRESS M. Meusel et al. / Solar Energy Materials & Solar Cells 90 (2006) 3268–3275 BOT
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0.88
0.92 TOP
Jphoto
(E(λ))
0.96 /
1.00
1.04
1.08
TOP
Jphoto
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Fig. 5. Spectrometric characterization of a 2J cell at BOL (solid symbols) and after irradiation with 3 MeV electrons at a fluence of 1 1015 cm2 (hollow symbols). The lines connecting the symbols are fits of a series connection of two two-diode models including parasitic resistances.
90
FF [%]
88 86 84 82 FF, red-rich spectrum
80
FF, matched spectrum η, red-rich spectrum η, matched spectrum
32 η [%]
η = 32.0 %
30 28
η = 31.1%
26 1
10 100 Concentration [suns]
1000
Fig. 6. Efficiency and fill factor vs. concentration of a GaInP/GaInAs 2J cell under a spectrum matched to AM1.5d direct and under a red-rich spectrum [9].
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3.8 3.6
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photo
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AM
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pho B O T to (
1.2 1.1 1.0
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d)
3.0
pho
PMPP [W]
3.4
Fig. 7. Outdoor measurement of a concentrator module using 2J cells [10]. The maximum power is plotted versus spectrum. Each spectrum is represented by a point in the spectral metric by calculation of the generated photocurrents in both subcells normalized by their photocurrent under AM1.5d at 850 W/m2.
characterization (see Figs. 3 and 5). Consequently, the measured fill factor and efficiency are too high using the red-rich spectrum (see also Ref. [9]). Considering the terrestrial application of MJ concentrator cells it is important to estimate the realistic annual power output of a concentrator system. Due to the everchanging terrestrial sun spectrum and the effect of current limitation in monolithic MJ cells, this prediction is much more difficult compared to flat-plate modules using single junction cells. Fig. 7 shows, how the power output of a concentrator system can be recorded in dependence on the spectrum using the formalism of spectrometric characterization. The measurements shown were performed outdoors [10]. While the characterization shown in Fig. 7 consists of only a few measuring points, a more extensive characterization could be used as a characteristic matrix of the MJ concentrator system. The idea is to characterize the solarization at a specific site by a set of spectra. These spectra can either consist of directly measured data or spectral data simulated from weather data. These spectra can then be located in the spectral metric and the respective power output can be determined. 7. Summary On the one hand, calibration techniques for monolithic 2J and 3J cells have been well established. On the other hand, emerging new cell concepts like the 5J cell pose new challenges for characterization techniques. Also the analysis in respect to the application in space and in terrestrial concentrator systems is demanding. The formalism of spectrometric characterization seems well suited as a basis for this analysis. The profound experience of ISE CalLab in characterizing MJ cells is also available in form of measurement soft- and hardware developed in co-operation with Aescusoft GmbH Automation. The product
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range covers spectral response measurement setups, multi-source solar simulators, outdoor measurement setups, etc. Acknowledgments The authors wish to acknowledge Gerhard Willeke as head of the department Solar Cells—Materials and Technology at Fraunhofer ISE for his continued interest and support of this work. This work was supported by the European Space Agency (ESA) under contract no. 15409/01 and by the German Ministry of Economy and technology (BMWi) under contract no. 0328554E. The authors are responsible for the contents. References [1] R.R. King, C.M. Fetzer, P.C. Colter, K.M. Edmondson, J.H. Ermer, H.L. Cotal, H. Yoon, A.P. Stavrides, G. Kinsey, D.D. Krut, N.H. Karam, in: Proceedings of the 29th IEEE Photovoltaic Specialists Conference, New Orleans, Louisiana, 2002, p. 776. [2] M.A. Green, K. Emery, D.L. King, S. Igari, W. Warta, Prog. Photovolt. 11 (2003) 347. [3] T. Takamoto, T. Agui, K. Kanimura, M. Kaneiwa, M. Imaitzumi, S. Matzuda, M. Yamaguchi, in: Proceedings of the Third World Conference on Photovoltaic Energy Conversion, Osaka, Japan, 2003, to be published. [4] J. Burdick, T. Glatfelter, Sol. Cells 18 (1986) 301. [5] M. Meusel, C. Baur, G. Le´tay, A.W. Bett, W. Warta, Prog. Photovolt. 11 (2003) 499. [6] F. Dimroth, C. Baur, M. Meusel, S.V. Riesen, A. W. Bett, in: Proceedings of the Third World Conference on Photovoltaic Solar Energy Conversion, Osaka, Japan, 2003, to be published. [7] K. Emery, M. Meusel, R. Beckert, F. Dimroth, A.W. Bett, W. Warta, in: Proceedings of the 28th IEEE Photovoltaic Specialists Conference, Anchorage, Alaska, 2000, p. 1126. [8] M. Meusel, R. Adelhelm, F. Dimroth, A.W. Bett, W. Warta, Prog. Photovolt. 10 (2002) 243. [9] G. Siefer, C. Baur, M. Meusel, F. Dimroth, A.W. Bett, W. Warta, in: Proceedings of the 29th IEEE Photovoltaic Specialists Conference, New Orleans, Louisiana, 2002, p. 836. [10] M. Hein, M. Meusel, C. Baur, F. Dimroth, G. Lange, G. Siefer, T. Tibbits, A. W. Bett, V. M. Andreev, V. D. Rumyantsev, in: Proceedings of the 17th European Photovoltaic Solar Energy Conference, Munich, Germany, 2001, p. 496.
Solar Energy Materials & Solar Cells 90 (2006) 3268–3275 www.elsevier.com/locate/solmat
Characterization of monolithic III–V multi-junction solar cells—challenges and application M. Meusela,b,, C. Baura, G. Siefera, F. Dimrotha, A.W. Betta, W. Wartaa a
Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstr. 2, D-79110 Freiburg, Germany b Freiburger Materialforschungszentrum FMF, Stefan-Meier-Str. 21, 79104 Freiburg, Germany Received 24 May 2005; accepted 19 September 2005 Available online 28 August 2006
Abstract The characterization of monolithic III–V multi-junction solar cells is still a challenging task. In this paper we show that quantum efficiency measurements have to be performed under optimized lightand voltage-bias conditions to minimize measurement artifacts. They appear, if the subcell to be measured has a low shunt resistance or a low reverse breakdown voltage. Moreover, cells with increasing number of junctions, such as our five-junction cell, pose new challenges to this characterization technique. Accurate I–V measurements of multi-junction cells under correct spectral conditions are usually very time consuming. However, ISE CalLab has developed a faster procedure for spectral correction. It is used in the methodology of spectrometric characterization. This characterization technique can be used to determine the current matching of the subcells. Applying spectrometric characterization in degradation experiments yields the degradation of the individual subcells additionally to the degradation of the total multi-junction device. Furthermore, it can be used to give a realistic estimation of the annual power output of terrestrial concentrator systems. r 2006 Published by Elsevier B.V.
1. Introduction Monolithic III–V multi-junction (MJ) solar cells are widely applied in space owing to their high efficiency of upto 30% (AM0, BOL) [1] and their high radiation hardness. Corresponding author. Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstr. 2, D-79110 Freiburg, Germany. Tel.: +49 761 4588 5223; fax: +49 761 4588 9250. E-mail address: [email protected] (M. Meusel).
0927-0248/$ - see front matter r 2006 Published by Elsevier B.V. doi:10.1016/j.solmat.2006.06.025
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Further on, they have a high potential to reduce terrestrial PV-electricity costs, if used in concentrator systems. Efficiencies of 35.2%(low AOD AM1.5 direct, C ¼ 66) [2] and 36%(AM1.5 G, 100–500 suns) [3], respectively, have already been reached by MJ concentrator cells and prototype concentrator modules and systems are being built by several groups. The I–V parameters of monolithic MJ cells are very dependent on the incident spectrum due to the internal series connection of the subcells. This has to be carefully considered for the calibration of such devices. Further on, these effects have to be taken into account for the application of these cells in space and in terrestrial concentrator systems. The presented paper discusses issues and challenges for analyzing monolithic MJ cells. 2. EQE measurements and measurement artifacts The basic procedure for measuring the external quantum efficiency (EQE) of MJ cells was established several years ago [4]. Recently, the ISE CalLab has worked on refining this procedure in order to minimize a certain measurement artifact often encountered when measuring the Ge bottom cell of a GaInP/GaInAs/Ge triple junction (3J) cell (see Fig. 1). The origin of this measurement artifact lies in the changing current mismatch of the subcells during measurement caused by the chopped monochromatic light. Consequently, the operating voltages of the subcells are shifting. This leads to artifact signals if the subcell to be measured has sloped characteristics in the voltage range close to short circuit, as for example caused by a low shunt resistance or a low reverse breakdown voltage. A detailed discussion of this effect as well as a guideline for how to adjust bias light and voltage in order to minimize this artifact can be found in Ref. [5]. In general, the measurement artifact can easily be detected in the case of GaInP/ GaInAs/Ge 3J cells (see Fig. 1). However, in the case of the recently developed 5J cell [6], the subcells have similar bandgaps and consequently overlapping response bands (Fig. 2). In this case a measurement artifact might be difficult to detect. 100
2.0
80
1.6
60
1.2
40
0.8
20
0.4
measured AC current [mA]
EQE [%]
measurement with artifacts measurement without artifacts
0.0
0 400
600
800 1000 1200 1400 1600 1800 wavelength [nm]
Fig. 1. EQE measurements of the Ge-bottom cell of a GaInP/GaInAs/Ge 3J cell. The measurement with artifacts shows response in the range of the top cell, a too-low response of the bottom cell and a distorted shape of the curve correlated to the signal height [5].
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3. I–V measurements and calibration For accurate I–V measurements of MJ cells, the spectrum of the solar simulator has to be adjusted such that each subcell generates the same photocurrent as under the reference spectrum. This can be achieved by using spectrally adjustable or multi-source solar simulators. The mathematical procedure usually used is a generalization of the procedure for single-junction cells (see e.g. Ref. [7]). This procedure involves the calculation of the socalled spectral mismatch factor for each junction. Following this approach leads to an iterative procedure, because after each adjustment of the simulator the spectrum has to be remeasured and the spectral mismatch factors recalculated. At ISE CalLab an alternative procedure was developed. We use a three-source simulator with independent light sources. The intensity of each source can be adjusted without changing its spectral distribution. The total simulator spectrum can therefore be calculated as the additive mixture of the individual sources. Thus, the iterative procedure can be avoided and the time for the measurement is significantly reduced. A detailed discussion of this procedure can be found in Ref. [8]. This mathematical approach can be well used for the calibration of 5J cells. However, the practical problems exists, that no five-source simulators are yet available. Therefore, it is still a challenge to measure a calibrated I–V curve of our 5J cell.
4. Spectrometric characterization Spectrometric characterization describes the behavior of the MJ cell under different spectral conditions. The I–V curve is recorded while the multi-source simulator is adjusted to a set of different spectra by the mathematical approach mentioned above. A detailed discussion in of 2J cells can be found in Ref. [8]. Figs. 3 and 4 show the spectrometric characterization of a 3J cell. In principle, spectrometric characterization of a 3J cell can be performed in analogy to 2J cells (Fig. 3), because the Ge-bottom cell usually has a large excess photocurrent and therefore acts only as a ‘‘voltage booster’’. 100
EQE [%]
80
60 AlGaInP
40
GaInP AlGaInAs GaInAs
20
Ge 0
400
600
800
1000
1200
1400
1600
1800
wavelength [nm]
Fig. 2. EQE of a five-junction cell fabricated at Fraunhofer ISE and measured at ISE CalLab [6].
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However, an additional measurement can be performed to analyze the Ge bottom cell as shown in Fig. 4. From these measurements it can be observed, that the Ge-bottom cell becomes current limiting at the maximum-power point (MPP), if its photocurrent is reduced to less than 65% of its value under AM0. At short circuit this cannot be observed because of the low reverse breakthrough voltage of the Ge-bottom cell.
5. Characterization of space MJ cells Considering the application of MJ cells in space, it is known, that the spectral response of the individual subcells degrades differently under the high-energy radiation in space. Therefore, the goal is to achieve maximum end-of-life (EOL) performance by current matching the subcells for EOL conditions. Consequently, a precise determination of the current matching is very important, which can be performed by spectrometric characterization. Fig. 5 shows the spectrometric characterization of a 2J cell at begin-oflife (BOL) and after irradiation with 3 MeV electrons at a fluence of 1 1015 cm2. It can be observed, that the measured cell is current matched at BOL but strongly mismatched Jphoto 1.08
MID
(E(λ)) / Jphoto
1.04
1.00
MID
(AM0)
0.96
0.92
FF [%]
88 87 FF
86 AM0 1367 W/m2
36
32
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17
matching
34 current
P [mW/cm2]
85
PMPP JSC
16 15
JMPP
14
V [V]
2.52 2.50 VOC
2.48
V [V]
2.28 2.26 2.24 2.22
VMPP
0.92
0.96 TOP
Jphoto (E(λ))
1.00 / Jphoto
1.04
1.08
TOP
(AM0)
Fig. 3. Spectrometric characterization of a 3J cell. Starting from the reference spectrum (here AM0), the spectra E(l) are varied to change the photocurrent of the top and middle cell, while the photocurrent of the bottom cell is kept constant. Spectra are blue rich on the right of AM0 (x-value ¼ 1) and red rich on the left. Since the sum of top- and middle-cell photocurrent is kept constant, the JSC is maximum when top and middle cell are currentmatched. On the right of this maximum the middle cell is limiting the current of the 3J cell, on the left the top cell is current limiting.
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FF [%]
Jphoto
TOP
/ Jphoto
TOP
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MID
/ Jphoto
MID
(AM0) = 1
85 FF
80
V [V]
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75 34 32 30 28
PMPP JSC
16 15 14 13
JMPP
2.52
VOC
2.50 2.48
V [V]
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2.24 2.22 2.20 0.5
0.6
0.7
0.8
Jphoto BOT(E(λ)) / Jphoto
0.9
1.0
BOT
(AM0)
Fig. 4. Spectrometric characterization of a 3J cell. The spectra E(l) are varied to change the photocurrent of the bottom cell, while the photocurrents of the top and middle cell are kept constant.
after irradiation. Additionally, the model of a series connection of two two-diode models, including parasitic resistances was fitted to the measurements (see Fig. 5). Therefore the I–V curves of the individual subcells can be calculated from the fitted parameters. Thus, spectrometric characterization applied in degradation experiments yields the degradation of the individual subcells additionally to the degradation of the total multi-junction device.
6. Characterization of terrestrial concentrator MJ cells and modules Procedures for calibrating MJ concentrator cells are described in Ref. [7]. However, it is important to point out that the one-sun calibration of MJ concentrator cells does not eliminate the need to use a simulator with correctly adjusted spectrum for the measurement under high light intensity. For illustration, Fig. 6 shows the measurement of a 2J cell under high light intensity and two different simulator spectra. The subcells are current matched for the AM1.5 direct reference spectrum. Therefore, only the measurement under a simulator spectrum spectrally matched to AM1.5 direct ensures that both subcells are irradiated by the same concentration calculated as C ¼ I SC =I SC ð1 sunÞ. Only this measurement yields the correct efficiency, in this case 31.1%. The measurement under the red-rich spectrum irradiates the bottom cell at a higher concentration than calculated. Although this does not affect the ISC of the 2J cell, it still causes the FF to increase (see Fig. 6). In general, current mismatch between the subcells increases the fill factor of the MJ cell, which is a well known effect and can well be observed in spectrometric
ARTICLE IN PRESS M. Meusel et al. / Solar Energy Materials & Solar Cells 90 (2006) 3268–3275 BOT
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TOP
Jphoto
(AM0)
Fig. 5. Spectrometric characterization of a 2J cell at BOL (solid symbols) and after irradiation with 3 MeV electrons at a fluence of 1 1015 cm2 (hollow symbols). The lines connecting the symbols are fits of a series connection of two two-diode models including parasitic resistances.
90
FF [%]
88 86 84 82 FF, red-rich spectrum
80
FF, matched spectrum η, red-rich spectrum η, matched spectrum
32 η [%]
η = 32.0 %
30 28
η = 31.1%
26 1
10 100 Concentration [suns]
1000
Fig. 6. Efficiency and fill factor vs. concentration of a GaInP/GaInAs 2J cell under a spectrum matched to AM1.5d direct and under a red-rich spectrum [9].
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3.8 3.6
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/J
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d)
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T
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BO
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J
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/ J
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J
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AM
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pho B O T to (
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d)
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pho
PMPP [W]
3.4
Fig. 7. Outdoor measurement of a concentrator module using 2J cells [10]. The maximum power is plotted versus spectrum. Each spectrum is represented by a point in the spectral metric by calculation of the generated photocurrents in both subcells normalized by their photocurrent under AM1.5d at 850 W/m2.
characterization (see Figs. 3 and 5). Consequently, the measured fill factor and efficiency are too high using the red-rich spectrum (see also Ref. [9]). Considering the terrestrial application of MJ concentrator cells it is important to estimate the realistic annual power output of a concentrator system. Due to the everchanging terrestrial sun spectrum and the effect of current limitation in monolithic MJ cells, this prediction is much more difficult compared to flat-plate modules using single junction cells. Fig. 7 shows, how the power output of a concentrator system can be recorded in dependence on the spectrum using the formalism of spectrometric characterization. The measurements shown were performed outdoors [10]. While the characterization shown in Fig. 7 consists of only a few measuring points, a more extensive characterization could be used as a characteristic matrix of the MJ concentrator system. The idea is to characterize the solarization at a specific site by a set of spectra. These spectra can either consist of directly measured data or spectral data simulated from weather data. These spectra can then be located in the spectral metric and the respective power output can be determined. 7. Summary On the one hand, calibration techniques for monolithic 2J and 3J cells have been well established. On the other hand, emerging new cell concepts like the 5J cell pose new challenges for characterization techniques. Also the analysis in respect to the application in space and in terrestrial concentrator systems is demanding. The formalism of spectrometric characterization seems well suited as a basis for this analysis. The profound experience of ISE CalLab in characterizing MJ cells is also available in form of measurement soft- and hardware developed in co-operation with Aescusoft GmbH Automation. The product
ARTICLE IN PRESS M. Meusel et al. / Solar Energy Materials & Solar Cells 90 (2006) 3268–3275
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range covers spectral response measurement setups, multi-source solar simulators, outdoor measurement setups, etc. Acknowledgments The authors wish to acknowledge Gerhard Willeke as head of the department Solar Cells—Materials and Technology at Fraunhofer ISE for his continued interest and support of this work. This work was supported by the European Space Agency (ESA) under contract no. 15409/01 and by the German Ministry of Economy and technology (BMWi) under contract no. 0328554E. The authors are responsible for the contents. References [1] R.R. King, C.M. Fetzer, P.C. Colter, K.M. Edmondson, J.H. Ermer, H.L. Cotal, H. Yoon, A.P. Stavrides, G. Kinsey, D.D. Krut, N.H. Karam, in: Proceedings of the 29th IEEE Photovoltaic Specialists Conference, New Orleans, Louisiana, 2002, p. 776. [2] M.A. Green, K. Emery, D.L. King, S. Igari, W. Warta, Prog. Photovolt. 11 (2003) 347. [3] T. Takamoto, T. Agui, K. Kanimura, M. Kaneiwa, M. Imaitzumi, S. Matzuda, M. Yamaguchi, in: Proceedings of the Third World Conference on Photovoltaic Energy Conversion, Osaka, Japan, 2003, to be published. [4] J. Burdick, T. Glatfelter, Sol. Cells 18 (1986) 301. [5] M. Meusel, C. Baur, G. Le´tay, A.W. Bett, W. Warta, Prog. Photovolt. 11 (2003) 499. [6] F. Dimroth, C. Baur, M. Meusel, S.V. Riesen, A. W. Bett, in: Proceedings of the Third World Conference on Photovoltaic Solar Energy Conversion, Osaka, Japan, 2003, to be published. [7] K. Emery, M. Meusel, R. Beckert, F. Dimroth, A.W. Bett, W. Warta, in: Proceedings of the 28th IEEE Photovoltaic Specialists Conference, Anchorage, Alaska, 2000, p. 1126. [8] M. Meusel, R. Adelhelm, F. Dimroth, A.W. Bett, W. Warta, Prog. Photovolt. 10 (2002) 243. [9] G. Siefer, C. Baur, M. Meusel, F. Dimroth, A.W. Bett, W. Warta, in: Proceedings of the 29th IEEE Photovoltaic Specialists Conference, New Orleans, Louisiana, 2002, p. 836. [10] M. Hein, M. Meusel, C. Baur, F. Dimroth, G. Lange, G. Siefer, T. Tibbits, A. W. Bett, V. M. Andreev, V. D. Rumyantsev, in: Proceedings of the 17th European Photovoltaic Solar Energy Conference, Munich, Germany, 2001, p. 496.