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Silicon nitride passivated bifacial Cz-silicon solar cells
ARTICLE IN PRESS Solar Energy Materials & Solar Cells 93 (2009) 1435–1439
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Silicon nitride passivated bifacial Cz-silicon solar cells L. Janßen a,c,, H. Windgassen a, D.L. Ba¨tzner a, B. Bitnar b, H. Neuhaus b a b c
Institute of Semiconductor Electronics, RWTH Aachen University, Sommerfeldstr. 24, 52074 Aachen, Germany Deutsche Cell GmbH, Berthelsdorfer Str. 111a, 09599 Freiberg, Germany Solland Solar Cells GmbH, Bohr 12, 52072 Aachen, Germany
a r t i c l e in fo
abstract
Article history: Received 30 January 2009 Accepted 19 March 2009 Available online 29 April 2009
A new process for all silicon nitride passivated silicon solar cells with screen printed contacts is analysed in detail. Since the contacts are fired through the silicon nitride layers on both sides, the process is easy to adapt to industrial production. The potential and limits of the presented bifacial design are simulated and discussed. The effectiveness of the presented process depends strongly on the base doping of the substrate, but only the open circuit voltage is affected. The current is mainly determined by the rear surface passivation properties. Thus, using a low resistivity ðo1:5 O cmÞ base material higher efficiencies compared to an aluminium back surface field can be achieved. & 2009 Elsevier B.V. All rights reserved.
Keywords: Surface passivation Silicon nitride PECVD Bifacial
1. Introduction Bifacial silicon solar cells and more generally double side passivated designs are developed very intensively in the last few years [1–4]. High open circuit voltages are only achieved in the processes, when using a dedicated back surface field [1] or with local opening of the passivation layer [2]. The metal contact firing through approach, pursued by [3,4], yields only modest open circuit voltages. The limitations of this firing through silicon nitride process, the earlier developed process for passivating thin bifacial silicon solar cells was transferred from multicrystalline to Czochralski silicon [3]. Only small modifications on the rear side collecting grid were carried out compared to the design for the multicrystalline solar cells. Especially, the influence of the base resistivity and the contact formation were investigated in this study. The findings are complemented with results on multicrystalline silicon solar cells to confirm the simulations on the potential and limitation of the process.
2. Experimental The developed double side silicon nitride passivation process is very simple, the details on the process were published elsewhere [3]. After the saw damage removal, a phosphorus diffusion is carried out. Beside an anti-reflection coating the silicon nitride on Corresponding author at: Solland Solar Cells GmbH, Bohr 12, 52072 Aachen, Germany. Tel.: +49 45 8001201; fax: +49 45 8800605. E-mail address: [email protected] (L. Janßen).
0927-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2009.03.015
the front provides also an etching mask for the rear emitter removal. Subsequently, a rear silicon nitride layer is deposited and the contacts are screen printed and co-fired. Based on this process the dependence of the open circuit voltage on the base resistivity is analysed in the first part of this paper. In the second part the contact formation of aluminium and silver–aluminium pastes with silicon will be investigated in presence of a silicon nitride layer. To determine the contact resistance with the transfer length method, stripes of 5 mm width were cut from the solar cells and evaluated as described in [9]. The specific contact resistance ðrc Þ is approximated as rc ¼ Rc =A with Rc the contact resistance and A the area.
3. Results and discussion 3.1. Bifacial solar cells As described above a first set of monocrystalline solar cells was processed with the bifacial and a standard aluminium back surface field (Al-BSF) process. A 2 O cm Cz silicon was used for the first set of solar cells. For the bifacial process two different commercially available aluminium pastes were tested, denoted as Al1 and Al2. In accordance to the earlier results [3] the short circuit current density is increased for the solar cells with the bifacial processes in comparison to the solar cells with the Al-BSF process (see Table 1). Noticeable is the difference in the open circuit voltage, which was not observed on multicrystalline silicon [3]. With approximately 75% the fill factors of the bifacial cells are rather low and fitting the light I2V curves to the two diode model yields a higher series resistance for these cells ðDRs ¼ þ0:2 O cm2 Þ.
ARTICLE IN PRESS 1436
L. Janßen et al. / Solar Energy Materials & Solar Cells 93 (2009) 1435–1439
0.8
Rear contact
J sc ðmA=cm2 Þ
V oc (mV)
Fill factor (%)
Z (%)
Al-BSF Bifacial Al1 Bifacial Al2
36:2 0:2 36:9 0:2 36:5 0:1
619:2 1:0 612:1 1:3 603:5 1:6
76:8 0:4 75:1 0:2 74:6 0:3
17:2 0:2 17:0 0:1 16:4 0:1
Rear side Bifacial Al1 Bifacial Al2
22:2 0:3 19:4 0:5
602:3 1:4 593:5 3:1
77:1 0:5 77:3 0:5
10:3 0:1 8:9 0:3
Different aluminium pastes were used. Values are averaged over sets of at least four solar cells. Errors indicate the standard deviation for each group of at least four solar cells.
Rear internal quantum efficiency
Table 1 Solar cell parameters for different rear side contacts on 2 O cm silicon.
Value
Unit
Thickness Sfront Rsheet;Emitter Peak doping
200 55 000 60
mm cm/s O=&
7 1019 45 (60)
cm3 ms cm3 mm
BSF depth
1 1019 7.5
0.5 0.4
Values in brackets for 6 O cm silicon.
Comparing the different rear contact pastes in the bifacial process, it can be seen that all I2V parameters for front side illumination of paste Al2 are lower compared to paste Al1. This indicates worse surface passivation properties on the rear side of the solar cell. Measuring the I2V characteristics with rear side illumination confirms that assumption, since the short circuit current density at rear illumination is nearly 3 mA=cm2 lower for paste Al2 than for paste Al1. With rear internal quantum efficiency (IQE) measurements the surface recombination velocity can be quantified [5]. The rear IQE was fitted with PC1D [6] using the input parameters in Table 2. Even though the fixed charges of the silicon nitride layers were measured in metal nitride silicon capacitances with C2V measurements to 1 1012 cm2 , it is not possible to include them properly in PC1D since then the simulations do not converge anymore. The results of the simulations are plotted together with the rear IQE measurements in Fig. 1. The deviation of the fit and the measured IQEs in the short wave length range is caused by the absorption of the silicon rich SiN layer on the rear. In contrast to the front side the rear IQE is not curved in the short wave length region, since the doping level near the surface is low. The simulated effective rear surface recombination velocity is 350 cm/s for the solar cells with aluminium paste Al1 and 750 cm/s for the solar cells with paste Al2. Since the solar cells were processed identically in one batch, the difference in the rear surface passivation is due to the contact formation, respectively, the difference in widening of the different aluminium pastes. The higher metal coverage of paste Al2 causes more recombination at the surface. The electronic and rheologic properties of the rear contact pastes are discussed in more detail in the second part of this paper. The loss in open circuit voltage in the bifacial process does not result from a poor rear surface passivation, since surface recombination velocity of 350 cm/s and even 750 cm/s would be low enough to conserve the open circuit voltage in Al-BSF solar cells.
bifacial Al1 bifacial Al2 Seff = 350 cm/s Seff = 750 cm/s
0.3 0.2 0.1
400
500
600 700 800 900 1000 1100 1200 Wavelength [nm]
Fig. 1. Rear internal quantum efficiency, measured and fitted with PC1D.
0.66 Open circuit voltage [V]
Parameter
BSF doping
0.6
0.0 300
Table 2 Input parameters for the PC1D simulations.
tbulk
0.7
0.64
0.62
Srear, eff = 250 cm/s, no BSF Srear, eff = 1500 cm/s, BSF Srear, eff = 250 cm/s, BSF
0.60 1E16
1E17
1E18
Base doping [cm-3] Fig. 2. Dependence of the open circuit voltage on the base doping for three different scenarios. Simulations were carried out with PC1D.
Assuming that no aluminium back surface field is formed in the bifacial process, which would be consistent with SIMS analysis performed by Lenkeit [7], simulations explain the observed voltage loss (see Fig. 2). Three different scenarios were simulated. First a well-passivated rear surface, but without back surface field, second a worse passivated surface including a BSF and third the best case of a well-passivated surface with BSF. As expected the third case yields the highest open circuit voltages over the whole range of simulated base doping levels. The modest surface passivation on the rear has only small effect on the open circuit voltage and is quite comparable to the best case. But the case without BSF and good surface passivation shows a strong drop in the open circuit voltage for lower base doping levels. So if there is no back surface field the open circuit voltage depends mainly on the emitter and base doping. To prove these findings, 6 O cm Cz silicon material was processed, which should amplify the observed effect. The open circuit voltage should drop further 10 mV compared to the 2 O cm Cz silicon. The lower short circuit current density of these solar cells are due to worse alkaline texture (see Table 3). Since the rear contact design was not further optimised with respect to the high base resistivity, the fill factors of the bifacial cells are limited due to a high series resistance caused by spreading resistance in the base
ARTICLE IN PRESS L. Janßen et al. / Solar Energy Materials & Solar Cells 93 (2009) 1435–1439
J sc ðmA=cm Þ
V oc (mV)
Fill factor (%)
Z (%)
Al-BSF Bifacial Al1 Bifacial Al2
34:1 0:2 33:8 0:2 34:0 0:2
621:4 0:8 596:9 0:2 594:2 0:2
76:8 0:4 71:7 0:4 72:2 0:8
16:3 0:1 14:5 0:1 14:6 0:1
Rear side Bifacial Al1 Bifacial Al2
23:4 0:6 22:7 0:6
589:3 2:4 587:6 3:7
73:9 0:2 74:4 1:1
9:6 0:3 9:4 0:3
Rear contact
2
Values are averaged over sets of at least four solar cells. Errors indicate the standard deviation for each group of at least four solar cells.
Table 4 Solar cell parameters for different rear side contacts on 2 O cm Cz silicon using silver and aluminium pastes for the rear contact.
0.8 Rear internal quantum efficiency
Table 3 Solar cell parameters for different rear side contacts on 6 O cm Cz silicon using different aluminium pastes.
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0.7 0.6 0.5 0.4 Paste Ag/Al Al1
0.3 0.2 0.1 0.0 300
400
500
600
700
800
900 1000 1100 1200
Wavelength [nm] Fig. 3. Rear internal quantum efficiency for an Ag/Al and a pure Al paste.
Rear contact
J sc ðmA=cm2 Þ
V oc (mV)
Fill factor (%)
Z (%)
Bifacial Al1 Bifacial Ag/Al
36:6 0:4 36:7 0:2
605:6 1:8 604:0 0:9
73:0 1:2 73:3 1:3
16:2 0:1 16:2 0:3
Rear side Bifacial Al1 Bifacial Ag/Al
21:9 0:8 22:9 0:4
598:8 2:8 596:0 1:3
76:5 0:1 73:7 0:4
10:0 0:4 10:1 0:2
Values are averaged over sets of at least four solar cells. Errors indicate the standard deviation for each group of at least four solar cells.
region [8]. But not only the rear contact design, also the front contact resistance is higher and so contributing to the low fill factors. The higher front contact resistance is due to the lower texturing and a non-optimal firing process. All three groups yield nearly comparable short circuit current densities, but the main interest is in the open circuit voltage which shows the predicted behaviour of an even lower open circuit voltage compared to the 2 O cm Cz silicon. The Al-BSF process is not affected since the silicon aluminium alloying forms an effective high–low junction on the rear surface [7]. Since no effective back surface field is formed in the bifacial process, the aluminium pastes can be substituted with solderable silver–aluminium pastes. Thus definitely no back surface field will be formed. Additionally the number of process steps can be reduced to two instead of three screen printing steps for industrial cell processing. Aluminium paste Al1 and a silver–aluminium paste (Ag/Al) were compared to show the proof of concept. The aluminium content of the investigated paste was 2 percent and therefore low enough to ensure that no back surface field is formed. Alkaline textured 2 O cm Cz silicon was used as substrate. The front I2V parameters are identical for the two pastes considering the measurement accuracy (compare Table 4). Only the rear I2V parameters reveal a slight difference of these cells. Even though the conductivity of the silver–aluminium paste is higher, the measured grid line resistance is higher due to a worse aspect ratio of the printed lines. This leads to an increased series resistance in the rear I2V curve and to a lower fill factor. Since the lines printed with the Ag/Al paste are narrower, the shading is reduced which explains, together with the inherent lower surface recombination (see Fig. 3), the higher short circuit current density. This experiment validates the assumption that no effective back surface field is formed. Thus the lower open circuit voltage for the bifacial process on 2 and 6 O cm Cz silicon is caused by the lower potential caused by the base doping.
Table 5 Solar cell parameters for different rear side contacts on 1 O cm multicrystalline silicon. Rear contact
J sc ðmA=cm2 Þ
V oc (mV)
Fill factor (%)
Z (%)
Al-BSF Bifacial Al1
32:4 0:2 33:8 0:3
605:9 0:5 617:1 3:5
75:9 0:5 74:3 0:8
14:9 0:2 15:5 0:2
Rear side Bifacial Al1
19:3 0:6
601:9 5:2
75:4 0:8
8:3 0:3
Values are averaged over sets of at least four solar cells. Errors indicate the standard deviation for each group of at least four solar cells.
For 1 O cm silicon the open circuit voltage should be higher than for the aluminium back surface field process. Unfortunately no large area Cz silicon was available with such a low base doping. Therefore mutlicrystalline silicon was used. The processing was the same as mentioned before for the Cz wafers. The IV data of these cells show the expected increase in open circuit voltage (see Table 5). The voltage is 11 mV higher then for the reference process on multicrystalline silicon and only slightly lower then the open circuit voltage of the Cz reference process. The increase in the short circuit current density is with DJsc ¼ 1:4 mA=cm2 very high and leads to an efficiency increase of DZ ¼ 0:6%. Even though the choice of the mutlicrystalline silicon is not optimal the same trend as before could be observed. 3.2. Contact formation Since the rear contact formation is a critical process step for the bifacial process, the contact formation with a silicon nitride layer between the printed paste and the silicon is investigated in detail. A dependence of the contact resistance on the used rear side metallisation paste is presented in Table 6. Only aluminium paste Al1 forms low ohmic contacts, whereas the other pastes result in two to three times higher contact resistance values. With 41:2 m O cm2 the contact resistance of the investigated Ag/Al paste is also twice as high as for aluminium paste Al1. Nevertheless the solar cell properties depend only weakly on the contact resistance in this range, as can be seen in the prior results. The contact formation potential of most commercial pastes is limited to penetrate thin silicon nitride layers (e.g. 30 nm), since
ARTICLE IN PRESS L. Janßen et al. / Solar Energy Materials & Solar Cells 93 (2009) 1435–1439
Table 6 Measured contact resistances for different screen printing pastes and different silicon nitride layer thickness on Cz silicon. Contact paste
Layer thickness (nm)
rc ðm O cm2 Þ
Ag/Al Al1 Al2 Al3 Al3
30 30 30 30 70
41.2 20.5 60.3 58.2 49.3
Table 7 Line resistance, aspect ration and specific resistivity of different screen printing pastes. Screen printing paste
RL ðO=mÞ
Width ðmmÞ
Aspect ratio
r ðO cmÞ
Al1
46.0
263
0.11
2:7 105
Al2
46.0
398
0.05
3:0 105
Al3
93.4
317
0.12
6:5 105
Ag/Al
61.2
236
0.06
1:6 105
Ag (VS)
41.9
126
0.13
5:7 106
no additives for silicon nitride etching are included. The development paste Al3 is capable to form contacts even through 70 nm of silicon nitride. The contact resistance of this paste is comparable to paste Al2 and should yield comparable solar cell performance with thin silicon nitride layers, but also on anti-reflection coating like thick layers of around 70 nm. The rear I2V measurements reveal the impact of the line conductivity of the rear contact. Even though all contacts are printed with the same screen the line properties are very different due to the different rheological properties of the screen printing pastes. Beside the line resistance, the line width, the aspect ratio and the specific resistance were determined for the aluminium and silver–aluminium pastes. A front side metallisation paste was also included in the comparison. The line resistance is identical for pastes Al1 and Al2, but there are major differences in the resulting line width and aspect ratio (see Table 7). The lines printed with paste Al2 are 50 percent wider and the aspect ratio is less then half of the aspect ratio of paste Al1. The widening of the fingers on the rear explains the difference in the cell performance, since the passivated surface area is smaller when using paste Al2. Paste Al3 has a high line resistance which can be ascribed to the low conductivity of the paste. The rheologic properties are satisfying in the sense that the aspect ratio is above 0.1 and the line widens approximately 50 percent in comparison to the screen openings. Surprisingly high is the line resistance of the silver–aluminium paste. This result is explained by the low aspect ratio together with a low widening of the lines. The specific resistance is half of that of the aluminium pastes as expected, but not as low as for front contact pastes. Solar cells with a 70 nm thick silicon nitride layer on the rear were produced to verify the previous measurements on the contact formation for paste Al3. Two sets of references were processed on alkaline textured 2 O cm Cz silicon; one set with a 30 nm SiN layer as reference to the prior results and one set with Al-BSF as overall reference. The open circuit voltage for the 30 nm SiN layer reference solar cells is comparable to the previous results (see Table 8) and is 8 mV lower than for the Al-BSF reference. For the 70 nm SiN layer cells the open circuit voltage drops further to 601 mV ðDV oc ¼ 18 mVÞ. Since the contact resistance and the line
Table 8 Solar cell parameters for different rear side contacts on 2 O cm silicon. Rear contact
J sc ðmA=cm2 Þ
V oc (mV)
Fill factor (%)
Z (%)
Al-BSF (0 nm SiN) bifacial (30 nm SiN) bifacial (70 nm SiN)
35:9 0:1 35:8 0:2 35:1 0:1
619:9 2:4 611:6 2:8 601:6 1:0
76:8 0:6 74:9 0:1 74:9 0:2
17:1 0:1 16:4 0:1 15:8 0:1
Rear side bifacial 30 nm bifacial 70 nm
18:1 0:3 20:5 0:5
591:7 0:6 590:6 0:1
75:8 0:3 75:5 0:2
8:1 0:1 9:2 0:1
Aluminium paste Al3 was used for the rear contact formation through different silicon nitride layer thicknesses. Values are averaged over sets of at least four solar cells. Errors indicate the standard deviation for each group of at least four solar cells.
0.7 Rear internal quantum efficiency
1438
0.6 0.5 0.4 0.3
30 nm 70 nm Seff = 1000 cm/s Seff = 1250 cm/s
0.2 0.1 0.0 300
400
500
600 700 800 900 1000 1100 1200 Wavelength [nm]
Fig. 4. Rear internal quantum efficiency for different silicon nitride layer thickness, simulated IQEs with PC1D.
conductivity are comparable, the difference has to result from different surface passivation properties. To determine the rear surface recombination velocity, rear internal quantum efficiencies were measured and fitted. The rear IQEs are displayed in Fig. 4 together with PC1D simulations. The difference due to the extracted surface recombination velocity would change the open circuit voltage only about 2 mV, if all other parameters remain constant. Therefore, other effects may explain the further open circuit voltage loss compared to the 30 nm SiN layer reference. Since the short circuit current density is also reduced, inversion layer shunting could be a possible explanation [10] or additionally a difference in the fixed charges of the silicon nitride layers [11]. Since the modelling in PC1D is limited to one dimension, a more detailed analysis is very difficult. For the first assumption, the border of the contacted and the passivated area has to be included, which is not possible in PC1D. The second possibility is in fact just an extension of the first theory and is rather unlikely, since the deposition parameters of both layers were the same except for the deposition time. The layer thickness should not influence the amount of fixed charges after an initial increase in the first 20–30 nm [12].
4. Conclusions A process for passivating silicon solar cells with silicon nitride on the rear side was successfully transferred from multicrystalline to Czochralski silicon. The observed open circuit voltage loss is
ARTICLE IN PRESS L. Janßen et al. / Solar Energy Materials & Solar Cells 93 (2009) 1435–1439
identified to depend on the base doping of the wafer material. Since the presented bifacial process does not form an aluminium back surface field, the open circuit voltage does strongly depend on the doping concentration in the emitter and bulk region. This fact allows the use of solderable silver–aluminium pastes instead of aluminium pastes with no loss in efficiency. With a base resistivity below 1:5 O cm the open circuit voltage of bifacial cells compared to Al-BSF solar cells can be maintained or even exceeded, as extracted from PC1D simulations. This is supported by the fact that on 1 O cm multicrystalline silicon open circuit voltages above 620 mV have already been achieved. Thus the benefit of the concept can be used on multicrystalline silicon, since the base resistivity is typically 1:0 O cm. The contact formation was investigated in detail. Rear contact resistivities as low as 20 m O cm2 and aspect ratios of up to 0.12 were achieved for contacting through a 30 nm thick silicon nitride layer. The feasibility of contacting through 70 nm thick silicon nitride layers was demonstrated. However, the shunting effect of the inversion layer affects the open circuit voltage adversely.
Acknowledgements The funding of this project by the Ministry of Innovation, Science, Research and Technology of the State of North RhineWestphalia, Germany, no. 313-206 002 04, is gratefully acknowledged.
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References [1] A. Kra¨nzel, M. Ka¨s, K. Peter, E. Enebakk, Future cell concepts for mc solar grade silicon feedstock material, in: Proceedings of the 21st European Photovoltaic Solar Energy Conference, Dresden, 2006, pp. 1038–1041. [2] G. Agostinelli, P. Choulat, H.F.W. Dekkers, Y. Ma, G. Beaucarne, Silicon solar cells on ultra-thin substrates for large scale production, in: Proceedings of the 21st European Photovoltaic Solar Energy Conference, Dresden, 2006, pp. 601–604. [3] L. Janßen, M. Rinio, D. Borchert, H. Windgassen, D.L. Ba¨tzner, H. Kurz, Passivating thin bifacial silicon solar cells for industrial production, Progress in Photovoltaics: Research and Applications 15 (6) (2007) 469–475. [4] I.G. Romijn, M. Koppes, E.J. Kossen, C.J.J. Tool, A.W. Weeber, High efficiencies on mc-Si solar cells enabled by industrial firing through rear side passivating SiNx:H, in: Proceedings of the 21st European Photovoltaic Solar Energy Conference and Exhibition, Dresden, 2006, pp. 717–720. [5] J.M. Gee, A simple procedure to analyze rear-surface internal quantum efficiency, in: Conference Record of the 25th IEEE Photovoltaic Specialists Conference, May 1996, pp. 557–560, doi: 10.1109/PVSC.1996.564067. [6] P.A. Basore, D.A. Clugston, PC1D Version 5: 32-bit solar cell modeling on personal computers, in: Conference Record of the 26th IEEE Photovoltaic Specialists Conference, Anaheim, September–October 1997, pp. 207–210, doi: 10.1109/PVSC.1997.654065. [7] B. Lenkeit, Ph.D. Thesis, University Hannover, 2002. [8] B. Fischer, Ph.D. Thesis, University Konstanz, 2003. [9] H.H. Berger, Contact resistance and contact resistivity, Journal of the Electrochemical Society 119 (4) (1972) 507–514. [10] S. Dauwe, L. Mittelsta¨dt, A. Metz, R. Hezel, Experimental evidence of parasitic shunting in silicon nitride rear surface passivated solar cells, Progress in Photovoltaics: Research and Applications 10 (4) (2002) 271–278. [11] S. Dauwe, J. Schmidt, A. Metz, R. Hezel, Fixed charge density in silicon nitride films on crystalline silicon surfaces under illumination, in: Conference Record of the 29th IEEE Photovoltaic Specialists Conference, May 2002, pp. 162–165. [12] J.R. Elmiger, M. Kunst, Investigation of charge carrier injection in silicon nitride/silicon junctions, Applied Physics Letters 69 (4) (1996) 517–519.
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Silicon nitride passivated bifacial Cz-silicon solar cells L. Janßen a,c,, H. Windgassen a, D.L. Ba¨tzner a, B. Bitnar b, H. Neuhaus b a b c
Institute of Semiconductor Electronics, RWTH Aachen University, Sommerfeldstr. 24, 52074 Aachen, Germany Deutsche Cell GmbH, Berthelsdorfer Str. 111a, 09599 Freiberg, Germany Solland Solar Cells GmbH, Bohr 12, 52072 Aachen, Germany
a r t i c l e in fo
abstract
Article history: Received 30 January 2009 Accepted 19 March 2009 Available online 29 April 2009
A new process for all silicon nitride passivated silicon solar cells with screen printed contacts is analysed in detail. Since the contacts are fired through the silicon nitride layers on both sides, the process is easy to adapt to industrial production. The potential and limits of the presented bifacial design are simulated and discussed. The effectiveness of the presented process depends strongly on the base doping of the substrate, but only the open circuit voltage is affected. The current is mainly determined by the rear surface passivation properties. Thus, using a low resistivity ðo1:5 O cmÞ base material higher efficiencies compared to an aluminium back surface field can be achieved. & 2009 Elsevier B.V. All rights reserved.
Keywords: Surface passivation Silicon nitride PECVD Bifacial
1. Introduction Bifacial silicon solar cells and more generally double side passivated designs are developed very intensively in the last few years [1–4]. High open circuit voltages are only achieved in the processes, when using a dedicated back surface field [1] or with local opening of the passivation layer [2]. The metal contact firing through approach, pursued by [3,4], yields only modest open circuit voltages. The limitations of this firing through silicon nitride process, the earlier developed process for passivating thin bifacial silicon solar cells was transferred from multicrystalline to Czochralski silicon [3]. Only small modifications on the rear side collecting grid were carried out compared to the design for the multicrystalline solar cells. Especially, the influence of the base resistivity and the contact formation were investigated in this study. The findings are complemented with results on multicrystalline silicon solar cells to confirm the simulations on the potential and limitation of the process.
2. Experimental The developed double side silicon nitride passivation process is very simple, the details on the process were published elsewhere [3]. After the saw damage removal, a phosphorus diffusion is carried out. Beside an anti-reflection coating the silicon nitride on Corresponding author at: Solland Solar Cells GmbH, Bohr 12, 52072 Aachen, Germany. Tel.: +49 45 8001201; fax: +49 45 8800605. E-mail address: [email protected] (L. Janßen).
0927-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2009.03.015
the front provides also an etching mask for the rear emitter removal. Subsequently, a rear silicon nitride layer is deposited and the contacts are screen printed and co-fired. Based on this process the dependence of the open circuit voltage on the base resistivity is analysed in the first part of this paper. In the second part the contact formation of aluminium and silver–aluminium pastes with silicon will be investigated in presence of a silicon nitride layer. To determine the contact resistance with the transfer length method, stripes of 5 mm width were cut from the solar cells and evaluated as described in [9]. The specific contact resistance ðrc Þ is approximated as rc ¼ Rc =A with Rc the contact resistance and A the area.
3. Results and discussion 3.1. Bifacial solar cells As described above a first set of monocrystalline solar cells was processed with the bifacial and a standard aluminium back surface field (Al-BSF) process. A 2 O cm Cz silicon was used for the first set of solar cells. For the bifacial process two different commercially available aluminium pastes were tested, denoted as Al1 and Al2. In accordance to the earlier results [3] the short circuit current density is increased for the solar cells with the bifacial processes in comparison to the solar cells with the Al-BSF process (see Table 1). Noticeable is the difference in the open circuit voltage, which was not observed on multicrystalline silicon [3]. With approximately 75% the fill factors of the bifacial cells are rather low and fitting the light I2V curves to the two diode model yields a higher series resistance for these cells ðDRs ¼ þ0:2 O cm2 Þ.
ARTICLE IN PRESS 1436
L. Janßen et al. / Solar Energy Materials & Solar Cells 93 (2009) 1435–1439
0.8
Rear contact
J sc ðmA=cm2 Þ
V oc (mV)
Fill factor (%)
Z (%)
Al-BSF Bifacial Al1 Bifacial Al2
36:2 0:2 36:9 0:2 36:5 0:1
619:2 1:0 612:1 1:3 603:5 1:6
76:8 0:4 75:1 0:2 74:6 0:3
17:2 0:2 17:0 0:1 16:4 0:1
Rear side Bifacial Al1 Bifacial Al2
22:2 0:3 19:4 0:5
602:3 1:4 593:5 3:1
77:1 0:5 77:3 0:5
10:3 0:1 8:9 0:3
Different aluminium pastes were used. Values are averaged over sets of at least four solar cells. Errors indicate the standard deviation for each group of at least four solar cells.
Rear internal quantum efficiency
Table 1 Solar cell parameters for different rear side contacts on 2 O cm silicon.
Value
Unit
Thickness Sfront Rsheet;Emitter Peak doping
200 55 000 60
mm cm/s O=&
7 1019 45 (60)
cm3 ms cm3 mm
BSF depth
1 1019 7.5
0.5 0.4
Values in brackets for 6 O cm silicon.
Comparing the different rear contact pastes in the bifacial process, it can be seen that all I2V parameters for front side illumination of paste Al2 are lower compared to paste Al1. This indicates worse surface passivation properties on the rear side of the solar cell. Measuring the I2V characteristics with rear side illumination confirms that assumption, since the short circuit current density at rear illumination is nearly 3 mA=cm2 lower for paste Al2 than for paste Al1. With rear internal quantum efficiency (IQE) measurements the surface recombination velocity can be quantified [5]. The rear IQE was fitted with PC1D [6] using the input parameters in Table 2. Even though the fixed charges of the silicon nitride layers were measured in metal nitride silicon capacitances with C2V measurements to 1 1012 cm2 , it is not possible to include them properly in PC1D since then the simulations do not converge anymore. The results of the simulations are plotted together with the rear IQE measurements in Fig. 1. The deviation of the fit and the measured IQEs in the short wave length range is caused by the absorption of the silicon rich SiN layer on the rear. In contrast to the front side the rear IQE is not curved in the short wave length region, since the doping level near the surface is low. The simulated effective rear surface recombination velocity is 350 cm/s for the solar cells with aluminium paste Al1 and 750 cm/s for the solar cells with paste Al2. Since the solar cells were processed identically in one batch, the difference in the rear surface passivation is due to the contact formation, respectively, the difference in widening of the different aluminium pastes. The higher metal coverage of paste Al2 causes more recombination at the surface. The electronic and rheologic properties of the rear contact pastes are discussed in more detail in the second part of this paper. The loss in open circuit voltage in the bifacial process does not result from a poor rear surface passivation, since surface recombination velocity of 350 cm/s and even 750 cm/s would be low enough to conserve the open circuit voltage in Al-BSF solar cells.
bifacial Al1 bifacial Al2 Seff = 350 cm/s Seff = 750 cm/s
0.3 0.2 0.1
400
500
600 700 800 900 1000 1100 1200 Wavelength [nm]
Fig. 1. Rear internal quantum efficiency, measured and fitted with PC1D.
0.66 Open circuit voltage [V]
Parameter
BSF doping
0.6
0.0 300
Table 2 Input parameters for the PC1D simulations.
tbulk
0.7
0.64
0.62
Srear, eff = 250 cm/s, no BSF Srear, eff = 1500 cm/s, BSF Srear, eff = 250 cm/s, BSF
0.60 1E16
1E17
1E18
Base doping [cm-3] Fig. 2. Dependence of the open circuit voltage on the base doping for three different scenarios. Simulations were carried out with PC1D.
Assuming that no aluminium back surface field is formed in the bifacial process, which would be consistent with SIMS analysis performed by Lenkeit [7], simulations explain the observed voltage loss (see Fig. 2). Three different scenarios were simulated. First a well-passivated rear surface, but without back surface field, second a worse passivated surface including a BSF and third the best case of a well-passivated surface with BSF. As expected the third case yields the highest open circuit voltages over the whole range of simulated base doping levels. The modest surface passivation on the rear has only small effect on the open circuit voltage and is quite comparable to the best case. But the case without BSF and good surface passivation shows a strong drop in the open circuit voltage for lower base doping levels. So if there is no back surface field the open circuit voltage depends mainly on the emitter and base doping. To prove these findings, 6 O cm Cz silicon material was processed, which should amplify the observed effect. The open circuit voltage should drop further 10 mV compared to the 2 O cm Cz silicon. The lower short circuit current density of these solar cells are due to worse alkaline texture (see Table 3). Since the rear contact design was not further optimised with respect to the high base resistivity, the fill factors of the bifacial cells are limited due to a high series resistance caused by spreading resistance in the base
ARTICLE IN PRESS L. Janßen et al. / Solar Energy Materials & Solar Cells 93 (2009) 1435–1439
J sc ðmA=cm Þ
V oc (mV)
Fill factor (%)
Z (%)
Al-BSF Bifacial Al1 Bifacial Al2
34:1 0:2 33:8 0:2 34:0 0:2
621:4 0:8 596:9 0:2 594:2 0:2
76:8 0:4 71:7 0:4 72:2 0:8
16:3 0:1 14:5 0:1 14:6 0:1
Rear side Bifacial Al1 Bifacial Al2
23:4 0:6 22:7 0:6
589:3 2:4 587:6 3:7
73:9 0:2 74:4 1:1
9:6 0:3 9:4 0:3
Rear contact
2
Values are averaged over sets of at least four solar cells. Errors indicate the standard deviation for each group of at least four solar cells.
Table 4 Solar cell parameters for different rear side contacts on 2 O cm Cz silicon using silver and aluminium pastes for the rear contact.
0.8 Rear internal quantum efficiency
Table 3 Solar cell parameters for different rear side contacts on 6 O cm Cz silicon using different aluminium pastes.
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0.7 0.6 0.5 0.4 Paste Ag/Al Al1
0.3 0.2 0.1 0.0 300
400
500
600
700
800
900 1000 1100 1200
Wavelength [nm] Fig. 3. Rear internal quantum efficiency for an Ag/Al and a pure Al paste.
Rear contact
J sc ðmA=cm2 Þ
V oc (mV)
Fill factor (%)
Z (%)
Bifacial Al1 Bifacial Ag/Al
36:6 0:4 36:7 0:2
605:6 1:8 604:0 0:9
73:0 1:2 73:3 1:3
16:2 0:1 16:2 0:3
Rear side Bifacial Al1 Bifacial Ag/Al
21:9 0:8 22:9 0:4
598:8 2:8 596:0 1:3
76:5 0:1 73:7 0:4
10:0 0:4 10:1 0:2
Values are averaged over sets of at least four solar cells. Errors indicate the standard deviation for each group of at least four solar cells.
region [8]. But not only the rear contact design, also the front contact resistance is higher and so contributing to the low fill factors. The higher front contact resistance is due to the lower texturing and a non-optimal firing process. All three groups yield nearly comparable short circuit current densities, but the main interest is in the open circuit voltage which shows the predicted behaviour of an even lower open circuit voltage compared to the 2 O cm Cz silicon. The Al-BSF process is not affected since the silicon aluminium alloying forms an effective high–low junction on the rear surface [7]. Since no effective back surface field is formed in the bifacial process, the aluminium pastes can be substituted with solderable silver–aluminium pastes. Thus definitely no back surface field will be formed. Additionally the number of process steps can be reduced to two instead of three screen printing steps for industrial cell processing. Aluminium paste Al1 and a silver–aluminium paste (Ag/Al) were compared to show the proof of concept. The aluminium content of the investigated paste was 2 percent and therefore low enough to ensure that no back surface field is formed. Alkaline textured 2 O cm Cz silicon was used as substrate. The front I2V parameters are identical for the two pastes considering the measurement accuracy (compare Table 4). Only the rear I2V parameters reveal a slight difference of these cells. Even though the conductivity of the silver–aluminium paste is higher, the measured grid line resistance is higher due to a worse aspect ratio of the printed lines. This leads to an increased series resistance in the rear I2V curve and to a lower fill factor. Since the lines printed with the Ag/Al paste are narrower, the shading is reduced which explains, together with the inherent lower surface recombination (see Fig. 3), the higher short circuit current density. This experiment validates the assumption that no effective back surface field is formed. Thus the lower open circuit voltage for the bifacial process on 2 and 6 O cm Cz silicon is caused by the lower potential caused by the base doping.
Table 5 Solar cell parameters for different rear side contacts on 1 O cm multicrystalline silicon. Rear contact
J sc ðmA=cm2 Þ
V oc (mV)
Fill factor (%)
Z (%)
Al-BSF Bifacial Al1
32:4 0:2 33:8 0:3
605:9 0:5 617:1 3:5
75:9 0:5 74:3 0:8
14:9 0:2 15:5 0:2
Rear side Bifacial Al1
19:3 0:6
601:9 5:2
75:4 0:8
8:3 0:3
Values are averaged over sets of at least four solar cells. Errors indicate the standard deviation for each group of at least four solar cells.
For 1 O cm silicon the open circuit voltage should be higher than for the aluminium back surface field process. Unfortunately no large area Cz silicon was available with such a low base doping. Therefore mutlicrystalline silicon was used. The processing was the same as mentioned before for the Cz wafers. The IV data of these cells show the expected increase in open circuit voltage (see Table 5). The voltage is 11 mV higher then for the reference process on multicrystalline silicon and only slightly lower then the open circuit voltage of the Cz reference process. The increase in the short circuit current density is with DJsc ¼ 1:4 mA=cm2 very high and leads to an efficiency increase of DZ ¼ 0:6%. Even though the choice of the mutlicrystalline silicon is not optimal the same trend as before could be observed. 3.2. Contact formation Since the rear contact formation is a critical process step for the bifacial process, the contact formation with a silicon nitride layer between the printed paste and the silicon is investigated in detail. A dependence of the contact resistance on the used rear side metallisation paste is presented in Table 6. Only aluminium paste Al1 forms low ohmic contacts, whereas the other pastes result in two to three times higher contact resistance values. With 41:2 m O cm2 the contact resistance of the investigated Ag/Al paste is also twice as high as for aluminium paste Al1. Nevertheless the solar cell properties depend only weakly on the contact resistance in this range, as can be seen in the prior results. The contact formation potential of most commercial pastes is limited to penetrate thin silicon nitride layers (e.g. 30 nm), since
ARTICLE IN PRESS L. Janßen et al. / Solar Energy Materials & Solar Cells 93 (2009) 1435–1439
Table 6 Measured contact resistances for different screen printing pastes and different silicon nitride layer thickness on Cz silicon. Contact paste
Layer thickness (nm)
rc ðm O cm2 Þ
Ag/Al Al1 Al2 Al3 Al3
30 30 30 30 70
41.2 20.5 60.3 58.2 49.3
Table 7 Line resistance, aspect ration and specific resistivity of different screen printing pastes. Screen printing paste
RL ðO=mÞ
Width ðmmÞ
Aspect ratio
r ðO cmÞ
Al1
46.0
263
0.11
2:7 105
Al2
46.0
398
0.05
3:0 105
Al3
93.4
317
0.12
6:5 105
Ag/Al
61.2
236
0.06
1:6 105
Ag (VS)
41.9
126
0.13
5:7 106
no additives for silicon nitride etching are included. The development paste Al3 is capable to form contacts even through 70 nm of silicon nitride. The contact resistance of this paste is comparable to paste Al2 and should yield comparable solar cell performance with thin silicon nitride layers, but also on anti-reflection coating like thick layers of around 70 nm. The rear I2V measurements reveal the impact of the line conductivity of the rear contact. Even though all contacts are printed with the same screen the line properties are very different due to the different rheological properties of the screen printing pastes. Beside the line resistance, the line width, the aspect ratio and the specific resistance were determined for the aluminium and silver–aluminium pastes. A front side metallisation paste was also included in the comparison. The line resistance is identical for pastes Al1 and Al2, but there are major differences in the resulting line width and aspect ratio (see Table 7). The lines printed with paste Al2 are 50 percent wider and the aspect ratio is less then half of the aspect ratio of paste Al1. The widening of the fingers on the rear explains the difference in the cell performance, since the passivated surface area is smaller when using paste Al2. Paste Al3 has a high line resistance which can be ascribed to the low conductivity of the paste. The rheologic properties are satisfying in the sense that the aspect ratio is above 0.1 and the line widens approximately 50 percent in comparison to the screen openings. Surprisingly high is the line resistance of the silver–aluminium paste. This result is explained by the low aspect ratio together with a low widening of the lines. The specific resistance is half of that of the aluminium pastes as expected, but not as low as for front contact pastes. Solar cells with a 70 nm thick silicon nitride layer on the rear were produced to verify the previous measurements on the contact formation for paste Al3. Two sets of references were processed on alkaline textured 2 O cm Cz silicon; one set with a 30 nm SiN layer as reference to the prior results and one set with Al-BSF as overall reference. The open circuit voltage for the 30 nm SiN layer reference solar cells is comparable to the previous results (see Table 8) and is 8 mV lower than for the Al-BSF reference. For the 70 nm SiN layer cells the open circuit voltage drops further to 601 mV ðDV oc ¼ 18 mVÞ. Since the contact resistance and the line
Table 8 Solar cell parameters for different rear side contacts on 2 O cm silicon. Rear contact
J sc ðmA=cm2 Þ
V oc (mV)
Fill factor (%)
Z (%)
Al-BSF (0 nm SiN) bifacial (30 nm SiN) bifacial (70 nm SiN)
35:9 0:1 35:8 0:2 35:1 0:1
619:9 2:4 611:6 2:8 601:6 1:0
76:8 0:6 74:9 0:1 74:9 0:2
17:1 0:1 16:4 0:1 15:8 0:1
Rear side bifacial 30 nm bifacial 70 nm
18:1 0:3 20:5 0:5
591:7 0:6 590:6 0:1
75:8 0:3 75:5 0:2
8:1 0:1 9:2 0:1
Aluminium paste Al3 was used for the rear contact formation through different silicon nitride layer thicknesses. Values are averaged over sets of at least four solar cells. Errors indicate the standard deviation for each group of at least four solar cells.
0.7 Rear internal quantum efficiency
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0.6 0.5 0.4 0.3
30 nm 70 nm Seff = 1000 cm/s Seff = 1250 cm/s
0.2 0.1 0.0 300
400
500
600 700 800 900 1000 1100 1200 Wavelength [nm]
Fig. 4. Rear internal quantum efficiency for different silicon nitride layer thickness, simulated IQEs with PC1D.
conductivity are comparable, the difference has to result from different surface passivation properties. To determine the rear surface recombination velocity, rear internal quantum efficiencies were measured and fitted. The rear IQEs are displayed in Fig. 4 together with PC1D simulations. The difference due to the extracted surface recombination velocity would change the open circuit voltage only about 2 mV, if all other parameters remain constant. Therefore, other effects may explain the further open circuit voltage loss compared to the 30 nm SiN layer reference. Since the short circuit current density is also reduced, inversion layer shunting could be a possible explanation [10] or additionally a difference in the fixed charges of the silicon nitride layers [11]. Since the modelling in PC1D is limited to one dimension, a more detailed analysis is very difficult. For the first assumption, the border of the contacted and the passivated area has to be included, which is not possible in PC1D. The second possibility is in fact just an extension of the first theory and is rather unlikely, since the deposition parameters of both layers were the same except for the deposition time. The layer thickness should not influence the amount of fixed charges after an initial increase in the first 20–30 nm [12].
4. Conclusions A process for passivating silicon solar cells with silicon nitride on the rear side was successfully transferred from multicrystalline to Czochralski silicon. The observed open circuit voltage loss is
ARTICLE IN PRESS L. Janßen et al. / Solar Energy Materials & Solar Cells 93 (2009) 1435–1439
identified to depend on the base doping of the wafer material. Since the presented bifacial process does not form an aluminium back surface field, the open circuit voltage does strongly depend on the doping concentration in the emitter and bulk region. This fact allows the use of solderable silver–aluminium pastes instead of aluminium pastes with no loss in efficiency. With a base resistivity below 1:5 O cm the open circuit voltage of bifacial cells compared to Al-BSF solar cells can be maintained or even exceeded, as extracted from PC1D simulations. This is supported by the fact that on 1 O cm multicrystalline silicon open circuit voltages above 620 mV have already been achieved. Thus the benefit of the concept can be used on multicrystalline silicon, since the base resistivity is typically 1:0 O cm. The contact formation was investigated in detail. Rear contact resistivities as low as 20 m O cm2 and aspect ratios of up to 0.12 were achieved for contacting through a 30 nm thick silicon nitride layer. The feasibility of contacting through 70 nm thick silicon nitride layers was demonstrated. However, the shunting effect of the inversion layer affects the open circuit voltage adversely.
Acknowledgements The funding of this project by the Ministry of Innovation, Science, Research and Technology of the State of North RhineWestphalia, Germany, no. 313-206 002 04, is gratefully acknowledged.
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