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Recent results obtained at IMEC on multicrystalline silicon solar cells
~)
Pergamon 0960-1481 (95)00050-X
Renewable Energy, Vol. 6, No. 5-6. pp. 573 578, 1995 Copyright ~) I995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0960-1481/95 $9.50+0.00
R E C E N T RESULTS O B T A I N E D AT IMEC ON M U L T I C R Y S T A L L I N E SILICON SOLAR CELLS J. N I J S , H. E. E L G A M E L , J. S Z L U F C I K , S. S I V O T H T H A M A N , O. E V R A R D , K . D E C L E R C Q , P. D E S C H E P P E R , J. P O O R T M A N S , M. GHANNAM a n d R. M E R T E N S IMEC, Leuven, Belgium and P. F A T H a n d G . W I L L E K E University of Konstanz, Konstanz, Germany
Abstract--In order to optimize the efficiency of multicrystalline silicon solar cells, the influence of specific process steps and sequences were studied. Therefore clean-room high efficiency as well as industrial screenprinted cells were fabricated. Benefits are found in choosing a substrate with lower base resistivity, using front and rear oxide passivation, using hydrogen passivation for bulk and surfaces, the use of Si3N4with a double function i.e. as an anti-reflection and passivation layer and the use of mechanical V-grooving. Efficiencies of 17% are found on 4 cm 2 clean-room fabricated cells and 15.2% has been obtained on 100 cm2 V-grooved screenprinted industrial cells.
INTRODUCTION
long time. However, in the case of multicrystalline silicon there are many other factors that also control the lifetime, making it not solely dependent and therefore less sensitive to the doping concentration. In addition to the grain boundaries, multi-Si materials have a large quantity of defects such as dislocations. It has been proven experimentally [I] that in low resistivity multicrystalline material, the dependence of lifetime on defect density is very weak, whereas in the case of high resistivity multi-Si, the lifetime has a strong dependence on defect density. Polix multicrystalline material (from Photowatt, France) shows good improvements after treatments such as gettering [2]. Therefore low resistivity ( < 0 . 2 cm) Polix material is used in this study in order to enhance the open-circuit voltage, while at the same time not lowering too much the bulk diffusion length : in gettered 0.2 ~ cm Polix material the bulk diffusion length is still around 200/~m [3]. The process flow of cells with selective emitter on these substrates is as follows :
The PV market has risen to a yearly volume of about 60 M W p , as we learn from P V N e w s and PVlnsider's Report. A b o u t 75% of that amount is based on crystalline silicon solar cells, of which about 45% is single crystalline and 30% is multicrystalline silicon. A lot of solar cell manufacturers make this material inhouse and the main issues are uniformity, reproducibility, material quality, material yield and efficiency of the fabricated solar cells. In the framework of a European project and also thanks to many informal international contacts, I M E C a.o. practices R & D in order to increase the efficiency of multicrystalline silicon solar cells, as well as on the relation between final solar cell efficiency and starting material quality. Clean-room fabricated cells are made to study the theoretical limits of solar energy conversion on such materials. Industrial screen-printed cells are developed to look for practical, cost-effective goals.
1. HIGHEFFICIENCYCLEAN-ROOM FABRICATED CELLS
saw damage removal ; phosphorous gettering by POC13 diffusion (900°C) ; removal of n ++ layers; boron diffusion from Bdoped A P C V D oxide (940°C) ;
1.1. Influence o f base resistivity In single crystalline silicon the doping concentration/lifetime relationship has been known for a 573
574
J. N I J S
phosphorous diffusion from P~05 solid source O0ff'C) ; mask oxide by wet oxidation on front side (920°C) ; photolithography to define finger pattern ; n + + removal between fingers ; NaOH texturing between fingers ; 2nd blanket diffusion of phosphorus (900°C) ; emitter etch-back (80 ~/sq.) ; surface passivation by oxidation (900°C) ; hydrogen plasma passivation ; lithography + metallization + lift-off; sintering of metal contacts (forming gas) ; evaporation of double antireflection coating ZnS/MgF2 ; annealing in H2 to passivate the e-beam damage. Although the wafer quality of high resistivity wafers (2.5 f~ cm) is slightly higher, they are penalized by their relatively low open-circuit voltage (see Table 1). The best low resistivity cell was also independently measured by NREL. They measured an open-circuit voltage of 639.8 mV (almost 640 mV) being the world record Voc for multi-Si cells made by any technology.
1.2. Influence of rear oxide passivation Attempts were also made to improve the Voc of cells on high resistivity Polix wafers although their bulk saturation current was found to be quite high. As the high resistivity wafers have a rather large diffusion length, the use of a heavily doped BSF (p+) layer complemented by a thin passivating oxide is very beneficial. The back side aluminium contacts are made by opening holes in the rear passivating oxide. In the final structure 70% of the outer p+ surface remains oxide-passivated. This structure has helped to improve the Voc by about 8-10 mV and a highest Voc of 601-602 mV was achieved on 2.5 fl cm Polix wafers with a very simple homogeneous emitter structure [4] (unlike under Section 1.1 where a selective emitter was used). The infrared spectral response is also increased when the outer p+ BSF layer is oxide-passivated. Table 2 shows the full results,
Table 1. Illuminated (AM1.5) /~V parameters of high efficiency 4 c m 2 multicrystalline Si (Polix) selective emitter solar cells made from different base resistivity substrates Wafer J~c resistivity (mA/cm2) 0.2 0.7 2.5
34.1 34.7 35.5
Vo~ (mY)
FF (%)
Efficiency (%)
638 620 596
77.3 78.2 78.3
16.8 16.8 16,5
et al. 1.3. Influence of bulk and surface passivation techniques, a.o. usin9 different H-passivation schemes For this work, multicrystalline silicon produced by Eurosolare (directionally solidificated material : 1.6 cm, 360 /~m thick) and Sumitomo-Sitix (electromagnetically casted material : 1 ~ cm, 300 ~tm thick) are used. The process consists of forming a selective emitter structure by doing first a heavy phosphorus diffusion at 900°C to obtain a sheet resistance of 20 ~/sq. under the front side contact fingers, removing the phosphorus diffused layer and texturing between the fingers. Then a light phosphorus diffusion at 900°C is done, followed by an etch-back in a weak acid solution to obtain an active emitter sheet resistance of 70-80 ~/sq. Four passivation schemes are investigated. A first passivation scheme, the reference group, A, does not involve any hydrogen passivation but includes oxide passivation of the emitter surface. Note that the oxide passivation step is done at 900°C for 6 min and results in an oxide thickness around 80-90 A. In a second scheme, B, both front and back sides received hydrogen rf plasma passivation, without surface oxide passivation. For group B the emitter is etched back after the hydrogen plasma step in order to etch off the emitter surface rf plasma-damaged layer. In a third passivation scheme, C, the front side has a thin oxide layer thermally grown in dry oxygen using the same conditions used in the cells of group A, and the back side is subjected to a hydrogen rf plasma treatment similar to the cells of group B. The front contact metal fingers are realized by evaporation of a triple metal layer of Ti/Pd/Ag (500 ~/500 A/6 ktm) on top of a defined photoresist pattern, followed by lift-off. The back side metal contact is obtained by A1 (2 pm) evaporation on the whole back side of the substrates. The sintering of the contacts is performed at 400°C for 20 rain in forming gas. In the last passivation scheme, D, some solar cells from group C are exposed to a microwave-induced remote (mir) plasma treatment for 1 h at a pressure of 2.0 torr, a temperature of 350°C, and a power of 75 W. The advantage of a remote plasma is that it avoids damage of the cell surface. Finally, the evaporation of a double antireflection coating consists of 106 nm MgF2 on top of 56 nm ZnS. Table 3 shows the illuminated 1-V characteristics, measured under standard illumination conditions (AM 1.5, 25"C). Averages of 4 cells are presented for each condition. For the conventionally casted Eurosolare material, the comparison between group A and group C indicates that treating the back side of the cells with a hydrogen rf plasma results in an improvement of up
Multicrystalline silicon solar cells
575
Table 2. Effect of rear passivation on 2.5 f~ cm multi-Si wafers. The cells had simple (and relatively deeper) homogeneous emitters and had a size of 4 cm 2 Cell structure BSF + rear SiO2 BSF, no SiO2
J~ (mA/cm2)
G~ (mY)
FF (%)
Efficiency (%)
34.4 33.0
601 588
78.2 79.3
16.2 15.4
Table 3. The illuminated ~ V measurements of evaporated contact Eurosolare and SSC EMC multicrystalline silicon solar cells before and after exposing to hydrogen mir plasma passivation from the emitter side. The cell area is 4 cm 2 Material Eurosolare
SSC-EMC
Group A B C D A B C D
J~ Voc (mA/cm 2) (mY) 33 33.8 34.2 34.6 31 33 33.9 34.2
600 603 610 616 584 600 604 610
FF (%)
q (%)
77.6 77.9 78 78.2 75.4 76 76.4 77
15.4 15.9 16.3 16.7 13.6 15.0 15.5 16.0
to 1.2 m A / c m 2 in J~c and up to 10 mV in Voc. In addition, the implementation of hydrogen mir plasma from the emitter side (group D cells) boosts the efficiency from 16.3% up to 16.7%. However, for the case of the S S C - E M C material, by comparing groups A and C, one can see that treating the back side of the cells with a hydrogen rf plasma results in Jsc and Voc improvements up to 3 m A / c m 2 and 20 mV, respectively. This yields a 2% absolute enhancement in the efficiency which is double what is achieved in the Eurosolare multicrystalline silicon solar cells. In addition, the implementation of hydrogen mir plasma (group D) boosts the efficiency from 15.5 up to 16%. This confirms that the electrical parameter improvement of the E M C cells due to hydrogenation is more significant compared to Eurosolare cells. The external quantum efficiencies of the ceils of groups A, C and D measured under white bias light for the S S C - E M C material are displayed in Fig. 1. Note that the improvement in the short wavelength response of the cells of group D compared to the cells of groups A and C upon performing remote hydrogen plasma is mainly due to the efficient passivation by the hydrogen atoms of the dangling bonds at the silicon/silicon dioxide interface. This passivation, of course, will result in reducing the surface recombination velocity and hence the enhancement of the short-circuit current density and open-circuit voltage. Moreover, it is clear that although the starting quality of the S C C - E M C multicrystalline material is
100
'
.....
20
'
"'
Group A (front oxide)
~
(
~&
font o~de and back rf H)
\.
Group D (front oxid&remotc H and back rf H)~, 0
0.4
.
l
0.5
,
I,
0.6
,
I
0.7
,
'
'
'
0.8
0.9
1.0
1.1
Wavelength (jJ.m)
Fig. 1. External quantum efficiency of the 4 c m 2 solar cells fabricated on SSC-EMC multicrystalline silicon material for the different passivation schemes.
inferior to the Eurosolare material because S C C E M C material has smaller grains and therefore more grain boundaries, the bulk hydrogen passivation has resulted in S C C - E M C cells with efficiencies very close to the efficiencies achieved by the Eurosolare cells.
1.4. Combination of low resistivity substrates, rear oxide passivation and optimal hydroyen passivation scheme Thin ( < 200 pro) good quality Polix material with 0.2 f~ cm base resistivity is used in the fabrication process. The fabrication process implemented here for achieving high efficiency multicrystalline silicon solar cells is roughly the same as for the cells of group D in Section 1.3, but performing additionally at the beginning of the process (before the formation of the emitter) a drive-in step on A P C V D boron-doped oxide to form a BSF at the back side. The passivation steps for bulk and surfaces are : oxide passivation at temperature of 900~C in a dry oxygen ambient for 6 min (the resulting oxide thicknesses are in the range of 60-90 A on the front side and 200-250/~ on the back side) ; hydrogen mir plasma passivation from emitter and back sides of the cells for 1 h, with a microwave
J. NIJS et al.
576
Table 4. The illuminated ~ V measurements of the Polix solar cells (with BSF) fabricated using different passivation schemes Material Polix
Oxide mir hydrogen passivation passivation No Yes Yes Yes
No No Yes Yes
H2 annealing
J~c (mA/cm 2)
Vo~ (mV)
FF (%)
Efficiency (%)
No No No Yes
31.2 32.4 33.6 34.1
615 624 634 640
77.5 77.6 78.0 78.0
14.9 15.7 16.6 17.0*
* Independently confirmed at N REL as 16.93 % using calibrated measurements.
plasma power of 75 W, pressure of 2.0 torr and a temperature of 350°C ; sintering the front and back contacts at 400°C for 20 min in forming gas (Hz/N2) in order to ensure a good ohmic contact and to recover the damage occurring in the oxide during the e-beam evaporation of the metal. Then the substrates are diced into 2 cm x 2 cm solar cells; after the double A R C evaporation comes an annealing of the cells in forming gas or H2 at 350°C for 15 rain. The cross-section of these multicrystalline silicon solar cells is depicted in Fig. 2. The illuminated I--V characteristics of these Polix multicrystalline silicon solar cells (4 cm 2) measured under standard illumination conditions (AMI.5, 25°C) are shown in Table 4. 2. PILOT-LINE CELLS USING SCREEN-PRINTING TECHNOLOGY
2.1. Combination o f passivation schemes and the use of Si3N4
The plasma-enhanced chemical vapour deposition (PECVD) of a silicon nitride layer seems to be a natural choice. It can serve as an anti-reflection coating layer, passivating layer and can be deposited with a
Z n S / M g F2
Si02
high throughput in existing commercial systems. The significant improvement in multicrystalline solar cell performance after P E C V D silicon nitride deposition has been reported by many authors [5-12]. This improvement is often explained by a surface and bulk passivation effect due to the diffusion of atomic hydrogen from the deposited layer. However, there are contradictory reports about the passivation stability if a post-deposition thermal treatment is applied in the temperature range above 500°C [5,7,9]. Experiments were done with multicrystalline silicon substrates (100 cm 2) from different suppliers using different sequences of metallization by screenprinting and P E C V D deposition of Si3N 4 [13]. After the initial saw damage etching, texturing and cleaning, the emitter region was formed by screenprinting a phosphorous paste and diffusion in a belt furnace. Next, a thin passivating oxide 10-15 nm thick was grown at 800°C in an open tube furnace. Additional passivation by a hydrogen plasma with and without dry SiO2 and combinations of both were applied before the plasma nitride deposition to further improve the cell parameters. The best process sequence consists of growing a thin (15 nm) dry SIO2, deposition of 80 nm of P E C V D SiNx, screenprinting metallization followed by a fast firing in the I R furnace at 650-700°C. The results are shown in Table 5. 2.2. Influence of texturization : chemical etchin9 versus mechanical V-grooving The advanced technology of crystalline silicon solar cells requires cell structures that are able to reduce the
Table 5. Best PECVD Si3N4 process on multicrystalline materials from different suppliers ; the cell area equals 100cm2
Fig. 2. Cross-section of the multicrystalline silicon solar cell with optimal passivation scheme.
Material
J~c (mA/cm2)
Vo¢ (mV)
FF (%)
Eft. (%)
A B C
30.9 31.5 31.8
602 603 604
77.5 77.4 77.3
14.5 14.7 14.9
Multicrystalline silicon solar cells high reflectivity o f silicon a n d at the same time e n h a n c e the optical p a t h length. The well-known stand a r d m e t h o d , applied in mass p r o d u c t i o n , relies o n an anisotropic etching. This technique is r a t h e r ineffective when applied to multicrystalline silicon substrates due to a r a n d o m distribution o f different grain orientations. Therefore there is a strong need for an orient a t i o n - i n d e p e n d e n t a n d efficient texturing method, especially one t h a t can easily be i m p l e m e n t e d in a n industrial e n v i r o n m e n t . Several techniques such as laser scribing [14], use o f defect etching [15], silicon m i c r o - m a c h i n i n g [16], a n d mechanical g r o o v i n g [17] are currently u n d e r investigation. O u r work is focused o n mechanical V-grooving using a c o n v e n t i o n a l dicing saw a n d bevelled blades [18-20]. Multicrystalline Silso silicon substrates, Vgrooved with a 35 ~ blade tip angle showed excellent light t r a p p i n g properties a n d effectively reduced front surface reflection. A m i n i m u m total reflectance o f 5.6% at 950 n m a n d an average R = 6.6% in the range 500 1000 n m has been o b t a i n e d on bare Si surface [18]. Multicrystalline, as-cut wafers with a size 10 x 10 cm 2 f r o m different suppliers, which were denoted as A, B a n d C, were used for solar cell processing in the optimised process sequence. The specific resistivity o f all wafers was a r o u n d 1 ~ cm. Wafer thicknesses of 280 (__+15) /~m, 350 (_+20) /~m a n d 360 ( + 1 6 ) /~m were m e a s u r e d for the A, B a n d C materials respectively. The substrates were V-grooved o n the f r o n t side as described above. The m e a s u r e d groove d e p t h after saw d a m a g e etching was a r o u n d 50/ma. N o n grooved wafers were added to evaluate the benefit o f mechanical grooving. The emitter was formed by diffusion from a solid P2Os source at 9 0 0 C for 10 rain. The emitter sheet resistance m e a s u r e d on n o n grooved wafers was 30 D/sq. A thin dry silicon dioxide was g r o w n for 15 min at 800~C. A P E C V D silicon nitride with a thickness o f 750-800 • was deposited at 350~C. Silver paste was screen-printed on the f r o n t side a n d s i l v e r - a l u m i n i u m paste on the back side o f the cells. A front c o n t a c t p a t t e r n optimised for Vgroove structures was used. O n some cells 1300 A, of MgF2 was e v a p o r a t e d as a second antireflection coating. Both contacts were co-fired at 685°C in a n I R furnace. T h e illuminated I - V p a r a m e t e r s were m e a s u r e d u n d e r s t a n d a r d A M 1.5 (100 m W / c m 2) radiation. A reference cell calibrated at the F r a u n h o f e r Institute for Solar Energy was used for simulator adjustment. The results are presented in Table 6 [21]. C o m p a r i s o n between V-grooved a n d n o n - g r o o v e d chemically textured solar cells with a double A R C reveals a difference in efficiency up to 0.9% absolute. This i m p r o v e m e n t comes from a 2 m A / c m 2 increase
577
Table 6. Results of illuminated I - V measurements of large area multicrystalline solar cells with a mechanically Vgrooved surface and different base materials
A B C C C
V-groove
DARC
Yes Yes No Yes Yes
Yes Yes Yes No Yes
J~, Voc (mA/cm 2) (mV) 32.7 32.3 30.3 31.6 32.3
602 598 603 598 599
FF (%)
q (%)
77.1 77.0 77.4 78.0 77.7
15.2 14.9 14. l 14.7 15.0
in the short-circuit current in the case of V-grooved cells. This can clearly be seen from external q u a n t u m efficiency m e a s u r e m e n t s at all wavelengths. However, internal q u a n t u m efficiency m e a s u r e m e n t s show t h a t the blue response of the V-grooved cell is s o m e w h a t inferior to the n o n - g r o o v e d cell [21]. CONCLUSION It is p r o v e n in this work h o w i m p o r t a n t the impact of certain specific process steps on the final multicrystalline silicon solar cell efficiency is. Efficiencies up to 17% are o b t a i n e d on 4 cm 2 clean-room fabricated cells a n d 15.2% has been o b t a i n e d on 100 cm 2 V-grooved screen-printed industrial cells. Acknowledgements--The authors acknowledge the financial support for this work from the Multichess project in the Joule programme of the Commission of the European Communities DG Xll, under contract number JOU2 CT92 0179. They also like to thank W. Laureys and P. Laermans, both from IMEC, for their contribution in the cell processing, and H. Nussbaumer and M. Steiner from University of Konstanz for their help during the measurements. The authors also would like to thank L. Frisson for his contribution in the optimisation of the fast-firing screen-printed contacts process. The authors also want to thank different multicrystalline silicon material manufacturers, such as Photowatt, Eurosolare, Bayer, Wacker and Sumitomo Sitix for providing us with the necessary substrates. REFERENCES
1. B. L. Sopori, Influence of substrate resistivity on the degradation of silicon solar cell performance due to crystal defects. Proc. 20th IEEE P V S C , p. 1412 (1988). 2. I. P~richaud and S. Martinuzzi, Additivity of phosphorus gettering and hydrogenation in multicrystalline silicon cells. Proc. 22nd IEEE P V S C , Las Vegas, pp. 877 882 (1991). 3. S. Sivoththaman, M. Rodot, L. Q. Nam, D. Sarti, M. Ghannam and J. Nijs, Proc. 23rd IEEE P V S C , Louisville, p. 335 (1993). 4. S. Sivoththaman, Doctoral Thesis, University of Paris XII (France), December 1993. 5. P. P. Michiels, L. A. Verhoef, J. C. Stroom, W. C. Sinke, R. J. C. van Zolingen, C. M. M. Denisse and M. Hen-
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6. 7. 8. 9. 10. 11. 12. 13.
J. NIJS et al. driks Proc. 21st. IEEE Photovoltaic Spec. Conf., Kissimmee, U.S.A., p. 638 (1990). K. Kimura, Techn. Digest of Int'l PVSEC-1, Kobe, Japan, p. 37 (1984). M. Takayama, H. Yamashita, K. Fukui, K. Masuri, K. Shirasawa and H. Watanabe, Tech. Doest of Int'l PVSEC-5, Kyoto, Japan, p. 319 (1990). J. Coppye, J. Szlufcik, H. Elgamel, M. Ghannam, P. De Schepper, J. Nijs and R. Mertens, Proc. 22nd IEEE Photovoltaic Spec. Conf., Las Vegas, p. 873 (1991). J. C. Muller, B. Hartiti, E. Hussian, J. P. Schunck, P. Siffert and D. Sarti, Proe. 22nd IEEE Photovoltaic Spec. Conf., Las Vegas, p. 883 (1991). S. Narayanan, S. Roncin and J. Wohlgemuth, Tech. Digest of lnt'l PVSEC-6, New Delhi, India, p. 133 (1992). T. Bickl, U. Creutzburg, D. Silber, W. Ebner, M. Eyckmans and G. Wandel, Proc. lOth Eur. Photovoltaic Solar Energy Conf., Lisbon, Portugal, p. 651 (1991). A. Rohatgi, Z. Chen, P. Sana, R. Ramanachalam, J. Crotty and J. Salami, Techn. Doest of Int'l PVSEC-7, Nagoya, Japan, p. 93 (1993). J. Szlufcik, K. Declercq, P. De Schepper, J. Poortmans,
14. 15. 16. 17. 18. 19. 20. 21.
A. Buczkowski, J. Nijs and R. Mertens, Proc. 12th European Photovoltaic Solar Energy Conf., Amsterdam, pp. 1018 1021 (1994). S. Narayanan, J. Zolper, F. Yun, S. Wenham, A. Sproul, C. Chong and M. Green, Proc. 21st IEEE P V Specialists Conf., Kissime, p. 678 (1990). U. Kaiser, M. Kaiser and R. Schindler, Proc. lOth. EC Photovoltaic Energy Conf., Lisbon, (Kluwer Ed.) 293 (1991), J. Culick, E. Jackson and A. Barnett, Proc. 21st. IEEE P V Specialists Conf., Kissime, p. 251 (1990). T. Saitoh, R. Shimokawa and Y. Hayashi, Proc. llth EC Photovoltaic Energy Conf., Harwood Academic, Switzerland, p. 393 (1993). G. Willeke, H. Nussbaumer, H. Bender and E. Bucher, ibid., p. 480. G. Willeke, H. Nussbaumer, H. Bender and E. Bucher, Solar Energy Materials and Solar Cells 26, 345 (1992). H. Nussbaumer, G. Willeke and E. Bucher, J. Appl. Phys. 75 (4), 2202 (1994). J. Szlufcik, P. Fath, J. Nijs, R. Mertens, G. Willeke and E. Bucher, Proc. 12th European Photovoltaic Solar Energy Conf., Amsterdam, pp. 769 772 (1994).
Pergamon 0960-1481 (95)00050-X
Renewable Energy, Vol. 6, No. 5-6. pp. 573 578, 1995 Copyright ~) I995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0960-1481/95 $9.50+0.00
R E C E N T RESULTS O B T A I N E D AT IMEC ON M U L T I C R Y S T A L L I N E SILICON SOLAR CELLS J. N I J S , H. E. E L G A M E L , J. S Z L U F C I K , S. S I V O T H T H A M A N , O. E V R A R D , K . D E C L E R C Q , P. D E S C H E P P E R , J. P O O R T M A N S , M. GHANNAM a n d R. M E R T E N S IMEC, Leuven, Belgium and P. F A T H a n d G . W I L L E K E University of Konstanz, Konstanz, Germany
Abstract--In order to optimize the efficiency of multicrystalline silicon solar cells, the influence of specific process steps and sequences were studied. Therefore clean-room high efficiency as well as industrial screenprinted cells were fabricated. Benefits are found in choosing a substrate with lower base resistivity, using front and rear oxide passivation, using hydrogen passivation for bulk and surfaces, the use of Si3N4with a double function i.e. as an anti-reflection and passivation layer and the use of mechanical V-grooving. Efficiencies of 17% are found on 4 cm 2 clean-room fabricated cells and 15.2% has been obtained on 100 cm2 V-grooved screenprinted industrial cells.
INTRODUCTION
long time. However, in the case of multicrystalline silicon there are many other factors that also control the lifetime, making it not solely dependent and therefore less sensitive to the doping concentration. In addition to the grain boundaries, multi-Si materials have a large quantity of defects such as dislocations. It has been proven experimentally [I] that in low resistivity multicrystalline material, the dependence of lifetime on defect density is very weak, whereas in the case of high resistivity multi-Si, the lifetime has a strong dependence on defect density. Polix multicrystalline material (from Photowatt, France) shows good improvements after treatments such as gettering [2]. Therefore low resistivity ( < 0 . 2 cm) Polix material is used in this study in order to enhance the open-circuit voltage, while at the same time not lowering too much the bulk diffusion length : in gettered 0.2 ~ cm Polix material the bulk diffusion length is still around 200/~m [3]. The process flow of cells with selective emitter on these substrates is as follows :
The PV market has risen to a yearly volume of about 60 M W p , as we learn from P V N e w s and PVlnsider's Report. A b o u t 75% of that amount is based on crystalline silicon solar cells, of which about 45% is single crystalline and 30% is multicrystalline silicon. A lot of solar cell manufacturers make this material inhouse and the main issues are uniformity, reproducibility, material quality, material yield and efficiency of the fabricated solar cells. In the framework of a European project and also thanks to many informal international contacts, I M E C a.o. practices R & D in order to increase the efficiency of multicrystalline silicon solar cells, as well as on the relation between final solar cell efficiency and starting material quality. Clean-room fabricated cells are made to study the theoretical limits of solar energy conversion on such materials. Industrial screen-printed cells are developed to look for practical, cost-effective goals.
1. HIGHEFFICIENCYCLEAN-ROOM FABRICATED CELLS
saw damage removal ; phosphorous gettering by POC13 diffusion (900°C) ; removal of n ++ layers; boron diffusion from Bdoped A P C V D oxide (940°C) ;
1.1. Influence o f base resistivity In single crystalline silicon the doping concentration/lifetime relationship has been known for a 573
574
J. N I J S
phosphorous diffusion from P~05 solid source O0ff'C) ; mask oxide by wet oxidation on front side (920°C) ; photolithography to define finger pattern ; n + + removal between fingers ; NaOH texturing between fingers ; 2nd blanket diffusion of phosphorus (900°C) ; emitter etch-back (80 ~/sq.) ; surface passivation by oxidation (900°C) ; hydrogen plasma passivation ; lithography + metallization + lift-off; sintering of metal contacts (forming gas) ; evaporation of double antireflection coating ZnS/MgF2 ; annealing in H2 to passivate the e-beam damage. Although the wafer quality of high resistivity wafers (2.5 f~ cm) is slightly higher, they are penalized by their relatively low open-circuit voltage (see Table 1). The best low resistivity cell was also independently measured by NREL. They measured an open-circuit voltage of 639.8 mV (almost 640 mV) being the world record Voc for multi-Si cells made by any technology.
1.2. Influence of rear oxide passivation Attempts were also made to improve the Voc of cells on high resistivity Polix wafers although their bulk saturation current was found to be quite high. As the high resistivity wafers have a rather large diffusion length, the use of a heavily doped BSF (p+) layer complemented by a thin passivating oxide is very beneficial. The back side aluminium contacts are made by opening holes in the rear passivating oxide. In the final structure 70% of the outer p+ surface remains oxide-passivated. This structure has helped to improve the Voc by about 8-10 mV and a highest Voc of 601-602 mV was achieved on 2.5 fl cm Polix wafers with a very simple homogeneous emitter structure [4] (unlike under Section 1.1 where a selective emitter was used). The infrared spectral response is also increased when the outer p+ BSF layer is oxide-passivated. Table 2 shows the full results,
Table 1. Illuminated (AM1.5) /~V parameters of high efficiency 4 c m 2 multicrystalline Si (Polix) selective emitter solar cells made from different base resistivity substrates Wafer J~c resistivity (mA/cm2) 0.2 0.7 2.5
34.1 34.7 35.5
Vo~ (mY)
FF (%)
Efficiency (%)
638 620 596
77.3 78.2 78.3
16.8 16.8 16,5
et al. 1.3. Influence of bulk and surface passivation techniques, a.o. usin9 different H-passivation schemes For this work, multicrystalline silicon produced by Eurosolare (directionally solidificated material : 1.6 cm, 360 /~m thick) and Sumitomo-Sitix (electromagnetically casted material : 1 ~ cm, 300 ~tm thick) are used. The process consists of forming a selective emitter structure by doing first a heavy phosphorus diffusion at 900°C to obtain a sheet resistance of 20 ~/sq. under the front side contact fingers, removing the phosphorus diffused layer and texturing between the fingers. Then a light phosphorus diffusion at 900°C is done, followed by an etch-back in a weak acid solution to obtain an active emitter sheet resistance of 70-80 ~/sq. Four passivation schemes are investigated. A first passivation scheme, the reference group, A, does not involve any hydrogen passivation but includes oxide passivation of the emitter surface. Note that the oxide passivation step is done at 900°C for 6 min and results in an oxide thickness around 80-90 A. In a second scheme, B, both front and back sides received hydrogen rf plasma passivation, without surface oxide passivation. For group B the emitter is etched back after the hydrogen plasma step in order to etch off the emitter surface rf plasma-damaged layer. In a third passivation scheme, C, the front side has a thin oxide layer thermally grown in dry oxygen using the same conditions used in the cells of group A, and the back side is subjected to a hydrogen rf plasma treatment similar to the cells of group B. The front contact metal fingers are realized by evaporation of a triple metal layer of Ti/Pd/Ag (500 ~/500 A/6 ktm) on top of a defined photoresist pattern, followed by lift-off. The back side metal contact is obtained by A1 (2 pm) evaporation on the whole back side of the substrates. The sintering of the contacts is performed at 400°C for 20 rain in forming gas. In the last passivation scheme, D, some solar cells from group C are exposed to a microwave-induced remote (mir) plasma treatment for 1 h at a pressure of 2.0 torr, a temperature of 350°C, and a power of 75 W. The advantage of a remote plasma is that it avoids damage of the cell surface. Finally, the evaporation of a double antireflection coating consists of 106 nm MgF2 on top of 56 nm ZnS. Table 3 shows the illuminated 1-V characteristics, measured under standard illumination conditions (AM 1.5, 25"C). Averages of 4 cells are presented for each condition. For the conventionally casted Eurosolare material, the comparison between group A and group C indicates that treating the back side of the cells with a hydrogen rf plasma results in an improvement of up
Multicrystalline silicon solar cells
575
Table 2. Effect of rear passivation on 2.5 f~ cm multi-Si wafers. The cells had simple (and relatively deeper) homogeneous emitters and had a size of 4 cm 2 Cell structure BSF + rear SiO2 BSF, no SiO2
J~ (mA/cm2)
G~ (mY)
FF (%)
Efficiency (%)
34.4 33.0
601 588
78.2 79.3
16.2 15.4
Table 3. The illuminated ~ V measurements of evaporated contact Eurosolare and SSC EMC multicrystalline silicon solar cells before and after exposing to hydrogen mir plasma passivation from the emitter side. The cell area is 4 cm 2 Material Eurosolare
SSC-EMC
Group A B C D A B C D
J~ Voc (mA/cm 2) (mY) 33 33.8 34.2 34.6 31 33 33.9 34.2
600 603 610 616 584 600 604 610
FF (%)
q (%)
77.6 77.9 78 78.2 75.4 76 76.4 77
15.4 15.9 16.3 16.7 13.6 15.0 15.5 16.0
to 1.2 m A / c m 2 in J~c and up to 10 mV in Voc. In addition, the implementation of hydrogen mir plasma from the emitter side (group D cells) boosts the efficiency from 16.3% up to 16.7%. However, for the case of the S S C - E M C material, by comparing groups A and C, one can see that treating the back side of the cells with a hydrogen rf plasma results in Jsc and Voc improvements up to 3 m A / c m 2 and 20 mV, respectively. This yields a 2% absolute enhancement in the efficiency which is double what is achieved in the Eurosolare multicrystalline silicon solar cells. In addition, the implementation of hydrogen mir plasma (group D) boosts the efficiency from 15.5 up to 16%. This confirms that the electrical parameter improvement of the E M C cells due to hydrogenation is more significant compared to Eurosolare cells. The external quantum efficiencies of the ceils of groups A, C and D measured under white bias light for the S S C - E M C material are displayed in Fig. 1. Note that the improvement in the short wavelength response of the cells of group D compared to the cells of groups A and C upon performing remote hydrogen plasma is mainly due to the efficient passivation by the hydrogen atoms of the dangling bonds at the silicon/silicon dioxide interface. This passivation, of course, will result in reducing the surface recombination velocity and hence the enhancement of the short-circuit current density and open-circuit voltage. Moreover, it is clear that although the starting quality of the S C C - E M C multicrystalline material is
100
'
.....
20
'
"'
Group A (front oxide)
~
(
~&
font o~de and back rf H)
\.
Group D (front oxid&remotc H and back rf H)~, 0
0.4
.
l
0.5
,
I,
0.6
,
I
0.7
,
'
'
'
0.8
0.9
1.0
1.1
Wavelength (jJ.m)
Fig. 1. External quantum efficiency of the 4 c m 2 solar cells fabricated on SSC-EMC multicrystalline silicon material for the different passivation schemes.
inferior to the Eurosolare material because S C C E M C material has smaller grains and therefore more grain boundaries, the bulk hydrogen passivation has resulted in S C C - E M C cells with efficiencies very close to the efficiencies achieved by the Eurosolare cells.
1.4. Combination of low resistivity substrates, rear oxide passivation and optimal hydroyen passivation scheme Thin ( < 200 pro) good quality Polix material with 0.2 f~ cm base resistivity is used in the fabrication process. The fabrication process implemented here for achieving high efficiency multicrystalline silicon solar cells is roughly the same as for the cells of group D in Section 1.3, but performing additionally at the beginning of the process (before the formation of the emitter) a drive-in step on A P C V D boron-doped oxide to form a BSF at the back side. The passivation steps for bulk and surfaces are : oxide passivation at temperature of 900~C in a dry oxygen ambient for 6 min (the resulting oxide thicknesses are in the range of 60-90 A on the front side and 200-250/~ on the back side) ; hydrogen mir plasma passivation from emitter and back sides of the cells for 1 h, with a microwave
J. NIJS et al.
576
Table 4. The illuminated ~ V measurements of the Polix solar cells (with BSF) fabricated using different passivation schemes Material Polix
Oxide mir hydrogen passivation passivation No Yes Yes Yes
No No Yes Yes
H2 annealing
J~c (mA/cm 2)
Vo~ (mV)
FF (%)
Efficiency (%)
No No No Yes
31.2 32.4 33.6 34.1
615 624 634 640
77.5 77.6 78.0 78.0
14.9 15.7 16.6 17.0*
* Independently confirmed at N REL as 16.93 % using calibrated measurements.
plasma power of 75 W, pressure of 2.0 torr and a temperature of 350°C ; sintering the front and back contacts at 400°C for 20 min in forming gas (Hz/N2) in order to ensure a good ohmic contact and to recover the damage occurring in the oxide during the e-beam evaporation of the metal. Then the substrates are diced into 2 cm x 2 cm solar cells; after the double A R C evaporation comes an annealing of the cells in forming gas or H2 at 350°C for 15 rain. The cross-section of these multicrystalline silicon solar cells is depicted in Fig. 2. The illuminated I--V characteristics of these Polix multicrystalline silicon solar cells (4 cm 2) measured under standard illumination conditions (AMI.5, 25°C) are shown in Table 4. 2. PILOT-LINE CELLS USING SCREEN-PRINTING TECHNOLOGY
2.1. Combination o f passivation schemes and the use of Si3N4
The plasma-enhanced chemical vapour deposition (PECVD) of a silicon nitride layer seems to be a natural choice. It can serve as an anti-reflection coating layer, passivating layer and can be deposited with a
Z n S / M g F2
Si02
high throughput in existing commercial systems. The significant improvement in multicrystalline solar cell performance after P E C V D silicon nitride deposition has been reported by many authors [5-12]. This improvement is often explained by a surface and bulk passivation effect due to the diffusion of atomic hydrogen from the deposited layer. However, there are contradictory reports about the passivation stability if a post-deposition thermal treatment is applied in the temperature range above 500°C [5,7,9]. Experiments were done with multicrystalline silicon substrates (100 cm 2) from different suppliers using different sequences of metallization by screenprinting and P E C V D deposition of Si3N 4 [13]. After the initial saw damage etching, texturing and cleaning, the emitter region was formed by screenprinting a phosphorous paste and diffusion in a belt furnace. Next, a thin passivating oxide 10-15 nm thick was grown at 800°C in an open tube furnace. Additional passivation by a hydrogen plasma with and without dry SiO2 and combinations of both were applied before the plasma nitride deposition to further improve the cell parameters. The best process sequence consists of growing a thin (15 nm) dry SIO2, deposition of 80 nm of P E C V D SiNx, screenprinting metallization followed by a fast firing in the I R furnace at 650-700°C. The results are shown in Table 5. 2.2. Influence of texturization : chemical etchin9 versus mechanical V-grooving The advanced technology of crystalline silicon solar cells requires cell structures that are able to reduce the
Table 5. Best PECVD Si3N4 process on multicrystalline materials from different suppliers ; the cell area equals 100cm2
Fig. 2. Cross-section of the multicrystalline silicon solar cell with optimal passivation scheme.
Material
J~c (mA/cm2)
Vo¢ (mV)
FF (%)
Eft. (%)
A B C
30.9 31.5 31.8
602 603 604
77.5 77.4 77.3
14.5 14.7 14.9
Multicrystalline silicon solar cells high reflectivity o f silicon a n d at the same time e n h a n c e the optical p a t h length. The well-known stand a r d m e t h o d , applied in mass p r o d u c t i o n , relies o n an anisotropic etching. This technique is r a t h e r ineffective when applied to multicrystalline silicon substrates due to a r a n d o m distribution o f different grain orientations. Therefore there is a strong need for an orient a t i o n - i n d e p e n d e n t a n d efficient texturing method, especially one t h a t can easily be i m p l e m e n t e d in a n industrial e n v i r o n m e n t . Several techniques such as laser scribing [14], use o f defect etching [15], silicon m i c r o - m a c h i n i n g [16], a n d mechanical g r o o v i n g [17] are currently u n d e r investigation. O u r work is focused o n mechanical V-grooving using a c o n v e n t i o n a l dicing saw a n d bevelled blades [18-20]. Multicrystalline Silso silicon substrates, Vgrooved with a 35 ~ blade tip angle showed excellent light t r a p p i n g properties a n d effectively reduced front surface reflection. A m i n i m u m total reflectance o f 5.6% at 950 n m a n d an average R = 6.6% in the range 500 1000 n m has been o b t a i n e d on bare Si surface [18]. Multicrystalline, as-cut wafers with a size 10 x 10 cm 2 f r o m different suppliers, which were denoted as A, B a n d C, were used for solar cell processing in the optimised process sequence. The specific resistivity o f all wafers was a r o u n d 1 ~ cm. Wafer thicknesses of 280 (__+15) /~m, 350 (_+20) /~m a n d 360 ( + 1 6 ) /~m were m e a s u r e d for the A, B a n d C materials respectively. The substrates were V-grooved o n the f r o n t side as described above. The m e a s u r e d groove d e p t h after saw d a m a g e etching was a r o u n d 50/ma. N o n grooved wafers were added to evaluate the benefit o f mechanical grooving. The emitter was formed by diffusion from a solid P2Os source at 9 0 0 C for 10 rain. The emitter sheet resistance m e a s u r e d on n o n grooved wafers was 30 D/sq. A thin dry silicon dioxide was g r o w n for 15 min at 800~C. A P E C V D silicon nitride with a thickness o f 750-800 • was deposited at 350~C. Silver paste was screen-printed on the f r o n t side a n d s i l v e r - a l u m i n i u m paste on the back side o f the cells. A front c o n t a c t p a t t e r n optimised for Vgroove structures was used. O n some cells 1300 A, of MgF2 was e v a p o r a t e d as a second antireflection coating. Both contacts were co-fired at 685°C in a n I R furnace. T h e illuminated I - V p a r a m e t e r s were m e a s u r e d u n d e r s t a n d a r d A M 1.5 (100 m W / c m 2) radiation. A reference cell calibrated at the F r a u n h o f e r Institute for Solar Energy was used for simulator adjustment. The results are presented in Table 6 [21]. C o m p a r i s o n between V-grooved a n d n o n - g r o o v e d chemically textured solar cells with a double A R C reveals a difference in efficiency up to 0.9% absolute. This i m p r o v e m e n t comes from a 2 m A / c m 2 increase
577
Table 6. Results of illuminated I - V measurements of large area multicrystalline solar cells with a mechanically Vgrooved surface and different base materials
A B C C C
V-groove
DARC
Yes Yes No Yes Yes
Yes Yes Yes No Yes
J~, Voc (mA/cm 2) (mV) 32.7 32.3 30.3 31.6 32.3
602 598 603 598 599
FF (%)
q (%)
77.1 77.0 77.4 78.0 77.7
15.2 14.9 14. l 14.7 15.0
in the short-circuit current in the case of V-grooved cells. This can clearly be seen from external q u a n t u m efficiency m e a s u r e m e n t s at all wavelengths. However, internal q u a n t u m efficiency m e a s u r e m e n t s show t h a t the blue response of the V-grooved cell is s o m e w h a t inferior to the n o n - g r o o v e d cell [21]. CONCLUSION It is p r o v e n in this work h o w i m p o r t a n t the impact of certain specific process steps on the final multicrystalline silicon solar cell efficiency is. Efficiencies up to 17% are o b t a i n e d on 4 cm 2 clean-room fabricated cells a n d 15.2% has been o b t a i n e d on 100 cm 2 V-grooved screen-printed industrial cells. Acknowledgements--The authors acknowledge the financial support for this work from the Multichess project in the Joule programme of the Commission of the European Communities DG Xll, under contract number JOU2 CT92 0179. They also like to thank W. Laureys and P. Laermans, both from IMEC, for their contribution in the cell processing, and H. Nussbaumer and M. Steiner from University of Konstanz for their help during the measurements. The authors also would like to thank L. Frisson for his contribution in the optimisation of the fast-firing screen-printed contacts process. The authors also want to thank different multicrystalline silicon material manufacturers, such as Photowatt, Eurosolare, Bayer, Wacker and Sumitomo Sitix for providing us with the necessary substrates. REFERENCES
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