- Email: [email protected]
Low temperature passivation of silicon surfaces by polymer films
Solar Energy Materials & Solar Cells 71 (2002) 369–374
Low temperature passivation of silicon surfaces by polymer films D. Biro*, W. Warta Fraunhofer Institute for Solar Energy Systems, Heidenhofstrasse 2, D-79110 Freiburg, Germany Received 7 July 2000; received in revised form 15 February 2001; accepted 22 May 2001
Abstract A novel surface passivation method for silicon carrier lifetime measurements and solar cells using a polymer film is introduced. It is easy to apply, no special pre-treatment, e.g. no hydrofluoric acid (HF)-treatment, is necessary. The surfaces to be passivated are covered with the polymer solution, dried at 901C and encapsulated. Surface recombination velocities (S) as low as S ¼ 30 cm/s for various doping concentrations have been observed, nearly independent of the bulk injection level. The passivation is stable for at least 6 h. For a polymer-passivated rear contact solar cell the same open circuit voltage is achieved as for a cell with thermally grown oxide. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Surface passivation; Lifetime measurement; Silicon solar cell
1. Introduction Reducing the carrier recombination at silicon surfaces is a very demanding task for both bulk carrier lifetime (tb ) measurements and development of efficient solar cells. In recent years, various methods for surface passivation have been developed and improved continuously. For bulk lifetime measurements high surface passivation can be achieved by immersing the wafer in hydrofluoric acid (HF) [1] during measurement, or keeping it in an alcoholic iodine solution after etching in HF [2,3]. However, the need for an HF-dip and a careful preparation of the samples are the major drawbacks of these methods. Whereas HF provides a stable passivation, its use is quite inconvenient under laboratory measurement conditions since it is a hazardous chemical. Iodine solutions exhibit a fast degradation in their passivation *Corresponding author. Tel. +49-761-4588-5246; fax: +49-761-4588-9250. E-mail address: [email protected] (D. Biro). 0927-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 1 ) 0 0 0 9 4 - 0
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quality, if the wafer surface is not protected from oxidation by measures like bubbling N2 through the passivating solution [4]. To overcome the additional problem that the passivating medium is a liquid, an iodine solution containing varnish has been used to cover the wafer after the HF-dip [5,6]. Whereas for a 1.5 O cm wafer from the FZ grown material, low surface recombination with effective lifetimes teff of about 300 ms has been reported, data on the degradation behaviour are given only for the lower lifetime level (teff E60 ms) of a 1 O cm Cz sample and lower doped (7 O cm) FZ wafer. On low doped p-type and n-type materials a good, stable surface passivation is readily achieved [7]. The critical point is the transfer to the 1 O cm doping range, which is especially interesting for photovoltaic applications. Another way to stabilise the passivation effect is suggested in Ref. [8]. The wafer is coated with an organic overlayer after the HF-dip to protect the surface from further oxidation. As in the previous methods an HF-treatment for the passivation is needed when applying this method of passivating the silicon surface. Field effect passivation by means of corona charges was frequently applied for lifetime measurements [9–11]. In these approaches an insulating layer on which the corona charges are deposited is used to build up a space charge region beneath the silicon surface. However, for insulating layers other than thermal oxide and SiN films a stability not exceeding 30 min has been reported. In order to achieve a durable and a very high passivation of silicon surfaces, thermal oxidation [12] or PECVD deposition of SiN films [13] can be applied, but a costly apparatus as well as careful surface preparation is needed in both cases. In the first case, the process temperatures are at about 950–10501C, in the second around 350–4001C. In this paper, a novel passivation method by means of a transparent polymer film is introduced [14]. The polymer film is formed from a solution of a perfluorsulphonic acid. Excellent film quality can be achieved for the poly(tetrafluoroethylene) based polymer Nafions (DuPont). The method is easy to apply, needs no special chemical pre-cleaning steps like RCA-cleaning or HF-dips, and no expensive instrumentation. A shiny etched surface is fully sufficient for a good passivation quality. It also involves no high temperature steps and is stable for at least 6 h which is sufficient for high resolution lifetime mappings. After measurements the film can be washed off by ethanol or acetone.
2. Sample preparation and measurement set-up In this study, boron doped (1 0 0)-oriented high quality acid etched FZ-wafers in the resistivity range 0.5–10 O cm and an approximate thickness of 250 mm have been passivated on both sides with the polymer film in the following way. On one surface a small amount of commercially available Nafions solution, diluted with an additional 10 volume percent of DI-water, is deposited as a thick liquid film. After drying at 901C for 10 min, the other side of the wafer is covered and the wafer is dried for additional 60 min at 901C. After coating both sides of the wafer a polymer film of several micrometer thickness remains. Subsequently, the wafer is covered with a commercially available adhesive foil to stabilise passivation.
D. Biro, W. Warta / Solar Energy Materials & Solar Cells 71 (2002) 369–374
371
Additionally, we passivated a rear contact solar cell, by applying the described procedure to the front side of the cell. The surface recombination velocities (SRV) have been extracted from lifetime measurements performed with a light biased microwave-detected photoconductance decay (MW-PCD) set-up similar to the one used by Kane and Swanson [15]. The measurements have been carried out under low injection conditions. Since for all known passivation methods the quality of the passivation depends more or less strongly on the minority carrier concentration in the material [12,16–18], the minority carrier lifetimes teff have been measured for several bias-light intensities in order to characterise the new method. The bias light of the MW-PCD set-up is supplied from a halogen lamp with calibrated intensities in the range of 1–100 mW/cm2. For all measurements the carrier injection produced by the laser pulse has been much smaller than the one produced by the bias-light. From the measured lifetimes an upper limit for the SRV has been calculated. While doing this we assume that the bulk carrier lifetimes of the material used is limited by Coulomb-enhanced Auger recombination [19]. The measured differential SRVs have been integrated to obtain the actual SRVs [20,21].
3. Results and discussion First the stability of the passivation has been investigated. Upper limits for the differential surface recombination values are given in Fig. 1. The wafer has been illuminated during the entire measurement time with a bias light intensity of 50 mW/ cm2. After approximately 20 min a stable passivation quality is observed for at least
Fig. 1. Time dependence of the differential SRV of Nafions-passivated FZ-wafers for different boron concentrations NA (bias illumination 50 mW/cm2). The lines are guides to the eye.
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6 h independent of the doping concentration. The differential SRVs range from about 90 cm/s for 0.5 O cm material to values as low as 25 cm/s for 10 O cm material. The effect of the decreasing surface recombination velocity within the first 20 min of the measurement is not yet well understood. However, since it occurs while the sample is illuminated, the bias light seems to be responsible for this effect. Interestingly such an effect has also been observed for silicon surfaces passivated with organic monolayers after immersion in HF [22]. Measurements on wafers which have been dried for more than 60 min showed lower lifetimes than the ones presented here. From this it can be concluded that the drying time of 60 min is appropriate and cannot explain the behaviour observed in Fig. 1. Next the injection dependence of the integral SRV for different doping concentrations has been determined from MWPCD measurements. The results are shown in Fig. 2. These values are obtained 6 h after the sealing of the wafers. All displayed SRVs are values obtained by integration from the differential SRVs, except for the sample with the highest resistivity. For this sample an integration would not lead to accurate results, since only a few points could be measured in low injection. Especially for the two higher doped samples an extraordinarily small dependence of the SRV on the injection level is observed. For over more than three orders of magnitude in carrier concentration, the SRV varies only around 75 cm/s. Due to this behaviour the measured differential SRVs and the integrated SRVs differ only slightly for the higher doped samples. Finally, we applied the new passivation method to the front side of a rear contact solar cell (RCC) fabricated in-house [23]. The open circuit voltage (Voc ), the short circuit current (Isc ) and the solar cell efficiency (Z) have been measured at AM1.5
Fig. 2. Effective surface recombination velocities (SRV) of Nafions-passivated surfaces on silicon with various boron concentrations, 6 h after drying and sealing of the polymer film. The open symbols indicate differential SRVs. The lines are guides to the eye.
D. Biro, W. Warta / Solar Energy Materials & Solar Cells 71 (2002) 369–374
373
Table 1 Passivation of a rear contact solar cell
(i) (ii) (iii)
Thermal oxide Unpassivated Nafions-passivated
Voc (mV)
Is (mA/cm2)
Z (%)
661 618 666
40.0 35.8 40.0
20.3 17.2 20.3
(100 mW/cm2). For measurement reasons the illumination was performed through the contacted rear surface. The fact that the bulk diffusion length in these solar cells is at least 3 times the cell thickness makes the cell very sensitive to front surface recombination. The obtained values are compared to the case in which the front side of the same cell is passivated by a high quality standard antireflection thermal oxide and with the case after removal of the oxide with HF (see Table 1). After oxide removal the cell can be considered to be unpassivated. Measurements on plain wafers show that the surface recombination velocity of wafers which have been immersed into HF rises very quickly up to several thousand cm/s after DI-water rinse and drying in air atmosphere. Table 1 clearly shows that the passivation quality of the polymer-passivation reaches the quality of the state-of-the-art thermal oxide. It is clearly superior to the unpassivated case which exhibits a Voc value of nearly 50 mV lower.
4. Conclusions In conclusion, a polymer film formed from a perfluorsulphonic acid solution is shown to passivate silicon surfaces very well. The passivation is easy to apply because no HF-dips or expensive equipment is needed for the passivation process. Furthermore, no high temperature steps are involved in the passivation procedure, it can be carried out at 901C. Surface recombination values in the range of 30–90 cm/s are reached for a wide range of doping concentrations, and the passivation quality is stable for at least 6 h. A weak injection dependence of the SRV values is found. This is particularly important for the determination of injection dependent bulk lifetimes as can be found for e.g. in multicrystalline or Czochralski silicon. The excellent passivation quality achieved for the front side of a rear contacted solar cell demonstrates the application potential of this new low temperature passivation technique. In this study Nafions has been chosen as passivation polymer due to its excellent film forming properties. In principle, other perfluorinated sulphonic acids should also be able to passivate silicon surfaces as well. In measurements performed in this study, this has been confirmed for wafers covered with perfluoroctan sulphonic acid with the same procedure as described for Nafions. Optimisation of the used polymer solutions and detailed investigation of the mechanism of the passivation should further improve the passivation quality. Recent experiments show that the stability
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D. Biro, W. Warta / Solar Energy Materials & Solar Cells 71 (2002) 369–374
can be further increased by an improved sealing procedure. Surface recombination velocities of below S ¼ 30 cm/s have been measured during a period of 5 days after passivation.
Acknowledgements This work was supported by the German Federal Ministry for Research and Technology (BMBF). The authors would like to thank Dr. J. Knobloch for the solar cells supplied for this study, E. Sch.affer for measurements, and Dr. A. Nolte for fruitful discussions. The Stadtwerke Karlsruhe, Germany gave financial support to one of us (D. Biro).
References [1] E. Yablonovitch, D.L. Allara, C.C. Chang, T. Gmitter, T.B. Bright, Phys. Rev. Lett. 57 (2) (1986) 249–252. [2] T.S. Hor!anyi, T. Pavelka, P. Tu. tto. , Appl. Surf. Sci. 63 (1993) 306–311. [3] A. Scho. necker, K. Heasman, J. Schmidt, J. Poortmans, T. Bruton, W. Koch, Proceedings of the 14th EC PVSEC, Barcelona, Spain, 1997, pp. 666–671. [4] H. M’Saad, J. Michel, J.J. Lappe, L.C. Kimerling, J. Electron. Mater. 23 (5) (1994) 487–491. [5] W. Arndt, K. Graff, P. Heim, Proceedings of the ALTECH ‘95, Den Haag, Netherlands, 1995. [6] J. Poortmans, T. Vermeulen, J. Nijs, R. Mertens, Proceedings of the 25th IEEE PVSC, Washington, DC, USA, 1996, pp. 721–724. [7] T. Pavelka, Problems and possibilities of comparing different lifetime measuring instruments and techniques, in: Recombination Lifetime Measurements in Silicon, D. C. Gupta, F.R. Bacher, W.M. Hughes (Eds.), Vol. ASTM STP 1340, American Society for Testing and Materials, Chelsea, Michigan, USA, 1998, 206–216. [8] H.M. Deckmann, B.R. Weinberger, E. Yablonovitch, European Patent, No. 0171278, (1985). [9] M. Scho. fthaler, U. Rau, G. Langguth, M. Hirsch, R. Brendel, J.H. Werner, Proceedings of the 12th EC PVSEC, Amsterdam, The Netherlands, 1994, pp. 533–536. [10] S.W. Glunz, D. Biro, S. Rein, W. Warta, J. Appl. Phys. 86 (1) (1999) 683–691. [11] J. Schmidt, A.G. Aberle, Progr. Photovolt. 6 (4) (1998) 259–263. [12] S.W. Glunz, A.B. Sproul, W. Warta, W. Wettling, J. Appl. Phys. 75 (3) (1994) 1611–1615. [13] A.G. Aberle, R. Hezel, Progr. Photovolt. 5 (1997) 29–50. [14] W. Warta, D. Biro, German Patent, No. P 198 57 064.3–33 Patent Pending, 1999. [15] D.E. Kane, R.M. Swanson, Proceedings of the 18th IEEE PVSC, New Orleans, Louisiana, USA, 1985 pp. 578–583. [16] R.B.M. Girisch, R.P. Mertens, R.F. De Keersmaecker, IEEE Trans. Electron Devices 35 (2) (1988) 203–222. [17] W.D. Eades, R.M. Swanson, J. Appl. Phys. 58 (11) (1985) 4267–4276. [18] A.G. Aberle, T. Lauinger, J. Schmidt, R. Hezel, Appl. Phys. Lett. 66 (21) (1995) 2828–2830. [19] R. H.acker, A. Hangleiter, J. Appl. Phys. 75 (11) (1994) 7570–7572. [20] R. Brendel, Appl. Phys. A 60 (1995) 523–524. [21] A.G. Aberle, J. Schmidt, R. Brendel, J. Appl. Phys. 79 (3) (1996) 1491–1496. [22] A.B. Sieval, H. Zuilhof, E.J.R. Sudh.alter, Proceedings of the second WCPEC, Vienna, Austria, 1998, pp. 322–325. [23] J. Dicker, J.O. Schumacher, S.W. Glunz, W. Warta, Proceedings of the 2nd WCPEC, Vienna, Austria, 1998, pp. 95–99.
Low temperature passivation of silicon surfaces by polymer films D. Biro*, W. Warta Fraunhofer Institute for Solar Energy Systems, Heidenhofstrasse 2, D-79110 Freiburg, Germany Received 7 July 2000; received in revised form 15 February 2001; accepted 22 May 2001
Abstract A novel surface passivation method for silicon carrier lifetime measurements and solar cells using a polymer film is introduced. It is easy to apply, no special pre-treatment, e.g. no hydrofluoric acid (HF)-treatment, is necessary. The surfaces to be passivated are covered with the polymer solution, dried at 901C and encapsulated. Surface recombination velocities (S) as low as S ¼ 30 cm/s for various doping concentrations have been observed, nearly independent of the bulk injection level. The passivation is stable for at least 6 h. For a polymer-passivated rear contact solar cell the same open circuit voltage is achieved as for a cell with thermally grown oxide. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Surface passivation; Lifetime measurement; Silicon solar cell
1. Introduction Reducing the carrier recombination at silicon surfaces is a very demanding task for both bulk carrier lifetime (tb ) measurements and development of efficient solar cells. In recent years, various methods for surface passivation have been developed and improved continuously. For bulk lifetime measurements high surface passivation can be achieved by immersing the wafer in hydrofluoric acid (HF) [1] during measurement, or keeping it in an alcoholic iodine solution after etching in HF [2,3]. However, the need for an HF-dip and a careful preparation of the samples are the major drawbacks of these methods. Whereas HF provides a stable passivation, its use is quite inconvenient under laboratory measurement conditions since it is a hazardous chemical. Iodine solutions exhibit a fast degradation in their passivation *Corresponding author. Tel. +49-761-4588-5246; fax: +49-761-4588-9250. E-mail address: [email protected] (D. Biro). 0927-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 1 ) 0 0 0 9 4 - 0
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quality, if the wafer surface is not protected from oxidation by measures like bubbling N2 through the passivating solution [4]. To overcome the additional problem that the passivating medium is a liquid, an iodine solution containing varnish has been used to cover the wafer after the HF-dip [5,6]. Whereas for a 1.5 O cm wafer from the FZ grown material, low surface recombination with effective lifetimes teff of about 300 ms has been reported, data on the degradation behaviour are given only for the lower lifetime level (teff E60 ms) of a 1 O cm Cz sample and lower doped (7 O cm) FZ wafer. On low doped p-type and n-type materials a good, stable surface passivation is readily achieved [7]. The critical point is the transfer to the 1 O cm doping range, which is especially interesting for photovoltaic applications. Another way to stabilise the passivation effect is suggested in Ref. [8]. The wafer is coated with an organic overlayer after the HF-dip to protect the surface from further oxidation. As in the previous methods an HF-treatment for the passivation is needed when applying this method of passivating the silicon surface. Field effect passivation by means of corona charges was frequently applied for lifetime measurements [9–11]. In these approaches an insulating layer on which the corona charges are deposited is used to build up a space charge region beneath the silicon surface. However, for insulating layers other than thermal oxide and SiN films a stability not exceeding 30 min has been reported. In order to achieve a durable and a very high passivation of silicon surfaces, thermal oxidation [12] or PECVD deposition of SiN films [13] can be applied, but a costly apparatus as well as careful surface preparation is needed in both cases. In the first case, the process temperatures are at about 950–10501C, in the second around 350–4001C. In this paper, a novel passivation method by means of a transparent polymer film is introduced [14]. The polymer film is formed from a solution of a perfluorsulphonic acid. Excellent film quality can be achieved for the poly(tetrafluoroethylene) based polymer Nafions (DuPont). The method is easy to apply, needs no special chemical pre-cleaning steps like RCA-cleaning or HF-dips, and no expensive instrumentation. A shiny etched surface is fully sufficient for a good passivation quality. It also involves no high temperature steps and is stable for at least 6 h which is sufficient for high resolution lifetime mappings. After measurements the film can be washed off by ethanol or acetone.
2. Sample preparation and measurement set-up In this study, boron doped (1 0 0)-oriented high quality acid etched FZ-wafers in the resistivity range 0.5–10 O cm and an approximate thickness of 250 mm have been passivated on both sides with the polymer film in the following way. On one surface a small amount of commercially available Nafions solution, diluted with an additional 10 volume percent of DI-water, is deposited as a thick liquid film. After drying at 901C for 10 min, the other side of the wafer is covered and the wafer is dried for additional 60 min at 901C. After coating both sides of the wafer a polymer film of several micrometer thickness remains. Subsequently, the wafer is covered with a commercially available adhesive foil to stabilise passivation.
D. Biro, W. Warta / Solar Energy Materials & Solar Cells 71 (2002) 369–374
371
Additionally, we passivated a rear contact solar cell, by applying the described procedure to the front side of the cell. The surface recombination velocities (SRV) have been extracted from lifetime measurements performed with a light biased microwave-detected photoconductance decay (MW-PCD) set-up similar to the one used by Kane and Swanson [15]. The measurements have been carried out under low injection conditions. Since for all known passivation methods the quality of the passivation depends more or less strongly on the minority carrier concentration in the material [12,16–18], the minority carrier lifetimes teff have been measured for several bias-light intensities in order to characterise the new method. The bias light of the MW-PCD set-up is supplied from a halogen lamp with calibrated intensities in the range of 1–100 mW/cm2. For all measurements the carrier injection produced by the laser pulse has been much smaller than the one produced by the bias-light. From the measured lifetimes an upper limit for the SRV has been calculated. While doing this we assume that the bulk carrier lifetimes of the material used is limited by Coulomb-enhanced Auger recombination [19]. The measured differential SRVs have been integrated to obtain the actual SRVs [20,21].
3. Results and discussion First the stability of the passivation has been investigated. Upper limits for the differential surface recombination values are given in Fig. 1. The wafer has been illuminated during the entire measurement time with a bias light intensity of 50 mW/ cm2. After approximately 20 min a stable passivation quality is observed for at least
Fig. 1. Time dependence of the differential SRV of Nafions-passivated FZ-wafers for different boron concentrations NA (bias illumination 50 mW/cm2). The lines are guides to the eye.
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D. Biro, W. Warta / Solar Energy Materials & Solar Cells 71 (2002) 369–374
6 h independent of the doping concentration. The differential SRVs range from about 90 cm/s for 0.5 O cm material to values as low as 25 cm/s for 10 O cm material. The effect of the decreasing surface recombination velocity within the first 20 min of the measurement is not yet well understood. However, since it occurs while the sample is illuminated, the bias light seems to be responsible for this effect. Interestingly such an effect has also been observed for silicon surfaces passivated with organic monolayers after immersion in HF [22]. Measurements on wafers which have been dried for more than 60 min showed lower lifetimes than the ones presented here. From this it can be concluded that the drying time of 60 min is appropriate and cannot explain the behaviour observed in Fig. 1. Next the injection dependence of the integral SRV for different doping concentrations has been determined from MWPCD measurements. The results are shown in Fig. 2. These values are obtained 6 h after the sealing of the wafers. All displayed SRVs are values obtained by integration from the differential SRVs, except for the sample with the highest resistivity. For this sample an integration would not lead to accurate results, since only a few points could be measured in low injection. Especially for the two higher doped samples an extraordinarily small dependence of the SRV on the injection level is observed. For over more than three orders of magnitude in carrier concentration, the SRV varies only around 75 cm/s. Due to this behaviour the measured differential SRVs and the integrated SRVs differ only slightly for the higher doped samples. Finally, we applied the new passivation method to the front side of a rear contact solar cell (RCC) fabricated in-house [23]. The open circuit voltage (Voc ), the short circuit current (Isc ) and the solar cell efficiency (Z) have been measured at AM1.5
Fig. 2. Effective surface recombination velocities (SRV) of Nafions-passivated surfaces on silicon with various boron concentrations, 6 h after drying and sealing of the polymer film. The open symbols indicate differential SRVs. The lines are guides to the eye.
D. Biro, W. Warta / Solar Energy Materials & Solar Cells 71 (2002) 369–374
373
Table 1 Passivation of a rear contact solar cell
(i) (ii) (iii)
Thermal oxide Unpassivated Nafions-passivated
Voc (mV)
Is (mA/cm2)
Z (%)
661 618 666
40.0 35.8 40.0
20.3 17.2 20.3
(100 mW/cm2). For measurement reasons the illumination was performed through the contacted rear surface. The fact that the bulk diffusion length in these solar cells is at least 3 times the cell thickness makes the cell very sensitive to front surface recombination. The obtained values are compared to the case in which the front side of the same cell is passivated by a high quality standard antireflection thermal oxide and with the case after removal of the oxide with HF (see Table 1). After oxide removal the cell can be considered to be unpassivated. Measurements on plain wafers show that the surface recombination velocity of wafers which have been immersed into HF rises very quickly up to several thousand cm/s after DI-water rinse and drying in air atmosphere. Table 1 clearly shows that the passivation quality of the polymer-passivation reaches the quality of the state-of-the-art thermal oxide. It is clearly superior to the unpassivated case which exhibits a Voc value of nearly 50 mV lower.
4. Conclusions In conclusion, a polymer film formed from a perfluorsulphonic acid solution is shown to passivate silicon surfaces very well. The passivation is easy to apply because no HF-dips or expensive equipment is needed for the passivation process. Furthermore, no high temperature steps are involved in the passivation procedure, it can be carried out at 901C. Surface recombination values in the range of 30–90 cm/s are reached for a wide range of doping concentrations, and the passivation quality is stable for at least 6 h. A weak injection dependence of the SRV values is found. This is particularly important for the determination of injection dependent bulk lifetimes as can be found for e.g. in multicrystalline or Czochralski silicon. The excellent passivation quality achieved for the front side of a rear contacted solar cell demonstrates the application potential of this new low temperature passivation technique. In this study Nafions has been chosen as passivation polymer due to its excellent film forming properties. In principle, other perfluorinated sulphonic acids should also be able to passivate silicon surfaces as well. In measurements performed in this study, this has been confirmed for wafers covered with perfluoroctan sulphonic acid with the same procedure as described for Nafions. Optimisation of the used polymer solutions and detailed investigation of the mechanism of the passivation should further improve the passivation quality. Recent experiments show that the stability
374
D. Biro, W. Warta / Solar Energy Materials & Solar Cells 71 (2002) 369–374
can be further increased by an improved sealing procedure. Surface recombination velocities of below S ¼ 30 cm/s have been measured during a period of 5 days after passivation.
Acknowledgements This work was supported by the German Federal Ministry for Research and Technology (BMBF). The authors would like to thank Dr. J. Knobloch for the solar cells supplied for this study, E. Sch.affer for measurements, and Dr. A. Nolte for fruitful discussions. The Stadtwerke Karlsruhe, Germany gave financial support to one of us (D. Biro).
References [1] E. Yablonovitch, D.L. Allara, C.C. Chang, T. Gmitter, T.B. Bright, Phys. Rev. Lett. 57 (2) (1986) 249–252. [2] T.S. Hor!anyi, T. Pavelka, P. Tu. tto. , Appl. Surf. Sci. 63 (1993) 306–311. [3] A. Scho. necker, K. Heasman, J. Schmidt, J. Poortmans, T. Bruton, W. Koch, Proceedings of the 14th EC PVSEC, Barcelona, Spain, 1997, pp. 666–671. [4] H. M’Saad, J. Michel, J.J. Lappe, L.C. Kimerling, J. Electron. Mater. 23 (5) (1994) 487–491. [5] W. Arndt, K. Graff, P. Heim, Proceedings of the ALTECH ‘95, Den Haag, Netherlands, 1995. [6] J. Poortmans, T. Vermeulen, J. Nijs, R. Mertens, Proceedings of the 25th IEEE PVSC, Washington, DC, USA, 1996, pp. 721–724. [7] T. Pavelka, Problems and possibilities of comparing different lifetime measuring instruments and techniques, in: Recombination Lifetime Measurements in Silicon, D. C. Gupta, F.R. Bacher, W.M. Hughes (Eds.), Vol. ASTM STP 1340, American Society for Testing and Materials, Chelsea, Michigan, USA, 1998, 206–216. [8] H.M. Deckmann, B.R. Weinberger, E. Yablonovitch, European Patent, No. 0171278, (1985). [9] M. Scho. fthaler, U. Rau, G. Langguth, M. Hirsch, R. Brendel, J.H. Werner, Proceedings of the 12th EC PVSEC, Amsterdam, The Netherlands, 1994, pp. 533–536. [10] S.W. Glunz, D. Biro, S. Rein, W. Warta, J. Appl. Phys. 86 (1) (1999) 683–691. [11] J. Schmidt, A.G. Aberle, Progr. Photovolt. 6 (4) (1998) 259–263. [12] S.W. Glunz, A.B. Sproul, W. Warta, W. Wettling, J. Appl. Phys. 75 (3) (1994) 1611–1615. [13] A.G. Aberle, R. Hezel, Progr. Photovolt. 5 (1997) 29–50. [14] W. Warta, D. Biro, German Patent, No. P 198 57 064.3–33 Patent Pending, 1999. [15] D.E. Kane, R.M. Swanson, Proceedings of the 18th IEEE PVSC, New Orleans, Louisiana, USA, 1985 pp. 578–583. [16] R.B.M. Girisch, R.P. Mertens, R.F. De Keersmaecker, IEEE Trans. Electron Devices 35 (2) (1988) 203–222. [17] W.D. Eades, R.M. Swanson, J. Appl. Phys. 58 (11) (1985) 4267–4276. [18] A.G. Aberle, T. Lauinger, J. Schmidt, R. Hezel, Appl. Phys. Lett. 66 (21) (1995) 2828–2830. [19] R. H.acker, A. Hangleiter, J. Appl. Phys. 75 (11) (1994) 7570–7572. [20] R. Brendel, Appl. Phys. A 60 (1995) 523–524. [21] A.G. Aberle, J. Schmidt, R. Brendel, J. Appl. Phys. 79 (3) (1996) 1491–1496. [22] A.B. Sieval, H. Zuilhof, E.J.R. Sudh.alter, Proceedings of the second WCPEC, Vienna, Austria, 1998, pp. 322–325. [23] J. Dicker, J.O. Schumacher, S.W. Glunz, W. Warta, Proceedings of the 2nd WCPEC, Vienna, Austria, 1998, pp. 95–99.