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Interface recombination in heterojunctions of amorphous and crystalline silicon
Journal of Non-Crystalline Solids 299–302 (2002) 663–667 www.elsevier.com/locate/jnoncrysol
Interface recombination in heterojunctions of amorphous and crystalline silicon A. Froitzheim *, K. Brendel, L. Elstner, W. Fuhs, K. Kliefoth, M. Schmidt Hahn-Meitner-Institut Berlin, Abt. Silizium-Photovoltaik, Kekul estr. 5, D-12489 Berlin, Germany
Abstract Heterojunction solar cells consisting of an n-type a-Si:H(n) emitter and a p-type monocrystalline silicon wafer have been studied with particular emphasis on the role of interface recombination. It is shown that the form of the I–V characteristics and the effective interface recombination velocity depend on the treatment of the Si-wafer prior to the deposition of the amorphous emitter. Numerical simulation suggests that the non-exponential (S-shape) dependence of the I–V curves under illumination arises from a high density of interface states which results in enhanced recombination via interface states. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 85.30.De; 73.70.Lq
1. Introduction Heterojunctions of amorphous and crystalline silicon are promising candidates for highly efficient solar cells processed at low temperatures. High efficiencies have been obtained in the laboratory and industrialization has already been announced [1,2]. Inspite of this technological progress surprisingly little is known about the properties of the heterojunction interface. Particularly interesting problems are the passivation and role of interface states. Optimization of the device performance requires proper pretreatment procedures of the Siwafers and optimized deposition conditions of the amorphous emitter. In a recent paper we reported
that the best result was obtained if the deposition occurs on flattened and hydrogen-terminated surfaces [3]. Another important question concerns the magnitude and influence of the bandoffsets DEC and DEV in the conduction and valence band. The values quoted in the literature differ appreciably [3–5]. In this study we investigate the system aSi:H(n)/c-Si(p) consisting of an n-type amorphous silicon emitter deposited by PECVD onto floatzone silicon wafers. We discuss the influence of the wafer pretreatment and show by simulations of the device that the interface recombination has a pronounced influence on the I–V characteristics once the density of interface states exceeds a value of 1012 cm2 . 2. Experimental procedure
*
Corresponding author. Tel.: +49-30 67053 305; fax: +49-30 67053 333. E-mail address: [email protected] (A. Froitzheim).
We prepared heterojunction solar cells by depositing n-type a-Si:H onto p-type c-Si substrates
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 1 0 2 9 - 8
664
A. Froitzheim et al. / Journal of Non-Crystalline Solids 299–302 (2002) 663–667
(FZ, 0.5–2 X cm, (1 1 1), 330 lm) with back surface field. The nþ -a-Si:H(n) films were deposited by plasma enhanced chemical vapour deposition (PECVD) at a substrate temperature of 250 °C using standard conditions for high quality films (p ¼ 400 mTorr, 0.6% gas phase concentration phosphine/silane, P ¼ 15 W). The thickness of the nþ -a-Si:H(n) emitter was kept at 30–40 nm. Prior to the deposition, the p-type c-Si wafers were treated in three different ways: (a) HF-dip which causes roughening, (b) wet chemical oxidation and etching in buffered NH4 F which is considered to smoothen the surface and to saturate dangling bonds (H-termination), (c) growth of an ultrathin oxide layer for passivation. Contact to the a-Si:H emitter was made by a 80 nm thick ZnO(Al) ðR ¼ 50 XÞ deposited by sputtering. Evaporated aluminium was used for the grid and rear contacts. I–V characteristics were measured at 300 K with under AM1.5 conditions. The measurements of the temperature dependence between 100 and 300 K were performed in a liquid nitrogen cryostat using a tungsten halogen lamp for illumination. Numerical simulations were performed which consisted of the simultaneous solution of the Poisson equation and the continuity equations for electrons and holes with a quasi-continuum of gap states in the amorphous layer and a single gap state in the crystalline wafer.
3. Results Table 1 summarizes previously published results of a measurement of the effective interface recombination velocity S for the various wafer pretreatment procedures (see [3] for details). We conclude that the interface recombination is strongly influenced by the density of interface states Dit which is determined by the pretreatment
Table 1 Effective recombination velocity [3] Wafer pretreatment
Seff (cm/s)
Tunnel-oxide HF-Dip H-termination
6000 1000 600
Fig. 1. I–V characteristics at 300 K under AM 1.5 – illumination.
procedure [6]. S is smallest for the sample where the deposition of the amorphous emitter was performed after careful H-termination of the Siwafer. This result is consistent with our experience that this treatment leads to highest cell efficiencies of above 13%. The I–V characteristics, too, exhibit a characteristic dependence on the wafer pretreatment (Fig. 1). Whereas the behaviour of the H-terminated sample is close to normal, the other pretreatments led to S-shaped I–V characteristics. This effect is most pronounced in case where a thin tunnel oxide has been formed which is known to lead to high Dit . Such effects have been observed only under illumination, there was no anomaly in the dark characteristics. Similar S-shaped characteristics of a-Si:H(n)/c-Si(p) junctions have been reported before by Unold et al. [4] who related this effect to a large bandoffset in the conduction band of DEC ¼ 0:3 eV. However, we found much smaller values of less than 0.1 eV [3]. In situ constantfinal-state photoemission led to DEC ¼ 0:16 eV which might be considered to be the most reliable value at present [5]. It is interesting to notice that generally at low temperature the S-shape form of the I–V characteristics becomes much more pronounced. Fig. 2 gives an example for the case of the HF-dipped specimen. The results presented so far show that the value of S as well as the form of the I–V curves can considerably be influenced by the pretreatment of the surface of the Si-wafer. Since the pretreatment procedure is known to vary the density of interface
A. Froitzheim et al. / Journal of Non-Crystalline Solids 299–302 (2002) 663–667
Fig. 2. Temperature dependence of the I–V characteristics for samples when illuminated (tungsten halogen lamp). The sample that is shown has an HF-dip as wafer pretreatment.
states [6] we argue that interface recombination is responsible for both features. To further investigate these effects we have performed numerical simulations using a program that allows to model the device with special regard to the interface. The Poisson equation and the transport equations for electrons and holes have been solved with the following boundary conditions for the front (0) and back side ðwÞ of the cell: uð0Þ ¼ 0;
uðwÞ ¼ Vbi V ;
ð1Þ
jn ð0Þ ¼ Snf Dnð0Þ;
jn ðwÞ ¼ Snr DnðwÞ;
ð2Þ
jp ð0Þ ¼ Spf Dpð0Þ;
jp ðwÞ ¼ Spr DpðwÞ:
ð3Þ
665
In these expressions u denotes the electrostatic potential, jn and jp are the electron and hole current densities, Dn and Dp are the excess conf=r centrations under illumination, Sn=p are the recombination velocities of the electrons (n) and holes (p) at the front (f) and rear (r) contact. All f=r Sn=p were chosen to 107 cm=s. Vbi is the built in voltage and V is the applied voltage. We assume flat band conditions on both contacts. For the aSi:H(n) the position of the Fermi level at the surface was kept at EC EF ¼ 0:25 eV which is the activation energy measured for thick a-Si:H(n) samples. The interface is assumed to be abrupt with bandoffsets DEC and DEV . The boundary conditions for the potential at the interface are uðx1 Þ uðx2 Þ ¼ 0;
ð4Þ
e0 ec-Si Eðx2 Þ e0 ea-Si Eðx1 Þ Qit ¼ 0:
ð5Þ
Physically this means that the potential is continuous across the interface, where x1 is at the interface on the amorphous side and x2 is at the interface on the crystalline side, such that dipoles are excluded (Eq. (4)). The Gauss equation for the dielectric displacement (Eq. (5)) has to be fulfilled including Qit which is the charge localized at the interface. e0 ; ea-Si and ec-Si are the dielectric constants and E is the electric field. Transport across the interface is simulated assuming thermionic emission of holes and electrons. The interface states have been positioned at midgap of the c-Si.
Table 2 Parameters used for simulation (data for a-Si:H(n) similar to [8]) Material parameters
a-Si:H(n)/c-Si(p)
Thickness (cm) EG (eV) ln and lp ðcm2 =V sÞ NC and NV ðcm3 Þ Doping [P], [B] ðcm3 Þ
3 105 =3:3 102 1.7/1.12 5/1400 and 1/480 1 1020 =2:86 1019 and 1 1020 =3:19 1019 1 1019 =1 1016
Tail states parameters a-Si:H(n) DOS at CB/VB edge ðcm3 =eVÞ CB/VB Urbach energy (eV) cn;p charged/neutral states ðcm3 =sÞ
1 1021 =1 1021 0.080/0.170 1 108 =1 1010
Dangling bond states in a-Si:H(n) ND ðcm3 Þ Eþ=0 =E0= above EV (eV) Gaussian distribution cn;p charged/neutral states ðcm3 =sÞ
1:5 1019 0.4/0.6 r ¼ 0:15 1 108 =1 1010
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A. Froitzheim et al. / Journal of Non-Crystalline Solids 299–302 (2002) 663–667
The defect structure of the a-Si:H(n) is modelled similar to [8] by two Gaussian distributions of the dangling bond states and exponential tail states at both bandedges. For c-Si a single trap at midgap is assumed. The most important parameters used are listed in Table 2. Here we present results for which the only varied parameters were the density Dit of acceptorlike states, their electron and hole capture rate coefficients cn=p and the temperature. Fig. 3 shows the effect of increasing Dit . The cn;p were assumed to be strongly asymmetric, cn ¼ 108 cm3 =s and cp ¼ 1012 cm3 =s, in accordance with literature data for the SiO2 =Si interface [7]. The bandoffset is
Fig. 3. Simulation of I–V characteristics at 300 K. Parameter is the density of interface states Dit : (a) 0 1012 cm2 , (b) 1 1012 cm2 , (c) 3 1012 cm2 (d) 4 1012 cm2 , (e) 5 1012 cm2 , (f) 6 1012 cm2 .
Fig. 4. Simulation of the I–V characteristics at various temperatures ðDit ¼ 4 1012 cm2 Þ.
kept constant at DEC ¼ 150 meV. As a result, the fill factor of the I–V curve is reduced and the Sshape evolves when the Dit exceeds a value of about 1012 cm2 . Fig. 4 displays the simulation of the temperature dependence using a value for Dit of 4 1012 cm2 . While at 300 K the I–V curve is almost normal, the S-shape develops rapidly with decreasing temperature. These results of the numerical simulation thus show that the S-shape of the I–V characteristics can arise from enhanced interface recombination.
4. Discussion For a qualitative understanding of the physical reasons for the occurrence of an S-shaped I–V curve (double exponential) we show in Fig. 5 the band diagram under short circuit conditions with the Dit as parameter. If Dit is smaller than 1012 cm2 the band bending is not affected by the Dit because the Qit in these states is small compared to the space charge QS . If Qit becomes comparable with QS , the bands are rearranged to keep charge balance. As a result, the band bending in the crystalline layer is reduced, while the band bending in the amorphous layer is enhanced, such that the built-in voltage Vbi remains constant. The reduction of the band bending results in an enhancement of the concentration of holes at the interface and a reduction of the electron concentration in
Fig. 5. Band bending diagrams of the heterojunction under illumination for short circuit conditions (V ¼ 0 V). Note that the x-axis has logarithmical scaling. Details see text.
A. Froitzheim et al. / Journal of Non-Crystalline Solids 299–302 (2002) 663–667
the amorphous layer. This allows the interface states to be more efficient for recombination of the minorities which reduces the electron current in the amorphous layer. With a high Dit it is possible to reach an accumulation layer in the crystalline wafer if a forward bias V is applied to the junction for V close to VOC . This means that the recombination at the interface changes drastically and a significant loss current flows across the interface thus reducing the photocurrent. For V > VOC the current flow occurs in the opposite direction. The recombination current at the interface then does not appear as a loss current such that an exponential increase of the I–V characteristic is obtained. It should be noted that such a behaviour is not obtained for defect states of donor type, because occupied donor states are neutral and do not result in a modified band bending. In this simple picture the temperature dependence, too, can qualitatively be explained. As the temperature is reduced Vbi increases. This gain in Vbi drops off almost entirely in the crystalline wafer due to the high density of states in the amorphous silicon. This results in an enhanced electron concentration at the interface and, again, if forward bias is applied, these electrons recombine with the increasing number of holes at the interface, resulting in a loss of the photocurrent. Since more electrons are available for recombination, the recombination current is enhanced and the S-shape becomes more pronounced. 5. Conclusion Modifications of the wafer surface which enhance interface recombination may have consid-
667
erable influence on the properties of a-Si/c-Si heterojunctions. S-shaped I–V characteristics can result if the charge in interface states becomes comparable with the space charge in the Si-wafer. In the present case this critical value of the density of interface states is at about Dit ¼ 1012 cm2 .
Acknowledgements We thank H. Angermann, G. Keiler, F. Fenske for preparation and the Bundesministerium f€ ur Bildung und Forschung (BMBF) for partial financial support (contract number 01SF0012).
References [1] M. Tanaka, M. Taguchi, T. Matsuyama, T. Sawada, S. Tsuda, S. Nakano, H. Hanafusa, Y. Kuwano, Jpn. J. Appl. Phys. 31 (1992) 3518. [2] H. Sakata, T. Nakai, T. Baba, M. Taguchi, S. Tsuge, K. Uchihashi, S. Kiyama, in: Proceedings of the 20th IEEE PVSEC, Anchorage, 2000, p. 7. [3] A. Froitzheim, H. Angermann, L. Elstner, W. F€ ussel, K. Kliefoth, J. Knechtel, M. Schmidt, N. Sinh, H. Weiser, W. Fuhs, in: Proceedings of the 16th European PVSEC, Glasgow, 2000, p. 1580. [4] T. Unold, M. R€ osch, G.H. Bauer, J. Non-Cryst. Solids 266–269 (2000) 1033. [5] M. Sebastiani, C. RiGaspare, G. Capellini, C. Bittencourt, F. Evangelisti, Phys. Rev. Lett. 75 (1995) 3352. [6] H. Angermann, W. Henrion, A. R€ oseler, M. Rebien, Mater. Sci. Eng. B 73 (2000) 178. [7] M.A. Green, in: Silicon Solar Cells Advanced Principles and Practice, UNSW, Sydney, 1995, p. 177. [8] R.E.I. Schropp, M. Zeeman, in: Amorphous and Microcrystalline Silicon Solar Cells, Kluwer Academic, Dordrecht, 1998, p. 183.
Interface recombination in heterojunctions of amorphous and crystalline silicon A. Froitzheim *, K. Brendel, L. Elstner, W. Fuhs, K. Kliefoth, M. Schmidt Hahn-Meitner-Institut Berlin, Abt. Silizium-Photovoltaik, Kekul estr. 5, D-12489 Berlin, Germany
Abstract Heterojunction solar cells consisting of an n-type a-Si:H(n) emitter and a p-type monocrystalline silicon wafer have been studied with particular emphasis on the role of interface recombination. It is shown that the form of the I–V characteristics and the effective interface recombination velocity depend on the treatment of the Si-wafer prior to the deposition of the amorphous emitter. Numerical simulation suggests that the non-exponential (S-shape) dependence of the I–V curves under illumination arises from a high density of interface states which results in enhanced recombination via interface states. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 85.30.De; 73.70.Lq
1. Introduction Heterojunctions of amorphous and crystalline silicon are promising candidates for highly efficient solar cells processed at low temperatures. High efficiencies have been obtained in the laboratory and industrialization has already been announced [1,2]. Inspite of this technological progress surprisingly little is known about the properties of the heterojunction interface. Particularly interesting problems are the passivation and role of interface states. Optimization of the device performance requires proper pretreatment procedures of the Siwafers and optimized deposition conditions of the amorphous emitter. In a recent paper we reported
that the best result was obtained if the deposition occurs on flattened and hydrogen-terminated surfaces [3]. Another important question concerns the magnitude and influence of the bandoffsets DEC and DEV in the conduction and valence band. The values quoted in the literature differ appreciably [3–5]. In this study we investigate the system aSi:H(n)/c-Si(p) consisting of an n-type amorphous silicon emitter deposited by PECVD onto floatzone silicon wafers. We discuss the influence of the wafer pretreatment and show by simulations of the device that the interface recombination has a pronounced influence on the I–V characteristics once the density of interface states exceeds a value of 1012 cm2 . 2. Experimental procedure
*
Corresponding author. Tel.: +49-30 67053 305; fax: +49-30 67053 333. E-mail address: [email protected] (A. Froitzheim).
We prepared heterojunction solar cells by depositing n-type a-Si:H onto p-type c-Si substrates
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 1 0 2 9 - 8
664
A. Froitzheim et al. / Journal of Non-Crystalline Solids 299–302 (2002) 663–667
(FZ, 0.5–2 X cm, (1 1 1), 330 lm) with back surface field. The nþ -a-Si:H(n) films were deposited by plasma enhanced chemical vapour deposition (PECVD) at a substrate temperature of 250 °C using standard conditions for high quality films (p ¼ 400 mTorr, 0.6% gas phase concentration phosphine/silane, P ¼ 15 W). The thickness of the nþ -a-Si:H(n) emitter was kept at 30–40 nm. Prior to the deposition, the p-type c-Si wafers were treated in three different ways: (a) HF-dip which causes roughening, (b) wet chemical oxidation and etching in buffered NH4 F which is considered to smoothen the surface and to saturate dangling bonds (H-termination), (c) growth of an ultrathin oxide layer for passivation. Contact to the a-Si:H emitter was made by a 80 nm thick ZnO(Al) ðR ¼ 50 XÞ deposited by sputtering. Evaporated aluminium was used for the grid and rear contacts. I–V characteristics were measured at 300 K with under AM1.5 conditions. The measurements of the temperature dependence between 100 and 300 K were performed in a liquid nitrogen cryostat using a tungsten halogen lamp for illumination. Numerical simulations were performed which consisted of the simultaneous solution of the Poisson equation and the continuity equations for electrons and holes with a quasi-continuum of gap states in the amorphous layer and a single gap state in the crystalline wafer.
3. Results Table 1 summarizes previously published results of a measurement of the effective interface recombination velocity S for the various wafer pretreatment procedures (see [3] for details). We conclude that the interface recombination is strongly influenced by the density of interface states Dit which is determined by the pretreatment
Table 1 Effective recombination velocity [3] Wafer pretreatment
Seff (cm/s)
Tunnel-oxide HF-Dip H-termination
6000 1000 600
Fig. 1. I–V characteristics at 300 K under AM 1.5 – illumination.
procedure [6]. S is smallest for the sample where the deposition of the amorphous emitter was performed after careful H-termination of the Siwafer. This result is consistent with our experience that this treatment leads to highest cell efficiencies of above 13%. The I–V characteristics, too, exhibit a characteristic dependence on the wafer pretreatment (Fig. 1). Whereas the behaviour of the H-terminated sample is close to normal, the other pretreatments led to S-shaped I–V characteristics. This effect is most pronounced in case where a thin tunnel oxide has been formed which is known to lead to high Dit . Such effects have been observed only under illumination, there was no anomaly in the dark characteristics. Similar S-shaped characteristics of a-Si:H(n)/c-Si(p) junctions have been reported before by Unold et al. [4] who related this effect to a large bandoffset in the conduction band of DEC ¼ 0:3 eV. However, we found much smaller values of less than 0.1 eV [3]. In situ constantfinal-state photoemission led to DEC ¼ 0:16 eV which might be considered to be the most reliable value at present [5]. It is interesting to notice that generally at low temperature the S-shape form of the I–V characteristics becomes much more pronounced. Fig. 2 gives an example for the case of the HF-dipped specimen. The results presented so far show that the value of S as well as the form of the I–V curves can considerably be influenced by the pretreatment of the surface of the Si-wafer. Since the pretreatment procedure is known to vary the density of interface
A. Froitzheim et al. / Journal of Non-Crystalline Solids 299–302 (2002) 663–667
Fig. 2. Temperature dependence of the I–V characteristics for samples when illuminated (tungsten halogen lamp). The sample that is shown has an HF-dip as wafer pretreatment.
states [6] we argue that interface recombination is responsible for both features. To further investigate these effects we have performed numerical simulations using a program that allows to model the device with special regard to the interface. The Poisson equation and the transport equations for electrons and holes have been solved with the following boundary conditions for the front (0) and back side ðwÞ of the cell: uð0Þ ¼ 0;
uðwÞ ¼ Vbi V ;
ð1Þ
jn ð0Þ ¼ Snf Dnð0Þ;
jn ðwÞ ¼ Snr DnðwÞ;
ð2Þ
jp ð0Þ ¼ Spf Dpð0Þ;
jp ðwÞ ¼ Spr DpðwÞ:
ð3Þ
665
In these expressions u denotes the electrostatic potential, jn and jp are the electron and hole current densities, Dn and Dp are the excess conf=r centrations under illumination, Sn=p are the recombination velocities of the electrons (n) and holes (p) at the front (f) and rear (r) contact. All f=r Sn=p were chosen to 107 cm=s. Vbi is the built in voltage and V is the applied voltage. We assume flat band conditions on both contacts. For the aSi:H(n) the position of the Fermi level at the surface was kept at EC EF ¼ 0:25 eV which is the activation energy measured for thick a-Si:H(n) samples. The interface is assumed to be abrupt with bandoffsets DEC and DEV . The boundary conditions for the potential at the interface are uðx1 Þ uðx2 Þ ¼ 0;
ð4Þ
e0 ec-Si Eðx2 Þ e0 ea-Si Eðx1 Þ Qit ¼ 0:
ð5Þ
Physically this means that the potential is continuous across the interface, where x1 is at the interface on the amorphous side and x2 is at the interface on the crystalline side, such that dipoles are excluded (Eq. (4)). The Gauss equation for the dielectric displacement (Eq. (5)) has to be fulfilled including Qit which is the charge localized at the interface. e0 ; ea-Si and ec-Si are the dielectric constants and E is the electric field. Transport across the interface is simulated assuming thermionic emission of holes and electrons. The interface states have been positioned at midgap of the c-Si.
Table 2 Parameters used for simulation (data for a-Si:H(n) similar to [8]) Material parameters
a-Si:H(n)/c-Si(p)
Thickness (cm) EG (eV) ln and lp ðcm2 =V sÞ NC and NV ðcm3 Þ Doping [P], [B] ðcm3 Þ
3 105 =3:3 102 1.7/1.12 5/1400 and 1/480 1 1020 =2:86 1019 and 1 1020 =3:19 1019 1 1019 =1 1016
Tail states parameters a-Si:H(n) DOS at CB/VB edge ðcm3 =eVÞ CB/VB Urbach energy (eV) cn;p charged/neutral states ðcm3 =sÞ
1 1021 =1 1021 0.080/0.170 1 108 =1 1010
Dangling bond states in a-Si:H(n) ND ðcm3 Þ Eþ=0 =E0= above EV (eV) Gaussian distribution cn;p charged/neutral states ðcm3 =sÞ
1:5 1019 0.4/0.6 r ¼ 0:15 1 108 =1 1010
666
A. Froitzheim et al. / Journal of Non-Crystalline Solids 299–302 (2002) 663–667
The defect structure of the a-Si:H(n) is modelled similar to [8] by two Gaussian distributions of the dangling bond states and exponential tail states at both bandedges. For c-Si a single trap at midgap is assumed. The most important parameters used are listed in Table 2. Here we present results for which the only varied parameters were the density Dit of acceptorlike states, their electron and hole capture rate coefficients cn=p and the temperature. Fig. 3 shows the effect of increasing Dit . The cn;p were assumed to be strongly asymmetric, cn ¼ 108 cm3 =s and cp ¼ 1012 cm3 =s, in accordance with literature data for the SiO2 =Si interface [7]. The bandoffset is
Fig. 3. Simulation of I–V characteristics at 300 K. Parameter is the density of interface states Dit : (a) 0 1012 cm2 , (b) 1 1012 cm2 , (c) 3 1012 cm2 (d) 4 1012 cm2 , (e) 5 1012 cm2 , (f) 6 1012 cm2 .
Fig. 4. Simulation of the I–V characteristics at various temperatures ðDit ¼ 4 1012 cm2 Þ.
kept constant at DEC ¼ 150 meV. As a result, the fill factor of the I–V curve is reduced and the Sshape evolves when the Dit exceeds a value of about 1012 cm2 . Fig. 4 displays the simulation of the temperature dependence using a value for Dit of 4 1012 cm2 . While at 300 K the I–V curve is almost normal, the S-shape develops rapidly with decreasing temperature. These results of the numerical simulation thus show that the S-shape of the I–V characteristics can arise from enhanced interface recombination.
4. Discussion For a qualitative understanding of the physical reasons for the occurrence of an S-shaped I–V curve (double exponential) we show in Fig. 5 the band diagram under short circuit conditions with the Dit as parameter. If Dit is smaller than 1012 cm2 the band bending is not affected by the Dit because the Qit in these states is small compared to the space charge QS . If Qit becomes comparable with QS , the bands are rearranged to keep charge balance. As a result, the band bending in the crystalline layer is reduced, while the band bending in the amorphous layer is enhanced, such that the built-in voltage Vbi remains constant. The reduction of the band bending results in an enhancement of the concentration of holes at the interface and a reduction of the electron concentration in
Fig. 5. Band bending diagrams of the heterojunction under illumination for short circuit conditions (V ¼ 0 V). Note that the x-axis has logarithmical scaling. Details see text.
A. Froitzheim et al. / Journal of Non-Crystalline Solids 299–302 (2002) 663–667
the amorphous layer. This allows the interface states to be more efficient for recombination of the minorities which reduces the electron current in the amorphous layer. With a high Dit it is possible to reach an accumulation layer in the crystalline wafer if a forward bias V is applied to the junction for V close to VOC . This means that the recombination at the interface changes drastically and a significant loss current flows across the interface thus reducing the photocurrent. For V > VOC the current flow occurs in the opposite direction. The recombination current at the interface then does not appear as a loss current such that an exponential increase of the I–V characteristic is obtained. It should be noted that such a behaviour is not obtained for defect states of donor type, because occupied donor states are neutral and do not result in a modified band bending. In this simple picture the temperature dependence, too, can qualitatively be explained. As the temperature is reduced Vbi increases. This gain in Vbi drops off almost entirely in the crystalline wafer due to the high density of states in the amorphous silicon. This results in an enhanced electron concentration at the interface and, again, if forward bias is applied, these electrons recombine with the increasing number of holes at the interface, resulting in a loss of the photocurrent. Since more electrons are available for recombination, the recombination current is enhanced and the S-shape becomes more pronounced. 5. Conclusion Modifications of the wafer surface which enhance interface recombination may have consid-
667
erable influence on the properties of a-Si/c-Si heterojunctions. S-shaped I–V characteristics can result if the charge in interface states becomes comparable with the space charge in the Si-wafer. In the present case this critical value of the density of interface states is at about Dit ¼ 1012 cm2 .
Acknowledgements We thank H. Angermann, G. Keiler, F. Fenske for preparation and the Bundesministerium f€ ur Bildung und Forschung (BMBF) for partial financial support (contract number 01SF0012).
References [1] M. Tanaka, M. Taguchi, T. Matsuyama, T. Sawada, S. Tsuda, S. Nakano, H. Hanafusa, Y. Kuwano, Jpn. J. Appl. Phys. 31 (1992) 3518. [2] H. Sakata, T. Nakai, T. Baba, M. Taguchi, S. Tsuge, K. Uchihashi, S. Kiyama, in: Proceedings of the 20th IEEE PVSEC, Anchorage, 2000, p. 7. [3] A. Froitzheim, H. Angermann, L. Elstner, W. F€ ussel, K. Kliefoth, J. Knechtel, M. Schmidt, N. Sinh, H. Weiser, W. Fuhs, in: Proceedings of the 16th European PVSEC, Glasgow, 2000, p. 1580. [4] T. Unold, M. R€ osch, G.H. Bauer, J. Non-Cryst. Solids 266–269 (2000) 1033. [5] M. Sebastiani, C. RiGaspare, G. Capellini, C. Bittencourt, F. Evangelisti, Phys. Rev. Lett. 75 (1995) 3352. [6] H. Angermann, W. Henrion, A. R€ oseler, M. Rebien, Mater. Sci. Eng. B 73 (2000) 178. [7] M.A. Green, in: Silicon Solar Cells Advanced Principles and Practice, UNSW, Sydney, 1995, p. 177. [8] R.E.I. Schropp, M. Zeeman, in: Amorphous and Microcrystalline Silicon Solar Cells, Kluwer Academic, Dordrecht, 1998, p. 183.