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Open circuit voltage in homojunction and heterojunction silicon solar cells grown by VHF-PECVD
Journal of Non-Crystalline Solids 299–302 (2002) 1203–1207 www.elsevier.com/locate/jnoncrysol
Open circuit voltage in homojunction and heterojunction silicon solar cells grown by VHF-PECVD R. Rizzoli
a,*
c, C. Summonte a, A. Migliori a, , E. Centurioni b, J. Pla A. Desalvo b, F. Zignani b a
CNR-Lamel, via Gobetti 101, I-40129 Bologna, Italy Dip. Chimica Applicata e Scienza dei Materiali, University of Bologna, I-40136 Bologna, Italy Grupo Energıa Solar, CAC-CNEA, Av. Gral. Paz 1499, 1650 San Martın, Buenos Aires, Argentine b
c
Abstract We present homojunction and lc-Si/a-Si:H/c-Si heterojunction silicon solar cells fabricated by PECVD. The H2 dilution used during the i-layer growth strongly affects the device efficiency. While intermediate H2 dilution of the gas mixture results in Voc degradation, the best Voc is obtained under zero or very high ( ¼ 99.4%) H2 dilution, resulting in totally amorphous or epitaxial i-layer respectively. A maximum value of 638 mV, with 13.7% efficiency, is observed in the case of an amorphous i-layer, indicating an improvement of interface quality. If the i-layer is deposited using a 99.4% H2 dilution, a 608 mV Voc is observed and for homojunction solar cells a 13.1% efficiency is obtained. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 52.75.R; 81.15; 61.72; 84.60.J; 68.65
1. Introduction The silicon p/n junction formed at low temperature (< 300 °C) by plasma deposition finds an important application in the fabrication of heterojunction photovoltaic devices [1], that present the double advantage of avoiding the degradation of electronic properties of the c-Si base produced by thermal processing, and a low thermal budget, that reflects on reduced cost of the device. The top
*
Corresponding author. Tel.: +39-051 639 9131; fax: +39051 639 9216. E-mail address: [email protected] (R. Rizzoli).
efficiency reported for this kind of device is 20.1% on texturized silicon [1]. However, little detail is reported about the deposition conditions needed to obtain such good results. In this paper, we study the performance of hetero- and homo-junction photovoltaic devices as a function of the deposition parameters used to deposit the i-layer, for different (amorphous, microcrystalline or epitaxial) structure of the p-type emitter. The p-type lc-Si was deposited by very high frequency (VHF) [2], with advantages in terms of stability of the deposited phase and improvement of crystalline quality [3]. However, as the VHF plasma can etch or completely recrystallize the underlying a-Si:H layer [4], in turn
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 8 8 - 2
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R. Rizzoli et al. / Journal of Non-Crystalline Solids 299–302 (2002) 1203–1207
Table 1 PECVD gas flow rates and deposition times, and p-layer plasma power and frequency, and substrate temperature Sample
H1 H2 H3 H4 H5
p/i structure
epi/epi amorphous/epi micro/amorphous
p-layer
i-layer
SiH4 (sccm)
H2 (sccm)
B2 H6 (sccm)
tp (s)
P (W/cm2 )
m (MHz)
T (°C)
SiH4 (sccm)
H2 (sccm)
ti (s)
3 3 2.5 2.5 3
197 197 21.5 21.5 197
0.008 0.008 0.04 0.04 0.008
240 240 170 170 240
0.156 0.156 0.028 0.028 0.156
100 100 13.56 13.56 100
170 170 120 120 170
0.6 3.5 0.6 6 20
94 80 94 94 –
375 180 125 120 25
All i-layers were deposited at 200 °C, 13.56 MHz, 0:028 W=cm2 .
affecting the growth quality, additional care in the double (p/i) layer design is needed.
2. Experimental The 1 1 cm2 solar cells were fabricated by PECVD on flat n-type c-Si, CZ, 1 X cm, 4 in. wafers. Three types of cells were fabricated: epi/epi: these cells are pþ epi-c-Si/i epi-c-Si/n c-Si homojunction devices. The hydrogen dilution H-dil ¼ [H2 ]/([SiH4 ]+[H2 ]) used to deposit the i-layer was varied in the range 94–99.4%. amorphous/epi: these cells are pþ a-Si:H/i epic-Si/n c-Si heterojunction devices. micro/amorphous: these cells are pþ lc-Si/i a-Si:H/n c-Si heterojunction devices. All deposition conditions are reported in Table 1. The condition for lc-Si and epi c-Si deposition are the same, yet, epi c-Si is obtained only on c-Si substrate. Before junction formation, the c-Si substrates were etched in 0.5% HF solution in deionized water for 60 s and rapidly inserted in vacuum with no rinse. All cells incorporate a microcrystalline nþ back surface layer, deposited at 13.56 MHz, 110 mW=cm2 , 170 °C, PH3 :SiH4 : H2 ¼ 0:04 : 2 : 94, that improves the back contact quality. After the formation of the emitter and nþ back surface layers, the solar cells were completed with the deposition of indium tin oxide (ITO) by magnetron RF sputtering at 250 °C, that acts as an antireflecting coating and transparent front contact, and Al contact evaporation on both sides. Further details on solar cells fabrication are reported in [5].
3. Results For each sample, four 1 1 cm2 cells were fabricated. The homogeneity over the four samples was very good, with a deviation of Voc among the four cell lower than 1% (3% for sample H4). The structure (amorphous or microcrystalline) of all samples was determined by reflectance and transmittance (R&T) spectroscopy, not reported here. Details on data elaboration as well as on the very good reliability of results obtained with this technique are reported in [2]. R&T spectroscopy shows that 94% dilution produces some amorphous fraction at the interface with c-Si. For higher dilution (96% and larger) no optically detectable amorphous fraction is present in the devices. In Figs. 1 and 2, we report the transmission electron microscopy (TEM) cross-sections of samples H1 and H2, that include an i-layer deposited with 99.4% and 96% hydrogen dilution, respectively. In agreement with R&T spectroscopy, the figures show that in both cases the deposited layers are epitaxial, although a more disordered interface is observable for the lower hydrogen dilution (Fig. 2). The J–V characteristics under illumination (AM1, 100 mW=cm2 ) of all devices are reported in Fig. 3. The i- and p-layer thickness, as well as the device electrical characteristics (open circuit voltage Voc , short circuit current, fill factor and efficiency) are reported in Table 2. The J–V characteristics of the amorphous/epi devices H3 and H4 show that a slightly lower hydrogen dilution severely affects the Voc , indicating the presence of a high density of interfacial states. The same trend is
R. Rizzoli et al. / Journal of Non-Crystalline Solids 299–302 (2002) 1203–1207
Fig. 1. High resolution transmission electron microscopy image taken on sample H1, at the epi film/silicon substrate interface.
observed for the two epi/epi devices H1 and H2, again including the same p-layer, and an i-layer deposited with different hydrogen dilution. The larger short circuit current density observed for the epi/epi with respect to the amorphous/epi devices reflects the lower absorption of crystalline silicon with respect to a-Si:H, which allows a larger fraction of the incident light to be absorbed in the c-Si base. A larger spectral response signal (reported in [6]) is indeed observed for epi/epi compared to amorphous/epi solar cells in the UV–Vis range. In the figure, the light J–V characteristic of the micro/amorphous device H5 is also reported. The amorphous i-layer is deposited with no hydrogen dilution (see Table 1). Correspondingly, a 638 mV Voc is observed (see Table 2). The Voc as a function of hydrogen dilution of the i-layer is reported in Fig. 4 for all devices. It is seen that, as the hydrogen dilution increases from
1205
Fig. 2. High resolution transmission electron microscopy image taken on sample H2, at the epi film/silicon substrate interface.
Fig. 3. Solar cell J–V characteristics under 100 mW=cm2 AM1 illumination. The curves are labeled with the hydrogen dilution used to deposit the i-layer.
94% to 99.4%, a marked increase of the Voc is observed, irrespective to the structure (amorphous or crystalline) of the emitter.
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R. Rizzoli et al. / Journal of Non-Crystalline Solids 299–302 (2002) 1203–1207
Table 2 H2 dilution used for i-layer deposition, i-and p-layer thickness, and solar cells parameters Sample H1 H2 H3 H4 H5
p/i structure epi/epi amorphous/epi micro/amorphous
p-layer
i-layer
d(p) (nm)
H dil (%)
d(i) (nm)
46 46 14 14 50
99.4 96.0 99.4 94.0 0
13 8 4 10 5
Fig. 4. Open circuit voltage, Voc , as a function of the hydrogen dilution used during the deposition of the i-layer.
4. Discussion The Voc of solar cell devices is mainly governed by the diffusion voltage VD , and by the reverse saturation current J0 , that is affected by interfacial defects. In heterojunction devices, the larger energy gap of a-Si:H with respect to c-Si results in a larger VD for amorphous/epi over epi/epi devices, as confirmed from the larger Voc of sample H3 with respect to H1. Still, Fig. 4 shows that the Voc is dominated by the quality of the intrinsic buffer layer rather than by the emitter structure (either epitaxial, microcrystalline or amorphous), confirming that the interfacial defect density is a crucial parameter affecting the device performance. It is well known that high hydrogen dilution during PECVD of silicon produces growth under conditions close to equilibrium, and results in microcrystalline growth [7]. The growth is epitaxial on oxide free c-Si surfaces. If hydrogen dilution is moderately decreased, epitaxy is preserved, yet the growth is faster and therefore more defective. A
Voc (mV)
Jsc (mA=cm2 )
FF (%)
gext: (%)
558 448 608 334 638
32.0 32.1 29.0 26.9 28.1
73 67 72 47 76
13.1 8.7 12.8 4.3 13.7
degradation of electronic properties (lifetime, mobility) is supposed to follow from such deviation from equilibrium deposition, with consequent departure of the i-layer properties from those of an ideal buffer layer [8]. For even lower hydrogen dilution, a partially amorphous layer forms at the interface with the emitter. In this case, the formation of parallel conduction paths between grains and gap defects in the amorphous regions is likely to be responsible for further Voc degradation. However, if hydrogen dilution is eliminated, a purely amorphous layer grows on c-Si. A maximum Voc of 638 mV is observed in this case (sample H5), in agreement with the well-known passivating properties of a-Si:H [9]. The larger Voc measured on the micro/amorphous cells if compared to epi/epi cells is attributed to the larger diffusion voltage consequent to the lower activation energy and larger energy gap of the microcrystalline compared to epitaxial emitter, although the definition and determination of energy gap in a thin microcrystalline Si layer is controversial [10,11]. This consideration suggests that an amorphous i-layer is preferable over epi layers. However, the epitaxial i-layer, if deposited with large hydrogen dilution, produces good interface passivation (sample H1), with the additional advantage of allowing for epitaxial growth of the emitter, and consequent increase of the device short circuit current density, due to the improved transparency and superior electrical characteristics. In fact, the overall efficiency is almost as good as the efficiency obtained in micro/ amorphous devices. One additional comment can be made. The extremely high H2 dilution, as that reported in this paper, might suggest that some chemical transport deposition (CTD) exists during the i-layer depo-
R. Rizzoli et al. / Journal of Non-Crystalline Solids 299–302 (2002) 1203–1207
sition, in the sense that the actual SiH4 concentration could include some silane etched by the H2 from the deposition chamber walls. However, in a previous study about chemical transport [12] we have shown that one of the conditions for CTD to occur is the long gas residence time, and therefore low gas flow rates, that is not our case. However, the possibility of having or not CTD during a pure H2 plasma, depending on H2 flow rate, can explain the opposite observations that an H2 plasma improves [13] or degrades [14] the surface quality in heterojunction devices. 5. Conclusions We observed that the performance of solar cell devices fabricated by PECVD is strongly affected by the quality of the intrinsic buffer layer. Although both amorphous and epitaxial buffer i-layers produce good passivation of the interface, the deposition conditions of the epitaxial layer seriously affect the Voc of the devices, that drops from 606 mV for the high H2 dilution deposition, down to less than 350 mV for lower H2 dilution. The result is correlated with the partially amorphous growth in this last case. In the case of an amorphous buffer layer, however, the passivation properties of the interface are restored, and a maximum 638 mV Voc is observed. In this last case, a 13.7% efficiency is observed, while a maximum 13.1% efficiency is measured for the case of an epitaxial buffer layer. Acknowledgements This work was partially supported by the Italian Ministero dell’Istruzione, dell’Universit a e della Ricerca.
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References [1] M. Taguchi, K. Kawamoto, S. Tsuge, T. Baba, H. Sakata, M. Morizane, K. Uchihasci, N. Nakamura, S. Kiyama, O. Oota, Prog. Photovolt: Res. Appl. 8 (2000) 503. [2] C. Summonte, R. Rizzoli, A. Desalvo, F. Zignani, E. Centurioni, R. Pinghini, M. Gemmi, J. Non-Cryst. Solids 266–269 (2000) 624. [3] A.A. Howling, L. Sansonnens, J. Ballutaud, F. Grangeon, T. Delachaux, C. Hollenstein, V. Daudrix, U. Kroll, in: Proceedings of the 16th European Photovoltaic Solar Energy Conference, Glasgow, 2000, p. 518. [4] R. Rizzoli, C. Summonte, J. Pla, E. Centurioni, R. Pinghini, A. Desalvo, F. Zignani, in: Proceedings of the 16th European Photovoltaic Solar Energy Conference, Glasgow, 2000, p. 593. [5] J. Pla, E. Centurioni, R. Pinghini, C. Summonte, R. Rizzoli, F. Zignani, A. Desalvo, in: Proceedings of the 16th European Photovoltaic Solar Energy Conference, Glasgow, 2000, p. 597. [6] J. Pla, E. Centurioni, C. Summonte, R. Rizzoli, A. Migliori, A. Desalvo, F. Zignani, Thin Solid Films (2002), in press. [7] R. Rizzoli, C. Summonte, J. Pla, E. Centurioni, G. Ruani, A. Desalvo, F. Zignani, Thin Solid Films 383 (2001) 7. [8] B. Von Roedern, G.H. Bauer, Mater. Res. Soc. Symp. Proc. 557 (1999) 761. [9] D. della Sala, F. Roca, G. Fameli, P. Grillo, F. Pascarella, F. Ferrazza, in: Proceedings of the 12th European Photovoltaic Solar Energy Conference, Amsterdam (NL), April 11–15, 1994, p. 799. [10] S. Guha, J. Yang, P. Nath, M. Hack, Appl. Phys. Lett. 49 (1986) 218. [11] X. Xu, J. Yang, A. Banerjee, S. Guha, K. Vasanth, S. Wagner, Appl. Phys. Lett. 67 (1995) 2323. [12] E. Centurioni, A. Desalvo, R. Pinghini, R. Rizzoli, C. Summonte, F. Zignani, MRS Proc. 536 (1999) 517. [13] T. Sawada, N. Terada, S. Tsuge, T. Baba, T. Takahama, K. Wakisaka, S. Tsuda, S. Nakano, in: Proceedings of the 1st World Conference on Photovoltaic Energy Conversion, Hawaii, 1994, p. 1219. [14] M. Tucci, R. De Rosa, F. Roca, D. Caputo, G. de Cesare, F. Palma, in: Proceedings of the 16th European Photovoltaic Solar Energy Conference, Glasgow, 2000, p. 589.
Open circuit voltage in homojunction and heterojunction silicon solar cells grown by VHF-PECVD R. Rizzoli
a,*
c, C. Summonte a, A. Migliori a, , E. Centurioni b, J. Pla A. Desalvo b, F. Zignani b a
CNR-Lamel, via Gobetti 101, I-40129 Bologna, Italy Dip. Chimica Applicata e Scienza dei Materiali, University of Bologna, I-40136 Bologna, Italy Grupo Energıa Solar, CAC-CNEA, Av. Gral. Paz 1499, 1650 San Martın, Buenos Aires, Argentine b
c
Abstract We present homojunction and lc-Si/a-Si:H/c-Si heterojunction silicon solar cells fabricated by PECVD. The H2 dilution used during the i-layer growth strongly affects the device efficiency. While intermediate H2 dilution of the gas mixture results in Voc degradation, the best Voc is obtained under zero or very high ( ¼ 99.4%) H2 dilution, resulting in totally amorphous or epitaxial i-layer respectively. A maximum value of 638 mV, with 13.7% efficiency, is observed in the case of an amorphous i-layer, indicating an improvement of interface quality. If the i-layer is deposited using a 99.4% H2 dilution, a 608 mV Voc is observed and for homojunction solar cells a 13.1% efficiency is obtained. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 52.75.R; 81.15; 61.72; 84.60.J; 68.65
1. Introduction The silicon p/n junction formed at low temperature (< 300 °C) by plasma deposition finds an important application in the fabrication of heterojunction photovoltaic devices [1], that present the double advantage of avoiding the degradation of electronic properties of the c-Si base produced by thermal processing, and a low thermal budget, that reflects on reduced cost of the device. The top
*
Corresponding author. Tel.: +39-051 639 9131; fax: +39051 639 9216. E-mail address: [email protected] (R. Rizzoli).
efficiency reported for this kind of device is 20.1% on texturized silicon [1]. However, little detail is reported about the deposition conditions needed to obtain such good results. In this paper, we study the performance of hetero- and homo-junction photovoltaic devices as a function of the deposition parameters used to deposit the i-layer, for different (amorphous, microcrystalline or epitaxial) structure of the p-type emitter. The p-type lc-Si was deposited by very high frequency (VHF) [2], with advantages in terms of stability of the deposited phase and improvement of crystalline quality [3]. However, as the VHF plasma can etch or completely recrystallize the underlying a-Si:H layer [4], in turn
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 8 8 - 2
1204
R. Rizzoli et al. / Journal of Non-Crystalline Solids 299–302 (2002) 1203–1207
Table 1 PECVD gas flow rates and deposition times, and p-layer plasma power and frequency, and substrate temperature Sample
H1 H2 H3 H4 H5
p/i structure
epi/epi amorphous/epi micro/amorphous
p-layer
i-layer
SiH4 (sccm)
H2 (sccm)
B2 H6 (sccm)
tp (s)
P (W/cm2 )
m (MHz)
T (°C)
SiH4 (sccm)
H2 (sccm)
ti (s)
3 3 2.5 2.5 3
197 197 21.5 21.5 197
0.008 0.008 0.04 0.04 0.008
240 240 170 170 240
0.156 0.156 0.028 0.028 0.156
100 100 13.56 13.56 100
170 170 120 120 170
0.6 3.5 0.6 6 20
94 80 94 94 –
375 180 125 120 25
All i-layers were deposited at 200 °C, 13.56 MHz, 0:028 W=cm2 .
affecting the growth quality, additional care in the double (p/i) layer design is needed.
2. Experimental The 1 1 cm2 solar cells were fabricated by PECVD on flat n-type c-Si, CZ, 1 X cm, 4 in. wafers. Three types of cells were fabricated: epi/epi: these cells are pþ epi-c-Si/i epi-c-Si/n c-Si homojunction devices. The hydrogen dilution H-dil ¼ [H2 ]/([SiH4 ]+[H2 ]) used to deposit the i-layer was varied in the range 94–99.4%. amorphous/epi: these cells are pþ a-Si:H/i epic-Si/n c-Si heterojunction devices. micro/amorphous: these cells are pþ lc-Si/i a-Si:H/n c-Si heterojunction devices. All deposition conditions are reported in Table 1. The condition for lc-Si and epi c-Si deposition are the same, yet, epi c-Si is obtained only on c-Si substrate. Before junction formation, the c-Si substrates were etched in 0.5% HF solution in deionized water for 60 s and rapidly inserted in vacuum with no rinse. All cells incorporate a microcrystalline nþ back surface layer, deposited at 13.56 MHz, 110 mW=cm2 , 170 °C, PH3 :SiH4 : H2 ¼ 0:04 : 2 : 94, that improves the back contact quality. After the formation of the emitter and nþ back surface layers, the solar cells were completed with the deposition of indium tin oxide (ITO) by magnetron RF sputtering at 250 °C, that acts as an antireflecting coating and transparent front contact, and Al contact evaporation on both sides. Further details on solar cells fabrication are reported in [5].
3. Results For each sample, four 1 1 cm2 cells were fabricated. The homogeneity over the four samples was very good, with a deviation of Voc among the four cell lower than 1% (3% for sample H4). The structure (amorphous or microcrystalline) of all samples was determined by reflectance and transmittance (R&T) spectroscopy, not reported here. Details on data elaboration as well as on the very good reliability of results obtained with this technique are reported in [2]. R&T spectroscopy shows that 94% dilution produces some amorphous fraction at the interface with c-Si. For higher dilution (96% and larger) no optically detectable amorphous fraction is present in the devices. In Figs. 1 and 2, we report the transmission electron microscopy (TEM) cross-sections of samples H1 and H2, that include an i-layer deposited with 99.4% and 96% hydrogen dilution, respectively. In agreement with R&T spectroscopy, the figures show that in both cases the deposited layers are epitaxial, although a more disordered interface is observable for the lower hydrogen dilution (Fig. 2). The J–V characteristics under illumination (AM1, 100 mW=cm2 ) of all devices are reported in Fig. 3. The i- and p-layer thickness, as well as the device electrical characteristics (open circuit voltage Voc , short circuit current, fill factor and efficiency) are reported in Table 2. The J–V characteristics of the amorphous/epi devices H3 and H4 show that a slightly lower hydrogen dilution severely affects the Voc , indicating the presence of a high density of interfacial states. The same trend is
R. Rizzoli et al. / Journal of Non-Crystalline Solids 299–302 (2002) 1203–1207
Fig. 1. High resolution transmission electron microscopy image taken on sample H1, at the epi film/silicon substrate interface.
observed for the two epi/epi devices H1 and H2, again including the same p-layer, and an i-layer deposited with different hydrogen dilution. The larger short circuit current density observed for the epi/epi with respect to the amorphous/epi devices reflects the lower absorption of crystalline silicon with respect to a-Si:H, which allows a larger fraction of the incident light to be absorbed in the c-Si base. A larger spectral response signal (reported in [6]) is indeed observed for epi/epi compared to amorphous/epi solar cells in the UV–Vis range. In the figure, the light J–V characteristic of the micro/amorphous device H5 is also reported. The amorphous i-layer is deposited with no hydrogen dilution (see Table 1). Correspondingly, a 638 mV Voc is observed (see Table 2). The Voc as a function of hydrogen dilution of the i-layer is reported in Fig. 4 for all devices. It is seen that, as the hydrogen dilution increases from
1205
Fig. 2. High resolution transmission electron microscopy image taken on sample H2, at the epi film/silicon substrate interface.
Fig. 3. Solar cell J–V characteristics under 100 mW=cm2 AM1 illumination. The curves are labeled with the hydrogen dilution used to deposit the i-layer.
94% to 99.4%, a marked increase of the Voc is observed, irrespective to the structure (amorphous or crystalline) of the emitter.
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R. Rizzoli et al. / Journal of Non-Crystalline Solids 299–302 (2002) 1203–1207
Table 2 H2 dilution used for i-layer deposition, i-and p-layer thickness, and solar cells parameters Sample H1 H2 H3 H4 H5
p/i structure epi/epi amorphous/epi micro/amorphous
p-layer
i-layer
d(p) (nm)
H dil (%)
d(i) (nm)
46 46 14 14 50
99.4 96.0 99.4 94.0 0
13 8 4 10 5
Fig. 4. Open circuit voltage, Voc , as a function of the hydrogen dilution used during the deposition of the i-layer.
4. Discussion The Voc of solar cell devices is mainly governed by the diffusion voltage VD , and by the reverse saturation current J0 , that is affected by interfacial defects. In heterojunction devices, the larger energy gap of a-Si:H with respect to c-Si results in a larger VD for amorphous/epi over epi/epi devices, as confirmed from the larger Voc of sample H3 with respect to H1. Still, Fig. 4 shows that the Voc is dominated by the quality of the intrinsic buffer layer rather than by the emitter structure (either epitaxial, microcrystalline or amorphous), confirming that the interfacial defect density is a crucial parameter affecting the device performance. It is well known that high hydrogen dilution during PECVD of silicon produces growth under conditions close to equilibrium, and results in microcrystalline growth [7]. The growth is epitaxial on oxide free c-Si surfaces. If hydrogen dilution is moderately decreased, epitaxy is preserved, yet the growth is faster and therefore more defective. A
Voc (mV)
Jsc (mA=cm2 )
FF (%)
gext: (%)
558 448 608 334 638
32.0 32.1 29.0 26.9 28.1
73 67 72 47 76
13.1 8.7 12.8 4.3 13.7
degradation of electronic properties (lifetime, mobility) is supposed to follow from such deviation from equilibrium deposition, with consequent departure of the i-layer properties from those of an ideal buffer layer [8]. For even lower hydrogen dilution, a partially amorphous layer forms at the interface with the emitter. In this case, the formation of parallel conduction paths between grains and gap defects in the amorphous regions is likely to be responsible for further Voc degradation. However, if hydrogen dilution is eliminated, a purely amorphous layer grows on c-Si. A maximum Voc of 638 mV is observed in this case (sample H5), in agreement with the well-known passivating properties of a-Si:H [9]. The larger Voc measured on the micro/amorphous cells if compared to epi/epi cells is attributed to the larger diffusion voltage consequent to the lower activation energy and larger energy gap of the microcrystalline compared to epitaxial emitter, although the definition and determination of energy gap in a thin microcrystalline Si layer is controversial [10,11]. This consideration suggests that an amorphous i-layer is preferable over epi layers. However, the epitaxial i-layer, if deposited with large hydrogen dilution, produces good interface passivation (sample H1), with the additional advantage of allowing for epitaxial growth of the emitter, and consequent increase of the device short circuit current density, due to the improved transparency and superior electrical characteristics. In fact, the overall efficiency is almost as good as the efficiency obtained in micro/ amorphous devices. One additional comment can be made. The extremely high H2 dilution, as that reported in this paper, might suggest that some chemical transport deposition (CTD) exists during the i-layer depo-
R. Rizzoli et al. / Journal of Non-Crystalline Solids 299–302 (2002) 1203–1207
sition, in the sense that the actual SiH4 concentration could include some silane etched by the H2 from the deposition chamber walls. However, in a previous study about chemical transport [12] we have shown that one of the conditions for CTD to occur is the long gas residence time, and therefore low gas flow rates, that is not our case. However, the possibility of having or not CTD during a pure H2 plasma, depending on H2 flow rate, can explain the opposite observations that an H2 plasma improves [13] or degrades [14] the surface quality in heterojunction devices. 5. Conclusions We observed that the performance of solar cell devices fabricated by PECVD is strongly affected by the quality of the intrinsic buffer layer. Although both amorphous and epitaxial buffer i-layers produce good passivation of the interface, the deposition conditions of the epitaxial layer seriously affect the Voc of the devices, that drops from 606 mV for the high H2 dilution deposition, down to less than 350 mV for lower H2 dilution. The result is correlated with the partially amorphous growth in this last case. In the case of an amorphous buffer layer, however, the passivation properties of the interface are restored, and a maximum 638 mV Voc is observed. In this last case, a 13.7% efficiency is observed, while a maximum 13.1% efficiency is measured for the case of an epitaxial buffer layer. Acknowledgements This work was partially supported by the Italian Ministero dell’Istruzione, dell’Universit a e della Ricerca.
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