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Nanocrystalline silicon as intrinsic layer in thin film solar cells
SSC 4452
PERGAMON
Solid State Communications 109 (1999) 125±128
Nanocrystalline silicon as intrinsic layer in thin ®lm solar cells Sukti Hazra*, Swati Ray Energy Research Unit, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700 032, India Received 22 July 1998; accepted 23 September 1998 by C. N. R. Rao
Abstract Nanocrystalline silicon (nc-Si) material is a potential candidate in photoluminescence and electroluminescence devices. So far, nc-Si thin ®lms have had no applications in solar cells as intrinsic layers because of their non-photoconductive nature. In the present work, a new type of photoconductive nc-Si thin ®lm has been developed by utilizing commercially compatible plasma enhanced chemical vapour deposition technique. Transmission electron micrographs of the new material gave evidence for the presence of scattered nanocrystallites of average size t 10 nm embedded in the hydrogenated amorphous silicon matrix. The photosensitivity of the nc-Si ®lms is . 1 £ 10 5. To test the viability of the new material in devices, a p±i±n solar cell has been fabricated using the nc-Si material of optical gap 1.89eV, as the intrinsic layer. The ef®ciency of this single junction solar cell in 8.7%. q 1998 Published by Elsevier Science Ltd. Keywords: D. Photoconductivity
1. Introduction The work on nanocrystalline silicon (nc-Si) material is generally focused on their photoluminescence [1] and electroluminesence [2] properties at room temperature, because of their potential applications in the electroluminescent and photoluminescent devices based on Si. So far, the non-photoconductive nature of nc-Si ®lms would play as a constraint to use them, as the active layers in solar cells. However, In the present work, the photovoltaic properties of nanocrystalline silicon ®lms have been demonstrated. As a result of their wide gap, new photovoltaic nanocrystalline silicon ®lms are good alternative to the high bandgap amorphous silicon carbide alloys (a-SiC:H) which are essential intrinsic layers of the top cells of tandem structure of amorphous silicon solar cells. The * Corresponding author. Tel.: 1 91-33-473-6612; Fax: 1 9133-473-2805; e-mail: [email protected].
photovoltaic quality of a nc-Si ®lm is superior than that of a a-SiC:H ®lm at the same optical gap. Further advantage of this new material is that it has been developed in convenstional rf powered (13.56 MHz) plasma enhanced chemical vapor deposition (RFPECVD) which is most compatible deposition system for the preparation of Si thin ®lms from the technological point of view. 2. Experimental Nanocrystalline silicon ®lms were developed in the ultrahigh vacuum ( t 10 29 Torr) plasma enhanced chemical vapor deposition system using process gases silane and hydrogen. Chamber pressure, r.f. power density and substrate temperature range were 1.8 Torr, 15 mW/cm 2 and 2008C±2508C respectively. The substrates used in this study were Corning glass (7059) for the electrical and optical measurements and crystalline silicon wafer for the determination of
0038-1098/99/$ - see front matter q 1998 Published by Elsevier Science Ltd. PII: S 0038-109 8(98)00522-5
126
S. Hazra, S. Ray / Solid State Communications 109 (1999) 125±128
Table 1 Optoelectronic properties of nc-Si and normal bandgap a-Si:H materials Rf power density (mW/Cm 2)
Chamber pressure (Torr)
15
1.8
30
0.5
Substrate temperature (8C)
Eg
CH
sD
s ph
(eV)
(at%)
(S cm 21)
(S cm 21)
200 220 250 250
1.92 1.89 1.80 1.72
6.72 7.23 9.54 12.34
8.66 £ 10 213 3.58 £ 10 212 9.21 £ 10 212 6.72 £ 10 211
3.58 £ 7.36 £ 1.56 £ 1.23 £
bonding con®gurations of hydrogen atoms in the silicon matrix. The thickness of the ®lms were measured by a stylus type instrument (Planer Product, UK) with a resolution of 15 nm. Optical absorption and re¯ection data in the wavelength range 185 to 2600 nm were taken from the UV-VIS-NIR double beam spectrophotometer (Hitachi 330) and optical gap (Eg) of ncSi ®lms were determined by the Tauc's method [3] using these data. Fourier Transform Infrared Spectrophotometer (Perkin Elmer, FTIR 1750) carried out the study on the bonding con®guration of hydrogen atoms in the Si thin ®lms. The bonded hydrogen contents (CH) were estimated from integrated absorption of the peak at 630 cm 21 [4] in the infrared spectra. The photoluminescent studies were done under the Xe lamp of 150 Watt. Nanocrystallites in the a-Si:H matrix were characterized by the transmission electron micrographs (TEM) and transmission electron
10 26 10 26 10 25 10 25
diffraction (TED) patterns. The dark and photoconductivity of the samples have been measured after annealing at 1508C for 1 hour in a cryostat under vacuum ( t 10 26 Torr). The secondary photoconductivity measurements were performed by using white light illumination of 50 mW/cm 2 from a tungsten lamp. Single junction p±i±n solar cell was fabricated using the nc-Si intrinsic layer on textured TCO coated glass substrate (7% haze). The structure of the cell (area 1 cm 2) was glass/TCO/p±a±SiC/nc-Si/n-aSi:H/Al. I±V characteristic was measured under AM 1.5 solar radiation of 100 mW/cm 2 from a WACOM solar Simulator. The measurement of quantum ef®ciency of the solar cell was carried out under the short circuit condition using a lock-in-ampli®er and chopped monochromatic light. 3. Results
Fig. 1. Photoluminescence spectra for nc-Si and a-Si:H at room temperature
Table 1 shows the variations of optoelectronic properties of the new material with substrate temperature at constant rf power density (15 mW/cm 2) and chamber pressure (1.8 Torr). This table also displays the optoelectronic properties of hydrogenated amorphous silicon (a-Si:H) along with its deposition parameters like, if power density (30 mW/cm 2) and chamber pressure (0.5 Torr). It is distinct from the table that the deposition regime for the new material is different from that of the normal a-Si:H. With the decrease of substrate temperature, Eg increases and CH decreases unlike to the general fact. Fig. 1 demonstrates the photoluminiscence spectra of new material with optical gap 1.89 eV and normal bandgap a-Si:H at room temperature. As observed in the ®gure, broad photoluminescent peak has been appeared for the
S. Hazra, S. Ray / Solid State Communications 109 (1999) 125±128
127
photoconductivity for nc-Si thin ®lm has been tested in the Fig. 2 As shown in the Fig.2. under light soaking, photoconductivity of nc-Si ®lm deteriorates only upto 10 hours while photoconductivity of the normal bandgap a-Si:H degrades even after 250 h. Finally Table 2 demonstrates the photovoltaic device performance of nc-Si material as a intrinsic layer. Open circuit voltage (Voc) of the single of the junction solar cell having the nc-Si intrinsic layer (0.93 volt) is higher than that of the normal bandgap solar cell (0.90 volt). The ef®ciency obtained so far with such high bandgap nc-Si cell is 8.7%. Fig. 2. Photoconductivity Vs Light soaking time. Curves 1 and 2 correspond to nc-Si (1.92 eV) and a-Si:H (1.72 eV) respectively.
nc-Si ®lm. However no such peak has been observed for the normal a-Si:H ®lm. Transmission electron micrograph of the wide bandgap Si thin ®lm has been displayed in Ref. [5]. In this micrograph, scattered nanocrystals are embedded in the amorphous silicon matrix and average size of nanocrystals is t 10 nm. Owing to the presence of scattered nanocrystallites, ®lms are called nanocrystalline silicon (nc-Si). So far, nc-Si thin ®lms were not photoconductive and hence those were not used as active layers of the solar cells. However, Table 1 shows that the new nc-Si samples are photoconductive and their photosensitivity i.e., the ratio of photoconductivity to darkconductivity is . 1 £ 10 5. In fact, the electronic properties of nc-Si ®lms are superior than those of the normal high bandgap a-SiC:H alloys. For solar cell applications, not only the electronic properties of the materials but also the stability of electronic properties under prolonged illumination are important. So light induced degradation of Table 2 Comparison of cell performances of single junction solar cells having I-layers with different materials Nature Of i-layer (eV)
Voc (Volt)
Isc (mA/cm 2)
F.F.
h (%)
nc-Si a-Si
0.93 0.90
14.0 15.2
0.65 0.68
8.7 9.4
4. Discussion For the general nc-Si ®lms, average extent of amorphous tissue between the neigh-bouring nanocrystallites is only 2±3 atomic spacing and the volume fraction of crys-tallinity is high (60±80%) [6]. According to Yoshida [7], the effective conductivity (s D) of such two phase (crystalline and amorphous) material can be expressed as
sD 2 sa s 2 sa fc c sD 1 2 sa s c 1 2 sa whereas s a and s c are the conductivities of amorphous and crystalline phase respectively and fc is the crystalline volume fraction. fc determines which play dominating role in s D and s c. In the normal nc-Si ®lms, since the crystalline volumefraction is greater than 60%, s c dominates in s D and hence conductivity is . 10 25 Scm 21. The charge carriers are transferred between the neighboring crystallites by tunneling through the thin amorphous tissue. However, the structure of the present nc-Si ®lms deposited at high chamber pressure and high hydrogen dilution, is different from the general nc-Si ®lms. Fig. 3 schematically describes the morphological difference between the general and present nanocrystalline silicon thin ®lms. In the present case nanocrystals are widely separated by the amorphous tissue and the distance between neighboring crystallites is too large for the transferring of charge carriers form one crystallite to other. Moreover, infrared spectra of the nc-Si ®lms are dominated by silicon monohydride [5] and so the large amorphous phase is more ordered
128
S. Hazra, S. Ray / Solid State Communications 109 (1999) 125±128
5. Conclusions
Fig. 3. Schematic diagrams of morphology of the general and the present nanocrystalline silicon ®lms
and compact compared to that in general nano or microcrystalline silicon material. As at high chamber pressure, average energy of the electrons increases [8]and the large amount of nascent hydrogen atoms has been generated in the plasma. Those energetic hydrogen atoms can supply huge amount of energy to the surface of the growing ®lms. Hence structurally relaxed amorphous matrix, which governs the electronic transport, has been developed. Nanocrystallites, embedded in the amorphous matrix, play another interesting role. Quantum con®nement of the charge carriers in the nc-Si ®lms extent the conduction band and valence band both and the optical gap of such materials increases [9]. Quantum size effect produces phtoluminescence at room temperature which is an unusual phenomenon for normal amorphous silicon. Thus we have obtained the wide gap photoconductive nanocrystalline silicon thin ®lms and single junction solar cell has been successfully fabricated by using the new type of ®lm as an intrinsic layer. Attempts are being made to fabricate double triple junction solar cells using this material as intrinsic layer of the top cell. Here it may be stated that Meier et al., [10]. developed compensated microcrystalline silicon ®lm which was used as alternative to low bandgap alloy (a-SiGe:H) ®lm. They fabricated double junction solar cell using this compensated microcrystalline ®lm as intrinsic layer of the bottom cell. This nanocrystalline layer may be used as a seed layer for the growth of polycrystalline Si ®lms at a low temperature [11]. The separated crystallites will help to form larger polycrystalline growth on it. It may be mentioned that thin ®lm polycrystalline Si solar cells are being widely investigated now a days because of its high performance, stability and low cost.
Nanocrystalline silicon (nc-Si) thin ®lms have been developed by plasma enhanced chemical vapor deposition at substrate temperature range 2008C± 2508C. The average size of nanocrystals embedded in the amorphous matrix is t 10nm. Optical gaps of the nc-Si ®lms are greater than 1.80 eV. The present nc-Si ®lms are photosensitive unlike the general nc-Si ®lms. Photovoltaic properties of a ncSi thin ®lm is superior than those of a common wide band gap alloy ®lm (a-SiC:H) at the same optical gap. Moreover electronic properties of these ®lm deteriorate under illumination upto 10 h only while electronic properties of amorphous silicon ®lms degrade even after 250 hours. It has been successfully used as an intrinsic layer of the p±i±n solar cell. This ncSi layer can be used as an intrinsic layer of the top cell of multijunction solar cell or a seed layer for polycrystalline Si growth at low temperature.
Acknowledgements This work is completed under MNES project.
References [1] M. Ruchschloss, B. Landkammer, S. Veprek, Appl. Phys. Lett. 63 (1993) 1474. [2] X. Liu, X. Wu, X. Bao, Y. He, Appl. Phys. Lett. 64 (1994) 220. [3] J. Tauc, Optical properties of solid, Plenum Press, New York, 1974 p. 159. [4] H. Shanks, C.J. Fang, L. Ley, M. Cardona, F.J. Demon, S. Kabbitzer, Phys. Stat. Solidi B 100 (1980) 43. [5] Ray, S., Hazra, S., 25th IEEE, (Washington, USA),1996 p. 1077. [6] S. Tong, X. Liu, L. Wang, F. Yau, X. Bao, Appl. Phys. Lett. 69 (1994) 596. [7] K. Yoshida, Phil. Mag. B 53 (1986) 55. [8] V.A. Godyak, R.B. Piejak, Phys. Rev. Lett. 65 (1990) 996. [9] S. Furukawa, T. Miyasato, Phys. Rev. B 38 (1988) 5726. [10] Meier, J., Rubail, S., Fluckiger, R., Fischer, D., Kappner, H., Shah, A., First WCPEC, Dec. 5-9, Hawii, 1994 p. 409. [11] J.R. Heath, S.M. Gates, C. A. Chess, Appl. Phys. Lett. 64 (1994) 27.
PERGAMON
Solid State Communications 109 (1999) 125±128
Nanocrystalline silicon as intrinsic layer in thin ®lm solar cells Sukti Hazra*, Swati Ray Energy Research Unit, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700 032, India Received 22 July 1998; accepted 23 September 1998 by C. N. R. Rao
Abstract Nanocrystalline silicon (nc-Si) material is a potential candidate in photoluminescence and electroluminescence devices. So far, nc-Si thin ®lms have had no applications in solar cells as intrinsic layers because of their non-photoconductive nature. In the present work, a new type of photoconductive nc-Si thin ®lm has been developed by utilizing commercially compatible plasma enhanced chemical vapour deposition technique. Transmission electron micrographs of the new material gave evidence for the presence of scattered nanocrystallites of average size t 10 nm embedded in the hydrogenated amorphous silicon matrix. The photosensitivity of the nc-Si ®lms is . 1 £ 10 5. To test the viability of the new material in devices, a p±i±n solar cell has been fabricated using the nc-Si material of optical gap 1.89eV, as the intrinsic layer. The ef®ciency of this single junction solar cell in 8.7%. q 1998 Published by Elsevier Science Ltd. Keywords: D. Photoconductivity
1. Introduction The work on nanocrystalline silicon (nc-Si) material is generally focused on their photoluminescence [1] and electroluminesence [2] properties at room temperature, because of their potential applications in the electroluminescent and photoluminescent devices based on Si. So far, the non-photoconductive nature of nc-Si ®lms would play as a constraint to use them, as the active layers in solar cells. However, In the present work, the photovoltaic properties of nanocrystalline silicon ®lms have been demonstrated. As a result of their wide gap, new photovoltaic nanocrystalline silicon ®lms are good alternative to the high bandgap amorphous silicon carbide alloys (a-SiC:H) which are essential intrinsic layers of the top cells of tandem structure of amorphous silicon solar cells. The * Corresponding author. Tel.: 1 91-33-473-6612; Fax: 1 9133-473-2805; e-mail: [email protected].
photovoltaic quality of a nc-Si ®lm is superior than that of a a-SiC:H ®lm at the same optical gap. Further advantage of this new material is that it has been developed in convenstional rf powered (13.56 MHz) plasma enhanced chemical vapor deposition (RFPECVD) which is most compatible deposition system for the preparation of Si thin ®lms from the technological point of view. 2. Experimental Nanocrystalline silicon ®lms were developed in the ultrahigh vacuum ( t 10 29 Torr) plasma enhanced chemical vapor deposition system using process gases silane and hydrogen. Chamber pressure, r.f. power density and substrate temperature range were 1.8 Torr, 15 mW/cm 2 and 2008C±2508C respectively. The substrates used in this study were Corning glass (7059) for the electrical and optical measurements and crystalline silicon wafer for the determination of
0038-1098/99/$ - see front matter q 1998 Published by Elsevier Science Ltd. PII: S 0038-109 8(98)00522-5
126
S. Hazra, S. Ray / Solid State Communications 109 (1999) 125±128
Table 1 Optoelectronic properties of nc-Si and normal bandgap a-Si:H materials Rf power density (mW/Cm 2)
Chamber pressure (Torr)
15
1.8
30
0.5
Substrate temperature (8C)
Eg
CH
sD
s ph
(eV)
(at%)
(S cm 21)
(S cm 21)
200 220 250 250
1.92 1.89 1.80 1.72
6.72 7.23 9.54 12.34
8.66 £ 10 213 3.58 £ 10 212 9.21 £ 10 212 6.72 £ 10 211
3.58 £ 7.36 £ 1.56 £ 1.23 £
bonding con®gurations of hydrogen atoms in the silicon matrix. The thickness of the ®lms were measured by a stylus type instrument (Planer Product, UK) with a resolution of 15 nm. Optical absorption and re¯ection data in the wavelength range 185 to 2600 nm were taken from the UV-VIS-NIR double beam spectrophotometer (Hitachi 330) and optical gap (Eg) of ncSi ®lms were determined by the Tauc's method [3] using these data. Fourier Transform Infrared Spectrophotometer (Perkin Elmer, FTIR 1750) carried out the study on the bonding con®guration of hydrogen atoms in the Si thin ®lms. The bonded hydrogen contents (CH) were estimated from integrated absorption of the peak at 630 cm 21 [4] in the infrared spectra. The photoluminescent studies were done under the Xe lamp of 150 Watt. Nanocrystallites in the a-Si:H matrix were characterized by the transmission electron micrographs (TEM) and transmission electron
10 26 10 26 10 25 10 25
diffraction (TED) patterns. The dark and photoconductivity of the samples have been measured after annealing at 1508C for 1 hour in a cryostat under vacuum ( t 10 26 Torr). The secondary photoconductivity measurements were performed by using white light illumination of 50 mW/cm 2 from a tungsten lamp. Single junction p±i±n solar cell was fabricated using the nc-Si intrinsic layer on textured TCO coated glass substrate (7% haze). The structure of the cell (area 1 cm 2) was glass/TCO/p±a±SiC/nc-Si/n-aSi:H/Al. I±V characteristic was measured under AM 1.5 solar radiation of 100 mW/cm 2 from a WACOM solar Simulator. The measurement of quantum ef®ciency of the solar cell was carried out under the short circuit condition using a lock-in-ampli®er and chopped monochromatic light. 3. Results
Fig. 1. Photoluminescence spectra for nc-Si and a-Si:H at room temperature
Table 1 shows the variations of optoelectronic properties of the new material with substrate temperature at constant rf power density (15 mW/cm 2) and chamber pressure (1.8 Torr). This table also displays the optoelectronic properties of hydrogenated amorphous silicon (a-Si:H) along with its deposition parameters like, if power density (30 mW/cm 2) and chamber pressure (0.5 Torr). It is distinct from the table that the deposition regime for the new material is different from that of the normal a-Si:H. With the decrease of substrate temperature, Eg increases and CH decreases unlike to the general fact. Fig. 1 demonstrates the photoluminiscence spectra of new material with optical gap 1.89 eV and normal bandgap a-Si:H at room temperature. As observed in the ®gure, broad photoluminescent peak has been appeared for the
S. Hazra, S. Ray / Solid State Communications 109 (1999) 125±128
127
photoconductivity for nc-Si thin ®lm has been tested in the Fig. 2 As shown in the Fig.2. under light soaking, photoconductivity of nc-Si ®lm deteriorates only upto 10 hours while photoconductivity of the normal bandgap a-Si:H degrades even after 250 h. Finally Table 2 demonstrates the photovoltaic device performance of nc-Si material as a intrinsic layer. Open circuit voltage (Voc) of the single of the junction solar cell having the nc-Si intrinsic layer (0.93 volt) is higher than that of the normal bandgap solar cell (0.90 volt). The ef®ciency obtained so far with such high bandgap nc-Si cell is 8.7%. Fig. 2. Photoconductivity Vs Light soaking time. Curves 1 and 2 correspond to nc-Si (1.92 eV) and a-Si:H (1.72 eV) respectively.
nc-Si ®lm. However no such peak has been observed for the normal a-Si:H ®lm. Transmission electron micrograph of the wide bandgap Si thin ®lm has been displayed in Ref. [5]. In this micrograph, scattered nanocrystals are embedded in the amorphous silicon matrix and average size of nanocrystals is t 10 nm. Owing to the presence of scattered nanocrystallites, ®lms are called nanocrystalline silicon (nc-Si). So far, nc-Si thin ®lms were not photoconductive and hence those were not used as active layers of the solar cells. However, Table 1 shows that the new nc-Si samples are photoconductive and their photosensitivity i.e., the ratio of photoconductivity to darkconductivity is . 1 £ 10 5. In fact, the electronic properties of nc-Si ®lms are superior than those of the normal high bandgap a-SiC:H alloys. For solar cell applications, not only the electronic properties of the materials but also the stability of electronic properties under prolonged illumination are important. So light induced degradation of Table 2 Comparison of cell performances of single junction solar cells having I-layers with different materials Nature Of i-layer (eV)
Voc (Volt)
Isc (mA/cm 2)
F.F.
h (%)
nc-Si a-Si
0.93 0.90
14.0 15.2
0.65 0.68
8.7 9.4
4. Discussion For the general nc-Si ®lms, average extent of amorphous tissue between the neigh-bouring nanocrystallites is only 2±3 atomic spacing and the volume fraction of crys-tallinity is high (60±80%) [6]. According to Yoshida [7], the effective conductivity (s D) of such two phase (crystalline and amorphous) material can be expressed as
sD 2 sa s 2 sa fc c sD 1 2 sa s c 1 2 sa whereas s a and s c are the conductivities of amorphous and crystalline phase respectively and fc is the crystalline volume fraction. fc determines which play dominating role in s D and s c. In the normal nc-Si ®lms, since the crystalline volumefraction is greater than 60%, s c dominates in s D and hence conductivity is . 10 25 Scm 21. The charge carriers are transferred between the neighboring crystallites by tunneling through the thin amorphous tissue. However, the structure of the present nc-Si ®lms deposited at high chamber pressure and high hydrogen dilution, is different from the general nc-Si ®lms. Fig. 3 schematically describes the morphological difference between the general and present nanocrystalline silicon thin ®lms. In the present case nanocrystals are widely separated by the amorphous tissue and the distance between neighboring crystallites is too large for the transferring of charge carriers form one crystallite to other. Moreover, infrared spectra of the nc-Si ®lms are dominated by silicon monohydride [5] and so the large amorphous phase is more ordered
128
S. Hazra, S. Ray / Solid State Communications 109 (1999) 125±128
5. Conclusions
Fig. 3. Schematic diagrams of morphology of the general and the present nanocrystalline silicon ®lms
and compact compared to that in general nano or microcrystalline silicon material. As at high chamber pressure, average energy of the electrons increases [8]and the large amount of nascent hydrogen atoms has been generated in the plasma. Those energetic hydrogen atoms can supply huge amount of energy to the surface of the growing ®lms. Hence structurally relaxed amorphous matrix, which governs the electronic transport, has been developed. Nanocrystallites, embedded in the amorphous matrix, play another interesting role. Quantum con®nement of the charge carriers in the nc-Si ®lms extent the conduction band and valence band both and the optical gap of such materials increases [9]. Quantum size effect produces phtoluminescence at room temperature which is an unusual phenomenon for normal amorphous silicon. Thus we have obtained the wide gap photoconductive nanocrystalline silicon thin ®lms and single junction solar cell has been successfully fabricated by using the new type of ®lm as an intrinsic layer. Attempts are being made to fabricate double triple junction solar cells using this material as intrinsic layer of the top cell. Here it may be stated that Meier et al., [10]. developed compensated microcrystalline silicon ®lm which was used as alternative to low bandgap alloy (a-SiGe:H) ®lm. They fabricated double junction solar cell using this compensated microcrystalline ®lm as intrinsic layer of the bottom cell. This nanocrystalline layer may be used as a seed layer for the growth of polycrystalline Si ®lms at a low temperature [11]. The separated crystallites will help to form larger polycrystalline growth on it. It may be mentioned that thin ®lm polycrystalline Si solar cells are being widely investigated now a days because of its high performance, stability and low cost.
Nanocrystalline silicon (nc-Si) thin ®lms have been developed by plasma enhanced chemical vapor deposition at substrate temperature range 2008C± 2508C. The average size of nanocrystals embedded in the amorphous matrix is t 10nm. Optical gaps of the nc-Si ®lms are greater than 1.80 eV. The present nc-Si ®lms are photosensitive unlike the general nc-Si ®lms. Photovoltaic properties of a ncSi thin ®lm is superior than those of a common wide band gap alloy ®lm (a-SiC:H) at the same optical gap. Moreover electronic properties of these ®lm deteriorate under illumination upto 10 h only while electronic properties of amorphous silicon ®lms degrade even after 250 hours. It has been successfully used as an intrinsic layer of the p±i±n solar cell. This ncSi layer can be used as an intrinsic layer of the top cell of multijunction solar cell or a seed layer for polycrystalline Si growth at low temperature.
Acknowledgements This work is completed under MNES project.
References [1] M. Ruchschloss, B. Landkammer, S. Veprek, Appl. Phys. Lett. 63 (1993) 1474. [2] X. Liu, X. Wu, X. Bao, Y. He, Appl. Phys. Lett. 64 (1994) 220. [3] J. Tauc, Optical properties of solid, Plenum Press, New York, 1974 p. 159. [4] H. Shanks, C.J. Fang, L. Ley, M. Cardona, F.J. Demon, S. Kabbitzer, Phys. Stat. Solidi B 100 (1980) 43. [5] Ray, S., Hazra, S., 25th IEEE, (Washington, USA),1996 p. 1077. [6] S. Tong, X. Liu, L. Wang, F. Yau, X. Bao, Appl. Phys. Lett. 69 (1994) 596. [7] K. Yoshida, Phil. Mag. B 53 (1986) 55. [8] V.A. Godyak, R.B. Piejak, Phys. Rev. Lett. 65 (1990) 996. [9] S. Furukawa, T. Miyasato, Phys. Rev. B 38 (1988) 5726. [10] Meier, J., Rubail, S., Fluckiger, R., Fischer, D., Kappner, H., Shah, A., First WCPEC, Dec. 5-9, Hawii, 1994 p. 409. [11] J.R. Heath, S.M. Gates, C. A. Chess, Appl. Phys. Lett. 64 (1994) 27.