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a-C superlattice structures for solar cell applications
ARTICLE IN PRESS
Solar Energy Materials & Solar Cells 90 (2006) 3394–3398 www.elsevier.com/locate/solmat
Optical properties of C60/a-C superlattice structures for solar cell applications Nobuaki Kojima, Yusuke Sugiura, Masafumi Yamaguchi Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468-8511, Japan Received 24 May 2005; accepted 3 January 2006 Available online 30 August 2006
Abstract C60/amorphous carbon superlattice structures were fabricated by shutter-controlled molecular beam deposition. The periodic structure of resulted films was confirmed by X-ray diffraction measurements. From the UV–vis reflectance/transmittance measurements, the energy shift of absorption edge was observed in the superlattice structures as a function of their well width. The carbon-based superlattice structure is the useful technique to control the band gap energy of carbon materials. r 2006 Elsevier B.V. All rights reserved.
1. Introduction Semiconducting carbon materials such as fullerene, nanotubes and amorphous carbon (a-C) have a wide range of electrical and optical properties depending on the microscopic structures of carbon atom network. From such variety of character and environmentfriendly nature, carbon solar cells have been investigated [1–5]. The conversion efficiency of a-C solar cells has reached 2.2% [3]. To increase the cell efficiency of carbon material which has complicated bonding structure, the control of band gap energy and electrical conductivity, and reduction of defect density are still key technologies. We have proposed C60/a-C superlattice structures as absorption layers [6]. Superlattice structure has advantages to control the optical and electrical properties easily. The band gap energy of superlattice can be controlled by the well width. Therefore, graded band structure solar cells can be produced by changing the well width with depth in the film, Corresponding author. Tel.: +81 52 809 1877; fax: +81 52 809 1879.
E-mail address: [email protected] (N. Kojima). 0927-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2006.01.003
ARTICLE IN PRESS N. Kojima et al. / Solar Energy Materials & Solar Cells 90 (2006) 3394–3398
3395
which effectively utilize wider wavelength range of sunlight. In this paper, we will report the fabrication of C60/a-C superlattice structure and their optical properties. 2. Experimental A shutter-controlled molecular beam deposition chamber equipped with a radio frequency (rf) plasma source was used for the deposition of C60 and a-C films. Base pressure of the chamber was 3 107 Pa. Si(1 0 0) or quartz glass was used as a substrate. Pure (99.98%) C60 powder was evaporated from a Knudsen cell. Films of a-C were deposited by simultaneous supply of C60 and nitrogen ions dissociated by rf plasma source. The supplied nitrogen ions bombard C60 film surface, and transform them into a-C. The superlattice structure was fabricated by intermittent supply of nitrogen ions during C60 deposition. Fig. 1 shows the shutter sequence for the fabrication of superlattice structures. C60 layers are deposited during T1 (C60: on, N: off), and a-C layers are formed during T2 (C60: on, N: on) due to the C60 cage breaking. The growth rate of C60 and a-C layers was around 2.2 nm/min. The periodic structure of resulted films was confirmed by X-ray diffraction (XRD) measurements. The optical absorption coefficient spectra were calculated from the UV–vis reflectance/transmittance measurements. 3. Results and discussion The periodic structure of resulted films was confirmed by XRD measurements. Fig. 2 shows XRD pattern of low angle 2y–o scan of the films grown with shutter sequence of T1 ¼ T2 for (a), (b) and T1 ¼ 6T2 for (c) and 45 cycles. Diffraction peaks from periodic layer structure were observed, corresponding to the superlattice period (C60 layer+a-C layer) of 7.96, 5.51 and 4.03 nm for ]1, ]2 and ]3 sample, respectively. These values are very close to the designed value. The thickness of each layer of C60 and a-C is unknown from this measurement. Designed thickness from shutter sequence is a-C(4.0 nm)/ C60(4.0 nm), a-C(2.8 nm)/C60(2.8 nm), and a-C(0.6 nm)/C60(3.4 nm) for ]1, ]2 and ]3 sample, respectively.
Fig. 1. Shutter sequence for the fabrication of C60/a-C superlattice structure.
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N. Kojima et al. / Solar Energy Materials & Solar Cells 90 (2006) 3394–3398
(a)
superlattice #1 (a-C(4.0nm)/C60(4.0nm)) x45 7.96 nm 7.48 nm 7.05 nm
8.52 nm
Intensity (a.u.)
(b)
superlattice #2 (a-C(2.8nm)/C60(2.8nm)) x45 5.95 nm
5.51 nm 5.06 nm
(c)
superlattice #3 (a-C(0.6nm)/C60(3.4nm)) x45 4.03 nm
0.5
1.0
1.5 2.0 Diffraction Angle 2θ (deg.)
2.5
3.0
Fig. 2. XRD pattern of low angle 2y–o scan of the C60/a-C superlattice structure.
Furthermore, diffraction peaks show splitting for ]1 and ]2 sample. The differences between the splitting are around 0.4–0.5 nm, which is near to the C60(2 2 0) or (3 1 1) spacing. Therefore, it is thought that the splitting of diffraction peak is related to the molecular arrangement. The optical absorption edge shift, which corresponds to band gap energy shift, was examined as a function of thickness of C60 and a-C layers. The optical absorption coefficient spectra are shown in Fig. 3. The C60 single-layer film has two strong absorption bands at around 3.6 and 4.6 eV, which are assigned to p-p* transitions, and one broad absorption band at around 2.6 eV, which is considered as a charge transfer exciton-related transition, while band edge absorption at around 1.7 eV is weak due to the optically forbidden transition. The singlelayer film of a-C, which is formed by simultaneous supplies of C60 and nitrogen ions, shows typical absorption spectrum of amorphous semiconductors and its Tauc gap was estimated at around 0.8 eV. Therefore, a-C is the well layer, and C60 is the barrier layer in superlattice structure. In the case of the a-C and C60 double-layer film, the absorption spectrum can be divided in two regions at around 2.4 eV. The main contribution in the absorption spectrum is the C60 layer in the range above 2.4 eV and the a-C layer in the range below 2.4 eV, respectively. The Tauc gap of the double-layer film was estimated at around 0.8 eV, same as that of a-Carbon single-layer film.
ARTICLE IN PRESS N. Kojima et al. / Solar Energy Materials & Solar Cells 90 (2006) 3394–3398
3397
Absorption Coefficient (cm-1)
106
105 C60 (single layer) a-C (single layer) double layers (a-C(130nm)/C60(130nm)) superlattice #1 (a-C(4.0nm)/C60(4.0nm)) x45 superlattice #3 (a-C(0.6nm)/C60(3.4nm)) x45
104
103 0.5
1.0
1.5
2.0 2.5 3.0 3.5 Photon Energy (eV)
4.0
4.5
5.0
Fig. 3. Optical absorption coefficient spectra.
On the other hand, the shape of absorption edge spectra of superlattice samples was changed from that of a-C films, and the blue shift of absorption edge was observed in the narrow well width superlattice. These results suggest that the band structure is modified by superlattice structure and that the effective band gap energy changes as a function of well width of a-C layers. 4. Conclusion C60/a-C superlattice structures were proposed as a new solar cell material. The superlattice structure was fabricated by intermittent supply of nitrogen ions during C60 deposition using a shutter-controlled molecular beam deposition. The periodic structure of the resulted films was confirmed by XRD measurements. From the UV-vis reflectance/ transmittance measurements, the energy shift of absorption edge was observed in the superlattice structures as a function of their well width. The carbon-based superlattice structure is the useful technique to control the band gap energy of carbon materials. Acknowledgements This work was supported in part by the JSPS as a program entitled Research for the Future (JSPS- RFTF97P00902: Study of New Carbon-Based Materials and Solar Cells), and by the MEXT as a Private University Academic Frontier Center Program, and by the JSPS, Grant-in-Aid for Scientific Research (B) 15360175. References [1] K.M. Krishna, T. Soga, K. Mukhopadhyay, M. Sharon, M. Umeno, Sol. Energy Mater. Sol. Cells 48 (1997) 25.
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N. Kojima et al. / Solar Energy Materials & Solar Cells 90 (2006) 3394–3398
[2] N. Kojima, M. Yamaguchi, N. Ishikawa, Jpn. J. Appl. Phys. 39 (2000) 1176. [3] K.M. Krishna, M. Umeno, Y. Nukaya, T. Soga, M. Umeno, Appl. Phys. Lett. 77 (2000) 1472. [4] K.L. Narayanan, O. Goetzberger, A. Khan, N. Kojima, M. Yamaguchi, Sol. Energy Mater. Sol. Cells 65 (2001) 29. [5] K.L. Narayanana, M. Yamaguchi, H. Azuma, Appl. Phys. Lett. 80 (2002) 1285. [6] N. Kojima, O. Goetzberger, Y. Ohshita, M. Yamaguchi, in: Proceedings of the 28th IEEE Photovoltaic Specialists Conference, 2000.
Solar Energy Materials & Solar Cells 90 (2006) 3394–3398 www.elsevier.com/locate/solmat
Optical properties of C60/a-C superlattice structures for solar cell applications Nobuaki Kojima, Yusuke Sugiura, Masafumi Yamaguchi Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468-8511, Japan Received 24 May 2005; accepted 3 January 2006 Available online 30 August 2006
Abstract C60/amorphous carbon superlattice structures were fabricated by shutter-controlled molecular beam deposition. The periodic structure of resulted films was confirmed by X-ray diffraction measurements. From the UV–vis reflectance/transmittance measurements, the energy shift of absorption edge was observed in the superlattice structures as a function of their well width. The carbon-based superlattice structure is the useful technique to control the band gap energy of carbon materials. r 2006 Elsevier B.V. All rights reserved.
1. Introduction Semiconducting carbon materials such as fullerene, nanotubes and amorphous carbon (a-C) have a wide range of electrical and optical properties depending on the microscopic structures of carbon atom network. From such variety of character and environmentfriendly nature, carbon solar cells have been investigated [1–5]. The conversion efficiency of a-C solar cells has reached 2.2% [3]. To increase the cell efficiency of carbon material which has complicated bonding structure, the control of band gap energy and electrical conductivity, and reduction of defect density are still key technologies. We have proposed C60/a-C superlattice structures as absorption layers [6]. Superlattice structure has advantages to control the optical and electrical properties easily. The band gap energy of superlattice can be controlled by the well width. Therefore, graded band structure solar cells can be produced by changing the well width with depth in the film, Corresponding author. Tel.: +81 52 809 1877; fax: +81 52 809 1879.
E-mail address: [email protected] (N. Kojima). 0927-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2006.01.003
ARTICLE IN PRESS N. Kojima et al. / Solar Energy Materials & Solar Cells 90 (2006) 3394–3398
3395
which effectively utilize wider wavelength range of sunlight. In this paper, we will report the fabrication of C60/a-C superlattice structure and their optical properties. 2. Experimental A shutter-controlled molecular beam deposition chamber equipped with a radio frequency (rf) plasma source was used for the deposition of C60 and a-C films. Base pressure of the chamber was 3 107 Pa. Si(1 0 0) or quartz glass was used as a substrate. Pure (99.98%) C60 powder was evaporated from a Knudsen cell. Films of a-C were deposited by simultaneous supply of C60 and nitrogen ions dissociated by rf plasma source. The supplied nitrogen ions bombard C60 film surface, and transform them into a-C. The superlattice structure was fabricated by intermittent supply of nitrogen ions during C60 deposition. Fig. 1 shows the shutter sequence for the fabrication of superlattice structures. C60 layers are deposited during T1 (C60: on, N: off), and a-C layers are formed during T2 (C60: on, N: on) due to the C60 cage breaking. The growth rate of C60 and a-C layers was around 2.2 nm/min. The periodic structure of resulted films was confirmed by X-ray diffraction (XRD) measurements. The optical absorption coefficient spectra were calculated from the UV–vis reflectance/transmittance measurements. 3. Results and discussion The periodic structure of resulted films was confirmed by XRD measurements. Fig. 2 shows XRD pattern of low angle 2y–o scan of the films grown with shutter sequence of T1 ¼ T2 for (a), (b) and T1 ¼ 6T2 for (c) and 45 cycles. Diffraction peaks from periodic layer structure were observed, corresponding to the superlattice period (C60 layer+a-C layer) of 7.96, 5.51 and 4.03 nm for ]1, ]2 and ]3 sample, respectively. These values are very close to the designed value. The thickness of each layer of C60 and a-C is unknown from this measurement. Designed thickness from shutter sequence is a-C(4.0 nm)/ C60(4.0 nm), a-C(2.8 nm)/C60(2.8 nm), and a-C(0.6 nm)/C60(3.4 nm) for ]1, ]2 and ]3 sample, respectively.
Fig. 1. Shutter sequence for the fabrication of C60/a-C superlattice structure.
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N. Kojima et al. / Solar Energy Materials & Solar Cells 90 (2006) 3394–3398
(a)
superlattice #1 (a-C(4.0nm)/C60(4.0nm)) x45 7.96 nm 7.48 nm 7.05 nm
8.52 nm
Intensity (a.u.)
(b)
superlattice #2 (a-C(2.8nm)/C60(2.8nm)) x45 5.95 nm
5.51 nm 5.06 nm
(c)
superlattice #3 (a-C(0.6nm)/C60(3.4nm)) x45 4.03 nm
0.5
1.0
1.5 2.0 Diffraction Angle 2θ (deg.)
2.5
3.0
Fig. 2. XRD pattern of low angle 2y–o scan of the C60/a-C superlattice structure.
Furthermore, diffraction peaks show splitting for ]1 and ]2 sample. The differences between the splitting are around 0.4–0.5 nm, which is near to the C60(2 2 0) or (3 1 1) spacing. Therefore, it is thought that the splitting of diffraction peak is related to the molecular arrangement. The optical absorption edge shift, which corresponds to band gap energy shift, was examined as a function of thickness of C60 and a-C layers. The optical absorption coefficient spectra are shown in Fig. 3. The C60 single-layer film has two strong absorption bands at around 3.6 and 4.6 eV, which are assigned to p-p* transitions, and one broad absorption band at around 2.6 eV, which is considered as a charge transfer exciton-related transition, while band edge absorption at around 1.7 eV is weak due to the optically forbidden transition. The singlelayer film of a-C, which is formed by simultaneous supplies of C60 and nitrogen ions, shows typical absorption spectrum of amorphous semiconductors and its Tauc gap was estimated at around 0.8 eV. Therefore, a-C is the well layer, and C60 is the barrier layer in superlattice structure. In the case of the a-C and C60 double-layer film, the absorption spectrum can be divided in two regions at around 2.4 eV. The main contribution in the absorption spectrum is the C60 layer in the range above 2.4 eV and the a-C layer in the range below 2.4 eV, respectively. The Tauc gap of the double-layer film was estimated at around 0.8 eV, same as that of a-Carbon single-layer film.
ARTICLE IN PRESS N. Kojima et al. / Solar Energy Materials & Solar Cells 90 (2006) 3394–3398
3397
Absorption Coefficient (cm-1)
106
105 C60 (single layer) a-C (single layer) double layers (a-C(130nm)/C60(130nm)) superlattice #1 (a-C(4.0nm)/C60(4.0nm)) x45 superlattice #3 (a-C(0.6nm)/C60(3.4nm)) x45
104
103 0.5
1.0
1.5
2.0 2.5 3.0 3.5 Photon Energy (eV)
4.0
4.5
5.0
Fig. 3. Optical absorption coefficient spectra.
On the other hand, the shape of absorption edge spectra of superlattice samples was changed from that of a-C films, and the blue shift of absorption edge was observed in the narrow well width superlattice. These results suggest that the band structure is modified by superlattice structure and that the effective band gap energy changes as a function of well width of a-C layers. 4. Conclusion C60/a-C superlattice structures were proposed as a new solar cell material. The superlattice structure was fabricated by intermittent supply of nitrogen ions during C60 deposition using a shutter-controlled molecular beam deposition. The periodic structure of the resulted films was confirmed by XRD measurements. From the UV-vis reflectance/ transmittance measurements, the energy shift of absorption edge was observed in the superlattice structures as a function of their well width. The carbon-based superlattice structure is the useful technique to control the band gap energy of carbon materials. Acknowledgements This work was supported in part by the JSPS as a program entitled Research for the Future (JSPS- RFTF97P00902: Study of New Carbon-Based Materials and Solar Cells), and by the MEXT as a Private University Academic Frontier Center Program, and by the JSPS, Grant-in-Aid for Scientific Research (B) 15360175. References [1] K.M. Krishna, T. Soga, K. Mukhopadhyay, M. Sharon, M. Umeno, Sol. Energy Mater. Sol. Cells 48 (1997) 25.
ARTICLE IN PRESS 3398
N. Kojima et al. / Solar Energy Materials & Solar Cells 90 (2006) 3394–3398
[2] N. Kojima, M. Yamaguchi, N. Ishikawa, Jpn. J. Appl. Phys. 39 (2000) 1176. [3] K.M. Krishna, M. Umeno, Y. Nukaya, T. Soga, M. Umeno, Appl. Phys. Lett. 77 (2000) 1472. [4] K.L. Narayanan, O. Goetzberger, A. Khan, N. Kojima, M. Yamaguchi, Sol. Energy Mater. Sol. Cells 65 (2001) 29. [5] K.L. Narayanana, M. Yamaguchi, H. Azuma, Appl. Phys. Lett. 80 (2002) 1285. [6] N. Kojima, O. Goetzberger, Y. Ohshita, M. Yamaguchi, in: Proceedings of the 28th IEEE Photovoltaic Specialists Conference, 2000.