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Silicon thin film solar cells deposited under 80°C
Thin Solid Films 383 Ž2001. 129᎐131
Silicon thin film solar cells deposited under 80⬚C Manabu Ito a,U , Christian Kochb , Vlado Svrcek c , Markus B. Schubert b , Jurgen H. Werner b ¨ a
Technical Research Institute, Toppan Printing Co., Ltd. 4-2-3, Takanodai-Minami, Sugito, Kitakatsushika, Saitama, Japan b Institut fur ¨ Physikalische Elektronik, Uni¨ ersitat ¨ Stuttgart, Pfaffenwaldring 47, D-70569 Stuttgart, Germany c Institute of Physics, Czech Academy of Sciences, Cukro¨ arnicka 10, 162 000 Prague 6, Czech Republic
Abstract We deposited silicon thin films by plasma-enhanced chemical vapor deposition ŽPECVD. at very low substrate temperatures of 75 and 40⬚C. Even at these low deposition temperatures, the protocrystalline Si Žpc-Si:H. exhibits a high photosensitivity and remarkably enhanced stability against light saturation. This material grows at the borderline between amorphous Ža-Si:H. and nanocrystalline Žnc-Si:H. phases in the deposition parameter space. Structural and optical characterization revealed a small fraction of crystallites embedded in an amorphous matrix. Thickness-dependent morphology of silicon films was revealed by absolute constant photocurrent method ŽCPM.. We demonstrated the effect of the amorphous-to-nanocrystalline transition on the solar cell performance. The cells with a protocrystalline absorber layer showed an improved fill factor. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Protocrystalline; Low-temperature deposition; Annealing; Fill factor
1. Introduction Recently, pc-Si:H thin film has attracted much attention. This material grows just before the onset of the nanocrystalline Žnc-Si:H. phase and exhibits superior electronic properties and extraordinary stability against light saturation w1᎐3x. Guha et al. report that the improved electronic properties are related to the ordered medium-range structure and the associated lower defect density w1x. Mahan et al. suggest that few nmsized crystallites exist within the amorphous matrix, with the majority of the H bonded on these crystallites surfaces w2x. However, the nature and properties of pc-Si:H are not yet well understood. In this study, we report on the formation of pc-Si:H
U
Corresponding author. Tel.: q81-480-33-9061; fax: q81-480-339022. E-mail address: [email protected] ŽM. Ito..
at very low deposition temperatures ŽT s. under 80⬚C, which is applicable to low-cost polymeric substrates like polyethylene terephtalate ŽPET.. We studied the effect of T s on the amorphous-to-nanocrystalline transition and examined the electronic properties of the deposited films against thermal annealing and light saturation. We also investigated the effect of this phase transition on the solar cell performance. 2. Experimental details Our samples are deposited by PECVD at T s s 75 and 40⬚C. Hydrogen dilution ratios r H s ŽwSiH 4 x q wH 2 xrwSiH 4 x. were changed over a wide range, from r H s 1 to 127, which determines a-Si:H, pc-Si:H, or nc-Si:H growth. The excitation frequency was kept at 54 MHz. As a substrate, we used Corning 7059 glass for single layer deposition and non-textured indium tin oxide- ŽITO. coated glass for the solar cell. The mobility᎐lifetime product of the photo carriers was de-
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 5 9 0 - X
130
M. Ito et al. r Thin Solid Films 383 (2001) 129᎐131
termined at a photon energy of 1.96 eV and a photon flux of 1.3= 10 15 cmy2 sy1 . We measured the electrical properties after thermal annealing in vacuum at 110⬚C for 12 h, and after light saturation under AM1.5-like illumination for 100 h. The constant photocurrent method ŽCPM. exhibited the Urbach energy of the films. We measured Raman spectra and derived the crystalline volume fraction Xc from the relative intensity ratio of the crystalline peak Žat 520 cmy1 . to the amorphous peak Žat 480᎐500 cmy1 .. 3. Results and discussions 3.1. Single layers The rd ratios of various r H values are depicted in Fig. 1 at T s s 75 and 40⬚C for initial, annealed, and light-saturated stages. The rd ratio is an index of absorber layer property of solar cells w3x. As shown in Fig. 1, rd ratios show a dramatic rise on the edge of crystallinity for both deposition temperatures. The borderline of this transition increases from 29 to 45 with a decrease in T s from 75 to 40⬚C. Especially at 75⬚C deposition, pc-Si:H shows extraordinary stability against light saturation. At this temperature, pc-Si:H is obtained within quite a narrow deposition-parameter window, and a slight deviation of r H from this borderline leads to a significant decrease in rd by three orders of magnitude. Fig. 2 shows the Urbach energy and crystalline volume fraction for T s s 75⬚C-deposited samples with a thickness of 1 m. In the a-Si:H range, Urbach energy decreases with increasing r H , indicating a more ordered silicon network. At the transition of a-Si:H to nc-Si:H, r H s 29 and Urbach energy is no longer defined, which indicates the start of nc-Si:H formation. However, a crystalline peak is not detected by Raman scattering or X-ray diffraction ŽXRD.. To reconcile these results, we assume that pc-Si:H is composed of nm-sized crystallites, which are too small to be detected by Raman scattering or XRD, embedded in an amorphous matrix. At constant r H s 39 and T s s 75⬚C, we deposited a series of samples with different thickness Ž0.25᎐2.0
Fig. 1. rd ratios for films as-deposited, after annealing at 110⬚C and AM1.5 light saturation.
Fig. 2. The Urbach energy and the crystalline volume fraction.
m. and analyzed the thickness-dependent morphology by absolute CPM w4x. All the deposition parameters except the deposition time were kept constant to obtain layers of various thickness. As depicted in Fig. 3, the 0.25-m thick film shows a typical a-Si:H CPM spectrum, indicating that the early stage of film formation starts with an amorphous seed layer. For the 0.5-m thick film, strong oscillation is observed in the spectrum, attributed to an inhomogeneous optical absorption in the film growth direction. This reveals the nanocrystalline formation in the amorphous matrix, which we consider pc-Si:H. For 1.0- and 2.0-m thick films, absorption by the nc-Si:H component dominates the CPM spectra. 3.2. Solar cell performance We investigated the amorphous-to-nanocrystalline transition of the absorber layer on the p-i-n solar cell performance. Fig. 4 depicts the initial fill factor of the solar cells at T s s 75⬚C. The absorber layer thickness was kept at 200 nm and only r H of this layer was varied. In the region of r H values smaller than 35, it is obvious that an amorphous layer grows. For r H values exceeding 45, we confirmed that the nc-Si:H layer increases in the absorber layer from the external quan-
Fig. 3. Absolute CPM spectra of four samples. The absorption coefficient of crystalline silicon Žc-Si. is inserted.
M. Ito et al. r Thin Solid Films 383 (2001) 129᎐131
131
gations suggest that a small fraction of crystallites are embedded in an amorphous-matrix. Absolute CPM measurements reveal the inhomogeneous morphology of silicon growing at the edge of crystallinity. The benefit of pc-Si:H as an absorber layer in a p-i-n solar cell is reflected in an improved fill factor and stability against light saturation. Acknowledgements Fig. 4. Fill factors of a p-i-n-structured solar cell.
tum efficiency measurement with reverse bias voltage. Between these two regions, we obtained a pc-Si:H absorber layer, which shows a peak in fill factor by more than 10%. In addition to the high material-quality of these as-deposited pc-Si:H solar cells, we observed an enhanced stability against light saturation w5x. 4. Conclusion Even at very low deposition temperatures under 80⬚C, we can obtain a pc-Si:H with high electronic quality and surprising stability. Structural and optical investi-
The authors would like to thank C. Kohler for tech¨ nical support. References w1x S. Guha, J. Yang, D.L. Williamson, Y. Lubianiker, J.D. Cohen, A.H. Mahan, Appl. Phys. Lett. 74 Ž1999. 1860. w2x A.H. Mahan, J. Yang, S. Guha, D.L. Williamson, Phys. Rev. B 61 Ž2000. 1677. w3x F. Finger, U. Kroll, V. Viret, A. Shah, W. Beyer, X.-M. Tang, J. Weber, A. Howling, C. Hollenstein, J. Appl. Phys. 71 Ž1992. 5665. w4x A. Fejfar, A. Poruba, M. Vanecek, J. Kocka, J. Non-Cryst. Solids 198᎐200 Ž1996. 304. w5x C. Koch, M. Ito, M.B. Schubert, J.H. Werner, 16th PVSEC, Glasgow, unpublished.
Silicon thin film solar cells deposited under 80⬚C Manabu Ito a,U , Christian Kochb , Vlado Svrcek c , Markus B. Schubert b , Jurgen H. Werner b ¨ a
Technical Research Institute, Toppan Printing Co., Ltd. 4-2-3, Takanodai-Minami, Sugito, Kitakatsushika, Saitama, Japan b Institut fur ¨ Physikalische Elektronik, Uni¨ ersitat ¨ Stuttgart, Pfaffenwaldring 47, D-70569 Stuttgart, Germany c Institute of Physics, Czech Academy of Sciences, Cukro¨ arnicka 10, 162 000 Prague 6, Czech Republic
Abstract We deposited silicon thin films by plasma-enhanced chemical vapor deposition ŽPECVD. at very low substrate temperatures of 75 and 40⬚C. Even at these low deposition temperatures, the protocrystalline Si Žpc-Si:H. exhibits a high photosensitivity and remarkably enhanced stability against light saturation. This material grows at the borderline between amorphous Ža-Si:H. and nanocrystalline Žnc-Si:H. phases in the deposition parameter space. Structural and optical characterization revealed a small fraction of crystallites embedded in an amorphous matrix. Thickness-dependent morphology of silicon films was revealed by absolute constant photocurrent method ŽCPM.. We demonstrated the effect of the amorphous-to-nanocrystalline transition on the solar cell performance. The cells with a protocrystalline absorber layer showed an improved fill factor. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Protocrystalline; Low-temperature deposition; Annealing; Fill factor
1. Introduction Recently, pc-Si:H thin film has attracted much attention. This material grows just before the onset of the nanocrystalline Žnc-Si:H. phase and exhibits superior electronic properties and extraordinary stability against light saturation w1᎐3x. Guha et al. report that the improved electronic properties are related to the ordered medium-range structure and the associated lower defect density w1x. Mahan et al. suggest that few nmsized crystallites exist within the amorphous matrix, with the majority of the H bonded on these crystallites surfaces w2x. However, the nature and properties of pc-Si:H are not yet well understood. In this study, we report on the formation of pc-Si:H
U
Corresponding author. Tel.: q81-480-33-9061; fax: q81-480-339022. E-mail address: [email protected] ŽM. Ito..
at very low deposition temperatures ŽT s. under 80⬚C, which is applicable to low-cost polymeric substrates like polyethylene terephtalate ŽPET.. We studied the effect of T s on the amorphous-to-nanocrystalline transition and examined the electronic properties of the deposited films against thermal annealing and light saturation. We also investigated the effect of this phase transition on the solar cell performance. 2. Experimental details Our samples are deposited by PECVD at T s s 75 and 40⬚C. Hydrogen dilution ratios r H s ŽwSiH 4 x q wH 2 xrwSiH 4 x. were changed over a wide range, from r H s 1 to 127, which determines a-Si:H, pc-Si:H, or nc-Si:H growth. The excitation frequency was kept at 54 MHz. As a substrate, we used Corning 7059 glass for single layer deposition and non-textured indium tin oxide- ŽITO. coated glass for the solar cell. The mobility᎐lifetime product of the photo carriers was de-
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 5 9 0 - X
130
M. Ito et al. r Thin Solid Films 383 (2001) 129᎐131
termined at a photon energy of 1.96 eV and a photon flux of 1.3= 10 15 cmy2 sy1 . We measured the electrical properties after thermal annealing in vacuum at 110⬚C for 12 h, and after light saturation under AM1.5-like illumination for 100 h. The constant photocurrent method ŽCPM. exhibited the Urbach energy of the films. We measured Raman spectra and derived the crystalline volume fraction Xc from the relative intensity ratio of the crystalline peak Žat 520 cmy1 . to the amorphous peak Žat 480᎐500 cmy1 .. 3. Results and discussions 3.1. Single layers The rd ratios of various r H values are depicted in Fig. 1 at T s s 75 and 40⬚C for initial, annealed, and light-saturated stages. The rd ratio is an index of absorber layer property of solar cells w3x. As shown in Fig. 1, rd ratios show a dramatic rise on the edge of crystallinity for both deposition temperatures. The borderline of this transition increases from 29 to 45 with a decrease in T s from 75 to 40⬚C. Especially at 75⬚C deposition, pc-Si:H shows extraordinary stability against light saturation. At this temperature, pc-Si:H is obtained within quite a narrow deposition-parameter window, and a slight deviation of r H from this borderline leads to a significant decrease in rd by three orders of magnitude. Fig. 2 shows the Urbach energy and crystalline volume fraction for T s s 75⬚C-deposited samples with a thickness of 1 m. In the a-Si:H range, Urbach energy decreases with increasing r H , indicating a more ordered silicon network. At the transition of a-Si:H to nc-Si:H, r H s 29 and Urbach energy is no longer defined, which indicates the start of nc-Si:H formation. However, a crystalline peak is not detected by Raman scattering or X-ray diffraction ŽXRD.. To reconcile these results, we assume that pc-Si:H is composed of nm-sized crystallites, which are too small to be detected by Raman scattering or XRD, embedded in an amorphous matrix. At constant r H s 39 and T s s 75⬚C, we deposited a series of samples with different thickness Ž0.25᎐2.0
Fig. 1. rd ratios for films as-deposited, after annealing at 110⬚C and AM1.5 light saturation.
Fig. 2. The Urbach energy and the crystalline volume fraction.
m. and analyzed the thickness-dependent morphology by absolute CPM w4x. All the deposition parameters except the deposition time were kept constant to obtain layers of various thickness. As depicted in Fig. 3, the 0.25-m thick film shows a typical a-Si:H CPM spectrum, indicating that the early stage of film formation starts with an amorphous seed layer. For the 0.5-m thick film, strong oscillation is observed in the spectrum, attributed to an inhomogeneous optical absorption in the film growth direction. This reveals the nanocrystalline formation in the amorphous matrix, which we consider pc-Si:H. For 1.0- and 2.0-m thick films, absorption by the nc-Si:H component dominates the CPM spectra. 3.2. Solar cell performance We investigated the amorphous-to-nanocrystalline transition of the absorber layer on the p-i-n solar cell performance. Fig. 4 depicts the initial fill factor of the solar cells at T s s 75⬚C. The absorber layer thickness was kept at 200 nm and only r H of this layer was varied. In the region of r H values smaller than 35, it is obvious that an amorphous layer grows. For r H values exceeding 45, we confirmed that the nc-Si:H layer increases in the absorber layer from the external quan-
Fig. 3. Absolute CPM spectra of four samples. The absorption coefficient of crystalline silicon Žc-Si. is inserted.
M. Ito et al. r Thin Solid Films 383 (2001) 129᎐131
131
gations suggest that a small fraction of crystallites are embedded in an amorphous-matrix. Absolute CPM measurements reveal the inhomogeneous morphology of silicon growing at the edge of crystallinity. The benefit of pc-Si:H as an absorber layer in a p-i-n solar cell is reflected in an improved fill factor and stability against light saturation. Acknowledgements Fig. 4. Fill factors of a p-i-n-structured solar cell.
tum efficiency measurement with reverse bias voltage. Between these two regions, we obtained a pc-Si:H absorber layer, which shows a peak in fill factor by more than 10%. In addition to the high material-quality of these as-deposited pc-Si:H solar cells, we observed an enhanced stability against light saturation w5x. 4. Conclusion Even at very low deposition temperatures under 80⬚C, we can obtain a pc-Si:H with high electronic quality and surprising stability. Structural and optical investi-
The authors would like to thank C. Kohler for tech¨ nical support. References w1x S. Guha, J. Yang, D.L. Williamson, Y. Lubianiker, J.D. Cohen, A.H. Mahan, Appl. Phys. Lett. 74 Ž1999. 1860. w2x A.H. Mahan, J. Yang, S. Guha, D.L. Williamson, Phys. Rev. B 61 Ž2000. 1677. w3x F. Finger, U. Kroll, V. Viret, A. Shah, W. Beyer, X.-M. Tang, J. Weber, A. Howling, C. Hollenstein, J. Appl. Phys. 71 Ž1992. 5665. w4x A. Fejfar, A. Poruba, M. Vanecek, J. Kocka, J. Non-Cryst. Solids 198᎐200 Ž1996. 304. w5x C. Koch, M. Ito, M.B. Schubert, J.H. Werner, 16th PVSEC, Glasgow, unpublished.