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Effects of RF power and pressure on performance of HF-PECVD silicon thin-film solar cells
Thin Solid Films 518 (2010) 7233–7235
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Effects of RF power and pressure on performance of HF-PECVD silicon thin-film solar cells Shui-Yang Lien a,⁎, Chao-Chun Wang b, Chau-Te Shen a, Yu-Chih Ou a, Yun-Shao Cho a, Ko-Wei Weng a, Ching-Hsun Chao a, Chia-Fu Chen a, Dong-Sing Wuu a a b
Department of Materials Science and Engineering, MingDao University, ChungHua 52345, Taiwan, ROC Department of Materials Science and Engineering, Nation Chung Hsing University, Taichung 402, Taiwan, ROC
a r t i c l e
i n f o
Available online 19 May 2010 Keywords: Plasma-enhanced chemical vapor deposition Amorphous silicon films Thin-film solar cells
a b s t r a c t High-frequency plasma-enhanced chemical vapor deposition (HF-PECVD) is a widely applicable method of deposition over a large area at a high rate for fabricating silicon thin-film solar cells. This investigation presents the properties of hydrogenated amorphous silicon (a-Si:H) films and the preparation of highlyefficient p–i–n solar cells using an RF (27.1 MHz) excitation frequency. The influence of the power (10– 40 W) and pressure (20–50 Pa) used during the deposition of absorber layers in p–i–n solar cells on the properties and mechanism of growth of the a-Si:H thin films and the solar cells is studied. The a-Si:H thin films prepared under various deposition conditions have widely varying deposition rates, optical-electronic properties and microstructures. When the deposition parameters were optimized, amorphous silicon-based thin-film silicon solar cells with efficiency of 7.6% were fabricated by HF-PECVD. These results are very encouraging for the future fabrication of highly-efficient thin-film solar cells by HF-PECVD. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental details
Hydrogenated amorphous silicon (a-Si:H), microcrystalline silicon (μc-Si) and polycrystalline silicon (poly-Si) thin films are still commonly produced using the first method of incorporating hydrogen in the materials [1–3]. a-Si:H based thin-film semiconductors are currently used in solar cells. Many deposition approaches are now available for synthesizing a-Si:H thin films. In these techniques, only plasma-enhanced chemical vapor deposition (PECVD) has been developed for industrial applications. However, the deposition rate of a-Si:H films of device quality, prepared by PECVD with the conventional frequency of 13.56 MHz with optimal deposition parameters is low. A low deposition rate is associated with a long process time and high production cost. In industrial applications, high rates of deposition of device-quality films with optical-electronic properties are required. Therefore, high power, high pressure and high plasma excitation frequency are applied. In this investigation, the high-frequency growth of a-Si:H singlejunction thin-film solar cells under various deposition conditions is studied. The effect of the power (10–40 W) and pressure (20–50 Pa) used during the deposition of absorber layers in p–i–n solar cells on the properties of the films and the solar cells is discussed. An efficiency of 7.6% was obtained.
In this work, a plasma excitation frequency of 27.1 MHz was adopted to deposit a-Si:H thin films. Intrinsic and doped silicon-based films were prepared in a single-chamber PECVD system. To achieve high growth rates of a-Si:H films of high-quality, the deposition regimes of RF power PRF (10–40 W) and deposition pressure Pdep (20–50 Pa) were investigated. The area of each electrode was 625 cm2. The dark conductivity (σd), photo conductivity (σph), optical band gap (Eg), absorption coefficients at a wavelength of 600 nm (α600), hydrogen content (CH), crystalline volume fraction (Rc) and microstructure factor (RIR) of intrinsic and doped silicon films for use in solar cells were examined, as described by the authors elsewhere [4–6]. The ability to deposit both pand n-type doped layers by PECVD following after the addition of B2H6 and PH3, respectively, was demonstrated. Table 1 presents deposition parameters of the intrinsic and doped silicon films. Glass substrate with an SnO2 film (ASAHI Type-U) was used to produce a thin-film solar cell. Fig. 1a schematically depicts the structures of the silicon thin-film solar cell. The p-layer, buffer layer i-layer and n-layer were deposited on the ASAHI substrate in that order. The 0.5 μm-thick Ag layer was deposited as a back contact by sputtering on the cell. The structure of the solar cells is as follows: ASAHI Type-U/p-type a-SiC:H (8 nm)/a-Si:H buffer (∼6 nm)/intrinsic a-Si absorber (250 nm)/n-type a-Si:H (30 nm)/Ag back contact (250 nm). The front electrode (SnO2:F) with a rough surface increases the optical path of the light by scattering it, increasing absorption in the active layers. The current–voltage and external quantum efficiency (EQE) characteristics of the solar cell (1× 1 cm2) were measured at 100 mW/cm2 using an AM 1.5 solar simulator.
⁎ Corresponding author. Fax: +886 48871020. E-mail address: [email protected] (S.-Y. Lien). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.04.083
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Table 1 Deposition parameters of intrinsic, n-type and p-type Si thin films by HF-PECVD. Deposition condition
p-type a-SiC Buffer layer Intrinsic a-Si n-type a-Si
Power (W) 10 Pressure (Pa) 80 E/S (mm) 20 Substrate temperature (°C) 200 20 SiH4 flow rate (sccm) 50 H2 flow rate (sccm) CH4 flow rate (sccm) 40 20 B2H6 flow rate (sccm) – PH3 flow rate (sccm) Layer thickness (nm) 8
15 60 17 200 20 200 40 to 0 – – 6
10–40 20–50 17 200 40 – – – – 250
10 60 25 200 20 40 – – 4 30
3. Results and discussion Silicon is a very useful material, as it can be modified from its purest, single crystalline state, via a two-phase microcrystalline or nano-crystalline state, to an almost perfectly disordered, amorphous state. Many features intrinsic to silicon films are exploited in its incorporation as an active layer in photovoltaic devices [7]. Fig. 2 plots the properties of intrinsic Si films, including Rd, Rc, α600, Eg, σd, σd, CH and RIR as functions of RF power and deposition pressure. Fig. 2a indicates that both Rd and Rc increase with the power (10–30 W) and pressure (20–50 Pa). Clearly, power and pressure reduce the deposition rate, which is explained by the fact that an increase in power or pressure increases the amount of atomic silicon that is available for film growth and crystallization, increasing both the Rd and Rc. The results show that α600 and Eg declines as power and pressure increases, indicating that the crystallinity increases, as presented in Fig. 2b. Fig. 2c plots the achieved quality of the device, in terms of dark and photo conductivities. The ratio of photo conductivity to dark conductivity is easily obtained to be 105. Fig. 2d demonstrates that the total CH of the film decreases and the microstructure factor increases as the power and pressure increase. A high value of the RIR corresponds to poor quality of the a-Si films. The a-Si films deposited at high power (N40 W) exhibit amorphous phase behavior, mainly because of the bombardment effect. Fig. 3 plots the performance of a solar cell with an ASAHI Type-U/ptype a-SiC:H (8 nm)/a-Si:H buffer (∼ 6 nm)/intrinsic a-Si absorber (250 nm)/n-type a-Si:H (30 nm)/Ag back contact (250 nm) structure for various deposition conditions of the a-Si absorber. The solar cell parameters are shown as functions of RF power and pressure. The efficiency of the solar cell reaches a maximum at a power of 20 W and a pressure of 40 Pa. Fig. 4a plots the best current–voltage characteristic measured under the one-sun condition (100 mA/cm2). When the deposition parameters (inset in Fig. 4a) of the solar cell were optimized, a thin-film solar cell Fig. 2. (a) Deposition rate Rd and crystalline fraction Rc, (b) absorption coefficient at wavelength of 600 nm α600 and band gap Eg, (c) dark conductivity σd and photo conductivity σp, and (d) hydrogen content CH and microstructure factor RIR as functions of RF power and deposition pressure.
Fig. 1. Schematic structure of the prepared silicon-based thin-film solar cell with a buffer layer.
with short circuit current density (Jsc) = 12.59 mA/cm2, open circuit voltage (Voc) = 0.819 V, fill factor (FF) = 0.739, and efficiency = 7.6% was fabricated. The performance of a solar cell can be significantly improved by manipulating the p+/i interface. A graded buffer layer was inserted between the p+- and i-layers to increase the efficiency of the solar cell. The increased Voc and FF have typically been attributed to a reduced interface density of recombination centers near the junction. Fig. 4b shows the EQE curve of the best solar cell. The effect of EQE can be elucidated by discussing two wavelength regions — the short wavelength region (Region A) from 350 nm to 550 nm and the long wavelength region (Region B) from 550 nm to 900 nm. In Region A, most of the absorption in the TCO (SnO2) and doped layers (p-SiC) is in the short wavelength region and is enhanced by scattering at the TCO/p
S.-Y. Lien et al. / Thin Solid Films 518 (2010) 7233–7235
7235
Fig. 4. (a) I–V curve and (b) EQE spectrum of solar cells measured under solar simulator with 100 mW/cm2 and AM1.5 conditions.
silicon films are investigated. High-quality intrinsic and doped a-Si layers were obtained. The deposition conditions of a-Si for use in p–i– n a-Si thin-film solar cells based on ASAHI (type-U) glass were optimized. When the deposition parameters of the solar cell were optimized, a thin-film solar cell with J sc = 12.59 mA/cm 2 , Voc = 0.819 V, FF = 0.739, and an efficiency of 7.6% was achieved. The details of this work reveal the great potential in the industrial production of highly-efficient silicon thin-film solar cells. Fig. 3. Experimental Voc, Jsc, FF and efficiency of thin-film solar cells as functions of RF power and pressure.
textured interface. In Region B, the major loss in the long wavelength region involves absorption in the metal contact layer. Some studies have demonstrated that the texture that is imparted to the metallic interface may result in a significant decrease in the reflectance of the real contact [8–10]. The insertion of a thin TCO layer between the n-layer and the contact metal critically traps scattered light and thus reduces the absorption of light in the real contact. The low response at long wavelength region further optimizes the highly reflective TCO/metal back contacts for solar cells. 4. Conclusions Hydrogenated amorphous silicon films were deposited for photovoltaic applications by high-frequency PECVD (27.1 MHz). The effects of power and pressure on the growth mechanisms and properties of
Acknowledgments This work is sponsored by Helius Power Company and the National Science Council of the Republic of China under contract nos. 98-2221E-451-003 and 98-2622-E-451-001-cc3. References [1] V. Patrick, Ph.D thesis, Universiteit Utrecht, Netherlands, 2002. [2] M. Fukawa, S. Suzuki, L. Guo, M. Kondo, A. Matsuda, Solar Energy Mater. Solar Cells 66 (2001) 217. [3] K. Yamamoto, et al., Progress in Photovoltaics: Research and Application 13 (2005) 489. [4] S.Y. Lien, H.Y. Mao, D.S. Wuu, Chem. Vapor Deposition 13 (2007) 247. [5] S.Y. Lien, B.R. Wu, D.S. Wuu, Thin Solid Films 516 (2008) 747. [6] S.Y. Lien, B.R. Wu, D.S. Wuu, Thin Solid Films 516 (2008) 765. [7] R.E.I. Schropp, M. Zeman, Kluwer Academic Publishers, 1998. [8] J. Morris, R.R. Arya, J.G. O'Dowd, S. Wiedeman, J. Appl. Phys. 67 (1990) 1079. [9] G. Tao, B.S. Girwar, G.E.N. Landweer, M. Zeman, J.W. Metselear, Proc. of the 11th European Photovoltaic Solar Energy Conference, 1992, p. 605, Montreux. [10] W. Beyer, J. Hupkes, H. Stiebig, Thin Solid Films 516 (2007) 147.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Effects of RF power and pressure on performance of HF-PECVD silicon thin-film solar cells Shui-Yang Lien a,⁎, Chao-Chun Wang b, Chau-Te Shen a, Yu-Chih Ou a, Yun-Shao Cho a, Ko-Wei Weng a, Ching-Hsun Chao a, Chia-Fu Chen a, Dong-Sing Wuu a a b
Department of Materials Science and Engineering, MingDao University, ChungHua 52345, Taiwan, ROC Department of Materials Science and Engineering, Nation Chung Hsing University, Taichung 402, Taiwan, ROC
a r t i c l e
i n f o
Available online 19 May 2010 Keywords: Plasma-enhanced chemical vapor deposition Amorphous silicon films Thin-film solar cells
a b s t r a c t High-frequency plasma-enhanced chemical vapor deposition (HF-PECVD) is a widely applicable method of deposition over a large area at a high rate for fabricating silicon thin-film solar cells. This investigation presents the properties of hydrogenated amorphous silicon (a-Si:H) films and the preparation of highlyefficient p–i–n solar cells using an RF (27.1 MHz) excitation frequency. The influence of the power (10– 40 W) and pressure (20–50 Pa) used during the deposition of absorber layers in p–i–n solar cells on the properties and mechanism of growth of the a-Si:H thin films and the solar cells is studied. The a-Si:H thin films prepared under various deposition conditions have widely varying deposition rates, optical-electronic properties and microstructures. When the deposition parameters were optimized, amorphous silicon-based thin-film silicon solar cells with efficiency of 7.6% were fabricated by HF-PECVD. These results are very encouraging for the future fabrication of highly-efficient thin-film solar cells by HF-PECVD. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental details
Hydrogenated amorphous silicon (a-Si:H), microcrystalline silicon (μc-Si) and polycrystalline silicon (poly-Si) thin films are still commonly produced using the first method of incorporating hydrogen in the materials [1–3]. a-Si:H based thin-film semiconductors are currently used in solar cells. Many deposition approaches are now available for synthesizing a-Si:H thin films. In these techniques, only plasma-enhanced chemical vapor deposition (PECVD) has been developed for industrial applications. However, the deposition rate of a-Si:H films of device quality, prepared by PECVD with the conventional frequency of 13.56 MHz with optimal deposition parameters is low. A low deposition rate is associated with a long process time and high production cost. In industrial applications, high rates of deposition of device-quality films with optical-electronic properties are required. Therefore, high power, high pressure and high plasma excitation frequency are applied. In this investigation, the high-frequency growth of a-Si:H singlejunction thin-film solar cells under various deposition conditions is studied. The effect of the power (10–40 W) and pressure (20–50 Pa) used during the deposition of absorber layers in p–i–n solar cells on the properties of the films and the solar cells is discussed. An efficiency of 7.6% was obtained.
In this work, a plasma excitation frequency of 27.1 MHz was adopted to deposit a-Si:H thin films. Intrinsic and doped silicon-based films were prepared in a single-chamber PECVD system. To achieve high growth rates of a-Si:H films of high-quality, the deposition regimes of RF power PRF (10–40 W) and deposition pressure Pdep (20–50 Pa) were investigated. The area of each electrode was 625 cm2. The dark conductivity (σd), photo conductivity (σph), optical band gap (Eg), absorption coefficients at a wavelength of 600 nm (α600), hydrogen content (CH), crystalline volume fraction (Rc) and microstructure factor (RIR) of intrinsic and doped silicon films for use in solar cells were examined, as described by the authors elsewhere [4–6]. The ability to deposit both pand n-type doped layers by PECVD following after the addition of B2H6 and PH3, respectively, was demonstrated. Table 1 presents deposition parameters of the intrinsic and doped silicon films. Glass substrate with an SnO2 film (ASAHI Type-U) was used to produce a thin-film solar cell. Fig. 1a schematically depicts the structures of the silicon thin-film solar cell. The p-layer, buffer layer i-layer and n-layer were deposited on the ASAHI substrate in that order. The 0.5 μm-thick Ag layer was deposited as a back contact by sputtering on the cell. The structure of the solar cells is as follows: ASAHI Type-U/p-type a-SiC:H (8 nm)/a-Si:H buffer (∼6 nm)/intrinsic a-Si absorber (250 nm)/n-type a-Si:H (30 nm)/Ag back contact (250 nm). The front electrode (SnO2:F) with a rough surface increases the optical path of the light by scattering it, increasing absorption in the active layers. The current–voltage and external quantum efficiency (EQE) characteristics of the solar cell (1× 1 cm2) were measured at 100 mW/cm2 using an AM 1.5 solar simulator.
⁎ Corresponding author. Fax: +886 48871020. E-mail address: [email protected] (S.-Y. Lien). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.04.083
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Table 1 Deposition parameters of intrinsic, n-type and p-type Si thin films by HF-PECVD. Deposition condition
p-type a-SiC Buffer layer Intrinsic a-Si n-type a-Si
Power (W) 10 Pressure (Pa) 80 E/S (mm) 20 Substrate temperature (°C) 200 20 SiH4 flow rate (sccm) 50 H2 flow rate (sccm) CH4 flow rate (sccm) 40 20 B2H6 flow rate (sccm) – PH3 flow rate (sccm) Layer thickness (nm) 8
15 60 17 200 20 200 40 to 0 – – 6
10–40 20–50 17 200 40 – – – – 250
10 60 25 200 20 40 – – 4 30
3. Results and discussion Silicon is a very useful material, as it can be modified from its purest, single crystalline state, via a two-phase microcrystalline or nano-crystalline state, to an almost perfectly disordered, amorphous state. Many features intrinsic to silicon films are exploited in its incorporation as an active layer in photovoltaic devices [7]. Fig. 2 plots the properties of intrinsic Si films, including Rd, Rc, α600, Eg, σd, σd, CH and RIR as functions of RF power and deposition pressure. Fig. 2a indicates that both Rd and Rc increase with the power (10–30 W) and pressure (20–50 Pa). Clearly, power and pressure reduce the deposition rate, which is explained by the fact that an increase in power or pressure increases the amount of atomic silicon that is available for film growth and crystallization, increasing both the Rd and Rc. The results show that α600 and Eg declines as power and pressure increases, indicating that the crystallinity increases, as presented in Fig. 2b. Fig. 2c plots the achieved quality of the device, in terms of dark and photo conductivities. The ratio of photo conductivity to dark conductivity is easily obtained to be 105. Fig. 2d demonstrates that the total CH of the film decreases and the microstructure factor increases as the power and pressure increase. A high value of the RIR corresponds to poor quality of the a-Si films. The a-Si films deposited at high power (N40 W) exhibit amorphous phase behavior, mainly because of the bombardment effect. Fig. 3 plots the performance of a solar cell with an ASAHI Type-U/ptype a-SiC:H (8 nm)/a-Si:H buffer (∼ 6 nm)/intrinsic a-Si absorber (250 nm)/n-type a-Si:H (30 nm)/Ag back contact (250 nm) structure for various deposition conditions of the a-Si absorber. The solar cell parameters are shown as functions of RF power and pressure. The efficiency of the solar cell reaches a maximum at a power of 20 W and a pressure of 40 Pa. Fig. 4a plots the best current–voltage characteristic measured under the one-sun condition (100 mA/cm2). When the deposition parameters (inset in Fig. 4a) of the solar cell were optimized, a thin-film solar cell Fig. 2. (a) Deposition rate Rd and crystalline fraction Rc, (b) absorption coefficient at wavelength of 600 nm α600 and band gap Eg, (c) dark conductivity σd and photo conductivity σp, and (d) hydrogen content CH and microstructure factor RIR as functions of RF power and deposition pressure.
Fig. 1. Schematic structure of the prepared silicon-based thin-film solar cell with a buffer layer.
with short circuit current density (Jsc) = 12.59 mA/cm2, open circuit voltage (Voc) = 0.819 V, fill factor (FF) = 0.739, and efficiency = 7.6% was fabricated. The performance of a solar cell can be significantly improved by manipulating the p+/i interface. A graded buffer layer was inserted between the p+- and i-layers to increase the efficiency of the solar cell. The increased Voc and FF have typically been attributed to a reduced interface density of recombination centers near the junction. Fig. 4b shows the EQE curve of the best solar cell. The effect of EQE can be elucidated by discussing two wavelength regions — the short wavelength region (Region A) from 350 nm to 550 nm and the long wavelength region (Region B) from 550 nm to 900 nm. In Region A, most of the absorption in the TCO (SnO2) and doped layers (p-SiC) is in the short wavelength region and is enhanced by scattering at the TCO/p
S.-Y. Lien et al. / Thin Solid Films 518 (2010) 7233–7235
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Fig. 4. (a) I–V curve and (b) EQE spectrum of solar cells measured under solar simulator with 100 mW/cm2 and AM1.5 conditions.
silicon films are investigated. High-quality intrinsic and doped a-Si layers were obtained. The deposition conditions of a-Si for use in p–i– n a-Si thin-film solar cells based on ASAHI (type-U) glass were optimized. When the deposition parameters of the solar cell were optimized, a thin-film solar cell with J sc = 12.59 mA/cm 2 , Voc = 0.819 V, FF = 0.739, and an efficiency of 7.6% was achieved. The details of this work reveal the great potential in the industrial production of highly-efficient silicon thin-film solar cells. Fig. 3. Experimental Voc, Jsc, FF and efficiency of thin-film solar cells as functions of RF power and pressure.
textured interface. In Region B, the major loss in the long wavelength region involves absorption in the metal contact layer. Some studies have demonstrated that the texture that is imparted to the metallic interface may result in a significant decrease in the reflectance of the real contact [8–10]. The insertion of a thin TCO layer between the n-layer and the contact metal critically traps scattered light and thus reduces the absorption of light in the real contact. The low response at long wavelength region further optimizes the highly reflective TCO/metal back contacts for solar cells. 4. Conclusions Hydrogenated amorphous silicon films were deposited for photovoltaic applications by high-frequency PECVD (27.1 MHz). The effects of power and pressure on the growth mechanisms and properties of
Acknowledgments This work is sponsored by Helius Power Company and the National Science Council of the Republic of China under contract nos. 98-2221E-451-003 and 98-2622-E-451-001-cc3. References [1] V. Patrick, Ph.D thesis, Universiteit Utrecht, Netherlands, 2002. [2] M. Fukawa, S. Suzuki, L. Guo, M. Kondo, A. Matsuda, Solar Energy Mater. Solar Cells 66 (2001) 217. [3] K. Yamamoto, et al., Progress in Photovoltaics: Research and Application 13 (2005) 489. [4] S.Y. Lien, H.Y. Mao, D.S. Wuu, Chem. Vapor Deposition 13 (2007) 247. [5] S.Y. Lien, B.R. Wu, D.S. Wuu, Thin Solid Films 516 (2008) 747. [6] S.Y. Lien, B.R. Wu, D.S. Wuu, Thin Solid Films 516 (2008) 765. [7] R.E.I. Schropp, M. Zeman, Kluwer Academic Publishers, 1998. [8] J. Morris, R.R. Arya, J.G. O'Dowd, S. Wiedeman, J. Appl. Phys. 67 (1990) 1079. [9] G. Tao, B.S. Girwar, G.E.N. Landweer, M. Zeman, J.W. Metselear, Proc. of the 11th European Photovoltaic Solar Energy Conference, 1992, p. 605, Montreux. [10] W. Beyer, J. Hupkes, H. Stiebig, Thin Solid Films 516 (2007) 147.