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p-type crystalline silicon heterojunction solar cells with a microcrystalline silicon buffer layer
Solar Energy Materials & Solar Cells 64 (2000) 241}249
Amorphous silicon/p-type crystalline silicon heterojunction solar cells with a microcrystalline silicon bu!er layer Y.J. Song, W.A. Anderson* Department of Electrical and Computer Engineering, State University of New York at Buwalo, Box 601900, Bonner Hall, Amherst, NY 14260-1900, USA
Abstract Undoped hydrogenated amorphous silicon (a-Si:H)/p-type crystalline silicon (c-Si) structures with and without a microcrystalline silicon (lc-Si) bu!er layer have been investigated as a potential low-cost heterojunction (HJ) solar cell. Unlike the conventional HJ silicon solar cell with a highly doped window layer, the undoped a-Si:H emitter was photovoltaically active, and a thicker emitter layer was proven to be advantageous for more light absorption, as long as the carriers generated in the layer are e!ectively collected at the junction. In addition, without using heavy doping and transparent front contacts, the solar cell exhibited a "ll factor comparable to the conventional HJ silicon solar cell. The optimized con"guration consisted of an undoped a-Si:H emitter layer (700 As ), providing an excellent light absorption and defect passivation, and a thin lc-Si bu!er layer (200 As ), providing an improved carrier collection by lowering barrier height at the interface, resulting in a maximum conversion e$ciency of 10% without an anti-re#ective coating. 2000 Elsevier Science B.V. All rights reserved. Keywords: Heterojunction solar cell; Amorphous silicon; Interface; Spectral response
1. Introduction Recently, thin-"lm silicon/crystalline silicon (c-Si) heterojunction (HJ)-type solar cells, which combine the low cost, low temperature and light weight of thin-"lm silicon, with the high e$ciency and high stability of c-Si, have been studied extensively
* Corresponding author. Tel.: #716-645-2422; fax: #716-645-5964. E-mail address: [email protected]!alo.edu (W.A. Anderson). 0927-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 2 2 3 - 3
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due to the increased need for both terrestrial and satellite applications [1]. In this type of cell, a thin emitter layer having a relatively large optical gap, like hydrogenated amorphous silicon (a-Si:H) [2], hydrogenated amorphous silicon carbide (a-SiC:H) [3] or microcrystalline silicon (lc-Si) [4], is normally deposited by plasma-assisted decomposition of silane to form a p-n junction at low temperatures ((4003C). It has been known that the performance of the cell is strongly dependent on the electrooptical properties of the emitter layer and the quality of the hetero-interface between thin-"lm silicon and c-Si [5]. The emitter layer in the HJ cell is usually highly doped to reduce the series resistance, and very thin ((300 As ) to minimize the recombination of photogenerated carriers in the layer. This layer is considered as a window, which is photovoltaically inactive, because the carrier lifetime drops as the doping level increases [6]. E!orts to improve the thin-"lm silicon material itself have focused on the development of various deposition techniques, such as hot-wire chemical vapor deposition (HW-CVD) [7], very high-frequency plasma CVD (VHF-CVD) [8], and electron cyclotron resonance CVD (ECR-CVD) [9]. Among them, ECR-CVD is particularly important in the growth of a very thin lc-Si "lm because it provides a better control of energetic ion bombardment [10,11]. In addition, it has been reported that the interface condition, such as band o!set and interface defects, signi"cantly a!ects the carrier transport and photovoltaic properties of these solar cells [12,13]. Techniques for obtaining a high-quality interface include bandgap grading [14] and hydrogen passivation of the interface states by chemical HF etching [5], atomic hydrogen treatment [14], and deposition of an a-Si:H passivation layer [15]. This paper describes the fabrication and characterization of low-cost HJ silicon solar cells. First, the photovoltaic properties of undoped a-Si:H/p-type c-Si HJ solar cells were investigated and then compared with the conventional HJ silicon solar cell, emphasizing the in#uence of the emitter layer thickness on the solar cell parameters. After that, a thin lc-Si bu!er layer was inserted into the a-Si:H/c-Si interface to examine the role of the interface condition, such as bandgap grading or defect pro"le, on the solar cell performance. Finally, an optimized solar cell design is proposed based on the preceding investigation. The emitter design is suitable for application to a thin-"lm silicon base and is accomplished without high-temperature processing such as di!usion.
2. Experiment All silicon thin "lms used in this study were deposited by microwave (2.45 GHz) ECR-CVD. The reactant gas was a 2% SiH /He mixture, while a minimum amount of pure Ar formed the background gas for plasma generation. The deposition of a-Si:H "lms was done without hydrogen dilution at a substrate temperature of 2503C, a chamber pressure of 5}10 mTorr, an input power of 375}400 W, and a deposition rate of 70}80 As /min. Meanwhile, the lc-Si "lm was grown with H dilution (R "0.58, where R "H /(2% SiH /He#H #Ar)) using the conditions of a sub& & strate temperature of 4003C, a chamber pressure of 1 mTorr, and an input power of
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Fig. 1. Heterojunction silicon solar cell structure used in this study.
350 W. The deposition rate of the lc-Si "lm was around 20 As /min. Flat Corning glass substrates were used for the measurement of dark (p ) and photoconductivity (p ), excess carrier lifetime, and optical gap (E ) of silicon thin "lms. The carrier lifetime was measured by a rf-conductivity method at a wavelength of 532 nm for excitation at the National Renewable Energy Laboratory (NREL). The E was deduced from a plot of (ahl) vs. photon energy (hl), where the light absorption coe$cient (a) spectra was obtained by transmission measurement of the "lm. The e!ective doping density (N ) of the silicon "lm was obtained from the high-frequency (1 MHz) ' capacitance}voltage characteristic of the a-Si:H or lc-Si/c-Si diode [2], where an HF-treated p-type (1 0 0) silicon wafer (&1.3 ) cm) was used as the substrate. An identical c-Si substrate was also used for the fabrication of undoped a-Si:H/(lc-Si)/c-Si solar cells, to give a cell area of 0.3 cm. A schematic diagram of the cell structure is shown in Fig. 1. Regarding the front-side grid and backside contact, evaporated Mg/Al (100 As /1000 As ) and evaporated Al (1000 As ) were used, respectively. The Al was sintered at 6003C to form an ohmic contact. Photoconductivity and photovoltaic response tests were performed under a 100 mW/cm, AM1.0 spectrum, from a quartzhalogen lamp calibrated with a cell previously tested at NREL.
3. Photovoltaic properties 3.1. Amorphous silicon/crystalline silicon structure The material properties of an emitter layer in the HJ solar cell, are critical because they directly a!ect the cell's energy band pro"le, bulk and interface defect densities, and light absorption ability. Table 1 includes the electro-optical properties of the
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Table 1 Properties of a-Si:H and lc-Si layers used in this study Layer
p (S/cm)
p (S/cm)
E (eV)
E (eV) N (cm\) '
Excess carrier lifetime (ns)
a-Si:H emitter lc-Si bu!er
8.6;10\ 3.2;10\
9.5;10\ 8.5;10\
0.86 0.51
1.73 1.23
1345 (180
2.5;10 2.7;10
Fig. 2. Dependence of solar cell parameters on the a-Si:H emitter thickness in undoped a-Si:H/p-type c-Si heterojunction solar cells.
a-Si:H used as an emitter in this study. The p , activation energy (E ), carrier lifetime, and N data show that this "lm is device quality undoped a-Si:H, which acts as ' a lightly doped n-type material. The relatively high value of E in the table, compared to that for a-Si:H grown by the standard plasma-enhanced CVD (typically, 1.5}1.6 eV), is often observed in ECR-CVD resulting from the di!erent "lm growth mechanism [9]. Fig. 2 shows the in#uence of emitter layer thickness (200}1200 As ) on the photovoltaic properties of n-type (undoped) a-Si:H/p-type c-Si solar cells. These cells did not have an antire#ective coating. From the "gure, it is seen that the cell's e$ciency increases gradually to about 9 % with the increase of emitter layer thickness (t ) up to around 850 As , but it drops for t beyond 900 As . This result indicates that the
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emitter layer is photovoltaically active, unlike the highly doped window layer in the conventional HJ silicon solar cell. For reference, in the conventional cell, the e$ciency normally decreases monotonically as the window layer thickness increases due to carrier recombination in the layer [15]. However, the a-Si:H emitter thickness providing the maximum e$ciency was much thinner than the emitter thickness in the conventional c-Si homojunction solar cell, which is typically 0.25 lm [16]. It is obvious that the much shorter minority carrier di!usion length and the lower conductivity of a-Si:H than for c-Si leads to this observation, addressing the use of a very thin emitter layer in the cell. However, the superior light absorption of the a-Si:H layer with a larger E suggests the use of an a-Si:H layer as thick as possible, only if most of the photogenerated carriers in the emitter are collected at the junction. Hence, a better quality a-Si:H "lm will permit a thicker emitter layer in the cell. Unlike the short-circuit current density (J ) and "ll factor (FF), the open-circuit voltage (< ) in Fig. 2 does not strongly depend on the emitter layer thickness. It seems that < is dominated by the interface conditions, which are independent of the layer thickness, such as band o!sets and interface defect pro"le. In spite of the junction formation at a quite low temperature (2503C), compared to the di!usion process in the conventional homojunction c-Si cell (&10003C), the resulting < values were com parable to the < obtained in the homojunction structure (0.55}0.60 V). The plasma deposited a-Si:H naturally provides surface passivation of the c-Si with hydrogen atoms, leading to a relatively high-quality junction at the decreased ¹ . The relatively rapid drops in J and FF seem to result from the increased recombination rate of the excess carriers generated by high-energy photons (i.e. carriers generated near the emitter surface) and increased series resistance of the emitter layer. Finally, it is observed that the g vs. thickness characteristic is dominated by the J vs. thickness characteristic. 3.2. Amorphous silicon/crystalline silicon with a thin microcrystalline buwer layer A thin lc-Si layer (70}400 As ) was inserted into the interface of the a-Si:H/c-Si structure, as a bu!er layer, where the a-Si:H thickness was "xed at 700 As . The Raman spectra in Fig. 3 indicates that a 70 As thick lc-Si "lm can be grown by ECR-CVD. The electrical data of the lc-Si "lm in Table 1 indicates that this "lm is more defective and has a narrower bandgap, compared to the previous a-Si:H "lm, since less hydrogen is incorporated in the lc-Si "lm which reduces the bandgap energy and increases the number of dangling bonds in the material. The e!ects of a lc-Si bu!er layer on the photovoltaic properties of the HJ cell is shown in Fig. 4. It shows that the insertion of a lc-Si layer into the a-Si:H/c-Si interface either enhanced or degraded the e$ciency of the cells, depending on the bu!er layer thickness. Meanwhile, all the cells with a bu!er layer in the "gure exhibited relatively high J values of 28.2}31.8 mA/cm, which are typically larger than those of c-Si homojunction solar cells. The e!ective combination of the a-Si:H emitter layer, providing excellent light trapping, and the lc-Si bu!er layer, improving the interface condition by bandgap grading, seems to contribute to the result. Despite
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Fig. 3. Raman spectra of a-Si:H and lc-Si (70 As ) on the a-Si:H underlayer.
Fig. 4. In#uence of the lc-Si bu!er layer thickness on the undoped a-Si:H/p-type c-Si heterojunction solar cells.
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Fig. 5. Comparison of J}V characteristics of the heterojunction solar cells with and without a lc-Si bu!er layer.
the slight degradation of < in the cell due to bandgap grading, the enhanced values of FF, up to 0.63, were measured for the cell with a bu!er layer and without using a transparent front contact such as indium tin oxide (ITO). The enhanced carrier collection rate, together with the higher conductivity of the lc-Si "lm seems to be responsible for this result. In fact, the FF values obtained here were comparable to those obtained for the conventional HJ cell with an ITO front contact layer. This suggests the favorable use of a photovoltaically active emitter layer in the cell structure providing an additional photon absorption in the solar cell, as long as most of the photogenerated carriers in this layer are collected at the junction. Fig. 5 compares the dark log J}V plots of the three di!erent cells without, and with a 200 and 400 As thick lc-Si bu!er layer at 3003K. It is seen that at least three di!erent conduction mechanisms exist in the forward bias (one region for 0.3 V'V, a second region for 0.3(V(0.55 V, and a space charge limited region for 0.55 V(V), whereas the conduction mechanism for lower voltages diminishes as the bu!er layer becomes thicker. Simultaneously, the thicker bu!er layer has generally increased the forward current level, which is attributed to the lowered potential barrier height at the interface. A detailed analysis of the conduction mechanism of the HJ is described in a separate paper [17]. The highest e$ciency of 9.9% occurs with the bu!er layer thickness of 200 As , whereas the cells with a bu!er layer thicker than 200 As show lower values of e$ciency due to the shorter carrier lifetime in the lc-Si (i.e. shorter di!usion length), which accelerates the recombination of photogenerated carriers moving towards the junction. Furthermore, it seems that the cell with a lc-Si bu!er layer thicker than 200 As su!ers from defects, even after the subsequent a-Si:H deposition, because hydrogen
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Fig. 6. Spectral response of the heterojunction solar cells having various emitter designs.
atoms in a-Si:H cannot completely di!use through a thicker lc-Si underlayer, leaving defects unpassivated near the hetero-interface. This was indicated by lower < 's in the cells having a thicker bu!er layer. 4. Spectral response Fig. 6 compares the spectral response of the cells with an emitter thickness of 250, 700 and 1100 As . The increase of the emitter thickness from 250 to 700 As improved the spectral response for all wavelengths because the thicker a-Si:H emitter absorbs more incident photons. This con"rms that the a-Si:H layer is photovoltaically active and contributes to the response. In contrast, the cell with an 1100 As thick emitter showed a poorer response, especially for shorter wavelengths, because of the enhanced recombination of the carriers generated by high-energy photons. Meanwhile, the cells with a bu!er layer exhibited a slightly higher red response compared to the cell without a bu!er layer, proposing an additional photon absorption in the bu!er layer. However, the bu!er layer thicker than 200 As showed a relatively low blue response due to recombination as found in the cell with a thick a-Si:H emitter or reduced utilization of high energy photons. Finally, the insertion of a 200 As thick lc-Si bu!er layer into the a-Si:H/c-Si structure revealed the best spectral response of all cells through the additional absorption of photons of less than 2.0 eV, without a degradation of blue response, and modi"cation of the hetero-interface. This corresponds to the highest J value for this solar cell. 5. Conclusion Photovolatic properties of undoped a-Si:H/p-type c-Si heterojunction solar cells with and without a lc-Si bu!er layer were studied. The a-Si:H was e!ectively used for
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better light absorption and defect passivation, whereas the lc-Si was utilized for bandgap grading. The emitter layer was photovoltaically active and strongly in#uenced the solar cell performance. The optimization of the emitter layer was conducted in terms of layer thickness, interface (bandgap grading), and defect passivation. Finally, an optimized solar cell structure of a-Si:H (700 As )/lc-Si (200 As )/c-Si provided the maximum e$ciency of 10% without an antire#ective coating. This is potentially suitable for a low-cost, low-temperature and light-weight solar cell design by replacing the c-Si with thin "lm polycrystalline Si [18].
Acknowledgements This research was sponsored by in part the National Renewable Energy Laboratory, the New York State Energy Research and Development Authority, and National Science Foundation.
References [1] W.A. Anderson, B. Jagannathan, E. Klementieva, Progr. Photovoltaics 5 (1997) 433. [2] B. Jagannathan, W.A. Anderson, J. Coleman, Sol. Energy Matter Sol. Cells 46 (1997) 289. [3] M. van Cleef, F.A. Rubinelli, R. Rizzoli, R. Pinghini, R.E.I. Schropp, W.F. van der Weg, Jpn. J. Appl. Phys. 37 (1998) 3926. [4] M. van Cleef, J.K. Rath, F.A. Rubinelli, C.H.M. van der Werf, R.E.I. Schropp, W.F. van der Weg, J. Appl. Phys. 82 (1997) 6089. [5] B. Jagannathan, W.A. Anderson, Sol. Energy Matter Sol. Cells 44 (1996) 165. [6] B.G. Streetman, Solid State Electronic Devices, 4th edition, Prentice-Hall, Englewood Cli!s, N J, 1995. [7] P. Brogueira, S. Grebner, F. Wang, R. Schwarz, V. Chu, J.P. Conde, Mat. Res. Soc. Symp. Proc. 297 (1998) 121. [8] F. Finger, P. Hapke, M. Luysberg, R. Carius, H. Wagner, M. Scheib, Appl. Phys. Lett. 65 (1994) 2588. [9] M. Kitagawa, S. Ishihara, K. Setsune, Y. Manabe, T. Hirao, Jpn. J. Appl. Phys. 26 (1987) L231. [10] J.L. Rogers, P.S. Andry, W.L. Varhue, P. McGaughnea, E. Adams, R. Kontra, Appl. Phys. Lett. 67 (1995) 971. [11] B. Jagannathan, R.L. Wallace, W.A. Anderson, J. Vac. Sci. Technol. A16 (1998) 2751. [12] M.W.M. van Cleef, F.A. Rubinelli, R.E.I. Schropp, Mat. Res. Soc. Symp. Proc. 507 (1998) 125. [13] B. Jagannathan, W.A. Anderson, J. Appl. Phys. 82 (1997) 1930. [14] B. Jagannathan, Ph.D Dissertation, State University of New York at Bu!alo, 1997. [15] M. Tanaka, M. Taguchi, T. Matsuyama, T. Sawada, S. Tsuda, S. Nakano, H. Hanafusa, Y. Kuwano, Jpn. J. Appl. Phys. 31 (1992) 3518. [16] S.M. Sze, Physics of Semiconductor Devices, 2nd edition, Wiley, New York, 1981. [17] Y.J. Song, M.R. Park, E. Guliants, W.A. Anderson, Sol. Energy Matter Sol. Cells, this issue. [18] Y.J. Song, E. Guliants, W.A. Anderson, Proceedings of 16th Space Photovoltaic Research and Technology Conference, John Glenn Research Center, Cleveland, OH, 1999.
Amorphous silicon/p-type crystalline silicon heterojunction solar cells with a microcrystalline silicon bu!er layer Y.J. Song, W.A. Anderson* Department of Electrical and Computer Engineering, State University of New York at Buwalo, Box 601900, Bonner Hall, Amherst, NY 14260-1900, USA
Abstract Undoped hydrogenated amorphous silicon (a-Si:H)/p-type crystalline silicon (c-Si) structures with and without a microcrystalline silicon (lc-Si) bu!er layer have been investigated as a potential low-cost heterojunction (HJ) solar cell. Unlike the conventional HJ silicon solar cell with a highly doped window layer, the undoped a-Si:H emitter was photovoltaically active, and a thicker emitter layer was proven to be advantageous for more light absorption, as long as the carriers generated in the layer are e!ectively collected at the junction. In addition, without using heavy doping and transparent front contacts, the solar cell exhibited a "ll factor comparable to the conventional HJ silicon solar cell. The optimized con"guration consisted of an undoped a-Si:H emitter layer (700 As ), providing an excellent light absorption and defect passivation, and a thin lc-Si bu!er layer (200 As ), providing an improved carrier collection by lowering barrier height at the interface, resulting in a maximum conversion e$ciency of 10% without an anti-re#ective coating. 2000 Elsevier Science B.V. All rights reserved. Keywords: Heterojunction solar cell; Amorphous silicon; Interface; Spectral response
1. Introduction Recently, thin-"lm silicon/crystalline silicon (c-Si) heterojunction (HJ)-type solar cells, which combine the low cost, low temperature and light weight of thin-"lm silicon, with the high e$ciency and high stability of c-Si, have been studied extensively
* Corresponding author. Tel.: #716-645-2422; fax: #716-645-5964. E-mail address: [email protected]!alo.edu (W.A. Anderson). 0927-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 2 2 3 - 3
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due to the increased need for both terrestrial and satellite applications [1]. In this type of cell, a thin emitter layer having a relatively large optical gap, like hydrogenated amorphous silicon (a-Si:H) [2], hydrogenated amorphous silicon carbide (a-SiC:H) [3] or microcrystalline silicon (lc-Si) [4], is normally deposited by plasma-assisted decomposition of silane to form a p-n junction at low temperatures ((4003C). It has been known that the performance of the cell is strongly dependent on the electrooptical properties of the emitter layer and the quality of the hetero-interface between thin-"lm silicon and c-Si [5]. The emitter layer in the HJ cell is usually highly doped to reduce the series resistance, and very thin ((300 As ) to minimize the recombination of photogenerated carriers in the layer. This layer is considered as a window, which is photovoltaically inactive, because the carrier lifetime drops as the doping level increases [6]. E!orts to improve the thin-"lm silicon material itself have focused on the development of various deposition techniques, such as hot-wire chemical vapor deposition (HW-CVD) [7], very high-frequency plasma CVD (VHF-CVD) [8], and electron cyclotron resonance CVD (ECR-CVD) [9]. Among them, ECR-CVD is particularly important in the growth of a very thin lc-Si "lm because it provides a better control of energetic ion bombardment [10,11]. In addition, it has been reported that the interface condition, such as band o!set and interface defects, signi"cantly a!ects the carrier transport and photovoltaic properties of these solar cells [12,13]. Techniques for obtaining a high-quality interface include bandgap grading [14] and hydrogen passivation of the interface states by chemical HF etching [5], atomic hydrogen treatment [14], and deposition of an a-Si:H passivation layer [15]. This paper describes the fabrication and characterization of low-cost HJ silicon solar cells. First, the photovoltaic properties of undoped a-Si:H/p-type c-Si HJ solar cells were investigated and then compared with the conventional HJ silicon solar cell, emphasizing the in#uence of the emitter layer thickness on the solar cell parameters. After that, a thin lc-Si bu!er layer was inserted into the a-Si:H/c-Si interface to examine the role of the interface condition, such as bandgap grading or defect pro"le, on the solar cell performance. Finally, an optimized solar cell design is proposed based on the preceding investigation. The emitter design is suitable for application to a thin-"lm silicon base and is accomplished without high-temperature processing such as di!usion.
2. Experiment All silicon thin "lms used in this study were deposited by microwave (2.45 GHz) ECR-CVD. The reactant gas was a 2% SiH /He mixture, while a minimum amount of pure Ar formed the background gas for plasma generation. The deposition of a-Si:H "lms was done without hydrogen dilution at a substrate temperature of 2503C, a chamber pressure of 5}10 mTorr, an input power of 375}400 W, and a deposition rate of 70}80 As /min. Meanwhile, the lc-Si "lm was grown with H dilution (R "0.58, where R "H /(2% SiH /He#H #Ar)) using the conditions of a sub& & strate temperature of 4003C, a chamber pressure of 1 mTorr, and an input power of
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Fig. 1. Heterojunction silicon solar cell structure used in this study.
350 W. The deposition rate of the lc-Si "lm was around 20 As /min. Flat Corning glass substrates were used for the measurement of dark (p ) and photoconductivity (p ), excess carrier lifetime, and optical gap (E ) of silicon thin "lms. The carrier lifetime was measured by a rf-conductivity method at a wavelength of 532 nm for excitation at the National Renewable Energy Laboratory (NREL). The E was deduced from a plot of (ahl) vs. photon energy (hl), where the light absorption coe$cient (a) spectra was obtained by transmission measurement of the "lm. The e!ective doping density (N ) of the silicon "lm was obtained from the high-frequency (1 MHz) ' capacitance}voltage characteristic of the a-Si:H or lc-Si/c-Si diode [2], where an HF-treated p-type (1 0 0) silicon wafer (&1.3 ) cm) was used as the substrate. An identical c-Si substrate was also used for the fabrication of undoped a-Si:H/(lc-Si)/c-Si solar cells, to give a cell area of 0.3 cm. A schematic diagram of the cell structure is shown in Fig. 1. Regarding the front-side grid and backside contact, evaporated Mg/Al (100 As /1000 As ) and evaporated Al (1000 As ) were used, respectively. The Al was sintered at 6003C to form an ohmic contact. Photoconductivity and photovoltaic response tests were performed under a 100 mW/cm, AM1.0 spectrum, from a quartzhalogen lamp calibrated with a cell previously tested at NREL.
3. Photovoltaic properties 3.1. Amorphous silicon/crystalline silicon structure The material properties of an emitter layer in the HJ solar cell, are critical because they directly a!ect the cell's energy band pro"le, bulk and interface defect densities, and light absorption ability. Table 1 includes the electro-optical properties of the
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Table 1 Properties of a-Si:H and lc-Si layers used in this study Layer
p (S/cm)
p (S/cm)
E (eV)
E (eV) N (cm\) '
Excess carrier lifetime (ns)
a-Si:H emitter lc-Si bu!er
8.6;10\ 3.2;10\
9.5;10\ 8.5;10\
0.86 0.51
1.73 1.23
1345 (180
2.5;10 2.7;10
Fig. 2. Dependence of solar cell parameters on the a-Si:H emitter thickness in undoped a-Si:H/p-type c-Si heterojunction solar cells.
a-Si:H used as an emitter in this study. The p , activation energy (E ), carrier lifetime, and N data show that this "lm is device quality undoped a-Si:H, which acts as ' a lightly doped n-type material. The relatively high value of E in the table, compared to that for a-Si:H grown by the standard plasma-enhanced CVD (typically, 1.5}1.6 eV), is often observed in ECR-CVD resulting from the di!erent "lm growth mechanism [9]. Fig. 2 shows the in#uence of emitter layer thickness (200}1200 As ) on the photovoltaic properties of n-type (undoped) a-Si:H/p-type c-Si solar cells. These cells did not have an antire#ective coating. From the "gure, it is seen that the cell's e$ciency increases gradually to about 9 % with the increase of emitter layer thickness (t ) up to around 850 As , but it drops for t beyond 900 As . This result indicates that the
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emitter layer is photovoltaically active, unlike the highly doped window layer in the conventional HJ silicon solar cell. For reference, in the conventional cell, the e$ciency normally decreases monotonically as the window layer thickness increases due to carrier recombination in the layer [15]. However, the a-Si:H emitter thickness providing the maximum e$ciency was much thinner than the emitter thickness in the conventional c-Si homojunction solar cell, which is typically 0.25 lm [16]. It is obvious that the much shorter minority carrier di!usion length and the lower conductivity of a-Si:H than for c-Si leads to this observation, addressing the use of a very thin emitter layer in the cell. However, the superior light absorption of the a-Si:H layer with a larger E suggests the use of an a-Si:H layer as thick as possible, only if most of the photogenerated carriers in the emitter are collected at the junction. Hence, a better quality a-Si:H "lm will permit a thicker emitter layer in the cell. Unlike the short-circuit current density (J ) and "ll factor (FF), the open-circuit voltage (< ) in Fig. 2 does not strongly depend on the emitter layer thickness. It seems that < is dominated by the interface conditions, which are independent of the layer thickness, such as band o!sets and interface defect pro"le. In spite of the junction formation at a quite low temperature (2503C), compared to the di!usion process in the conventional homojunction c-Si cell (&10003C), the resulting < values were com parable to the < obtained in the homojunction structure (0.55}0.60 V). The plasma deposited a-Si:H naturally provides surface passivation of the c-Si with hydrogen atoms, leading to a relatively high-quality junction at the decreased ¹ . The relatively rapid drops in J and FF seem to result from the increased recombination rate of the excess carriers generated by high-energy photons (i.e. carriers generated near the emitter surface) and increased series resistance of the emitter layer. Finally, it is observed that the g vs. thickness characteristic is dominated by the J vs. thickness characteristic. 3.2. Amorphous silicon/crystalline silicon with a thin microcrystalline buwer layer A thin lc-Si layer (70}400 As ) was inserted into the interface of the a-Si:H/c-Si structure, as a bu!er layer, where the a-Si:H thickness was "xed at 700 As . The Raman spectra in Fig. 3 indicates that a 70 As thick lc-Si "lm can be grown by ECR-CVD. The electrical data of the lc-Si "lm in Table 1 indicates that this "lm is more defective and has a narrower bandgap, compared to the previous a-Si:H "lm, since less hydrogen is incorporated in the lc-Si "lm which reduces the bandgap energy and increases the number of dangling bonds in the material. The e!ects of a lc-Si bu!er layer on the photovoltaic properties of the HJ cell is shown in Fig. 4. It shows that the insertion of a lc-Si layer into the a-Si:H/c-Si interface either enhanced or degraded the e$ciency of the cells, depending on the bu!er layer thickness. Meanwhile, all the cells with a bu!er layer in the "gure exhibited relatively high J values of 28.2}31.8 mA/cm, which are typically larger than those of c-Si homojunction solar cells. The e!ective combination of the a-Si:H emitter layer, providing excellent light trapping, and the lc-Si bu!er layer, improving the interface condition by bandgap grading, seems to contribute to the result. Despite
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Fig. 3. Raman spectra of a-Si:H and lc-Si (70 As ) on the a-Si:H underlayer.
Fig. 4. In#uence of the lc-Si bu!er layer thickness on the undoped a-Si:H/p-type c-Si heterojunction solar cells.
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Fig. 5. Comparison of J}V characteristics of the heterojunction solar cells with and without a lc-Si bu!er layer.
the slight degradation of < in the cell due to bandgap grading, the enhanced values of FF, up to 0.63, were measured for the cell with a bu!er layer and without using a transparent front contact such as indium tin oxide (ITO). The enhanced carrier collection rate, together with the higher conductivity of the lc-Si "lm seems to be responsible for this result. In fact, the FF values obtained here were comparable to those obtained for the conventional HJ cell with an ITO front contact layer. This suggests the favorable use of a photovoltaically active emitter layer in the cell structure providing an additional photon absorption in the solar cell, as long as most of the photogenerated carriers in this layer are collected at the junction. Fig. 5 compares the dark log J}V plots of the three di!erent cells without, and with a 200 and 400 As thick lc-Si bu!er layer at 3003K. It is seen that at least three di!erent conduction mechanisms exist in the forward bias (one region for 0.3 V'V, a second region for 0.3(V(0.55 V, and a space charge limited region for 0.55 V(V), whereas the conduction mechanism for lower voltages diminishes as the bu!er layer becomes thicker. Simultaneously, the thicker bu!er layer has generally increased the forward current level, which is attributed to the lowered potential barrier height at the interface. A detailed analysis of the conduction mechanism of the HJ is described in a separate paper [17]. The highest e$ciency of 9.9% occurs with the bu!er layer thickness of 200 As , whereas the cells with a bu!er layer thicker than 200 As show lower values of e$ciency due to the shorter carrier lifetime in the lc-Si (i.e. shorter di!usion length), which accelerates the recombination of photogenerated carriers moving towards the junction. Furthermore, it seems that the cell with a lc-Si bu!er layer thicker than 200 As su!ers from defects, even after the subsequent a-Si:H deposition, because hydrogen
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Fig. 6. Spectral response of the heterojunction solar cells having various emitter designs.
atoms in a-Si:H cannot completely di!use through a thicker lc-Si underlayer, leaving defects unpassivated near the hetero-interface. This was indicated by lower < 's in the cells having a thicker bu!er layer. 4. Spectral response Fig. 6 compares the spectral response of the cells with an emitter thickness of 250, 700 and 1100 As . The increase of the emitter thickness from 250 to 700 As improved the spectral response for all wavelengths because the thicker a-Si:H emitter absorbs more incident photons. This con"rms that the a-Si:H layer is photovoltaically active and contributes to the response. In contrast, the cell with an 1100 As thick emitter showed a poorer response, especially for shorter wavelengths, because of the enhanced recombination of the carriers generated by high-energy photons. Meanwhile, the cells with a bu!er layer exhibited a slightly higher red response compared to the cell without a bu!er layer, proposing an additional photon absorption in the bu!er layer. However, the bu!er layer thicker than 200 As showed a relatively low blue response due to recombination as found in the cell with a thick a-Si:H emitter or reduced utilization of high energy photons. Finally, the insertion of a 200 As thick lc-Si bu!er layer into the a-Si:H/c-Si structure revealed the best spectral response of all cells through the additional absorption of photons of less than 2.0 eV, without a degradation of blue response, and modi"cation of the hetero-interface. This corresponds to the highest J value for this solar cell. 5. Conclusion Photovolatic properties of undoped a-Si:H/p-type c-Si heterojunction solar cells with and without a lc-Si bu!er layer were studied. The a-Si:H was e!ectively used for
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better light absorption and defect passivation, whereas the lc-Si was utilized for bandgap grading. The emitter layer was photovoltaically active and strongly in#uenced the solar cell performance. The optimization of the emitter layer was conducted in terms of layer thickness, interface (bandgap grading), and defect passivation. Finally, an optimized solar cell structure of a-Si:H (700 As )/lc-Si (200 As )/c-Si provided the maximum e$ciency of 10% without an antire#ective coating. This is potentially suitable for a low-cost, low-temperature and light-weight solar cell design by replacing the c-Si with thin "lm polycrystalline Si [18].
Acknowledgements This research was sponsored by in part the National Renewable Energy Laboratory, the New York State Energy Research and Development Authority, and National Science Foundation.
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