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Graphene based Schottky junction solar cells on patterned silicon-pillararray substrate Tingting Feng, Dan Xie, Yuxuan Lin, Yongyuan Zang, Tianling Ren et al. Citation: Appl. Phys. Lett. 99, 233505 (2011); doi: 10.1063/1.3665404 View online: http://dx.doi.org/10.1063/1.3665404 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v99/i23 Published by the AIP Publishing LLC.
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APPLIED PHYSICS LETTERS 99, 233505 (2011)
Graphene based Schottky junction solar cells on patterned silicon-pillar-array substrate Tingting Feng,1 Dan Xie,1,a) Yuxuan Lin,1 Yongyuan Zang,2 Tianling Ren,1,b) Rui Song,1 Haiming Zhao,1 He Tian,1 Xiao Li,3 Hongwei Zhu,3,4,c) and Litian Liu1
1 Tsinghua National Laboratory for Information Science and Technology (TNList), Institute of Microelectronics, Tsinghua University, Beijing 100084, P. R. China 2 Electrical and Computer Engineering, McGill University, Montreal, Quebec, H3A 2T8, Canada 3 Department of Mechanical Engineering, Tsinghua University, Beijing 100084, P. R. China 4 Center for Nano and Micro Mechanics (CNMM), Tsinghua University, Beijing 100084, P. R. China
(Received 26 September 2011; accepted 13 November 2011; published online 7 December 2011) Graphene-on-silicon Schottky junction solar cells were prepared with pillar-array-patterned silicon substrate. Such patterned substrate showed an anti-reflective characteristic and led to an absorption enhancement of the solar cell, which showed enhanced performance with short-circuit current density, open-circuit voltage, fill factor, and energy conversion efficiency of 464.86 mV, 14.58 mA/cm2, 0.29, and 1.96%, respectively. Nitric acid was used to dope graphene film and the cell performance showed a great improvement with efficiency increasing to 3.55%. This is due to the p-type chemical doping effect of HNO3 which increases the work function and the carrier C 2011 American Institute of Physics. [doi:10.1063/1.3665404] density of graphene. V Graphene, as a 2-dimensional carbon material,1–3 has great potential in electro-optical applications due to its low sheet resistance,4 high transmittance within the ultravioletvisible spectrum,4,5 and outstanding mechanical strength.5,6 Since graphene was first prepared by mechanical exfoliation from highly oriented pyrolytic graphite (HOPG),1,2 large-area single- or few-layer graphene has been synthesized through chemical vapor deposition (CVD),4,7 solution processing,8 epitaxy of SiC substrate,9 etc., which made it especially feasible for graphene to be used as transparent electrodes,4,5,8 display screens,10 transistors,11 and solar cells.12,13 Graphene/ silicon structure has been studied and shows diode characteristics as a Schottky junction.14–17 Using the Schottky effect between graphene and silicon, the graphene/silicon solar cell has been reported.16,17 The graphene/silicon Schottky junction solar cells show great potential in light harvesting and conversion application with the advantage of low cost, facile processibility, and environmental amity. Although the first reported energy conversion efficiency is only about 1.65%,17 far below the requirements for practical application, the performance of such graphene/silicon Schottky junction solar cell could be improved through the optimization of graphene quality and device architecture. In this paper, we designed a patterned silicon substrate which could improve the cell performance of graphene/silicon Schottky junction. It is found that the patterned silicon-pillar-array (SiPA) substrate shows better anti-reflective effect in graphene/silicon Schottky solar cells. Meanwhile, the chemical doping effect of nitric acid (HNO3) on graphene films was studied. It is expected that HNO3 could enhance the conductivity as well as the work function of graphene. Therefore, the cell performances of graphene/ SiPA (G/SiPA) solar cells before and after HNO3 treatment a)
Electronic mail: [email protected]. Electronic mail: [email protected]. c) Electronic mail: [email protected]. b)
0003-6951/2011/99(23)/233505/3/$30.00
were also investigated. Finally, the application of the prepared G/SiPA solar cell used as a photodetector to drive a liquid crystal display (LCD) screen has been demonstrated. SiO2(300 nm)/Si(n(100), 2-3 Xcm) substrate was patterned to obtain 3 3 mm2 windows to define the illumination area of solar cell. Then photolithography and inductive couple plasma (ICP) etching were employed to fabricate the pillar array in the window area. The pillar height was controlled through the etching time. Ti(10 nm)/Au(50 nm) was evaporated around the window as front electrodes while Ti (10 nm)/ Pd (5 nm)/Au(30 nm) on the backside as back electrodes. Graphene film was prepared by CVD method on copper foil, the detailed process could be referred to Ref. 17. Then graphene film was transferred to the top of the SiPA substrate to form a G/SiPA solar cell. The prepared SiPA substrates and the graphene on SiPA were observed by scanning electron microscopy (SEM, Hitachi, s-5500). The reflectance characteristics of SiPA substrates with different pillar height were measured by UV-Vis spectrometer (TU-1901). The photovoltaic properties of solar cells were characterized by a solar simulator (Newport 91195 class A), under air mass 1.5 (AM 1.5G) at an illumination intensity of 100 mW/cm2. Fig. 1(a) shows the top-view SEM image of prepared SiPA substrate, where the diameter of the silicon pillar is 2 lm. Fig. 1(b) shows the graphene transferred on the 200nm-SiPA substrate. The wrinkles of graphene around the circumference are clearly seen. Fig. 1(c) compares the reflectance spectra of the planar silicon and patterned substrates with various pillar height. The results confirm that the SiPA substrates show anti-reflective effect in ultraviolet-visible region, and the substrates with larger pillar height show better reflective suppression performance. The reason is proposed in the inset, where planar silicon substrate has a relatively high reflectance for its mirror-like surface, while the SiPA substrate could make the incident light repeatedly reflected through the side walls of silicon pillars and results
99, 233505-1
C 2011 American Institute of Physics V
Downloaded 31 Aug 2013 to 18.189.37.30. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions
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FIG. 1. (Color online) (a) Top SEM view of prepared silicon pillar with 2 lm diameter. (b) Top SEM view of graphene transferred on the SiPA substrate with the pillar height of 200 nm. (c) The reflectance of SIPA with various pillar height as well as planar silicon substrate. Inset shows the principle of antireflective effect of SiPA substrate. (d) Schematic view of a G/SiPA Schottky solar cell. (e) Photograph of a G/SiPA solar cell with 0.09 cm2 junction area. Silver wire was glued to the front electrode for test with silver paste.
in the enhancement of light absorption. Figs. 1(d) and 1(e) show the schematic diagram and photograph of G/SiPA solar cell, respectively. The current-voltage (I-V) curves in dark and under illumination of one typical solar cell are shown in Fig. 2(a), with the conversion efficiency of 1.96%, higher than that of graphene/planar-Si solar cells (1.65%).17 The Voc is 464.9 mV, which is consistent with the results reported in other works.14,17 The I-V relation of Schottky junction can be described as Eq. (1) eV I ¼ Is exp 1 ; (1) nkT where Is is the reverse saturation current, k is the Boltzmann constant, n is the ideality factor, and e is the electron charge, respectively. n can be approximately extracted from the measured curve by Eq. (2) I e ¼ V: (2) log Is nkT Through calculation, the estimated n is about 7.738. The Jsc is 14.58 mA/cm2, about two folds of the graphene/planar-Si
Appl. Phys. Lett. 99, 233505 (2011)
FIG. 2. (Color online) (a) The I-V curves of one typical G/SiPA solar cell in dark and under illumination. (b) Comparison of I-V curves of the solar cells under illumination with different HNO3 treatment durations. (c) Jsc, (d) Voc, (e) FF, (f) g of three G/SiPA solar cells as a function of HNO3 doping time. All of them increase with the doping time and then reach a saturation state.
cells under the same test conditions in Ref. 17. The improvement of G/SiPA solar cell is attributed to the reflective suppression effect brought about by the patterning of the substrate, which is, as a result, more advantageous for light absorption compared with its planar counterpart. Another possible reason is the increase of the junction area that is attributed to the contact of the graphene with the side walls of silicon pillars, as Fig. 1(b) indicates. HNO3 was usually used for chemical doping of carbon nanotube18,19 and graphene.20 HNO3 treatment has a p-type doping effect and serves to modulate the work function of graphene,21 which would both attenuate series resistance and enhance the built-in potential. The I-V curves with different HNO3 treatment time are shown in Fig. 2(b). The Jsc Voc, fill factor (FF), and g as a function of HNO3 treatment time are shown in Figs. 2(c)–2(f). All the parameters gradually TABLE I. Data for another three G/SiPA samples before and after HNO3 treatment. Sample #4 #5 #6
HNO3 treatment
Voc (mV)
Jsc (mA/cm2)
FF
g (%)
Before After (15 s) Before After (15 s) Before After (15 s)
427.18 434.71 359.33 389.49 434.71 464.87
14.68 16.61 18.04 17.71 11.73 17.55
0.29 0.31 0.36 0.38 0.29 0.37
1.82 2.22 2.33 2.63 1.49 3.03
Downloaded 31 Aug 2013 to 18.189.37.30. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions
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Feng et al.
Appl. Phys. Lett. 99, 233505 (2011)
FIG. 3. (Color online) Application of a G/SiPA device used as a light sensor. The device is connected to a signal amplifier circuit to drive a liquid crystal display screen, which is transparent under high voltage and opaque under low voltage. (a) The screen is opaque when the illumination is weak and (b) the screen becomes transparent when the illumination is strong.
increase with the treatment time before reaching a saturation state. After HNO3 treatment for 100 s, the Voc, Jsc, FF, and g are 487.47 mV, 16.03 mA/cm2, 0.45, and 3.55%, respectively. Our results demonstrate the p-type doping effect of HNO3 treatment: the increase in Voc proves the increase of work function for graphene through HNO3 doping; and the increase of the Jsc reveals the decreased resistivity of the graphene film. Table I shows the cell performances of another three samples before and after HNO3 treatment for 15 s, all of which have shown similar enhancement. A similar work of graphene/planar silicon Schottky junction doped by AuCl3 solution has been reported by Li’s group.14 They demonstrated that the surface potential (work function) of the graphene film can be adjusted by as large as 0.5 eV. The maximum power conversion efficiency of the AuCl3 doped graphene/planar silicon Schottky junction is less than 1%. Compared with their results, our HNO3 doped G/SiPA shows a much greater value of Jsc and cell efficiency. Moreover, it is much more convenient and cheaper to process with HNO3. Figs. 3(a) and 3(b) show the application of our G/SiPA solar cell as a photodetector. One G/SiPA cell is connected with a signal amplifier circuit to drive the LCD screen whose transparency can be controlled by driving voltage. The lamp was employed as a light source with adjustable light intensity. When the light intensity is weak, the G/SiPA photodetector could not supply enough voltage to drive the LCD screen, so it is opaque, as shown in Fig. 3(a). When gradually increasing the light intensity, the LCD screen turns transparent due to the enlargement of the output voltage of G/SiPA cell and the characters behind the screen are observable, as shown in Fig. 3(b). This is attributed to the increase of the voltage generated by G/SiPA cells under increasing light intensity. To conclude, anti-reflective SiPA substrate was introduced in graphene/silicon Schottky junction solar cells, showing energy conversion efficiencies of 1.96%. Chemical doping on graphene with HNO3 is an effective method to improve the cell performance. After treated by HNO3, the energy conversion efficiency of G/SiPA Schottky solar cell increases to 3.55%. Our study demonstrates such graphenebased solar cells possess promising potential in energy harvesting and photo-sensitive applications. This work was supported by the China NSF (61025021, 60936002, 60729308, 51072089, 61011130296, and
61020106006), National Key Projects of Science and Technology (2009ZX02023-001-3, 2011ZX02403-002), Tsinghua National Laboratory for Information Science and Technology (TNList) Cross-discipline Foundation, and Foundation of State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu (KFJJ200904), the International Cooperation Project of the Ministry of Science and Technology of China (2008DFA12000). 1
K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, Science 306, 666 (2004). 2 K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsneison, I. V. Grigorieva, S. V. Dubonos, and A. A. Firsov, Nature 438, 197 (2005). 3 K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov, and A. K. Geim, Proc. Natl. Acad. Sci. U.S.A. 102, 10451 (2005). 4 S. Bae, H. Kim, Y. Lee, X. Xu, J. S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. R. Kim, Y. I. Song et al., Nature Nanotech. 5, 574 (2010). 5 K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J. H. Ahn, P. Kim, J. Y. Choi, and B. H. Hong, Nature 457, 706 (2009). 6 C. Lee, X. Wei, J. W. Kysar, and J. Hone, Science 321, 385 (2008). 7 Y. Lee, S. Bae, H. Jang, S. Jang, S. E. Zhu, S. H. Sim, Y. I. Song, B. H. Hong, and J. H. Ahn. Nano Lett. 10, 490 (2010). 8 H. A. Bacerril, J. Mao, Z. Liu, R. M. Stoltenberg, Z. Bao, and Y. Chen, ACS Nano 2, 463 (2008). 9 C. Virojanadara, R. Yakimova, J. R. Osiecki, M. Syva¨ja¨rvi, R. I. G. Uhrberg, L. I. Johansson, and A. A. Zakharov, Surf. Sci. 603, L87 (2009). 10 C.-K. Huang, Y. Ou, Y. Bie, Q. Zhao, and D. Yu, Appl. Phys. Lett. 98, 263104 (2011). 11 M. P. Levendorf, C. S. Ruiz-Vargas, S. Garg, and J. Park, Nano Lett. 9, 4479 (2009). 12 L. G. D. Arco, Y. Zhang, C. W. Schlenker, K. Ryu, M. E. Thompson, and C. Zhou, ACS Nano 4, 2865 (2010). 13 Q. Liu, Z. Liu, X. Zhang, L. Yang, N. Zhang, G. Pan, S. Yin, Y. Chen, and J. Wei, Adv. Funct. Mater. 19, 894 (2009). 14 Y. Shi, K. K. Kim, A. Reina, M. Hofmann, L.-J. Li, and J. Kong, Acs Nano 4, 2689 (2010). 15 C.-C. Chen, M. Aykol, C.-C. Chang, A. F. J. Levi, and S. B. Cronin, Nano Lett. 11, 1863 (2011). 16 K. Ihm, J. T. Lim, K.-J. Lee, J. W. Kwon, T.-H. Kang, S. Chung, S. Bae, J. H. Kim, B. H. Hong, and G. Y. Yeom, Appl. Phys. Lett. 97, 032113 (2010). 17 X. Li, H. Zhu, K. Wang, A. Cao, J. Wei, C. Li, Y. Jia, Z. Li, X. Li, and D. Wu, Adv. Mater. 22, 2743 (2010). 18 D.-W. Shin, J. H. Lee, Y.-H. Kim, S. M. Yu, S.-Y. Park, and J.-B. Yoo, Nanotechnology 20, 475703 (2009). 19 G.-H. Xu, J.-Q. Huang, Q. Zhang, M.-Q. Zhao, and F. Wei, Appl. Phys. A 103, 403 (2011). 20 A. Kasry, M. A. Kuroda, G. J. Martyna, G. S. Tulevski, and A. A. Bol, ACS Nano 4, 3839 (2010). 21 Y. Shi, K. K. Kim, A. Reina, M. Hofmann, L.-J. Li, and J. Kong, ACS Nano 4, 2689 (2010).
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APPLIED PHYSICS LETTERS 99, 233505 (2011)
Graphene based Schottky junction solar cells on patterned silicon-pillar-array substrate Tingting Feng,1 Dan Xie,1,a) Yuxuan Lin,1 Yongyuan Zang,2 Tianling Ren,1,b) Rui Song,1 Haiming Zhao,1 He Tian,1 Xiao Li,3 Hongwei Zhu,3,4,c) and Litian Liu1
1 Tsinghua National Laboratory for Information Science and Technology (TNList), Institute of Microelectronics, Tsinghua University, Beijing 100084, P. R. China 2 Electrical and Computer Engineering, McGill University, Montreal, Quebec, H3A 2T8, Canada 3 Department of Mechanical Engineering, Tsinghua University, Beijing 100084, P. R. China 4 Center for Nano and Micro Mechanics (CNMM), Tsinghua University, Beijing 100084, P. R. China
(Received 26 September 2011; accepted 13 November 2011; published online 7 December 2011) Graphene-on-silicon Schottky junction solar cells were prepared with pillar-array-patterned silicon substrate. Such patterned substrate showed an anti-reflective characteristic and led to an absorption enhancement of the solar cell, which showed enhanced performance with short-circuit current density, open-circuit voltage, fill factor, and energy conversion efficiency of 464.86 mV, 14.58 mA/cm2, 0.29, and 1.96%, respectively. Nitric acid was used to dope graphene film and the cell performance showed a great improvement with efficiency increasing to 3.55%. This is due to the p-type chemical doping effect of HNO3 which increases the work function and the carrier C 2011 American Institute of Physics. [doi:10.1063/1.3665404] density of graphene. V Graphene, as a 2-dimensional carbon material,1–3 has great potential in electro-optical applications due to its low sheet resistance,4 high transmittance within the ultravioletvisible spectrum,4,5 and outstanding mechanical strength.5,6 Since graphene was first prepared by mechanical exfoliation from highly oriented pyrolytic graphite (HOPG),1,2 large-area single- or few-layer graphene has been synthesized through chemical vapor deposition (CVD),4,7 solution processing,8 epitaxy of SiC substrate,9 etc., which made it especially feasible for graphene to be used as transparent electrodes,4,5,8 display screens,10 transistors,11 and solar cells.12,13 Graphene/ silicon structure has been studied and shows diode characteristics as a Schottky junction.14–17 Using the Schottky effect between graphene and silicon, the graphene/silicon solar cell has been reported.16,17 The graphene/silicon Schottky junction solar cells show great potential in light harvesting and conversion application with the advantage of low cost, facile processibility, and environmental amity. Although the first reported energy conversion efficiency is only about 1.65%,17 far below the requirements for practical application, the performance of such graphene/silicon Schottky junction solar cell could be improved through the optimization of graphene quality and device architecture. In this paper, we designed a patterned silicon substrate which could improve the cell performance of graphene/silicon Schottky junction. It is found that the patterned silicon-pillar-array (SiPA) substrate shows better anti-reflective effect in graphene/silicon Schottky solar cells. Meanwhile, the chemical doping effect of nitric acid (HNO3) on graphene films was studied. It is expected that HNO3 could enhance the conductivity as well as the work function of graphene. Therefore, the cell performances of graphene/ SiPA (G/SiPA) solar cells before and after HNO3 treatment a)
Electronic mail: [email protected]. Electronic mail: [email protected]. c) Electronic mail: [email protected]. b)
0003-6951/2011/99(23)/233505/3/$30.00
were also investigated. Finally, the application of the prepared G/SiPA solar cell used as a photodetector to drive a liquid crystal display (LCD) screen has been demonstrated. SiO2(300 nm)/Si(n(100), 2-3 Xcm) substrate was patterned to obtain 3 3 mm2 windows to define the illumination area of solar cell. Then photolithography and inductive couple plasma (ICP) etching were employed to fabricate the pillar array in the window area. The pillar height was controlled through the etching time. Ti(10 nm)/Au(50 nm) was evaporated around the window as front electrodes while Ti (10 nm)/ Pd (5 nm)/Au(30 nm) on the backside as back electrodes. Graphene film was prepared by CVD method on copper foil, the detailed process could be referred to Ref. 17. Then graphene film was transferred to the top of the SiPA substrate to form a G/SiPA solar cell. The prepared SiPA substrates and the graphene on SiPA were observed by scanning electron microscopy (SEM, Hitachi, s-5500). The reflectance characteristics of SiPA substrates with different pillar height were measured by UV-Vis spectrometer (TU-1901). The photovoltaic properties of solar cells were characterized by a solar simulator (Newport 91195 class A), under air mass 1.5 (AM 1.5G) at an illumination intensity of 100 mW/cm2. Fig. 1(a) shows the top-view SEM image of prepared SiPA substrate, where the diameter of the silicon pillar is 2 lm. Fig. 1(b) shows the graphene transferred on the 200nm-SiPA substrate. The wrinkles of graphene around the circumference are clearly seen. Fig. 1(c) compares the reflectance spectra of the planar silicon and patterned substrates with various pillar height. The results confirm that the SiPA substrates show anti-reflective effect in ultraviolet-visible region, and the substrates with larger pillar height show better reflective suppression performance. The reason is proposed in the inset, where planar silicon substrate has a relatively high reflectance for its mirror-like surface, while the SiPA substrate could make the incident light repeatedly reflected through the side walls of silicon pillars and results
99, 233505-1
C 2011 American Institute of Physics V
Downloaded 31 Aug 2013 to 18.189.37.30. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions
233505-2
Feng et al.
FIG. 1. (Color online) (a) Top SEM view of prepared silicon pillar with 2 lm diameter. (b) Top SEM view of graphene transferred on the SiPA substrate with the pillar height of 200 nm. (c) The reflectance of SIPA with various pillar height as well as planar silicon substrate. Inset shows the principle of antireflective effect of SiPA substrate. (d) Schematic view of a G/SiPA Schottky solar cell. (e) Photograph of a G/SiPA solar cell with 0.09 cm2 junction area. Silver wire was glued to the front electrode for test with silver paste.
in the enhancement of light absorption. Figs. 1(d) and 1(e) show the schematic diagram and photograph of G/SiPA solar cell, respectively. The current-voltage (I-V) curves in dark and under illumination of one typical solar cell are shown in Fig. 2(a), with the conversion efficiency of 1.96%, higher than that of graphene/planar-Si solar cells (1.65%).17 The Voc is 464.9 mV, which is consistent with the results reported in other works.14,17 The I-V relation of Schottky junction can be described as Eq. (1) eV I ¼ Is exp 1 ; (1) nkT where Is is the reverse saturation current, k is the Boltzmann constant, n is the ideality factor, and e is the electron charge, respectively. n can be approximately extracted from the measured curve by Eq. (2) I e ¼ V: (2) log Is nkT Through calculation, the estimated n is about 7.738. The Jsc is 14.58 mA/cm2, about two folds of the graphene/planar-Si
Appl. Phys. Lett. 99, 233505 (2011)
FIG. 2. (Color online) (a) The I-V curves of one typical G/SiPA solar cell in dark and under illumination. (b) Comparison of I-V curves of the solar cells under illumination with different HNO3 treatment durations. (c) Jsc, (d) Voc, (e) FF, (f) g of three G/SiPA solar cells as a function of HNO3 doping time. All of them increase with the doping time and then reach a saturation state.
cells under the same test conditions in Ref. 17. The improvement of G/SiPA solar cell is attributed to the reflective suppression effect brought about by the patterning of the substrate, which is, as a result, more advantageous for light absorption compared with its planar counterpart. Another possible reason is the increase of the junction area that is attributed to the contact of the graphene with the side walls of silicon pillars, as Fig. 1(b) indicates. HNO3 was usually used for chemical doping of carbon nanotube18,19 and graphene.20 HNO3 treatment has a p-type doping effect and serves to modulate the work function of graphene,21 which would both attenuate series resistance and enhance the built-in potential. The I-V curves with different HNO3 treatment time are shown in Fig. 2(b). The Jsc Voc, fill factor (FF), and g as a function of HNO3 treatment time are shown in Figs. 2(c)–2(f). All the parameters gradually TABLE I. Data for another three G/SiPA samples before and after HNO3 treatment. Sample #4 #5 #6
HNO3 treatment
Voc (mV)
Jsc (mA/cm2)
FF
g (%)
Before After (15 s) Before After (15 s) Before After (15 s)
427.18 434.71 359.33 389.49 434.71 464.87
14.68 16.61 18.04 17.71 11.73 17.55
0.29 0.31 0.36 0.38 0.29 0.37
1.82 2.22 2.33 2.63 1.49 3.03
Downloaded 31 Aug 2013 to 18.189.37.30. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions
233505-3
Feng et al.
Appl. Phys. Lett. 99, 233505 (2011)
FIG. 3. (Color online) Application of a G/SiPA device used as a light sensor. The device is connected to a signal amplifier circuit to drive a liquid crystal display screen, which is transparent under high voltage and opaque under low voltage. (a) The screen is opaque when the illumination is weak and (b) the screen becomes transparent when the illumination is strong.
increase with the treatment time before reaching a saturation state. After HNO3 treatment for 100 s, the Voc, Jsc, FF, and g are 487.47 mV, 16.03 mA/cm2, 0.45, and 3.55%, respectively. Our results demonstrate the p-type doping effect of HNO3 treatment: the increase in Voc proves the increase of work function for graphene through HNO3 doping; and the increase of the Jsc reveals the decreased resistivity of the graphene film. Table I shows the cell performances of another three samples before and after HNO3 treatment for 15 s, all of which have shown similar enhancement. A similar work of graphene/planar silicon Schottky junction doped by AuCl3 solution has been reported by Li’s group.14 They demonstrated that the surface potential (work function) of the graphene film can be adjusted by as large as 0.5 eV. The maximum power conversion efficiency of the AuCl3 doped graphene/planar silicon Schottky junction is less than 1%. Compared with their results, our HNO3 doped G/SiPA shows a much greater value of Jsc and cell efficiency. Moreover, it is much more convenient and cheaper to process with HNO3. Figs. 3(a) and 3(b) show the application of our G/SiPA solar cell as a photodetector. One G/SiPA cell is connected with a signal amplifier circuit to drive the LCD screen whose transparency can be controlled by driving voltage. The lamp was employed as a light source with adjustable light intensity. When the light intensity is weak, the G/SiPA photodetector could not supply enough voltage to drive the LCD screen, so it is opaque, as shown in Fig. 3(a). When gradually increasing the light intensity, the LCD screen turns transparent due to the enlargement of the output voltage of G/SiPA cell and the characters behind the screen are observable, as shown in Fig. 3(b). This is attributed to the increase of the voltage generated by G/SiPA cells under increasing light intensity. To conclude, anti-reflective SiPA substrate was introduced in graphene/silicon Schottky junction solar cells, showing energy conversion efficiencies of 1.96%. Chemical doping on graphene with HNO3 is an effective method to improve the cell performance. After treated by HNO3, the energy conversion efficiency of G/SiPA Schottky solar cell increases to 3.55%. Our study demonstrates such graphenebased solar cells possess promising potential in energy harvesting and photo-sensitive applications. This work was supported by the China NSF (61025021, 60936002, 60729308, 51072089, 61011130296, and
61020106006), National Key Projects of Science and Technology (2009ZX02023-001-3, 2011ZX02403-002), Tsinghua National Laboratory for Information Science and Technology (TNList) Cross-discipline Foundation, and Foundation of State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu (KFJJ200904), the International Cooperation Project of the Ministry of Science and Technology of China (2008DFA12000). 1
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