- Email: [email protected]
amorphous silicon tandem solar cell
Journal of Non-Crystalline Solids 352 (2006) 1847–1850 www.elsevier.com/locate/jnoncrysol
Fabrication of a n–p–p tunnel junction for a protocrystalline silicon multilayer/amorphous silicon tandem solar cell Joonghwan Kwak *, Seong Won Kwon, Koeng Su Lim Department of Electrical Engineering and Computer Science, KAIST, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea Available online 10 March 2006
Abstract We have fabricated a n-lc-Si:H/p-a-Si:H/p-a-SiC:H tunnel junction for a protocrystalline silicon (pc-Si:H) multilayer/amorphous silicon (a-Si:H) tandem solar cell. In order to improve the hole injection from the a-Si:H bottom cell, we insert a thin conductive p-a-Si:H layer between the n-lc-Si:H and p-a-SiC:H layers. We deposit all layers by the photo-CVD method. Due to ion-damage free characteristics, we could obtain high quality films. We measure the current–voltage (I–V) characteristic and activation energy in order to characterize the fabricated tunnel junction. We have applied this n–p–p tunnel junction to a pc-Si:H multilayer/a-Si:H tandem solar cell and achieved 9.24% energy conversion efficiency whereas only 7.84% efficiency was obtained for a tandem solar cell with no p-a-Si:H insertion layer. 2006 Elsevier B.V. All rights reserved. PACS: 84.60.h; 84.60.Jt; 81.15.z; 81.15.Gh; 73.40.c; 73.40.Lq Keywords: Amorphous semiconductors; Silicon; Solar cells; Devices
1. Introduction Tandem solar cells have attracted strong interest owing to their high conversion efficiency and better stability compared to amorphous silicon based single junction solar cells. The tandem solar cell incorporates one or more tunnel junctions at the interface of the n or p layer of the adjacent top and bottom cells. In order to fabricate an efficient tandem solar cell, it is important to design and fabricate a good tunnel junction. The tunnel junction is the region that connects the top and bottom cells. It should show ohmic, not rectifying, behavior to recombine electrons of the top cell and holes of the bottom cell [1]. All photogenerated electrons of the top cell and photogenerated holes of the bottom cell must be recombined at the tunnel junction. If the recombination process does not proceed properly, piled charges will corrupt the electric field inside the cell and the cell performance will be degraded. *
Corresponding author. Tel.: +82 42 869 8027; fax: +82 42 869 8530. E-mail address: [email protected] (J. Kwak).
0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.11.085
In this work, we have fabricated a n-lc-Si:H/p-a-Si:H/ p-a-SiC:H tunnel junction for a pc-Si:H multilayer/a-Si:H tandem cell. In our previous publications, we have reported on a pc-Si:H multilayer solar cell prepared by a mercury (Hg)-sensitized photo-chemical vapor deposition (photoCVD) technique. The pc-Si:H multilayer solar cell has attracted much interest for its excellent light-induced metastability and fast annealing behavior [2,3]. The lightinduced degradation of the amorphous silicon solar cell (Staebler–Wronsky effect) has been a major obstacle to realizing a high conversion efficiency solar cell [4]. Therefore, instead of using a-Si:H solar cell, we have employed a pc-Si:H multilayer solar cell as the top cell for better stability. In order to improve the hole injection from the bottom cell, we insert a conductive p-a-Si:H thin layer between n-lc-Si:H and p-a-SiC:H layers. Because the optimum thickness of the p-a-Si:H insertion layer is very small (<10 nm) so as to prevent severe optical losses [5], a deposition method to control the thickness precisely is necessary. We deposit all layers by the photo-CVD method. In
J. Kwak et al. / Journal of Non-Crystalline Solids 352 (2006) 1847–1850
the photo-CVD method, a UV-light source is used to dissolve the reaction gases [6]. As the deposition rate of photo-CVD is slower than that of the conventional plasma enhanced CVD (PECVD) method, precise control of film thickness is possible. In addition, due to ion-damage free characteristic, high quality films can be obtained. 2. Experimental 2.1. The n–p–p tunnel junction All layers were deposited in a Hg-sensitized photo-CVD system. A low-pressure Hg-sensitized lamp (160 W · 2) with resonance lines of 184.9 nm (intensity at the window: 5 mW/cm2) and 253.7 nm (30 mW/cm2) was used as a UV light source to dissociate the mixture gases. The n-lc-Si:H layer was deposited from SiH4, H2, and PH3 reaction gases at a chamber pressure of 0.798 Torr and a substrate temperature (Tsub) of 250 C. Phosphine doping ratio (PH3/ SiH4) and hydrogen dilution ratio (H2/SiH4) were kept at 3600 ppm and 17, respectively. The p-a-SiC:H layer was deposited from Si2H6, C2H4, and B2H6 reaction gases at a chamber pressure of 0.46 Torr and a Tsub of 250 C. Boron doping ratio (B2H6/Si2H6) and ethylene flow ratio (C2H4/Si2H6) were kept at 30 000 ppm and 0.26, respectively. The p-a-Si:H insertion layer was prepared at a chamber pressure of 0.46 Torr and a Tsub of 250 C from Si2H6 and B2H6. The boron doping ratio corresponded with that of the p-a-SiC:H layer. In order to determine the optimum thickness of the p-a-Si:H insertion layer, we varied the deposition time from 0 to 50 s. In order to investigate the properties of the tunnel junction, we deposited n-lc-Si:H/p-a-Si:H/p-a-SiC:H films onto SnO2:F coated glass substrates, and measured dark current–voltage (I–V) and activation energy (Ea). The sample area of the devices was 0.09 cm2 (3 mm · 3 mm). In order to determine the thickness and refractive index of the p-a-Si:H insertion layer, we fitted experimental Psi– Delta (W, D) data measured by a spectroscopic ellipsometer using the Tauc–Lorentz dispersion model [7]. 2.2. The pc-Si:H multilayer/a-Si:H tandem solar cell The intrinsic layer (i-layer) of the pc-Si:H multilayer top cell consists of undiluted a-Si:H sublayers and highly H2diluted a-Si:H sublayers [2]. These sublayers were deposited by toggling the SiH4/H2 flow rate from 18/0 to 1/19 sccm alternately. The chamber pressure, substrate temperature, and Hg bath temperature were 0.65 Torr, 250 C, and 20 C, respectively. The deposition time of undiluted and highly H2-diluted a-Si:H sublayers was 1 and 5 min, respectively. We repeated this process twice. The i-layer thickness ˚ . The of the pc-Si:H multilayer top cell was about 800 A i-layer of the a-Si:H bottom cell was deposited at a chamber pressure of 0.24 Torr and a SiH4 flow rate of 12 sccm. The i-layer thickness of the a-Si:H bottom cell was about ˚ . The thicknesses of the cells were chosen such that 6000 A
they would provide appropriate short-circuit current density for the tandem solar cell. The n, p layer deposition conditions of the pc-Si:H multilayer top cell and a-Si:H bottom cell corresponded with those of the n-lc-Si:H and p-a-SiC:H layers of the tunnel junction. In order to enhance the electric field inside the cell, we employed a buffer layer between the p and i-layer. The final structure of the tandem cell is glass/SnO2:F/p-a-SiC:H/p, i-buffer/i-pc-Si:H multilayer/n-lc-Si:H/p-a-Si:H/p-a-SiC:H/ p,i-buffer/i-a-Si:H/n-lc-Si:H/Al. The bold-faced writing represents the n–p–p tunnel junction. The sample area of the devices is 0.09 cm2. The cell characteristics were measured under 100 mW/ cm2 (AM1.5) solar simulator irradiation. 3. Results 3.1. The n–p–p tunnel junction We fabricated four samples, varying the deposition time of the p-a-Si:H insertion layer from 0 to 50 s. The thicknesses of the p-a-Si:H insertion layers deposited for 20, ˚ , respectively. 30, and 50 s were 54.1, 71.5, and 78.6 A Fig. 1 shows the voltage dependence of normalized resistance (R/Rmax V) at 25 C for the four samples. This indicates the degree of non-ohmic behavior of the samples. The sample with p-a-Si:H deposited for 30 s shows nearly ohmic behavior. The maximum resistance was reduced by about 93% from 34.6 to 2.27 X/cm2 by inserting the p-aSi:H layer. Fig. 2 shows the temperature dependence of conductivity. The conductivity of the samples with the p-a-Si:H insertion layer is higher than that of the sample with no p-a-Si:H insertion layer by a factor of two. The highest conductivity was obtained from the sample with p-a-Si:H deposited for 30 s. Fig. 3 shows the Ea of each sample derived from Fig. 2. The Ea indicates the level of the carrier transport barrier crossing the tunnel junction. The lowest Ea was obtained Normalized Resistance (R/Rmax )
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0 sec 20 sec 30 sec 50 sec
0.1
-0.4
-0.2
0.0
0.2
0.4
Voltage (V) Fig. 1. Voltage dependence of normalized resistance (R/Rmax V) of n-lc-Si:H/p-a-Si:H/p-a-SiC:H tunnel junction according to p-a-Si:H insertion layer deposition time. This value indicates the degree of non-ohmic behavior of samples.
J. Kwak et al. / Journal of Non-Crystalline Solids 352 (2006) 1847–1850 1.0
Collection Efficiency
ln (conductivity)
9 8 7
0 sec 20 sec 30 sec 50 sec
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1/T (K )
Activation Energy (eV)
0.25
0.20
0.15
10
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30
0.4 0.2
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600
700
800
Wavelength (nm)
Fig. 2. Conductivity dependence of temperature (ln(conductivity) vs. 1/T) of n-lc-Si:H/p-a-Si:H/p-a-SiC:H tunnel junction according to p-a-Si:H insertion layer deposition time.
0
2
0 sec (7.59 mA/cm ) 2 20 sec (7.39 mA/cm ) 2 30 sec (7.36 mA/cm ) 2 50 sec (7.23 mA/cm )
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0.0028 0.0029 0.0030 0.0031 0.0032 0.0033
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Fig. 5. Collection efficiency of a-Si:H bottom cell according to p-a-Si:H insertion layer deposition time. The current density of a-Si:H bottom cells is indicated.
(Voc = 1.81 V, Jsc = 7.13 mA/cm2, FF = 0.715). This is much higher than that of the cell with no p-a-Si:H insertion layer, i.e., 7.84% (Voc = 1.65 V, Jsc = 7.06 mA/cm2, FF = 0.674). Fig. 5 depicts the collection efficiency of the a-Si:H bottom cell. As the thickness of the p-a-Si:H insertion layer is increased, the light that can reach the bottom cell is decreased and Jsc of bottom cell is decreased from 7.59 mA/cm2 to 7.23 mA/cm2.
Deposition Time (sec) Fig. 3. Activation energy of n-lc-Si:H/p-a-Si:H/p-a-SiC:H tunnel junction according to p-a-Si:H insertion layer deposition time.
for the sample with p-a-Si:H deposited for 30 s. This value is almost half of the Ea of the sample with no p-a-Si:H insertion layer. 3.2. The pc-Si:H multilayer/a-Si:H tandem solar cells
Current Density (mA/cm2)
We have applied a n–p–p tunnel junction to pc-Si:H multilayer/a-Si:H tandem solar cells. Fig. 4 displays current density–voltage (J–V) characteristics of the fabricated tandem solar cells under 100 mW/cm2 (AM1.5) solar simulator irradiation. The cell with a p-a-Si:H layer deposited for 30 s achieved the highest conversion efficiency, 9.24% 8 6 0 sec 20 sec 30 sec 50 sec
4 2 0 0.0
0.3
0.6
0.9
1.2
1.5
1.8
Voltage (V) Fig. 4. Current density–voltage characteristics of pc-Si:H multilayer/ a-Si:H tandem solar cell according to p-a-Si:H insertion layer deposition time under 100 mW/cm2 (AM 1.5) solar simulator irradiation.
4. Discussion From Fig. 1, we can identify that the R/Rmax values are symmetric about 0 V. This indicates that the fabricated tunnel junction is not operating as a n/p diode. The low activation energy of samples with a thin p-a-Si:H layer insertion layer indicates that the fabricated tunnel junction improves the carrier transport at the conjunction of the top and bottom cell. In Fig. 4, the weak inflection near Voc, which indicates a junction opposing the photocurrent, is displayed for the sample with no p-a-Si:H insertion layer. In contrast, this inflection is not observed for the samples with a p-a-Si:H insertion layer. We have improved the cell performance by inserting a conductive p-a-Si:H thin layer between the n-lc-Si:H and p-a-SiC:H layers. This improvement is explained as follows. The p-a-Si:H insertion layer has higher conductivity than the p-a-SiC:H layer because of the absence of carbon addition. Therefore, it can provide many more holes to recombine with the electrons from the n-layer of the top cell [8]. The inserted p-a-Si:H layer enhances the injection of holes. We measured the refractive index of p-a-Si:H and p-a-SiC:H using a spectroscopic ellipsometer. Cody proposed the application of a constant dipole matrix element rather than a constant momentum matrix element as a modification of Tauc theory. It is known that the resulting {a(E)n(E)/E}1/2 / (E Eg) equation fits the absorption onset of amorphous silicon better than a Tauc plot [9].
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J. Kwak et al. / Journal of Non-Crystalline Solids 352 (2006) 1847–1850
method, which can be utilized to precisely control the thickness, is suitable for fabricating the tunnel junction.
( (E)n(E) / E)
1/2
1000 800 600
p-a-SiC:H fitting line p-a-Si:H fitting line
5. Conclusions
400 200 1.83 eV
0 1
2.14 eV
2
3
4
5
Photon Energy (eV) Fig. 6. Optical band gap comparison between p-a-Si:H and p-a-SiC:H from Cody’s plot. The Cody gap equation {a(E)n(E)/E}1/2 / (E Eg) fits the absorption onset of amorphous silicon better than the Tauc plot.
We drew a Cody’s plot to fit the optical band gap of p-aSi:H and p-a-SiC:H. The optical band gap of p-a-Si:H is 1.82 eV. This is smaller than that of p-a-SiC:H, i.e., 2.13 eV (see Fig. 6). This band gap difference between p-a-Si:H and p-a-SiC:H results in a graded valence band edge. This valence band edge grading drives the holes toward p-a-Si:H and enhances recombination [10]. As a result, the recombination rate increases and the number of piled charges that corrupt the electric field inside the cell decreases and Voc, Jsc, and FF of the cell are improved. It should be noted that the efficiencies are very sensitive to the p-a-Si:H insertion layer thickness. The thicknesses of p-a-Si:H layers deposited for 20, 30, and 50 s were 54.1, ˚ , respectively. When the p-a-Si:H layer 71.5, and 78.6 A ˚ ) or decreased thickness is slightly increased (80 A ˚ ˚ ), the cell (55 A) relative to the optimum thickness (70 A performance starts to degrade. Therefore, the photo-CVD
We have fabricated a n-lc-Si:H/p-a-Si:H/p-a-SiC:H tunnel junction for a pc-Si:H multilayer/a-Si:H tandem solar cell. In order to improve the hole injection from the bottom cell, we inserted a conductive p-a-Si:H layer between the n-lc-Si:H and p-a-SiC:H layers. We have applied this n–p–p tunnel junction to a pc-Si:H multilayer/a-Si:H tandem solar cell and obtained 9.24% energy conversion efficiency. This is much higher than the efficiency realized for a tandem solar cell with no p-a-Si:H insertion layer, i.e., 7.84%. References [1] X. Deng, E.A. Schiff, Handbook of Photovoltaic Engineering, John Wiley, 2002, p. 45 (Chapter: Amorphous Silicon Related Solar Cells). [2] K.H. Jun, J.D. Ouwens, R.E.I. Schropp, J.Y. Lee, J.H. Choi, H.S. Lee, K.S. Lim, J. Appl. Phys. 88 (2000) 4881. [3] S.Y. Myong, S.W. Kwon, K.S. Lim, M. Konagai, Sol. Energy Mater. Sol. Cells 59 (1999) 133. [4] D.L. Staebler, C.R. Wronski, Appl. Phys. Lett. 31 (1977) 292. [5] F.A. Rubinelli, J.K. Rath, R.E.I. Schropp, J. Appl. Phys. 89 (2001) 4010. [6] C.H. Lee, PhD thesis, Korea Institute of Science and Technology, 2000. [7] G.E. Jellison Jr., F.A. Modine, Appl. Phys. Lett. 69 (1996) 371. [8] S.S. Hegedus, F. Kampas, J. Xi, Appl. Phys. Lett. 67 (1995) 813. [9] G.D. Cody, in: J.I. Pankove (Ed.), In Semiconductors and Semimetals, Academic, Orlando, FL, 1984, p. 11. [10] J.Y. Hou, thesis in Engineering Science and Mechanics, The Pennsylvania State University, 1993.
Fabrication of a n–p–p tunnel junction for a protocrystalline silicon multilayer/amorphous silicon tandem solar cell Joonghwan Kwak *, Seong Won Kwon, Koeng Su Lim Department of Electrical Engineering and Computer Science, KAIST, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea Available online 10 March 2006
Abstract We have fabricated a n-lc-Si:H/p-a-Si:H/p-a-SiC:H tunnel junction for a protocrystalline silicon (pc-Si:H) multilayer/amorphous silicon (a-Si:H) tandem solar cell. In order to improve the hole injection from the a-Si:H bottom cell, we insert a thin conductive p-a-Si:H layer between the n-lc-Si:H and p-a-SiC:H layers. We deposit all layers by the photo-CVD method. Due to ion-damage free characteristics, we could obtain high quality films. We measure the current–voltage (I–V) characteristic and activation energy in order to characterize the fabricated tunnel junction. We have applied this n–p–p tunnel junction to a pc-Si:H multilayer/a-Si:H tandem solar cell and achieved 9.24% energy conversion efficiency whereas only 7.84% efficiency was obtained for a tandem solar cell with no p-a-Si:H insertion layer. 2006 Elsevier B.V. All rights reserved. PACS: 84.60.h; 84.60.Jt; 81.15.z; 81.15.Gh; 73.40.c; 73.40.Lq Keywords: Amorphous semiconductors; Silicon; Solar cells; Devices
1. Introduction Tandem solar cells have attracted strong interest owing to their high conversion efficiency and better stability compared to amorphous silicon based single junction solar cells. The tandem solar cell incorporates one or more tunnel junctions at the interface of the n or p layer of the adjacent top and bottom cells. In order to fabricate an efficient tandem solar cell, it is important to design and fabricate a good tunnel junction. The tunnel junction is the region that connects the top and bottom cells. It should show ohmic, not rectifying, behavior to recombine electrons of the top cell and holes of the bottom cell [1]. All photogenerated electrons of the top cell and photogenerated holes of the bottom cell must be recombined at the tunnel junction. If the recombination process does not proceed properly, piled charges will corrupt the electric field inside the cell and the cell performance will be degraded. *
Corresponding author. Tel.: +82 42 869 8027; fax: +82 42 869 8530. E-mail address: [email protected] (J. Kwak).
0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.11.085
In this work, we have fabricated a n-lc-Si:H/p-a-Si:H/ p-a-SiC:H tunnel junction for a pc-Si:H multilayer/a-Si:H tandem cell. In our previous publications, we have reported on a pc-Si:H multilayer solar cell prepared by a mercury (Hg)-sensitized photo-chemical vapor deposition (photoCVD) technique. The pc-Si:H multilayer solar cell has attracted much interest for its excellent light-induced metastability and fast annealing behavior [2,3]. The lightinduced degradation of the amorphous silicon solar cell (Staebler–Wronsky effect) has been a major obstacle to realizing a high conversion efficiency solar cell [4]. Therefore, instead of using a-Si:H solar cell, we have employed a pc-Si:H multilayer solar cell as the top cell for better stability. In order to improve the hole injection from the bottom cell, we insert a conductive p-a-Si:H thin layer between n-lc-Si:H and p-a-SiC:H layers. Because the optimum thickness of the p-a-Si:H insertion layer is very small (<10 nm) so as to prevent severe optical losses [5], a deposition method to control the thickness precisely is necessary. We deposit all layers by the photo-CVD method. In
J. Kwak et al. / Journal of Non-Crystalline Solids 352 (2006) 1847–1850
the photo-CVD method, a UV-light source is used to dissolve the reaction gases [6]. As the deposition rate of photo-CVD is slower than that of the conventional plasma enhanced CVD (PECVD) method, precise control of film thickness is possible. In addition, due to ion-damage free characteristic, high quality films can be obtained. 2. Experimental 2.1. The n–p–p tunnel junction All layers were deposited in a Hg-sensitized photo-CVD system. A low-pressure Hg-sensitized lamp (160 W · 2) with resonance lines of 184.9 nm (intensity at the window: 5 mW/cm2) and 253.7 nm (30 mW/cm2) was used as a UV light source to dissociate the mixture gases. The n-lc-Si:H layer was deposited from SiH4, H2, and PH3 reaction gases at a chamber pressure of 0.798 Torr and a substrate temperature (Tsub) of 250 C. Phosphine doping ratio (PH3/ SiH4) and hydrogen dilution ratio (H2/SiH4) were kept at 3600 ppm and 17, respectively. The p-a-SiC:H layer was deposited from Si2H6, C2H4, and B2H6 reaction gases at a chamber pressure of 0.46 Torr and a Tsub of 250 C. Boron doping ratio (B2H6/Si2H6) and ethylene flow ratio (C2H4/Si2H6) were kept at 30 000 ppm and 0.26, respectively. The p-a-Si:H insertion layer was prepared at a chamber pressure of 0.46 Torr and a Tsub of 250 C from Si2H6 and B2H6. The boron doping ratio corresponded with that of the p-a-SiC:H layer. In order to determine the optimum thickness of the p-a-Si:H insertion layer, we varied the deposition time from 0 to 50 s. In order to investigate the properties of the tunnel junction, we deposited n-lc-Si:H/p-a-Si:H/p-a-SiC:H films onto SnO2:F coated glass substrates, and measured dark current–voltage (I–V) and activation energy (Ea). The sample area of the devices was 0.09 cm2 (3 mm · 3 mm). In order to determine the thickness and refractive index of the p-a-Si:H insertion layer, we fitted experimental Psi– Delta (W, D) data measured by a spectroscopic ellipsometer using the Tauc–Lorentz dispersion model [7]. 2.2. The pc-Si:H multilayer/a-Si:H tandem solar cell The intrinsic layer (i-layer) of the pc-Si:H multilayer top cell consists of undiluted a-Si:H sublayers and highly H2diluted a-Si:H sublayers [2]. These sublayers were deposited by toggling the SiH4/H2 flow rate from 18/0 to 1/19 sccm alternately. The chamber pressure, substrate temperature, and Hg bath temperature were 0.65 Torr, 250 C, and 20 C, respectively. The deposition time of undiluted and highly H2-diluted a-Si:H sublayers was 1 and 5 min, respectively. We repeated this process twice. The i-layer thickness ˚ . The of the pc-Si:H multilayer top cell was about 800 A i-layer of the a-Si:H bottom cell was deposited at a chamber pressure of 0.24 Torr and a SiH4 flow rate of 12 sccm. The i-layer thickness of the a-Si:H bottom cell was about ˚ . The thicknesses of the cells were chosen such that 6000 A
they would provide appropriate short-circuit current density for the tandem solar cell. The n, p layer deposition conditions of the pc-Si:H multilayer top cell and a-Si:H bottom cell corresponded with those of the n-lc-Si:H and p-a-SiC:H layers of the tunnel junction. In order to enhance the electric field inside the cell, we employed a buffer layer between the p and i-layer. The final structure of the tandem cell is glass/SnO2:F/p-a-SiC:H/p, i-buffer/i-pc-Si:H multilayer/n-lc-Si:H/p-a-Si:H/p-a-SiC:H/ p,i-buffer/i-a-Si:H/n-lc-Si:H/Al. The bold-faced writing represents the n–p–p tunnel junction. The sample area of the devices is 0.09 cm2. The cell characteristics were measured under 100 mW/ cm2 (AM1.5) solar simulator irradiation. 3. Results 3.1. The n–p–p tunnel junction We fabricated four samples, varying the deposition time of the p-a-Si:H insertion layer from 0 to 50 s. The thicknesses of the p-a-Si:H insertion layers deposited for 20, ˚ , respectively. 30, and 50 s were 54.1, 71.5, and 78.6 A Fig. 1 shows the voltage dependence of normalized resistance (R/Rmax V) at 25 C for the four samples. This indicates the degree of non-ohmic behavior of the samples. The sample with p-a-Si:H deposited for 30 s shows nearly ohmic behavior. The maximum resistance was reduced by about 93% from 34.6 to 2.27 X/cm2 by inserting the p-aSi:H layer. Fig. 2 shows the temperature dependence of conductivity. The conductivity of the samples with the p-a-Si:H insertion layer is higher than that of the sample with no p-a-Si:H insertion layer by a factor of two. The highest conductivity was obtained from the sample with p-a-Si:H deposited for 30 s. Fig. 3 shows the Ea of each sample derived from Fig. 2. The Ea indicates the level of the carrier transport barrier crossing the tunnel junction. The lowest Ea was obtained Normalized Resistance (R/Rmax )
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0 sec 20 sec 30 sec 50 sec
0.1
-0.4
-0.2
0.0
0.2
0.4
Voltage (V) Fig. 1. Voltage dependence of normalized resistance (R/Rmax V) of n-lc-Si:H/p-a-Si:H/p-a-SiC:H tunnel junction according to p-a-Si:H insertion layer deposition time. This value indicates the degree of non-ohmic behavior of samples.
J. Kwak et al. / Journal of Non-Crystalline Solids 352 (2006) 1847–1850 1.0
Collection Efficiency
ln (conductivity)
9 8 7
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6 5 4 3
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Activation Energy (eV)
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0.20
0.15
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20
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0.4 0.2
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600
700
800
Wavelength (nm)
Fig. 2. Conductivity dependence of temperature (ln(conductivity) vs. 1/T) of n-lc-Si:H/p-a-Si:H/p-a-SiC:H tunnel junction according to p-a-Si:H insertion layer deposition time.
0
2
0 sec (7.59 mA/cm ) 2 20 sec (7.39 mA/cm ) 2 30 sec (7.36 mA/cm ) 2 50 sec (7.23 mA/cm )
0.6
0.0 400
0.0028 0.0029 0.0030 0.0031 0.0032 0.0033
0.10
0.8
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Fig. 5. Collection efficiency of a-Si:H bottom cell according to p-a-Si:H insertion layer deposition time. The current density of a-Si:H bottom cells is indicated.
(Voc = 1.81 V, Jsc = 7.13 mA/cm2, FF = 0.715). This is much higher than that of the cell with no p-a-Si:H insertion layer, i.e., 7.84% (Voc = 1.65 V, Jsc = 7.06 mA/cm2, FF = 0.674). Fig. 5 depicts the collection efficiency of the a-Si:H bottom cell. As the thickness of the p-a-Si:H insertion layer is increased, the light that can reach the bottom cell is decreased and Jsc of bottom cell is decreased from 7.59 mA/cm2 to 7.23 mA/cm2.
Deposition Time (sec) Fig. 3. Activation energy of n-lc-Si:H/p-a-Si:H/p-a-SiC:H tunnel junction according to p-a-Si:H insertion layer deposition time.
for the sample with p-a-Si:H deposited for 30 s. This value is almost half of the Ea of the sample with no p-a-Si:H insertion layer. 3.2. The pc-Si:H multilayer/a-Si:H tandem solar cells
Current Density (mA/cm2)
We have applied a n–p–p tunnel junction to pc-Si:H multilayer/a-Si:H tandem solar cells. Fig. 4 displays current density–voltage (J–V) characteristics of the fabricated tandem solar cells under 100 mW/cm2 (AM1.5) solar simulator irradiation. The cell with a p-a-Si:H layer deposited for 30 s achieved the highest conversion efficiency, 9.24% 8 6 0 sec 20 sec 30 sec 50 sec
4 2 0 0.0
0.3
0.6
0.9
1.2
1.5
1.8
Voltage (V) Fig. 4. Current density–voltage characteristics of pc-Si:H multilayer/ a-Si:H tandem solar cell according to p-a-Si:H insertion layer deposition time under 100 mW/cm2 (AM 1.5) solar simulator irradiation.
4. Discussion From Fig. 1, we can identify that the R/Rmax values are symmetric about 0 V. This indicates that the fabricated tunnel junction is not operating as a n/p diode. The low activation energy of samples with a thin p-a-Si:H layer insertion layer indicates that the fabricated tunnel junction improves the carrier transport at the conjunction of the top and bottom cell. In Fig. 4, the weak inflection near Voc, which indicates a junction opposing the photocurrent, is displayed for the sample with no p-a-Si:H insertion layer. In contrast, this inflection is not observed for the samples with a p-a-Si:H insertion layer. We have improved the cell performance by inserting a conductive p-a-Si:H thin layer between the n-lc-Si:H and p-a-SiC:H layers. This improvement is explained as follows. The p-a-Si:H insertion layer has higher conductivity than the p-a-SiC:H layer because of the absence of carbon addition. Therefore, it can provide many more holes to recombine with the electrons from the n-layer of the top cell [8]. The inserted p-a-Si:H layer enhances the injection of holes. We measured the refractive index of p-a-Si:H and p-a-SiC:H using a spectroscopic ellipsometer. Cody proposed the application of a constant dipole matrix element rather than a constant momentum matrix element as a modification of Tauc theory. It is known that the resulting {a(E)n(E)/E}1/2 / (E Eg) equation fits the absorption onset of amorphous silicon better than a Tauc plot [9].
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method, which can be utilized to precisely control the thickness, is suitable for fabricating the tunnel junction.
( (E)n(E) / E)
1/2
1000 800 600
p-a-SiC:H fitting line p-a-Si:H fitting line
5. Conclusions
400 200 1.83 eV
0 1
2.14 eV
2
3
4
5
Photon Energy (eV) Fig. 6. Optical band gap comparison between p-a-Si:H and p-a-SiC:H from Cody’s plot. The Cody gap equation {a(E)n(E)/E}1/2 / (E Eg) fits the absorption onset of amorphous silicon better than the Tauc plot.
We drew a Cody’s plot to fit the optical band gap of p-aSi:H and p-a-SiC:H. The optical band gap of p-a-Si:H is 1.82 eV. This is smaller than that of p-a-SiC:H, i.e., 2.13 eV (see Fig. 6). This band gap difference between p-a-Si:H and p-a-SiC:H results in a graded valence band edge. This valence band edge grading drives the holes toward p-a-Si:H and enhances recombination [10]. As a result, the recombination rate increases and the number of piled charges that corrupt the electric field inside the cell decreases and Voc, Jsc, and FF of the cell are improved. It should be noted that the efficiencies are very sensitive to the p-a-Si:H insertion layer thickness. The thicknesses of p-a-Si:H layers deposited for 20, 30, and 50 s were 54.1, ˚ , respectively. When the p-a-Si:H layer 71.5, and 78.6 A ˚ ) or decreased thickness is slightly increased (80 A ˚ ˚ ), the cell (55 A) relative to the optimum thickness (70 A performance starts to degrade. Therefore, the photo-CVD
We have fabricated a n-lc-Si:H/p-a-Si:H/p-a-SiC:H tunnel junction for a pc-Si:H multilayer/a-Si:H tandem solar cell. In order to improve the hole injection from the bottom cell, we inserted a conductive p-a-Si:H layer between the n-lc-Si:H and p-a-SiC:H layers. We have applied this n–p–p tunnel junction to a pc-Si:H multilayer/a-Si:H tandem solar cell and obtained 9.24% energy conversion efficiency. This is much higher than the efficiency realized for a tandem solar cell with no p-a-Si:H insertion layer, i.e., 7.84%. References [1] X. Deng, E.A. Schiff, Handbook of Photovoltaic Engineering, John Wiley, 2002, p. 45 (Chapter: Amorphous Silicon Related Solar Cells). [2] K.H. Jun, J.D. Ouwens, R.E.I. Schropp, J.Y. Lee, J.H. Choi, H.S. Lee, K.S. Lim, J. Appl. Phys. 88 (2000) 4881. [3] S.Y. Myong, S.W. Kwon, K.S. Lim, M. Konagai, Sol. Energy Mater. Sol. Cells 59 (1999) 133. [4] D.L. Staebler, C.R. Wronski, Appl. Phys. Lett. 31 (1977) 292. [5] F.A. Rubinelli, J.K. Rath, R.E.I. Schropp, J. Appl. Phys. 89 (2001) 4010. [6] C.H. Lee, PhD thesis, Korea Institute of Science and Technology, 2000. [7] G.E. Jellison Jr., F.A. Modine, Appl. Phys. Lett. 69 (1996) 371. [8] S.S. Hegedus, F. Kampas, J. Xi, Appl. Phys. Lett. 67 (1995) 813. [9] G.D. Cody, in: J.I. Pankove (Ed.), In Semiconductors and Semimetals, Academic, Orlando, FL, 1984, p. 11. [10] J.Y. Hou, thesis in Engineering Science and Mechanics, The Pennsylvania State University, 1993.