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Characterization of laser carved micro channel polycrystalline silicon solar cell
Solid-State Electronics 61 (2011) 23–28
Contents lists available at ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Characterization of laser carved micro channel polycrystalline silicon solar cell Hsin-Chien Chen a,b,⇑, Liann-Be Chang a, Ming-Jer Jeng a, Chao-Sung Lai a a b
Department of Electronic Engineering & Green Technology Research Center, Chang-Gung University, #259, Winhwa 1st Rd., Kueisan-Taoyuan 333, Taiwan, ROC Department of Computer & Communication, Army Accident ROC, CO32092 No. 113, Sec. 4, Jungshan E. Rd., Jungli City, Taoyuan County, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 27 May 2010 Received in revised form 1 January 2011 Accepted 25 February 2011 Available online 23 March 2011 The review of this paper was arranged by Prof. E. Calleja Keywords: Micro channel Polycrystalline silicon Solar cell Laser carving
a b s t r a c t In this study the efficiency of polycrystalline silicon solar cells was increased carving micro channel structures using a laser. In research to date, micro channel structures on the surface of polycrystalline silicon solar cells have been manufactured and studied. In an experiment polycrystalline silicon solar cell with micro channel structures on the surface demonstrated an increase in efficiency of 0.23–1.50%, as the radius of the micro channel structures varied from 15 lm to 35 lm. Micro channels also improved the Fill Factor of polycrystalline silicon solar cells. However, the efficiency started to decrease when the radius of the micro channel structures was greater than 40 lm. Detailed features of the variation in current voltage of polycrystalline silicon solar cells with micro channels are discussed. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Increasing efficiency and reducing costs are the major studies for solar cell research now. Many reports have claimed to obtain increased efficiency of solar cells, for example by changing material [1–3], through different kinds of layer structures [4–8], solar concentration [9–11] and hybrid type photovoltaic devices [12,13]. Recently there have been several articles discussing the results obtained by changing the surface of solar cells. These articles include a discussion of optimization of natural lithography [14], efficiency enhancement using a textured photonic crystal back reflector [15], a comparison of short current output confirming the correlation between p–n junction depth and texturing [16] and a study of a new texturization process for multicrystalline silicon solar cells [17–19]. Polycrystalline silicon is a key component in solar panel construction. Because the manufacturing process of the polycrystalline silicon solar cell is simple, the cost is relatively low and therefore the importance of the poly silicon solar cell has gradually exceeded the single crystalline silicon solar cell [20]. The efficiency of commercially available polycrystalline silicon solar cells is approximately 13–19%. The micro channel structure is some regular holes which are made on the device surface. The structures are often applied to in⇑ Corresponding author at: Department of Electronic Engineering & Green Technology Research Center, Chang-Gung University, #259, Winhwa 1st Rd., Kueisan-Taoyuan 333, Taiwan, ROC. E-mail addresses: [email protected], [email protected] (H.-C. Chen). 0038-1101/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2011.02.005
crease the luminous efficiency of the Light Emitter Diode [21]. The structure can lead the inner light source of the LED, overcome the total reflection effect, and increase the brightness of the light’s output. If this structure is used on the solar cell surface, we expect that the structure will increase the roughness of the solar cell surface and let sunlight into the solar cell’s inner part to improve the characteristic of the solar cell. This study used laser carving to manufacture micro channel structures on the surface of polycrystalline silicon solar cells, and studied the influence of structure parameters such as channel aperture size, channel arrays and distance between channels on the output characteristics of polycrystalline silicon solar cells. 2. Experiment The solar cell substrate was a p-type polycrystalline silicon; the anti-reflection layer was coated with Si3N4(85 nm). The top electrode was a 2.0 mm wide silver bus bar. The back electrode was a full aluminum back surface together with 4.0 mm wide silver/ aluminum soldering pads. The polycrystalline silicon solar cells were provided by Solartech Energy Corp. The circular micro channel structures were shaped using a laser carving machine (YAG laser, Rofin-Baasel Inc.) on the top surface of the solar cells. The conversion efficiency was measured using a solar simulator under AM 1.5. Fig. 1 shows the solar cell structure with micro channels. Twelve different samples were created using laser carving to fabricate the micro channels. Firstly, Coldraw software was used to design different shapes and sizes of micro channel structures. The second step was to set the laser machine’s parameter. Thirdly,
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H.-C. Chen et al. / Solid-State Electronics 61 (2011) 23–28
Fig. 1. Schematic diagram of the solar cell with micro channel structures.
laser carving was used to shape the micro channel structures on the polycrystalline silicon solar cells. The output characteristics of the carved solar cells were then measured. Fig. 2 shows the surface morphology of the carved samples. The samples (a)–(f) had different micro channel radii. The largest micro channel radius was 50 lm while the smallest radius was 15 lm. The lower radius limit was determined by the carving machine’s light source resolution. There was one micro channel array between the two top electrodes, and the distance between the micro channels was 225 lm, center to center. The sample (g) had micro channels with a radius of 15 lm, two number arrays of micro channels between the two
Fig. 3. Measured efficiency deviation of cells with varying micro channel radii.
electrodes, and the distance between the micro channels was 225 lm, center to center. The samples (h)–(i) also had micro channels with a radius of 25 lm and the distance between the micro channels was 225 lm, center to center. However, two or three arrays of micro channels between two electrodes were present. The
Fig. 2. The surface morphology of each sample (a) micro channel radius is 50 lm, (b) micro channel radius is 45 lm, (c) micro channel radius is 40 lm, (d) micro channel radius is 35 lm, (e) micro channel radius is 25 lm, (f) micro channel radius is 15 lm, (g) micro channel radius is 15 lm, two channel arrays, (h) micro channel radius is 25 lm, two channel arrays, (i) micro channel radius is 25 lm, three channel arrays, (j) micro channel radius is 25 lm, distance between two channels is 137 lm, (k) micro channel radius is 25 lm, distance between two channels is 356 lm, and (l) micro channel radius is 25 lm, distance between two channels is 456 lm.
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H.-C. Chen et al. / Solid-State Electronics 61 (2011) 23–28 Table 1 The cell efficiency and Fill Factor of samples (a)–(f). Sample
a b c d e f
Solar cell without micro channel Solar cell with micro channe1 Efficiency (%)
Standard Deviation of Efficiency r (%)
FF
Radius of channel (lm)
Efficiency (%)
Standard Deviation of Efficiency r
FF
14.69 15.25 14.32 14.35 13.61 14.28
0.0027 0.0006 0.0001 0.0006 0.0007 0.0009
0.707 0.710 0.695 0.702 0.689 0.709
50 45 40 35 25 15
13.01 14.45 13.50 14.58 14.55 15.78
0.0042 0.0005 0.0066 0.0148 0.0002 0.0265
0.633 0.653 0.647 0.676 0.711 0.722
Fig. 4. Measured reflection rate of fabricated solar cell, before and after 15 lm micro channel carving. Fig. 5b. Measured JV characteristics of fabricated solar cells, before and after 25 lm micro channel carving.
Fig. 5a. Measured JV characteristics of the fabricated solar cell, before and after 15 lm micro channel carving.
samples (j)–(k) had micro channels with a radius of 25 lm and one micro channel between the two electrodes, however the distance between micro channels varied from 137 lm to 456 lm. The channel depth was approximately 3.9 lm in all samples. Because the sample is part of coming from different batches and the polycrystalline silicon solar cells have been cut, making the battery inside the uneven distribution of grain boundary, resulting in the efficiency of each sample is different. Therefore, this article is only for the purpose of improving the discussion and research in respect of the relative efficiency of solar cells. We repeat some measurements a few more times to confirm the accuracy of the efficiency of the sample, not coming from mea-
Fig. 5c. Measured JV characteristics of fabricated solar cells, before and after 35 lm micro channel carving.
surement procedures. And we also shows that the Standard Deviation of Efficiency (r) in all Table of the paper.
3. Results and discussion The radius of the micro channel structures is 15 lm on the solar cells, the efficiency displayed optimum results. As shown in Fig. 3 and Table 1, when the carved radius is less than 40 lm, the samples show an increased efficiency. Sample (f), with a 15 lm carved
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H.-C. Chen et al. / Solid-State Electronics 61 (2011) 23–28
will strengthen. Eq. (1) demonstrates the proportional relationship of light intensity to the carrier’s generation rate. Subsequently, Eq. (2) shows that the light current is proportional to the generation rate. Therefore, the efficiency of the solar cell will increase due to the increase of the light current when the trapped photon rate is increased. Simulation results similarly indicated that the current–voltage (IV) characteristic of a solar cell can be increased by using similar structures on the solar cell surface [16]. These results, previously published, also support the experiment’s results
GðxÞ ¼ ca/0 eax
ð1Þ
where G is the generation rate, /0 is the incident light intensity, c is the electro-optic coefficient, a is the absorption coefficient, and x is the depth of the p–n junction
Fig. 5d. Measured JV characteristics of fabricated solar cells, before and after 50 lm micro channel carving.
radius, exhibited the greatest efficiency increase from 14.28% to 15.78%, and its Fill Factor (FF) also improved from 0.709 to 0.722. The improved efficiency of the solar cell was possibly caused by the fabricated micro channel structures on the solar cell, which increased the light absorption (raised the trapped photon rate) due to the textured surface. This can be proven via the reduction of the solar cells reflection rate, as shown in Fig. 4 and the short current density of the solar cell (Jsc) which increased from 0.0344 A/ cm2 to 0.03718 A/cm2, as shown in Figs. 5a, 5b and 5c. When the rate of photon capture increases, the intensity of the incident light
Z Isc ¼ Iph ¼ J pn A ¼ ðJ d þ J p þ J n Þ A ¼ q Gdx þ J p þ J n A
ð2Þ
where Isc is the short current, Ipn is the light current, Jpn is the light current density, A is the area of solar cell, and q is the single electron charge quantity. However, as the radius of the micro channel increased beyond 35 lm, efficiency was found to decrease by 0.82–1.68%. Furthermore, the efficiency was reduced by 1.67% (from 14.69% to 13.01%) when the radius of the micro channel structures were as large as 50 lm and the corresponding FF decreased from 0.707 to 0.633. This phenomena was possibly caused by the aperture size of the micro channel structures being too large and destroying the surface structure of the cell, thereby decreasing the short current (Isc) of the solar cell as shown in Fig. 4d. Moreover, Eq. (3)
Table 2 The efficiency and Fill Factor for varying micro channel arrays per block of electrodes, before and after micro channel carving, with both 15 lm and 25 lm radii. Sample
f g e h 1
Solar cell without micro channel Solar cell with micro channel Efficiency (%)
Standard Deviation of Efficiency r (%)
FF
Line(s) of channel Between two electrode (lm)
Radius of channel (lm)
Efficiency (%)
Standard Deviation of Efficiency r (%)
FF
14.28 14.02 13.62 14.02 14.79
0.0009 0.0010 0.0007 0.0005 0.0012
0.709 0.671 0.689 0.701 0.709
1 2 1 2 3
15 15 25 25 25
15.78 15.35 14.55 14.85 15.36
0.0265 0.003 0.0002 0.0003 0.0094
0.722 0.682 0.711 0.717 0.722
Fig. 6. Measured efficiency deviation with lines per block of micro channels and the corresponding efficiency for a radius of (a) 15 lm and (b) 25 lm.
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H.-C. Chen et al. / Solid-State Electronics 61 (2011) 23–28
Fig. 7. Measured JV characteristic of solar cells before and after each carving (a) with 15 lm radius, single array per block, (b) with 25 lm radius, two arrays per block, and (c) with 25 lm radius, three arrays per block.
Table 3 The cell efficiency and Fill Factor for varying micro channel distances with a 25 lm radius and before and after laser carving. Sample
1 k e J
Solar cell without micro channel Solar cell with micro channe1 Efficiency
Standard Deviation of Efficiency r (%)
FF
Distance between two micro channels (lm)
Radius of channel (lm)
Efficiency (%)
Standard Deviation of Efficiency a1 r (%)
FF
13.97 13.94 13.62 15.12
0.0014 0.0008 0.0007 0.0023
0.705 0.693 0.689 0.712
495 356 225 137
25 25 25 25
14.53 14.54 14.55 14.92
0.0007 0.0053 0.0002 0.0188
0.716 0.694 0.712 0.683
shows that the efficiency of a solar cell is proportional to the Isc of the cell [22]. Therefore, when the Isc was decreased, the efficiency of a solar cell decreased accordingly
g¼
FF V oc Isc 100% Pmax
ð3Þ
Fig. 4a–d show the I–V curves of the solar cells before and after laser carving, with micro channel radii of 15–50 lm. As shown in Fig. 5a, the polycrystalline silicon solar cell with a 15 lm radius micro channel structure had an open voltage (Voc) of 0.59 V and a short current (Isc) that increased from 0.0344 A/cm2 before carving to 0.03718 A/cm2 after carving. When the carved radius of the micro channel structures was 50 lm, the open circuit voltage of the solar cell was 0.58 V but the short current only increased from 0.3562 A/cm2 to 0.3613 A/cm2 as shown in Fig. 5d. The efficiency and the Fill Factor of fabricated solar cells with varying numbers of micro channel arrays between two electrodes
before and after micro channel carving have been obtained. Table 2 shows that regardless of whether the micro channel radius was 15 lm or 25 lm, the Fill Factor decreased with the number of micro channel arrays. Fig. 6a and b report the same result – the efficiency of the polycrystalline silicon solar cell decreased with the number of micro channel arrays. The reason being, the surface roughness is less textured when the density of the micro channel structures increased over a certain value on the solar cell surface. This takes effect once the radius goes above a certain value. Up until this point, as the micro channel structures increased, so does the surface recombination rate, resulting in the light generated electron hole pair partially recombining again and an increase in the parallel resistance is apparent. This is supported by Fig. 7 which shows the current–voltage (IV) curves of the solar cells. A comparison of the efficiency and Fill Factor of solar cells with different distances, center to center, between two micro channels with a 25 lm radius before and after laser carving have been obtained. As shown
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silicon solar cells. According to the research results, when the radius of the micro channel structures is 15 lm and there is one channel array between the two electrodes and the distance between two channels is 225 lm, polycrystalline silicon solar cells display optimum efficiency, IV and FF characteristics. In addition, the method used to produce polycrystalline silicon solar cells with micro channel structures is simple and cheap. References
Fig. 8. Measured efficiency deviation with varied center to center distances between two micro channels and its correspondent efficiency under the radius of 25 lm.
in Table 3 and Fig. 8, both the Fill Factor and the efficiency improved as a result of laser carving. The optimal outcome was found to be when the radius of the micro channel structure was 25 lm, and the distance between two micro channels was 225 lm. The longer the distance between two micro channels, the lower the efficiency of the polycrystalline silicon solar cell. This is due to the reduced surface texture when the micro channel structures on the surface are decreased. On the other hand, as the density increased with the amount of micro channel structures, the antireflection layer was destroyed. Comparatively, the texture density of the solar cell will be reduced, resulting in a lower efficiency and Fill Factor characteristics of polycrystalline silicon solar cells. The results of these experiments once again support the beginning of the experiment outcome. 4. Conclusions This paper analyzed the influence of various micro channel structure laser carving features (channel aperture, channel lines, and channel distance) on the efficiency and FF of polycrystalline
[1] Terakawa Takeshi, Wang Dong, Nakashima Hiroshi. Jpn J Appl Phys 2005;44:4060–1. [2] Moriya Katsuhiko, Tanaka Kunihiko, Uchiki Hisao. Jpn J Appl Phys 2007;46:5780–1. [3] Inazu Takuya, Bhandari Ramesh Kumar, Kadowaki Yuji, Hashimoto Yoshio, Ito Kentaro. Jpn J Appl Phys 2005;44:1204–7. [4] Hwang Jun-Dar, Chi Tzu-Yi, Liu Jun-Chin, Kung Chung-Yuan, Hsein In-Cha. Jpn J Appl Phys 2006;45:7675–6. [5] Grigors K, Krotkus A, Pacentas V, Simkiene I. SPIE, p. 3580158–67. [6] Borchert D, Grabosch G, Fahrner WR. Solar Energy Mater Solar Cells 1997;49:53–9. [7] Ning Cai, Ying Zhao, Chang Chun Wei, Jian Sun, Shaozhen Xing. Proc SPIE 6841Q1-6841Q6. [8] Kiravittaya S, Manmontri U, Sopitpan S, Ratanathamnaphan S, Antarasen C, Sawadsaringkarn M, et al. Solar Energy Mater Solar Cells 2001;68:85–9. [9] Feuermann Daniel, Gordon Jeffrey M. Solar Energy 2001;70:423–30. [10] Yamaguchi M, Luque A. IEEE Trans Electron Dev Electron Dev 1999;46: 2139–44. [11] Chen HC, Chang LB. Electron Lett 2010;46:162–4. [12] Kois J, Bereznev S, Raudoja J, Mellikov E, Opik A. Solar Energy Mater Solar Cells 2005;87:657–65. [13] Huynh Wendy U, Dittmer Janke J, Paul Alivisatos A. Science 2002;295:2425–7. [14] Manshanden P, Burgers AR, Nositschka WA, Voigt O, Weeber AW. In: 29th IEEE PVSC 1P2.9. [15] Zeng L, Yi Y, Hong C, Liu J, Feng N, Duan X, Kimerling LC. Appl Phys Lett 2006;89. [16] Jahanshah F, Sopian K, Zaidi SH, Othman MY, Asim NA. Am J Appl Sci 2009;6:667–71. [17] Basu PK, Dhasmana H, Vdayakumar N, Thakur DK. Renew Energy 2009;34:2571–6. [18] Macdonald DH, Cuevas A, Kerr MJ, Samundsett C, Ruby D, Winderbaum S, et al. Solar Energy 2004;76:277–83. [19] Gangopadhyay U, Dhungel SK, Basn PK, Dutta SK, Saha H, Yi J. Solar Energy Mater Solar Cells 2007;91:285–9. [20] Lin MS. The technology of solar cell. Taiwan: CHWA Ltd.; 2008. [chapter 1]. [21] Chang LB, Chang YH, Jeng MJ. IEEE Technol Lett 2007;19:1175–7. [22] Tsai JI. Phys ROC 2005;27:701–19.
Contents lists available at ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Characterization of laser carved micro channel polycrystalline silicon solar cell Hsin-Chien Chen a,b,⇑, Liann-Be Chang a, Ming-Jer Jeng a, Chao-Sung Lai a a b
Department of Electronic Engineering & Green Technology Research Center, Chang-Gung University, #259, Winhwa 1st Rd., Kueisan-Taoyuan 333, Taiwan, ROC Department of Computer & Communication, Army Accident ROC, CO32092 No. 113, Sec. 4, Jungshan E. Rd., Jungli City, Taoyuan County, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 27 May 2010 Received in revised form 1 January 2011 Accepted 25 February 2011 Available online 23 March 2011 The review of this paper was arranged by Prof. E. Calleja Keywords: Micro channel Polycrystalline silicon Solar cell Laser carving
a b s t r a c t In this study the efficiency of polycrystalline silicon solar cells was increased carving micro channel structures using a laser. In research to date, micro channel structures on the surface of polycrystalline silicon solar cells have been manufactured and studied. In an experiment polycrystalline silicon solar cell with micro channel structures on the surface demonstrated an increase in efficiency of 0.23–1.50%, as the radius of the micro channel structures varied from 15 lm to 35 lm. Micro channels also improved the Fill Factor of polycrystalline silicon solar cells. However, the efficiency started to decrease when the radius of the micro channel structures was greater than 40 lm. Detailed features of the variation in current voltage of polycrystalline silicon solar cells with micro channels are discussed. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Increasing efficiency and reducing costs are the major studies for solar cell research now. Many reports have claimed to obtain increased efficiency of solar cells, for example by changing material [1–3], through different kinds of layer structures [4–8], solar concentration [9–11] and hybrid type photovoltaic devices [12,13]. Recently there have been several articles discussing the results obtained by changing the surface of solar cells. These articles include a discussion of optimization of natural lithography [14], efficiency enhancement using a textured photonic crystal back reflector [15], a comparison of short current output confirming the correlation between p–n junction depth and texturing [16] and a study of a new texturization process for multicrystalline silicon solar cells [17–19]. Polycrystalline silicon is a key component in solar panel construction. Because the manufacturing process of the polycrystalline silicon solar cell is simple, the cost is relatively low and therefore the importance of the poly silicon solar cell has gradually exceeded the single crystalline silicon solar cell [20]. The efficiency of commercially available polycrystalline silicon solar cells is approximately 13–19%. The micro channel structure is some regular holes which are made on the device surface. The structures are often applied to in⇑ Corresponding author at: Department of Electronic Engineering & Green Technology Research Center, Chang-Gung University, #259, Winhwa 1st Rd., Kueisan-Taoyuan 333, Taiwan, ROC. E-mail addresses: [email protected], [email protected] (H.-C. Chen). 0038-1101/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2011.02.005
crease the luminous efficiency of the Light Emitter Diode [21]. The structure can lead the inner light source of the LED, overcome the total reflection effect, and increase the brightness of the light’s output. If this structure is used on the solar cell surface, we expect that the structure will increase the roughness of the solar cell surface and let sunlight into the solar cell’s inner part to improve the characteristic of the solar cell. This study used laser carving to manufacture micro channel structures on the surface of polycrystalline silicon solar cells, and studied the influence of structure parameters such as channel aperture size, channel arrays and distance between channels on the output characteristics of polycrystalline silicon solar cells. 2. Experiment The solar cell substrate was a p-type polycrystalline silicon; the anti-reflection layer was coated with Si3N4(85 nm). The top electrode was a 2.0 mm wide silver bus bar. The back electrode was a full aluminum back surface together with 4.0 mm wide silver/ aluminum soldering pads. The polycrystalline silicon solar cells were provided by Solartech Energy Corp. The circular micro channel structures were shaped using a laser carving machine (YAG laser, Rofin-Baasel Inc.) on the top surface of the solar cells. The conversion efficiency was measured using a solar simulator under AM 1.5. Fig. 1 shows the solar cell structure with micro channels. Twelve different samples were created using laser carving to fabricate the micro channels. Firstly, Coldraw software was used to design different shapes and sizes of micro channel structures. The second step was to set the laser machine’s parameter. Thirdly,
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H.-C. Chen et al. / Solid-State Electronics 61 (2011) 23–28
Fig. 1. Schematic diagram of the solar cell with micro channel structures.
laser carving was used to shape the micro channel structures on the polycrystalline silicon solar cells. The output characteristics of the carved solar cells were then measured. Fig. 2 shows the surface morphology of the carved samples. The samples (a)–(f) had different micro channel radii. The largest micro channel radius was 50 lm while the smallest radius was 15 lm. The lower radius limit was determined by the carving machine’s light source resolution. There was one micro channel array between the two top electrodes, and the distance between the micro channels was 225 lm, center to center. The sample (g) had micro channels with a radius of 15 lm, two number arrays of micro channels between the two
Fig. 3. Measured efficiency deviation of cells with varying micro channel radii.
electrodes, and the distance between the micro channels was 225 lm, center to center. The samples (h)–(i) also had micro channels with a radius of 25 lm and the distance between the micro channels was 225 lm, center to center. However, two or three arrays of micro channels between two electrodes were present. The
Fig. 2. The surface morphology of each sample (a) micro channel radius is 50 lm, (b) micro channel radius is 45 lm, (c) micro channel radius is 40 lm, (d) micro channel radius is 35 lm, (e) micro channel radius is 25 lm, (f) micro channel radius is 15 lm, (g) micro channel radius is 15 lm, two channel arrays, (h) micro channel radius is 25 lm, two channel arrays, (i) micro channel radius is 25 lm, three channel arrays, (j) micro channel radius is 25 lm, distance between two channels is 137 lm, (k) micro channel radius is 25 lm, distance between two channels is 356 lm, and (l) micro channel radius is 25 lm, distance between two channels is 456 lm.
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H.-C. Chen et al. / Solid-State Electronics 61 (2011) 23–28 Table 1 The cell efficiency and Fill Factor of samples (a)–(f). Sample
a b c d e f
Solar cell without micro channel Solar cell with micro channe1 Efficiency (%)
Standard Deviation of Efficiency r (%)
FF
Radius of channel (lm)
Efficiency (%)
Standard Deviation of Efficiency r
FF
14.69 15.25 14.32 14.35 13.61 14.28
0.0027 0.0006 0.0001 0.0006 0.0007 0.0009
0.707 0.710 0.695 0.702 0.689 0.709
50 45 40 35 25 15
13.01 14.45 13.50 14.58 14.55 15.78
0.0042 0.0005 0.0066 0.0148 0.0002 0.0265
0.633 0.653 0.647 0.676 0.711 0.722
Fig. 4. Measured reflection rate of fabricated solar cell, before and after 15 lm micro channel carving. Fig. 5b. Measured JV characteristics of fabricated solar cells, before and after 25 lm micro channel carving.
Fig. 5a. Measured JV characteristics of the fabricated solar cell, before and after 15 lm micro channel carving.
samples (j)–(k) had micro channels with a radius of 25 lm and one micro channel between the two electrodes, however the distance between micro channels varied from 137 lm to 456 lm. The channel depth was approximately 3.9 lm in all samples. Because the sample is part of coming from different batches and the polycrystalline silicon solar cells have been cut, making the battery inside the uneven distribution of grain boundary, resulting in the efficiency of each sample is different. Therefore, this article is only for the purpose of improving the discussion and research in respect of the relative efficiency of solar cells. We repeat some measurements a few more times to confirm the accuracy of the efficiency of the sample, not coming from mea-
Fig. 5c. Measured JV characteristics of fabricated solar cells, before and after 35 lm micro channel carving.
surement procedures. And we also shows that the Standard Deviation of Efficiency (r) in all Table of the paper.
3. Results and discussion The radius of the micro channel structures is 15 lm on the solar cells, the efficiency displayed optimum results. As shown in Fig. 3 and Table 1, when the carved radius is less than 40 lm, the samples show an increased efficiency. Sample (f), with a 15 lm carved
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H.-C. Chen et al. / Solid-State Electronics 61 (2011) 23–28
will strengthen. Eq. (1) demonstrates the proportional relationship of light intensity to the carrier’s generation rate. Subsequently, Eq. (2) shows that the light current is proportional to the generation rate. Therefore, the efficiency of the solar cell will increase due to the increase of the light current when the trapped photon rate is increased. Simulation results similarly indicated that the current–voltage (IV) characteristic of a solar cell can be increased by using similar structures on the solar cell surface [16]. These results, previously published, also support the experiment’s results
GðxÞ ¼ ca/0 eax
ð1Þ
where G is the generation rate, /0 is the incident light intensity, c is the electro-optic coefficient, a is the absorption coefficient, and x is the depth of the p–n junction
Fig. 5d. Measured JV characteristics of fabricated solar cells, before and after 50 lm micro channel carving.
radius, exhibited the greatest efficiency increase from 14.28% to 15.78%, and its Fill Factor (FF) also improved from 0.709 to 0.722. The improved efficiency of the solar cell was possibly caused by the fabricated micro channel structures on the solar cell, which increased the light absorption (raised the trapped photon rate) due to the textured surface. This can be proven via the reduction of the solar cells reflection rate, as shown in Fig. 4 and the short current density of the solar cell (Jsc) which increased from 0.0344 A/ cm2 to 0.03718 A/cm2, as shown in Figs. 5a, 5b and 5c. When the rate of photon capture increases, the intensity of the incident light
Z Isc ¼ Iph ¼ J pn A ¼ ðJ d þ J p þ J n Þ A ¼ q Gdx þ J p þ J n A
ð2Þ
where Isc is the short current, Ipn is the light current, Jpn is the light current density, A is the area of solar cell, and q is the single electron charge quantity. However, as the radius of the micro channel increased beyond 35 lm, efficiency was found to decrease by 0.82–1.68%. Furthermore, the efficiency was reduced by 1.67% (from 14.69% to 13.01%) when the radius of the micro channel structures were as large as 50 lm and the corresponding FF decreased from 0.707 to 0.633. This phenomena was possibly caused by the aperture size of the micro channel structures being too large and destroying the surface structure of the cell, thereby decreasing the short current (Isc) of the solar cell as shown in Fig. 4d. Moreover, Eq. (3)
Table 2 The efficiency and Fill Factor for varying micro channel arrays per block of electrodes, before and after micro channel carving, with both 15 lm and 25 lm radii. Sample
f g e h 1
Solar cell without micro channel Solar cell with micro channel Efficiency (%)
Standard Deviation of Efficiency r (%)
FF
Line(s) of channel Between two electrode (lm)
Radius of channel (lm)
Efficiency (%)
Standard Deviation of Efficiency r (%)
FF
14.28 14.02 13.62 14.02 14.79
0.0009 0.0010 0.0007 0.0005 0.0012
0.709 0.671 0.689 0.701 0.709
1 2 1 2 3
15 15 25 25 25
15.78 15.35 14.55 14.85 15.36
0.0265 0.003 0.0002 0.0003 0.0094
0.722 0.682 0.711 0.717 0.722
Fig. 6. Measured efficiency deviation with lines per block of micro channels and the corresponding efficiency for a radius of (a) 15 lm and (b) 25 lm.
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Fig. 7. Measured JV characteristic of solar cells before and after each carving (a) with 15 lm radius, single array per block, (b) with 25 lm radius, two arrays per block, and (c) with 25 lm radius, three arrays per block.
Table 3 The cell efficiency and Fill Factor for varying micro channel distances with a 25 lm radius and before and after laser carving. Sample
1 k e J
Solar cell without micro channel Solar cell with micro channe1 Efficiency
Standard Deviation of Efficiency r (%)
FF
Distance between two micro channels (lm)
Radius of channel (lm)
Efficiency (%)
Standard Deviation of Efficiency a1 r (%)
FF
13.97 13.94 13.62 15.12
0.0014 0.0008 0.0007 0.0023
0.705 0.693 0.689 0.712
495 356 225 137
25 25 25 25
14.53 14.54 14.55 14.92
0.0007 0.0053 0.0002 0.0188
0.716 0.694 0.712 0.683
shows that the efficiency of a solar cell is proportional to the Isc of the cell [22]. Therefore, when the Isc was decreased, the efficiency of a solar cell decreased accordingly
g¼
FF V oc Isc 100% Pmax
ð3Þ
Fig. 4a–d show the I–V curves of the solar cells before and after laser carving, with micro channel radii of 15–50 lm. As shown in Fig. 5a, the polycrystalline silicon solar cell with a 15 lm radius micro channel structure had an open voltage (Voc) of 0.59 V and a short current (Isc) that increased from 0.0344 A/cm2 before carving to 0.03718 A/cm2 after carving. When the carved radius of the micro channel structures was 50 lm, the open circuit voltage of the solar cell was 0.58 V but the short current only increased from 0.3562 A/cm2 to 0.3613 A/cm2 as shown in Fig. 5d. The efficiency and the Fill Factor of fabricated solar cells with varying numbers of micro channel arrays between two electrodes
before and after micro channel carving have been obtained. Table 2 shows that regardless of whether the micro channel radius was 15 lm or 25 lm, the Fill Factor decreased with the number of micro channel arrays. Fig. 6a and b report the same result – the efficiency of the polycrystalline silicon solar cell decreased with the number of micro channel arrays. The reason being, the surface roughness is less textured when the density of the micro channel structures increased over a certain value on the solar cell surface. This takes effect once the radius goes above a certain value. Up until this point, as the micro channel structures increased, so does the surface recombination rate, resulting in the light generated electron hole pair partially recombining again and an increase in the parallel resistance is apparent. This is supported by Fig. 7 which shows the current–voltage (IV) curves of the solar cells. A comparison of the efficiency and Fill Factor of solar cells with different distances, center to center, between two micro channels with a 25 lm radius before and after laser carving have been obtained. As shown
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silicon solar cells. According to the research results, when the radius of the micro channel structures is 15 lm and there is one channel array between the two electrodes and the distance between two channels is 225 lm, polycrystalline silicon solar cells display optimum efficiency, IV and FF characteristics. In addition, the method used to produce polycrystalline silicon solar cells with micro channel structures is simple and cheap. References
Fig. 8. Measured efficiency deviation with varied center to center distances between two micro channels and its correspondent efficiency under the radius of 25 lm.
in Table 3 and Fig. 8, both the Fill Factor and the efficiency improved as a result of laser carving. The optimal outcome was found to be when the radius of the micro channel structure was 25 lm, and the distance between two micro channels was 225 lm. The longer the distance between two micro channels, the lower the efficiency of the polycrystalline silicon solar cell. This is due to the reduced surface texture when the micro channel structures on the surface are decreased. On the other hand, as the density increased with the amount of micro channel structures, the antireflection layer was destroyed. Comparatively, the texture density of the solar cell will be reduced, resulting in a lower efficiency and Fill Factor characteristics of polycrystalline silicon solar cells. The results of these experiments once again support the beginning of the experiment outcome. 4. Conclusions This paper analyzed the influence of various micro channel structure laser carving features (channel aperture, channel lines, and channel distance) on the efficiency and FF of polycrystalline
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