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Ultra-low reflection loss for silicon nanowire-array-textured based photovoltaic devices
Accepted Manuscript Title: Ultra-low reflection loss for silicon nanowire-array-textured based photovoltaic devices Author: Bohr-Ran Huang Shang-Chao Hung Chun-Hao Hsu Chao-Wei Tu Wen-Luh Yang PII: DOI: Reference:
S0025-5408(16)30121-0 http://dx.doi.org/doi:10.1016/j.materresbull.2016.03.020 MRB 8713
To appear in:
MRB
Received date: Revised date: Accepted date:
19-8-2015 14-3-2016 16-3-2016
Please cite this article as: Bohr-Ran Huang, Shang-Chao Hung, Chun-Hao Hsu, Chao-Wei Tu, Wen-Luh Yang, Ultra-low reflection loss for silicon nanowire-array-textured based photovoltaic devices, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2016.03.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ultra-low reflection loss for silicon nanowire-array-textured based photovoltaic devices
Bohr-Ran Huang1, Shang-Chao Hung2*, Chun-Hao Hsu1, Chao-Wei Tu1 and Wen-Luh Yang3 1
Graduate Institute of Electro-Optical Engineering & Department of Electronic and
Computer Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan. 2
Department of Information Technology & Communication,
Shih Chien University Kaohsiung Campus, Neimen, Kaohsiung 845, Taiwan. 3
Departments of Electronic Engineering, Feng Chia University, Taichung 407, Taiwan.
*Corresponding Author: Shang-Chao Hung E-mail: [email protected] TEL: +886-76678888-4331 Fax: +886-76678888-4332
1
Graphical Abstract
Ultra-low reflection loss for silicon nanowire-array-textured based photovoltaic devices
2
Highlight
Ultra-low reflection loss for silicon nanowire-array-textured based photovoltaic devices
Si-NW arrays on pyramid surface show the lowest reflectivity of ~1.11% with an aspect ratio of ~5.4. A photovoltaic characteristic of the PCE shows ~10.26% with a Jsc of 28.2mA/cm2 and a Voc of 540mV. The PCE shows a dependence on Rsh while the Rs and the NWs aspect ratio are below 2.41 Ω and 10.
3
Abstract A two-step, metal-assisted, electroless etching technique based on the sputtering of silver particles is described, using phosphorus silicate glass, doping, and screen printing processes to fabricate photovoltaic devices. Si-NW arrays on pyramid surfaces show a low reflectivity of ~1.11%, while the average length of the Si-NW arrays is ~375 nm, with an aspect ratio of ~5.4. Superior photovoltaic characteristics of the PCE (~10.26%) with a Jsc of 28.2mA/cm2 and a Voc of 540mV are achieved. Power conversion efficiency shows a dependence on shunt resistance while the series resistance and the NWs aspect ratio are respectively below 2.41 Ω and 10.
KEYWORDS: A. electronic materials, A. nanostructures, B. optical properties, C. electron microscopy, D. electrical properties, D. surface properties.
1. Introduction
The efficiency of solar cells is mainly defined by the optical absorption characteristics of nanowire arrays in the solar spectrum. Anti-reflection and efficient light trapping of incident light play an important role in improving optical absorption. Anti-reflection 4
coatings on solar cells consisting of one or more thin layers of dielectric material with a specially chosen thickness result in zero net reflected light and more optical absorption. However, the manufacturing of multilayer coatings with precise thickness is highly complex. Recently, interest has increased in the use of silicon nanowire (Si-NW) nanostructures for anti-reflection layers in photovoltaic devices due to their excellent anti-reflection and light trapping effects [1, 2]. The incident light will be scattered in the Si-NW arrays decorated surface. Therefore, the scattering effect decreased the transmission and the reflection. Numerical simulations have been used to conduct detailed investigations of the optical absorption of periodic nanowire arrays to facilitate the design and performance optimization of photovoltaic devices [3–5]. Meanwhile, many improvements have been made to the anti-reflection capability of vertically-aligned Si-NW-based photovoltaic devices [6–10] through methods
including
chemical
vapor
deposition
(CVD)
growth
[11–13],
vapor–liquid–solid (VLS) growth [14-15], reactive-ion etching (RIE) [16–18], and metal-assisted electroless etching [19–23]. The Si-NWs produced by these methods exhibit highly different photo-electron conversion characteristics. Among them, Chen et al. reported that the reflectance ratio of the etched Si-NW arrays was below 5% with etching duration more than 2 hours [23]. Huang et al. announced that the reflectivity of solar cells decreased to 3.1% as the length of PSG-doped SiNW arrays 5
increased to 2.93 µm while the electroless etching time is 20 minutes [9]. Longer SiNW arrays produced improved antireflection properties. However, the longer etching duration required process reduces time–efficiency, and longer nanowire structrures could reduce the mechanical strength and increase surface recombination. On the other hand, several studies have found that structurally random Si-NW arrays have more optical absorbability than regularly patterned wire arrays because a randomly distributed wire can produce stronger optical scattering [24, 25]. In addition, many successful approaches are based on the use of textured silicon surfaces [26, 27] in photovoltaic devices. Processing has been reported to reduce optical reflections and thus enhance the efficiency of photovoltaic energy conversion. Therefore, nanowire-based photovoltaic devices with textured surfaces which will reduce the length of Si-NW arrays and time cost, and increase the mechanical strength are extremely promising for the development of low-cost, high-efficiency light harvesting devices.
This study demonstrates the optical and electrical performance of the proposed Si-NWs on textured pyramids in c-Si photovoltaic devices using a two-step, metal-assisted, electroless etching (MAEE) method which is cost-efficient for the fabrication of randomly positioned, non-patterned Si-NWs. Prior to electroless 6
etching, we deposited Ag particles on the textured c-Si surface to control the NW’s length and coverage. The ultralow reflectivities of the Si-NWs/pyramids c-Si indicate that they are promising for use in the low-cost mass-production of photovoltaic devices.
2. Experiment First, the Si (100) wafers (p-type, Boron-doped, resistivity = 1–10Ωcm, thickness = 525+25μm) were cleaned ultrasonically in acetone and isopropyl alcohol for 30 minutes each. The cleaned silicon wafer was then anisotropically etched using a 80°C solution mixture of Potassium hydroxide (KOH), isopropyl alcohol (IPA), and distilled water (DI) (KOH : IPA : DI water = 0.5 g: 3 ml: 27.5 ml) for 1 hour to produce the pyramid structures.
To uniformly cover the silver particles on the surface, the silicon wafer with the pyramid structures is placed in the sputter system with respective settings for DC power, chamber pressure, Argon flow rate, and sputtering time of 20 Watts, 20 mTorr, 20 sccm and 1 minute. Afterwards, the silicon wafer with silver particles is sectioned 7
into small samples (1x1 cm2) and immersed in a solution mixture of 4.8M hydrofluoric acid (HF) aqueous and 0.5M hydrogen peroxide (H2O2) with etching time periods of 30, 60, 90, and 120 seconds.
Following the wet electroless etching process, the as-prepared samples were dipped into a 30 wt.% HNO3 aqueous solution for 60 seconds to remove the capped silver. Finally, the samples were rinsed with distilled water and blown dry using air guns with nitrogen gas. The PSG solution was prepared using the sol-gel method, mixing ethanol (C2H5OH), de-ionized water, phosphorus pentoxide powder (C2H5OH：DI water：P2O5 = 10 ml：0.8 ml：0.7 g) and 2.3 ml of tetraethoxysilane (TEOS) followed by the spin coating of the Si-NW arrays. In the fabrication of solar cells, the PSG/Si-NW samples were heated at 965°C for 10 minutes to form the PSG-doped Si-NW arrays and parasitic layer. The parasitic layer was then removed by immersion in a 10% HF aqueous solution for 2 minutes.
Following the phosphorus diffusion process, the front silver grid electrode was formed by screen-printing and the rear layer was coated with aluminum paste. The samples were then subjected to a co-firing process in a chamber with N2 ambient (100sccm, 5 Torr) at 720°C for 15 minutes to form the back surface field (BSF). 8
Figure 1 presents a schematic drawing of the fabrication of solar cells with Si-NW arrays with pyramid structures (NWs/pyramids c-Si). Following fabrication, the samples’ surface morphologies and microstructures were characterized by field-emission scanning electron microscopy (FESEM, JEOL JSM–6700F) operated at 15kV. Optical reflectance spectra measurements of the wavelength range 300–1100 nm were performed using a UV/VIS/NIR Spectrophotometer (Jasco V–670) equipped with an integrating sphere. The quantum efficiency spectra were examined by using a monochromator and chopped-light system made by Newport.
3. Results and discussion Figures 2(a) to 2(h) shows top-view (left) and cross-sectional (right) FESEM images of the PSG-doped NWs/pyramids c-Si samples of different lengths fabricated with etching times of 30, 60, 90, and 120 seconds. The length of the nanowires increased from a hundred nanometers to near micrometers with increasing etching time. High-density, well-aligned nanowires fully covered the surface of all the pyramid c-Si samples, which suggests that the sputter system could uniformly cover silver particles 9
over the pyramids’ entire surfaces.
Figure 3 shows the reflectance spectra measurements of PSG-doped pyramids c-Si surfaces with and without NW arrays. Obviously, all the NWs/pyramids c-Si samples have lower reflectance than the pyramids c-Si samples, which exhibit relatively high reflectance (14.91%) within the wavelength range 300–1,100 nm (black line in Fig. 3). The NWs/pyramids c-Si significantly reduced the pyramid surface reflection and demonstrated a low average reflection loss of <1.7% (colored line in Fig. 3) within the same wavelength range. In addition, the lowest reflectance was shown in the range of ultraviolet light. These results suggest that the highest sunlight absorption could be attributed to the decreasing reflectance of nanowires at short wavelengths.
Table I and Fig. 4 display the etching time, average length aspect ratio, and corresponding reflectance spectra of the PSG-doped Si-NW arrays. The diameter and average length of these Si-NW arrays were around 68~78 nm and 175-759 nm, respectively. The average length of Si-NWs increased linearly with etching time, with an etching rate ~6.5 nm/s. The reflectance of the pyramid’s c-Si specimen was ~14.91%, which was much greater than all the NWs/pyramid c-Si samples. The data indicates that the extremely large surface area of the Si-NW arrays could produce 10
good antireflection properties [28]. Moreover, the reflectance decreased to ~1.11% while the average length of the Si-NW arrays increased to ~375 nm with an aspect ratio (A.R.) of ~5.4. These results prove that the lowest reflectance of the NWs/pyramid surface can be achieved by modulating the length of Si-NWs by sputtering silver particles.
Figure 5 depicts the illuminated (AM 1.5G) current-voltage (I-V) characteristics of the Si-based photovoltaic devices with and without Si-NW arrays. The short-circuit current density (Jsc) of the pyramids c-Si samples was 22.24 mA/cm2 and the Jsc of the NWs/pyramids c-Si samples climbed to 28.2 mA/cm2 as expected. An increase of 26.8% in Jsc is attributed to reduced reflection loss from the Si-NW arrays, which improves light absorption and results in increased photo generated current.
Figure 6 shows the relationship of the PCE, series resistance (Rs), with the shunt resistance (Rsh) as a function of the Si-NWs array solar cells with various aspect ratios. The Rs value was calculated using the corrected light and dark I-V curve comparison method [29]. The estimated series resistance increased linearly from 1.32 Ω to 2.41 Ω with the Si-NW length. This result suggests increasing the length of the Si-NW arrays weakens the electrical contacts, which might break the connection between the front 11
electrode and the PSG-doped Si-NW arrays. In addition, the minority carrier lifetime decreased as the length increased, so the length of the Si-NW arrays is also a principal determinant of cell performance [28, 30]. This indicates that low aspect ratio (5.4) straight-aligned Si-NW arrays on pyramid structure-based photovoltaic devices provide better photovoltaic characteristics than the pyramid structure one. This finding corresponds with results from other study [9] which indicate that straight-aligned Si-NW arrays structure-based solar cells exhibit better photovoltaic characteristics than those based on a porous or bundled (high aspect ratio) structure. Similar results were also shown by Baeket. al., who achieved a 7.1% PCE performance improvement for silicon NWs solar cells with an aspect ratio of 6.1 [31].
In Fig. 6, the shunt resistance (Rsh) increased from 781 Ω to 1909 Ω while the aspect ratio of NWs increased to ~5.4. Then, the Rsh decreased to 500 Ω while the aspect ratio continues toincrease to ~10.8. Furthermore the PCE was found to be positively correlated with the Rsh. These results suggest that Rsh could have a strong impact on cell performance while the series resistance (Rs) and the NWs aspect ratio are below 2.41 Ω and 10, respectively. The exact reasons for these observations are not clear yet. It is worth mentioning that acid wash before Phosphorous diffusion process can reduce the current leakage at the edges and therefore increase the shunt resistance. 12
This may be the reason why the shunt resistance of pyramid based solar cell is small in this study.
In Fig. 7, the internal quantum efficiency (IQE) spectra were derived from the external quantum efficiency (EQE) in the equation IQE=EQE/(1-Rλ). In the short wavelength region, the IQE of the pyramids c-Si solar cells is higher than that of the NWs/pyramids c-Si photovoltaic devices. Also, the IQE spectra decreased as the aspect ratio increased. This suggests that longer length results in a greater surface recombination loss because of the larger surface area [32, 33]. In the long wavelength region, the IQEs for the NWs/pyramids c-Si photovoltaic devices are increased, indicating that the Si-NW arrays have good absorption properties via the diffraction mechanism [8]. This finding is consistent with reflectance spectra of Fig. 3.
Table II shows the evaluation results for the fabricated photovoltaic device based on the pyramid surface with and without Si-NW arrays. Among these five representative photovoltaic devices, Sample 1 without Si-NW arrays showed the lowest conversion efficiency, confirming that Si-NW arrays on a pyramid surface decrease reflectivity. The fill factor (FF) of the NWs/pyramids c-Si samples was substantially enhanced from 59.91% to 67.42%, and the estimated power conversion efficiency (PCE) 13
dramatically increased from 7.18% to 10.26%, an improvement of 42.90%. Sample 3 showed the best cell efficiency. The fill factor (FF) and open circuit voltage (Voc) for Samples 3 and 4 were similar, but Sample 3 had a higher short circuit current (Jsc) of about 2.71 mA/cm2, indicating improved photo-generation, which could be predicted from the lowest reflectance observed in Fig. 3. Therefore, we could increase Jsc by decreasing reflectance. These results suggest that using a proper aspect ratio Si-NW array on a pyramid surface could enhance the conversion efficiency by increasing the effectiveness of photon trapping.
4. Conclusion This study fabricates Si-NW array-based photovoltaic devices using a two-step metal-assisted electroless etching (MAEE) technique, PSG doping and screen printing. The resulting specimens exhibited a significant reduction in reflectivity (<1.7%) due to the silver particles being uniformly deposited on the pyramid surface by sputtering. The lowest reflectivity (~1.11%) resulted from Si-NW array structures on a pyramid surface with a length of ~375 nm and an aspect ratio of ~5.4.Superior photovoltaic PCE characteristics (~10.26%) were achieved with a Jsc of 28.2mA/cm2 and a Voc of 14
540mV. Moreover, the power conversion efficiency shows a dependence on shunt resistance while the Rs and the NWs aspect ratio are respectively below 2.41 Ω and 10. These results could assist in the development of low-cost, high efficiency c-Si based photovoltaic devices.
Acknowledgements This work was partly supported by the Ministry of Science and Technology of Taiwan under grant No. MOST-104-2221-E-158-003, MOST-105-2221-E-158-005, and Shih Chien University Kaohsiung Campus under contract number of USC-104-05-05008.
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20
Table I. The relationship of the length, aspect ratio, reflectance of PSG-doped Si-NW arrays with diameter of 68-78 nm under various etching time. Electroless
Average lengthof
Aspect ratio
Reflectance
etching time
PSG-doped Si-NW
(A. R.)
(%)
( second )
arrays ( nm )
0
-
-
～14.91
30
～175
～2.5
～1.6
60
～375
～5.4
～1.11
90
～563
～8.1
～1.47
120
～759
～10.8
～1.7
21
Table II. Extractedthe PCE, open-circuitvoltage (Voc), short-circuit current (Jsc), series resistance (Rs), and shunt resistance (Rsh) to the growth time of Si-NW arrays. Sampl e#
Electrole ss
Voc
Jsc
FF 2
(mV)
(mA/cm )
PCE
Rs
Rsh
(%)
(Ω)
(Ω)
etching time (s)
1
0
520±2. 5
22.24±2. 20
59.91±5. 2
7.18±0.0 5
1.32±0. 15
781±176
2
30
530±2. 5
24.82±1. 05
65.47±3. 5
8.61±0.0 9
1.58±0. 22
806.45±15 4
3
60
540±5. 0
28.2±0.8 5
67.42±0. 65
10.26±0. 11
1.8±0.5 6
1909.09±1 05
4
90
530±5. 0
25.49±0. 75
67.03±0. 45
9.03±0.0 9
2.24±0. 1
1470.59±7 4
5
120
525±2.
24.94±0.
58.43±0.
7.56±0.0
2.41±0.
500±55
5
55
35
2
4
22
Figure Captions Figure 1. The schematic drawing for the fabrication of the Si-NW arrays based with pyramid structures (NWs/pyramids c-Si) solar cells. Figure 2. Top-view FESEM images of PSG-doped Si-NW arrays with different length: (a) 175 nm, (c) 375nm, (e) 563nm, (g) 759nm.(b), (d), (f), (h), shows the corresponding cross-sectional FESEM images. Figure 3. The reflectance spectra measurements of PSG-doped pyramid c-Si surfaces and NWs/pyramids c-Si surfaces with various aspect ratios. Inset is the blow-up figure of overlap regions. Figure 4. The relationship of the reflectivity, length and aspect ratio of PSG-doped Si-NW arrays respect to etching time. Figure 5. Illuminated (AM 1.5G) current–voltage (I-V) characteristics of the photovoltaic devices based on pyramid surface with and without Si-NW arrays for different length of 175, 375, 563 and 759 nm with different aspect ratio of 2.5, 5.4, 8.1, 10.8, respectively. Figure 6. The relationships between PCE, series resistance (Rs), and shunt resistance (Rsh) as a function of the aspect ratio of Si-NW arrays. The series resistance is increased as the Si-NW length increases. The PCE shows a dependence on shunt resistance. 23
Figure 7. The internal quantum efficiency (IQE) spectra of the fabricated photovoltaic device based on pyramid surface with and without Si-NW arrays.
24
Figure 1.
25
Figure2.
26
Reflectance (%)
50 40
Pyramid SiNW/Pyramid Aspect ratio = 2.5 Aspect ratio = 5.4 Aspect ratio = 8.1 Aspect ratio = 10.8
30 20 10 0 400
600
800
1000
Wavelength (nm) Figure 3.
27
Figure 4.
28
Figure 5.
29
Figure 6.
30
Figure 7.
31
S0025-5408(16)30121-0 http://dx.doi.org/doi:10.1016/j.materresbull.2016.03.020 MRB 8713
To appear in:
MRB
Received date: Revised date: Accepted date:
19-8-2015 14-3-2016 16-3-2016
Please cite this article as: Bohr-Ran Huang, Shang-Chao Hung, Chun-Hao Hsu, Chao-Wei Tu, Wen-Luh Yang, Ultra-low reflection loss for silicon nanowire-array-textured based photovoltaic devices, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2016.03.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ultra-low reflection loss for silicon nanowire-array-textured based photovoltaic devices
Bohr-Ran Huang1, Shang-Chao Hung2*, Chun-Hao Hsu1, Chao-Wei Tu1 and Wen-Luh Yang3 1
Graduate Institute of Electro-Optical Engineering & Department of Electronic and
Computer Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan. 2
Department of Information Technology & Communication,
Shih Chien University Kaohsiung Campus, Neimen, Kaohsiung 845, Taiwan. 3
Departments of Electronic Engineering, Feng Chia University, Taichung 407, Taiwan.
*Corresponding Author: Shang-Chao Hung E-mail: [email protected] TEL: +886-76678888-4331 Fax: +886-76678888-4332
1
Graphical Abstract
Ultra-low reflection loss for silicon nanowire-array-textured based photovoltaic devices
2
Highlight
Ultra-low reflection loss for silicon nanowire-array-textured based photovoltaic devices
Si-NW arrays on pyramid surface show the lowest reflectivity of ~1.11% with an aspect ratio of ~5.4. A photovoltaic characteristic of the PCE shows ~10.26% with a Jsc of 28.2mA/cm2 and a Voc of 540mV. The PCE shows a dependence on Rsh while the Rs and the NWs aspect ratio are below 2.41 Ω and 10.
3
Abstract A two-step, metal-assisted, electroless etching technique based on the sputtering of silver particles is described, using phosphorus silicate glass, doping, and screen printing processes to fabricate photovoltaic devices. Si-NW arrays on pyramid surfaces show a low reflectivity of ~1.11%, while the average length of the Si-NW arrays is ~375 nm, with an aspect ratio of ~5.4. Superior photovoltaic characteristics of the PCE (~10.26%) with a Jsc of 28.2mA/cm2 and a Voc of 540mV are achieved. Power conversion efficiency shows a dependence on shunt resistance while the series resistance and the NWs aspect ratio are respectively below 2.41 Ω and 10.
KEYWORDS: A. electronic materials, A. nanostructures, B. optical properties, C. electron microscopy, D. electrical properties, D. surface properties.
1. Introduction
The efficiency of solar cells is mainly defined by the optical absorption characteristics of nanowire arrays in the solar spectrum. Anti-reflection and efficient light trapping of incident light play an important role in improving optical absorption. Anti-reflection 4
coatings on solar cells consisting of one or more thin layers of dielectric material with a specially chosen thickness result in zero net reflected light and more optical absorption. However, the manufacturing of multilayer coatings with precise thickness is highly complex. Recently, interest has increased in the use of silicon nanowire (Si-NW) nanostructures for anti-reflection layers in photovoltaic devices due to their excellent anti-reflection and light trapping effects [1, 2]. The incident light will be scattered in the Si-NW arrays decorated surface. Therefore, the scattering effect decreased the transmission and the reflection. Numerical simulations have been used to conduct detailed investigations of the optical absorption of periodic nanowire arrays to facilitate the design and performance optimization of photovoltaic devices [3–5]. Meanwhile, many improvements have been made to the anti-reflection capability of vertically-aligned Si-NW-based photovoltaic devices [6–10] through methods
including
chemical
vapor
deposition
(CVD)
growth
[11–13],
vapor–liquid–solid (VLS) growth [14-15], reactive-ion etching (RIE) [16–18], and metal-assisted electroless etching [19–23]. The Si-NWs produced by these methods exhibit highly different photo-electron conversion characteristics. Among them, Chen et al. reported that the reflectance ratio of the etched Si-NW arrays was below 5% with etching duration more than 2 hours [23]. Huang et al. announced that the reflectivity of solar cells decreased to 3.1% as the length of PSG-doped SiNW arrays 5
increased to 2.93 µm while the electroless etching time is 20 minutes [9]. Longer SiNW arrays produced improved antireflection properties. However, the longer etching duration required process reduces time–efficiency, and longer nanowire structrures could reduce the mechanical strength and increase surface recombination. On the other hand, several studies have found that structurally random Si-NW arrays have more optical absorbability than regularly patterned wire arrays because a randomly distributed wire can produce stronger optical scattering [24, 25]. In addition, many successful approaches are based on the use of textured silicon surfaces [26, 27] in photovoltaic devices. Processing has been reported to reduce optical reflections and thus enhance the efficiency of photovoltaic energy conversion. Therefore, nanowire-based photovoltaic devices with textured surfaces which will reduce the length of Si-NW arrays and time cost, and increase the mechanical strength are extremely promising for the development of low-cost, high-efficiency light harvesting devices.
This study demonstrates the optical and electrical performance of the proposed Si-NWs on textured pyramids in c-Si photovoltaic devices using a two-step, metal-assisted, electroless etching (MAEE) method which is cost-efficient for the fabrication of randomly positioned, non-patterned Si-NWs. Prior to electroless 6
etching, we deposited Ag particles on the textured c-Si surface to control the NW’s length and coverage. The ultralow reflectivities of the Si-NWs/pyramids c-Si indicate that they are promising for use in the low-cost mass-production of photovoltaic devices.
2. Experiment First, the Si (100) wafers (p-type, Boron-doped, resistivity = 1–10Ωcm, thickness = 525+25μm) were cleaned ultrasonically in acetone and isopropyl alcohol for 30 minutes each. The cleaned silicon wafer was then anisotropically etched using a 80°C solution mixture of Potassium hydroxide (KOH), isopropyl alcohol (IPA), and distilled water (DI) (KOH : IPA : DI water = 0.5 g: 3 ml: 27.5 ml) for 1 hour to produce the pyramid structures.
To uniformly cover the silver particles on the surface, the silicon wafer with the pyramid structures is placed in the sputter system with respective settings for DC power, chamber pressure, Argon flow rate, and sputtering time of 20 Watts, 20 mTorr, 20 sccm and 1 minute. Afterwards, the silicon wafer with silver particles is sectioned 7
into small samples (1x1 cm2) and immersed in a solution mixture of 4.8M hydrofluoric acid (HF) aqueous and 0.5M hydrogen peroxide (H2O2) with etching time periods of 30, 60, 90, and 120 seconds.
Following the wet electroless etching process, the as-prepared samples were dipped into a 30 wt.% HNO3 aqueous solution for 60 seconds to remove the capped silver. Finally, the samples were rinsed with distilled water and blown dry using air guns with nitrogen gas. The PSG solution was prepared using the sol-gel method, mixing ethanol (C2H5OH), de-ionized water, phosphorus pentoxide powder (C2H5OH：DI water：P2O5 = 10 ml：0.8 ml：0.7 g) and 2.3 ml of tetraethoxysilane (TEOS) followed by the spin coating of the Si-NW arrays. In the fabrication of solar cells, the PSG/Si-NW samples were heated at 965°C for 10 minutes to form the PSG-doped Si-NW arrays and parasitic layer. The parasitic layer was then removed by immersion in a 10% HF aqueous solution for 2 minutes.
Following the phosphorus diffusion process, the front silver grid electrode was formed by screen-printing and the rear layer was coated with aluminum paste. The samples were then subjected to a co-firing process in a chamber with N2 ambient (100sccm, 5 Torr) at 720°C for 15 minutes to form the back surface field (BSF). 8
Figure 1 presents a schematic drawing of the fabrication of solar cells with Si-NW arrays with pyramid structures (NWs/pyramids c-Si). Following fabrication, the samples’ surface morphologies and microstructures were characterized by field-emission scanning electron microscopy (FESEM, JEOL JSM–6700F) operated at 15kV. Optical reflectance spectra measurements of the wavelength range 300–1100 nm were performed using a UV/VIS/NIR Spectrophotometer (Jasco V–670) equipped with an integrating sphere. The quantum efficiency spectra were examined by using a monochromator and chopped-light system made by Newport.
3. Results and discussion Figures 2(a) to 2(h) shows top-view (left) and cross-sectional (right) FESEM images of the PSG-doped NWs/pyramids c-Si samples of different lengths fabricated with etching times of 30, 60, 90, and 120 seconds. The length of the nanowires increased from a hundred nanometers to near micrometers with increasing etching time. High-density, well-aligned nanowires fully covered the surface of all the pyramid c-Si samples, which suggests that the sputter system could uniformly cover silver particles 9
over the pyramids’ entire surfaces.
Figure 3 shows the reflectance spectra measurements of PSG-doped pyramids c-Si surfaces with and without NW arrays. Obviously, all the NWs/pyramids c-Si samples have lower reflectance than the pyramids c-Si samples, which exhibit relatively high reflectance (14.91%) within the wavelength range 300–1,100 nm (black line in Fig. 3). The NWs/pyramids c-Si significantly reduced the pyramid surface reflection and demonstrated a low average reflection loss of <1.7% (colored line in Fig. 3) within the same wavelength range. In addition, the lowest reflectance was shown in the range of ultraviolet light. These results suggest that the highest sunlight absorption could be attributed to the decreasing reflectance of nanowires at short wavelengths.
Table I and Fig. 4 display the etching time, average length aspect ratio, and corresponding reflectance spectra of the PSG-doped Si-NW arrays. The diameter and average length of these Si-NW arrays were around 68~78 nm and 175-759 nm, respectively. The average length of Si-NWs increased linearly with etching time, with an etching rate ~6.5 nm/s. The reflectance of the pyramid’s c-Si specimen was ~14.91%, which was much greater than all the NWs/pyramid c-Si samples. The data indicates that the extremely large surface area of the Si-NW arrays could produce 10
good antireflection properties [28]. Moreover, the reflectance decreased to ~1.11% while the average length of the Si-NW arrays increased to ~375 nm with an aspect ratio (A.R.) of ~5.4. These results prove that the lowest reflectance of the NWs/pyramid surface can be achieved by modulating the length of Si-NWs by sputtering silver particles.
Figure 5 depicts the illuminated (AM 1.5G) current-voltage (I-V) characteristics of the Si-based photovoltaic devices with and without Si-NW arrays. The short-circuit current density (Jsc) of the pyramids c-Si samples was 22.24 mA/cm2 and the Jsc of the NWs/pyramids c-Si samples climbed to 28.2 mA/cm2 as expected. An increase of 26.8% in Jsc is attributed to reduced reflection loss from the Si-NW arrays, which improves light absorption and results in increased photo generated current.
Figure 6 shows the relationship of the PCE, series resistance (Rs), with the shunt resistance (Rsh) as a function of the Si-NWs array solar cells with various aspect ratios. The Rs value was calculated using the corrected light and dark I-V curve comparison method [29]. The estimated series resistance increased linearly from 1.32 Ω to 2.41 Ω with the Si-NW length. This result suggests increasing the length of the Si-NW arrays weakens the electrical contacts, which might break the connection between the front 11
electrode and the PSG-doped Si-NW arrays. In addition, the minority carrier lifetime decreased as the length increased, so the length of the Si-NW arrays is also a principal determinant of cell performance [28, 30]. This indicates that low aspect ratio (5.4) straight-aligned Si-NW arrays on pyramid structure-based photovoltaic devices provide better photovoltaic characteristics than the pyramid structure one. This finding corresponds with results from other study [9] which indicate that straight-aligned Si-NW arrays structure-based solar cells exhibit better photovoltaic characteristics than those based on a porous or bundled (high aspect ratio) structure. Similar results were also shown by Baeket. al., who achieved a 7.1% PCE performance improvement for silicon NWs solar cells with an aspect ratio of 6.1 [31].
In Fig. 6, the shunt resistance (Rsh) increased from 781 Ω to 1909 Ω while the aspect ratio of NWs increased to ~5.4. Then, the Rsh decreased to 500 Ω while the aspect ratio continues toincrease to ~10.8. Furthermore the PCE was found to be positively correlated with the Rsh. These results suggest that Rsh could have a strong impact on cell performance while the series resistance (Rs) and the NWs aspect ratio are below 2.41 Ω and 10, respectively. The exact reasons for these observations are not clear yet. It is worth mentioning that acid wash before Phosphorous diffusion process can reduce the current leakage at the edges and therefore increase the shunt resistance. 12
This may be the reason why the shunt resistance of pyramid based solar cell is small in this study.
In Fig. 7, the internal quantum efficiency (IQE) spectra were derived from the external quantum efficiency (EQE) in the equation IQE=EQE/(1-Rλ). In the short wavelength region, the IQE of the pyramids c-Si solar cells is higher than that of the NWs/pyramids c-Si photovoltaic devices. Also, the IQE spectra decreased as the aspect ratio increased. This suggests that longer length results in a greater surface recombination loss because of the larger surface area [32, 33]. In the long wavelength region, the IQEs for the NWs/pyramids c-Si photovoltaic devices are increased, indicating that the Si-NW arrays have good absorption properties via the diffraction mechanism [8]. This finding is consistent with reflectance spectra of Fig. 3.
Table II shows the evaluation results for the fabricated photovoltaic device based on the pyramid surface with and without Si-NW arrays. Among these five representative photovoltaic devices, Sample 1 without Si-NW arrays showed the lowest conversion efficiency, confirming that Si-NW arrays on a pyramid surface decrease reflectivity. The fill factor (FF) of the NWs/pyramids c-Si samples was substantially enhanced from 59.91% to 67.42%, and the estimated power conversion efficiency (PCE) 13
dramatically increased from 7.18% to 10.26%, an improvement of 42.90%. Sample 3 showed the best cell efficiency. The fill factor (FF) and open circuit voltage (Voc) for Samples 3 and 4 were similar, but Sample 3 had a higher short circuit current (Jsc) of about 2.71 mA/cm2, indicating improved photo-generation, which could be predicted from the lowest reflectance observed in Fig. 3. Therefore, we could increase Jsc by decreasing reflectance. These results suggest that using a proper aspect ratio Si-NW array on a pyramid surface could enhance the conversion efficiency by increasing the effectiveness of photon trapping.
4. Conclusion This study fabricates Si-NW array-based photovoltaic devices using a two-step metal-assisted electroless etching (MAEE) technique, PSG doping and screen printing. The resulting specimens exhibited a significant reduction in reflectivity (<1.7%) due to the silver particles being uniformly deposited on the pyramid surface by sputtering. The lowest reflectivity (~1.11%) resulted from Si-NW array structures on a pyramid surface with a length of ~375 nm and an aspect ratio of ~5.4.Superior photovoltaic PCE characteristics (~10.26%) were achieved with a Jsc of 28.2mA/cm2 and a Voc of 14
540mV. Moreover, the power conversion efficiency shows a dependence on shunt resistance while the Rs and the NWs aspect ratio are respectively below 2.41 Ω and 10. These results could assist in the development of low-cost, high efficiency c-Si based photovoltaic devices.
Acknowledgements This work was partly supported by the Ministry of Science and Technology of Taiwan under grant No. MOST-104-2221-E-158-003, MOST-105-2221-E-158-005, and Shih Chien University Kaohsiung Campus under contract number of USC-104-05-05008.
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20
Table I. The relationship of the length, aspect ratio, reflectance of PSG-doped Si-NW arrays with diameter of 68-78 nm under various etching time. Electroless
Average lengthof
Aspect ratio
Reflectance
etching time
PSG-doped Si-NW
(A. R.)
(%)
( second )
arrays ( nm )
0
-
-
～14.91
30
～175
～2.5
～1.6
60
～375
～5.4
～1.11
90
～563
～8.1
～1.47
120
～759
～10.8
～1.7
21
Table II. Extractedthe PCE, open-circuitvoltage (Voc), short-circuit current (Jsc), series resistance (Rs), and shunt resistance (Rsh) to the growth time of Si-NW arrays. Sampl e#
Electrole ss
Voc
Jsc
FF 2
(mV)
(mA/cm )
PCE
Rs
Rsh
(%)
(Ω)
(Ω)
etching time (s)
1
0
520±2. 5
22.24±2. 20
59.91±5. 2
7.18±0.0 5
1.32±0. 15
781±176
2
30
530±2. 5
24.82±1. 05
65.47±3. 5
8.61±0.0 9
1.58±0. 22
806.45±15 4
3
60
540±5. 0
28.2±0.8 5
67.42±0. 65
10.26±0. 11
1.8±0.5 6
1909.09±1 05
4
90
530±5. 0
25.49±0. 75
67.03±0. 45
9.03±0.0 9
2.24±0. 1
1470.59±7 4
5
120
525±2.
24.94±0.
58.43±0.
7.56±0.0
2.41±0.
500±55
5
55
35
2
4
22
Figure Captions Figure 1. The schematic drawing for the fabrication of the Si-NW arrays based with pyramid structures (NWs/pyramids c-Si) solar cells. Figure 2. Top-view FESEM images of PSG-doped Si-NW arrays with different length: (a) 175 nm, (c) 375nm, (e) 563nm, (g) 759nm.(b), (d), (f), (h), shows the corresponding cross-sectional FESEM images. Figure 3. The reflectance spectra measurements of PSG-doped pyramid c-Si surfaces and NWs/pyramids c-Si surfaces with various aspect ratios. Inset is the blow-up figure of overlap regions. Figure 4. The relationship of the reflectivity, length and aspect ratio of PSG-doped Si-NW arrays respect to etching time. Figure 5. Illuminated (AM 1.5G) current–voltage (I-V) characteristics of the photovoltaic devices based on pyramid surface with and without Si-NW arrays for different length of 175, 375, 563 and 759 nm with different aspect ratio of 2.5, 5.4, 8.1, 10.8, respectively. Figure 6. The relationships between PCE, series resistance (Rs), and shunt resistance (Rsh) as a function of the aspect ratio of Si-NW arrays. The series resistance is increased as the Si-NW length increases. The PCE shows a dependence on shunt resistance. 23
Figure 7. The internal quantum efficiency (IQE) spectra of the fabricated photovoltaic device based on pyramid surface with and without Si-NW arrays.
24
Figure 1.
25
Figure2.
26
Reflectance (%)
50 40
Pyramid SiNW/Pyramid Aspect ratio = 2.5 Aspect ratio = 5.4 Aspect ratio = 8.1 Aspect ratio = 10.8
30 20 10 0 400
600
800
1000
Wavelength (nm) Figure 3.
27
Figure 4.
28
Figure 5.
29
Figure 6.
30
Figure 7.
31