l O P Publishing | Vietnam Academy of Science and Technology Advances in Natural Sciences: Nanoscience and Nanotechnology Adv. Nat Sci.; Nanosci. Nanolechnol 5 (2014) 045014 (5pp) doiil 0.1088/2043-6262/5/4/045014
Effect of nanowire length on the
performance of silicon nanowires based solar cell
Van Trinh Pham^'^, Mrinal Dutta^, Hung Thang Bui^ and Naoki Fukata^
' Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Road, Cau tjiay Distnct, Hanoi, Vietnam
^Intemational Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
E-mail: [email protected],ac vn and FUKATA,Naoki@ nims.go.jp Received 6 October 2014
Accepted for pubhcation 21 October 2014 Pubhshed 14 November 2014 Abstract
Currently, silicon nanowu^es (SiNWs) are attracting attention as promising candidate matenals for developing the next-generation solar cells to realize both low cost and high efficiency due to their unique structural, electrical, and optical properties. In this paper, a vertical-aUgned SiNWs array has been prepared by metal-assistant chemical etching technique and implemented on SiNW array textured solar cells for photovoltaic apphcation. The shape and size of SiNWs were contiolled by etching time of 30 min, 45 rran and 60 min with the length of SiNWs of 4 p,m, 6 p.m and 8 /im, respectively The etching rate was estimated to be about 133 nm per minute. The optical properties of a SiNWs array with different lengths were investigated in terms of optical reflection property. Less than 6% reflection ratio from 300 nm to 800 nm wavelength was achieved. In addition, I-V characteristic was used to estimate the dependence of the SiNWs length on the performance of SiNWs based solar cell. Conservation efficiencies were achieved of 1,71%, 2.19%, and 2.39% corresponding to 4/im, 6pim and 8 fim SiNWs m length, respectively.
Keywords: sihcon nanowires, SiNW array, solar ceUs, SiNWs based solar cell Mathematics Subject Classiflcation: 4.08, 6.03
1. Introduction SiNWs can be prepared by two mam methods including bottom-up and top-down fabrication Therein, the bottom-up There is gieat demand for the development of the next genera- method usually uses chenucal vapor deposition (CVD) tech- tions of solar cell with higher efficiency, cheaper price and niques with vapor-fiquid-solid (VLS) process for fabricating longer life. Nanomaterials and nanostructures based solar cells the dense, high aspect ratio, and vertically aligned SiNWs [1].
hold promising potency to enhance the performance of solar Moreover, the formation of pn junction and preparation of cells by improvmg both hght trappmg and photo-camer collec- SiNWs can be easily prepared together by VLS process tion. Many ^preaches have been taken to lower the production [10, 11] However, this method always requires a highly cost of Si photovoltaics, among which fiun-fflm Si solar cells protected quality clean room, and expensive CVD systems offer a promising low-cost solution [1, 2], however, dun-film Si lead to an increase of the production cost of material. In solar cells have lower efficiencies than bulk Si, due to then- contrast, the top-down meUiod wiUi chemical etching tech- limited absorption thickness. Si nanowues (SiNWs) have been nique is promoted as a simple technique to prepare the uni- considered as novel class of nanostmctinred materials for high- form, high aspect ratio as well as vertically aligned SiNWs performance devices [3, 4] due to thek unique stmctural, elec- similar to the VLS process, but it seems to remain highly cost- tncal, optical, and tiiermoelectric properties. Recentiy, SiNWs effective in comparison with VLS process [12-14]. To be have been widely mvestigation for developmg next-generation used for solar cell application, SiNWs prepared by chemical solar ceils to reatize both low cost and high efficiency [5-9], etching need doping to form pn junction; doping techniques 2043-6262/14/045014+05$33 00 5 @ 2014 Vietnam Academy of Science & Technology
Adv, Nat. Sci,. Nanosci Nanotechnol 5 (2014)045014
Figure 1. Schematic diagram of the fabncation process of SiNWs based solar cell by wet diffusion process.
such as VLS process [15, 16] and wet diffusion process [17, 18] have been used for this goal. Therein, wet diffusion process using dopant solution has become a promising approach for lowering the cost manufacturing of solar cells.
Therefore, in this paper, we present the results of the investigation of using SiNWs for low cost solar cell with the combination of metal-assisted chemical etching for preparing SiNWs and wet diffusion process for fabncating the radial pn junction. The optical property and l-V characteristics of SiNWs based solar cell are also presented in this paper.
2. Experimental procedure
2.1. Materials
Acetone, etiianol, sulfunc acid (H2SO4), hydrogen peroxide (H2O2), hydro fluonc (HF) acid and nitric acid (HNO3) were supplied by Wako Pure Chemical Industries Co, Silver nitrate (AgNOs) were purchased ft'om Sigma-Aldrich. The p-type Si (100) wafers were produced by Ferrotec Silicon Co. Ohka coating diffusion-source (OCD) solution containing phos- phorus manufactured by Tokyo Chemical Industry Co. Ltd.
Silver and aluminum targets were purchased Irom Kojundo Chemical Laboratory Co.
2.3. Fabrication of solar cell
The fabrication process of SiNW radial pn junction airays shown in figure 1 consists of two main steps: (a) spin-coating OCD solution onto the surface of syndiesized SiNW arrays at a speed of 2000 rpm; (b) annealing of the samples by a thermal annealing system in argon atmosphere at 850 °C for 45 min followed by removal of remaining OCD solution in piranha solution for 4 min and Si02 fihn in I % HF solution for 2 min. After radial pn junction structure was formed, a thin layer of aluminum film with thickness of 200 nm was deposited on the rear side of samples. Then a Ag film with thickness of 600 nm was deposited on the surface of SiNW radial pn junction arrays to form the front electrode via a shadow mask evaporation process in thermal evaporation system.
2.4. Characterization
The morphologies of the samples were characterized by Hitachi S4800 held emission scanning electron microscope (FESEM). Optical reflectance spectra were recorded by Jac- cob V-570 UVA'is/NIR spectrophotometer. The I-V char- acteristic measurement of based solar cell was performed using a solar simulator under Air Mass (AM) 1.5 G illumi- nation with intensity of 100 mW cm"^
2.2. Preparation of SiNWs
SiNW arrays were prepared witii Ag-assisted etching metiiod.
p-type (100) silicon wafers were cleaned widi acetone (5 min) and ethanol (5 mm), rinsed with deionized water (lOmin) 3 times, tiien immersed in a 3:1 mixture of H2SO4 (97%) and H2O2 (30%) for 10 mm, Uioroughly rinsed with deionized water for 10 min, and tiien dipped in HF solution for I min.
The cleaned silicon wafers were immersed into an aqueous HF solution (4.6 M) containing AgNOs (0.02 M) and treated for 30 min, 45 min and 60 min at room temperature. As-pre- pared SiNW samples were rinsed in deionized water and dried at room temperamre. SiNW arrays were treated in HNO3 (35%) for desired durations, and SiNW array of vaned nanowire densities could be obtamed.
3. Results and discussion
Morphologic observation of SiNWs is shown in figure 2, figure 2(a) shows the top-view SEM image of SiNWs deter- mining that the diameter of SiNWs is in the range of 50 to 180 nm. Cross-sectional SEM images of vertically aligned SiNW arrays with different lengths of 4/(m, 6/im and 8/ini were prepared by Ag assistant etching method for 30 min, 45 min and 60 min, respectively. As a result, the lengtiis of SiNWs depended linearly on the etching time with the con- stant etching rate of 133 nm per minute. With the above constant etching rate, SiNWs of deshed length could be prepared easily by controlling the etching time.
Adv Nat. SCI • Nanosci Nanotechnol. S (2014) 045014 V T Pham et al
Figure 2. (a) Top-view SEM image of p-type SiNWs; cross-sectional SEM images of SiNWs widi different length (b) 4 fim. (c) 6 fim and (d) 8 fim corresponding to 30 min, 45 mm and 60 min etching nmes, respectively.
The doping diffiision profile of radial pn juncbon was estimated by the following equation [19]
where D is the diffusivity of P into Si, I is the diffusion time and L is the diffusion depth. Dopant diffusion process was carried out at 850 °C and diffiision time of 45 mm with Z ) = I . 5 x IO"'^cm^s~', the diffiision length was estimated about 20 nm. In addition, the diameter distributions of SiNWs are shown in figure 3 almost remaining from 50 to 180 nm.
As a result, this mdicated that the radial pn junction was formed on SiNWs with n-typed Si about 20 nm acting as shell layer coated p-typed Si core as shown m figure 3(d), There- fore, SiNWs not only acted as a non-reflecting electrode but also acted as radial pn junction array for solar cell structure.
Figure 4 shows Uie spectral reflectivity of SiNW arrays of planar silicon surface and SiNWs with different lengths in wavelength ranging from 200 to 1300 nm. Obtaining from reflective spectrum, the refection of SiNWs sample decreases strongly compared with the planar Si substrate. The reflection of SiNW array remained on average about 10% for the wave lengtii from 200 to 1300 nm From tins result it was demon- strated that the texture stiiicture of SiNWs remained vitally important for reducmg the light reflection on the surface of samples. The reflectance of an SiNW array is dependent on the length of SiNWs; the reflection becomes lower witii
higher length. The low reflectivity from an SiNW array's surface has been attributed to the structural morphology of SiNW arrays, which closely resembles the sub-wavelength structured (SWS) surfaces. In addition, because of the unique morphology tiiere is porosity variation from top to bottom in the arrays, which results in refractive index gradient with depth and, therefore, SiNW arrays effectively act as a multi- layer anti-reflection surface.
To analyze the electrical properties, the l-V curve of the fabricated SiNWs solar cell was measured by solar simulator under AM 1.5 (lOOmAcra"^) illumination. Figure 5 shows the typical light current-voltage curves of SiNWs along with planar Si based solar cell in the 1 cm^ effective area. Solar cell parameters consist of short circuit current density (J^c)' Ofien circuit voltage (Voc), fill factor (FF) and efficiency (tf), pre- sented in table 1, The measured results show that the per- formance of SiNWs based solar cell increases since greater length of SiNWs was used. Among these SiNW-based solar cells, the 8//m SiNW-based solar cells had the highest effi- ciency of 2.39%, which is not only nearly twice higher than our planar Si solar cell but also is one of the highest efficiency compared with other SiNWs based solar cells. Additionally, in this case, the corresponding /^c = 11 -76 mA, approximating twice higher dian that of the single-side pohshed short circuit controls (in J^).
Adv, Nat. SCI Nanosci. Nanolechnol, 5 (2014)045014
p typed Si 100 120 140 160
SINW diameters (nm)
Figure 3. Diameter distribution of SiNWs with different etching times (a) 30 min, (b) 45 min, (c) 60 min and (d) core-shell pn junction structure of individual SiNWs after using wet diffusion process.
90 • 80 • 70 -
^ 60 •
§ 50 •
! « > •
20 • 10 •
"
z,
"\ A *' , , « ^ -
" " n«.
planar S I 4 [imSiNWs 6 iimSiNWs StimSiNWs
^^
^
JT-, "" ""
200 400 600 800 1000 Wavelength (nm)
- * - p l a n a r Si, eff = 1 27%
- » - 4 | i m SiNWs. e t f = 1 . 7 1
— • — 6 M m S i N W s , e f f = 2 - 1 9 - * - 8 M m S i N V l / s , e f f = 2 . 3 9
0.2 0.3 Voltage (V) Figure 4. Reflectance of planar Si and SiNWs with different lengtiis
measured by UV-vis spectroscopy in wavelengdi ranging from 200 Figure 5. Measured I-V characteristics of planar Si and different to 1300 nm. l^n^ih SiNWs based solar cell.
Adv. Nat, Sci-: Nanosci. Nanotechnol 5(2014)045014 V T Pham et al Table 1. Solar cell parameters- short cucuit current density (Jsc)^
open cucult voltage (Vo^), fill facior (FF), efficiency (»;).
Cell samples J,^ (mA cm"^) Voc (V) FF (%) >} (%) 0.47 47.02 1.27 0.47 38.54 1.76 0 44 47.62 2,19 0.42 48,91 2.39 Planar Si 5.8
SiNWs (4;jm) 9.73 SiNWs (6 fim) 10.57 SiNWs (%nm) 11.76
4. Conclusion
SiNWs synthesized by metal-assisted chemical etching method can easily be integrated into sihcon solar cell fabn- cation technology and can be a key issue for improving the conversation efficiency by enhancing the hght absorption with texture structures. The reflection is dependent on the length of SiNWs and decreases with higher lengths. The conversation efficiency of an SiNWs based solar cell was improved by reducing tiie light refiection, and achieved 1.71%, 2.19%, and 2.39% cortesponding to 4/im, 6/im and 8/im SiNWs in length, respiectively, which is higher in comparison with the planar Si based solar cell. Wet diffusion process using dopant solution is a cost effective approach to reduce the cost of solar cell energy in fumre. However, the application of SiNWs to solar cell using wet diffusion for making pn juncnon still needs further optimization of various parameters related with SiNW formation, diffusion etc, to improve the performance of the SiNW based solar cell.
Acknowledgments
The audior (VTP) would hke to thank the support of National Institute for Materials Science, Japan for the opportunity to perform this research through NIMS internship fellow program.
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