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Efficient light trapping nanopyramid structures for solar cells patterned using UV nanoimprint lithography
Materials Science in Semiconductor Processing 57 (2017) 54–58
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Short communication
Efficient light trapping nanopyramid structures for solar cells patterned using UV nanoimprint lithography
crossmark
⁎
Amalraj Peter Amalathas , Maan M Alkaisi MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Electrical and Computer Engineering, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand
A R T I C L E I N F O
A BS T RAC T
Keywords: Inverted nanopyramid Light trapping Nanoimprint lithography Photovoltaics Self-cleaning Solar cells
In this paper, we demonstrate that periodic inverted nanopyramid structures can enhance the power conversion efficiency of monocrystalline silicon solar cells by minimizing reflections, improving light trapping process in addition to its self-cleaning functionality. The periodic inverted nanopyramid structures were fabricated on monocrystalline silicon solar cell surfaces using a UV nanoimprint lithography. By introducing inverted nanopyramid structures on the front side of the monocrystalline silicon solar cells, the power conversion efficiency was improved by 11.73% compared to identical solar cells without the texturing. The inverted nanopyramid coating decreased the reflectance and increased the external quantum efficiency over a broad wavelength range. Moreover, the surface of the solar cells exhibited hydrophobic properties due to increased contact angle caused by the nanostructure patterns and the self-assembled monolayer coating. The enhanced hydrophobicity provided the solar cells with an added self-cleaning functionality. These results suggest that the periodic inverted nanopyramid structures has high potential in improving the performance of silicon solar cells and may be applied to different types of solar cells.
1. Introduction Reducing optical losses in the solar cells has always been a key challenge in enhancing the conversion efficiency. In general, efficient light management has been achieved by textured surfaces that enhance the light collection and increasing the effective optical path length of the light within the absorber layer of a solar cell [1]. Various texturing studies have been carried out, such as texturing at the back side [2] or at the front side of a solar cell [3] or pre-texturing the solar cell substrates [4,5] where a wide variety of light management schemes have been investigated to enhance the power conversion efficiency of a solar cell. The use of nanostructures for improving the light absorption and trapping in solar cells is a more promising method compared with the traditional micro-sized surface texturing [6,7]. Nanostructures can be fabricated by various techniques including electron beam lithography (EBL) [8], laser interference lithography (LIL) [9], nanoimprint lithography (NIL) [10,11], nanosphere lithography (NSL) [12] and Block copolymer lithography (BCPL) [13]. Nanoimprint lithography is one of the most promising low-cost methods for nanostructure patterning over a large area with high throughput, high resolution, and high fidelity. Various nanostructures such as nanowires [14–16], nanorods [17], nanocones [18], nanopyr-
⁎
amids [19], nanopillars [20] and metal nanostructures such as nanogrooves [21] and nanoparticles arrays [22–24] have been extensively studied. Despite their excellent light trapping properties, texturing the active solar cell region or introducing metal nanostructures results in poor charge carrier collection due to increased surface recombination. Fang Jiao et al. [25] demonstrated that the imprinting of moth-eye-like structures on the front side of monocrystalline Si solar cell surface enhanced the conversion efficiency by 19% compared to the reference solar cell by coupling incident light into the absorber layer. This approach of surface texturing differs from other approaches such as texturing the active material or using metal nanostructures. This approach enhances solar cell performance without introducing additional surface recombination and also provides excellent solar cell selfcleaning functionality. However, the periodic inverted nanopyramid structures have not been demonstrated on solar cells by means of UV nanoimprint lithography. The objective of this work was to determine the enhancement of monocrystalline Si solar cell performance employing periodic inverted nanopyramid structures using low cost and scalable approach. In this work, the periodic inverted nanopyramid structures were fabricated on the front side of monocrystalline Si solar cell surface by means of UV nanoimprint lithography. The reflectance, external
Corresponding author. E-mail addresses: [email protected] (A. Peter Amalathas), [email protected] (M.M. Alkaisi).
http://dx.doi.org/10.1016/j.mssp.2016.09.032 Received 20 July 2016; Accepted 27 September 2016 1369-8001/ © 2016 Elsevier Ltd. All rights reserved.
Materials Science in Semiconductor Processing 57 (2017) 54–58
A. Peter Amalathas, M.M. Alkaisi
built imprint tool was attached to the UV illumination source of the Karl Suss Mask Aligner (MA-6) system. This created a vacuum environment in order to reduce the air bubbles trapped in-between the mold and resist during the imprint process. The Si master mold/UV curable resist/glass plate was loaded into the imprint tool. A vacuum pressure of 4 mbar and the Mask Aligner (MA-6) system were then activated, and the resist was cured under a UV exposure for 4 min using 4.4 mW/cm2 illumination intensity with 365 nm UV source at room temperature. Finally, a manual de-molding process was utilized by applying gradual force using a scalpel at one corner of the mold in order to delaminate between the two material surfaces. The upright nanopyramid pattern was successfully replicated from the Si mold to the glass substrate with high fidelity, which was used as a stamp in the second imprint process. After the UV nanoimprint process, F13-TCS based SAM was coated onto the upright nanopyramid patterned glass substrate in order to enhance the anti-sticking property. In order to measure the effect of the inverted nanopyramid structure for improving the photo-current conversion efficiency, the inverted nanopyramid structures were replicated onto a monocrystalline Si solar cell front surface by the same UV imprinting method using upright nanopyramid patterned glass substrate as a master stamp. Fig. 1 illustrates the schematic diagram of overall imprint process steps. In this study, the solar cells were fabricated in our lab. Details of the solar cells fabrication are given in the Supporting Information.
quantum efficiency, current-voltage measurements and wettability of the monocrystalline Si solar cells with and without the inverted nanopyramid structures were investigated. The monocrystalline Si solar cells with inverted nanopyramid structure show enhancement of light trapping and power conversion efficiency in addition to high hydrophobic surfaces suitable for self-cleaning purposes. 2. Material and methods 2.1. Master mold preparation As a stamp, the periodic inverted pyramid structures were formed on Si mold and subsequently replicated to form upright nanopyramid using UV curable resist coated mold. An OrmoStamp UV curable resist (micro resist technology GmbH) was used to form nanostructures by means of UV nanoimprint lithography. The periodic inverted pyramid structure master Si mold was fabricated by laser interference lithography and subsequent pattern transfer by combined reactive ion etching and KOH wet etching. Additional details on the fabrication process of master mold can be found in the reference [6]. A (1H, 1H, 2H, 2H-Perfluorooctyl) trichlorosilane also known as F13-TCS solution from Sigma-Aldrich was used as an anti-sticking layer placed on molds and imprint resists sides respectively. 2.2. UV imprinting process
2.3. Characterization techniques Prior to coating, the glass substrate was cleaned with acetone, methanol, and isopropyl alcohol (IPA) solvents in an ultrasonic bath and then rinsed with deionized water and finally dried with Nitrogen gas. Next, the substrate surface was treated using oxygen plasma to enhance optimum adhesion at the interface between the glass and the OrmoPrime08. The OrmoPrime08 from micro resist technology was used as an adhesion promoter solution based on organofunctional silanes. After that, the substrate was baked using an oven at 200 °C for 30 min and cooled down to room temperature immediately before coating. OrmoPrime08 was deposited onto the glass substrate by spin coating at a spin speed of 4000 rpm for 60 s. The spin-coated film was then baked on a hot plate at 150 °C for 5 min and cooled down to room temperature. Finally, a liquid and transparent UV curable resist was spun coated onto the OrmoPrime08 layer coated substrate at a spinning speed of 6000 rpm for 60 s and prebaked on a hot plate at 80 °C for 2 min. To perform the imprint experiments, a vacuum operated home-
The nanostructures of the samples were examined by scanning electron microscope (SEM) (JEOL 7000F FE-SEM) and atomic force microscope (AFM) to determine the pattern of the master stamp was replicated uniformly on the top of the solar cell surface. The reflectance of the monocrystalline Si cell with and without the inverted nanopyramid structures was obtained using UV–visible spectrophotometer at room temperature, with an integrating sphere over the wavelength range of 300–1200 nm. The current density-voltage characteristics were measured for the solar cells using a Keithley 2400 source meter and a solar simulator (ABET Sun3000) with AM 1.5G filter under illumination of 1 sun. The external quantum efficiency (EQE) was also performed using tungsten –halogen lamp in combination with a monochromator (CS130 1/8m). In addition, for hydrophobicity evaluation, the contact angle of water droplets on Si surface with and without pattern were measured using a contact angle goniometer with Edmund Scientific Camera.
Fig. 1. The schematic diagram of the imprint process steps.
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Materials Science in Semiconductor Processing 57 (2017) 54–58
A. Peter Amalathas, M.M. Alkaisi
Fig. 2. Top view of SEM image of (a) the inverted nanopyramid Si master mold, (b) the upright nanopyramid replica stamp (c) the periodic inverted nanopyramid imprinted onto the surface of the solar cells.
Fig. 3. 3D view of AFM image of (a) the inverted nanopyramid Si master mold, (b) the upright nanopyramid replica stamp (c) the periodic inverted nanopyramid imprinted onto the surface of the solar cells.
Fig. 5. Current density – voltage (J-V) characteristics of monocrystalline Si solar cell with and without the inverted nanopyramid structures under AM 1.5 illumination. Fig. 4. Reflectance of the monocrystalline Si surface with and without the inverted nanopyramid structure measured as a function of wavelength.
Table 1 The device performance of the monocrystalline Si solar cell with and without the inverted nanopyramid structures measured under AM 1.5G conditions (100 mW cm−2).
3. Results and discussion Fig. 2(a) shows the top view SEM images of the periodic inverted nanopyramid Si master stamp. The upright nanopyramid replica stamp (Fig. 2b) was imprinted from the master Si stamp (Fig. 2a) and the periodic inverted nanopyramid imprinted onto the surface of the solar cells (Fig. 2c) from the upright nanopyramid replica stamp. It can be clearly seen that the inverted nanopyramid structures were transferred to the surface of the solar cell with high fidelity. Fig. 3(a–c) show the 3D view AFM images of the periodic inverted nanopyramid Si master stamp, the negative replica on the glass and positive replica on solar cells, respectively. The periodic inverted nanopyramid structure had features with a width of about 450 nm, a height of about 310 nm and separation of about 125 nm and was transferred uniformly over the large area of 10×10 mm after two
Device
Voc (V)
Jsc (mAcm−2)
FF (%)
Efficiency (%)
Bare monocrystalline Si solar cell Monocrystalline Si solar cell with inverted nanopyramid structure
0.525
29.442
52.55
8.122
0.525
32.793
52.71
9.075
imprint processes. Fig. 4 shows the reflectance of the monocrystalline Si surface with and without coating with inverted nanopyramid structures measured as a function of wavelength. From Fig. 4, it can be observed that the 56
Materials Science in Semiconductor Processing 57 (2017) 54–58
A. Peter Amalathas, M.M. Alkaisi
the Fig. 4. These results demonstrate that the periodic inverted nanopyramid structures reduced reflections, increased short circuit current and improved the efficiency of the silicon solar cells under this study. This is due to the formation of a gradual refractive index gradient between air and the solar cell, which can reduce the Fresnel reflectance and direct more incident light inside the solar cell active material. The combined light trapping and antireflection effect have been improved, and the optical path length has been prolonged by the inverted nanopyramid structures, resulting in increasing the overall conversion efficiency of the monocrystalline Si solar cells. In addition, the nanopyramid coating can be applied after cell fabrication to eliminate any losses due to surface damage by the etching processes [26]. In outdoor environments, the solar cells can be easily contaminated by dust particles, which interfere with incident light affecting cell light absorption and thus reducing the device performance. Therefore, a selfcleaning ability at the top surface of the solar cell would maintain the cell performance when exposed to practical environments [27,28]. Fig. 7 shows the contact angles values of water droplets on the bare solar cell, inverted nanopyramid patterned solar cell, and SAM-coated inverted nanopyramid patterned solar cell. As shown in the Fig. 7, the contact angle of the solar cell was increased from 35 to 96° after the formation of inverted nanopyramid structure, which exhibited a hydrophobic behavior. Moreover, the hydrophobicity was enhanced with SAM-coated inverted nanopyramid structures. In this case, the contact angle of the SAM-coated patterned solar cell was increased to 125°. As a result, solar cells with inverted nanopyramids can utilize the self-cleaning functionality induced by the high hydrophobic surface properties in addition to their antireflection properties.
Fig. 6. The EQE values of monocrystalline Si solar cell with and without inverted nanopyramid layer measured as a function of wavelength.
surface reflectance of the monocrystalline Si with the inverted nanopyramid layer was significantly decreased over broad wavelength range compared to the planar solar cells due to the gradual change in the refractive index between the air and Si surfaces obtained by the inverted nanopyramid layer. Fig. 5 shows the J-V characteristics of the monocrystalline Si solar cell with and without the inverted nanopyramid structures under AM 1.5 illumination. The photovoltaic parameters of the monocrystalline Si solar cell with and without nanopyramid structures extracted from these J-V curves are summarized in Table 1. The characteristics show that there was no significant change in the open circuit voltage (VOC). However, the short-circuit current density (JSC) has increased from 29.422 to 32.793 mA cm−2 in the monocrystalline Si solar cell with the inverted nanopyramid structures. This JSC increment was mainly due to the reduced reflectance resulting from the inverted nanopyramid structure over a broad wavelength range from 300 to 1200 nm as shown in the Fig. 4. As a result, the conversion efficiency of the monocrystalline Si solar cell with inverted nanopyramid was increased significantly from 8.122% to 9.075%, which is 11.73% higher than the result obtained for the planar solar cell. The EQE values of monocrystalline Si solar cell with and without inverted nanopyramid layer are shown in the Fig. 6. This EQE measurement was carried out under a monochromatic illumination by a tungsten – halogen lamp coupled to a monochromator. The EQE of the monocrystalline Si solar cell with inverted nanopyramid layer was significantly improved over the entire wavelength range compared to the non-imprinted solar cell. For instance, the EQE value increased by about 8% at 450 nm. This higher EQE values for the solar cell with inverted nanopyramid indicates enhanced light trapping, which is due to the imprinted nanostructures on top of the solar cell surface. This result was precisely matched with the reflectance value result shown in
4. Conclusions In conclusion, the periodic inverted nanopyramid structures were fabricated on the monocrystalline solar cell surface using a UV nanoimprint lithography. The pyramid coating can be applied after cell fabrication to eliminate any losses due to surface damage by the etching processes. The periodic inverted nanopyramid structure has successfully reduced the Fresnel reflection and led to directing and trapping more incident light into the monocrystalline Si solar cells, thereby improving the short circuit current density and enhancing the power conversion efficiency. The power conversion efficiency of the solar cells with inverted nanopyramid structure was improved by 11.73% compared to the identical solar cells without the surface treatment. In addition, the high hydrophobicity of the solar cells with inverted nanopyramid structures provided a self-cleaning functionality. These results suggest that the periodic inverted nanopyramid structures with antireflection properties and self-cleaning properties could be useful in different types of solar cells.
Fig. 7. Photographs of a water droplet on (a) bare solar cell, (b) patterned solar cell and (c) SAM-coated patterned solar cell. θc is a water contact angle.
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Materials Science in Semiconductor Processing 57 (2017) 54–58
A. Peter Amalathas, M.M. Alkaisi
Acknowledgment [12]
The authors would like to thank Annette Koo, Callaghan Innovation for transmittance measurements, Jonathan Halpert and Elijah Peach, Victoria University of Wellington, New Zealand for EQE measurements, Hari Murthy for SEM measurements, Gary Turner and Helen Devereux from the Nanofabrication Laboratory, University of Canterbury, New Zealand for providing technical assistance. Amalraj PA acknowledges UC Doctoral Scholarship, University of Canterbury, New Zealand.
[13]
[14] [15]
[16]
Appendix A. Supporting information [17]
Supplementary data associated with this article can be found in the online version at doi:10.1016/j.mssp.2016.09.032.
[18]
References
[19]
[1] S. Chattopadhyay, Y. Huang, Y.-J. Jen, A. Ganguly, K. Chen, L. Chen, Antireflecting and photonic nanostructures, Mater. Sci. Eng. R: Rep. 69 (2010) 1–35. [2] K. Söderström, J. Escarré, O. Cubero, F.J. Haug, S. Perregaux, C. Ballif, UV-nano‐ imprint lithography technique for the replication of back reflectors for n-i‐p thin film silicon solar cells, Prog. Photovolt.: Res. Appl. 19 (2011) 202–210. [3] B. Wang, T. Gao, P.W. Leu, Broadband light absorption enhancement in ultrathin film crystalline silicon solar cells with high index of refraction nanosphere arrays, Nano Energy 19 (2016) 471–475. [4] C.M. Hsu, C. Battaglia, C. Pahud, Z. Ruan, F.J. Haug, S. Fan, C. Ballif, Y. Cui, Highefficiency amorphous silicon solar cell on a periodic nanocone back reflector, Adv. Energy Mater. 2 (2012) 628–633. [5] M. Moreno, D. Daineka, P.R. i Cabarrocas, Plasma texturing for silicon solar cells: from pyramids to inverted pyramids-like structures, Sol. Energy Mater. Sol. Cells 94 (2010) 733–737. [6] S. Sivasubramaniam, M.M. Alkaisi, Inverted nanopyramid texturing for silicon solar cells using interference lithography, Microelectron. Eng. 119 (2014) 146–150. [7] Y. Li, J. Zhang, B. Yang, Antireflective surfaces based on biomimetic nanopillared arrays, Nano Today 5 (2010) 117–127. [8] Y. Kanamori, M. Sasaki, K. Hane, Broadband antireflection gratings fabricated upon silicon substrates, Opt. Lett. 24 (1999) 1422–1424. [9] C. Eisele, C. Nebel, M. Stutzmann, Periodic light coupler gratings in amorphous thin film solar cells, J. Appl Phys. 89 (2001) 7722–7726. [10] K.-S. Han, J.-H. Shin, H. Lee, Enhanced transmittance of glass plates for solar cells using nano-imprint lithography, Sol. Energy Mater. Sol. Cells 94 (2010) 583–587. [11] W. Wu, M. Hu, F.S. Ou, Z. Li, R.S. Williams, Cones fabricated by 3D nanoimprint
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Short communication
Efficient light trapping nanopyramid structures for solar cells patterned using UV nanoimprint lithography
crossmark
⁎
Amalraj Peter Amalathas , Maan M Alkaisi MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Electrical and Computer Engineering, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand
A R T I C L E I N F O
A BS T RAC T
Keywords: Inverted nanopyramid Light trapping Nanoimprint lithography Photovoltaics Self-cleaning Solar cells
In this paper, we demonstrate that periodic inverted nanopyramid structures can enhance the power conversion efficiency of monocrystalline silicon solar cells by minimizing reflections, improving light trapping process in addition to its self-cleaning functionality. The periodic inverted nanopyramid structures were fabricated on monocrystalline silicon solar cell surfaces using a UV nanoimprint lithography. By introducing inverted nanopyramid structures on the front side of the monocrystalline silicon solar cells, the power conversion efficiency was improved by 11.73% compared to identical solar cells without the texturing. The inverted nanopyramid coating decreased the reflectance and increased the external quantum efficiency over a broad wavelength range. Moreover, the surface of the solar cells exhibited hydrophobic properties due to increased contact angle caused by the nanostructure patterns and the self-assembled monolayer coating. The enhanced hydrophobicity provided the solar cells with an added self-cleaning functionality. These results suggest that the periodic inverted nanopyramid structures has high potential in improving the performance of silicon solar cells and may be applied to different types of solar cells.
1. Introduction Reducing optical losses in the solar cells has always been a key challenge in enhancing the conversion efficiency. In general, efficient light management has been achieved by textured surfaces that enhance the light collection and increasing the effective optical path length of the light within the absorber layer of a solar cell [1]. Various texturing studies have been carried out, such as texturing at the back side [2] or at the front side of a solar cell [3] or pre-texturing the solar cell substrates [4,5] where a wide variety of light management schemes have been investigated to enhance the power conversion efficiency of a solar cell. The use of nanostructures for improving the light absorption and trapping in solar cells is a more promising method compared with the traditional micro-sized surface texturing [6,7]. Nanostructures can be fabricated by various techniques including electron beam lithography (EBL) [8], laser interference lithography (LIL) [9], nanoimprint lithography (NIL) [10,11], nanosphere lithography (NSL) [12] and Block copolymer lithography (BCPL) [13]. Nanoimprint lithography is one of the most promising low-cost methods for nanostructure patterning over a large area with high throughput, high resolution, and high fidelity. Various nanostructures such as nanowires [14–16], nanorods [17], nanocones [18], nanopyr-
⁎
amids [19], nanopillars [20] and metal nanostructures such as nanogrooves [21] and nanoparticles arrays [22–24] have been extensively studied. Despite their excellent light trapping properties, texturing the active solar cell region or introducing metal nanostructures results in poor charge carrier collection due to increased surface recombination. Fang Jiao et al. [25] demonstrated that the imprinting of moth-eye-like structures on the front side of monocrystalline Si solar cell surface enhanced the conversion efficiency by 19% compared to the reference solar cell by coupling incident light into the absorber layer. This approach of surface texturing differs from other approaches such as texturing the active material or using metal nanostructures. This approach enhances solar cell performance without introducing additional surface recombination and also provides excellent solar cell selfcleaning functionality. However, the periodic inverted nanopyramid structures have not been demonstrated on solar cells by means of UV nanoimprint lithography. The objective of this work was to determine the enhancement of monocrystalline Si solar cell performance employing periodic inverted nanopyramid structures using low cost and scalable approach. In this work, the periodic inverted nanopyramid structures were fabricated on the front side of monocrystalline Si solar cell surface by means of UV nanoimprint lithography. The reflectance, external
Corresponding author. E-mail addresses: [email protected] (A. Peter Amalathas), [email protected] (M.M. Alkaisi).
http://dx.doi.org/10.1016/j.mssp.2016.09.032 Received 20 July 2016; Accepted 27 September 2016 1369-8001/ © 2016 Elsevier Ltd. All rights reserved.
Materials Science in Semiconductor Processing 57 (2017) 54–58
A. Peter Amalathas, M.M. Alkaisi
built imprint tool was attached to the UV illumination source of the Karl Suss Mask Aligner (MA-6) system. This created a vacuum environment in order to reduce the air bubbles trapped in-between the mold and resist during the imprint process. The Si master mold/UV curable resist/glass plate was loaded into the imprint tool. A vacuum pressure of 4 mbar and the Mask Aligner (MA-6) system were then activated, and the resist was cured under a UV exposure for 4 min using 4.4 mW/cm2 illumination intensity with 365 nm UV source at room temperature. Finally, a manual de-molding process was utilized by applying gradual force using a scalpel at one corner of the mold in order to delaminate between the two material surfaces. The upright nanopyramid pattern was successfully replicated from the Si mold to the glass substrate with high fidelity, which was used as a stamp in the second imprint process. After the UV nanoimprint process, F13-TCS based SAM was coated onto the upright nanopyramid patterned glass substrate in order to enhance the anti-sticking property. In order to measure the effect of the inverted nanopyramid structure for improving the photo-current conversion efficiency, the inverted nanopyramid structures were replicated onto a monocrystalline Si solar cell front surface by the same UV imprinting method using upright nanopyramid patterned glass substrate as a master stamp. Fig. 1 illustrates the schematic diagram of overall imprint process steps. In this study, the solar cells were fabricated in our lab. Details of the solar cells fabrication are given in the Supporting Information.
quantum efficiency, current-voltage measurements and wettability of the monocrystalline Si solar cells with and without the inverted nanopyramid structures were investigated. The monocrystalline Si solar cells with inverted nanopyramid structure show enhancement of light trapping and power conversion efficiency in addition to high hydrophobic surfaces suitable for self-cleaning purposes. 2. Material and methods 2.1. Master mold preparation As a stamp, the periodic inverted pyramid structures were formed on Si mold and subsequently replicated to form upright nanopyramid using UV curable resist coated mold. An OrmoStamp UV curable resist (micro resist technology GmbH) was used to form nanostructures by means of UV nanoimprint lithography. The periodic inverted pyramid structure master Si mold was fabricated by laser interference lithography and subsequent pattern transfer by combined reactive ion etching and KOH wet etching. Additional details on the fabrication process of master mold can be found in the reference [6]. A (1H, 1H, 2H, 2H-Perfluorooctyl) trichlorosilane also known as F13-TCS solution from Sigma-Aldrich was used as an anti-sticking layer placed on molds and imprint resists sides respectively. 2.2. UV imprinting process
2.3. Characterization techniques Prior to coating, the glass substrate was cleaned with acetone, methanol, and isopropyl alcohol (IPA) solvents in an ultrasonic bath and then rinsed with deionized water and finally dried with Nitrogen gas. Next, the substrate surface was treated using oxygen plasma to enhance optimum adhesion at the interface between the glass and the OrmoPrime08. The OrmoPrime08 from micro resist technology was used as an adhesion promoter solution based on organofunctional silanes. After that, the substrate was baked using an oven at 200 °C for 30 min and cooled down to room temperature immediately before coating. OrmoPrime08 was deposited onto the glass substrate by spin coating at a spin speed of 4000 rpm for 60 s. The spin-coated film was then baked on a hot plate at 150 °C for 5 min and cooled down to room temperature. Finally, a liquid and transparent UV curable resist was spun coated onto the OrmoPrime08 layer coated substrate at a spinning speed of 6000 rpm for 60 s and prebaked on a hot plate at 80 °C for 2 min. To perform the imprint experiments, a vacuum operated home-
The nanostructures of the samples were examined by scanning electron microscope (SEM) (JEOL 7000F FE-SEM) and atomic force microscope (AFM) to determine the pattern of the master stamp was replicated uniformly on the top of the solar cell surface. The reflectance of the monocrystalline Si cell with and without the inverted nanopyramid structures was obtained using UV–visible spectrophotometer at room temperature, with an integrating sphere over the wavelength range of 300–1200 nm. The current density-voltage characteristics were measured for the solar cells using a Keithley 2400 source meter and a solar simulator (ABET Sun3000) with AM 1.5G filter under illumination of 1 sun. The external quantum efficiency (EQE) was also performed using tungsten –halogen lamp in combination with a monochromator (CS130 1/8m). In addition, for hydrophobicity evaluation, the contact angle of water droplets on Si surface with and without pattern were measured using a contact angle goniometer with Edmund Scientific Camera.
Fig. 1. The schematic diagram of the imprint process steps.
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Materials Science in Semiconductor Processing 57 (2017) 54–58
A. Peter Amalathas, M.M. Alkaisi
Fig. 2. Top view of SEM image of (a) the inverted nanopyramid Si master mold, (b) the upright nanopyramid replica stamp (c) the periodic inverted nanopyramid imprinted onto the surface of the solar cells.
Fig. 3. 3D view of AFM image of (a) the inverted nanopyramid Si master mold, (b) the upright nanopyramid replica stamp (c) the periodic inverted nanopyramid imprinted onto the surface of the solar cells.
Fig. 5. Current density – voltage (J-V) characteristics of monocrystalline Si solar cell with and without the inverted nanopyramid structures under AM 1.5 illumination. Fig. 4. Reflectance of the monocrystalline Si surface with and without the inverted nanopyramid structure measured as a function of wavelength.
Table 1 The device performance of the monocrystalline Si solar cell with and without the inverted nanopyramid structures measured under AM 1.5G conditions (100 mW cm−2).
3. Results and discussion Fig. 2(a) shows the top view SEM images of the periodic inverted nanopyramid Si master stamp. The upright nanopyramid replica stamp (Fig. 2b) was imprinted from the master Si stamp (Fig. 2a) and the periodic inverted nanopyramid imprinted onto the surface of the solar cells (Fig. 2c) from the upright nanopyramid replica stamp. It can be clearly seen that the inverted nanopyramid structures were transferred to the surface of the solar cell with high fidelity. Fig. 3(a–c) show the 3D view AFM images of the periodic inverted nanopyramid Si master stamp, the negative replica on the glass and positive replica on solar cells, respectively. The periodic inverted nanopyramid structure had features with a width of about 450 nm, a height of about 310 nm and separation of about 125 nm and was transferred uniformly over the large area of 10×10 mm after two
Device
Voc (V)
Jsc (mAcm−2)
FF (%)
Efficiency (%)
Bare monocrystalline Si solar cell Monocrystalline Si solar cell with inverted nanopyramid structure
0.525
29.442
52.55
8.122
0.525
32.793
52.71
9.075
imprint processes. Fig. 4 shows the reflectance of the monocrystalline Si surface with and without coating with inverted nanopyramid structures measured as a function of wavelength. From Fig. 4, it can be observed that the 56
Materials Science in Semiconductor Processing 57 (2017) 54–58
A. Peter Amalathas, M.M. Alkaisi
the Fig. 4. These results demonstrate that the periodic inverted nanopyramid structures reduced reflections, increased short circuit current and improved the efficiency of the silicon solar cells under this study. This is due to the formation of a gradual refractive index gradient between air and the solar cell, which can reduce the Fresnel reflectance and direct more incident light inside the solar cell active material. The combined light trapping and antireflection effect have been improved, and the optical path length has been prolonged by the inverted nanopyramid structures, resulting in increasing the overall conversion efficiency of the monocrystalline Si solar cells. In addition, the nanopyramid coating can be applied after cell fabrication to eliminate any losses due to surface damage by the etching processes [26]. In outdoor environments, the solar cells can be easily contaminated by dust particles, which interfere with incident light affecting cell light absorption and thus reducing the device performance. Therefore, a selfcleaning ability at the top surface of the solar cell would maintain the cell performance when exposed to practical environments [27,28]. Fig. 7 shows the contact angles values of water droplets on the bare solar cell, inverted nanopyramid patterned solar cell, and SAM-coated inverted nanopyramid patterned solar cell. As shown in the Fig. 7, the contact angle of the solar cell was increased from 35 to 96° after the formation of inverted nanopyramid structure, which exhibited a hydrophobic behavior. Moreover, the hydrophobicity was enhanced with SAM-coated inverted nanopyramid structures. In this case, the contact angle of the SAM-coated patterned solar cell was increased to 125°. As a result, solar cells with inverted nanopyramids can utilize the self-cleaning functionality induced by the high hydrophobic surface properties in addition to their antireflection properties.
Fig. 6. The EQE values of monocrystalline Si solar cell with and without inverted nanopyramid layer measured as a function of wavelength.
surface reflectance of the monocrystalline Si with the inverted nanopyramid layer was significantly decreased over broad wavelength range compared to the planar solar cells due to the gradual change in the refractive index between the air and Si surfaces obtained by the inverted nanopyramid layer. Fig. 5 shows the J-V characteristics of the monocrystalline Si solar cell with and without the inverted nanopyramid structures under AM 1.5 illumination. The photovoltaic parameters of the monocrystalline Si solar cell with and without nanopyramid structures extracted from these J-V curves are summarized in Table 1. The characteristics show that there was no significant change in the open circuit voltage (VOC). However, the short-circuit current density (JSC) has increased from 29.422 to 32.793 mA cm−2 in the monocrystalline Si solar cell with the inverted nanopyramid structures. This JSC increment was mainly due to the reduced reflectance resulting from the inverted nanopyramid structure over a broad wavelength range from 300 to 1200 nm as shown in the Fig. 4. As a result, the conversion efficiency of the monocrystalline Si solar cell with inverted nanopyramid was increased significantly from 8.122% to 9.075%, which is 11.73% higher than the result obtained for the planar solar cell. The EQE values of monocrystalline Si solar cell with and without inverted nanopyramid layer are shown in the Fig. 6. This EQE measurement was carried out under a monochromatic illumination by a tungsten – halogen lamp coupled to a monochromator. The EQE of the monocrystalline Si solar cell with inverted nanopyramid layer was significantly improved over the entire wavelength range compared to the non-imprinted solar cell. For instance, the EQE value increased by about 8% at 450 nm. This higher EQE values for the solar cell with inverted nanopyramid indicates enhanced light trapping, which is due to the imprinted nanostructures on top of the solar cell surface. This result was precisely matched with the reflectance value result shown in
4. Conclusions In conclusion, the periodic inverted nanopyramid structures were fabricated on the monocrystalline solar cell surface using a UV nanoimprint lithography. The pyramid coating can be applied after cell fabrication to eliminate any losses due to surface damage by the etching processes. The periodic inverted nanopyramid structure has successfully reduced the Fresnel reflection and led to directing and trapping more incident light into the monocrystalline Si solar cells, thereby improving the short circuit current density and enhancing the power conversion efficiency. The power conversion efficiency of the solar cells with inverted nanopyramid structure was improved by 11.73% compared to the identical solar cells without the surface treatment. In addition, the high hydrophobicity of the solar cells with inverted nanopyramid structures provided a self-cleaning functionality. These results suggest that the periodic inverted nanopyramid structures with antireflection properties and self-cleaning properties could be useful in different types of solar cells.
Fig. 7. Photographs of a water droplet on (a) bare solar cell, (b) patterned solar cell and (c) SAM-coated patterned solar cell. θc is a water contact angle.
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Acknowledgment [12]
The authors would like to thank Annette Koo, Callaghan Innovation for transmittance measurements, Jonathan Halpert and Elijah Peach, Victoria University of Wellington, New Zealand for EQE measurements, Hari Murthy for SEM measurements, Gary Turner and Helen Devereux from the Nanofabrication Laboratory, University of Canterbury, New Zealand for providing technical assistance. Amalraj PA acknowledges UC Doctoral Scholarship, University of Canterbury, New Zealand.
[13]
[14] [15]
[16]
Appendix A. Supporting information [17]
Supplementary data associated with this article can be found in the online version at doi:10.1016/j.mssp.2016.09.032.
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