December 1, 2013 / Vol. 38, No. 23 / OPTICS LETTERS
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High-efficiency light-trapping effect using silver nanoparticles on thin amorphous silicon subwavelength structure Chee Leong Tan1 and Yong Tak Lee1,2,* 1
Department of Information and Communications, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro (Oryong-dong), Buk-gu, Gwangju 500-712, South Korea 2 Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro (Oryong-dong), Buk-gu, Gwangju 500-712, South Korea *Corresponding author: [email protected] Received September 2, 2013; revised October 22, 2013; accepted October 24, 2013; posted October 25, 2013 (Doc. ID 196676); published November 19, 2013 In this Letter, we experimentally demonstrate a hybrid structure consisting of metal nanoparticles deposited onto a subwavelength structure (SWS), which further increases the absorption of thin amorphous silicon (a-Si) and can possibly lead to a reduction in the minimum required thickness of the a-Si layer. Experimental results show that backscattering of the silver nanoparticles (Ag NPs) deposited on the top surface can be suppressed dramatically (by 85.5%) by the Ag NPs deposited on the SWS. We also experimentally prove that the thin a-Si SWS only lowers the surface reflectivity and does not increase the absorption rate of the material. The absorption of the thin a-Si layer can be increased by depositing Ag NPs onto a thin a-Si SWS, which not only reduces the backscattering of the metal NPs but also increases the light-trapping effect within thin a-Si through localized surface plasmon resonance properties. This decrease of reflection and increase in the light-trapping effect of Ag NPs on cone-shaped thin a-Si SWSs leads to extremely high average absorption (86.14%) within a 400 nm thick a-Si layer. © 2013 Optical Society of America OCIS codes: (310.1210) Antireflection coatings; (240.6680) Surface plasmons; (290.5850) Scattering, particles; (160.4760) Optical properties. http://dx.doi.org/10.1364/OL.38.004943
Over the past decade, plasmonic structures using metal nanoparticles (NPs) and random nanotexturing using subwavelength structures (SWSs) have emerged as a promising route to improve light absorption in thin-film solar cells. SWSs are well known as antireflection layers [1,2], and are widely used in thin-film photovoltaic (PV) due to low cost, broadband antireflective properties, and also ease in fabrication [2–6]. SWSs can significantly suppress the surface reflection over a broad wavelength range. However, plasmonic structures composed of metal NPs have better performance in increasing the absorption properties of PV through efficient lighttrapping properties compared to SWS [7], due to their ability to confine light in spaces significantly shorter than one fourth of the wavelength of the incident light, thereby providing strong light absorption or scattering. In PV devices, metal NPs deposited on top surfaces have been reported to reduce reflectivity and increase light trapping within devices, thereby increasing the device efficiency [8–17]. However, there are also reports where the metal NPs decrease the device efficiency due to absorption and backscattering caused by the metal NPs themselves [9,12]. Light absorption due to NPs is relatively small and the amount of absorbed light is reduced when the size of the metal NPs becomes larger [12]. Backscattering of incident light by metal NPs can cause high reflection loss for PV devices decorated with metal NPs on the top. It is observed that about 30%–40% of the incident light is lost due to back reflections caused by Ag NPs at the resonant wavelengths. This greatly decreases the device efficiency by preventing the incident light from being transmitted into the PV devices. In this Letter, we experimentally demonstrate that Ag NPs deposited onto a thin amorphous silicon (a-Si) SWS 0146-9592/13/234943-03$15.00/0
allow maximum transmission of incident light into the absorbing substrate, while metal NPs further increase the absorption of the thin a-Si SWS through their ability to confine light in spaces significantly shorter than one fourth of the wavelength of the incident light, thereby providing strong light absorption or scattering. Ag NPs are deposited onto cone-shaped a-Si SWSs whose continuous or finely stepped increase of refractive index from external medium to the PV structures [5] leads to ultralow reflectivity. The cone shape of the SWS structure and Ag NPs randomize the incident light direction within the structures. Ag NPs on a cone-shaped thin a-Si SWS have high light-trapping efficiency while simultaneously exhibiting ultralow reflectivity by suppressing the backscattering of NPs fabricated on the top surface of the a-Si substrate. This leads to higher light collection and absorption of the incident light by creating multiple interactions of the incident photon within the surface of the device, and also increases the path length of the incident light compared to an a-Si SWS and bare a-Si substrate. This decrease of reflection due to antireflection structure and increase of transmission due to localized surface plasmon resonance (LSPR) effect of the Ag NPs on the cone-shaped thin a-Si SWS lead to a substantial enhancement in the average absorption of 27.98% over a broad wavelength range (200–700 nm). The fabrication process of Ag NPs on thin a-Si SWSs started by cleaning the glass substrate using acetone, methanol, and isopropanol solutions sequentially for 5 min each, followed by cleaning with a diluted Buffer Oxide Etchant in order to remove the native oxide on the surface of the samples. Then, 500 nm thick a-Si was deposited onto the glass substrate using electron beam evaporation at the rate of 5 Å s−1 . Ag film of thickness © 2013 Optical Society of America
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OPTICS LETTERS / Vol. 38, No. 23 / December 1, 2013
15 nm was deposited onto a thin a-Si layer and was annealed at 500°C to form Ag NPs (Step 1). The size and distribution of the metal NPs can be controlled by varying the thickness of silver film and annealing time. The estimated average diameter and average distance of the nearest particles of Ag NPs with film thicknesses of 15 nm were 89.74 and 116.8 nm, respectively. After the formation of the Ag NPs, the sample was etched by inductively coupled plasma (ICP) reactive ion etching to form SWSs. The Ag NPs formed during Step 1 act as a mask during dry etching. Thin a-Si film was etched by a gas mixture of SiCl4 and Ar in the ratio 1∶1 with RF power of 50 W and ICP power of 0 W at a pressure set to 2 mTorr for 20 min (optimized condition for forming a cone-shaped thin a-Si SWS glass structure). Excess metal NPs on SWSs are removed using HNO3 . Then, a thin metal film of Ag (10 nm) was deposited by e-beam evaporation onto a thin a-Si SWS formed in the previous step, and was annealed at 500°C, thereby forming Ag NPs on the cone-shaped surface of the thin a-Si SWS. Total reflection and transmission spectroscopy measurement was carried out to characterize the optical properties of the fabricated samples. Subsequently, the normalized optical absorption is calculated by subtracting the sum of normalized reflection and transmission from unity. The total reflectance and transmittance from all angles were measured over the wavelength range 300– 1000 nm. The optical reflectance at all angles was obtained using the standard UV/Vis–near-IR spectrophotometer (Cary 5000, Varian, USA) equipped with an integrating sphere. The transmittance was measured only for the normal incidence angle, since the measurement setup can measure transmission only along normal incidence. Ag NPs are fabricated on a thin a-Si SWS to show that such structures demonstrate a LSPR effect, which further enhances the absorption of the thin a-Si SWS. Figures 1(a)– 1(d) show the SEM image of thin a-Si deposited onto glass substrate, the cone-shaped thin a-Si SWSs after dry etching, Ag NPs deposited onto cone-shaped thin a-Si SWSs,
and the magnified view of Ag NPs deposited onto coneshaped thin a-Si SWSs, respectively. The average height of thin a-Si on glass (496 nm) is reduced to 407 nm when the cone-shaped thin a-Si SWSs are formed using dry etching. The average diameter of Ag NPs on cone-shaped thin a-Si SWSs is 22.3 nm. The light-trapping effect of the metal NPs can be demonstrated due to the decrease of thin a-Si thickness, which allows light penetration through the samples. Figure 2 shows the total reflection spectra of Ag NPs deposited onto cone-shaped a-Si SWSs compared to a thin a-Si structure, Ag NPs on a thin a-Si structure, and thin a-Si SWS. From Fig. 2, it can be seen that the average reflection of the Ag NPs on thin a-Si (22.84%) can be greatly suppressed (4.22%) by depositing Ag NPs onto a thin a-Si SWS for incident light over a wavelength from 200 to 700 nm. This reflectivity reduction is mainly due to four mechanisms; (1) cone-shaped a-Si SWSs, with continuous or finely stepped increase of refractive index from external medium to the substrate, lead to ultralow surface reflectivity; (2) the direction of the reflected light is randomized by the NPs and SWSs, which reduces the backscattering of the metal NPs; (3) the reduction of the average size of metal NPs on the SWSs also reduces the backscattering effect [9]; and (4) NPs and a-Si SWSs prevent the light reflected from the bottom side of the substrate from escaping [18]. However, the reflection of Ag NPs deposited onto cone-shaped a-Si SWSs is 6.7% higher compared to that of thin a-Si SWSs, especially at the LSPR peak (∼470 nm) of the Ag NPs, due to backscattering of the Ag NPs when the light is normally incident on Ag NPs. More importantly, the effective light-trapping effect due to the LSPR properties of the Ag NPs is clearly shown in Fig. 3. The reduction of the average transmission at the resonance peak of Ag NPs on cone-shaped thin a-Si SWSs (9.6%) compared to thin a-Si SWSs (16.6%) agrees well with published result of the transmission reduction of Ag NPs deposited onto glass substrate at the resonance wavelength [18]. This transmission reduction is most likely due to the high intensity electron resonances excited by the incident light, which helps in enhancing
Fig. 1. SEM image of (a) thin a-Si deposited onto glass substrate, (b) cone-shaped thin a-Si SWSs after dry etching, (c) Ag NPs deposited onto cone-shaped thin a-Si SWSs, and (d) magnified view of Ag NPs deposited onto cone-shaped thin a-Si SWSs.
Fig. 2. Reflectance spectra of Ag NPs fabricated onto coneshaped a-Si SWSs, cone-shaped a-Si SWSs, Ag NPs on thin a-Si substrate, and thin a-Si substrate (reference). We note that the backscattering of Ag NPs on thin a-Si substrate is suppressed efficiently.
December 1, 2013 / Vol. 38, No. 23 / OPTICS LETTERS
Fig. 3. Transmittance spectra of Ag NPs fabricated onto coneshaped a-Si SWSs, cone-shaped a-Si SWSs, Ag NPs on thin a-Si substrate, and thin a-Si substrate (reference). The increment of transmission in cone-shaped a-Si SWSs is due to the antireflection properties. Ag NPs on cone-shaped a-Si SWSs further increase the light-trapping effect through LSPR properties.
the absorption, and also the light scattering of the thin aSi layer. Also proven in Fig. 3 is that the thin a-Si SWS has a considerably lower light-trapping effect (high transmittance 16.78%) compared to Ag NPs on cone-shaped thin a-Si SWSs due to shorter light path and the incident light penetrating through the thin a-Si layer (thin a-Si has poor absorption). That the transmission of the thin a-Si SWS is higher compared to flat a-Si is mostly due to the antireflection properties, which allow high light transmission into the substrate. The decrease of surface reflection and total transmission in Ag NPs on cone-shaped thin a-Si SWSs leads to a substantial enhancement in the average absorption of 27.98% over a wavelength range (200–700 nm), as displayed in Fig. 4. The average absorption of Ag NPs on cone-shaped thin a-Si SWSs is 86.14% within the thickness of 400 nm a-Si. In comparison, the average absorption of thin a-Si SWSs is 7.6% lower compared to that of Ag NPs on thin a-Si SWSs. These experimental results prove that thin a-Si SWSs increase the light-trapping effect of the layer by lowering the surface reflectivity but do not increase the absorption of the material. On the other hand, Ag NPs
Fig. 4. Calculated absorption spectra of Ag NPs fabricated onto cone-shaped a-Si SWSs, cone-shaped a-Si SWSs, Ag NPs on thin a-Si substrate, and thin a-Si substrate (reference).
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on thin a-Si SWSs not only can reduce the backscattering of the metal NPs but can also increase the absorption rate of the thin a-Si through LSPR properties. In summary, we have proposed a hybrid structure of metal NPs deposited onto SWSs to increase the absorption of the thin a-Si that leads to reduction of the minimum required thickness of the thin a-Si layer. We have shown that backscattering of the Ag NPs can be suppressed dramatically (by 78.9%) for Ag NPs deposited onto a-Si SWSs compared to Ag NPs deposited onto thin a-Si. Our experimental results also proved that a-Si SWSs increase the light-trapping effect of the a-Si layer, by lowering the surface reflectivity, but do not increase the absorption of the material. The deposition of Ag NPs on a-Si SWSs does not merely reduce the backscattering of the metal NPs but also increases the absorption of the a-Si through LSPR properties. This decrease of reflection and transmission of Ag NPs onto cone-shaped a-Si SWSs leads to extremely high average absorption (86.14%) within the 400 nm thick a-Si layer, providing a technique to enhance the absorption and reduce the light reflection in thin-film solar cells. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the South Korean government (MEST) (No. 2011-0017606), and by the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the South Korean government Ministry of Trade, Industry and Energy (No. 20133010011750). The authors also wish to thank Dr. R. Sooraj for his devoted support. References 1. B. Sanden, Mater. Today 11(12), 22 (2008). 2. D. G. Stavenga, S. Foletti, G. Palasantzas, and K. Arikawa, Proc. Royal Soc., Biol. Sci. 273, 661 (2006). 3. Y. M. Song, S. J. Jang, J. S. Yu, and Y. T. Lee, Small 6, 984 (2010). 4. Y. M. Song, J. H. Jang, J. C. Lee, E. K. Kang, and Y. T. Lee, Sol. Energy Mater. Sol. Cells 101, 73 (2012). 5. P. Lalanne and G. M. Morris, Nanotechnology 8, 53 (1997). 6. Y. F. Huang, S. Chattopadhyay, Y. J. Jen, C. Y. Peng, T. A. Liu, Y. K. Hsu, C. L. Pan, H. C. Lo, C. H. Hsu, Y. H. Chang, C. S. Lee, K. H. Chen, and L. C. Chen, Nat. Nanotechnol. 2, 770 (2007). 7. H. Tan, R. Santbergen, A. H. M. Smets, and M. Zeman, Nano Lett. 12, 4070 (2012). 8. K. R. Catchpole and A. Polman, Opt. Express 16, 21793 (2008). 9. T. L. Temple, G. D. K. Mahanama, H. S. Reehal, and D. M. Bagnall, Sol. Energy Mater. Sol. Cells 93, 1978 (2009). 10. S. Pillai, K. R. Catchpole, T. Trupke, and M. A. Green, J. Appl. Phys. 101, 093105 (2007). 11. K. R. Catchpole, S. Mokkapati, F. Beck, E.-C. Wang, A. McKinley, A. Basch, and J. Lee, MRS Bull. 36, 461 (2011). 12. E. Moulin, J. Sukmanowski, P. Luo, R. Carius, F. X. Royer, and H. Stiebig, J. Non-Cryst. Solids 354, 2488 (2008). 13. P. R. Pudasaini and A. A. Ayon, Opt. Commun. 285, 4211 (2012). 14. P. R. Pudasaini and A. A. Ayon, Phys. Status Solidi A 209, 1475 (2012). 15. C. X. Lin and M. L. Povinelli, Appl. Phys. Lett. 97, 071110 (2010). 16. K. Q. Peng, X. Wang, X. L. Wu, and S. T. Lee, Nano Lett. 9, 3704 (2009). 17. R. Ren, Y. Guo, and R. Zhu, Opt. Lett. 37, 4245 (2012). 18. C. L. Tan, S. J. Jang, and Y. T. Lee, Opt. Express 20, 17448 (2012).
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High-efficiency light-trapping effect using silver nanoparticles on thin amorphous silicon subwavelength structure Chee Leong Tan1 and Yong Tak Lee1,2,* 1
Department of Information and Communications, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro (Oryong-dong), Buk-gu, Gwangju 500-712, South Korea 2 Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro (Oryong-dong), Buk-gu, Gwangju 500-712, South Korea *Corresponding author: [email protected] Received September 2, 2013; revised October 22, 2013; accepted October 24, 2013; posted October 25, 2013 (Doc. ID 196676); published November 19, 2013 In this Letter, we experimentally demonstrate a hybrid structure consisting of metal nanoparticles deposited onto a subwavelength structure (SWS), which further increases the absorption of thin amorphous silicon (a-Si) and can possibly lead to a reduction in the minimum required thickness of the a-Si layer. Experimental results show that backscattering of the silver nanoparticles (Ag NPs) deposited on the top surface can be suppressed dramatically (by 85.5%) by the Ag NPs deposited on the SWS. We also experimentally prove that the thin a-Si SWS only lowers the surface reflectivity and does not increase the absorption rate of the material. The absorption of the thin a-Si layer can be increased by depositing Ag NPs onto a thin a-Si SWS, which not only reduces the backscattering of the metal NPs but also increases the light-trapping effect within thin a-Si through localized surface plasmon resonance properties. This decrease of reflection and increase in the light-trapping effect of Ag NPs on cone-shaped thin a-Si SWSs leads to extremely high average absorption (86.14%) within a 400 nm thick a-Si layer. © 2013 Optical Society of America OCIS codes: (310.1210) Antireflection coatings; (240.6680) Surface plasmons; (290.5850) Scattering, particles; (160.4760) Optical properties. http://dx.doi.org/10.1364/OL.38.004943
Over the past decade, plasmonic structures using metal nanoparticles (NPs) and random nanotexturing using subwavelength structures (SWSs) have emerged as a promising route to improve light absorption in thin-film solar cells. SWSs are well known as antireflection layers [1,2], and are widely used in thin-film photovoltaic (PV) due to low cost, broadband antireflective properties, and also ease in fabrication [2–6]. SWSs can significantly suppress the surface reflection over a broad wavelength range. However, plasmonic structures composed of metal NPs have better performance in increasing the absorption properties of PV through efficient lighttrapping properties compared to SWS [7], due to their ability to confine light in spaces significantly shorter than one fourth of the wavelength of the incident light, thereby providing strong light absorption or scattering. In PV devices, metal NPs deposited on top surfaces have been reported to reduce reflectivity and increase light trapping within devices, thereby increasing the device efficiency [8–17]. However, there are also reports where the metal NPs decrease the device efficiency due to absorption and backscattering caused by the metal NPs themselves [9,12]. Light absorption due to NPs is relatively small and the amount of absorbed light is reduced when the size of the metal NPs becomes larger [12]. Backscattering of incident light by metal NPs can cause high reflection loss for PV devices decorated with metal NPs on the top. It is observed that about 30%–40% of the incident light is lost due to back reflections caused by Ag NPs at the resonant wavelengths. This greatly decreases the device efficiency by preventing the incident light from being transmitted into the PV devices. In this Letter, we experimentally demonstrate that Ag NPs deposited onto a thin amorphous silicon (a-Si) SWS 0146-9592/13/234943-03$15.00/0
allow maximum transmission of incident light into the absorbing substrate, while metal NPs further increase the absorption of the thin a-Si SWS through their ability to confine light in spaces significantly shorter than one fourth of the wavelength of the incident light, thereby providing strong light absorption or scattering. Ag NPs are deposited onto cone-shaped a-Si SWSs whose continuous or finely stepped increase of refractive index from external medium to the PV structures [5] leads to ultralow reflectivity. The cone shape of the SWS structure and Ag NPs randomize the incident light direction within the structures. Ag NPs on a cone-shaped thin a-Si SWS have high light-trapping efficiency while simultaneously exhibiting ultralow reflectivity by suppressing the backscattering of NPs fabricated on the top surface of the a-Si substrate. This leads to higher light collection and absorption of the incident light by creating multiple interactions of the incident photon within the surface of the device, and also increases the path length of the incident light compared to an a-Si SWS and bare a-Si substrate. This decrease of reflection due to antireflection structure and increase of transmission due to localized surface plasmon resonance (LSPR) effect of the Ag NPs on the cone-shaped thin a-Si SWS lead to a substantial enhancement in the average absorption of 27.98% over a broad wavelength range (200–700 nm). The fabrication process of Ag NPs on thin a-Si SWSs started by cleaning the glass substrate using acetone, methanol, and isopropanol solutions sequentially for 5 min each, followed by cleaning with a diluted Buffer Oxide Etchant in order to remove the native oxide on the surface of the samples. Then, 500 nm thick a-Si was deposited onto the glass substrate using electron beam evaporation at the rate of 5 Å s−1 . Ag film of thickness © 2013 Optical Society of America
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OPTICS LETTERS / Vol. 38, No. 23 / December 1, 2013
15 nm was deposited onto a thin a-Si layer and was annealed at 500°C to form Ag NPs (Step 1). The size and distribution of the metal NPs can be controlled by varying the thickness of silver film and annealing time. The estimated average diameter and average distance of the nearest particles of Ag NPs with film thicknesses of 15 nm were 89.74 and 116.8 nm, respectively. After the formation of the Ag NPs, the sample was etched by inductively coupled plasma (ICP) reactive ion etching to form SWSs. The Ag NPs formed during Step 1 act as a mask during dry etching. Thin a-Si film was etched by a gas mixture of SiCl4 and Ar in the ratio 1∶1 with RF power of 50 W and ICP power of 0 W at a pressure set to 2 mTorr for 20 min (optimized condition for forming a cone-shaped thin a-Si SWS glass structure). Excess metal NPs on SWSs are removed using HNO3 . Then, a thin metal film of Ag (10 nm) was deposited by e-beam evaporation onto a thin a-Si SWS formed in the previous step, and was annealed at 500°C, thereby forming Ag NPs on the cone-shaped surface of the thin a-Si SWS. Total reflection and transmission spectroscopy measurement was carried out to characterize the optical properties of the fabricated samples. Subsequently, the normalized optical absorption is calculated by subtracting the sum of normalized reflection and transmission from unity. The total reflectance and transmittance from all angles were measured over the wavelength range 300– 1000 nm. The optical reflectance at all angles was obtained using the standard UV/Vis–near-IR spectrophotometer (Cary 5000, Varian, USA) equipped with an integrating sphere. The transmittance was measured only for the normal incidence angle, since the measurement setup can measure transmission only along normal incidence. Ag NPs are fabricated on a thin a-Si SWS to show that such structures demonstrate a LSPR effect, which further enhances the absorption of the thin a-Si SWS. Figures 1(a)– 1(d) show the SEM image of thin a-Si deposited onto glass substrate, the cone-shaped thin a-Si SWSs after dry etching, Ag NPs deposited onto cone-shaped thin a-Si SWSs,
and the magnified view of Ag NPs deposited onto coneshaped thin a-Si SWSs, respectively. The average height of thin a-Si on glass (496 nm) is reduced to 407 nm when the cone-shaped thin a-Si SWSs are formed using dry etching. The average diameter of Ag NPs on cone-shaped thin a-Si SWSs is 22.3 nm. The light-trapping effect of the metal NPs can be demonstrated due to the decrease of thin a-Si thickness, which allows light penetration through the samples. Figure 2 shows the total reflection spectra of Ag NPs deposited onto cone-shaped a-Si SWSs compared to a thin a-Si structure, Ag NPs on a thin a-Si structure, and thin a-Si SWS. From Fig. 2, it can be seen that the average reflection of the Ag NPs on thin a-Si (22.84%) can be greatly suppressed (4.22%) by depositing Ag NPs onto a thin a-Si SWS for incident light over a wavelength from 200 to 700 nm. This reflectivity reduction is mainly due to four mechanisms; (1) cone-shaped a-Si SWSs, with continuous or finely stepped increase of refractive index from external medium to the substrate, lead to ultralow surface reflectivity; (2) the direction of the reflected light is randomized by the NPs and SWSs, which reduces the backscattering of the metal NPs; (3) the reduction of the average size of metal NPs on the SWSs also reduces the backscattering effect [9]; and (4) NPs and a-Si SWSs prevent the light reflected from the bottom side of the substrate from escaping [18]. However, the reflection of Ag NPs deposited onto cone-shaped a-Si SWSs is 6.7% higher compared to that of thin a-Si SWSs, especially at the LSPR peak (∼470 nm) of the Ag NPs, due to backscattering of the Ag NPs when the light is normally incident on Ag NPs. More importantly, the effective light-trapping effect due to the LSPR properties of the Ag NPs is clearly shown in Fig. 3. The reduction of the average transmission at the resonance peak of Ag NPs on cone-shaped thin a-Si SWSs (9.6%) compared to thin a-Si SWSs (16.6%) agrees well with published result of the transmission reduction of Ag NPs deposited onto glass substrate at the resonance wavelength [18]. This transmission reduction is most likely due to the high intensity electron resonances excited by the incident light, which helps in enhancing
Fig. 1. SEM image of (a) thin a-Si deposited onto glass substrate, (b) cone-shaped thin a-Si SWSs after dry etching, (c) Ag NPs deposited onto cone-shaped thin a-Si SWSs, and (d) magnified view of Ag NPs deposited onto cone-shaped thin a-Si SWSs.
Fig. 2. Reflectance spectra of Ag NPs fabricated onto coneshaped a-Si SWSs, cone-shaped a-Si SWSs, Ag NPs on thin a-Si substrate, and thin a-Si substrate (reference). We note that the backscattering of Ag NPs on thin a-Si substrate is suppressed efficiently.
December 1, 2013 / Vol. 38, No. 23 / OPTICS LETTERS
Fig. 3. Transmittance spectra of Ag NPs fabricated onto coneshaped a-Si SWSs, cone-shaped a-Si SWSs, Ag NPs on thin a-Si substrate, and thin a-Si substrate (reference). The increment of transmission in cone-shaped a-Si SWSs is due to the antireflection properties. Ag NPs on cone-shaped a-Si SWSs further increase the light-trapping effect through LSPR properties.
the absorption, and also the light scattering of the thin aSi layer. Also proven in Fig. 3 is that the thin a-Si SWS has a considerably lower light-trapping effect (high transmittance 16.78%) compared to Ag NPs on cone-shaped thin a-Si SWSs due to shorter light path and the incident light penetrating through the thin a-Si layer (thin a-Si has poor absorption). That the transmission of the thin a-Si SWS is higher compared to flat a-Si is mostly due to the antireflection properties, which allow high light transmission into the substrate. The decrease of surface reflection and total transmission in Ag NPs on cone-shaped thin a-Si SWSs leads to a substantial enhancement in the average absorption of 27.98% over a wavelength range (200–700 nm), as displayed in Fig. 4. The average absorption of Ag NPs on cone-shaped thin a-Si SWSs is 86.14% within the thickness of 400 nm a-Si. In comparison, the average absorption of thin a-Si SWSs is 7.6% lower compared to that of Ag NPs on thin a-Si SWSs. These experimental results prove that thin a-Si SWSs increase the light-trapping effect of the layer by lowering the surface reflectivity but do not increase the absorption of the material. On the other hand, Ag NPs
Fig. 4. Calculated absorption spectra of Ag NPs fabricated onto cone-shaped a-Si SWSs, cone-shaped a-Si SWSs, Ag NPs on thin a-Si substrate, and thin a-Si substrate (reference).
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on thin a-Si SWSs not only can reduce the backscattering of the metal NPs but can also increase the absorption rate of the thin a-Si through LSPR properties. In summary, we have proposed a hybrid structure of metal NPs deposited onto SWSs to increase the absorption of the thin a-Si that leads to reduction of the minimum required thickness of the thin a-Si layer. We have shown that backscattering of the Ag NPs can be suppressed dramatically (by 78.9%) for Ag NPs deposited onto a-Si SWSs compared to Ag NPs deposited onto thin a-Si. Our experimental results also proved that a-Si SWSs increase the light-trapping effect of the a-Si layer, by lowering the surface reflectivity, but do not increase the absorption of the material. The deposition of Ag NPs on a-Si SWSs does not merely reduce the backscattering of the metal NPs but also increases the absorption of the a-Si through LSPR properties. This decrease of reflection and transmission of Ag NPs onto cone-shaped a-Si SWSs leads to extremely high average absorption (86.14%) within the 400 nm thick a-Si layer, providing a technique to enhance the absorption and reduce the light reflection in thin-film solar cells. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the South Korean government (MEST) (No. 2011-0017606), and by the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the South Korean government Ministry of Trade, Industry and Energy (No. 20133010011750). The authors also wish to thank Dr. R. Sooraj for his devoted support. References 1. B. Sanden, Mater. Today 11(12), 22 (2008). 2. D. G. Stavenga, S. Foletti, G. Palasantzas, and K. Arikawa, Proc. Royal Soc., Biol. Sci. 273, 661 (2006). 3. Y. M. Song, S. J. Jang, J. S. Yu, and Y. T. Lee, Small 6, 984 (2010). 4. Y. M. Song, J. H. Jang, J. C. Lee, E. K. Kang, and Y. T. Lee, Sol. Energy Mater. Sol. Cells 101, 73 (2012). 5. P. Lalanne and G. M. Morris, Nanotechnology 8, 53 (1997). 6. Y. F. Huang, S. Chattopadhyay, Y. J. Jen, C. Y. Peng, T. A. Liu, Y. K. Hsu, C. L. Pan, H. C. Lo, C. H. Hsu, Y. H. Chang, C. S. Lee, K. H. Chen, and L. C. Chen, Nat. Nanotechnol. 2, 770 (2007). 7. H. Tan, R. Santbergen, A. H. M. Smets, and M. Zeman, Nano Lett. 12, 4070 (2012). 8. K. R. Catchpole and A. Polman, Opt. Express 16, 21793 (2008). 9. T. L. Temple, G. D. K. Mahanama, H. S. Reehal, and D. M. Bagnall, Sol. Energy Mater. Sol. Cells 93, 1978 (2009). 10. S. Pillai, K. R. Catchpole, T. Trupke, and M. A. Green, J. Appl. Phys. 101, 093105 (2007). 11. K. R. Catchpole, S. Mokkapati, F. Beck, E.-C. Wang, A. McKinley, A. Basch, and J. Lee, MRS Bull. 36, 461 (2011). 12. E. Moulin, J. Sukmanowski, P. Luo, R. Carius, F. X. Royer, and H. Stiebig, J. Non-Cryst. Solids 354, 2488 (2008). 13. P. R. Pudasaini and A. A. Ayon, Opt. Commun. 285, 4211 (2012). 14. P. R. Pudasaini and A. A. Ayon, Phys. Status Solidi A 209, 1475 (2012). 15. C. X. Lin and M. L. Povinelli, Appl. Phys. Lett. 97, 071110 (2010). 16. K. Q. Peng, X. Wang, X. L. Wu, and S. T. Lee, Nano Lett. 9, 3704 (2009). 17. R. Ren, Y. Guo, and R. Zhu, Opt. Lett. 37, 4245 (2012). 18. C. L. Tan, S. J. Jang, and Y. T. Lee, Opt. Express 20, 17448 (2012).
