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Energy Express

  • Editor: Bernard Kippelen
  • Vol. 20, Iss. S5 — Sep. 10, 2012
  • pp: A777–A787
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The tradeoff between plasmonic enhancement and optical loss in silicon nanowire solar cells integrated in a metal back reflector

Keya Zhou, Zhongyi Guo, Xiaopeng Li, Jin-Young Jung, Sang-Won Jee, Kwang-Tae Park, Han-Don Um, Ning Wang, and Jung-Ho Lee  »View Author Affiliations


Optics Express, Vol. 20, Issue S5, pp. A777-A787 (2012)
http://dx.doi.org/10.1364/OE.20.00A777


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Abstract

We perform a systematic numerical study to characterize the tradeoff between the plasmonic enhancement and optical loss in periodically aligned, silicon nanowire (SiNW) arrays integrated with a silver back reflector (Ag BR). Optimizing the embedded depth of the wire bottoms into a silver reflector achieved a highly efficient SiNW solar cell. Compared to the SiNW solar cell employing a flat back reflector, the embedded depth of ~20 nm resulted in the relative increase of ~5% in ultimate solar cell efficiency.

© 2012 OSA

1. Introduction

Plasmonic effects have recently gained much interest in solar cell research and have presented new opportunities for more efficient solar cell designs [1

1. M. A. Green and S. Pillai, “Harnessing plasmonics for solar cells,” Nat. Photonics 6(3), 130–132 (2012). [CrossRef]

]. Experimental results showing that the photogenerated current can be improved by introducing plasmonic nanostructures into photovoltaic devices have been reported [2

2. V. E. Ferry, L. A. Sweatlock, D. Pacifici, and H. A. Atwater, “Plasmonic Nanostructure Design for Efficient Light Coupling into Solar Cells,” Nano Lett. 8(12), 4391–4397 (2008). [CrossRef] [PubMed]

4

4. K. R. Catchpole and A. Polman, “Plasmonic solar cells,” Opt. Express 16(26), 21793–21800 (2008). [CrossRef] [PubMed]

]. Plasmonic back contacts (called back reflectors, or BR) that employ nanogratings and/or photonic crystals have been adopted in order to utilize the surface plasmonic wave scattering or localized surface plasmonic resonances focused mostly on the interfaces between semiconductors and metal [5

5. W. Wang, S. Wu, K. Reinhardt, Y. Lu, and S. Chen, “Broadband Light Absorption Enhancement in Thin-Film Silicon Solar Cells,” Nano Lett. 10(6), 2012–2018 (2010). [CrossRef] [PubMed]

8

8. R. Biswas and D. Zhou, “Simulation and modelling of photonic and plasmonic crystal back reflectors for efficient light trapping,” Phys. Status Solidi., A Appl. Mater. Sci. 207(3), 667–670 (2010). [CrossRef]

]. Unfortunately, optical losses are also caused by the metallic back contacts and are an unavoidable side effect that decreases the efficiency of the solar cell operation. Here, we theoretically analyze the tradeoff between the plasmonic enhancement and optical loss that appears when we introduce a metal back reflector into a SiNW solar cell.

Conceptual schematics comparing the distinctive features of light trapping between Si thin films and SiNW solar cells are shown in Fig. 1(a) and 1(b). In a SiNW array, we need to further consider the following three optical routes, which are absent in its thin film counterpart: i) Each Si wire acts as an individual cylindrical waveguide so that the incident light is gradually absorbed along the axial direction while coupling with guided modes inside a wire. The light remains until the wire bottom couples with the metallic back contacts, creating an ohmic loss. ii) Multiple scattering occurs in the SiNWs, and their periodicity excites surface plasmon polaritons (SPPs) travelling along the Ag surfaces that contacted to air or silicon under the phase matching condition. This process also causes light absorption loss at the corresponding wavelengths. iii) Localized surface plasmonic resonances (LSPR) appear in the enhanced local near-field for both the metal contacts and the SiNWs. Additionally, the localized near-field contributes not only to the enhancement of light absorption, but also to the optical loss over the spectral range where such resonances occur.

We theoretically investigated the optical properties of a SiNW array solar cell using the following three different geometries: a bare SiNW array, a SiNW array with a flat Ag BR, and a SiNW array with a wire-embedded Ag BR. In the last formation, the bottoms of a NW array were embedded into a BR, thus forming a nanohole-grating-structure, as shown in Fig. 1(c). A three-dimensional simulation was performed using a finite element method (FEM) implemented in the RF module of COMSOL Multi-Physics, which attempted to determine the optical relation between the plasmonic enhancement in SiNW solar cells and the light losses from the Ag back contact.

2. Optical models and simulation approach

As depicted in Fig. 1(c), we can divide the overall geometry into three layers: a NW array, a thin nanohole-contact-grating (NCG) layer, and a flat, thick Ag BR. Their corresponding, two-dimensional, top and cross-sectional views are shown in Fig. 1(d) and 1(e), where d, l, a, andpcorrespond to the wire diameter, wire length, lattice constant, and embedded depth of NWs into the Ag BR, respectively.

This geometrical modeling is based on the experimental fabrication of SiNW solar cells [18

18. M. D. Kelzenberg, S. W. Boettcher, J. A. Petykiewicz, D. B. Turner-Evans, M. C. Putnam, E. L. Warren, J. M. Spurgeon, R. M. Briggs, N. S. Lewis, and H. A. Atwater, “Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications,” Nat. Mater. 9(3), 239–244 (2010). [PubMed]

], in which the SiNW array is fabricated using vapor-liquid-solid (VLS) or vapor-solid-solid (VSS) growth. Note that a SiNW array can also be peeled off a grown substrate after immersion into a polydimethylsiloxane (PDMS) polymer [20

20. Y. Wu, Y. Cui, L. Huynh, C. J. Barrelet, D. C. Bell, and C. M. Lieber, “Controlled Growth and Structures of Molecular-Scale Silicon Nanowires,” Nano Lett. 4(3), 433–436 (2004). [CrossRef]

22

22. J. M. Weisse, D. R. Kim, C. H. Lee, and X. Zheng, “Vertical Transfer of Uniform Silicon Nanowire Arrays via Crack Formation,” Nano Lett. 11(3), 1300–1305 (2011). [CrossRef] [PubMed]

]. The metallic BR was easily formed via deposition onto the backside of a vertical SiNW array [19

19. K.-T. Park, Z. Guo, H.-D. Um, J.-Y. Jung, J. M. Yang, S. K. Lim, Y. S. Kim, and J.-H. Lee, “Optical properties of Si microwires combined with nanoneedles for flexible thin film photovoltaics,” Opt. Express 19(S1Suppl 1), A41–A50 (2011). [CrossRef] [PubMed]

]. The surface profile of a detached PDMS bottom face is generally concave in between nanowires because the wire bottoms normally protrude out of the PDMS, which causes the deposited metallic BR to always have a nanohole array grating morphology. In this work, we emulated such a nanograting structure to be a thin NCG layer in contact with an optically thick Ag BR in which the wire bottoms were embedded with a thickness p, as shown in Fig. 1(e). The process formed the nanohole layer.

In our simulation, the incident light was set to be an x-polarized plane wave propagating along the −z direction. Perfect magnetic and electric conductors (PMCs and PECs) were used on the sidewalls of a unit cell for generating either symmetric magnetic field or symmetric electric field on the two sides of the sidewalls [23

23. J. Kupec and B. Witzigmann, “Computational electromagnetics for nanowire solar cells,” J. Comput. Electron. 11(2), 153–165 (2012). [CrossRef]

]. On a PMC (or PEC) wall, there was no parallel magnetic field (or electric field) component. As a result, the simulated region was simplified to a quarter of its initial size, as shown in Fig. 1(f). Perfect matched layers (PMLs) were used at the top and bottom of a unit cell, which was capped by the scattering boundaries. The wavelength-dependent optical constants of crystalline silicon (c-Si) and Ag were obtained by fitting the experimental data taken from Ref [24

24. E. D. Palik, Handbook of Optical Constants of Solids (Academic, Orlando, Fla., 1985).

] using a linear interpolation. Our simulation has been carried out by COMSOL Multiphysics in frequency domain in which the Maxwell Equations were used for a sweep on wavelengths. The maximum mesh size was chosen to be 40 nm in air, and 10 nm in Si for accurate calculation. The grids near the Ag/air and Ag/Si interfaces were chosen to be ≤2 nm for observing the plasmonic effects. The portion of light absorbed by SiNW and the energy loss in the Ag BR was calculated via integration within their corresponding volumes:
ASi,Ag(λ)=QSi,Ag(λ,x,y,z)dVEin
(1)
where Einis the energy of incident light in the given unit cell. QSi and QAgare the electromagnetic power loss densities in the SiNW and Ag BR, respectively, calculated by
QSi,Ag(λ,x,y,z)=12εSi,Ag''(λ)|E(x,y,z)|2
(2)
εSi,Ag'' is the imaginary part of the permittivity of Si or Ag, and E(x,y,z)is the electric field calculated by COMSOL Multiphysics.” To evaluate the overall absorption performance of the different geometries in the solar spectrum, we calculated the ultimate efficiency, ηUE at Air Mass (AM) 1.5 direct normal and circumsolar conditions using the following equation:
ηUE=310nmλgI(λ)ASi(λ)λλgdλ310nm4000nmI(λ)dλ
(3)
where I(λ)is the spectral irradiance at AM 1.5 direct normal conditions in W/m2/nm. The λg=1127nmcorresponds to a bandgap energy (1.1 eV) of Si. The lower limit (310 nm) of the integral is 4 eV, and the upper limit (4000 nm) in the denominator indicates the limit of the data available for a solar spectrum. Equation (1) assumes that each photon with energy greater than the bandgap energy photogenerates only one electron-hole pair with an energy equal to λg, whereby the excess energy is converted to heat. Likewise, in a spectrum ranging from 310 to 1127 nm, the total energy losses by the Ag back contact could be expressed as:

ηLoss=310nmλgI(λ)AAg(λ)dλ310nm4000nmI(λ)dλ
(4)

3. Results and discussion

We set the length of NWs to 2.33 μm, comparable to the film thickness of thin film solar cells. Ag was chosen as the BR material because it possesses the lowest absorption coefficient among the candidate metals in the visible range; moreover, it is easily deposited onto the backside of SiNW arrays.

3.1 SiNW arrays with a flat Ag BR for small wire diameters and lattice constants

We investigated SiNW arrays that had relatively smaller values for both their wire diameters and lattice constants. The wire diameter was set to 60 nm. According to the optical waveguide theory [25

25. A. W. Snyder and J. D. Love, Optical Waveguide Theory (Chapman & Hall, New York, 1983).

], each SiNW can be treated as a step-profile fiber defined by so-called fiber parameter, V=2πρ/λnco2ncl2, where ρ, λ, nco, and ncl are the fiber radius, light wavelength, refractive index of core and cladding layers of a fiber, respectively. In each SiNW, a number of guided modes coexist, but in our case using a small wire diameter, the fiber parameter V is less than the cutoff value of 2.405. Consequently, the fundamental mode, HE11 only exists to make each NW behave as a ‘single-mode NW’. Under illumination, a fundamental mode is excited when phase matching occurs between the air and the SiNW array [26

26. K. Seo, M. Wober, P. Steinvurzel, E. Schonbrun, Y. Dan, T. Ellenbogen, and K. B. Crozier, “Multicolored Vertical Silicon Nanowires,” Nano Lett. 11(4), 1851–1856 (2011). [CrossRef] [PubMed]

]. For SiNW arrays with a diameter of 60 nm, such phase matching is calculated to occur at a wavelength of ~425 nm. For clarity, the electric field distribution of the HE11 mode at a wavelength of 425 nm is depicted in the inset of Fig. 2(a)
Fig. 2 (a) Absorptance spectra of SiNW arrays without Ag BR. (b) Absorptance spectra of SiNW arrays integrated with a flat Ag BR. (c) Optical properties of SiNW arrays that have a lattice constant of 100 nm and a wire diameter of 60 nm. The blue and black lines correspond to light absorption by Si with and without Ag BR, respectively. The solid red line corresponds to the absorption loss by the Ag BR with the same geometry, and the green line shows the reflectance. The dashed red line shows the absorption loss at the Ag/air interface calculated by Fresnel equations. (d) The line plots show the light intensities along the probe line ‘AB’ at four typical wavelengths labeled with (i-iv), as denoted in panel c. The insets show the two-dimensional electric field distribution as a function of different wavelengths for the probe rectangle ‘C’ depicted in Fig. 1(f).
using the formulas provided in Ref [25

25. A. W. Snyder and J. D. Love, Optical Waveguide Theory (Chapman & Hall, New York, 1983).

]. In fact, such phenomenon could also be interpreted semi-analytically to couple into a set of so-called key modes of a SiNW array, as shown in Refs [27

27. B. C. P. Sturmberg, K. B. Dossou, L. C. Botten, A. A. Asatryan, C. G. Poulton, C. M. de Sterke, and R. C. McPhedran, “Modal analysis of enhanced absorption in silicon nanowire arrays,” Opt. Express 19(S5Suppl 5), A1067–A1081 (2011). [CrossRef] [PubMed]

, 28

28. J. Kupec and B. Witzigmann, “Dispersion, wave propagation and efficiency analysis of nanowire solar cells,” Opt. Express 17(12), 10399–10410 (2009). [CrossRef] [PubMed]

]. The wavelength-dependent absorptances are plotted in Fig. 2(a) and 2(b) for the SiNW arrays without and with a flat Ag BR, respectively. The guided mode coupling between the air and the SiNW array reveals the presence of an evident peak around ~425 nm in the absorptance spectra.

For solar cell applications, it is desirable to use SiNW arrays with a higher filling ratio in order to obtain a high efficiency in energy conversion. Figure 2(c) shows the optical properties of SiNW arrays with the smallest lattice constant of 100 nm, where the blue and black lines correspond to the light absorption in Si with and without a Ag BR, respectively. The absorptance of the Ag substrate in air was also calculated for comparison using the Fresnel equationA=1(niκ1)2/(niκ+1)2, wheren andκ are the real and imaginary parts of the refractive index of Ag, respectively [24

24. E. D. Palik, Handbook of Optical Constants of Solids (Academic, Orlando, Fla., 1985).

]. Given the optical loss by the Ag BR, the broadband optical loss was observed for wavelengths from 450 to 700 nm, where the highest loss occurred at ~600 nm revealing the joule heating converted from ~60% of the incident photon energy. Nevertheless, the presence of the Ag BR improved ηUE to 12.2% compared to the bare SiNW array surrounded with vacuum (without Ag BR and a substrate), which gave a ηUE of 8.5%. Although the optical loss of only ~1.64% occurred at the air/Ag interface, ~8.1% of the total incident light was estimated to thermally dissipate in the Ag BR, where most of the optical loss occurred in the wavelengths from 450 to 700 nm. This feature is clearly different from the loss spectra observed in crystalline silicon thin-film solar cells [9

9. J. Springer, A. Poruba, L. Müllerova, M. Vanecek, O. Kluth, and B. Rech, “Absorption loss at nanorough silver back reflector of thin-film silicon solar cells,” J. Appl. Phys. 95(3), 1427–1429 (2004). [CrossRef]

11

11. U. W. Paetzold, F. Hallermann, B. E. Pieters, U. Rau, R. Carius, and G. von Plessen, “Localized plasmonic losses at metal back contacts of thin-film silicon solar cells,” Proc. SPIE 7725, 772517, 772517-9 (2010). [CrossRef]

], which only show some resonant absorption peaks in shorter wavelengths less than 500 nm.

To further clarify this feature, the electric field distribution along the center of the Si nanowire – i.e., the probe line ‘AB’ shown in Fig. 1(f) – was compared for the four different wavelengths (see Fig. 2(d)). At 400 nm, no optical losses occurred at the Ag back contact since the incident light was quickly absorbed within several hundred nanometers due to multiple scatterings in between the Si nanowires. At 600 nm, the light interacted strongly with the Ag BR because the NW length of 2.33 μm was not long enough to completely absorb the incident light. Note that the electric field intensity at the interface between the Si and Ag BR enhanced the amplitude of the incident light by at least three times its original value. The electric field distribution in the inset (ii) of Fig. 2(d) shows that a strong enhancement occurred in both the Si and Ag regions, which implies that the plasmonic absorption (positive) by the SiNWs accompanies the optical loss (negative) by the Ag BR. At longer wavelengths, such as 800 nm (iii) and 1000 nm (iv), the interactions between the incident light and the Ag BR are insignificant because almost all the incident light was back-reflected due to the low absorptance in both the SiNWs and the Ag BR.

3.2 SiNW arrays with a flat Ag BR for large wire diameters and lattice constants

Many of the evident loss peaks were distributed over the broadband spectrum while overlapping with their absorption resonances, as shown in the red curve of Fig. 3(a). This feature implies that the optical losses by the Ag BR were not only associated with the surface plasmon excitations, but also with the guide-mode excitations in SiNWs. Nevertheless, the SiNWs integrated with a flat Ag BR achieved a higher ηUE of 26.7%, compared to the ηUE of 22.7% in the bare SiNW array. The total amount of optical loss was calculated to be 3.5% from Eq. (3). Although this value is almost twice the loss portion calculated from the Ag/air interface, it is significantly less than the loss amount (8.1%) of the NW array with a = 100 nm and d = 60 nm. This reduction of optical loss in the wire array with a large lattice constant can be attributed to the enhanced light absorption in the SiNWs due to the guide-mode excitation and multiple scatterings. These processes then directly decrease the light coupling with the Ag BR because the amount of light that is left to interact with the Ag BR decreases.

In Fig. 3(b), the electric field distributions along the probe line ‘AB’ shown in Fig. 1(f) are plotted for three wavelengths of (i) 833 nm, (ii) 983 nm, and (iii) 1000 nm. At 833 nm, the energy loss by the Ag BR reached almost 50% of the total incident light, which shows the notable increase in the electric field intensity by a factor of ~15 at the Si/Ag BR interface. At 983 nm, the electric field was observed to increase by a factor of 5 in the Si nanowire compared to the intensity of incident light. At 1000 nm, however, the electric field intensity in the Si nanowire decreased to only ~20% of the value in 983 nm. In other words, the incident light for a wavelength of 983 nm appears to be highly concentrated in Si nanowires compared to other wavelengths, suggesting that the amount of light remaining is larger accordingly just before reaching the metal back reflector. Given the almost stationary extinction characteristics of Ag for long wavelengths, the optical loss is overall governed by the light intensity, which can explain why each absorption peak observed in a SiNW array accompanies the optical loss peak for long wavelengths.

3.3 SiNW arrays integrated with a thin NCG layer

Three different geometric conditions, i) a = 100 nm and d = 60 nm, ii) a = 150 nm and d = 100 nm, iii) a = 500 nm and d = 300 nm, are compared in Fig. 4
Fig. 4 (a) The ultimate efficiencies calculated using three different geometrical parameters are shown as a function of the embedded depth of Ag nanoholes. For reference, each dashed line (color-matched) denotes the ultimate efficiency value of bare SiNW arrays without a Ag BR. (b) Optical losses by the Ag BR are also shown for comparison.
in order to analyze the effects on the ultimate efficiency (Fig. 4(a)) and on the optical losses (Fig. 4(b)) by the Ag BR as a function of the embedded depth of Ag nanoholes. In bare SiNW arrays without a Ag BR, the ultimate efficiencies were calculated to be 8.5%, 15.2%, and 22.7% for wire diameters of 60, 100, and 300 nm, respectively, (see dashed lines in Fig. 4(a)). These ultimate efficiency values are in accordance with those from Ref [16

16. C. Lin and M. L. Povinelli, “Optical absorption enhancement in silicon nanowire arrays with a large lattice constant for photovoltaic applications,” Opt. Express 17(22), 19371–19381 (2009). [CrossRef] [PubMed]

], who reported that ηUE in SiNW arrays increases with increasing lattice constants without changing the filling ratio. To analyze the influence of the Ag NCG layer on the photovoltaic performances, the embedded depths of the nanoholes were varied from 10 to 50 nm in the Ag BR. After integrating the Ag BR with NCG, the ultimate efficiencies were found to have slightly increased by 3~6%, as shown in Fig. 4(a). The highest ηUE was obtained at the embedded depth of 20 nm. More specifically, the SiNW array (a = 100 nm and d = 60 nm) integrated with a 20-nm embedded depth NCG layer exhibited a ηUE of 13.3%, which is ~43% higher than the ultimate efficiency in a bare NW array and ~7% higher than that in a NW array with a flat Ag BR. Similar results were also observed for the SiNW arrays in the other two geometric conditions. In clear contrast to the ultimate efficiency behavior, the optical energy loss by the Ag BR was notably affected by the NCG depths, especially for smaller NW diameter values and lattice constants (Fig. 4(b)). The optical loss by the NCG layers was observed to dramatically increase upon initiation of embedding the nanoholes. Highest energy loss, ηLoss in AgBR reaches ~14.5% for the SiNW array (a=100 nm, d=60 nm) solar cells at the 20-nm embedded depth of a NCG layer. This energy loss value is then gradually decreased as the depths are deeper. Note that the SiNW array with a larger lattice constant recorded the highest ηUE value with the minimum amount (<5%) of optical loss. Due to the LSP resonances at the Ag/Si interface, the optical loss was found to be greater than or equal to the 1.64% loss calculated at the Ag/air interface.

The wavelength-dependent spectra of energy loss are presented in Fig. 5(a-c)
Fig. 5 (a-c) Optical loss spectra for three different geometrical conditions of SiNW arrays as a function of embedded depth of Ag nanoholes. (d-f) Absorption enhancement (AE) in SiNWs with different embedded depths of nanoholes where the AE in the SiNWs integrated with a flat BR (p = 0, dark blue lines) was set to 1 for comparison.
, and the wavelength-dependent absorption enhancements (AE) are shown in Fig. 5(d-e), where the AE is defined as the ratio of light absorptances by the SiNWs integrated with a NCG BR over the NWs using a flat BR. In terms of optical loss, only one wavelength zone (450~700 nm, denoted ‘I’ in Fig. 5(a)) was found for the NW array (a = 100 nm, d = 60 nm) using a flat BR (see the dark blue plot in Fig. 5(a)). After replacing the NCG BR, however, a second loss zone (denoted ‘II’ in Fig. 5(a)) appeared in long wavelengths. In addition, the first loss zones were redshifted, making their peak breadths sharper as a function of the increase in embedded depths (p). This feature can be understood based on the typical excitation behavior of LSPs. Theoretically, LSP peak positions are known to always redshift with an increase in metal film thickness, which guides the interference coupling between direct diffraction and a resonant contribution [31

31. C. Genet, M. P. van Exter, and J. P. Woerdman, “Fano-type interpretation of red shifts and red tails in hole array transmission spectra,” Opt. Commun. 225(4-6), 331–336 (2003). [CrossRef]

]. In our case, a similar coupling behavior at the interference between the diffracted light and the excited surface plasmon was expected to occur at the interface of the NCG BR, causing the LSP resonance peaks to red-shift toward longer wavelengths.

The AE spectra in Fig. 5(d) demonstrate that the absorption in the SiNW with a NCG was significantly enhanced compared to the SiNW with a flat BR. The regions showing the AE >1 are separated into the first zone (denoted ‘II’ in Fig. 5(d)) – centered on 700 nm – and the second zone (denoted ‘III’ in Fig. 5(d)), which ranged over 800−1000 nm. The region showing the AE <1 also exists over a narrow bandwidth around 600 nm (denoted ‘I’ in Fig. 5(d)). By comparison with the corresponding loss plots in Fig. 5(a), these absorption enhancement behaviors can be explained by the optical phase relationship, whereby the scattered light and the LSP-induced dipole waves are out of phase for short wavelengths below the resonance wavelength but became in phase at long wavelengths. As a result, the AE feature in SiNW arrays mostly occurred at long wavelengths owing to the constructive interferences; on the other hand, destructive interference was observed at wavelengths around 600 nm. Similar results were also found for the other SiNW array solar cells with a = 150 nm d = 100 nm, and a = 500 nm, d = 300 nm (see Fig. 5(b-c) and Fig. 5(e-f)). For the SiNW arrays with larger lattice constant and wire diameter values, more peaks of LSP resonance are needed to be further considered along with the increase in the number of guided modes, which makes the light interactions too complicated to be simply described. The common key features evident from the optical loss spectra are as follows: 1) The optical loss (plasmonic-induced) peaks were redshifted for increased nanohole embedded depths. 2) The absorption enhancement in SiNWs occurred at long wavelengths of >700 nm where constructive light interferences are feasible. 3) The optimal value of embedded depth was found to be ~20 nm, at which the light absorption by SiNWs was further enhanced by ~5%.

4. Conclusions

This work theoretically investigated the optical properties of SiNW array solar cells using a FEM method while specifically comparing the flat and NCG Ag BR contacts. Two main routes for optical loss were found to coexist in such SiNW geometries: 1) near-field absorption by localized surface plasmon, and 2) guided mode coupling of the Si nanowire to the Ag BR. Optical losses in the SiNW array solar cells mostly occurred at long wavelengths, which were remarkably larger than those in thin-film solar cells. Highest energy loss in AgBR was almost ~14.5% for the SiNW array (a=100 nm, d=60 nm) solar cells, but for the SiNW array with a=500 nm, d=300 nm, the energy loss was only ~5%. Despite the optical losses by the Ag BR, the highly enhanced near-field caused by the localized SPR at the bottom of the SiNW was helpful in efficiently promoting the ultimate efficiencies of the SiNW solar cells after integrating them with a NCG Ag BR, where the optimal embedded wire depth was ~20 nm.

Acknowledgments

This work was supported by the New & Renewable Energy R&D Program (No: 2009T100100614)) and Human Resources Development (No. 20104010100620) of the Korea Institute of Energy Technology Evaluation and Planning(KETEP) grant funded by the Korea government Ministry of Knowledge Economy. This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No 2011-0028604).

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M. D. Kelzenberg, S. W. Boettcher, J. A. Petykiewicz, D. B. Turner-Evans, M. C. Putnam, E. L. Warren, J. M. Spurgeon, R. M. Briggs, N. S. Lewis, and H. A. Atwater, “Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications,” Nat. Mater. 9(3), 239–244 (2010). [PubMed]

19.

K.-T. Park, Z. Guo, H.-D. Um, J.-Y. Jung, J. M. Yang, S. K. Lim, Y. S. Kim, and J.-H. Lee, “Optical properties of Si microwires combined with nanoneedles for flexible thin film photovoltaics,” Opt. Express 19(S1Suppl 1), A41–A50 (2011). [CrossRef] [PubMed]

20.

Y. Wu, Y. Cui, L. Huynh, C. J. Barrelet, D. C. Bell, and C. M. Lieber, “Controlled Growth and Structures of Molecular-Scale Silicon Nanowires,” Nano Lett. 4(3), 433–436 (2004). [CrossRef]

21.

Z. Zhang, T. Shimizu, L. Chen, S. Senz, and U. Gösele, “Bottom-Imprint Method for VSS Growth of Epitaxial Silicon Nanowire Arrays with an Aluminium Catalyst,” Adv. Mater. (Deerfield Beach Fla.) 21, 4701–4705 (2009).

22.

J. M. Weisse, D. R. Kim, C. H. Lee, and X. Zheng, “Vertical Transfer of Uniform Silicon Nanowire Arrays via Crack Formation,” Nano Lett. 11(3), 1300–1305 (2011). [CrossRef] [PubMed]

23.

J. Kupec and B. Witzigmann, “Computational electromagnetics for nanowire solar cells,” J. Comput. Electron. 11(2), 153–165 (2012). [CrossRef]

24.

E. D. Palik, Handbook of Optical Constants of Solids (Academic, Orlando, Fla., 1985).

25.

A. W. Snyder and J. D. Love, Optical Waveguide Theory (Chapman & Hall, New York, 1983).

26.

K. Seo, M. Wober, P. Steinvurzel, E. Schonbrun, Y. Dan, T. Ellenbogen, and K. B. Crozier, “Multicolored Vertical Silicon Nanowires,” Nano Lett. 11(4), 1851–1856 (2011). [CrossRef] [PubMed]

27.

B. C. P. Sturmberg, K. B. Dossou, L. C. Botten, A. A. Asatryan, C. G. Poulton, C. M. de Sterke, and R. C. McPhedran, “Modal analysis of enhanced absorption in silicon nanowire arrays,” Opt. Express 19(S5Suppl 5), A1067–A1081 (2011). [CrossRef] [PubMed]

28.

J. Kupec and B. Witzigmann, “Dispersion, wave propagation and efficiency analysis of nanowire solar cells,” Opt. Express 17(12), 10399–10410 (2009). [CrossRef] [PubMed]

29.

C. Genet and T. W. Ebbesen, “Light in tiny holes,” Nature 445(7123), 39–46 (2007). [CrossRef] [PubMed]

30.

J. Bhattacharya, N. Chakravarty, S. Pattnaik, W. D. Slafer, R. Biswas, and V. L. Dalal, “A photonic-plasmonic structure for enhancing light absorption in thin film solar cells,” Appl. Phys. Lett. 99(13), 131114 (2011). [CrossRef]

31.

C. Genet, M. P. van Exter, and J. P. Woerdman, “Fano-type interpretation of red shifts and red tails in hole array transmission spectra,” Opt. Commun. 225(4-6), 331–336 (2003). [CrossRef]

OCIS Codes
(350.6050) Other areas of optics : Solar energy
(310.6628) Thin films : Subwavelength structures, nanostructures

ToC Category:
Photovoltaics

History
Original Manuscript: July 20, 2012
Revised Manuscript: August 30, 2012
Manuscript Accepted: August 30, 2012
Published: September 7, 2012

Citation
Keya Zhou, Zhongyi Guo, Xiaopeng Li, Jin-Young Jung, Sang-Won Jee, Kwang-Tae Park, Han-Don Um, Ning Wang, and Jung-Ho Lee, "The tradeoff between plasmonic enhancement and optical loss in silicon nanowire solar cells integrated in a metal back reflector," Opt. Express 20, A777-A787 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-S5-A777


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References

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  15. E. Garnett and P. Yang, “Light Trapping in Silicon Nanowire Solar Cells,” Nano Lett.10(3), 1082–1087 (2010). [CrossRef] [PubMed]
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  17. L. Hu and G. Chen, “Analysis of Optical Absorption in Silicon Nanowire Arrays for Photovoltaic Applications,” Nano Lett.7(11), 3249–3252 (2007). [CrossRef] [PubMed]
  18. M. D. Kelzenberg, S. W. Boettcher, J. A. Petykiewicz, D. B. Turner-Evans, M. C. Putnam, E. L. Warren, J. M. Spurgeon, R. M. Briggs, N. S. Lewis, and H. A. Atwater, “Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications,” Nat. Mater.9(3), 239–244 (2010). [PubMed]
  19. K.-T. Park, Z. Guo, H.-D. Um, J.-Y. Jung, J. M. Yang, S. K. Lim, Y. S. Kim, and J.-H. Lee, “Optical properties of Si microwires combined with nanoneedles for flexible thin film photovoltaics,” Opt. Express19(S1Suppl 1), A41–A50 (2011). [CrossRef] [PubMed]
  20. Y. Wu, Y. Cui, L. Huynh, C. J. Barrelet, D. C. Bell, and C. M. Lieber, “Controlled Growth and Structures of Molecular-Scale Silicon Nanowires,” Nano Lett.4(3), 433–436 (2004). [CrossRef]
  21. Z. Zhang, T. Shimizu, L. Chen, S. Senz, and U. Gösele, “Bottom-Imprint Method for VSS Growth of Epitaxial Silicon Nanowire Arrays with an Aluminium Catalyst,” Adv. Mater. (Deerfield Beach Fla.)21, 4701–4705 (2009).
  22. J. M. Weisse, D. R. Kim, C. H. Lee, and X. Zheng, “Vertical Transfer of Uniform Silicon Nanowire Arrays via Crack Formation,” Nano Lett.11(3), 1300–1305 (2011). [CrossRef] [PubMed]
  23. J. Kupec and B. Witzigmann, “Computational electromagnetics for nanowire solar cells,” J. Comput. Electron.11(2), 153–165 (2012). [CrossRef]
  24. E. D. Palik, Handbook of Optical Constants of Solids (Academic, Orlando, Fla., 1985).
  25. A. W. Snyder and J. D. Love, Optical Waveguide Theory (Chapman & Hall, New York, 1983).
  26. K. Seo, M. Wober, P. Steinvurzel, E. Schonbrun, Y. Dan, T. Ellenbogen, and K. B. Crozier, “Multicolored Vertical Silicon Nanowires,” Nano Lett.11(4), 1851–1856 (2011). [CrossRef] [PubMed]
  27. B. C. P. Sturmberg, K. B. Dossou, L. C. Botten, A. A. Asatryan, C. G. Poulton, C. M. de Sterke, and R. C. McPhedran, “Modal analysis of enhanced absorption in silicon nanowire arrays,” Opt. Express19(S5Suppl 5), A1067–A1081 (2011). [CrossRef] [PubMed]
  28. J. Kupec and B. Witzigmann, “Dispersion, wave propagation and efficiency analysis of nanowire solar cells,” Opt. Express17(12), 10399–10410 (2009). [CrossRef] [PubMed]
  29. C. Genet and T. W. Ebbesen, “Light in tiny holes,” Nature445(7123), 39–46 (2007). [CrossRef] [PubMed]
  30. J. Bhattacharya, N. Chakravarty, S. Pattnaik, W. D. Slafer, R. Biswas, and V. L. Dalal, “A photonic-plasmonic structure for enhancing light absorption in thin film solar cells,” Appl. Phys. Lett.99(13), 131114 (2011). [CrossRef]
  31. C. Genet, M. P. van Exter, and J. P. Woerdman, “Fano-type interpretation of red shifts and red tails in hole array transmission spectra,” Opt. Commun.225(4-6), 331–336 (2003). [CrossRef]

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