Plasmon enhancement of optical absorption in ultra-thin film solar cells by rear located aluminum nanodisk arrays

Transcription

1 Opt Quant Electron (2017)49:161 DOI /s x Plasmon enhancement of optical absorption in ultra-thin film solar cells by rear located aluminum nanodisk arrays Debao Zhang 1 Yawei Kuang 1 Xuekun Hong 1 Yushen Liu 1 Xifeng Yang 1 Received: 25 August 2016 / Accepted: 27 January 2017 Springer Science+Business Media New York 2017 Abstract In this work, in order to enhance the light absorption in one micron thick crystalline silicon solar cells, a back reflecting and rear located plasmonic nanodisk scheme is proposed. We investigate the scattering properties of aluminum nanostructures located at the back side and optimize them for enhancing absorption in the silicon layer by using finite difference time domain simulations. The results indicate that the period and diameters of nanodisks, thickness of spacer layer have a strong impact on short circuit current enhancements. The optimized Al nanoparticle arrays embedded in rear located SiO 2 layer enhance J sc with an increase of 47% from the non-plasmonic case of 18.9 to 27.8 ma/cm 2 when comparing with a typical stack with a planar aluminum back reflector and a back reflector with plasmonic nanoparticles. This finding could lead to improved light trapping within a thin silicon solar cell device. Keywords Plasmon enhancement Rear nanoparticle array Surface plasmon Thin film solar cell 1 Introduction The low absorption coefficient in silicon thin-film solar cells for light in the red to near infrared (NIR) region results in poor efficiencies (Catchpole and Polman 2008). Plasmonic metal nanoparticles are of great practical interest in the field of thin film photovoltaics due This article is part of the Topical Collection on Numerical Simulation of Optoelectronic Devices Guest edited by Yuh-Renn Wu, Weida Hu, Slawomir Sujecki, Silvano Donati, Matthias Auf der Maur and Mohamed Swillam. & Xifeng Yang xfyang@cslg.cn 1 Department of Physics, Changshu Institute of Technology, No.99, Nansanhuan Rd., Changshu , Jiangsu, China

2 161 Page 2 of 8 D. Zhang et al. to their special scattering abilities of localized surface-plasmon resonances (LSPR) (Miao et al. 2015; Zhu et al. 2016) and resonant surface plasmon polaritons (SPP) excitations at the particle/substrate interface, as a means to increase coupling of light into silicon (Atwater and Polman 2010; Catchpole and Polman 2008). Extensive work by various groups has been put forth to investigate light enhancement using Ag metal particles on the front surface or back side of thin-film Si solar cells (Akimov and Koh 2011; Ouyang et al. 2010; Schaadt et al. 2005; Stuart and Hall 1998; Winans et al. 2015). Earlier studies (Beck et al. 2010) suggested that using plasmonic nanostructures at the front side of the solar cell results in a decrease in its energy conversion efficiency, attributed to the fact that the destructive interference between the incident and scattered light leads to suppressed absorption in the cell below the resonance wavelength of the particles. Therefore, the plasmons of the metal nanoparticles on the rear would be excited by only light that is not absorbed by the cell (transmitted light). Even thin film Si cells normally absorb the short wavelength light strongly, hence any likely absorption in the metals or suppression of below resonance wavelengths can be avoided. Silver can support a strong resonance, which is widely tunable by varying the size (d and h), shape (L) and dielectric function of the particle, but it appears to be difficult to blue shift the cross-over point below 500 nm while maintaining a sufficiently large nanoparticle. Aluminum, on the other hand, also supports a strong plasmon resonance, which has been shown to be tunable from the infrared to the ultraviolet by varying the shape and size of the particle (Langhammer et al. 2008). In this work, we propose a back reflecting and rear located plasmonic nanodisk array scheme and optimize the architecture by employing Al metallic nanostructures with varied geometry parameters, so as to maximize scattering light back into silicon. 2 Simulation models Finite-difference time-domain (FDTD) numerical simulations (Hu et al. 2013; Liang et al. 2014; Zhang et al. 2016) were performed to simulate the absorption in the active layer of the cell, from which we calculate the absorption enhancement factor G and the expected short circuit current density (J sc ) output in the wavelength range from 300 to 1200 nm. The cell structure consists of a 1 lm thick crystalline silicon absorber with 70 nm of silicon nitride for anti-reflection. Figure 1 illustrates a cross-section of the simulated geometry, with the Al nanodisk arrays on the rear of the cell, and a 100 nm thick Al mirror separated by a SiO 2 gap. The light source is located upon anti-reflecting coating. The dielectric parameters used for the silicon, aluminum, SiO 2 and Si 3 N 4 layers were taken from Piller and Palik (1985). For the FDTD simulations, the x and y axis boundary conditions are periodic and the z axis boundaries are defined by perfectly matched layers (PML). The PML boundary at the negative Z-axis was extended through the aluminum reflector. The aluminum nanodisk area is meshed by mesh cells of 1 nm. All other parts of the simulation are meshed by cells of 2 nm. Power monitors are placed on each side of the silicon layer and the absorption within the silicon is calculated as the difference in power flux between the two monitors. The light source used in the simulation is a short-pulse (broad wavelength) source allowing us to calculate the full spectral response in a single simulation. It is assumed that all absorption within the silicon leads to generated electron hole pairs that are then collected in the external circuit i.e. the external quantum efficiency (EQE) is equal to the absorption

3 Plasmon enhancement of optical absorption in ultra-thin Page 3 of 8161 Fig. 1 (Color online) Structure setup for the cell and FDTD simulations model within the silicon. The J sc is calculated by integrating the external quantum efficiency response over the wavelength range from 300 to 1200 nm multiplied by the corresponding spectral photon flux and the electronic charge. Z qk J sc ¼ hc EQEðkÞI AM1:5ðkÞdk where q is the elementary charge, k is the wavelength, h is the Planck constant, c is the speed of light, and I AM1.5 is the weighted sun spectrum (AM1.5 spectral irradiance). The absorption enhancement factor G for the wavelength range nm can be written as G = IQE np /IQE bare, where IQE nps and IQE bare are the integrated quantum efficiency of the whole wavelength range with and without Al nano-disks, respectively (Zhang et al. 2015). 3 Results and discussion A first rough scan was performed to get the optimum values for period and diameter of nanodisks without back aluminum reflector, as shown in Fig. 2. In practice, periodic arrays of metal NPs are deposited onto plasmonic thin film solar cells (Atwater and Polman 2010; Ferry et al. 2010). Therefore, it is essential to consider the impact of different periods on the enhancement of light absorption. It can be seen from Fig. 2 that both diameter and period have a significant impact on the absorption of the silicon in solar cell. The enhancement factor G increases first and then decreases significantly with increasing period, which indicates that the absorption enhancement of the aluminum nanodisk array with a closed period is degraded due to the interparticle coupling effects. As disk diameter increases, the plasmon resonance peak is spectrally red-shifting with increasing disk diameter due to the dependence of the LSPR energy on aspect ratio (and volume) (Schaadt et al. 2005) and there tends to be a reduction in absorption at shorter wavelengths, whereas at longer wavelengths there is an increase (Pillai et al. 2007).

4 161 Page 4 of 8 D. Zhang et al. Fig. 2 (Color online) The relation of absorption enhancement factor G with period at different radii of nanodisks G Diameters/nm Period(nm) Considering that there is a peak of the AM 1.5 solar spectrum at the wavelength near 500 nm, it is easily to understood that the enhancement factor G increase and then decrease with increasing nanoparticle size. Diameter of 220 nm, height of 50 nm, spacer thickness of 10 nm, and period of 3300 nm were chosen as starting parameters based on previous studies (Spinelli et al. 2011) and the first rough scan. The thickness of the spacer was kept constant at 10 nm while the distance between silicon and reflector, i.e. SiO 2 layer thickness, was scanned form 110 to 570 nm, with a step of 20 nm. From the J sc curve shown in Fig. 3a, 180 nm of SiO 2 layer scatters the light into silicon most effectively and gives a J sc value of ma/cm 2. The thickness of the nanodisks was scanned from 20 to 100 nm, for every 10 nm. From the J sc curve shown in Fig. 3b, it is observed that by decreasing the height of nanodisks below 30 nm, it decreases the scattering efficiency. A 30 nm height of nanodisks scatters the light into silicon most effectively and gives a J sc value of ma/cm 2. The diameter of aluminum nanodisks was scanned from 80 to 300 nm for every 10 nm, keeping their thickness constant at 50 nm. Decreasing the diameter below 180 nm increases the Ohmic absorption rapidly as the particle size becomes too small with respect to the wavelength of incident light. For the particles with bigger diameter (*300 nm), the resonance shifts away from the desired part of the spectrum. A 180 nm diameter of nanodisks scatters the light into silicon most effectively and gives a J sc value of ma/ cm 2. Compared with the optimum diameter of without back aluminum reflector, it is clearly that the optimum diameter of plasmonic nanodisk decrease due to back aluminum film reflecting more light. The effect of silicon on the scattering properties of the plasmonic nanostructures will be modified due to the presence of the layered substrate (Malinsky et al. 2001). The closer the structure is to the high index medium, the higher is the effect on the homogeneity of its surrounding and hence on the preferential scattering properties. The effect of the spacer thickness was studied on the structure from 0 nm (i.e. nanodisks directly on the silicon) to 60 nm, for every 1 nm. It can be seen in Fig. 3d that a 6 10 nm of spacer layer results in the highest enhancement in the silicon layer with J sc value about 26.2 ma/cm 2. After increasing the spacer thickness, silicon s ability to modify the surrounding environment around the plasmonic particle starts decreasing and keeps on decreasing as the plasmonic nanostructure moves away from silicon. Another reason is the evanescent nature of the

5 Plasmon enhancement of optical absorption in ultra-thin Page 5 of 8161 (a) (b) J sc (ma/cm 2 ) J sc (ma/cm 2 ) Thickness of SiO 2 layer (nm) Height of nanodisk(nm) (c) 28 (d) J sc (ma/cm 2 ) J sc (ma/cm 2 ) Diameter of nanodisk (nm) Thickness of spacer layer(nm) Fig. 3 The dependence of J sc on various parameters: a thickness of SiO 2 layer, b height of nanodisk, c diameter of nanodisk, d thickness of spacer surface plasmon field, which decreases very rapidly from the surface of the nanoparticle (Lim et al. 2007). Thus, the further the nanoparticle is from silicon, the weaker is the coupling of the enhanced field into the silicon. A significant enhancement is achieved, as shown in Fig. 4, which shows the comparison of absorption in silicon between the structure with aluminum nanodisk array (red dot) and without (black solid) nanodisk array as the reference structure. One can clearly see that there is broadband enhancement in absorption throughout the spectrum, especially from *600 to 1100 nm. The J sc is increased from 18.9 ma/cm 2 for the reference structure without nanodisk array, to 27.8 ma/cm 2 for the structure with aluminum nanodisk array, which amounts to *47.1% AM1.5 photon absorption enhancement in silicon. We also compare calculated electric field intensity to verify the absorption enhancement in the silicon layer. Figure 5 shows the electric field profiles ( E ) at the wavelengths of (a), (b), (c) 967 nm and (d), (e), (f) 1071 nm, which are the wavelengths of two peaks in Fig. 4. Data is shown for (a), (d) bare silicon and (b), (c), (e), (f) decorated with Al nanodisks, which has a base diameter of 180 nm and a height of 50 nm. The magnitude of the electric field, E, is calculated in the (a), (b), (d), (e) x z plane bisecting the cell and nanoparticle, or (c), (f) x y plane through the interface between Al nanodisks and silicon substrate. Similar field profiles are observed for reference cell without rear-located particles; however, the field plots clearly show that the electric field is pronounced enhanced in the silicon with rear-located nanodisk array, especially at the wavelength of 1071 nm (Fig. 5e). Conversely the electric field at the particle/substrate interface at the wavelength of 1071 nm (Fig. 5e, f) is reduced, compared with that at the wavelength of 967 nm

6 161 Page 6 of 8 D. Zhang et al. Fig. 4 (Color online) absorption in silicon without (black solid) or with aluminum nanodisk array (red dot) Absorption nm Al disks Reference(w/o nanodisks) Wavelength (nm) Si SiO 2 (a) (b) (c) Si SiO 2 (d) (e) (f) Fig. 5 (Color online) The electric field profiles ( E ) calculated in the (a), (b), (d), (e) x z plane bisecting the cell and nanoparticle, or (c), (f) x y plane through the interface between Al nanodisks and silicon substrate, with light incident from the silicon at the wavelengths of (a), (b), (c) 967 nm and (d), (e), (f) 1071 nm. Data is shown for Bare silicon (a), (d) and decorated (b), (c), (e), (f) with Al nanodisks, which has a base diameter of 180 nm and a height of 30 nm (Fig. 5b, c). The enhancement can be attributed to the excitation of resonant surface plasmon polaritons modes at the particle/substrate interface. These types of resonant modes have increased excitation efficiencies when the particle near-field overlaps significantly with the substrate. This can lead to very high scattering cross-sections for particles that support resonant SPP modes at the aluminum/substrate interface (Beck et al. 2011). 4 Conclusion In summary, we have studied numerically aluminum nanodisk arrays embedded in dielectric for light trapping on the rear of 1 lm thin film silicon solar cells with a reflector. The effects of plasmonic nanodisks height, diameter, and thickness of spacer layer on their

7 Plasmon enhancement of optical absorption in ultra-thin Page 7 of 8161 scattering behavior were investigated and then optimized the geometrical properties of the arrays to maximize the scattering and reflection into cell. The optimized Al nanoparticle arrays embedded in rear located SiO 2 layer enhance J sc with an increase of 47% from the non-plasmonic case of 18.9 to 27.8 ma/cm 2 when comparing with a typical stack with a planar aluminum back reflector and a back reflector with plasmonic nanoparticles. The results clearly demonstrate the advantages offered by aluminum nanoparticles for rear surface light trapping on thin film solar cells with back reflector. The optimized structure has a spacer thickness of 10 nm, aluminum nanodisks of height 30 nm, diameter 180 nm, and SiO 2 layer of thickness 180 nm. Acknowledgements This work was supported in part by the National Natural Science Foundation of China (Grant No and ). References Akimov, Y.A., Koh, W.S.: Design of plasmonic nanoparticles for efficient subwavelength light trapping in thin-film solar cells. Plasmonics 6(1), (2011) Atwater, H.A., Polman, A.: Plasmonics for improved photovoltaic devices. Nat. Mater. 9(3), (2010) Beck, F.J., Mokkapati, S., Polman, A., Catchpole, K.R.: Asymmetry in photocurrent enhancement by plasmonic nanoparticle arrays located on the front or on the rear of solar cells. Appl. Phys. Lett. 96(3), (2010) Beck, F.J., Verhagen, E., Mokkapati, S., Polman, A., Catchpole, K.R.: Resonant SPP modes supported by discrete metal nanoparticles on high-index substrates. Opt. Express 19(102), A146 A156 (2011) Catchpole, K.R., Polman, A.: Plasmonic solar cells. Opt. Express 16(26), (2008) Ferry, V.E., Munday, J.N., Atwater, H.A.: Design considerations for plasmonic photovoltaics. Adv. Mater. 22(43), (2010) Hu, W., Wang, L., Chen, X., Guo, N., Miao, J., Yu, A., Lu, W.: Room-temperature plasmonic resonant absorption for grating-gate GaN HEMTs in far infrared terahertz domain. Opt. Quantum Electron. 45, (2013) Langhammer, C., Schwind, M., Kasemo, B., Zoric, I.: Localized surface plasmon resonances in aluminum nanodisks. Nano Lett. 8(5), (2008) Liang, J., Hu, W., Ye, Z., Liao, L., Li, Z., Chen, X., Lu, W.: Improved performance of HgCdTe infrared detector focal plane arrays by modulating light field based on photonic crystal structure. J. Appl. Phys. 115(18), (2014) Lim, S.H., Mar, W., Matheu, P., Derkacs, D., Yu, E.T.: Photocurrent spectroscopy of optical absorption enhancement in silicon photodiodes via scattering from surface plasmon polaritons in gold nanoparticles. J. Appl. Phys. 101(10), (2007) Malinsky, M.D., Kelly, K.L., Schatz, G.C., Van Duyne, R.P.: Nanosphere lithography: effect of substrate on the localized surface plasmon resonance spectrum of silver nanoparticles. J. Phys. Chem. B 105(12), (2001) Miao, J., Hu, W., Jing, Y., Luo, W., Liao, L., Pan, A., Wu, S., Cheng, J., Chen, X., Lu, W.: Surface plasmonenhanced photodetection in few layer MoS2 phototransistors with Au nanostructure arrays. Small 11(20), (2015) Ouyang, Z., Pillai, S., Beck, F., Kunz, O., Varlamov, S., Catchpole, K.R., Campbell, P., Green, M.A.: Effective light trapping in polycrystalline silicon thin-film solar cells by means of rear localized surface plasmons. Appl. Phys. Lett. 96(26), (2010) Pillai, S., Catchpole, K.R., Trupke, T., Green, M.A.: Surface plasmon enhanced silicon solar cells. J. Appl. Phys. 101(9), (2007) Piller, H., Palik, E.D.: Handbook of Optical Constants of Solids. Academic Press, New York (1985) Schaadt, D.M., Feng, B., Yu, E.T.: Enhanced semiconductor optical absorption via surface plasmon excitation in metal nanoparticles. Appl. Phys. Lett. 86(6), (2005) Spinelli, P., Hebbink, M., De Waele, R., Black, L., Lenzmann, F., Polman, A.: Optical impedance matching using coupled plasmonic nanoparticle arrays. Nano Lett. 11(4), (2011) Stuart, H.R., Hall, D.G.: Island size effects in nanoparticle-enhanced photodetectors. Appl. Phys. Lett. 73(26), (1998)

8 161 Page 8 of 8 D. Zhang et al. Winans, J.D., Hungerford, C., Shome, K., Rothberg, L.J., Fauchet, P.M.: Plasmonic effects in ultrathin amorphous silicon solar cells: performance improvements with Ag nanoparticles on the front, the back, and both. Opt. Express 23(3), A92 A105 (2015) Zhang, D., Yang, X., Hong, X., Liu, Y., Feng, J.: Aluminum nanoparticles enhanced light absorption in silicon solar cell by surface plasmon resonance. Opt. Quantum Electron. 47(6), (2015) Zhang, D., Yang, X., Hong, X., Liu, Y., Feng, J.: Scattering of light into thin film solar cells by rear located hemispherical silver nanoparticles. Opt. Quantum Electron. 48(2), (2016) Zhu, Z., Zou, Y., Hu, W., Li, Y., Gu, Y., Cao, B., Guo, N., Wang, L., Song, J., Zhang, S., Gu, H.: Nearinfrared plasmonic 2D semimetals for applications in communication and biology. Adv. Funct. Mater. 26(11), (2016)