Biomimetic and plasmonic hybrid light trapping for ... - OSA Publishing

Biomimetic and plasmonic hybrid light trapping for highly efficient ultrathin crystalline silicon solar cells Y. Zhang, B. Jia, and M. Gu* Centre for Micro-Photonics, Faculty of Science, Engineering and Technology, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia * [email protected]

Abstract: Designing effective light-trapping structures for the insufficiently absorbed long-wavelength light in ultrathin silicon solar cells represents a key challenge to achieve low cost and highly efficient solar cells. We propose a hybrid structure based on the biomimetic silicon moth-eye structure combined with Ag nanoparticles to achieve advanced light trapping in 2 μm thick crystalline silicon solar cells approaching the Yablonovitch limit. By synergistically using the Mie resonances of the silicon moth-eye structure and the plasmonic resonances of the Ag nanoparticles, the integrated absorption enhancement achieved across the usable solar spectrum is 69% compared with the cells with the conventional light trapping design. This is significantly larger than both the silicon motheye structure (58%) and Ag nanoparticle (41%) individual light trapping. The generated photocurrent in the 2 μm thick silicon layer is as large as 33.4 mA/cm2, which is equivalent to that generated by a 30 μm single-pass absorption in the silicon. The research paves the way for designing highly efficient light trapping structures in ultrathin silicon solar cells. ©2016 Optical Society of America OCIS codes: (350.6050) Solar energy; (250.5403) Plasmonics; (310.6628) Subwavelength structures, nanostructures.

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Received 11 Nov 2015; revised 30 Dec 2015; accepted 29 Jan 2016; published 16 Feb 2016 21 Mar 2016 | Vol. 24, No. 6 | DOI:10.1364/OE.24.00A506 | OPTICS EXPRESS A506

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1. Introduction Ultrathin crystalline Si solar cells with micrometer thickness have attracted intensive research interest in photovoltaics in recent years due to their great potential in reducing the cost and increasing the electrical performance of the solar cells [1–6]. In such a thin Si layer, the conventional pyramid light trapping structures, with a feature size of 3 - 8 μm, are no longer applicable due to the wafer thickness limitation [7, 8]. Therefore, advanced light trapping strategies are essential to significantly enhance the light absorption to achieve high efficiency solar cells. A variety of nanostructures, including Si nanostructures, dielectric nanospheres and plasmonic nanoparticles, have been developed to enhance the light absorption in the solar cells [9–28]. To ensure a broadband light absorption, one needs to consider not only the antireflection design allowing the entire usable solar spectrum to enter the solar cells, but also the effective light trapping of the weakly-absorbed long-wavelength light inside solar cells in particular for ultrathin Si thickness to prevent the escape of the unabsorbed light. Biomimetic moth-eye structures of sub-wavelength scale have been demonstrated to be able to realize an almost perfect antireflection across a fairly wide solar spectrum [13–18]. The main mechanism is based on the graded refractive index matching between air and Si. Compared with the conventional pyramid textured surface and SiNx antireflection coating (ARC), this structure allows even better light coupling into Si. Furthermore, the sub-

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wavelength scale suits perfectly the ultrathin Si wafers with a few microns. However, the light trapping effect of this structure is unclear, in particular the resonant scattering property, which is critical for long-wavelength light when the structures are applied on ultrathin Si wafers. On the other hand, metallic nanoparticles, which support localized surface plasmon resonances, are known to have excellent light trapping performance due to strong far-field scattering and near-field light concentration with a tunable wavelength [19–28]. The top side located nanoparticles enhance the light absorption in the active layer by forward-scattering, which reduces the light reflection and improves the light path length. However, the nanoparticles in this case introduce unavoidable intrinsic plasmonic absorption and the Fano effect reduces transmission at wavelengths below the plasmon resonance. Compared with the top side located nanoparticle geometry, the bottom side located nanoparticle geometry potentially achieves a stronger solar cell absorption enhancement for the long-wavelength light with ignorable influence on the short-wavelengths light. In this paper, we propose a hybrid resonant structure, employing the top surface moth-eye sub-wavelength structure (paraboloid shape) and the bottom side located metallic Ag nanoparticles to maximize the light absorption of the large portion long-wavelength light in addition to the demonstrated perfect antireflection of the moth-eye structure. Theoretically, we firstly investigate the resonance-induced properties, including scattering and absorption of the paraboloid Si moth-eye element and the Ag nanoparticles by using the finite-difference time-domain (FDTD) method. Then we apply the hybrid structure on a 2 µm thick ultrathin Si wafer. The light absorption and the corresponding photocurrent are optimized. A bare solar cell with a 75 nm SiNx ARC on the top surface and an Al reflector at the bottom is calculated as a reference. We also calculate the absorption of the solar cells with the top moth-eye only and the bottom nanoparticle only structures for comparison. The hybrid configuration is shown in Figs. 1(a) and 1(b), which consist of paraboloid-shaped Si structure at the top surface and hemispherical Ag nanoparticles embedded in the SiO2 dielectric layer between the Si layer and the Al reflector layer at the bottom. Hemispherical nanoparticles are used due to the better coupling of the scattered light into the Si layer [23, 24]. The thickness of the SiO2 layer is 200 nm and the thickness of the spacing layer between the nanoparticles and the Si is 20 nm.

Fig. 1. (a) Schematic of the hybrid light trapping structure with the top surface moth-eye structure and the bottom hemispherical Ag nanoparticles. (b) Cross-sectional view of the hybrid light trapping structure. Grey: Si, red: Ag nanoparticles, green: Al reflector.

2. Methods To investigate the scattering and absorption properties of the Si moth-eye and the Ag nanoparticle, we calculate their normalized scattering/absorption cross-sections with Lumerical FDTD solutions, respectively [29]. Total field scattered-field (TFSF) source with a wavelength range of 300 - 1200 nm are used to cover the usable solar spectrum for Si. Perfectly matched layer (PML) boundary conditions are used in all directions in the scattering/absorption cross-section calculation. Two analysis groups, comprising 6 transmission monitors for each group in 3D, were placed inside and outside the light source #253724 © 2016 OSA


box. The inside and outside groups were used to obtain the absorption power and the scattering power, respectively. To investigate the light trapping performance by the hybrid structure, the absorption spectrum in the Si layer is calculated. In this case, a plane wave source with the same spectrum range is used. PML boundary conditions are also used in the incident direction to prevent interference effect whereas periodic boundary conditions are used in the lateral direction to simulate an ordered array. Two transmission monitors are placed slightly above the moth-eye structure and at the bottom surface of the Si layer. The dielectric constants of the metallic nanoparticles and Si are taken from Palik [30] and that of SiNx is measured by an ellipsometer from the commercial Si wafer solar cells. We show in Fig. 2 the refractive indices of the Si and the SiNx as a reference. For comparison, the singlepass absorption and the Yablonovitch limit [31, 32] are calculated, respectively. It has been found that the following four parameters play a key role in determining the light trapping performance: the base diameter D1 and the aspect ratio AR of the top moth-eye element; the bottom nanoparticle diameter D2 and the coverage density C. We conduct a comprehensive optimization process of the geometries to cover all the four parameters. A nested parameter sweep of the base diameter D1 is conducted in 100 nm steps from 300 nm to 1200 nm and the aspect ratio AR ranging from 1 to 2 with an interval of 0.1. The nanoparticle diameter D2 and the coverage density C are swept in the range of 50 - 300 nm and 5 - 50% with intervals of 50 nm and 5%, respectively. In the meantime, we tune the thickness of the flat Si layer to ensure an equivalent Si volume with that in the flat cell geometry.

Fig. 2. Complex refractive indices n + ki of (a) the Si used in the FDTD and (b) the measured SiNx.

The paraboloid of the moth-eye structure is generated according to the following equation. S=

4AR 2 r D1

(1)

where S indicates the height and r is the radius of the paraboloid; AR and D1 represent the aspect ratio and the base diameter of the moth-eye element, respectively.

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3. Results and discussion

Fig. 3. Normalized scattering/absorption cross-sections of (a) the moth-eye element with base diameter 1000 nm and aspect ratio 1.8 and (b) the hemispherical Ag nanoparticle embedded in SiO2 with a diameter of 250 nm, in comparison to that with the Si layer present.

Figures 3(a) and 3(b) present the normalized scattering/absorption cross-sections of the Si moth-eye on top of a flat Si layer with the optimized base diameter D1 = 1000 nm and the aspect ratio AR = 1.8 and a 250 nm hemispherical Ag nanoparticle embedded in the SiO2 medium. It can be observed multiple higher-order Mie resonances are excited in the Si motheye in the spectrum range 300 - 1200 nm, leading to strong scattering at particular wavelengths. The moth-eye acts as a cavity, selectively supporting the resonances. On the other hand, the absorption only occurs at the short wavelengths below 800 nm. As the Si moth-eye is part of the active layer of the solar cells, both the absorption and scattering contribute to the total absorption. However, only the scattering contributes to the longwavelength light trapping. Compared with the moth-eye, the plasmonic scattering of the Ag nanoparticles occurs in a relatively broad spectrum while the absorption is very low at the wavelengths above 600 nm. As the Si layer is out of the near-field range of the Ag nanoparticles, the near-field light concentration would contribute marginally to the Si absorption and the main mechanism is the plasmonic scattering. It should be noted that the presence of the Si layer affects the scattering and the absorption of the Ag nanoparticle, as shown in the dash lines of Fig. 3(b). It is expected the combination of these two structures potentially leads to a strong and broadband light trapping at long-wavelengths above 600 nm where the 2 μm thick Si is weakly light-absorbing through making use of two different mechanisms Figure 4 shows the optimized light absorption spectra of the Si layers and the calculated photocurrent density for the four different cell geometries, referenced to the Yablonovitch limit and the single-pass absorption. The absorption enhancements, relative to the bare solar cells are presented in Fig. 4(f). The absorption peaks, particularly the distinct peaks in Figs. 4(a)-4(c) stem from the guided resonances excited and coupled from the moth-eye structure and the Ag nanoparticles while the regular peaks in Fig. 4(d) are the results of Fabry-Perot optical interference. Figure 4(a) gives the light absorption spectrum for the hybrid structure, reaching the Yablonovitch limit when D1 = 1000 nm, AR = 1.8, D2 = 250 nm, and C = 20%. The flat part of the Si is 1.29 μm to ensure an equivalent thickness of 2 μm. It can be observed the absorption is strongly enhanced in the range 600 - 1200 nm and 300 - 1200 nm compared with that by the Si moth-eye structure and Ag nanoparticle individual light trapping structure in Figs. 4(b) and 4(c), respectively. This is due to both the Mie resonance of the moth-eye structure and the plasmonic Ag nanoparticle scattering. The corresponding photocurrent density is 32.7 mA/cm2, representing an enhancement of 66% compared with that (19.7 mA/cm2) in the bare structure. This is comparable to that of a 25 μm single-pass absorption. The absorption gap between the hybrid structure and the Yablonovitch limit is mainly a result

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of the light absorption losses in the bottom Ag nanoparticles and the Al reflector. It has been observed the absorption in the long wavelength range from1.1 μm to 1.2 μm slightly surpasses the Yablonovitch limit. This is possibly introduced by the material fitting of the Si refractive indices in the FDTD modelling in this range. In addition, the calculation considers the wave effect inside the Si layer while the Yablonovitch limit is based on the ray optics.

Fig. 4. Si absorption spectra (blue) for the four different structures (a) hybrid (b) top moth-eye only (c) bottom nanoparticle only (d) bare, compared with that in the single-pass (black) and the Yablonovitch limit (red). The inset diagrams present the corresponding solar cell structures. Grey: Si, red: Ag nanoparticles, green: Al reflector, blue: SiNx. The photocurrent density (e) of (a-d) and the single-pass absorption (black dotted line), the Yablonovitch limit (red dotted line) and the full absorption of the entire available solar spectrum from 300 nm to 1200 nm (green dotted line). (f) Absorption enhancement by the structures (a)-(c), relative to that by structure (d).

The optimized absorption spectrum of the top moth-eye only structure in Fig. 4(b) shows an entire wavelength enhancement, compared with the absorption of the bare cell in Fig. 4(d). The optimized photocurrent is 30.5 mA/cm2 with the base diameter D1 = 1000 nm and aspect ratio AR = 1.8. The broadband absorption enhancement is due to the wavelength-independent

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antireflection effect and the light trapping of the moth-eye structure. This structure creates a graded average refractive index from the air to Si for all the wavelengths, leading to a perfect impedance match between the air and Si. The higher the aspect ratio is, the better the light coupling is. For the base diameters from 300 nm to 1200 nm, it is observed the light coupling into Si can reach more than 99% when the aspect ratio AR is more than 2. The moth-eye structures at the same time exhibit scattering effect (Fig. 3(a)), redirecting the normal incident light with an oblique angle thereby a light path length enhancement for the long-wavelength light (light trapping). This leads to strong absorption enhancement at the long wavelengths, compared with the single-pass absorption. The optimized result is a trade-off between the light incoupling for the whole wavelengths and the light trapping for the long-wavelength light roughly above 600 nm where the 2 μm Si layer is weakly absorbing. The optimized light absorption in the Si by the bottom plasmonic Ag nanoparticles is shown in Fig. 4(c). The absorption enhancement can be as large as 28% for the optimized particle diameter D2 = 200 nm and coverage C = 20%, with a photocurrent of 25.3 mA/cm2. The optimum size is a trade-off between the scattering red-shift and the higher-order plasmon mode excitation while the particle diameter increases [23]. The coverage density is mainly determined by the scattering cross-sections of the particles. Calculation shows the scattering cross-section of the 200 nm Ag particle is approximately five times of the geometric crosssection at the long wavelengths, indicating a 20% coverage is sufficient to provide a full-area light-material interaction. As expected, the absorption enhancement is achieved only at the wavelengths longer than 600 nm where the light would otherwise easily escape from the top surface in the flat solar cells. The Ag nanoparticles at the bottom side scatter the long wavelength light travelling to the bottom side in a wide angle thereby enhancing the light path length. It should be noted that, in this geometry, around 10% light is firstly reflected at the top surface due to the narrowband antireflection of the SiNx ARC. Therefore, a proper antireflection design for the whole wavelengths is essential to achieve both the effective light trapping and the antireflection.

Fig. 5. Electric field distributions inside the Si layers of the four solar cell configurations (a) hybrid (b) top only (c) bottom only (d) bare at the wavelength 900 nm. (The red dash lines indicate the interfaces of the Si moth-eye structure and the air).

To understand the optical behaviors inside the Si layer, we calculate the electric field distribution illuminated with a 900 nm incident light beam, with the results shown in Fig. 5. The obvious wave effect inside the Si leads to the absorption peaks in Fig. 4. The strongest electric field in Fig. 5(a) also verifies the largest absorption enhancement achieved by the hybrid structure, compared with the other structures. The field pattern in Fig. 5(d) is due to the interference effect in the vertical direction between the incident light entering the Si and the reflected light from the bottom surface of the Si layer. However, the fields in the other structures are results of both the vertical interference modes and the horizontal guided modes, which are excited by the leaking of the Mie resonances from the moth-eye structure and the coupling of the scattered light by Ag particles, generating extra spectra absorption peaks in Figs. 4(a), 4(b) and 4(c).

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Introducing metallic structures at the bottom side of the solar cells means a certain amount of light loss. To evaluate the losses in the hybrid light trapping structure, the absorption in the Ag nanoparticles and the Al layer were calculated, with the results shown in Fig. 6(a). The absorption in the Ag nanoparticles and the Al both start from the wavelength around 600 nm, where the light starts to transmit to the bottom side of the solar cells. The loss in the Ag nanoparticles is more severe than that in the Al reflector, with the average value reaching around 10% from 800 nm to 1200 nm. It is interesting that the calculated normalized absorption cross-section of the Ag nanoparticles does not indicate obvious absorption in this spectrum range. This loss is probably induced by the Fabry-Perot and guided resonance in the SiO2 layer, which enhance the light absorption in the Ag nanoparticles. To maximize the light absorption in the Si layer, Ag reflector and perfect electrical conductor (PEC) are employed to replace the Al reflector, with their absorption also shown in Fig. 6(a). The PEC is an ideal material exhibiting infinite electrical conductivity and thus infinite permittivity. Using the back reflector made of PEC makes the back reflection 100%. Compared with the PEC mirror, both the Al and Ag reflectors introduce a certain proportion of light loss. The Al reflector exhibits an average loss of 5% whereas Ag reflector is much closer to the PEC mirror with an only 2% average loss. Figure 6(b) shows the light absorption in the Si layer by the different reflectors. We observe an increase of the photocurrent from 32.7 mA/cm2 to 33.3 mA/cm2 by the Ag reflector, with a slight spectrum enhancement from 600 nm to 900 nm. The current is further increased to 33.4 mA/cm2 by the PEC mirror. This is equivalent to a 30 μm single-pass light absorption. Clearly, less losses from the reflector lead to larger absorption enhancement in the Si layers.

Fig. 6. (a) The absorption losses in the Ag nanoparticles and the various reflectors (b) the Si absorption spectra of the hybrid structure with different reflector schemes: Al (black), Ag (red) and PEC (green).

Our approach is flexible for a wide range of Si thicknesses. We apply the hybrid structure to 10 μm, 5 μm, 2 μm and 1μm thick Si slabs and calculated their photocurrent, respectively, with the results shown in Fig. 7. The photocurrent induced by the hybrid structure is the closest to the Yablonovitch limit at each thickness and is larger than both the top and bottom only structures, respectively. Meanwhile, the solar cells with the top only moth-eye structures exhibit larger photocurrents than those with the bottom only Ag nanoparticles due to the simultaneous light trapping and antireflection effect.

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Fig. 7. Photocurrent generated by the four structures as a function of the Si thickness, referenced to the Yablonovitch limit.

As a final remark, the biomimetic moth-eye structures can be fabricated by the colloidal lithography and the nanoimprint lithography in a large-scale, followed by controllable Si wet etching or dry etching [16–18]. The plasmonic Ag nanoparticles are also easily accessible by evaporating a thin layer of Ag film, followed by high temperature annealing [23–25]. In terms of the electrical contact fabrication, the photolithograph-defined localized back contact is compatible with the bottom Ag nanoparticle integration [33]. 4. Conclusion In conclusion, we have proposed a hybrid light trapping strategy to effectively attract and localize light in ultrathin Si solar cells, with an enhancement up to 69% for the 2 μm thick cells. This enhancement originates from the strong light trapping at the long wavelength range between 600 - 1200 nm by both the Mie resonance of the Si moth-eye structure and plasmonic Ag nanoparticle scattering in addition to the prefect antireflection of the moth-eye structure across the whole wavelength range. This effective structure paves the way for achieving highly efficient ultrathin Si solar cells. Acknowledgment Min Gu acknowledges support from the Science and Industry Endowment Fund (SIEF) (Project No. 34798).

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