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Prospects of near-field plasmonic absorption enhancement in semiconductor materials using embedded Ag nanoparticles

P. Spinelli and A. Polman

Open Access Open Access

Abstract

Metal nanoparticles are efficient antennas for light. If embedded in a semiconductor material, they can enhance light absorption in the semiconductor, due to the strong plasmonic near-field coupling. We use numerical simulations to calculate the absorption enhancement in the semiconductor using Ag nanoparticles with diameters in the range 5–60 nm for crystalline Si, amorphous Si, a polymer blend, and Fe2O3. We study single Ag particles in a 100×100×100 nm semiconductor volume, as well as periodic arrays with 100 nm pitch. We find that in all cases Ohmic dissipation in the metal is a major absorption factor. In crystalline Si, while Ag nanoparticles cause a 5-fold enhancement of the absorbance in the weakly absorbing near-bandgap spectral range, Ohmic losses in the metal dominate the absorption. We conclude crystalline Si cannot be sensitized with Ag nanoparticles in a practical way. Similar results are found for Fe2O3. The absorbance in the polymer blend can be enhanced by up to 100% using Ag nanoparticles, at the expense of strong additional absorption by Ohmic losses. Amorphous Si cannot be sensitized with Ag nanoparticles due to the mismatch between the plasmon resonance and the bandgap of a-Si. By using sensitization with Ag nanoparticles the thickness of some semiconductor materials can be reduced while keeping the same absorbance, which has benefits for materials with short carrier diffusion lengths. Scattering mechanisms by plasmonic nanoparticles that are beneficial for enhanced light trapping in solar cells are not considered in this paper.

© 2012 Optical Society of America

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Figures (7)
Fig. 1
Fig. 1 Simulated absorption spectra of a bare substrate (black dashed line), absorption in the substrate with a 30-nm-diameter Ag NP (blue) and absorption in the Ag NP (red), for a c-Si (a) and a PF10TBT:PCBM (b) hosting semiconductors. A clear increase in absorption is observed for wavelengths around the NP plasmon resonance. In the case of a c-Si substrate, the near-field absorption in the NP is much stronger than in the semiconductor. The inset shows a sketch of the simulation geometry.

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Fig. 2
Fig. 2 Absorption enhancement (left axis) and average absorptance (right axis), weighted over the AM1.5 solar spectrum in the 300–1100 nm spectral range, as a function of the Ag NP diameter. The panels refer to c-Si (a), PF10TBT:PCBM (b), a-Si (c), and Fe2O3 (d) embedding media. In each panel, the inset shows the dipolar (D, black) and quadrupolar (Q, red) LSPR wavelengths. An absorption enhancement up to a factor of 2 can be achieved in the polymer substrate, due to the spectral match of the LSPR resonance with the spectral range where the polymer is strongly absorbing. In c-Si or Fe2O3, the LSPR resonance is in a spectral region where the material is poorly absorbing, and absorption is thus strongly limited by the losses in the metal NP. In a-Si, the resonant wavelength is larger than the bandgap wavelength, and no absorption enhancement is observed.

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Fig. 3
Fig. 3 Fraction of the incident power that is absorbed in the semiconductor (blue), in the metal NP (red) or not absorbed (green), as a function of particle diameter, for c-Si (a), PF10TBT:PCBM (b), a-Si (c) and Fe2O3 (d). All data are averaged by weighting over the AM1.5 solar spectrum in the 300–1100 nm spectral range. The reduction of the non-absorbed power (green) is associated with an increase of the absorption in the substrate and in the NP. For the polymer substrate, the absorption in the active layer is larger than the losses in the metal. For a c-Si and Fe2O3 substrates, the strong absorption in the NP strongly limits the plasmonic near-field absorption enhancement in the substrate. In an a-Si substrate no significant change is observed as a result of the resonant wavelength being larger than the bandgap wavelength.

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Fig. 4
Fig. 4 (a) LSPR dipolar resonance wavelength (black line, vertical axis) as a function of the shell thickness (bottom axis), for a Ag/SiO2 core-shell particle embedded in a c-Si layer. The Ag core diameter is 30 nm. A strong blue shift is observed as the silica shell thickness increases. Also shown is the absorption coefficient of c-Si (red line, top axis) as a function of wavelength. Increasing the shell thickness shifts the resonance into a spectral range where Si is more absorbing. (b) Absorption enhancement, averaged by weighting over the AM1.5 solar spectrum in the 300–1100 nm spectral range, in the c-Si substrate as a function of the shell thickness. A reduction in absorption is observed for larger shell thicknesses as a result of the reduced overlap of the near-field with the active material.

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Fig. 5
Fig. 5 AM1.5 averaged absorption in c-Si (a) and PF10TBT:PCBM (b) for oblate (circles) and prolate (crosses) embedded Ag nanoparticles. Absorption in the semiconductor (blue) and in the metal NP (red) is shown as a function of the ratio of the short radius over the long radius of the spheroid. The absorption of a bare semiconductor volume is shown for reference (dashed black line). The graph shows that an increase in the absorption in the semiconductor due to the presence of the NP is always associated with strong Ohmic losses in the metal NP.

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Fig. 6
Fig. 6 Average absorption for arrays of silver NPs embedded in a c-Si (a) and PF10TBT:PCBM (b) substrate. Data are plotted for particles with 5 nm (dash-dotted lines) and 40 nm (dashed lines) diameter, as a function of the ratio of the array pitch over the particle diameter. Absorption in the hosting material is shown in blue, absorption in the metal NPs is shown in red. The absorption in a substrate without NPs is shown for reference (black solid line). For both c-Si and PF10TBT:PCBM, the highest absorption in the semiconductor is achieved for NP arrays with short inter-particle distance compared to the NP size. In the case of c-Si (a), the increase in absorption in the c-Si layer comes at the expense of a significant increase in the ohmic losses in the metal NPs.

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Fig. 7
Fig. 7 (a) Sketch of the simulation geometry. A square array of 40-nm-diameter Ag NPs, spaced by 100 nm, is placed 40 nm below the surface of a semi-infinite semiconductor layer. (b, c) Absorptance per unit length (top axis) as a function of depth (right axis) for a bare substrate (black), in a substrate with embedded Ag NPs (blue) and in the Ag NP array (red), for c-Si (b) and PF10TBT:PCBM (c). Absorption in the active layer is enhanced by the presence of the NP in the proximity of the NP, due to the strong plasmonic near field. (d, e) Absorptance in a bare substrate (black) and in a substrate containing a Ag NP array (blue), for a c-Si (d) and PF10TBT:PCBM (e) substrates, as a function of layer thickness. For thinner layers, absorption is enhanced by the LSPR near field of the NP. For thicker layer, the bare substrate shows larger absorption than a substrate with NPs, as a result of the metal losses in the latter.

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Equations (1)

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P abs ( ω ) = 1 2 ω ∫ Im ( n ( x , y , z , ω ) ) E ( x , y , z , ω ) 2 d V
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