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Light Trapping Enhancement in a Thin Film with 2D Conformal Periodic Hexagonal Arrays.

Yang X, Zhou S, Wang D, He J, Zhou J, Li X, Gao P, Ye J - Nanoscale Res Lett (2015)

Bottom Line: Compared with the planar reference, the double-sided conformal periodic structures with all feature periodicities of sub-wavelength (300 nm), mid-wavelength (640 nm), and infrared wavelength (2300 nm) show significant broadband absorption enhancements under wide angles.The films with an optimum periodicity of 300 nm exhibit outstanding antireflection and excellent trade-off between light scattering performance and parasitic absorption loss.The average absorption of the optimum structure with a thickness of 160 nm is 64.8 %, which is much larger than the planar counterpart of 38.5 %.

View Article: PubMed Central - PubMed

Affiliation: Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, China, yangx@nimte.ac.cn.

ABSTRACT
Applying a periodic light trapping array is an effective method to improve the optical properties in thin-film solar cells. In this work, we experimentally and theoretically investigate the light trapping properties of two-dimensional periodic hexagonal arrays in the framework of a conformal amorphous silicon film. Compared with the planar reference, the double-sided conformal periodic structures with all feature periodicities of sub-wavelength (300 nm), mid-wavelength (640 nm), and infrared wavelength (2300 nm) show significant broadband absorption enhancements under wide angles. The films with an optimum periodicity of 300 nm exhibit outstanding antireflection and excellent trade-off between light scattering performance and parasitic absorption loss. The average absorption of the optimum structure with a thickness of 160 nm is 64.8 %, which is much larger than the planar counterpart of 38.5 %. The methodology applied in this work can be generalized to rational design of other types of high-performance thin-film photovoltaic devices based on a broad range of materials.

No MeSH data available.


Distributions of optical absorption. a The simulated absorption spectra of the 300-nm period structure and the planar reference over the wavelength range from 300 to 900 nm. Distributions of normalized electric field for TE polarization at the wavelengths bλ = 370 nm, cλ = 450 nm, and dλ = 640 nm. Distributions of normalized magnetic field for TM polarization at absorption peak wavelengths eλ = 370 nm, fλ = 450 nm, and gλ = 640 nm
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Fig7: Distributions of optical absorption. a The simulated absorption spectra of the 300-nm period structure and the planar reference over the wavelength range from 300 to 900 nm. Distributions of normalized electric field for TE polarization at the wavelengths bλ = 370 nm, cλ = 450 nm, and dλ = 640 nm. Distributions of normalized magnetic field for TM polarization at absorption peak wavelengths eλ = 370 nm, fλ = 450 nm, and gλ = 640 nm

Mentions: Taking into account the significant difference in the structural requirements for antireflection and light trapping, the optical performance of the double-sided periodic conformal structure with both the top and the bottom hexagonally periodic arrays is simulated. As shown in Fig. 7a, the absorption of the hexagonal configurations is obviously enhanced over the whole wavelength compared with the planar reference. The absorption patterns at typical wavelengths in Figs. 6g and 7b well illustrate the absorption inside the periodic structures and satisfactorily verify the previous observation and explanation. For TE polarization, three Bloch modes are clearly observed at absorption peak wavelengths, namely, λ = 370, 450, and 640 nm, whose electric field distributions are shown in Fig. 7b–d, respectively. For TM polarization, the situation is different, and we plot the magnetic field distributions at the absorption peak wavelengths in Fig. 7e–g, respectively. The first mechanism of absorption enhancement is related to cavity resonance and can be seen in Fig. 7b obviously. The interference within the thin film leads to a resonant FP cavity effect, depending on the thickness of the layer. Another resonance, which is shown in Fig. 7c, is introduced by adding the periodic patterns, whose mode strength is related to the parameters of the Ag back patterns. The second mechanism is related to SPPs that can be excited only by TM polarization, which can be inferred from the Hz field observation in Fig. 7g. The third mechanism will be associated with the coupling into waveguide modes, which can be found in the middle of the active layer. Because of these mechanisms, the absorption spectra are enhanced and broadened significantly for both TE and TM polarizations although the SPP waves still have higher losses due to the parasitic absorption in the metal film as shown from the red curve in Fig. 7a. To characterize the loss of parasitic absorption, a thin layer of ZnO film is placed between the active layer and Ag-BSR as a dielectric spacer to reduce metallic loss. Figure 8 shows the schematic of the optimized structures (a), the absorption of the active layer (b), and the parasitic loss of the metallic layer (c) as a function of wavelength and thickness (W) of the ZnO layer. When W < 50 nm, the metallic parasitic loss accompanied by the excitation of SPPs in the longer wavelength range is still high. With the increasing W, the grating at the bottom of the active layer becomes far from the metal surface and the parasitic absorption of the metal is reduced while the scattering effect of the back surface morphology also slackens. When W = 100 nm, the Ag-BSR induces no significant parasitic loss and keeps good light scattering properties. In this case, the ZnO layer not only acts as a diffusion buffer layer between a-Si and Ag but also optically separates the a-Si waveguide modes from the lossy SPP modes on the metallic interface.Fig. 7


Light Trapping Enhancement in a Thin Film with 2D Conformal Periodic Hexagonal Arrays.

Yang X, Zhou S, Wang D, He J, Zhou J, Li X, Gao P, Ye J - Nanoscale Res Lett (2015)

Distributions of optical absorption. a The simulated absorption spectra of the 300-nm period structure and the planar reference over the wavelength range from 300 to 900 nm. Distributions of normalized electric field for TE polarization at the wavelengths bλ = 370 nm, cλ = 450 nm, and dλ = 640 nm. Distributions of normalized magnetic field for TM polarization at absorption peak wavelengths eλ = 370 nm, fλ = 450 nm, and gλ = 640 nm
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4495099&req=5

Fig7: Distributions of optical absorption. a The simulated absorption spectra of the 300-nm period structure and the planar reference over the wavelength range from 300 to 900 nm. Distributions of normalized electric field for TE polarization at the wavelengths bλ = 370 nm, cλ = 450 nm, and dλ = 640 nm. Distributions of normalized magnetic field for TM polarization at absorption peak wavelengths eλ = 370 nm, fλ = 450 nm, and gλ = 640 nm
Mentions: Taking into account the significant difference in the structural requirements for antireflection and light trapping, the optical performance of the double-sided periodic conformal structure with both the top and the bottom hexagonally periodic arrays is simulated. As shown in Fig. 7a, the absorption of the hexagonal configurations is obviously enhanced over the whole wavelength compared with the planar reference. The absorption patterns at typical wavelengths in Figs. 6g and 7b well illustrate the absorption inside the periodic structures and satisfactorily verify the previous observation and explanation. For TE polarization, three Bloch modes are clearly observed at absorption peak wavelengths, namely, λ = 370, 450, and 640 nm, whose electric field distributions are shown in Fig. 7b–d, respectively. For TM polarization, the situation is different, and we plot the magnetic field distributions at the absorption peak wavelengths in Fig. 7e–g, respectively. The first mechanism of absorption enhancement is related to cavity resonance and can be seen in Fig. 7b obviously. The interference within the thin film leads to a resonant FP cavity effect, depending on the thickness of the layer. Another resonance, which is shown in Fig. 7c, is introduced by adding the periodic patterns, whose mode strength is related to the parameters of the Ag back patterns. The second mechanism is related to SPPs that can be excited only by TM polarization, which can be inferred from the Hz field observation in Fig. 7g. The third mechanism will be associated with the coupling into waveguide modes, which can be found in the middle of the active layer. Because of these mechanisms, the absorption spectra are enhanced and broadened significantly for both TE and TM polarizations although the SPP waves still have higher losses due to the parasitic absorption in the metal film as shown from the red curve in Fig. 7a. To characterize the loss of parasitic absorption, a thin layer of ZnO film is placed between the active layer and Ag-BSR as a dielectric spacer to reduce metallic loss. Figure 8 shows the schematic of the optimized structures (a), the absorption of the active layer (b), and the parasitic loss of the metallic layer (c) as a function of wavelength and thickness (W) of the ZnO layer. When W < 50 nm, the metallic parasitic loss accompanied by the excitation of SPPs in the longer wavelength range is still high. With the increasing W, the grating at the bottom of the active layer becomes far from the metal surface and the parasitic absorption of the metal is reduced while the scattering effect of the back surface morphology also slackens. When W = 100 nm, the Ag-BSR induces no significant parasitic loss and keeps good light scattering properties. In this case, the ZnO layer not only acts as a diffusion buffer layer between a-Si and Ag but also optically separates the a-Si waveguide modes from the lossy SPP modes on the metallic interface.Fig. 7

Bottom Line: Compared with the planar reference, the double-sided conformal periodic structures with all feature periodicities of sub-wavelength (300 nm), mid-wavelength (640 nm), and infrared wavelength (2300 nm) show significant broadband absorption enhancements under wide angles.The films with an optimum periodicity of 300 nm exhibit outstanding antireflection and excellent trade-off between light scattering performance and parasitic absorption loss.The average absorption of the optimum structure with a thickness of 160 nm is 64.8 %, which is much larger than the planar counterpart of 38.5 %.

View Article: PubMed Central - PubMed

Affiliation: Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, China, yangx@nimte.ac.cn.

ABSTRACT
Applying a periodic light trapping array is an effective method to improve the optical properties in thin-film solar cells. In this work, we experimentally and theoretically investigate the light trapping properties of two-dimensional periodic hexagonal arrays in the framework of a conformal amorphous silicon film. Compared with the planar reference, the double-sided conformal periodic structures with all feature periodicities of sub-wavelength (300 nm), mid-wavelength (640 nm), and infrared wavelength (2300 nm) show significant broadband absorption enhancements under wide angles. The films with an optimum periodicity of 300 nm exhibit outstanding antireflection and excellent trade-off between light scattering performance and parasitic absorption loss. The average absorption of the optimum structure with a thickness of 160 nm is 64.8 %, which is much larger than the planar counterpart of 38.5 %. The methodology applied in this work can be generalized to rational design of other types of high-performance thin-film photovoltaic devices based on a broad range of materials.

No MeSH data available.