October 1, 2013 / Vol. 38, No. 19 / OPTICS LETTERS
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Enhanced optical absorption in nanohole-textured silicon thin-film solar cells with rear-located metal particles Yankun Chen, Weihua Han,* and Fuhua Yang Engineering Research Center for Semiconductor Integrated Technology, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China *Corresponding author:
[email protected] Received April 10, 2013; revised August 13, 2013; accepted September 5, 2013; posted September 5, 2013 (Doc. ID 188585); published September 30, 2013 We report the computational modeling of Ag nanoparticles deposited on the rear of a nanohole-textured silicon thin film to achieve higher absorption for silicon solar cells. The silicon nanoholes and the rear-located Ag nanoparticles can enhance the absorption in the silicon thin film. The short circuit current density for nanohole-textured silicon thin film can be further improved by about 11.6% by Ag nanoparticles. The combination of silicon nanoholes and plasmonic metal nanoparticles provides a promising way to enhance the absorption of silicon thin-film solar cells. © 2013 Optical Society of America OCIS codes: (350.6050) Solar energy; (250.5403) Plasmonics. http://dx.doi.org/10.1364/OL.38.003973
Currently over 80% of the photovoltaic solar cells are made from crystalline silicon due to its natural abundance, nontoxicity, long-term stability, and mature technology. The thickness of the commercial Si solar cells is about 200–300 μm, which accounts for ∼40% of the total cost. Using thin-film solar cells with a thickness of a few micrometers is a promising way to reduce material costs, but transmission losses increase due to the thinner absorber layers. Light-trapping nanostructures, such as metal nanoparticles (NPs) [1–4], nanowires (NWs) [5–7], nanocones [8,9], and nanoholes (NHs) [10,11], have been developed to achieve high efficiencies. In particular, the light-scattering effect and near-field enhancement from metal NPs have emerged as a promising ways to provide light trapping. At the same time, Si NHs are found to exhibit higher efficiency and superior mechanical stability than Si NWs, appearing to be an alternative structure with great potential for photovoltaics [10,12,13]. To further enhance optical absorption, researchers proposed hybrid nanostructures. Zhou et al. found that a large fraction of light can be coupled into the Si NWs with the help of coated metal NPs to increase the optical absorption [14]. Ren et al. demonstrated that the hemisphere-Ag back reflector is able to enhance the absorption of the NW-textured silicon thin film, resulting from localized surface plasmon resonance induced scattering [15]. Pudasaini and Ayon deposited Au NPs on the surface of the Si NHs to increase the ultimate efficiency via the Au-NP scattering [16]. Therefore, optimized hybrid nanostructures for optical absorption enhancement are very effective in the application of photovoltaic devices. In this Letter, we demonstrate novel (to our knowledge) NH-textured silicon thin-film solar cells with rearlocated Ag NPs for enhancing optical absorption. The expected role of the Si NHs is to absorb the light at the short wavelengths, and that of the rear Ag NPs is to supply the backscattering of the transmitted light at the long wavelengths into the absorber layer. The light-trapping properties can be optimized by adjusting the parameters of NHs and Ag NPs independently. 0146-9592/13/193973-03$15.00/0
Figure 1 shows the schematics of the NH-textured silicon thin film with rear-located Ag NPs. The parameters of the Si NHs are represented by the periodicity (P 1 ), the radius (r), and the height (H). Then the Si-NH fill factor can be defined as f 1 πr 2 ∕P 21 . As shown in Fig. 1(b), the NH depth is 200 nm from the top of the 1 μm thick silicon film. Semi-spherical Ag NPs with diameter d and periodicity P 2 are located on the back of the thin film. The fill factor of Ag NPs is defined as f 2 πd2 ∕4P 22 . Numerical simulations are performed using the finite-difference time-domain solutions package from Lumerical. Periodic boundary conditions are used in the x and y directions, and perfectly matched layer boundary conditions are applied in the z direction for simulating the infinitely extended air medium. A normally incident electromagnetic wave within a photon energy range from 1.127 to 3.1 eV, corresponding to wavelengths from 1100 to 400 nm, illustrates the structure along the z direction. The dielectric functions are provided by a Drude model for Ag NPs and a Drude–Lorentz model for silicon substrate. A planar monitor at the interface of the silicon thin film and Ag NPs is applied to register the light that transmits through the Si film, and a second planar monitor above the surface of the Si substrate records the reflected light. After calculation of the transmission Tλ through the NH-textured silicon thin film and the reflection Rλ by
Fig. 1. (a) Schematic of the NH-textured Si thin film with rearlocated Ag nanoparticles for computational simulations and (b) cross-sectional view of the proposed structure. Structural parameters of the solar cells are nanohole periodicity (P 1 ), radius (r), height (H), Ag nanoparticle diameter (d), and periodicity (P 2 ). © 2013 Optical Society of America
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the Ag NPs, the absorption Aλ inside the silicon film is given by Aλ 1 − Tλ − Rλ. The absorption performance of the structure will be measured by the short circuit current density J sc , which is defined as J sc
e hc
Z
1100 400
λAλΦλdλ;
(1)
where Φλ is the solar energy density spectrum corresponding to the air mass 1.5 spectrum, e is the elementary charge, h is Planck’s constant, and c is the speed of light in a vacuum. It is assumed that all of the generated carriers are collected by their respective electrodes, which is useful for thin-film solar cells, because the distance traveled by each of the generated carriers is much shorter than the diffusion length of minority carriers. Figure 2(a) shows the absorption spectrum of the NHtextured silicon thin film. Due to the antireflection effect and light-trapping properties of Si NHs, the absorption is significantly increased by the resonance scattering in comparison with that in the planar silicon thin film. When the periodicity of Si NHs increases, the absorption edge has a redshift due to the strong optical scattering for the wavelengths comparable to the periodicity of Si NHs. The absorption peaks from the resonance scattering are located in photon energies of about 1.9 eV for P 1 400 nm, about 2.2 eV for P 1 500 nm, and about 2.6 eV for P 1 600 nm. However, with increasing P 1 , the absorption in the short wavelengths is reduced at the same time. There is a trade-off for the absorption between the short wavelengths and the long wavelengths. From Fig. 2(b), it can be seen that the short circuit current density increases with increasing Si fill factor as well as periodicity of Si NHs. The J sc tends to saturate at f 1 0.5. Considering that a large fill factor will make the NH wall too thin to support the silicon NH structure, we expect that the optimum fill factor and periodicity are 0.5 and 500 nm, respectively. Thus the NH-textured silicon thin film achieves the best J sc value of 22.75 mA∕cm2 , with P 1 500 nm and f 1 0.5. Next we turn to the optimization of the Ag-NP array located on the rear of the silicon thin film. Figure 3(a) depicts the absorption spectrum for the silicon thin film with rear-located Ag NPs of different diameters. The incident light photons in the short wavelengths are fully captured before they reach the Ag NPs. As a result, the absorption data in the short wavelengths are similar to those in the bare Si thin film. However, the longwavelength light transmits through the Si layer and interacts with the Ag NPs, then is backscattered into the Si layer by the Ag NPs, leading to the enhanced absorption
Fig. 2. (a) Absorption spectrum and (b) the short circuit current density for the Si thin film textured with Si NHs.
Fig. 3. (a) Absorption spectrum and (b) the short circuit current density for the Si thin film with rear Ag NPs.
in the long wavelengths. Figure 3(b) shows the short circuit current density for the silicon thin film with rearlocated Ag NPs by various fill factors. The best J sc value of 15.54 mA∕cm2 is produced for d 500 nm and f 2 0.5. We expect the optimum f 2 will be 0.5, since J sc tends to be saturated at this value. The analysis above has shown that the absorption of the Si thin film can be enhanced by the Si NHs and the rear-located Ag NPs. In order to determine the absorption enhancement for the hybrid nanostructure, Ag NPs with different diameters are deposited on the rear of the NH-textured silicon thin film. The periodicity and the fill factor for the Si NHs are fixed at 500 nm and 0.5, respectively. The periodicity of the Ag NPs is focused on 1000 nm. We calculate the absorption ratio, defined as the ratio between the absorption for the NH-textured silicon thin film with and without rear Ag NPs, as shown in Fig. 4(a). As large as 10.9-fold absorption enhancement at a photon energy of about 1.2 eV is observed by the Ag NPs with d 800 nm. Larger particles exhibit higher absorption enhancement due to the larger scattering cross section. From Fig. 4(b), the best J sc is achieved at d 800 nm, with f 2 0.5, which is in accordance with the conclusion made from Fig. 3(b). With rearlocated Ag NPs, the optimum J sc for NH-textured silicon thin film is increased from 22.75 to 25.40 mA∕cm2 , showing 11.6% improvement. The increase of J sc with the increase of Ag NP diameter results from the strong confinement of light by the Ag-NP reflection. To further understand the impact of Ag NPs on the absorption of NH-textured silicon thin film as seen in Fig. 4, we conduct the crosssectional electric intensity distribution at a photon energy of about 1.2 eV, as shown in Fig. 5. The stronger electric intensity inside the hybrid nanostructure benefits from the light-trapping properties of Si NHs and the excitation of the localized surface plasmons induced by Ag NPs. The resonance frequency of Ag NPs gets redshifted because of the ellipsoid shape
Fig. 4. (a) Absorption ratio and (b) the short circuit current density for the Si NH-textured thin film with different rear Ag NPs. The Si NHs have a periodicity of 500 nm and fill factor of 0.5. The periodicity of Ag NPs is focused on 1000 nm.
October 1, 2013 / Vol. 38, No. 19 / OPTICS LETTERS
Fig. 5. Electric intensity distributions at the cross section for silicon thin film with (a) Si NHs, (b) Ag NPs, and (c) Si NHs and Ag NPs at a photon energy of 1.2 eV. The structural parameters are P 1 500 nm, d 800 nm, and f 1 f 2 0.5.
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In conclusion, the hybrid nanostructure of the NHtextured silicon thin film with Ag NPs on its back is demonstrated in the optimized parameters. The Si NHs and the rear-located Ag NPs can enhance the absorption of the Si thin film. The short circuit current density for NH-textured silicon thin film can be further improved about 11.6% by rear-located Ag NPs. The optimized SiO2 passivation layer inserted between the Si layer and the Ag NPs to suppress the carrier recombination effect is found to be 40 nm thick. The combination of Si NHs and plasmonic metal NPs provides a promising way to improve the absorption of silicon thin-film solar cells. This work has been supported by the National Basic Research Program of China (973 Program) under grant number 2010CB934104. The authors acknowledge Prof. X. D. Wang’s help and thank Mr. Y. P. Shi for meaningful discussions.
Fig. 6. Passivation layer thickness dependence of short circuit current density for the optimized hybrid nanostructure. The Si NH-textured thin film with and without Ag NPs is plotted for reference, corresponding to the upper and lower dotted lines, respectively.
of Ag NPs and the dielectric function of the silicon substrate. The redshifting of the resonance results in the increase in the absorption at longer wavelengths [4]. Since the metal particles can serve as recombination centers in solar cells, a SiO2 passivation layer is inserted between the Si layer and the Ag NPs to suppress the carrier recombination effect. Figure 6 depicts the short circuit current density as a function of SiO2 thickness for the optimum hybrid structure. The current densities of Si NHs with and without Ag NPs are plotted as references, corresponding to the upper and lower dotted lines, respectively. Due to the decreasing coupling modes inside the silicon layer, the J sc is decreased with the insertion of the SiO2 layer, but still shows higher absorption than that of the NH-textured silicon thin film. The optimum SiO2 thickness is selected as 40 nm, corresponding to a J sc of 24.96 mA∕cm2 .
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