The performances of thin film solar cells are considerably limited by the low light absorption. Plasmonic nanostructures have been introduced in the thin film solar cells as a possible solution around this issue in recent years. Here, we propose a solar cell design, in which an ultrathin Si film covered by a periodic array of Ag strips is placed on a metallic nanograting substrate. The simulation results demonstrate that the designed structure gives rise to 170% light absorption enhancement over the full solar spectrum with respect to the bared Si thin film. The excited multiple resonant modes, including optical waveguide modes within the Si layer, localized surface plasmon resonance (LSPR) of Ag stripes, and surface plasmon polaritons (SPP) arising from the bottom grating, and the coupling effect between LSPR and SPP modes through an optimization of the array periods are considered to contribute to the significant absorption enhancement. This plasmonic solar cell design paves a promising way to increase light absorption for thin film solar cell applications.
The low conversion efficiencies and high production costs have been the major difficulties facing photovoltaic technology. For solar cells based on bulk crystalline silicon, around 40% of a solar cell module’s price comes from the silicon (Si) materials and its processing costs. To reduce the costs, thin film solar cells with an active layer thickness of about 1 to 2
In the past years, many light trapping techniques have been investigated for solar cell applications. A typical example is the use of micron-size pyramidal surface textures [
Since the excitation of plasmon resonances can capture and trap the sunlight into the active layer and increase absorption strength, multiple plasmon resonances are desired for the thin film solar cell with superior performance. Previous designs of plasmonic solar cells are mostly based on placing one- or two-dimensional metallic nanoparticle arrays on the top or buried inside the active layer [
(a) Schematic diagram of the proposed thin film Si solar cell structure. (b) Side view of the unit cell, where the structural parameters are defined.
The proposed structure with the defined structural parameters is illustrated in Figures
Numerical electromagnetic simulations were performed by the commercial finite element software of COMSOL Multiphysics 3.5. The periodic boundaries were employed to a unit cell for simulating an infinite array. Perfectly matched layers (PML) were applied in the propagation direction to eliminate the nonphysical reflections at domain boundaries. The whole structure was illuminated from the top surface at normal incidence with E-field along the metal stripes (TE wave) or H-field along the metal stripes (TM wave). The absorption inside the absorber layer for an incidence monochrome plane wave with certain wavelength was calculated by using
The absorption enhancement function (
Figures
(a) Absorption spectrum of the bared Si film. The insets show the normalized stimulated magnetic field distributions (
To explore the mechanisms of absorption enhancement and improve the light absorption capability of the proposed structure for solar cell applications, the absorption enhancement map (
Map of the absorption enhancement (
The normalized magnetic field distribution (
Figure
In Figure
Figure
Unlike TM polarizations, the SPP and LSPR modes are absent under TE plane wave incidence (E-field along the metal strips and H-field in this plane normal to the metal strips). Only the enhanced absorption related to the TE waveguide modes are observed in Figure
To perform a full evaluation on the performance of the proposed design, the total absorption enhancement under solar illuminations must be considered. Generally, the short circuit current in a solar cell is proportional to the number of absorbed photons if assuming unity internal quantum efficiency (every photoexcited electron-hole pair is collected). Therefore, the total absorption enhancement must be evaluated based on the number of photons being absorbed. Figure
(a) Photon number spectrum of the AM1.5 solar irradiations (AM(
Considering the equal contributions from TE and TM polarized light, the APNS of the proposed solar cells under randomly polarized sunlight can be described as
The wavelength range for integrating is selected from 400 nm to 1000 nm. Figure
In conclusion, the light absorption properties of a novel ultrathin film Si solar cell structure were discussed in detail by using the finite element method. As compared to a bared Si thin film, the total absorption enhancement of absorbed photon number can reach to 170% within a broad period range by introducing Ag strips on top and Ag nanograting as the back contacts in the solar cell structure. The absorption enhancement mechanisms are revealed by analyzing the enhanced field distributions and investigating the absorption enhancement spectra as a function of periods. These results pave a promising way for the realization of high efficiency thin-film solar cells.
This work is supported by the National Science Foundation of China (no. 10974183 and 11104252), the Ministry of Education of China (no. 20114101110003), the Aeronautical Science Foundation of China (no. 2011ZF55015), and the Basic and Frontier Technology Research Program of Henan Province (no. 122300410162 and 112300410264).