| Developing the solar cells with low cost, high efficiency and high stability is the key to large-scale photovoltaic generation. Accordingly, a variety of thin film solar cells and new concept solar cells arise at the historic moment. Microcrystalline silicon(μc-Si:H) has many advantages, such as rich raw materials, clean non-toxic and good light stability, etc. So that it is ideal absorbing material for thin film solar cells. However, μc-Si:H belongs to indirect band gap semiconductor and has low light absorption coefficient, which is largely limits the short circuit current and the photoelectric conversion efficiency of the solar cells. Therefore, employing effective light-trapping techniques to increase the light absorption of solar cells is of crucial importance for the industrialization of μc-Si:H thin film solar cells. Using surface plasmon technology widely attention in recent years to light-trapping of solar cells, can effectively enhance the thin film solar cells light absorption.We use the simulation software which is based on the finite element method to simulate the optical absorption of solar cells with different light-trapping structures. We first made a brief analysis for the theoretical basis and development history of surface plasmon technology, and then a three-dimensional model is proposed to simulate the light absorption of μc-Si:H thin film solar cells with periodic metal nanoparticle arrays located on front surface of the cells. We optimize their structural parameters and expound the physical mechanism of enhancement of light absorption with the electromagnetic field distribution. We next study the influence of different types nanoparticles, which are alternating distribution on the solar cells, to light absorption of solar cells, so as to analyze the effective ways to further improvement of light-trapping. Finally combining with the latest research results of other photovoltaic devices in recent years, we design new type of light-trapping solar cell structures to broaden the range of light-trapping. The main results of this paper are as follows:1. When particles are distribution on front surface of solar cells, the key parameter that influences light absorption in solar cell is period/radius ratio(P/R) or particle surface coverage. When P/R of Al nanosphere arrays is 4—5, the optimum integrated absorption enhancement(Eabs) is over 18% under AM1.5 illumination compared with the solar cell without nanoparticles, which are attributed to dipole resonance mode from nanoparticles and waveguide mode from solar cells. For Al hemispherical particle arrays, the Eabs is 24.5%, which is higher than that of the solar cell with optimized Al spherical particle arrays. But the former is very sensitive to the hemispherical particle radius. When radius is too large or too small, both of these are detrimental to light absorption enhancement of solar cells. For Ag nanosphere arrays, the Eabs is 26.4% with R = 110 nm, P = 500 nm because of the superior plasmon. Comparing Al and Ag nanoparticles, Ag particles have better light-trapping effect, but Al particles is more cost-effective, so that we need to choose the appropriate nanoparticles according to actual situation.2. Metal nanoparticles with different sizes or different composition being alternative distribution on front surface of solar cells can effectively enhance the light absorption of them. With alternating distribution of different sizes Al or Ag nanosphere arrays, the photon absorption rate of solar cells in the long wave band is determined by large size particles and the light absorption is improved in short wave area due to mutual coupling of the metal particles with different sizes. For alternative distribution of Ag nanosphere arrays with particles radius of 85 nm and 120 nm respectively, when P = 500 nm, the Eabs is 32.0%. Considering different regulation law caused by different composition Metal nanoparticles, we let them alternative distribution, which can use their regulate complementary advantages to broaden the spectral absorption scope of solar cells. When metal nanoparticles embedded the ITO layer of solar cells front surface to change the medium environment around the particles, the control law of solar cells light absorption can further optimized by nanoparticles. The optimized Eabs is 50.1% when Ag nanosphere of R = 110 nm embedded ITO layer of 70 nm thickness.3. Metal nanoparticles on back surface of solar cells can control and improve the light absorption in the long wavelengths. For Al nanosphere, the Eabs first increases and then decreases with the increase of R, and the enhancement is over 21% by making the P/R = 6—7 when R is 75—90 nm. Solar cell light absorption is affected by the particle composition and shape of solar cell back surface, where the light-trapping effect of hemisphere and cylindrical particle arrays is better than that of spherical particles. The light absorption spectrum characteristics of solar cells with Ag, Au and Cu particles on solar cell back surface is quite similarity, and among them Ag is the best. Synthesizing the above advantages of optical properties, we can design composite light-trapping structure of solar cells. For Ag nanosphere upper the μc-Si:H layer embedded ITO layer and Ag nanocylinder under the μc-Si:H layer inside ITO layer, the light absorption enhancement increases significantly both in short wave and long wave area, and the Eabs reach to 67.4%. The light absorption of solar cells have space to further improvement if the shape and size of particles continue to optimized. |
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