2015enhanced lighttrappingwithdouble-groovegratinginthin-film1.pdf

Full length articleEnhanced light trapping with double-groove grating in thin-film amorphous silicon solar cellsJun WuLab of Information Optics and Opto-electronic Technology, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P. O. Box 800-211, Shanghai 201800, PR Chinaa r t i c l e i n f oArticle history: Received 5 September 2015 Received in revised form 25 October 2015 Accepted 24 November 2015Keywords: Diffractive optics Gratings Solar cellsa b s t r a c tA design to enhance light absorption in thin-film amorphous silicon (a-Si) solar cells is proposed. It is achieved by patterning a double-groove grating with waveguide layer as the absorbing layer and coatinga double-groove grating anti-reflective layer in the front window of the cell. The broadband absorption under normal incidence can be achieved for both TE and TM polarizations. It is shown that the averaged integrated absorptions have very large angle independence for the optimized solar cell. An qualitative understanding of such broadband enhanced absorption effect, which is attributed to the guided mode resonance, is presented. The conclusions can be exploited to guide the design of solar cells based on a grating structure.h1is the thickness of anti-reflective layer; h3is the thickness of grating layer; h2¼h3?h1; h4is the thickness of wa- veguide layer; f1and f2are duty cycles of the two ridges, d1is the separation of the two ridges.A TM (with H-field pointed along the stripes) and a TE (withE-field pointed along the stripes) polarization plane wave are in- cident from the air with an incident angleθ. The absorbing layers (h3þh4) is selected as 150 nm. To evaluate the total performance of the devices over a wide wavelength range, the integrated absorption is employed as afigure of merit. The integrated absorptions over all the interesting wavelengths range for TM and TE polarizations are [23,29]:∫∫∫∫λλλλλλλλλλ=( ) ( )( )=( ) ( )( )( )AaSdSdAaSdSd, 1TM300800 TM300800TE300800 TE300800whereαTM(λ) (αTE(λ)) is the absorption spectrum of TM(TE) po- larization in the active layer, which are obtained from RCWA bythe reflection (R(λ)) and transmission spectrums (T(λ)):αTE(λ)¼ 1?R(λ)?T(λ), and S(λ) is the solar irradiance spectrum, which is selected as AM1.5g solar spectral irradiance. The integration is done from 300 nm to 800 nm. The averaged integrated absorptionis defined as the mean value of TE and TM polarizations. To obtain the optimized structure parameters, we employ the simulated annealing (SA) algorithm. The cost function is [23]:ϕ = −(+)( )AA1/22TMTEThe objective is to minimizeϕ(d, h1,h3,h4,f1,d1,f2) by selecting suitable grating parameters. After optimization, we obtained the optimized structure parameters of the solar cells, which are shown asfollows:d¼506 nm,h1¼58 nm,h3¼100 nm,h4¼50 nm, f1¼0.208, d1¼74 nm, f2¼0.341. Though the optimized structure parameters are obtained with the device under normal incidence, the integrated absorption is still large when the incident angle is nonzero, which will be shown later. The optical response of the solar cells under normal incidence is shown in Fig. 2. Broadband absorption is achieved for both TE and TM polarizations. At short wavelengths, the absorption for TM polarization is slightly higher than TE polarization. However, at long wavelengths, the situation is reversed due to the two large resonant absorption peaks for TE polarization. Therefore, the ab- sorption spectrum presents a large polarization dependence, which is the inherent characteristic of one-dimensional grating. To solve this problem, the one-dimensional grating structure should be replaced by a two-dimensional grating structure. For a two- dimensional square lattice of nanocuboid, the spectral response is the same for TE and TM polarizations at normal incidence due to the symmetry of cuboids. Thus the polarization-independent ab- sorption can be achieved by use of square lattice of nanocuboid. At normal incidence, the averaged integrated absorption is about 73.18% for the structure proposed in this paper, which is much larger than the integrated absorption of the corresponding planar structure (about 38% for the unpatterned a-Si layer with thickness of 150 nm). Thus, the solar cells achieve about 92.6% increases. To obtain the potential for solar cell applications, a short-circuit current density is employed. When the internalquantum efficiency is selected as 1, the short-circuit current den- sity Jsc(mA/cm2) can be defined as follows [30,31]:Fig. 1. Schematic of the solar cells based double-groove grating and anti-reflective grating structure.Fig. 2. Spectral absorptions in solar cells.J. Wu / Optics & Laser Technology 79 (2016) 95–9996∫λα λλλ=( ) ( )( )λλ Jq hcSd3sc minmaxwhere q is elementary charge,λis the wavelength, the integration is fromλmin¼300 nm toλmax¼800 nm, h is the Planck constant, c is the speed of light. The calculated short-circuit current density is about 18.8 mA/cm2for the structure propo。