Design And Simulation Of Photonic Crystal Superposition Structure In Silicon Thin Film Solar Cells

After decades of development,the industrial application of crystalline silicon solar cells has become increasingly perfect.However,with the shortage of raw materials and rising manufacturing costs,people turn their attention to silicon thin film solar cells.In addition to saving a large amount of silicon raw materials,it also has the advantages of lower requirements for raw materials,better electrical properties,wider application range and more sensitive to spectral response.However,due to the existence of silicon indirect band gap,the thickness of silicon thin film solar cells is not enough to absorb longwavelength photons,resulting in lower photocurrent and photoelectric conversion efficiency than crystalline silicon cells.In order to enhance the absorption of photons by the absorption layer and prolong the propagation path and residence time of photons in the absorption layer,the micro-nano optical structure is commonly used to regulate it.Among various types of optical structures,photonic crystal(PC)is widely used in solar cells due to its unique optical properties such as photonic band gap and slow photon effect.In this paper,a variety of PC multilayer structure anti-reflection layers and onedimensional(1D)+two-dimensional(2D)PC back reflection layers are designed by the finite element method.The propagation process of sunlight in silicon thin film solar cells with the above structure is simulated,and the transmittance and reflectivity of the antireflection layer and the back reflection layer in the working band(400-1100nm)of the cell are calculated respectively.The anti-reflection layer can reduce the reflectivity when the sunlight is incident so that more photons enter the absorption layer.In this paper,based on the optimized multilayer structure anti-reflection layer,the groove type,grating type,moth-eye hole type and moth-eye cone type PC structure are superimposed,so that the transmission effect of the anti-reflection layer is significantly improved compared with the original multilayer structure.The multilayer film materials are composed of silicon dioxide(SiO2),silicon dioxide-titanium dioxide(SiO2-TiO2)composite and titanium dioxide(TiO2),respectively.The grooves and moth-eye holes are filled with air,and the grating and moth-eye cone materials are SiO2.In our work,by changing the height(depth)or period of the unit cell,the transmission spectra of the anti-reflection layer of the PC multilayer film under different structural parameters are measured.It is calculated that the average transmitivity of 1D PC groove multilayer structure,1D PC grating multilayer structure,2D PC motheye hole multilayer structure and 2D PC moth-eye cone multilayer structure anti-reflection layer in 400-1100 nm solar cells under the optimal structural parameters is 98.71%,98.14%,98.65%and 98.30%respectively.Among them,the average transmittance of 1D PC groove multilayer structure with H=181 nm and T=80 nm is the largest,which is 1.33%higher than that of the multilayer structure anti-reflection layer,and the increase range accounts for 50.76%of the room for improvement.The back reflection layer can reflect the incident light through the absorption layer back to the absorption layer to realize the secondary capture of short-wave light and the absorption and utilization of long-wave light.Therefore,a back reflection layer composed of 1D PC and 2D PC is designed in this paper.A single 1D PC is composed of high and low refractive index materials alternately and periodically arranged,and the 2D PC unit cell is composed of crystalline silicon with etched periodic air holes.After superimposing 1D PC and 2D PC,a new back reflection layer acting on the full 400-1100 nm band is obtained.After calculation,the average reflectivity of the existing 1D+2D PC back reflector in the effective range of 470-1100 nm reaches 97.85%,which is higher than 68.23%,81.29%,83.75%,90.41 and 96.84%produced by 1D PC,2D PC,2D+1D PC,metal Al and Ag back reflectors.In addition,for the two narrow bands of 470～800nm and 800～1100nm,the average reflectivity of the designed 4-cycle 1D PC back reflector in the short and narrow bands is 97.55%.In the long and narrow bands,the designed four-layer air hole 2D PC back reflector has an average reflectivity of 99.32%.In summary,1D PC,2D PC,and 1D+2D PC back reflectors have excellent reflection effects for short-band,long-band,and full-band,respectively.