Silicon Based Thin Film Solar Cells' Interface Micro And Nano Structure Design And Optoelectrical Performance Analysis

Solar cells are a kind of semiconductor devices,which transfer the solar energy into the electrical energy.Owing to the extensive of the solar energy,solar cells have been paid attention by all the contries around the world.Silicon has been widely used in sem-iconductor industry and its mass amount is around 26.4%in the earth's crust,so silicon solar cells are quite important in the photovoltaic industry.However,the high manufac-ture cost of crystalline silicon and the low photoelectric conversion efficiency of amor-phous silicon,the thickness of silicon layers in the solar cells should be reduced.The low optical absorption efficiency and the low optical generated carrier's collection efficiency of the silicon based thin film solar cells?TFSCs?are two problems,which need to be solved urgently.With the development of micro and nano-fabrication technology,interface structure design at micro and nano scale provides a feasible approach to improve the performance of silicon based TFSCs.According to the optical waveguide mode's coupling theory,the geometrical averaged permittivity theory at sub-wavelength scale and the carrier's Schol-ley-Read-Hall?SRH?recombination theory etc.,four interface structures are designed for silicon based TFSCs.These designed silicon based TFSCs'optical absorption efficiency and electrical performance parameters are calculated by the finite-difference time-domain?FDTD?method and the finite elementary method?FEM?respectively.Through the opti-mization of the interface structures'parameters,the output performance parameters'en-hancing fractions are calculated.According to the optimal waveguide theory,the geomet-rical optics and the electromagnetic field's amplitude distribution etc.,the underlying mechanisms for the optical absorption efficiency's and the carrier's collection effi-ciency's improvement are discussed.These analysis will provide related theoretical and scientific basis for high efficiency silicon based TFSC's researches and developments.The main research works and results are listed as following:1)The bottom silver grating is designed as two layers of silver strips,which have a height difference?h.Taking advantage of FDTD method,the height difference‘s influ-ence on c-Si based TFSC's optical absorption efficiency and photonic current density.The electrical performance parameters are calculated by FEM method,and the designed c-Si based TFSC's output performance parameters are obtained.Results show that the designed bottom two layer of silver grating could improve the optical absorption effi-ciency of c-Si based TFSC.2)According to optical waveguide theory,a layer of bottom hybrid grating is designed for c-Si TFSC.The bottom hybrid grating consists of Ag strips and AZO strips,which arranged alternately.The optical absorption efficiency enhancement spectrum is calcu-lated for the HG c-Si based TFSC,and this enhancement spectrum is compared with the analytical dispersion map of the c-Si layer.We find that the designed HG c-Si based TFSC's optical absorption efficiency enhancement mainly comes from the coupled opti-cal waveguide modes.Then,this HG structure is applied for conformal bi-layer gratings c-Si TFSC.The optical absorption efficiency and output performance parameters are cal-culated.Results show that HG grating could improve the c-Si layer's optical absorption at long wavelength region efficiently.3)According to the geometrical averaged refraction index theory at sub-wavelength scale,a layer of interface photonic crystal?IPC?is designed at the interface between the AZO cover layer and the c-Si layer.A reduced power transmission efficiency formula T_in=T₁?T₂ is derived according to the light's reflection and transmission equations.Accord-ing to the deduced power transmission efficiency formula,the optimal parameters of the IPC layer are calculated.Through the analysis by the FDTD method,the IPC layer could efficiently improve the c-Si layer's optical absorption in wavelength region of 300 nm to600 nm.Through the analysis by FEM method,the c-Si TFSC with IPC layer's output performance parameters are improved efficiently.4)The generated carrier's distribution in 200 nm thick a-Si layer of TFSC is researched.It is found that around 36%generated carriers concentrated in top and bottom 20 nm thick a-Si regions.Owing to the extremely high carrier's recombination rates in the heavily doping a-Si region,we designed an interval doping method for the a-Si TFSC.Through the research on the generated carrier's distribution in the interval doping TFSC,it is found that more than half of the generated carriers in the top and bottom 20 nm thick a-Si region are moved from the heavily doping a-Si regions to the intrinsic a-Si region.The light's focalization by the top strip's structure improves the generated carrier's fraction in the intrinsic a-Si region.According to the dangling bond defect's and bandtail defect's den-sity of state?DOS?curves,the generated carrier's SRH recombination rate's model in the a-Si layer are built.Through the analysis of generated carrier's collection efficiency of the interval doping TFSC,it is found that the designed interval doping method could im-prove the electrical internal quantum efficiency?EIQE?of the a-Si based TFSC.