| Flexible thin-film silicon based solar cells,for its abundance and nontoxicity of the source materials,flexibility and lightweight with high power/weight ratio,are particularly promising for Military and Civilian application.For hydrogen nanocrystalline silicon(nc-Si:H),hydrogen amorphous siicon(a-Si:H)and hydrogen amorphous silicon germanium alloy(a-SiGex:H),due to their low long wavelength absorption coefficient,light trapping is crucial to these cell performance.So in this article,designing and optimizing new light trapping substrate for enhancing cell efficiency is one of our work.Another,for understand the mechanism of light trapping,optical simulation is often used to do it.In this article,light propagation in nc-Si:H solar cell was systematically investigated by using FDTD optical simulation method.Finally,designing new thin film solar cell structure is crucial to obtain higher cell efficiency and lower cost.In this article,the new cell structure was investigated by FDTD simulation.So for these research directions,we have conducted several research as followed.I.Self-textured Ag films were fabricated on flexible stainless steel substrate by thermal evaporation process,then forming back reflector(BR)by depositing ZnO:Al film.The back reflector RMS can be regulated by deposition temperature and deposition rates of Ag films respectively.The optical characteristics of BRs were measured.We have demonstrated that when RMS of BR is 23.6 nm,the highest nc-Si:H cell efficiency of 8.82%was obtained.The nc-Si:H cell efficiency began to decrease when the RMS of BR is higher or lower than 23.6 nm.Especially,when RMS of BR reach 49.7 nm,the nc-Si:H cell performace become deteriorated.In order to conform the poor nc-Si:H cell performance with large RMS,we measured the cell QE curves under-3 and 0 V reverse bias conditions and dark curves.When RMS of BR is 49.8 nm,the diode factor of nc-Si:H is 1.95,indicating the cell has high body defect density.But when RMS of BR is 23.6 nm,the diode factor of nc-Si:H is 1.56.Finally,the optimized BR was applied in a-Si:H/nc-Si:H tandem cell,the cell efficiency of 12.68%and 10.7%were realized respectively at small area(0.25 cm2)and large area(200 cm2)respectively.II.Ag nanoparticles(AgNPs)with suitable size has superior light scattering property,which can be used as light trapping in solar cell.In this article,AgNPs was fabricated on flexible SS substrate by high temperature annealing of thin Ag films and was applied in nip a-SiGex:H solar cell.The correlation between the size of AgNPs and cell performace was investigated in detail.When AgNPs size is about 110 nm,AgNPs BR has the strongest parasitic absorption,and the cell also has the highest average absorption,but the cell current is lowest in three AgNPs BRs cell.When AgNPs size is large with 680 nm,the AgNPs back reflector has lowest parasitic absorption in three AgNPs BRs,but the scattering intensity and large degree scattering density are lower than AgNPs BR with size of 320 nm,so the current density is low than cell with AgNPs back reflector with 320 nm Ag particle size.When AgNPs size is 320 nm,the BR has the strongest scattering intensity and large angle scattering intensity,the current density is the highest.Comparing the cell with specular BR,the current increase 3.0 mA/cm2,or 19.5%,and the cell efficiency increase 21%.It demonstrated that the size of Ag NPs is crucial to cell current.The interplay between scattering intensity,parasitic absorption and total reflector of BR will determine the cell performance.Finally,the weight of light trapping due to AgNPs scattering or cell surface RMS scattering was evaluated by FDTD simulation.III.For understanding the optical wave propagation in solar cell,it is necessary to use optical simulation to solve Maxwell's equations.In this article,period triangular grid substrate was studied to enhance the light trapping in nc-Si:H solar cell by FDTD simulation.Two parameters of So and k were used to define the structure of triangular grid.The correlation between the Sa,k and optical field distribution,current,and carrier generation rates distribution in nc-Si:H cell were studied in detail.The optimized triangulat substrate structure was obtained.When So and k are 400 nm and 1.0,the highest photocurrent of 27.0 mA/cm2 in nc-Si:H cell can be obtained with only 1-?m thick absorber and the high carrier generaton rate zone in nc-Si:H absorber focus at the top zone,and the low carrier generation rate located the middle zone.The optical losses in nc-Si:H with optimized triangular grid substrate(Sa = 400 nm,k= 1.0)were analyzed.In terms of optical losses,ITO and Ag dominated the optical losses,with the current losses of 4.07 mA/cm2 and 2.37 mA/cm2,or 54.5%and 31.7%of total loss,respectively.So it is crucial to reduce the optical losses of the ITO and Ag layers.Finally,a new light trapping substrate design was provided to decrease the Ag film parasitic absorption.IV.For improving the thin-film solar cell for further,it is necessary to design new cell structure.In this article,the new tandem solar cell with the high-perfomance perovskite material as top cell absorber and c-Ge as bottom cell absorber was studied by FDTD simulation.We analyzed the electric field intensity distribution,absorption,carrier generation rate distribution and optical losses in the tandem solar cell for understanding the electromagnetic wave propagation and absorption in the device.By optimizing the absorption layer thickness,the top cell short circuit current(Jsc)is achieved to as high as 23.70 mA/cm2 and bottom cell Jsc is achieved to as high as 24.0 mA/cm2 by using very thin absorption layers,as thin as 750 nm for each active material.For tandem cell optical losses,ZnO:Al has strong optical absorption due to its free carrier absorption.The optical loss due to reflection is another large optical loss with current loss can reach 5.6 mA/cm2.Finally,the perovskite/c-Ge tandem thin film cell design is capable of high potential efficiency 28.4%based on the simulation and the Voc of tandem cell is 1.50 V,FF is 80%and photocurrent is 23.7 mA/cm2. |
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