| The dopant-free crystalline silicon(c-Si)heterojunction solar cell has gained lots of scientific and technical interest recently due to its advantages such as low temperature process,simple fabrication,and no toxic and harmful gases applied.The passivated carrier selective contact layer is the key for dopant-free Si heterojunction solar cell with high conversion effieiceny.For p-type silicon(p-Si),cadmium sulfide(CdS)thin film can be a promising candidate as the electon selective layer.In this work,the dopant-free n-CdS/p-Si heterojunction solar cells are prepared successfully at low temperature.SCAPS simulation is ultilized for anylysising the physical properties of the lay stacks,including:the thickness of CdS,CdS electron affinity,impurity concentration,interface defect density/composite,and the effect of transparent conductive oxide(TCO)work function.Then the effects of annealing process,different backfield materials,silicon substrates and CdS film thickness on the solar cells were studied.The optical properties,material structure and surface morphology of CdS and ITO were studied by means of material characterization.Further analysis of the mechanism to verify the reasons for the improvement of solar cell performance.Finally,the paper focuses on the optimization of the electrical and optical aspects of the windows layer TCO.The author proposes an embedding metal grid structure,which can maintain the high transmittance of the TCO film while its resistivity is significantly reduced.The optical anti-reflection was optimized to prepare an n-CdS/p-Si heterojunction solar cell with an efficiency of 9.78%.It is concluded as following:(1)Given the SCAPS simulation data,the cell performance deteriorates as the CdS thickness more than 60 nm,and TCO of low work function is more suitable for electron collection by minimizing the band offset at the conduction band.As the doping concentration of the emitter material increases,the open circuit voltage(Voc),the fill factor(FF)and the current density(Jsc)are all significantly reduced.The optimal rang of the emitter electron affinity in contact with silicon is[-0.25,0.15];increasing the impurity concentration can enhance the conductivity of the material,but a slight excess of doping can aggravates current recombination.(2)The electrical conductivity of CdS and ITO films is improved obviously after annealing,and the work function is reduced by 0.27 eV and 0.62 eV,respectively,and the device performance is improved.The back field material MoO3 or gold(Au)with p-Si can form an ohmic contact,and the Voc of the solar cell with MoO3 back field is about 30 mV higher than that of Au due to the higher work function of MoO3.The textured silicon wafer has a lower reflectivity and light trapping effect than the polished silicon wafer,and can increase the current density to over 35 mA/cm2.By reducing the CdS thickness from 60 nm to 18 nm on the device,the cell conversion efficiency can be further improved.(3)The optical transmittance,conductivity and surface morphology of CdS and ITO film can be significantly improved by annealling at 300? in N2 atmosphere.The annealing results in the average CdS grain size increaseing from 41 nm to 63 nm,and the crystallization of amorphous ITO films.The grain size of the growth is favorable for carrier transport,which is confirmed by the electrical test results of the thin film.In addition,the roughness of the CdS and ITO films was significantly reduced after annealing.(4)A metal grid embedding structure is proposed to improve the conductivity of TCO.The ITO/Ag grid/ITO and AZO/Mg grid/AZO structures can not only maintain optical transmittance at around 85%in the range of 300-1200 nm,but also result in decreasing the sheet resistance into 91 ?/? and 1.94 ?/? respectively.Combined with the simulation of optical properties,a cell efficiency of 9.31%on the n-CdS/p-Si heterojunction solar cell was achieved by using the optimized ITO/Ag grid/ITO. |
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