| Because n-type silicon wafer has a higher lifetime,without light-induced degradation?LID?effect and higher tolerance to common impurity,n-type crystal silicon solar cell is more desirable than p-type to achieve higher efficiency.However,it exists several problems in n-type crystalline silicon solar cell,such as back-side contact and boron diffusing.Meanwhile,further efficiency improvement of c-Si solar cell needs to reduce surface recombination.The introduction of electron-selective-collection passivated contact structure can simultaneously improve the interface contact and surface passivation.At the same time,its simple one-dimensional structure can get higher fill factor.The study of electron-selective-collection passivated contact structure with superior performance is helpful for exploring n-type silicon cells with higher cell efficiency.In this paper,the structure with tunnel silicon oxide/low work function scandium is applied as an electron-selective-collection passivated contact structure in n-type crystal silicon solar cell.Tunnel silicon oxide?SiOx?can be used to saturate the suspension key on the surface and bring certain chemical passivation effect.The introduction of low work function metal scandium?Sc?can improve the interface contact and bring the field passivation effect.In this paper,the preparation of SiOx by wet chemical method and Sc film by E-gun electron beam?E-beam?vacuum evaporation is studied.Then,physical and chemical characterization is carried out.In addition,the electrical properties and application in c-Si solar cell of tunneling SiOx/low work function metal Sc film structure are explored.At last,the numerical simulation of tunneling SiOx/low work function metal as electron-selective-collection passivated contact structure is studied by using AFORS-HET software.The main results are as follows:?1?The thickness of SiOx prepared by hot nitric acid oxidation changes with the temperature of nitric acid and the immersion time in nitric acid and the thickness is in the range of 1-2 nm;Subsequent annealing will lead to SiOx thickness increase,the higher the temperature,the higher the thickness.At the same time,annealing can improve the passivation performance of SiOx which is explained as that the high temperature treatment improves the sauring degree of Si-O key and the ratio of Si 4+bond.When the temperature is 875?,the passivation of SiOx is the best.However,when the temperature exceeds 900?,passivation performance decreased.?2?The thickness of the Sc film prepared by E-beam vacuum evaporation method is uniform and controllable through the measurement of step profiler and TEM cross-section diagram.The 3D SPM topography shows that surface roughness of Sc film is very small.The distribution of elements on TEM section map shows that:due to the reaction of scandium and silicon oxide in the preparation process,ScOx is formed?By UV photoelectron spectroscopy results,the work function of n-Si/SiOx/Sc interface is ensured as about 3.65 eV,which is slightly higher than the theoretical value.?3?The interface shows a well ohmic contact instead of schottky contact when Sc is used to take place of aluminum?Al?and contacts directly with 1?.cm n-Si.Ohmic characteristics still maintain when SiOx annealed at 500?-900?is inserted between n-Si and Sc.When the annealing temperature is over 920?,the contact characteristics of the interface are turned into Schottky contact because of the decreasement of electron tunneling ability.When SiOx annealed at 700?,the chemical passivation and field passivation of SiOx/Sc structure reach the best coordination,and iVocc reaches 623 mV.?4?For c-Si solar cell,700?is the best annealing temperature of SiOx.When full-back SiOx/Sc passivation contact structure replace full-back Al contact,the efficiency improvement of solar cell reaches about 4%.Post-annealing is helpful to improve the contact between the front electrode and diffusion layer for the samples with back SiOx/Sc structure;250?+5 min annealing can upgrade FF to more than 75%.However,when the annealing temperature continues to rise,the passivation effect will decline sharply.?5?The numerical simulation results show that the passivation propertity of tunnel SiOx/low work function metal structure is affected by SiOx thickness and the work function of the metal.To get well passivation effect,the following requirements must be meet:SiOx thickness is in the range of 1.2-1.6 nm;Interface defect density(Dit)<1×1010cm-2/eV;the density of pinhole(Dph)<1×10-44 and the metal work function is less than or equal to 3.6 eV.When the metal work function is less than 3.6 eV,ability of promoting electron tunneling is close to saturation;when the thickness of SiOx is 1.4 nm,the work function of metal is less than 3.6 eV,the efficiency of simulated n-type c-Si solar cell containing tunnel SiOx/low work function metal back structure is more than 23%. |
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