Study On Molybdenum Oxide/Silicon Interface Passivation And Heterojunction Solar Cell

Solar photovoltaic power generation has attracted wide attention due to its advantages such as abundant resources,cleanliness and safety.Among them,crystalline silicon solar cells have the advantages of high photoelectric conversion efficiency and long life,and occupy a dominant position for a long time.With the increase of wafer bulk lifetime,reducing surface recombination plays a key role in improving solar cell efficiency.Typically,the industry uses highly efficient solar cell structures and uses various surface passivation techniques to reduce front and rear surface recombination,including thermal oxidation of SiO₂,plasma chemical vapor deposition of Si N_x,a-Si:H,and atomic layer deposition of Al₂O₃.However,serious Auger recombination in the heavily doped region of silicon solar cells and the pinning effect of Fermi level between metal electrode and doped silicon layer limit the efficiency improvement of current devices.Recently,contact passivation technology has attracted a lot of attention in the academic circle,which mainly uses thermal evaporation or atomic layer deposition of metal oxides with the selective contact performance of carrier to form undoped heterojunction solar cells with crystalline silicon.However,these passivation techniques still have some disadvantages,such as high temperature damage caused by thermal oxidation,vacuum deposition requiring high vacuum environment and dangerous and flammable precursor materials.Therefore,it is particularly important to explore a new preparation method of silicon surface passivation layer,improve the surface passivation effect and study the passivation mechanism for further improving the photoelectric conversion efficiency of silicon solar cells.High work function metal oxides,represented by molybdenum oxide（MoO_x）,form a strong anti-type layer with n-type silicon,which on the one hand provides excellent performance of hole transport and electron blocking,on the other hand,can reduce the carrier recombination in the silicon surface contact area.Previous studies focused on carrier selectivity,but the surface passivation performance was less studied.In this paper,we used solution method to deposit MoO_x thin films on the surface of monocrystalline silicon by spin-coated hydrogen molybdenum-oxide bronzes（H_zMoO_x）solution.Compared with the thermal evaporation method,the passivation quality of MoO_x films prepared by solution method is higher,and the effective lifetime(τ_eff)of n-type silicon is 110.6μs(Δn=10¹⁵ cm^-3).After annealing in low temperature atmosphere,the carrier life rises to 201.2μs.Further study shows that the dense amorphous film and the formation of Si-O-Si bond at the interface after annealing are the main reasons for the increased passivation of silicon surface.This work shows that it is feasible to use low-cost solution method to prepare transition metal oxides as effective carrier selective contact layer passivation.In this paper,a novel method of preparing ultra-thin aluminum oxide films by rapid thermal annealing（RTA）treatment for effective silicon surface passivation has been proposed.The high temperature RTA processing（750-825^oC）for tens of seconds in oxygen atmosphere completely converts the thermally evaporated aluminum metal film（1-7 nm）on crystalline silicon to aluminum oxide（Al₂O₃）,with a thin SiO_x layer formed at the interface between silicon and Al₂O₃ film.The generated Al₂O₃ film can provide superior passivation performance for silicon surface,and the effective lifetime(τ_eff)of n-type silicon is 121.9μs(Δn=10¹⁵ cm^-3),which is better than that of hot atom layer deposition technology..Moreover,the growth kinetics of Al₂O₃ passivating film indicates that it is a diffusion-controlled activation process,with an energy barrier of3.3 e V for aluminum ions diffusing across the metal/oxide interface.In addition,we deposited the MoO_x thin film and Al₂O₃ thin film into laminated structure on the surface of crystalline silicon,and found that the passivation performance was further improved.Compared with the monolayer MoO_x thin film,the effective lifetime was increased from 110.6μs to 276.4μs.The material cost of solar cells can be effectively reduced by using conventional transparent conductive electrodes as carrier collection and selective contact instead of high doping layer.In this study,an oxide/metal/oxide（OMO）multilayer film with MoO_x and Ag thin films was developed simultaneously by solution method and thermal evaporation to serve as an undoped hole transport transparent conductive electrode for n-type c-Si solar cells to produce undoped asymmetric heterojunction solar cells on n-type c-Si.The influence of the thickness of MoO_x layer on the performance of the film was studied.Based on the previous research results,the influence of low-temperature annealing and deposition methods on the photovoltaic characteristics of the prepared solar cells was explored.It was found that low temperature annealing can significantly improve the device performance of solar cells.The photoelectric conversion efficiency of c-Si solar cells with OMO multilayer structure prepared by thermal evaporation can be achieved by 6.1%after low temperature annealing at 150^oC.