Study On Surface Passivation Property Of Monocrystalline Silicon

Crystalline silicon solar cells are the mainstream of photovoltaic devices,accounting for95%of the market share at present.Improving the conversion efficiency of silicon solar cells can further save the cost of per kilowatt hour of photovoltaic power,providing a solid foundation for photovoltaic power generation to be used in a wide range.The device’s performance is determined on the features of crystalline silicon solar cell’s surface or interface area.For example,Passivated Emitter and Rear Cells(PERC),the passivation technology of current mainstream,has significant carrier compound loss because of the metal electrodes and the silicon on the back of direct contact,as a result,the limit efficiency of volume production of battery is limited to around 23%.Silicon Heterojunction solar cells have attracted great interest of people in the past decades due to their high efficiency potential.At present,the maximum efficiency of Heterojunction with Intrinsic thin-layer(HIT)cells is 26.7%.The key to the structure of HIT solar cell is the application of the surface passivation technology of intrinsic amorphous silicon(i-a-Si:H),which has a significant surface passivation effect and greatly reduces the surface recombination of crystalline silicon.Because of the symmetrical structure,HIT solar cells have fewer process steps compared with traditional crystalline silicon cells and can generate power on both sides of the cell.Therefore,HIT solar cells are the mainstream development direction of high-efficiency crystalline silicon solar cells in the future.In order to obtain a good surface passivation effect,the hydrogen dilution ratio(R_H=hydrogen/silane)is generally small during the growth process,which leads to a low conductivity of i-a-Si:H thin films,and ultimately reduces the efficiency of HIT solar cells.If the hydrogen dilution ratio is increased,the conductivity of i-a-Si:H films increases,but the passivation effect on the surface of crystalline silicon will decrease.Therefore,how to obtain good passivation effect as well as a high conductivity of i-a-Si:H thin film at the same time is the key to obtain high efficiency HIT solar cell.In this paper,the passivation effect on the surface of monocrystalline silicon is studied.The silicon substrate is n-type c-Si single crystal wafer while the passivation layer includes three types of intrinsic layer,n-doped layer and p-doped layer.Plasma enhanced chemical vapor deposition(PECVD)is a common method for deposition of i-a-Si:H thin films.In this paper,the influence of process parameters on the passivation performance of amorphous silicon thin films was explored,including hydrogen dilution ratio,film thickness,post-annealing temperature and so on.The conclusion was reached that the best hydrogen dilution ratio was 4,the best film thickness was 40nm,and the best annealing temperature was 400℃.In addition,in order to have both high conductivity and good passivation effect,this paper adopts a two-step method to deposit i-a-Si:H laminated films on the surface of monocrystalline silicon.Firstly,the first layer of i-a-Si:H thin film is grown through low hydrogen dilution ratio.This amorphous silicon layer shows excellent passivation effect on crystalline silicon.Then,the second layer of thicker i-a-Si:H thin film is grown under the condition of high hydrogen dilution ratio,which shows a higher conductivity.As a result,this paper studies the effect of the combination of deposition and annealing conditions on the quality of a-Si:H passivation in the two-layer structure,and finally obtains that the lifetime of the monocrystalline silicon passivated by i-a-Si:H laminated film reaches 7.36ms.Then on the second layer is doped with boron and phosphorus,at this time,the sample shows not only the chemical passivation but also the field effect passivation.The n-type sample shows better passivation effect before annealing and after annealing,both n-type and p-type samples shows lower lifetime compared with the un-doped sample,showing 3.27ms of n-type sample and 0.75ms of p-type sample.The reason for this is that annealing deepens the inward diffusion of doped atoms,increasing the number of defects,but the conductivity is further improved,which is conducive to the subsequent research of the solar cell.In addition,magnetron sputtering,a new method for deposition of amorphous silicon thin films on silicon substrates,is proposed in this paper.Compared with PECVD,it has the advantages of simpler and safer,but during sputtering,ion bombardment will cause great damage to the substrate surface.The effects of sputtering pressure,film thickness,hydrogen dilution ratio and post-annealing temperature on passivation effect were investigated in detail.Amorphous silicon thin films prepared by magnetron sputtering have a certain passivation effect.Although there is a certain gap between the films prepared by PECVD method and the magnetron sputtering method,it is worth confirming that the thin films prepared by the two methods have a high similarity in the characterization of passivation performance,which proves that the i-a-Si:H thin films prepared by magnetron sputtering method can be investigated.