Optical And Electrical Properties Of SiC_x(n~+) And Its Application In High Efficiency Crystalline Silicon Solar Cells

At present,the tunneling oxygen passivation contact(TOPCon)technology is widely considered as the main solution for the next generation of industrial high efficiency crystalline silicon solar cells.It consists of silicon substrate,ultra-thin tunneling silicon oxide and doped polysilicon.Doped polycrystalline silicon(PCS)thin films are used as functional layers for field passivation in TOPCon structures.They are usually prepared by in situ doped amorphous silicon thin films prepared by plasma enhanced chemical vapor deposition(PECVD)after high temperature annealing for crystallization.In both laboratory and industrial production,annealing in a conventional high temperature tubular furnace(800-1100?)is a standard annealing process and usually obtains very good passivation effect.However,after annealing at high temperatures,amorphous silicon films often blister,which results in serious silicon surface recombination and passivation performance degradation.At the same time,polycrystalline silicon film itself has severe parasitic absorption,which makes the device performance deteriorate seriously.In order to improve the passivation performance of amorphous silicon film after annealing,reduce parasitic absorption and blistering,and further improve the efficiency of crystalline silicon solar cells.In this paper,a novel carbon-doped polycrystalline silicon film material SiC_x(n⁺)was developed to replace the traditional doped polycrystalline silicon film.By crystallization,the control of interface stress was successfully achieved.The problem of film blistering during annealing was avoided,the excellent passivation effect was obtained,and the parasitic absorption of medium and long wave was basically eliminated.The effects of annealing temperature,carbon content(controlled by r=CH₄/Si H₄,the more r,the more C content)on the optical,electrical and passivation properties of n-type TOPCon contact and the effect of C content on the TOPCon interface were studied.The main research results are as follows:(1)The presence of element C enhances the ultraviolet parasitic absorption in the short-wave spectrum range of the films,which is detrimental to its use as a window layer;However,C element and higher annealing temperature help to reduce the parasitic absorption of carbon-doped polycrystalline silicon film SiC_x(n⁺)in the spectral range with wavelength greater than 600 nm.The higher r,the lower the infrared parasitic absorption and the value tends to zero at the same annealing temperature.Similarly,when R is the same,the higher the annealing temperature,the weaker the infrared parasitic absorption of the film.Using the novel TOPCon as the passivation structure on the back of solar cells can improve the infrared spectrum response of solar cells.(2)Element C inhibits the activation of donor impurities,resulting in the decrease of carrier concentration and mobility in the film,as well as the increase of the work function of the film.By increasing the annealing temperature,we reduced the film square resistance from up to 200?/sq to about 100?/sq,and controlled the contact resistance R_c from up to 66.5 m?/cm² to below 5 m?/cm²,which fully meets the electrical requirements of high efficiency crystalline silicon solar cells.(3)It is found that the passivation effect of doped polycrystalline silicon film SiC_x(n⁺)is better than that of doped polycrystalline silicon film.Among them,r=CH₄/Si H₄=3,annealing temperature 880?has the best passivation effect,implied open-circuit voltage(i V_oc)is higher than 750 m V,single-sided reverse saturated dark current(J₀)reaches lower than<2 f A/cm².(4)The crystallization rate and phase of the film were analyzed by Raman spectroscopy and XPS spectroscopy.It was found that the presence of carbon inhibited the crystallization of the film,and C existed in the amorphous phase,which also was confirmed by TEM results.The main structure of doped polycrystalline silicon film SiC_x(n⁺)is amorphous phase of Si C_x distributed in the crystalline silicon substrate.(5)C not only inhibits the crystallization of the film during crystallization annealing,but also passes through the ultra-thin tunneling oxide layer from the carbon-doped polycrystalline silicon film SiC_x(n⁺)to the monocrystalline silicon substrate by high-temperature diffusion,forming an amorphous disordered band at the interface substrate,which is called C-doping-induced amorphization.This phenomenon is the main reason that the carbon doped polycrystalline silicide film inhibits the blistering.(6)The carbon-doped polycrystalline silicide film SiC_x(n⁺)was validated at the device level.The efficiency of all the validated solar cells were above 23%,with the highest efficiency being 23.62%,V_oc=699.10 m V,J_sc=40.94 m A/cm²,FF=82.53%.In summary,based on the traditional high efficiency TOPCon crystalline silicon solar cells,this thesis develops a new type of carbon-doped silicon film material,which effectively solves the shortage of the traditional doped polycrystalline silicon film,greatly improves the efficiency of high efficiency TOPCon crystalline silicon solar cells,and provides an effective way to further improve the efficiency of the solar industry.