Study Of Impurities And Defects In Germanium Doped Czochralski Silicon For Photovoltaic Applications

The photovoltaic(PV)energy market is witnessed by a booming rate beyond 30%per year recently.Silicon(Si)continues to have a prominent place in the PV energy market,with about 90%share of the current PV production.A major factor in the rapid growth in PV energy production is a steady decline of production costs,resulting from increasing solar-cell efficiencies.The cost of the starting wafers has a strong influence on the final cost of＄/W.To cut the cost,thinner wafers are going to be utilized.New materials have been investigated for solar cells.Among them Si-rich SiGe alloys shows some potential to improve the efficiency in comparison to the normal silicon solar cell,but the vast use of germaium(Ge)raises the cost.Ge doped silicon also shows some advantages beyond normal silicon.It can strengthen up the wafers by locking disloactions, inhibit void-type defects and promote the oxygen precipitation etc.Low warpage and breakage can be gained for thin Ge-doped silicon wafers.To understand the properties of Ge-doped silicon from the view of PV application not only makes sense for the knowledges,but also can help to fabricate low-cost and high-efficiency solar cells.Up to now,there is no systematic study reported.In this thesis,the as-grown Ge-doped Czochralski silicon(CZ Si)character, the effects of Ge on the behavior of grown-in defects and the process induced defects,as well as intentionally induced Fe and Cu metallic impurites,the effect of Ge-doping on the texturisation of wafers and performance of solar cells were systematically studied by means of Microwave photoconductance Decay (MWPCD),Fourier-Transform Infrared(FTIR),Scanning Infrared Microscope (SIRM)and Field-Emitting Scanning Electron Microscope(FE-SEM)techniques.In as-grown silicon,MWPCD study revealed that the minority carrrier lifetime distribution varied with the ingot position due to the Ge-doping level.The main traps detected were thermal donors and Ge-related defects.The effect of Ge-doping level on the formation of thermal donors with and without pretreatment was also investigated.A short-time enhancement of thermal donors by lightly Go-doping was observed after RTA pretreatment.Based on the experimental results,a reaction that oxygen dimmer(O_2i)was captured ty Go-related defects was proposed to explain the inhibition of Go on thermal donors.Under illumination,Go-doping could stabilize the minority carrier lifetime of wafers by reduce light induced degradation(LID)attributed to the BO complex. Go-doping hardly changed the migration barrier of O_2i,while the stability of BO was slightly reduced at 100℃.The capture of O_2iby Go-related defects and the disturbance to BO configuration by Go-doping both contributed to the decrease of LID.In Go-doped CZ silicon with artificial Fe and Cu contaminations,it was found that the disovation and formation of FeB complex was retarded.At the meantime,the lifetime ratioÏ„_GCZ4/Ï„_CZ3of Cu contaminated wafers was smaller than 1 below critical temperature about 700℃while larger than 1 above 700℃. 1250℃/50 s RTA protreatment can increase the critical temperature to about 900℃.The interaction between Go-Vacancy-related defects and Cu resulted into the lifetime character below critical temperature.At higher temperature,the inhibition of Cu precipitation mostly duo to the locking of dislocations by Go-doping resulted into a higher lifetime in Go-doped silicon.During the texutrisation of silicon wafers in alkaline aqueous solution, pyramids grew up by peeling atom layers from {111} surfaces of pyramids. Larger ones tended to hold larger space while the smaller ones would decrease or even disappear.Based on the comparison of the experimental reflecting spectra to the calculation results,a dominating reflecting pathway was found out which shew a very low reflectance 0.083.In practice,the reflectance was rised up by nonideal structures of pyramids as well as the very small ones.Texture on wafers with high density of BMD also had higher reflectance.The BMD in wafers beyond 10⁷ cm^-3could change the nucleation of pyramids further resulted into the non-uniformity of them.Go-doping at 10¹⁶～5×10¹⁹cm^-3level had no obvious effect on the etching rate of silicon in alkaline aqueous solution.And the texturiation character had never been changed when BMD density was smaller than 10⁷ cm^-3in Ge-doped silicon.In comparison with conventional CZ Si,Ge doped CZ Si with Ge content 5×10¹⁹cm^-3was used to fabricate solar cells by the same processes.It was found that the short-circuit current I_scand conversion efficiency of Ge-doped Si solar cells(GSCs)was slightly higher than that of the conventional solar cells(SCs), while the open-circuit voltage and fill factor of both the solar cells was almost the same.The better saturation current I₀₂and shunting resistance R_shfor GSCs were gained.