Surface Light-trapping And Passivation Technique For Silicon Solar Cell

In decades,many advances in wafering technology have been made,resulting in an emerging new technique known as diamond wire sawing(DWS)which has become more and more popular.The introduction of DWS technique leads to the remarkable reduction of production cost.Besides,many new types of silicon solar cells(e.g.PERC)have been investigated,which improve the efficiency of solar cell greatly.In order to fabricate high-efficient silicon(Si)solar cell,it is important to keep the improvement of both optical and electrical performance.However,the DWS wafer brings the following problems to the photovoltaic(PV)industry,due to its special surface morphology with serious saw marks,it is hard to use the conventional alkali-based texturing process to form homogeneous pyramid texture on the surface.In addition,the serious defects and recombination caused by the remain saw marks after the conventional texturing process will have a negative influence on the passivation effect and cell performance.Therefore,according to the current surface characteristic of the DWS wafer,several works have been done as follows:the TMAH etch route was investigated to remove the saw marks and then a uniform pyramid texture was formed on the surface;the submicron-texture was fabricated on mono-Si wafer by using Ag metal-catalyzed chemical etching(Ag-MCCE);the passivation effect of ALD-Al2O3 on the sunbmicron-texture was demonstrated.We believe that those researches should benefit both optical and electrical performance of high-efficient solar cells.1.A TMAH pre-polishing route before the conventional alkali-based texturing process was studied to fabricate high-quality pyramid texture.The results show that TMAH has an obviously positive effect on the formation of excellent pyramid texture,saw marks removal,high-quality pn junction and Ag finger.The surface reflectance can be reduced to 13.1%(13.0%for conventional MWSS sc-Si).In addition,the efficiency of DWS sc-Si solar cell can reach to 19.15%,which can be kept as the same level of conventional MWSS sc-Si one(19.14%).2.The etching mechanism and technique of Ag-MCCE for single-crystalline silicon(sc-Si)wafer are presented.In this work,DWS sc-Si and multicrystalline(mc-Si)was experienced a conventional acid-based texturing process to remove a part of surface saw damage.However,it is difficult to remove the saw marks totally,and surface reflectance of sc/mc-Si is about 29.0%/30.3%.After MCCE,the nano holes are modified by the HF/HNO3 polishing step and the reflectances of sc-and mc-Si was reduced to 13.2%/18.4%,respectively.The respective 19.38%sc-Si and 18.72%mc-Si solar cells(18.17%for sc-Si and 18.16%for mc-Si ones with conventional acidic texturing process)were achieved,which are obviously higher than the conventional ones.It demonstrates that this Ag-MCCE technique has a significant improvement for both sc/mc-Si solar cells.3.For conventional Ag-MCCE process,the light-trapping property was investigated.The lowest reflectance is about 2.34%,even though it could be possible to fabricate a lower reflectance than 2.34%by improve the experiment specific recipe further,the experiment window is too narrow so that it is hard to keep it stable and repeatable.We improved the experiment process:texture was made in a higher-HF solution first to ensure its enough depth and then another solution with relevant lower HF was used to widen its diameter.Finally,we fabricated the nano-texture with the ultra-low reflectance(～1%),which can be used into the following ALD passivation.4.The passivation effect of ALD-Al2O3 on the surface with the MCCE nano-texture was investigated.Generally,with conventional silicon nitride passivation the Si surface recombination will be increased due to the increasing surface area.However,the results show the superior passivation effect of ALD-Al2O3.The ALD-Al2O3 passivation effect was studied from several aspects including AgNO3 concentration and acid modification time after MCCE process.In the case of high AgNO3 concentration,there is no need of the polishing step.The high lifetime was achieved without any loss of reflectance.In the case of low AgNO3 concentration,the lifetime was increased with the increase of the modification time.The highest lifetime is about 1 ms while the reflectance was also increased to around 19%.In addition,ALD-Al2O3 was deposited on the ultra-low reflectance(1%)Si surface to achieve high lifetime(～300 μs).In summary,this theise focused on the new mechanism and technique for the surface treatment of DWS sc-Si.Especially,the expansion of Ag-MCCE from mc-Si to sc-Si is helpful to achieve the universal texturing process for both sc/mc-Si solar cells.In addition,we demonstrated that it has great potential to fabricate efficient solar cells with the superor optical and electrical performance by combining the MCCE and ALD-Al2O3 techniques.