【CITPV】Development Of High Efficiency Passivated Contact Silicon Solar Cell

CSIQ NASDAQ Listed Development Of High Efficiency Passivated Contact Silicon Solar CellWeiwei Deng @ EU PVSEC 20192019 CITPV @ 杭州阿特斯阳光电力集团2019年9月10日 2Canadian Solar Inc. Motivationn Mono PERC cell efficiency is ~22.5% in mass production.n Industrial type PERC cell efficiency gradually reaches the limit.n The dominant loss is from contact recombination.n Passivated contact can solve the problem under the contact. n Passivated contact formed with tunnel oxide and doped poly-Si is under development widely. 3Canadian Solar Inc. Passivated contactn Traditional contact n Passivated contact n High recombination under metal contact (J0 = 500-1000 fA/cm2)n+ Si MetalSi n Very low J0metal (50 fA/cm2)n Low resistance lossSin+ poly-Si SiOxMetal 4Canadian Solar Inc. Process developmentWhy not combine passivated contact with PERC?n Limited cell efficiency gain. n AlOx shows excellent passivation quality, and local Al-Si contact area is limited.n No necessary to be replaced by SiOx/p-poly-Si.n Complicated process, especially p-type doping of poly-Si. P-type PERC cell withrear passivated contactP-type PERC PERC + TOPCon 5Canadian Solar Inc. Process developmentWhy not combine passivated contact with PERC?n Limited cell efficiency gain (Eta +0.3-0.4%) n Reduce contact recombination, but increase absorption loss.n Complicated process (formation of poly finger, precise alignment, etc.) PERC + TOPConP-type PERC cell withfront local passivated contactP-type PERC 6Canadian Solar Inc. Process developmentCell structure comparison N type celln Passivated contact is suitable of integrating with N-PERT cell structure.n Excellent passivation of SiOx/n-Poly-Si (vs. SiNx/n +-Si).n Reduced contact recombination (vs. Ag/n+-Si).n Not significant absorption loss by poly-Si on the rear side. N-PERT cell withrear passivated contact 7Canadian Solar Inc. Process development Process flow comparisonTexture POCl3 diffusionLDSEFront passivationLaser openingMetallizationP-type PERC TextureBoron diffusionRear etchRear passivationFront passivationMetallizationSiOx/i-poly-SiPoly-Si dopingN-type TOPCon cell123 Wet clean4Rear etchRear passivation 8Canadian Solar Inc.n J0 of boron doped emitter can be as lower as 20 fA/cm2.n Selective emitter seems not necessary for boron doped emitter. 1Ω·cm N type waferPolished surfaceRho_sheet=83ohm/squ.Process development Boron doped emitter 9Canadian Solar Inc. Process development Rear surface morphologyn Three types of typical rear morphology§ Acid etched surface§ Alkaline polished planar surface§ Pyramidsn Planar surface: lower J0, higher Rcn Acid etch: good contact and lower J0 n Textured surface with pyramids:§ Lower Rc§ Highest J0§ Higher bifaciality Acid etchedAlkaline polished Pyramids 10Canadian Solar Inc. Process developmentPoly-Si doping conditionn Doped by ex-situ POCl3 diffusionn Higher drive-in temperature § More doping in silicon§ Passivation quality deterioration at very high temperature§ More pinhole was observed n With sufficient doping in poly-Si, the contact resistance keeps constantly with the increasing of drive-in temperature. 11Canadian Solar Inc. Process developmentThickness of poly-Sin The absorption of rear poly-Si layer affects the current and cell efficiency.n The thickness of poly-Si layer was investigated (measured and simulated).n The thicker poly-Si layer, the lower reflectance at long wavelength.n Jsc can be increased ~0.13mA/cm2, if the thickness decrease from 200nm to 120nm. 12Canadian Solar Inc. Process developmentThickness of poly-Sin When changing the thickness of poly-Si, the change of J0 and Rc also should be considered.n J0 of passivated area is revealed to be stable. n Contact resistance increases with the increase of the thickness of poly-Si.n From the PL counts, it is obvious that the recombination under metal contact increases. 13Canadian Solar Inc. Process developmentProcess integrationn With optimized process, cells were fabricated on ~1Ω·cm N type wafer.n The average iVoc of 712mV was achieved before metallization.n With all these optimized process, the highest cell efficiency reaches 23.52% (In-house Sinton I-V measurement).Voc (mV) Jsc (mA/cm2) FF (%) Eta (%) J01 (fA/cm2) J02 (nA/cm2)705.7 40.51 82.26 23.52 45.12 2.58 14Canadian Solar Inc. Process developmentPower loss analysis For recombination loss:n The dominant is from the front emitter. For resistive loss:n Lateral transportation is the dominant.n Contact resistance for Ag/Al and emitter. 15Canadian Solar Inc. Process developmentCell efficiency roadmap n Based on screen printing process, 24% cell efficiency is expected combining all improvements. 24% 16Canadian Solar Inc. Challenge for industrializationProcess flow selectionTextureBoron diffusionRear etch Rear SiNxFront AlOx/SiNxMetallizationSiOx/i-Poly-Si (LPCVD)P implantAnnealWet clean Double-side polishSiOx/i-Poly-Si (LPCVD)POCl3 diffusionFront AlOx/SiNxMetallizationRear SiNxFront textureBoron diffusionWet clean TextureBoron diffusionRear etchFront AlOx/SiNxMetallizationSiOx/i-PolySi (LPCVD)POCl3 diffusionWet cleanRear SiNx TextureBoron diffusionRear etchFront AlOx/SiNxMetallizationSiOx/n-PolySi (PECVD)AnnealWet cleanRear SiNxImplant SiNx Mask LPCVD PECVDn Performance (cell efficiency, Voc, bifaciality, etc.), cost (yield, Irev, etc.) and manufacturability (process steps, process window, complexity, etc.) need to be considered thoroughly. 17Canadian Solar Inc. Challenge for industrializationProcess control issuesn Contamination control§ Cleanness before boron diffusion§ Cleanness before SiOx/poly-Si depositionn SiOx thickness control§ Too thick/thin SiOx both are not good. Process window is narrow. § Difficulty in measuring SiOx thickness leads to process control issue.n Edge isolation !! PL image for Cell before metallization 18Canadian Solar Inc. Challenge for industrializationCost analysisn Cell cost was evaluated based on the process parameters in CSI pilot line.n Assumption: Same Yield with PERC line. PERCcell costn PERC cell cost was calculated based on the data from the production lines.n N type wafer cost is ~10% higher with same thickness of 160μm.n Cost of boron diffusion is high, due to long diffusion time and low throughput.n Cost of SiOx/poly-Si deposition is relatively low.n Cost of Ag paste is high due to usage on both sides.n Cost of wet process can be varied largely based on methods to be used. BoronPasteClean Cost adder for passivated contactWet processwaferLPCVD 19Canadian Solar Inc. Summaryn Passivated contact is the next step for higher efficiency.n The performance of key processes (boron diffusion, rear morphology, SiOx/Poly-Si, etc.) are evaluated.n With optimized process, the highest cell efficiency reaches 23.52% (in-house Sinton measurement).n Further process optimization will drive the cell efficiency to 24% in mass production.n So far, the challenge of mass production of passivated contact solar cells relies on the process complexity and cost effectiveness. 20Canadian Solar Inc. WIN BY INNOVATION