Fraunhofer - Jochen Rentsch
© Fraunhofer ISE TOPCon – Poly-Si based Passivating Contacts Jochen Rentsch, Frank Feldmann, Martin Hermle, Ralf Preu, Stefan W. Glunz Fraunhofer Institute for Solar Energy Systems ISE PV CellTech Penang, March 12th, 2019 www.ise.fraunhofer.de © Fraunhofer ISE 2 Introduction Silicon is still the working horse of Photovoltaic Conversion efficiency is the key to further bring down the levelized costs of electricity and to survive competition. Learning curve for efficiency improvement at 0.6%abs/year Passivating contacts represent promising concept for next generation solar cells after PERC [1] [1] https://www.pv-magazine.com/2018/12/31/14-pv-trends-for-2019/ [2] M. Hermle, ETIP PV, PV Manufacturing in Europe, 2017, Brussels [2] © Fraunhofer ISE 3 Introduction Poly-Si as Passivating Contact is already an old story… SIPOS Hetero- contact[1] Voc = 720mV Polysilicon Emitters for IBC cells[2] Low J0 and c [1]Yablonovich, Applied Physics Letters (1985) [2] Gan and Swanson, IEEE PVSC (1990) [3] F. Feldmann et al., EU-PVSEC (2013) Tunnel Oxide Passivating Contact (TOPCon) by Fraunhofer ISE [4] Solar cell with Voc = 703 mV and = 23.7% 1985 1990 2013 © Fraunhofer ISE 4 TOPCon Process[1] Tunnel oxide (HNO3, UV/O3[2], O3[2], TO) Interface passivation PECVD deposition (single-sided) doped a-Si(Cx) layer Carrier-selectivity The TOPCon Approach Process n-base SiOx P-doped Si layer [1] F. Feldmann et al., SolMat (2014) [2] A. Moldovan et al., SolMat (2015) © Fraunhofer ISE 5 TOPCon Process[1] Tunnel oxide (HNO3, UV/O3, O3, TO) Interface passivation PECVD deposition (single-sided) doped a-Si(Cx) layer Carrier-selectivity Furnace Anneal partial crystallization dopant diffusion Hydrogenation (RPHP) Defect passivation The TOPCon Approach Process SiOx P-doped Si layer n-base [1] F. Feldmann et al., SolMat (2014) © Fraunhofer ISE 6 Record lifetimes on both p-type (0.095 s) and n-type (0.225 s) FZ Si TOPCon (Tunnel Oxide Passivated Contact) Surface Passivation with Hydrogenation Graph from Niewelt et al., SolMat (accepted) © Fraunhofer ISE 7 Improved TOPCon process: Excellent surface passivation with iVoc = 740 mV demonstrated Low contact resistivity c 10 mΩcm² Efficient passivating and carrier-selective contact TOPCon (Tunnel Oxide Passivated Contact) Electrical properties 0.1 1 10 700 710 720 730 740 900 °C 950 °C 900 °C Tunn el ox ide : HNO 3 O 3 TO Implied V oc [mV] Contact resistivity [m cm 2 ] 800 °C SiOx P-doped Si layer n-base © Fraunhofer ISE 8 2013 2014 2015 2016 2017 2018 2019 21 22 23 24 25 26 27 Hybrid Top/Rea r Efficiency [% ] Year Both Side Contacted Record Cells with TOPCon p+ (boron-diffused) Full-area rear contact (TOPCon) c-Si(n)p++ Hybrid ■ Lab scale cells on n-type Fz-Si © Fraunhofer ISE 9 Material Area Contact Voc Jsc FF η techn. [mV] [mA/cm2] [%] [%] n-type Mono 4 cm² (da) PL+Evap. 724 42.9 83.1 25.8*,1 n-type Multi 4 cm² (ap) PL+Evap. 674 41.1 80.5 22.3*,2 n-type Mono 100 cm² (ap) PL+Evap. 713 41.4 83.1 24.5*,3 n-type Mono 100 cm² (ap) LCO+Ni/Cu Plat. 697 41.4 81.2 23.4* Both Side Contacted Record Cells with TOPCon [3] F.Feldmann et al., Evaluation of TOPCon technology on large area solar cells EUPVSEC, Amsterdam, 2017 Cu-Plating [1] A. Richter et al., Tunnel oxide passivating electron contacts as full‐area rear emitter of high‐efficiency p‐type silicon solar cells, Prog Photovolt Res Appl. 2018;26:579–586 [2] J. Benick et al., High-Efficiency n-Type HP mc Silicon Solar Cells, IEEE JPV, Vol. 7, No. 5, 2017 *confirmed by Fraunhofer ISE CalLab PL: Contacts defined by Photolithography © Fraunhofer ISE 10 Both Side Contacted Record Cells with TOPCon Material Area Contact Voc Jsc FF η techn. [mV] [mA/cm2] [%] [%] n-type Mono 4 cm² (da) PL+Evap. 724 42.9 83.1 25.8*,1 n-type Multi 4 cm² (ap) PL+Evap. 674 41.1 80.5 22.3*,2 n-type Mono 100 cm² (ap) PL+Evap. 713 41.4 83.1 24.5*,3 n-type Mono 100 cm² (ap) LCO+Ni/Cu Plat. 697 41.4 81.2 23.4*,4 [4] B.Steinhauser et al., Large Area TOPCon Technology Achieving 23.4% Efficiency IEEE PVSC, Hawaii, 2018 [3] F.Feldmann et al., Evaluation of TOPCon technology on large area solar cells EUPVSEC, Amsterdam, 2017 [1] A. Richter et al., Tunnel oxide passivating electron contacts as full‐area rear emitter of high‐efficiency p‐type silicon solar cells, Prog Photovolt Res Appl. 2018;26:579–586 [2] J. Benick et al., High-Efficiency n-Type HP mc Silicon Solar Cells, IEEE JPV, Vol. 7, No. 5, 2017 *confirmed by Fraunhofer ISE CalLab © Fraunhofer ISE 11 Both Side Contacted Record Cells with TOPCon p+ (boron-diffused) Full-area rear contact (TOPCon) c-Si(n)p++ Hybrid ■ Lab scale cells on n-type Fz-Si ■ First industrial adoptions on large area n-type Cz-Si 2013 2014 2015 2016 2017 2018 2019 21 22 23 24 25 26 27 Hybrid Top/Rea r Efficiency [% ] Year [1] http://ir.jinkosolar.com/news-releases/news-release-details/ jinkosolar-large-area-n-type-topcon-monocrystalline-silicon [2] Presentation Duttagupta et.al., CSPV14 Xian, China 2018 [3] Presentation Zhifeng Liu, et.al. Jolywood, EU-PVSEC 2018 [1] [2][3] © Fraunhofer ISE 12 Challenges for Industrial Implementation of TOPCon ■ How to upgrade from existing PERC lines? Material Change from p- to n-type silicon material PERC TOPCon c-Si(p) c-Si(n) © Fraunhofer ISE 13 Challenges for Industrial Implementation of TOPCon ■ How to upgrade from existing PERC lines? Material Change from p- to n-type silicon material Front side Replace POCl3 with BBr3 diffusion, shift AlOx/SiNx passivation Diffusion LP-BBr3 SDE + texture Chemical edge isolation Diffusion LP-POCl3 SDE + texture Chemical edge isolation Laser Selective Emitter Laser Selective Emitter c-Si(p)n++ Al2O3 (front) PECVD SiNx (front) c-Si(n)p++ n+-emitter SiNx AlOx SiNxp+-emitter PECVD SiNx (front) © Fraunhofer ISE 14 Challenges for Industrial Implementation of TOPCon ■ How to upgrade from existing PERC lines? Material Change from p- to n-type silicon material Front side Replace POCl3 with BBr3 diffusion, shift AlOx/SiNx passivation Rear side Implement TOPCon layer formation (replacing LCO) Diffusion LP-BBr3 TOPCon: Oxidation SDE + texture Chemical edge isolation TOPCon: PECVD TOPCon: High-T anneal Al2O3 (front) PECVD SiNx (front) PECVD SiNx (rear) Diffusion LP-POCl3 SDE + texture Chemical edge isolation Al2O3 (rear) PECVD SiNx (rear) Laser Contact Opening Laser Selective Emitter Laser Selective Emitter PECVD SiNx (front) c-Si(p)n++ c-Si(n)p++ SiNxpc-Six TOLCO © Fraunhofer ISE 15 Challenges for Industrial Implementation of TOPCon ■ How to upgrade from existing PERC lines? Material Change from p- to n-type silicon material Front side Replace POCl3 with BBr3 diffusion, shift AlOx/SiNx passivation Rear side Implement TOPCon layer formation (replacing LCO) Adapt metallization grid Diffusion LP-BBr3 TOPCon: Oxidation SDE + texture Chemical edge isolation TOPCon: PECVD TOPCon: High-T anneal Al2O3 (front) PECVD SiNx (front) SP (front and back) PECVD SiNx (rear) FFO Diffusion LP-POCl3 SDE + texture Chemical edge isolation SP (front and back) FFO Laser Contact Opening Laser Selective Emitter Laser Selective Emitter c-Si(n)p++c-Si(p)n++ Ag-GridAl (finger) + Ag (Pads) Al2O3 (rear) PECVD SiNx (rear) PECVD SiNx (front) © Fraunhofer ISE 16 Research for Industrial Implementation PV-TEC pilot manufacturing platform Over 2000 m² for Highest efficiency solar cell processing Cutting edge automated pilot equipment from leading manufacturers, including © Fraunhofer ISE 17 Poly-Si deposition Batch PECVD – Thin n-TOPCon films Excellent homogeneity over the boat Textured wafers, 15 nm thin pc-Si film - 1 S D 1 S D 50% M a x M in 725 730 735 740 745 i V OC Performance over Boat i V OC [mV] Me an ± 1 SD Data 15 6×1 56 mm ², text ur ed 1 cm n- ty pe 15 0 µm thickn ess © Fraunhofer ISE 18 Poly-Si deposition Inline PECVD Comparison of two PECVD sources for n-TOPCon film deposition Both provide excellent passivation So far, only MW allows for deposition of thick films without blistering (~100 nm) which are essential for fire-through metallization 0 10 20 30 40 50 680 700 720 740 plana r t ex tured Impli ed V oc (mV) Nom inal thickness [nm] 0 20 40 60 80 100 120 140 680 700 720 740 plana r t ex tured Implied V oc [mV] Nomina l thickn ess [nm ] MW source RF source © Fraunhofer ISE 19 Benefits and Challenges for Industrial Implementation Challenges Fire-through metallization1-3 Very thick pc-Si needed for current generation of pastes Low Rsheet but high parasitic absorption for front/rear illumination Further paste development essential [1] R. Naber et al., EU PVSEC, 2016 [2] S. Mack et al., Phys. Status Solidi RLL, 2017 [3] H. E. Ciftpinar et al., Energy Proc., 2017 © Fraunhofer ISE 20 Screen printed based metallization Contacting poly-Si Suitable J0,met and 𝜌𝑐 for adapted Ag paste found Process window @ 830°C compatible also with front side (BBr3 emitter, J0,met = 600 fA/cm², 𝜌𝑐 = 3 mΩcm² ) n-base SiOx P-doped Si layer SiNx Sample test structure Ag contact © Fraunhofer ISE 21 Analysis of processing cost with Bottom-Up TCO approach Diffusion LP-BBr3 TOPCon: Oxidation SDE + texture Chemical edge isolation TOPCon: PECVD TOPCon: High-T anneal Al2O3 (front) PECVD SiNx (front) SP (front and back) PECVD SiNx (rear) FFO Diffusion LP-POCl3 SDE + texture Chemical edge isolation SP (front and back) FFO Laser Contact Opening Laser Selective Emitter Laser Selective Emitter c-Si(n)c-Si(p) Al2O3 (rear) PECVD SiNx (rear) PECVD SiNx (front) PERC TOPCon n-type wafer + 2.7 €ct/cell © Fraunhofer ISE 22 Analysis of processing cost with Bottom-Up TCO approach Diffusion LP-BBr3 TOPCon: Oxidation SDE + texture Chemical edge isolation TOPCon: PECVD TOPCon: High-T anneal Al2O3 (front) PECVD SiNx (front) SP (front and back) PECVD SiNx (rear) FFO Diffusion LP-POCl3 SDE + texture Chemical edge isolation SP (front and back) FFO Laser Contact Opening Laser Selective Emitter Laser Selective Emitter c-Si(n)c-Si(p) Al2O3 (rear) PECVD SiNx (rear) PECVD SiNx (front) PERC TOPCon BBr3 diffusion + 0.3 €ct/cell © Fraunhofer ISE 23 Analysis of processing cost with Bottom-Up TCO approach Diffusion LP-BBr3 TOPCon: Oxidation SDE + texture Chemical edge isolation TOPCon: PECVD TOPCon: High-T anneal Al2O3 (front) PECVD SiNx (front) SP (front and back) PECVD SiNx (rear) FFO Diffusion LP-POCl3 SDE + texture Chemical edge isolation SP (front and back) FFO Laser Contact Opening Laser Selective Emitter Laser Selective Emitter c-Si(n)c-Si(p) Al2O3 (rear) PECVD SiNx (rear) PECVD SiNx (front) PERC TOPCon TOPCon layer + 5.4 €ct/cell Passivation © Fraunhofer ISE 24 Analysis of processing cost with Bottom-Up TCO approach Diffusion LP-BBr3 TOPCon: Oxidation SDE + texture Chemical edge isolation TOPCon: PECVD TOPCon: High-T anneal Al2O3 (front) PECVD SiNx (front) SP (front and back) PECVD SiNx (rear) FFO Diffusion LP-POCl3 SDE + texture Chemical edge isolation SP (front and back) FFO Laser Contact Opening Laser Selective Emitter Laser Selective Emitter c-Si(n)c-Si(p) Al2O3 (rear) PECVD SiNx (rear) PECVD SiNx (front) PERC TOPCon alt. metallization scheme + 3.3 €ct/cell add. cost TOPcon + 12.4 €ct/cell Passivation © Fraunhofer ISE 25 Cost reduction potentials Short term solutions expected – TOPCon+ ■ n-type wafer: Reduction of cost difference to p-type from currently 10% to 5% due to Higher ingot yield due to narrower resistivity distribution (optimisation of continous Cz pulling technology) up to 2 €ct/wafer © Fraunhofer ISE 26 Cost reduction potentials Short term solutions expected – TOPCon+ ■ n-type wafer: Reduction of cost difference to p-type from currently 10% to 5% ■ Reducing process and automation complexity by using inline capable process technologies, e.g. Integration of oxide formation within edge isolation tool APCVD or PVD technology for poly-Si deposition Annealing within Inline high temperature furnaces up to 1-2 €ct/wafer © Fraunhofer ISE 27 Cost reduction potentials Short term solutions expected – TOPCon+ ■ n-type wafer: Reduction of cost difference to p-type from currently 10% to 5% ■ Reducing process and automation complexity by using inline capable process technologies, e.g. ■ Reduction of poly-Si thickness from currently 120 down to 30 nm with optimized screen printing pastes up to 1 €ct/wafer © Fraunhofer ISE 28 Cost reduction potentials Short term solutions expected – TOPCon+ ■ n-type wafer: Reduction of cost difference to p-type from currently 10% to 5% ■ Reducing process and automation complexity by using inline capable process technologies, e.g. ■ Reduction of poly-Si thickness from currently 120 down to 30 nm with optimized screen printing pastes ■ Alternative metallization schemes like NiCu plating instead of screen printed Ag up to 1.3 €ct/wafer © Fraunhofer ISE 29 Analysing TCO along the PV value chain Expected benefit of TOPCon approach All-in module cost comparison for 60cell Glass-glass modules PERC benchmark TOPCon (current status) TOPCon+ (incl. short term cost reduction potential) Current status TOPCon process TOPCon+ PERC benchmark Targeted efficiency range PERC TOPCon Modul type Glass-glass, EVA, 5 BB interconnection CTM - 3.2 % Cz PERC TOPCon TOPCon+ © Fraunhofer ISE 30 Analysing TCO along the PV value chain Expected benefit of TOPCon approach *1st year degradation 3% 2+ years degradation 0.5% p.a. Bifaciality advantage of TOPCon can over- compensate higher cost Current status affords ~0.6% abs. efficiency gain to PERC Targeted efficiency range PERC benchmark Irradiation: 1700 kWh/m²a Cz PERC TOPCon TOPCon+ PERC TOPCon Bifaciality 75 % 90 % Albedo 10 % System life* 25 years WACC (nom). 5 %
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