Fundamental Reactions During the Formation of Fired Silver Contacts and Solar Cell Results M. Hörteis, S. W. Glunz 2 nd workshop on Metallization for Crystalline Silicon Solar Cells Konstanz, 15.04.2010
Materials used for the front side metallization Ingredients Front side silver paste Silver Glas frit (Lead oxide, Bismuth oxide ) 2
Development of ink/pastes Preparation 3
Paste deposition on the front side Used technologies Printing techniques Screen printing Stencil printing Pad printing Direct write Inkjet Aerosol jet Extrusion/Dispensing 4
Formation of the contact Schematic Printed layer n-emitter Fired at T= 800 C d=18 µm optionally enhanced by a plating step p-base 5
Reactions during contact formation Ink Solar cell Deposited ink n-emitter Opening of the ARC (SiNx) Ink reacts with Silicon Nitride Contact formation Ink reacts with Silicon n-emitter 6
Investigated materials Related to the contact formation Silicon Silicon nitride (ARC) Glas frit (Lead oxide, Bismuth oxide ) Silver 7
Basic reactions TG-DTA measurement Si + PbO Sample TC2 TC1 Furnace Balance temperature [ C] 900 800 700 600 500 400 300 200 100 temperature Si + PbO mass-signal 100.0 99.8 99.6 99.4 99.2 0 10 20 30 40 50 60 70 80 90 100 110120 130 99.0 time [min] mass [%] M. Hörteis, Adv. Funct. Mater. 2010, 20, 476 484 8
Basic reactions TG-DTA measurement Si + PbO Sample TC2 TC1 Furnace Balance temperature [ C] 900 800 700 600 500 400 300 200 100 DTA-Signal temperature Si + PbO 656 C 0 10 20 30 40 50 60 70 80 90 100 110120 130 time [min] 0.5 0.0-0.5-1.0-1.5 DTA [μv/mg] 9
Basic reactions SEM + EDX analysis Si + PbO Intensity [a.u.] Si Energy [kev] 1: Silicon 2 1 Intensity [a.u.] Intensity [a.u.] Pb Energy [kev] Si Pb O Energy [kev] Pb 2: Lead 3: Glass Si 3 Glas Pb >600 C Si + 2 PbO SiO/PbO glas + 2 Pb 10
Basic reactions TG-DTA measurements Si + PbO + Ag Reaction under the presence of silver Re-crystallization during cooling Temperatur [ C] 900 800 700 600 500 400 300 200 Si + PbO + Ag 0.00-0.25 DTA-signal [μv/mg] 100 0-0.50 20 40 60 80 100 120 140 160 180 Zeit [min] 11
Basic reactions - Si + PbO + Ag Phase diagram Ag-Pb 962 C DTA-signal [a.u.] 600 C 300 C temperature [ C] 600 C 304 C Eutectic at ca. 304 C 800 700 600 500 400 300 200 100 temperature [ C] Ag [at. %] 12
Basic reactions - Si + PbO + Ag SEM + EDX analysis Intensity [a.u.] Ag 1 2 0 2 4 6 8 10 12 14 16 18 20 Energy [kev] Intensity [a.u.] Pb Ag Pb 0 2 4 6 8 10 12 14 16 18 20 Energy [kev] 13
Basic reactions Summing-up Reaction between Lead oxide and Silicon Lead oxide is reduced to lead, and simultaneously Si is oxidized Silver is liquefied far below its melting point Silver lead melt recrystallizes during cooling During the reaction glass is produced 14
Basic reactions Si 3 N 4 + PbO Opening of the passivation-layer Mass loss at T = 685 C mass [%] 100.0 SiN x - PbO 99.8 99.6 99.4 685 C 99.2 99.0 98.8 98.6 mass 98.4 98.2 98.0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 time [min] 900 800 700 600 500 400 300 200 100 temperature [ C] 15
Basic reactions Si 3 N 4 + PbO Opening of the passivation-layer Mass loss at T = 685 C Exothermic reaction mass [%] 100.0 SiN x - PbO 99.8 99.6 99.4 685 C 99.2-5.0 99.0-7.5 98.8-10.0 98.6 mass -12.5 98.4 PbO - SiN x -15.0 98.2 98.0-17.5 10 20 30 40 50 60 70 80 90 100 110 120 130 time [min] 5.0 2.5 0.0-2.5 DTA-Signal [µv/mg] 16
Basic reactions Si 3 N 4 + PbO Opening of the passivation-layer Mass loss at T = 685 C Exothermic reaction Evolution of nitrogen mass [%] 100.0 99.8 99.6 99.4 99.2 99.0 98.8 98.6 98.4 98.2 SiN x - PbO 685 C 98.0 10 20 30 40 50 60 70 80 90 100 110 120 130 time [min] 12 PbO + 2 Si 3 N 4 6 SiO 2 + 12 Pb + 4 N 2 ( ) mass PbO - SiN x N 2 (28) 12 10 8 6 MS-Signal [1x10-9 A] 17
Basic reactions Summing-up Reaction between Lead oxide and Silicon Lead oxide is reduced to lead, and simultaneously Si/SiN x are oxidized Silver is liquefied far below its melting point Silver lead melt recrystallizes during cooling, forming an el. contact During contact formation additional glass is produced Opening of the passivation Layer (SiN x ) Similar reaction as with pure Silicon SiN x is oxidized and glass is formed Contact formation Formation of an isolating glass layer 18
Ink Optimization Variation of the glass content Density of contactcrystallites is increased with increasing glass content For more than 10% of glass, the contact resistance is increased again, due to an increased glass layer contact resistance R c xw [Ωcm] 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0 10 20 30 glass content [%] 19
Optimized ink, applied on high efficiency solar cells Printed and fired contacts vs. evaporated contacts Is it possible to close the gap between screen printed contacts and high efficiency contacts? 20
Application on High efficiency solar cells Contact geometries 37 µm 55 µm industrial-type contact vs. high-efficiency contact 21
Application on High efficiency solar cells Printed and fired contacts vs. evaporated contacts Contact on shallow emitter fine line printing contact firing through ARC Light induced plating 3 process steps for the front side metallization Contact on a deep emitter photolithographically opening of passivation layer evaporation of seed contact TiPdAg Light induced plating 8 process steps for the front side metallization 22
Application on High efficiency solar cells Cell structure LIP-Silver Printed and fired contact / evaporated contacts SiN x -PECVD Antireflexion coating / thermal grown SiO 2 Shallow 110 Ω/sq. POCl 3 emitter / deep 120 Ω/sq. emitter LFC point contacts ALD-Al 2 O 3 / PECVD SiO 2 PVD - Aluminum 23
Application on High efficiency solar cells Solar cell results Contacts R sh [Ω/sq.] A [cm²] V OC [mv] J SC [ma/cm²] FF [%] η [%] industrial type 110 shallow 4 656 40.2 80.1 21.1 high efficiency type best cell results 120 deep 4 684 39.8 79.8 21.7 Main difference in open circuit voltage For higher solar cell efficiencies a passivated front side is necessary Gap between high efficiency solar cells and industrial-type solar cells is going to be closed 24
Fraunhofer-Institut für Solare Energiesysteme ISE Matthias Hörteis www.ise.fraunhofer.de matthias.hoerteis@ise.fraunhofer.de 25