Thick Film Metallization for Contacting Emitters with High Sheet Resistance

Transcription

1 Thick Film Metallization for Contacting Emitters with High Sheet Resistance Current Technologies and New Approaches 1 R. Hoenig, 1 M. Pospischil, 1 T. Fellmeth, 1 J. Bartsch, 1 D. Erath, 1 J. Specht, 1 F. Clement, 1 D. Biro, 2 M. Koenig, 2 M. Neidert, 2 A. Henning, 2 C. Mohr, 2 M. Hoerteis, 2 W. Zhang 1 Fraunhofer Institute for Solar Energy Systems ISE 2 Heraeus HPM, Business Unit PV 3rd Metallization Workshop Charleroi/Belgium,

2 Agenda Potential & challenges for high R sheet emitters Results of screen printed mc-/cz-si solar cells with R sheet 75 Ω/sq Physical aspects of contact improvement by Ag-LIP Grid optimization with GridSim2D, new approaches Conclusions 2

3 Emitters with high sheet resistance Potential & challenges Improved conversion efficiency IQE: Increased blue response higher j SC potential Less recombination losses ( j 0e ) higher V OC potential Challenges for metallization Contact formation N D / N D,Surface ρ C FF Thermal stability during co-firing possiblity of shunting (j 02,R P ) [International Technology Roadmap for Photovoltaics, Results 2010, itrpv.net] 3

4 Emitters with high sheet resistance Potential & challenges Improved conversion efficiency IQE: Increased blue response higher j SC potential Less recombination losses ( j 0e ) higher V OC potential Challenges for metallization Contact formation N D / N D,Surface ρ C FF Thermal stability during co-firing possiblity of shunting (j 02,R P ) Selective emitter Decoupling of requirements for light conversion and metallization Necessity of additional process steps [G. Hahn, 25 th EU PVSEC, Valencia, 2010] Direct contacting Improved metallization techniques Further paste development Subject of investigations here 4

5 Agenda Potential & challenges for high R sheet emitters Results of screen printed mc-/cz-si solar cells with R sheet 75 Ω/sq Physical aspects of contact improvement by Ag-LIP Grid optimization with GridSim2D, new approaches Conclusions 5

6 solar cell efficiency in % Screen printed mc-/cz-si solar cells IV results H-patterned Al-BSF solar cells, full Al coverage on rear surface Highest cell efficiencies 75 Ω/sq, SOL9410: 16.7% (mc-si) / 18.4% (Cz-Si) High R sheet emitters Efficiency level of standard 75 / 85 Ω/sq emitter not reached Significant efficiency loss for higher-ohmic emitters on Cz-Si BUT Same grid layout as for 75 / 85 Ω/sq emitter 19,2 18,8 18,4 18,0 17,6 17,2 16,8 16,4 16,0 15,6 15,2 14,8 14,4 14,0 Highest efficiency solar cells (out of best firing groups) mc-si (0.5-2 cm) 75 Highest cell efficiencies SOL9273MA SOL Cz-Si (1-3 cm) 85 emitter sheet resistance R sheet in /sq 135 6

7 fill factor FF in % Screen printed mc-/cz-si solar cells IV results High R sheet emitters η limited by fill factor FF η / FF loss explainable by too high series resistance R S Short Ag-LIP step carried out on cells with R S around group median m = mg/cell, w finger 2µm 80,4 80,0 79,6 79,2 78,8 78,4 78,0 77,6 77,2 76, Highest efficiency solar cells (out of best firing groups) mc-si (0.5-2 cm) 75 Highest cell efficiencies SOL9273MA SOL Cz-Si (1-3 cm) 85 emitter sheet resistance R sheet in /sq 135 7

8 V OC in mv J SC in ma/cm² Screen printed mc-/cz-si solar cells IV results influence of short Ag-LIP step Same V OC level of mc-si before & after Ag-LIP Significant V OC gain for Cz-Si by Ag-LIP 1-2 mv for 85 Ω/sq emitter 3-4 mv for 135 Ω/sq emitter V OC gain explainable by reduction of R S *? Significant J SC loss by Ag-LIP -0.2 to -0.3 ma/cm² for mc-si -0.5 to -0.9 ma/cm² for Cz-Si 4,8 4,4 4,0 3,6 3,2 2,8 2,4 2,0 1,6 1,2 0,8 0,4 0,0-0,4-0,8 Change of IV parameters by short Ag-LIP step (SOL9410) -0,10 mc-si (0.5-2 cm) Cz-Si (1-3 cm) -0,15-0,20-0,25-0,30-0,35-0,40-0,45-0,50-0,55-0,60-0,65-0,70-0,75 V OC J SC -0,80-0,85-0, emitter sheet resistance R sheet in /sq 8 [A. Cuevas et al, Solar Cells 11, , 1984]

9 Screen printed mc-/cz-si solar cells IV results influence of short Ag-LIP step Same efficiency range for mc-si solar cells after Ag-LIP FF gain can not compensate J SC /V OC loss for 75 Ω/sq emitter Slight gain in η only for 130 Ω/sq emitter Small η loss for 85 Ω/sq emitter on Cz-Si after Ag-LIP BUT... Significant η / FF gain for 135 Ω/sq emitter on Cz-Si in % abs Change of IV parameters by short Ag-LIP step (SOL9410) 3,0 18 2,8 mc-si (0.5-2 cm) Cz-Si (1-3 cm) 17 2,6 16 2,4 15 2,2 14 2,0 13 1,8 12 1,6 11 1,4 10 1,2 9 1,0 8 0,8 7 0,6 6 0,4 5 0,2 4 0,0 3-0,2 2-0,4 FF emitter sheet resistance R sheet in /sq FF in % abs 9

10 Screen printed mc-/cz-si solar cells TLM results influence of short Ag-LIP step TLM measurements on 1 cell with / without Ag-LIP 2 TLM measurements on group representative cell Specific contact resistance ρ C for mc-si on high R sheet emitters already sufficient before Ag-LIP Significant reduction of ρ C by Ag-LIP for higher-ohmic emitters on Cz-Si one order of magnitude lower! specific contact resistance C in m cm Contact improvement by short Ag-LIP step (SOL9410) 75 mc-si 130 as co-fired (SOL9410) after Ag-LIP (SOL9410) -36% 85 Cz-Si emitter sheet resistance R sheet in /sq -90%

11 Agenda Potential & challenges for high R sheet emitters Results of screen printed mc-/cz-si solar cells with R sheet 75 Ω/sq Physical aspects of contact improvement by Ag-LIP Grid optimization with GridSim2D, new approaches Conclusions 11

12 Screen printed mc-/cz-si solar cells Theory of contact improvement by Ag-LIP Current paths 4-7 possibly responsible for improvement of R C *w / ρ C by Ag-LIP * Reasons for improvement of ρ C a) Higher crystallite density / contact quality at finger edges and/or b) Improvement of contact quality underneath finger** plated silver n-si 5 6 SiNx layer SiNx layer scree * [D. Pysch et al, PIP 17, , 2009] screen-printed contact contact plated Precipitate Glass SiNx layer n-emitter p-base co What is the dominating mechanism? ** [A. Ebong, IEEE 32, , 2011] 12 SiNx layer plated silver crystallite glass

13 Screen printed Cz-Si solar cells SEM images front side contacts Cz-Si 135 Ω/sq after screen printing & co-firing 20 µm Cz-Si 135 Ω/sq after short Ag-LIP step 20 µm contact area improved by Ag-LIP contact finger (bulk Ag) contact area improved by Ag-LIP 20 µm 20 µm 13

14 Screen printed Cz-Si solar cells SEM images front side contacts (as processed) Cz-Si, 135 Ω/sq, before Ag-LIP Contact finger (bulk Ag) Ultra-thin glass layer, partly with holes contact improvement by Ag-LIP via a) Direct contact to Ag crystallites (4) or emitter (5) SiNx layer b) Direct or multiple tunneling along precipitates (6) possible screen-prin contact 4 screen-printed contact plated silver n-emitter p-base 10µm SiNx layer plated silver plat ed silver 5 n-si 6 precipitate glass SiNx layer 14 1

15 Screen printed Cz-Si solar cells SEM images front side contacts Cz-Si 135 Ω/sq bulk Ag etched back contact area improved by Ag-LIP 10 µm contact finger (bulk Ag) contact area improved by Ag-LIP 50 µm 15

16 Agenda Potential & challenges for high R sheet emitters Results of screen printed mc-/cz-si solar cells with R sheet 75 Ω/sq Physical aspects of contact improvement by Ag-LIP Grid optimization with GridSim2D, new approaches Conclusions 16

17 SP + Ag-LIP SP seed & plate dispensed R S in cm² H-patterned Cz-Si Al-BSF solar cells IV results without process optimization Blue = measured values (input parameters GridSim2D) Black = assumptions or output parameters GridSim2D 2,6 2,5 2,4 2,3 2,2 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0 R contact R busbar & finger R sheet R base & rear SP as processed 85 /sq 135 /sq w finger / h finger [µm] 97 / / 15.2 finger [µωcm] / C [mωcm²] 2.8 / / 126 V OC [mv] j SC [ma/cm²] FF [%] [%] /sq GridSim2D: to be published by T. Fellmeth 17

18 SP + Ag-LIP SP seed & plate dispensed R S in cm² H-patterned Cz-Si Al-BSF solar cells Optimization of grid (GridSim2D) & metallization Blue = measured values (input parameters GridSim2D) Black = assumptions or output parameters GridSim2D 18 2,6 2,5 2,4 2,3 2,2 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0 R contact R busbar & finger R sheet R base & rear SP as processed 85 /sq 135 /sq 135 /sq w finger / h finger [µm] 97 / / / 17.2 finger [µωcm] / C [mωcm²] 2.8 / / / 14.3 V OC [mv] j SC [ma/cm²] FF [%] [%] optimized grid (GridSim2D) Optimization of emitter necessary!

19 Phosphor atoms (cm -3 ) H-patterned Cz-Si Al-BSF solar cells High-performance PV-TEC emitter 1E22 1E22 Improved PV-TEC emitter 1E21 1E20 SIMS profiles 1E21 1E20 Tested / applied in several solar cell batches 1E19 1E18 1E19 1E18 Shallow, but relatively high N D,Surface Input parameter for GridSim2D Measured Calculated* R sheet [Ω/sq] ρ C,seed&plate / ρ C,SOL9273MA [mωcm²] 3.2 / j 0e-n+ [fa/cm²] j 0e-met,seed&plate / j 0e-met,SOL9273MA [fa/cm²] / E17 1E17 1E16 85 /sq 1E16 1E15 0,0 0,1 0,2 0,3 0,4 0,5 1E15 0,6 depth (µm) *[T. Fellmeth et al, 1 st SiliconPV, Freiburg, 2011]

20 SP + Ag-LIP SP seed & plate dispensed R S in cm² H-patterned Cz-Si Al-BSF solar cells Optimization of grid (GridSim2D) & metallization Blue = measured values (input parameters GridSim2D) Black = assumptions or output parameters GridSim2D 2,6 2,5 2,4 2,3 2,2 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0 R contact R busbar & finger R sheet R base & rear SP as processed 85 /sq 135 /sq 135 /sq w finger / h finger [µm] 97 / / / / 15.2 finger [µωcm] / C [mωcm²] 2.8 / / / / 8.4 V OC [mv] j SC [ma/cm²] FF [%] [%] optimized grid (GridSim2D) optimized emitter 20

21 New approaches for optimized metallization Seed & plate front side contacts Seed layers realized by screen printing applied & simulated here Seed layers 56 µm on Cz-Si Seed layer realized by screen printing... Seed layer paste provides very low ρ C on high-ohmic emitters ρ C 3.2 mωcm² measured on 110 Ω/sq emitter...and plated (Ag-LIP) after firing For following simulations: w seed (before Ag-LIP): 56 µm w finger after Ag-LIP: 82 µm 21

22 SP + Ag-LIP SP seed & plate dispensed R S in cm² H-patterned Cz-Si Al-BSF solar cells Optimization of grid (GridSim2D) & metallization Blue = measured values (input parameters GridSim2D) Black = assumptions or output parameters GridSim2D 2,6 2,5 2,4 2,3 2,2 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0 R contact R busbar & finger R sheet R base & rear SP as processed 85 /sq 135 /sq 135 /sq optimized grid (GridSim2D) optimized emitter w finger / h finger [µm] 97 / / / / 15.2 (56)82/ 12.1 finger [µωcm] / C [mωcm²] 2.8 / / / / / 3.2 V OC [mv] j SC [ma/cm²] FF [%] [%]

23 New approaches for optimized metallization Dispensed front side contacts Dispensing of some screen printing pastes possible Line widths down to 65µm & aspect ratios up to 0.5* Heraeus & ISE currently developing special pastes for dispensing Line widths of 50-60µm realized screen printed AR 0.25 d f 87 µm dispensed AR 0.5 d f 65 µm 23 *[J. Specht et al, 25 th EU-PVSEC, Valencia, 2010]

24 SP + Ag-LIP SP seed & plate dispensed R S in cm² H-patterned Cz-Si Al-BSF solar cells Optimization of grid (GridSim2D) & metallization Blue = measured values (input parameters GridSim2D) Black = assumptions or output parameters GridSim2D 24 2,6 2,5 2,4 2,3 2,2 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0 R contact R busbar & finger R sheet R base & rear SP as processed 85 /sq 135 /sq 135 /sq optimized grid (GridSim2D) optimized emitter w finger / h finger [µm] 97 / / / / 15.2 (56)82/ / 28 finger [µωcm] / C [mωcm²] 2.8 / / / / / / 8.4 V OC [mv] j SC [ma/cm²] FF [%] [%]

25 Conclusions Contact improvement by Ag-LIP shown (IV, TLM & SEM) mechanisms to be clarified [International Technology Roadmap for Photovoltaics, Results 2010, itrpv.net] 25

26 Conclusions Contact improvement by Ag-LIP shown (IV, TLM & SEM) mechanisms to be clarified Efficient contacting of emitters with R sheet 100 Ω/sq still problematic for front side screen printing pastes Pastes to be optimized! Short Ag-LIP step after co-firing Emitters (profiles) to be optimized / adapted to pastes! New metallization technologies (seed & plate, dispensing, aerosol,...) [International Technology Roadmap for Photovoltaics, Results 2010, itrpv.net] 26

27 Thank You Very Much for Your Attention! Fraunhofer Institute for Solar Energy Systems ISE Rene Hoenig R. Hoenig gratefully acknowledges the Deutsche Bundesstiftung Umwelt (DBU) for funding his works 27