METALLIZATION OF SILICON CARBIDE- AND SILICON OXIDE-BASED LAYER STACKS AS PASSIVATING CONTACTS FOR SILICON SOLAR CELLS

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1 METALLIZATION OF SILICON CARBIDE- AND SILICON OXIDE-BASED LAYER STACKS AS PASSIVATING CONTACTS FOR SILICON SOLAR CELLS Andrea Ingenito 1, G. Nogay 1, P. Wyss 1, J. Stuckelberger 1, I. Mack 1, Q. Jeangros 1, C. Allebé 2, J. Horzel 2, M. Despeisse 2, P. Löper 1, F.-J. Haug 1, C. Ballif 1,2 1 EPFL PVLAB, Rue de la Maladière 71b, 2002 Neuchâtel, Switzerland 2 CSEM SA, PV-Center, Jaquet-Droz 1, 2002 Neuchâtel, Switzerland SiliconPV J. P. 1 Seif et al. 7th Workshop on Metallization & Interconnection for Crystalline Silicon Solar Cells, Konstanz,

2 Metallization schemes for temperature-stable passivating contacts Temperature-stable passivating contacts: High surface passivation and charge carrier transport [1,2] In principle compatible with industrial processes at high temperature [1] A. Richter et al., Sol. Energy Mater Sol., (2017) [2] F. Haase et al. Jpn. J. Appl. Phys., (2017) Metallization schemes for passivating contacts: High temperature process Screen print and firing-through [3], [4] Industrially compatible Ag-frit etches through the poly-si degrading passivation (, thickness poly-si, Ag-paste optimization, ) Low temperature process PVD of Metals or TCO/metal stack [1,2,5-9] Potential for industrialization for low-cost metals and In-free TCOs [6] Sputtering damage not always fully recovered (soft-deposition techniques required e.g. LPCVD, evaporation, PECVD [6], ) [3] H. E. Çiftpnar., Energ. proc., (2017) [4] R. Naber et al. EUPVSEC 33rd, (2017) [5] Y.Tao et al., AIMS Materials Science, (2016), [7] D. Yan et al., Sol. Energy Mater Sol., (2017), [9] G. Nogay Sol. Energy Mater Sol., (2017) [6] R. Piebst et al., EUPVSEC 33rd, (2017), [8] G. Yang et al., Appl. Phys. Lett. (2016), [10] 2

3 Strategies for passivating contacts at PV-LAB Rear side upgrade Integration of passivating contact with treatment compatible with front side POCl 3 SiN x ARC Both side contacted [1-3] c-si solar cells with passivating contacts TCO n+ c-si(p) p+ POCl 3 - Emitter TCO TCO n+ p+ c-si(p) Co-annealed TCO still needed for: Improving rear optic Improving lateral transport in case of bifacial cells (depending on the R SH of the in-diffused region) TCO still needed for: Improving front and rear optics Improving lateral conductivity (high R SH of the thin front passivating contact) [1] F. Feldman et al., IEEE, Portlan, (2016) [2] U. Römer et al., Sol. Energy Mater Sol., (2014) [3] R. Piebst et al. EUPVSEC 33rd, (2017) 3

4 Strategies for passivating contacts at PV-LAB Rear side upgrade Integration of passivating contact with treatment compatible with front side POCl 3 SiN x ARC Both side contacted [1,2] c-si solar cells with passivating contacts TCO n+ c-si(p) p+ POCl 3 - Emitter TCO TCO n+ p+ c-si(p) Co-annealed Full area rear side passivating contact: SiC x (p), µc-sio x (p) TCO still needed for: Improving front and rear optics Improving lateral conductivity (high R SH of the thin front passivating contact) 4

5 SiC x (p) passivating hole contacts Advantages of alloying a-si:h with C: Less prone to blistering Better stability in standard wet chemistry a-si(i) buffer layer displaces Boron and Carbon from the chem-siox layer SiC x (p) a-si(i) c-si(p) G. Nogay et al., SOLMAT 2017, doi: /j.solmat thin SiO x 5

6 SiC x (p) passivating hole contacts i-v OC degrades for excessive annealing or doping SiC x (p) a-si(i) c-si(p) G. Nogay et al., SOLMAT 2017, doi: /j.solmat thin SiO x 6

7 SiC x (p) passivating hole contacts i-v OC degrades for excessive annealing or doping ρ C decreases with annealing and doping Possibly explained with decomposition of chem-sio x Two optimum conditions: 800 C, high 1.5 sscm iv OC =717mV, ρ c =16mΩcm C, 0.5 sccm iv OC =716mV, ρ c =27mΩcm 2 ITO/Ag SiC x (p) a-si(i) c-si(p) hard mask G. Nogay et al., SOLMAT 2017, doi: /j.solmat thin SiO x 7

8 Hybrid Cells with diffused rear side ITO a-si:h(i/n) Ag The effect of annealing T for two different TMB flows thin SiO x Si(i) SiC x (p) ITO Ag c-si(p) p + /p V oc follows the iv oc trend V oc up to709 mv iv oc -V oc 10 mv (sputtering damage) Lower doping higher J sc Less free carrier absorption Higher annealing T higher FF Higher doping higher FF FF up to 81% G. Nogay et al., SOLMAT 2017, doi: /j.solmat Best Cells TMB V oc FF J sc η [mv] [%] [ma cm -2 ] [%]

9 Full-area passivating hole rear contacts SiC x -based layer stack 800 C Short dwell time 5 min 850 C c-si(p) thin SiO x Tendency in literature: Thicker and denser SiO x beneficial for higher annealing temperature [1,2] 725 µc-sio x (p) Annealing at 900 C 25 POCl 3 integration Attain higher thermal budgets with chem-sio x grown in HNO 3 and PECVD? µc-si by PECVD, and introduce O in order to «support» the chem-sio x (µc- SiO x ) ivoc (mv) Dwell time A iv OC J 0 12x longer dwell time iv OC J J o (fa/cm 2 ) 575 1/8 CO2 3/8 CO2 4/8 CO2 6/8 CO2 1 CO2 0 [1] Moldovan et al., SOLMAT, 142, 123 (2015) [2] Larionova et al., PSSA, (2017) Good passivation for very broad dwell time range 9

10 µc-sio x (p) hole passivating contacts Jsc (ma*cm -2 ) P. Wyss et al., manuscript in preparation Annealead at 900 C Voc (mv) Jsc (ma/cm2) FF (%) Eff (%) Voltage (mv) ITO a-si:h(i/n) thin SiO x µc-sio x (p) ITO Ag Ag c-si(p) p + /p FF: efficient carrier transport for the µc-siox(p) possibly still limited by the front i/n J SC : Limited by the front contact V OC : > 10 mv lower than i-v OC η : > 22% 10

11 Strategies for passivating contacts at PV-LAB Rear side upgrade Integrate contact formation with standard thermal processes - Emitter diffusion Both side contacted c-si solar cells with passivating contacts Full-area front side passivating contact µc-sio x (n) SiC x (n) SiN x ARC n+ c-si(p) p+ POCl 3 -Emitter n+/p+ p+/n+ c-si(n/p) Co-annealed Full area rear side passivating contact: SiC x (p), µc-sio x (p) Full area rear side passivating contact: SiC x (p), µc-sio x (p) 11

12 µc-sio x (n) for front side passivating contacts Parasitic absorbtion of Si layers on the front side can cause large J SC losses [1] Replace part of the Si layer by SiO x Vertically oriented Si filaments within the SiO x matrix to ensure current transport Already present in the asdeposited state but also visible after annealing (850 C) J. Stuckelberger et al., Sol. Energy Mater. Sol. Cells, 158,(2016) Patent pending (PCT filed in April 2017) [1] S. Reiter et al., Energy Procedia 92, pp (2016) josua.stuckelberger@epfl.ch 12

13 µc-sio x (n) for front side passivating contacts Annealing at 900 C * ** * Sputtered ** Evaporated ρ c strongly decreasing with increasing doping ρ c for uc-siox(n) metallized with evaporated Al lower than ITO/Ag sputtered J. Stuckelberger et al., Sol. Energy Mater. Sol. Cells, 158,(2016) J. Stuckelberger et al., manuscript under review josua.stuckelberger@epfl.ch 13

14 ITO sputtering damage for uc-sio x (n) 900 C Lowly doped samples => strong sputtering damage (curing does not help) Highly doped samples => Lower sputtering damage Surface passivation less sensitive to Auger recombination rather than indusced sputtering damage J. Stuckelberger et al., manuscript under review 14

15 Surface passivation of SiC x (n) for front textured side (PCD measurements) 800 C; 8 min SiH 4 flow constant 735 mv 7 fa/cm 2 J 0 Annealing J 0 Hydrogenation i-v OC Annealing i-v OC Hydrogenation 800 C; 8 min: Auger-limited at low CH 4 /SiH 4 Massive improvement after hydrogenation towards higher CH 4 flow SiC x (n) ~13 nm c-si FZ (p-type) 15

16 Surface passivation of SiC x (n) for front textured side (PCD measurements) 800 C; 8 min SiH 4 flow constant 735 mv 7 fa/cm 2 J 0 Annealing J 0 Hydrogenation i-v OC Annealing i-v OC Hydrogenation 800 C; 8 min: Auger-limited at low CH 4 /SiH 4 Massive improvement after hydrogenation towards higher CH 4 flow 850 C; 8 min 15 fa/cm mv 850 C; 8 min: Auger-limited at low CH 4 /SiH 4 Massive improvement after hydrogenation towards higher CH 4 flow SiC x (n) ~13 nm c-si FZ (p-type) 16

17 Surface passivation of SiC x (n) for front textured side (PCD measurements) 800 C; 8 min SiH 4 flow constant 735 mv 7 fa/cm 2 J 0 Annealing J 0 Hydrogenation i-v OC Annealing i-v OC Hydrogenation 800 C; 8 min: Auger-limited at low CH 4 /SiH 4 Massive improvement after hydrogenation towards higher CH 4 flow 850 C; 8 min 900 C; 8 min 715 mv 15 fa/cm fa/cm mv 850 C; 8 min: Auger-limited at low CH 4 /SiH 4 Massive improvement after hydrogenation towards higher CH 4 flow 900 C; 8 min: No variations with CH 4 and hydrogenation Break-up of the oxide and/or high Auger recombination A. Ingenito et. al, manuscript in preparation SiC x (n) ~13 nm c-si FZ (p-type) 17

18 Co-annealed SiCx-based solar cells V OC : same trend as i-v OC iv OC -V OC > 20 mv FF: decreases with increasing of the temperature (p-ff?) J SC : peaks at 850 C (poor transparency at 800 C; high recombination at 900 C) η: highest at 800 C 800 C 850 C 900 C 800 C 850 C 900 C V OC [mv] FF [%] J SC [ma/cm 2 ] η [%] Sputtered ITO/ screen printed Ag paste front side Sputtered ITO/Ag rear side SiC x (n) High CH 4 SiC x (p) c-si(p) 18

19 Impact of the metallization process on V OC and FF 800 C; 8 min 850 C; 8 min p-v OC =709 mv p-ff=81.5% p-v OC =716 mv p-ff=82.7% i-v OC =728 mv i-ff=85.6% p-v OC =679 mv i-ff=80.3% p-v OC =685 mv i-ff=82.2% i-v OC =695 mv i-ff=84.4% i-v OC and i-ff almost untouched after ITO (no sputtering damage) Degradation at low injection after metallization (p-η=22.2%) Dark diode losses >7 mv on p-v OC and >1% on the p-ff (p-η=21.8%) i-v OC and i-ff untouched after ITO (no sputtering damage) Degradation at low injection after metallization (p-η=21.9%) Dark diode losses >6 mv on V OC and >2% on the p-ff (p-η=21.1%) 19

20 Current Density (ma/cm 2 ) Both side SiCx-based solar cells (planar) Best Cell: J-V Curve V oc = mv J sc = 33.5 ma.cm -2 FF = 84.0 % η = 20.5 % Voltage (V) iv oc before ITO [mv] Light J-V Pseudo J-V Total J 0 [fa/cm 2 ] Current Density (ma/cm 2 ) pv oc [mv] Voltage (V) pff [%] pη [%] R s [Ω.cm 2 ] Measured with ARF* V oc = mv J sc = 36.8 ma.cm -2 FF = 83.5 % η = 22.4 % ITO nc-sic x (n) nc-sic x (p) ITO FZ(p) 2Ω cm Co-annealed planar cell (850 C) Ag Ag 17 mv difference in iv oc and V oc after metalization due to sputtering damage ITO/low-T Ag-paste metallization is not FF-limiting! *Microtextured anti-reflective foil (ARF) Ulbrich, et al. Prog. Photovoltaics 2013, 21,

21 Summary Annealed rear side passivating contacts: SiC x (p), iv OC >715 mv, ρc<20 mωcm 2 µc-sio x (p), iv OC >720 mv, J 0 <15 fa/cm 2 Proof-of-principle cells with front side heterojunction: Annealed SiC x (p), η = 21.9% Annealed SiOx(p), η = 22.3% Annealed front side passivating contacts for better transparency SiO x (n) -> i-v OC > 720 mv SiC x (n) -> i-v OC > 735 mv (on textured) Co-annealed solar cells SiO x (n)/sio x (p), good J SC and FF (planar) SiC x (n)/sic x (p), η = 21.5 %, J SC to be improved, good FF and V OC (texured) SiC x (n)/ SiC x (p), reasonable J SC excellent FF V OC ( ) with low-t Ag paste 21

22 THANK YOU FOR YOUR ATTENTION! ACKNOWLEDGEMENT The authors gratefully acknowledge support by the Swiss National Science Foundation (SNF) under grant No _14588/1 and No. IZLIZ2_156641, by the Swiss Federeal Office for Energy (SFOE) under grant No. SI/ and by the European Union s Horizon 2020 research and innovation programme under Grant Agreements no (project DISC) We would like to thank all co-workers at PV-Lab & CSEM. SiliconPV J. P. 22 Seif et al. EUPVSEC 33rd, Septembre 26th

23 EQE co-annealed cells with front SiC x (n) SiCx(n): high SiH 4 EQE, R [-] EQE R 800 C 850 C 900 C Wavelength Blue response increases towards higher annealing temperature (higher crystallinity) At 900 C, the high recombination rate and FCA limit the EQE in the IR region 23

24 Cell results: Co-diffused SiOx-based solar cells 1.0 Current density [ma cm -2 ] V OC = mv J SC = 34.8 ma cm -2 FF = 79.6 % η = 19.0 % EQE / IQE / Refl. [.] EQE IQE Reflectance c-si(n) Voltage [V] ITO µc-sio x (n) µc-sio x (p) ITO wavelength [nm] FF: efficient carrier transport through SiOx layer J SC : promising high for planar cell V OC : improvable, compared to iv OC η : proof of concept for SiOx(n)/Si(n) passivating contact J. Stuckelberger et al., manuscript under review

25 March 18-22, on the Swiss EPFL campus in Lausanne in the amazing ROLEX Learning Center Conference chaired by EPFL (IMT/PV-lab) npv-workshop chaired by CSEM 25

26 Optical losses Alloying with C helps to widen the bandgap 6 5 a-si(n) Poly-Si Low C (900 C) High C (900 C) High C (800 C) OPAL 2 Increasing C-content shifts crystallization towards higher temperature Layers annealed at 900 C are more transparent than a-si(n) J A [ma/cm 2 ] Thickness[nm] n[-] k [-] High C (900 C) High C (850 C) High C (800 C) Low C (900 C) a-si(n) To decrease J A below 1 ma/cm 2, thickness of 5 nm Wavelength [nm] Wavelength [nm] 26

27 Testing of different TCOs on SiC x (p) Minority Carrier Lifetime (seconds) 2.0x x x x10-4 ITO ZTO IZO As After ITO After Annealing After Etching Minority Carrier Density (cm -3 ) As After ZTO After Annealing Minority Carrier Density (cm -3 ) As After IZO After Annealing After Etching Minority Carrier Density (cm -3 ) Contact Resistivity (mω.cm 2 ) ZTO IZO IO:H ITO Sputtering damage not fully recovered Conductivity [1/Ωcm] Mobility [cm 2 V -1 s -1 ] ITO ZTO IZO 1.4x x x

28 Process flow of co-annealed front and rear contacted SiC x -based c-si solar cells (patterning free) Single side texturing ITO SiC x (n) Ag H H H H «Chem-SiO x» grown in HNO 3 PECVD SiC x (n) front PECVD SiC x (p) rear Chem-SiO x SiC x (p) ITO n+ buried junction c-si(p) p+ buried junction Co-Annealing Ag H H H H Hydrogenation Metallization* *Sputtered ITO/Ag rear side *Sputtered ITO/ screen printed Ag paste front side Rear SiC x (p) [1] J 0 [fa/cm 2 ] i-v OC [mv] 800 C; 8 min C; 8 min C; 8 min [1] Optimized by G.Nogay 28

29 Surface passivation of SiC x (n) on planar and textured surfaces (PCD measurements) All samples after hydrogenation with a SiNx as donor layer -11 mv -8 mv 800 C; 8 min: Small losses when moving form planar to textured (5-10 mv) -22 mv -12 mv 850 C; 8 min: Moderate losses when moving form planar to textured (>10 mv ) -60 mv -35 mv 900 C; 8 min: Strong losses when moving form planar to textured (>50 mv) J 0 textured J 0 planar i-v OC textured i-v OC planar 29