HF last passivation for high efficiency a-si:h/c-si heterojunction solar cells Adrien Danel, F. Souche, PJ. Ribeyron : INES Y. Le Tiec : LETI T. Nolan : Akrion Systems 1 A. Danel, UCPSS 20-1
Heterojonction solar cells at INES 125 PSQ, CZ or FZ (0), n-type 1-5 Ohm.cm, PV wafers for 148.5 cm 2 cells 20µm 20µm Surface / Interface is (111) oriented pyramid facets Frontside grid electrode Transparent Conductive Oxide Frontside a-si:h stack c-si, n-type, 180µm Backside a-si:h stack Transparent Conductive Oxide rear grid 1) Texturing c-si substrates (KOH & IPA) Removal of damaged zone Good optical confinement 2) Surface cleaning Insure good interface quality 3) HF-last passivation SiO 2 removal As clean, as dense, as stable as possible Si-H termination 4) a-si:h low T deposition (PECVD) i,p & i,n stacks for emitter and BSF formation 5) TCO deposition Antireflective coating, charge collection 6) Grid electrode (screen printing) and edge isolation (laser cutting) Electrical tests : cell performance 2 A. Danel, UCPSS 20-2
Outline Impact of practical parameters on cell performance 1) HF concentration 2) Process time 3) Post rinse and drying 4) Delay before deposition steps 5) HF cleanliness and HCl spike 6) HF & NH 4 F RCA & HF or O 3 & HF based post texturization clean HF 2%w, HCl 1%w, 5min, 20 C, no rinse & dry reference process of this work + = Lifetime tests: texturing, cleaning, passivation, intrinsic a-si:h 40nm µpcd map (Semilab) and QSSPC (Sinton, transient mode) : Effective recombination lifetime (τ eff ) Implied Voc and effective surface recombination velocity (S eff ) at 1 sun HJ cell: 148.5 cm 2, full process module compatible I(V) under AM 1.5, 0 mw/cm 2 condition at 25 C : solar energy conversion efficiency (η) with Voc, Jsc, FF 3 A. Danel, UCPSS 20-3
1) HF concentration Time to dewet (s) 000 00 0 1 more than 30 min region 0.01 0.1 1 0 HF concentration (% by weight) less than 20 s region For good dewetting, fast and robust process: [HF] 2% is recommended As soon as a good dewet is achieved, only a slight improvement of passivation is seen with [HF] (QSSPC data) 750 Confirmation on cells Seff (cm/s) 9.5 9 8.5 8 7.5 7 a) 0 20 30 40 50 60 Implied Voc (mv) 745 740 735 730 b) 0 20 30 40 50 60 HF 2% HF 20% η (%) & (sigma) 18.4 (0.05) 18.5 (0.) Jsc (ma /cm 2 ) 35.1 35.1 Voc (mv) 686 689 FF (%) 76.4 76.5 HF concentration (% by weight) 4 A. Danel, UCPSS 20-4
2) Process time Seff (cm/s) 11.5 9.5 9 8.5 8 0 50 0 150 µpcd lifetime (µs) Process time (min) Full map, no edge exclusion, average data 900 850 800 750 700 650 600 550 500 0 50 0 150 A slight degradation is seen, mostly on data taken from the complete full wafer : likely time dependent roughening and contamination issues As soon as the deox is achieved, the shorter the better: with HF 2%, 2 5 min long process is recommended 5 A. Danel, UCPSS 20-5
3) Post rinse and drying For safety reasons, post HF rinse & drying is mandatory in industrial application Rinse: dump HF-HCl + overflow UPW rinse varied from 5min to 2h S eff stable Si (0): 2h stable Si-H x passivation in UPW reported by H. Morita et al, JAP 68, 1272 (1990) Drying: 5min UPW min hot N2 flow 6min slow drain with IPA vapor + 5min hot N2 (LUCID ) η (%) Jsc (ma /cm 2 ) Voc (mv) FF (%) η (%) Jsc (ma /cm 2 ) Voc (mv) FF (%) No rinse & dry Hot N2 16.9 (0.3) 17.0 (0.2) 34.2 34.1 678 683 73.0 73.3 Run #1 No rinse & dry IPA & hot N2 17.2 (0.07) 17.2 (0.16) 34.9 33.4 677 676 73.0 76.6 No critical issue More data needed for precise numbers Run #2 No rinse & dry IPA & hot N2 18.4 (0.08) 18.3 (0.30) 35.1 35.3 695 696 75.4 74.5 6 A. Danel, UCPSS 20-6
4) Delay before deposition steps Passivation tests (QSSPC) : q-time degradation starts at about 30 min 11 900 Seff (cm/s).5 9.5 9 8.5 8 30min. delay prior asi:h deposition 0 50 0 150 µpcd lifetime (µs) 850 800 750 700 650 600 550 500 30min. delay prior asi:h deposition 0 50 0 150 Effect is more visible on data from the complete full wafer Process time (min) Cell performance : 5 min to 30 min q-time = no visible impact 90 min q-time in clean room air = about 5% relative degradation Q-time < min (ref.) η (%) 17.8 (0.2) Jsc (ma /cm 2 ) 34.6 Voc (mv) 691 FF (%) 74.4 Impact depends on RH and on the effectiveness of the previous clean step 90 min 17.0 (0.2) 34.1 683 73.3 D. Graf et al, J. Appl. Phys. 69, 7620 (1991) H. Angerman et al, Solid State Phen. 92, 179 (2003) 7 A. Danel, UCPSS 20-7
5) HF cleanliness (1/2). Metals 000 ref HF & HCl 50.00 127 ref HF & HCl HF + 40ppb Fe HF + 40ppb Fe 40.00 Passivation degradation Effective lifetime (µs) 00 0 HF + 40ppb Fe & Cu HF + 40ppb Fe & Cu HF & HCl + 40ppb Fe & Cu HF & HCl + 40ppb Fe & Cu Seff (cm/s) 30.00 20.00.00 0.00 HF 2% Ref. Textured wafers Fe spike 20 ppb + 20 ppb + Cu spike 20 ppb + 20 ppb Recovery + HCl 1% 1E+14 1E+15 1E+16 1E+17 Polished (111) wafers Injection level (cm-3) 20ppb Cu ~ 30mV drop on Voc implied : > 4% relative degradation of cell perf. 40ppb Cu ~ 0mV drop on Voc implied : > 12% relative degradation of cell perf. 8 A. Danel, UCPSS 20-8
5) HF cleanliness (2/2) Surface contamination by VPD-DC-ICPMS (H. Fontaine, ) as-textured E at/cm 2 Na Mg Al K Ca Ti Cr Fe Ni Cu Zn Sum ctl #1 54 2900 370 220 10 630 95 30080 11760 30 47 300 ctl #2 27 1300 26 630 64 200 300 4600 2 1200 21 8 400 ctl #3 65 10 59 85 69 5 870 13000 8 8400 16 24 200 Texturization + SPM - HF - RCA + HF/HCl last ctl #1 4.3 0.8 4.2 11 5.2 <0.1 0.2 16 0.1 <0.5 13 54.8 ctl #2 2 0.3 1 5 2 0.2 0.1 0.3 < 0.1 0.6 0.5 12 Texturization + O3 & HF -based INES clean + HF/HCl last HF 2% <1 <0.2 <1 4.3 <1 1.2 1.5 4.2 <0.1 <0.5 0.2 11.4 HF 2%, 40ppb Fe <1 <0.2 <1 <1 <1 <0.1 <0.1 7.2 <0.1 <0.5 <0.1 7.2 HF 2%, 40ppb Fe & Cu <1 <0.2 <1 <1 <1 <0.1 <0.1 0 <0.1 31 <0.1 31 HF 2% & HCl 1%, 40ppb Fe & Cu <1 <02 <1 <1 <1 <0.1 <0.1 1.4 <0.1 <0.5 <0.1 1.4 As expected from deposition mechanisms: No impact of Fe in HF (verified @ 40ppb) Strong impact of Cu (20 and 40ppb) HCl 1% allows safe process (complexation of Cu) 9 A. Danel, UCPSS 20-9
6) HF & NH 4 F η (%) Jsc Voc (mv) FF (%) (ma /cm 2 ) HF 2% - HCl 1% 18.4 (0.05) 35.1 686 76.4 BOE 2% 18.0 (0.12) 35.2 680 75.7 HF alternative might be possible but couple of trials with NH 4 F (BOE, post HF dip in NH 4 F) always showed detrimental impact on our HJ technology A. Danel, UCPSS 20 -
Conclusion More extensive runs are needed for consolidation of the performance numbers, but this work allows some recommendations: HF concentration: 2% Process time: 2 to 5 min Contamination (metals): HCl spike helps Rinse and Dry: no critical issue using industrial equipment Q-time: 30 min OK if the cleaning steps are reliable might be critical if poor HF 2%-HCl 1% 5min step was OK for INES HJ ramp up 20 40 efficiency, η(%) 18 16 14 12 8 6 4 2 0 18.5 % 2009 20? June 20 Short-circuit current, Jsc, (ma/cm 2 ) 30 20 D. Muñoz et al. 35 th PVSC, Hawaï June 20 FZ substrate S=148.5cm 2 η= 19.6% Voc=718mV Jsc=35.6mA/cm 2 FF=76.9% 0 0 0 200 300 400 500 600 700 800 Voltage, V (mv) 11 A. Danel, UCPSS 20-11