contact for silicon solar cells SPREE Seminar Talk Udo Rmer - PowerPoint PPT Presentation

Polycrystalline silicon as carrier selective contact for silicon solar cells SPREE Seminar Talk Udo Römer 21.07.2016

Outline Theory / understanding Local overcompensation poly-Si contacts via ion implantation Process optimisation Combination of these poly-Si contacts technologies 1

Motivation • Removing front side metal shadowing  ~2.5 mA/cm² gain in J sc • Voltage can be enhanced by reducing PERC solar cell recombination:   J kT   gen   V ln 1 OC q  J  0 • SP-PERC cell has J 0 of ~300 fA/cm² • Poly-Si rear contact Poly-Si / c-Si-junction: solar cell 15 fA/cm² for p -type poly-Si 1 and 20 fA/cm² for n -type poly-Si 1  ~70 mV gain in V oc [1] J. Y. Gan and R. M. Swanson, IEEE Trans. Electron Devices , vol. ED-37, pp.245-250 (1990) 2

Poly-Si contact: working principle Band diagram of a n -typ poly-Si contact (sketch) electron transport few interface Energy [eV] many defects defects Passivation: Transport: • • Tunnel transport through the oxide 1 interface oxide: interface oxide Low D it at wafer surface due to high larger tunnel barrier quality oxide or for holes than for • • Field effect passivation due to holes in the oxide 2 electrons highly doped poly-Si layer Depth [nm] [1] Steinkemper, Feldmann, Bivour, and Hermle, IEEE Journal of Photovoltaics, vol. 5, no. 5, pp. 1348 (2015) [2] Peibst, Römer, Hofmann, Lim, Wietler, Krügener, Harder, and Brendel, IEEE Journal of Photovoltaics, vol. 4, no. 3, pp. 841 (2014) 3

Poly-Si contact: transport RCA-oxide after 10 min at 950 °C RCA-oxide after 10 min at 1100 °C • HR-TEM investigations show braking up of the oxide after high temperature processing Wolstenholme, Jorgensen, Ashburn, and Booker, J. Appl. Phys. 61, 225 (1987) 4

Process flow poly-Si test structures 1-step-process / 2-step-process Thin oxidation Deposition of doped poly-Si layers Annealing / cracking of the oxide 5

Process flow poly-Si test structures 1-step-process / 2-step-process Thin oxidation Deposition of undoped poly-Si layers Annealing / cracking of the oxide Doping 5

Measurement of the recombination characteristics Flash lamp 2.4 nm thermal oxide 30 min at 1050 °C J 0 -test structure Coil J 0 = 5 fA/cm² • J 0 -determination with photoconductance measurement and Kane & Swanson method 1 [1] Kane and Swanson, Proc.of the 18th IEEE PVSC, pp. 578 – 583 (1985) 6

Determination of the contact resistance R poly R abs R bulk R poly R poly R poly R bulk R bulk R poly R poly Inductive sheet resistance 4PP sheet resistance measurement measurement  If oxide is perfectly insulating: R 4PP ~ R poly  If oxide is perfectly insulating: R 4PP ~ R poly   rel ~ 1  If oxide is perfectly conducting: R 4PP ~ R abs  If oxide is perfectly conducting: R 4PP ~ R abs   rel ~ 0 Römer, Peibst, Ohrdes, Lim, Krügener, Bugiel, Wietler, and Brendel, Solar Energy Materials and Solar Cells, vol. 131, pp. 85-91, Dec. 2014. 7

Simulation of 4PP-measurements SENTAURUS-DEVICE 3D-simulation Input parameters: • Sample geometry • Resistivities of wafer, poly-Si layer and oxide Result: • “Measured" sheet resistance Römer, Peibst, Ohrdes, Lim, Krügener, Bugiel, Wietler, and Brendel, Solar Energy Materials and Solar Cells, vol. 131, pp. 85-91, Dec. 2014. 8

Simulation of 4PP-measurements • Calculation of relative contact resistance from simulated 4PP resistance • Plot vs. “real” (specified) contact resistance • Example: poly-Si layer with sheet resistance of 280 Ω/□ :  rel < 0.05 corresponds to contact resistance of < 0.5 Ωcm² Römer, Peibst, Ohrdes, Lim, Krügener, Bugiel, Wietler, and Brendel, Solar Energy Materials and Solar Cells, vol. 131, pp. 85-91, Dec. 2014. 9

Investigation of different oxides • All oxides reach J 0 -values between 5 and 20 fA/cm² • High temperatures needed for low contact resistance • Boron-doped poly-Si contacts comparable to Phosphorus-doped Römer, Peibst, Ohrdes, Lim, Krügener, Bugiel, Wietler, and Brendel, Solar Energy Materials and Solar Cells, vol. 131, pp. 85-91, Dec. 2014. 10

Investigation of different oxides • Passivation stable up to 60 min annealing at 1050 °C • Contact resistance decreases with increasing annealing duration • Good combination of low J 0 and  rel possible Römer, Peibst, Ohrdes, Lim, Krügener, Bugiel, Wietler, and Brendel, Solar Energy Materials and Solar Cells, vol. 131, pp. 85-91, Dec. 2014. 11

Influence of the metallization Metallized pieces Lifetime distribution measured via Infrared Solar cell demonstrator Lifetime Mapping (ILM) • ILM measurements before and after metallization show comparable lifetime level (apart from edge effects) • Planar solar cell demonstrators show V oc of 714 mV 1 • Series resistance of 0.6 Ωcm² not limited by poly -Si contact 1 [1] Römer, Peibst, Ohrdes, Lim, Krügener, Bugiel, Wietler, and Brendel, Solar Energy Materials and Solar Cells, vol. 131, pp. 85-91, Dec. 2014. 12

Theory / understanding Local overcompensation poly-Si contacts via ion implantation Process optimisation poly-Si contacts 13

Local counterdoping with in-situ patterned ion implantation • Counterdoping: Overcompensating one polarity of dopants with dopants of the other polarity • With in-situ patterned counterdoping local doping possible without structured dielectric layers • Counterdoping with in-situ masked ion implantation enables elegant process flow for back contacted solar cells • Process results in formation of lateral pn -junction with heavily doped p - and n -regions - Risk for band to band tunneling - Risk for trap-assisted tunneling 14

Full area counterdoping • Processing: - 1.5 x 10 15 cm -2 B implantation - 3 x 10 15 cm -2 P implantation - Annealing at 1050 °C • SIMS measurement: - Phosphorus profile covers boron profile over whole depth  Counterdoping works fine! Römer, Peibst, Ohrdes, Larionova, Harder, Brendel, Grohe, Stichtenoth, Wütherich, et al. , Proc. 39 th ,IEEE PVSC, pp. 1280 (2013) 15

Local counterdoping Local phosphorus Passivation layer implantation Position z [µm] p -type silicon Full area boron Aluminum contacts implantation Position x [µm] Simulations of the diode Measurements on test structures characteristics • Simulations featuring lateral pn -junction • Variation of the lifetime in the “implanted” area • Measurements on test structures and comparison with simulations Römer, Peibst, Ohrdes, Larionova, Harder, Brendel, Grohe, Stichtenoth, Wütherich, et al. , Proc. 39 th ,IEEE PVSC, pp. 1280 (2013) 16

Local counterdoping Simulations of the diode Measurements on test characteristics structures • Simulations show no detrimental influence of highly doped pn -junction: n = 1 as long as implant damage is well annealed • Otherwise strong recombination in space charge region with n = 2 • Measurements on test structures show n = 1  Everything is fine! Römer, Peibst, Ohrdes, Larionova, Harder, Brendel, Grohe, Stichtenoth, Wütherich, et al. , Proc. 39 th ,IEEE PVSC, pp. 1280 (2013) 17

Local counterdoping • Further investigations (incl. influence of the lateral doping profile & characteristics in reverse direction) published 1  Everything is fine! • Large area (156 mm x 156 mm psq.) ion implanted IBC solar cells featuring local counterdoping reach efficiencies of 22.1 % 2 [1] Römer, Peibst, Ohrdes, Larionova, Harder, Brendel, Grohe, Stichtenoth, et al. , Proc. 39 th ,IEEE PVSC, pp. 1280 (2013) [2] Bosch Solar Energy, ISFH, press release, Aug. 15th, 2013 18

Theory / understanding Local overcompensation poly-Si contacts via ion implantation Process optimisation Combination of these poly-Si contacts technologies 19

Process flow for ion implanted poly-Si with counterdoping Thermal LPCVD poly-Si Boron Annealing/ oxidation deposition implantation oxide break-up Boron implanted Phosphorus implanted test Test structure structure 20

Process flow for ion implanted poly-Si with counterdoping Thermal LPCVD poly-Si Boron Masked Annealing/ oxidation deposition implantation phosphorus oxide break-up implantation Boron implanted Phosphorus Test structure implanted test Test structure full area structure counterdoping 20

Process flow for ion implanted poly-Si with counterdoping Thermal LPCVD poly-Si Boron Masked Annealing/ oxidation deposition implantation phosphorus oxide break-up implantation Boron implanted Phosphorus Test structure Test structure implanted test local Test structure full area structure counterdoping counterdoping 20

Ion implantation in poly-Si • Decreasing J 0 with increasing dose • Very low values of 1.1 fA/cm² (phosphorus) and 4.4 fA/cm² (boron) • Increase at too high doses, especially for boron doping Römer, Peibst, Ohrdes, Lim, Krügener, Wietler, and Brendel, IEEE Journal of Photovoltaics, vol. 5, no. 2, pp. 507 (2015) 21

Ion implantation in poly-Si • Doping concentration constant inside poly-Si layer • Oxide acts as diffusion barrier, in particular for phosphorus • For high doses strong diffusion of boron into wafer Römer, Peibst, Ohrdes, Lim, Krügener, Wietler, and Brendel, IEEE Journal of Photovoltaics, vol. 5, no. 2, pp. 507 (2015) 22