EFFECT OF HYDROGEN, CERIUM AND TUNGSTEN DOPING ON INDIUM OXIDE THIN FILMS FOR HETEROJUNCTION SOLAR CELLS A. Valla, P. Carroy, F. Ozanne, G. Rodriguez & D. Muñoz 1
OVERVIEW Description of amorphous / crystalline Silicon Heterojunction solar cell (HET) Transparent Conducting Oxides (TCO) Hydrogen impact Experimental Simulation Doping W, Ce impact Experimental Simulation 2
DESCRIPTION OF AMORPHOUS / CRYSTALLINE SILICON HETEROJUNCTION SOLAR CELL AT CEA-INES Ag η = 25.1 % Kaneka η = 24.7 % Sanyo On large area devices TCO a-si:h (n/i) c-si(n) a-si:h (i/p) TCO M. Taguchi et al., IEEE J. Photovoltaics, 4 (2013) pp.1-3 Adachi et al., Appl. Phys. Lett. 107, 233506 (2015); http://dx.doi.org/10.1063/1.4937224 3
WHY A TRANSPARENT CONDUCTING OXIDE? a-si:h thin films demonstrate very high passivation levels (Voc > 740mV) BUT: Ag 1 TCO 1 Optical issue : No adapted as antireflection J sc 2 Electrical issue : Low conductivity Poor lateral collection FF 2 a-si:h (n/i) c-si(n) Need to add a layer with: - Good anti-reflection properties : n air < n TCO < n a Si:H - Good lateral conduction TCO! 4
REFERENCE TCO IN HETEROJUNCTION SOLAR CELLS: ITO @ CEA-INES: Indium Tin Oxide : ITO: In 2 O 3 95% - SnO 2 5% SEM observations As deposited Annealing 200ºC 20 min in air Post-annealing ρ [W.cm] 9.2.10-4 6.8.10-4 N [cm -3 ] 2.2.10 20 2.3.10 20 μ [cm².v -1.s -1 ] 32 40 1 µm a-si:h are modified at > 200 C Maximal T for TCO : challenging (Limitation of HET technology) Can we do better? 5
HOW TO IMPROVE THE PROPERTIES OF A TCO? 1 directly ρ = N. μ. q BUT ALSO! Electrical properties μ ρ Optical properties μ ω p in NIR by Drude Model α NIR graph obtained by calculations T.J. Coutts and al., MRS Bull. Vol. 25, Iss. 08 (2005), pp.58-65 6
μ HOW TO IMPROVE THE MOBILITY? Scattering mechanisms μ ii Ionized impurities μ ni Neutral impurities Structural elements Amorphous state V O (oxygen vacancies) Points defects Deposition parameters Low temperature deposition Hydrogen Doping μ gb Grain boundaries S. Calnan and al., Thin Solid Films, 518 (2010) pp.1839-1849 N. Mori and al., e-j Surf. Sci. Nanotech., 10 (2012) pp.471-475 E. Kobayashi and al., Proc. 29th Eur. PVSEC, Hamburg Ger., 2014 Z. Lu and al., J. Phys. D. Appl. Phys. 46 (2013) pp. 75103-75107 P.F. Newhouse and al., Appl. Phys. Lett.87 (2005) 1121081 Y. Abe and al., Mater. Lett. 61 (2007) 566 Lattice distortion Grain size Grain Boundaries Deposition energy Substrate Post deposition annealing 7
DEPOSITION CONDITIONS Alliance Concept CT-200 Jusung ITO reference New TCO Type PVD-DC PVD-DC Temperature 200 C Target In 2 O 3 : SnO 2 wt. 5% RT In 2 O 3 : SnO 2 wt. 1% In 2 O 3 : CeO 2 wt. 3% In 2 O 3 W wt. 1% Low temperature deposition Doping Atmosphere Ar (315 sccm) Ar / Ar:H (50 sccm) Hydrogen O 2 (6 sccm) O 2 (0.7-1.1 sccm) Pressure 1.7.10-3 mbar 2.10-3 mbar Power 2.5 kw 50 W 8
OVERVIEW 1 ST PART Deposition parameters μ Scattering mechanisms μ ii Ionized impurities μ ni Neutral impurities Structural elements Amorphous state V O (oxygen vacancies) Points defects Low temperature deposition Hydrogen Doping μ gb Grain boundaries S. Calnan and al., Thin Solid Films, 518 (2010) pp.1839-1849 N. Mori and al., e-j Surf. Sci. Nanotech., 10 (2012) pp.471-475 E. Kobayashi and al., Proc. 29th Eur. PVSEC, Hamburg Ger., 2014 Z. Lu and al., J. Phys. D. Appl. Phys. 46 (2013) pp. 75103-75107 P.F. Newhouse and al., Appl. Phys. Lett.87 (2005) 1121081 Y. Abe and al., Mater. Lett. 61 (2007) 566 Lattice distortion Grain size Grain Boundaries Deposition energy Substrate Post deposition annealing 9
r [W.cm] RESULTS ITO VS ITO:H ELECTRICAL PROPERTIES (1/2) SEM observations 3,0E-03 asdep. 2,5E-03 2,0E-03 ITO 99/1 ann. 1 µm 1,5E-03 1,0E-03 ITO 95/5 5,0E-04 0,0E+00 1,20% 1,70% 2,20% 2,70% Amorphous-ITO H ρ O 2 U behaviour for ITO 99/1 Flat tendancy for ITO:H 99/1 ITO:H 99/1 Annealing conditions : 200ºC 20 min in air K. Zhang and al., J. Appl. Phys., 86 (1999) pp.974-980 L. Barraud and al., Solar Energy Materials & Solar Cells, 115 (2013) pp.151-156 10
N [cm -3 ] m [cm².v -1.s -1 ] RESULTS ITO VS ITO:H ELECTRICAL PROPERTIES (2/2) SEM observations 8,E+20 60 7,E+20 6,E+20 5,E+20 4,E+20 ITO:H 99/1 asdep. 50 40 30 ann. 2 µm 3,E+20 20 2,E+20 ann. ITO 95/5 ITO 99/1 1,E+20 1,4% 1,9% 2,4% 10 asdep. 0 1,4% 1,9% 2,4% As-Deposited : H Trap O 2- V O N O 2 N After annealing : Oxidation V O N N ITO:H > N ITO J. Liu and al., J. Wuhan Univ. Technol.-Mater. Sci.Ed. 25 (2010) pp.753-759 R. Wang, Ph.D thesis, University of Hong-Kong (2005) M. Huang and al., Phys. Status Solidi A 212, 10 (2015), pp.2226-2232 As-Deposited : H Trap O 2- V O μ ii μ O 2 μ After annealing : a-ito:h polycrystalline ITO:H : Grain size ITO:H > grain size ITO μ gbito:h μ Annealing conditions : 200ºC 20 min in air 11
HOW TO CHARACTERIZE OPTICAL PROPERTIES OF TCO? TCO on glass substrate Spectrophotometer ( HET cell) TCO on c-si substrate Ellipsometer fitting Air (semi- ) R Reflected energy by front face (lost) n, k and thickness of TCO Optical simulation with HET stack TCO a-si:h(n+in) c-si (semi- ) A TCO Absorbed energy by TCO (lost) A a Si:H Absorbed energy by a-si:h (lost) Conditions for optical simulations (home made) - Method Matrix Transfer - No rear side (c-si semi-infinite) - Spectrum : 300-1200 nm - Textured regular pyramids T Transmitted energy to c-si 100% generated current 12
[ma.cm-2] RESULTS OPTICAL SIMULATIONS ITO VS ITO:H Results after annealing ITO 95/5 47 ITO 99/1 ITO:H 99/1 46 R Reflected energy by front face (lost) 45 44 43 42 A TCO Absorbed energy by TCO (lost) R A a Si:H Absorbed energy by a-si:h (lost) TCO a-si:h T T reference 41 40 1,9% 1,4% 1,6% 1,8% 2,0% 2,2% 1,8% 2,0% 2,2% 2,4% 2,6% T transmitted Energy to c-si Over 2% O 2 ITO 99/1 ITO:H 99/1 Lower absorption better than reference 13
SUMMARY Low temperature deposition Hydrogen Amorphous state V O (oxygen vacancies) μ ii μ Post deposition annealing Amorphous state Grain size (Polycrystalline) μ gb 14
OVERVIEW 2 ND PART Deposition parameters μ Scattering mechanisms μ ii Ionized impurities μ ni Neutral impurities Structural elements Amorphous state V O (oxygen vacancies) Points defects Low temperature deposition Hydrogen Doping μ gb Grain boundaries S. Calnan and al., Thin Solid Films, 518 (2010) pp.1839-1849 N. Mori and al., e-j Surf. Sci. Nanotech., 10 (2012) pp.471-475 E. Kobayashi and al., Proc. 29th Eur. PVSEC, Hamburg Ger., 2014 Z. Lu and al., J. Phys. D. Appl. Phys. 46 (2013) pp. 75103-75107 P.F. Newhouse and al., Appl. Phys. Lett.87 (2005) 1121081 Y. Abe and al., Mater. Lett. 61 (2007) 566 Lattice distortion Grain size Grain Boundaries Deposition energy Substrate Post deposition annealing 15
r [W.cm] RESULTS ICO:H - IWO:H ELECTRICAL PROPERTIES (1/2) Annealing conditions : 200ºC 20 min in air 1,E-03 9,E-04 ITO 95/5 8,E-04 asdep. 7,E-04 6,E-04 5,E-04 4,E-04 ann. IWO:H 3,E-04 ICO:H 2,E-04 1,6% 1,8% 2,0% 2,2% 2,4% 2,6% 2,8% As-Deposited : O 2 ρ After annealing : ρ Best results with ICO:H at 2.4% O 2 : 2.7.10-4 W.cm 16
N [cm -3 ] m [cm 2.V -1.s -1 ] RESULTS ICO:H - IWO:H ELECTRICAL PROPERTIES (2/2) SEM observations ICO:H =IWO:H 9,E+20 70 8,E+20 ICO:H 60 ann. 7,E+20 6,E+20 asdep. 50 40 ICO:H =IWO:H 2 µm 5,E+20 30 4,E+20 3,E+20 IWO:H ann. 20 10 asdep. ITO 95/5 2,E+20 1,8% 2,0% 2,2% 2,4% 2,6% As-Deposited : N similar and stable with O 2 After annealing : Oxidation V O N but N ICO:H less than N IWO:H P.F. Newhouse and al., Appl. Phys. Lett.87 (2005) 1121081 Y. Abe and al., Mater. Lett. 61 (2007) 566 0 1,8% 2,0% 2,2% 2,4% 2,6% As-Deposited : O 2 μ μ ICO:H > μ IWO:H After annealing : a-tco:h polycrystalline TCO:H : μ gb μ High μ IWO:H (62 cm².v -1.s -1 ) Annealing conditions : 200ºC 20 min in air 17
[ma.cm-2] RESULTS OPTICAL SIMULATIONS ICO:H - IWO:H Results after annealing ITO 95/5 47 ICO:H IWO:H 46 R Reflected energy by front face (lost) 45 A TCO Absorbed energy by TCO (lost) 44 43 42 41 40 1,9% 1,8% 2,0% 2,2% 2,4% 2,6% 1,8% 2,0% 2,2% 2,4% 2,6% AR a Si:H Absorbed energy by a- Si:H TCO (lost) a-si:h T T reference T transmitted to c-si ICO:H : Over 2.4% O 2 better than reference IWO:H : Over 2.2% O 2 better than reference Less absorption 18
SUMMARY Low temperature deposition Amorphous state μ ii Hydrogen Doping V O (oxygen vacancies) μ Treatment post deposition Amorphous state Grain size (Polycrystalline) μ gb 19
FINAL CONCLUSION Best candidates for HET solar cells integration with our tools : Results after annealing TCO ρ [W.cm] N [cm -3 ] μ [cm².v -1.s -1 ] T [ma.cm -2 ] ITO 95/5 6,8.10-4 2,3.10 20 40,1 41,71 ITO 99/1 1,9.10-3 1,4.10 20 23,0 41,87 ITO:H 99/1 4,3.10-4 2,6.10 20 55,9 42,49 ICO:H 2,7.10-4 4,8.10 20 48,6 42,16 IWO:H 3,5.10-4 2,9.10 20 61,8 42,41 The best candidate is IWO:H : better compromise between electrical and optical properties 20
ACKNOWLEDGEMENTS CEA-INES team: Perrine, Fabien, Wilfried, Charles and Delfina; LETI team: Guillaume, Magalie, Chiara, Frederic for the TCO deposition and XRD characterizations; PVTC organizers; HERCULES for the funding; Helen McEwan for the English speech language 21
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