Atomic layer deposition enabling higher efficiency solar cells-Bram Hoex
Faculty of Engineering School of Photovoltaic and Renewable Energy Engineering Atomic layer deposition enabling higher efficiency solar cells 15 th -CSPV, Shanghai, 21 November 2019 Bram Hoex 1 , Kean Thong Khoo 1 , Xin Cui 1 , Geedhika Poduval 1 , Tian Zhang 1 , Xiang Li 2 , Wei Min Li 2 , and Md. Anower Hossain 1 1 School of Photovoltaic and Renewable Energy Engineering, UNSW Sydney, Sydney, Australia 2 Jiangsu Leadmicro Nano-Equipment Technology Ltd., Wuxi, ChinaSPREE PV research overviewSPREE PV research overview4 • SPREE research overview • ALD enabling high-efficiency solar cells • Improving carrier selectivity on c-Si by ALD AlO x capping layers • Improving NiO hole contacts for Si and perovskite solar cells • Surface passivation of CZTS solar cells • Conclusions OutlineResearch Question • In principle, single sided deposition is very hard to achieve and parasitic deposition onto the front side of the solar cell can occur during fabrication. • This has been reported for both PECVD and ALD AlO x deposition processes. 1) What is the effect of AlO x wrap-around on screen-printed contact resistance (i.e., selectivity)? 2) What is the effect of AlO x wrap on p-PERC solar cell performance? Source: To, A. et al., IEEE JPV (2017) 99 p. 1-8 5Methodology: 1. Fabricate TLM test structures with intervening AlO x layers. Vary: Paste, Temperature, Speed. The effect of AlO x wrap-around Processing sequence of the PERC and PERT precursors. Schematic diagram of the p-type PERC test structures used in this experiment. Source: To, A. et al., IEEE JPV (2017) 99 p. 1-8Contact resistivity ρ c – PERC Structures • A clear ‘U’-shaped trend, representing an optimal firing temperature. Contact resistivity vs. temperature for screen-printed silver fingers fired through an AlO x /SiN x stack. 10 -3 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 Contact Resistivity [m Ω cm 2 ] 0 (25%~75%) Range within 1.5IQR Median Line Source: To, A. et al., IEEE JPV (2017) 99 p. 1-8Contact resistivity ρ c – PERC Structures • A clear ‘U’-shaped trend, representing an optimal firing temperature. • The addition of a 3 nm AlO x film on top of SiN x results in lower contact resistance. • Firing temperature window appears larger and allows for lower firing temperatures. Contact resistivity vs. temperature for screen-printed silver fingers fired through an AlO x /SiN x stack. 10 -3 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 Contact Resistivity [m Ω cm 2 ] 0 (25%~75%) 3 (25%~75%) Range within 1.5IQR Median Line Source: To, A. et al., IEEE JPV (2017) 99 p. 1-8Contact resistivity ρ c – PERC Structures • A flatter ‘U’-shaped trend, representing an optimal firing temperature. • The addition of a 5 nm AlO x film on top of SiN x still performs better than no AlO x capping layer. Contact resistivity vs. temperature for screen-printed silver fingers fired through an AlO x /SiN x stack. 10 -3 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 Contact Resistivity [m Ω cm 2 ] 0 (25%~75%) 3 (25%~75%) 5 (25%~75%) Range within 1.5IQR Median Line Source: To, A. et al., IEEE JPV (2017) 99 p. 1-8Contact resistivity ρ c – PERC Structures • 10 nm AlO x capping layer on top of SiN x results in poor contact resistance for the investigated paste and firing conditions. Contact resistivity vs. temperature for screen-printed silver fingers fired through an AlO x /SiN x stack. 10 -3 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 Contact Resistivity [m Ω cm 2 ] 0 (25%~75%) 3 (25%~75%) 5 (25%~75%) 10 (25%~75%) Range within 1.5IQR Median Line Source: To, A. et al., IEEE JPV (2017) 99 p. 1-811 • Bifacial ALD AlO x layers tried in high volume manufacturing • Significantly higher performance compared to monofacial counterpart • Lower cost of ownership while adding one layer! Implementation in high-volume manufacturing Source: Hossain et al., Nano Material Science, 10.1016/j.nanoms.2019.10.001 21.0 21.5 22.0 0 5000 10000 15000 20000 25000 30000 35000 40000 Count Efficiency (%) Bifacial PERC (ALD) Monofacial PERC (PECVD)12 • SPREE research overview • ALD enabling high-efficiency solar cells • Improving carrier selectivity on c-Si by ALD AlO x capping layers • Improving NiO hole contacts for Si and perovskite solar cells • Surface passivation of CZTS solar cells • Conclusions OutlineNovel Carrier Selective Contacts n-type Si E Fn E Fp J p J n NiO x TCO In 2O 3 ：H ITO E F Small ΔE v Large ΔE c Transition metal oxides as selective contacts Dopant free enables low temperature process Better optical properties Higher thermal stability than a-Si:H possible [1] Battaglia et al., Appl. Phys. Lett. 2014 [2] Greiner et al., NPG Asia Mat. 2013 • High work function ( 6 eV) • n-type • Low valence band offset between Si • p-type n-type Si E Fn E Fp D it d) duced ba d be d g se ect e J p J n High WF Metal oxide MoO x WO x VO x TCO In 2O 3 ：H ITO E F[1] Menchini et al., Phys. Status Solidi C, 2016. Motivation and Approach -5 -4 -3 -2 -1 0 1 2 3 4 5 -10 -5 0 5 10 Voltage(V) Current (mA) • RF sputtered NiO x • Undesired S-shape [1] • Atomic Layer Deposited (ALD) NiO x • Very high resistance, Zener diode performanceSource: Zhang et al., Appl. Phys. Lett. 113, 262102 (2018) Motivation and Approach DFT (density functional theory) calculation Yellow & Green atoms: Ni Purple atoms: O Blue atoms: Zn NiO, cubic rock-salt structure A 2x2x2 supercell, 64 atomsMotivation and Approach Zn incorporation into NiO • Additional electronic states contribution from the Zn 3d at the VBM • Decreased band gap (3.2 eV to 2.9 eV) DFT (density functional theory) calculation VBM VBM CBM CBM Source: Zhang et al., Appl. Phys. Lett. 113, 262102 (2018)ALD Process • Bis(N,N -di-t-butylacetamidinato) nickel(II), NiAMD, 125 ℃ • Diethyl Zinc, DEZ • H 2 O Source: Zhang et al., Appl. Phys. Lett. 113, 262102 (2018)ALD Process NiAMD DEZn H 2 O H 2 O Purge Time Supercycle ((A1B1)m (A2B2)n)x x supercycle m cycles n cycles Source: Zhang et al., Appl. Phys. Lett. 113, 262102 (2018) 10 11 12 13 3 4 5 Thickness (nm) Supercycle numberALD Process NiAMD DEZn H 2 O H 2 O Purge Time Supercycle ((A1B1)m (A2B2)n)x x supercycle m cycles n cycles 0 1 2 3 4 0 1 2 3 m:n=56:1 1cycle ZnO 56 cycle NiO Thickness (nm) 0 1 2 3 4 0 1 2 3 0 1 2 3 4 5 6 7 8 0 1 2 3 0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 m:n=56:1 1cycle ZnO 56 cycle NiO Thickness (nm) Thickness (nm) m:n=20:1 m:n=10:1 Thickness (nm) Supercycle number Source: Zhang et al., Appl. Phys. Lett. 113, 262102 (2018)Contact performance Cox & Strack method ~3.5 nm Zn x Ni 1-x O films with three different Zn concentrations -40 -20 0 20 40 -80 -40 0 40 80 Voltage (mV) Current (mA) increasing dot size Good Ohmic contact was obtained from the structure Contact resistivity: ~55 m Ω∙cm 2 Zn 0.62 Ni 0.38 O Source: Zhang et al., Appl. Phys. Lett. 113, 262102 (2018)Thermal stability Step-wise annealing in N 2 gas from room temperature to 500 o C for 10 min at each temperature 200 o C annealing improved the contact performance Lowest contact resistivity: ~21.5 m Ω∙cm 2 The films are highly thermal stable RT 200 220 250 300 400 500 20 30 40 50 60 Contact resistivity (mΩ.cm 2 ) Temperature (°C) Zn 0.46 Ni 0.54 O Zn 0.09 Ni 0.91 O Zn 0.62 Ni 0.38 O Source: Zhang et al., Appl. Phys. Lett. 113, 262102 (2018)22 • SPREE research overview • ALD enabling high-efficiency solar cells • Improving carrier selectivity on c-Si by ALD AlO x capping layers • Improving NiO hole contacts for Si and perovskite solar cells • Surface passivation of CZTS solar cells • Conclusions OutlineCurrent status and development of CZTS Benefits and Weakness • Earth-abundant, Environmentally-friendly, low cost material • Bandgap ~1.5 eV for tandem, absorption 10 4 cm -1 • Large V oc deficit Large V oc deficit from: • Bulk defects and disorder in the CZTS • Unfavourable heterojunction band alignment • Interface defects induced recombination [1] A. Polman et al. , Science, 2016. [1]Interface passivation via ALD-Al 2 O 3 [1]. Benick, J., et al., Applied Physics Letters, 2008. [2]. K. E. Roelofs, et al.,J. Phys. Chem. C, 2013. [3]. Koushik, D., et al., Energy Environ. Sci., 2017. Method – thermal ALD-Al 2 O 3 Motivation [1] [2] [3] Silicon solar cell Quantum dot sensitized solar cells Perovskite solar cell Al CH 3 O H TMA exposure Ar purge H 2 O exposure Ar purgeglass Mo CZTS ZnSnO i-ZnO ITO glass Mo CZTS CdS i-ZnO ITO Previous work on CZTS/ZnSnO solar cell CZTS E C ZnO E V ZnO ZnO SnO X = 0.13 X = 0.18 X = 0.20 X = 0.31 CdS Source: Cui, X., et al., Chemistry of Materials, 2018.Passivation effects – Different cycles of ALD Al 2 O 3 0 2 4 6 8 10 Efficiency (%) 20 30 40 50 60 70 Fill factor (%) 12 14 16 18 20 22 J SC (mA/cm 2 ) 560 600 640 680 720 V OC (mV) glass Mo CZTS ZnSnO i-ZnO ITO glass Mo CZTS ZnSnO i-ZnO ITO AlOx Source: Cui, X., et al., Energy Environ. Sci.,12, 2751 (2019)Passivation effects – Sub-steps of ALD Al 2 O 3 TMA exposure CZTS Ar purge CZTS Ar purge CZTS H 2 O exposure CZTS One cycle of ALD Al 2 O 3 150 o C vacuum heat Device performance 7 8 9 10 50 55 60 65 70 Fill factor (%) J SC (mA/cm 2 ) 660 680 700 720 740 V OC (mV) Efficiency (%) 18 20 22 Source: Cui, X., et al., Energy Environ. Sci.,12, 2751 (2019)0.0 0.2 0.4 0.6 0 4 8 12 16 20 0.0 0.2 0.4 0.6 0 4 8 12 16 20 0.0 0.2 0.4 0.6 0 4 8 12 16 20 Current density (mA/cm 2 ) Voltage (V) η V OC J SC FF (%) (mV) (mA/cm 2 ) (%) Al 2 O 3 10.2 736 21.0 65.8 (Full Area) ZnSnO 9.3 720 20.5 63.5 Current density (mA/cm 2 ) Voltage (V) Current density (mA/cm 2 ) Voltage (V) • Efficiency improve from 9.3% to 10.2%, record for Cd-free CZTS solar cell • Benefits mainly from increased V oc and FF Record efficiency in Cd-free CZTS solar cell Source: Cui, X., et al., Energy Environ. Sci.,12, 2751 (2019)29 • Thin ALD AlO x capping layers can decrease contact resistance • You can reduce total cost of ownership by adding one nanoscale thin film to a PERC solar cell • Doping of ALD NiO is straightforward and can significantly improve its conductance & performance as hole selective contact • ALD Al 2 O 3 enables world-record Cd-free CZTS solar cells Summary https://unswhoexgroup.com/ b.hoex@unsw.edu.au 10 -3 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 Contact Resistivity [m Ω cm 2 ] 0 (25%~75%) 3 (25%~75%) 5 (25%~75%) 10 (25%~75%) Range within 1.5IQR Median Line RT 200 220 250 300 400 500 20 30 40 50 60 Contact resistivity (mΩ.cm 2 ) Temperature (°C) Zn 0.46 Ni 0.54 O Zn 0.09 Ni 0.91 O Zn 0.62 Ni 0.38 O 0.0 0.2 0.4 0.6 0 4 8 12 16 20 0.0 0.2 0.4 0.6 0 4 8 12 16 20 0.0 0.2 0.4 0.6 0 4 8 12 16 20 Current density (mA/cm 2 ) Voltage (V) η V OC J SC FF (%) (mV) (mA/cm 2 ) (%) Al 2 O 3 10.2 736 21.0 65.8 (Full Area) ZnSnO 9.3 720 20.5 63.5 Current density (mA/cm 2 ) Voltage (V) Current density (mA/cm 2 ) Voltage (V)www.pvsec-31.com pvsec31@arinex.com.au 12 – 17 December 2021 We look forward to welcoming you to Sydney, Australia! 31st International Photovoltaic Science and Engineering Conference
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