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NEW APPROACHES IN THE SILICON HETEROJUNCTION SOLAR CELLS Middle East Technical University - 2019© Rasit Turan The Center for Solar Energy Research and Applications GÜ NAM, METU, Ankara, Turkey Department of Physics, METU, Ankara, Turkey OUTLINE 1 Introduction Silicon Heterojunction Solar Cell, old and new concepts Research directioons at GÜ NAM Use of carrier selective layers instead of doped a-Si Conclusions Center for Solar Energy Research and Applications GÜ NAM Middle East Technical University METU GÜ NAM Photovoltaic c-Sİ Solar Cell Pilot Production Line GPVL 2 Collaboration with Institute of Photovoltaics of Nanchang University Institute of Photovoltaics of Nanchang University Center for Solar Energy Research and Applications Prof. Lang Zhou, who is the director of Institute of Photovoltaics of Nanchang University China,was hosted by GÜNAM He delivered a talk titled “Development of solar cells based on heterojunction of amorphous silicon and crystalline silicon HACHIT and post HIT” on 26th November 2018. Conventional Al-BSF c-Si Solar Cell Technology 2 A standard solar cell is composed of a p-n junction on the front side and Al contact on the back side. Al diffusion creates a p-p barrier called back surface field on the back side Disadvantages Low carrier life time due to high recombination rate at Si/Al interface Works only with p-type substrate which suffers from low bulk life time Record efficiency 20.291 ARC/Passivation SiNx/SiOx n-emitter p-Si Al p-BSF Screen-printed Al Ag 1 Ki Hyung Kim, et al, JJAP, vol 56, 2017 Heterojunction Solar Cell 2 In a heterojunction solar cell, p-n juntion is formed externally by n- and p- type a-Si layers An i-a-Si layer provides excellent passivation leading to very high carrier life time values miliseconds Carrier collection takes place through n and p layers Disadvantages Extremely good control on layer thicknesses is needed Surface cleaning is critical High contact resistance Use of extremely toxic gases High cost Record efficiency 25.42 ITO p-a-SiH i-a-SiH n-type cSi wafer n-a-SiH Ag i-a-SiH Ag ITO 2 Masuko K et al., IEEE J Photovoltaics, 461433‐1435 2014 EC EV EF ΔEC1 ΔEV1 ΔEC2 ΔEV2 a-SiH p/i n-type c-Si a-SiH i/n MetalITO Metal ITO Dopant Free Carrier Selective Junctions 2 In principle, doped layers are not needed to seperate and collect e-h pairs generated by a photon Appropriate materials may provide band alignment/band bending to seperate electrons and holes and collect carriers selectively Proposed dopant free passivating carrier selective contacts Metal Flourides LiFx, KFx, MgFx, . Metal Oxides MoOx, TiOx, V2Ox, Promising candidates Hole selective layer MoOx Electron selective TiOx TCO Hole selective layer MoOx Pasivation Crystal Si wafer Electron selective layer TiOx Metal Pasivation Metal TCO Passivation might be needed 7 Best SHJ Solar Cell Efficiencies 0 5 10 15 20 25 30 1995 2000 2005 2010 2015 2020 Effi cien cy Year Panasonic Kaneka Enhanced passivation quality of a-SiH Large area application, Bifacial SHJ solar cell by Sanyo Optimized band alignments and ITO layer a clean c-Si surface before a- SiH deposition; deposition of a high-quality intrinsic a-SiH layer; lower plasma and/or thermal damage to the c-Si surface during a-SiH, TCO, and conductive electrode fabrication. IBC design; Further improvements on passivation Best Cell Efficiencies-SHJ IBC aK. Masuko et al., IEEE Journal of Photovoltaics, 4, 1433 2014 bD. Adachi et al., Applied Physics Letters, 107, 233506 2015 cM. Taguchi et al., IEEE Journal of Photovaoltaics, 4, 96 2014 dhttps//www.helmholtz-berlin.de/media/media/projekte/hercules/hercules-m36-workshop/yoshikawa-kaneka-2016-hercles-designated-version.pdf Voc [V] Jsc [mA/cm2] FF Efficiency Area*[cm2] Cell type aPanasonic 0.740 41.8 82.7 25.6 143.7 da SHJ-IBC dKaneka 0.740 42.5 84.65 26.63 179.74da SHJ-IBC cPanasonic 0.750 39.49 83.2 24.7 101.8 ta SHJ bKaneka 0.738 40.8 83.5 25.1 151.9 ap SHJ HAC Solar Cell Approach Heterojunction of Amorphous silicon and Crystalline silicon with Localized pn structure HACL* * designed and proposed by Prof. Zhou and his colleauges Simulation results obtained by ATLAS** ** Haibin Huang, Lang Zhou, Jire Yuan, Zhijue Quan PECVD Cluster System at GÜNAM 10 Manufactured by Vaksis Turkish Company ✓3 CCP chambers, 1 ICP chamber and two sputter units CCP intrinsic CCP n-type CCP p-type SPUTTER Ag AZO ICP CLUSTER PECVD SYSTEM SHJ Studies in GÜNAM 11 Wet Chemical Cleaning Processes RCA-1, RCA-2 and Piranha Deposition of i a-SiH on both sides Passivation layer Deposition of n a-SiH BSF Deposition of p a-SiH Surface Texturing Formation of random pyramids Oxide Removal HF/HCl Metal contact formation on both sides Sputtering of TCO on both sides ITO I-V characterization Different surface morphologies To improve passivation quality -Various deposition conditions -Hydrogen termination -Different passivation layers i aSiCH P type a-SiC window layer Different electron and hole selective layers Reactive sputtering Screen printing and electroplating Formation of inverted pyramids 12 Wet Chemical Cleaning Processes RCA-1, RCA-2 and Piranha Deposition of i a-SiH on both sides Passivation layer Deposition of n a-SiH BSF Deposition of p a-SiH Surface Texturing Formation of random pyramids Oxide Removal HF/HCl Metal contact formation on both sides Sputtering of TCO on both sides ITO I-V characterization Different surface morphologies Lithography free inverted pyramids on p and n type c-Si Inverted Pyramids 13 E. Donercark et al., PVCON2018, Ankara Single Step Lithography-Free Inverted Pyramids with Cu-assisted Etching Cu nanoparticle removal by Nitric acid CuNO32/𝐻𝐹 /𝐻2𝑂2 etchant Etching in 15 minutes {100} planes {110} planes The etch rate difference between 100 and 110 planes results in the formation of star shapes 0 200 400 600 800 0 10 20 30 40 C u r r e n t D e n s i ty m A / c m 2 Vol ta g e m V Exteremely low reflection 2.6 Upright vs. Inverted Pyramid Jsc [mA/cm2] 39.1 Jsc [mA/cm2] upward pyramids 37.7 Higher Jsc values have been obtained in solar cells with inverted pyramids Passivation of i a-SiH layer 14 Wet Chemical Cleaning Processes RCA-1, RCA-2 and Piranha Deposition of i a-SiH on both sides Passivation layer Deposition of n a-SiH BSF Deposition of p a-SiH Surface Texturing Formation of random pyramids Oxide Removal HF/HCl Metal contact formation on both sides Sputtering of TCO on both sides ITO I-V characterization To improve passivation quality -Various deposition conditions -Hydrogen termination -Different passivation layers i aSiCH Impact of SiH4/H2 ratio on lifetime 15 SiH4 /H2 Tau µs impliedVocmV 1/2 2300 725 1/3 2500 730 1/4 1600 700 1/5 460 600 1/9 50 550 0.00E00 5.00E-04 1.00E-03 1.50E-03 2.00E-03 1.00E14 2.10E15 4.10E15 Tau s Δn cm-3 1/9ratio 1/6ratio 1/2ratio 1/3ratio Impact of SiH4/H2 ratio on Dit as measured by MOS device 16 SiH4 /H2 Dit eV-1cm-2 1/2 2.62x1011 1/3 1.83x1011 1/4 3.94x1011 1/5 5.19x1011 1/9 4.84x1012 Impact of SiH4/H2 ratio 17 SiH4 /H2 Dit eV-1cm-2 CH impliedVocmV Tau µs 1/2 2.62x1011 7.82 725 2300 1/3 1.83x1011 8.14 730 2500 1/4 3.94x1011 10.28 700 1600 1/5 5.19x1011 11.97 600 460 1/9 4.84x1012 15.34 550 50 H plasma 7.62x1010 9.45 740 4000 After deposition of i-aSiH layer, H plasma treatment enhance passivation quality Dit CH impliedVoc Tau H plasma treatment Electron and hole selective layers 18 Wet Chemical Cleaning Processes RCA-1, RCA-2 and Piranha Deposition of i a-SiH on both sides Passivation layer Deposition of n a-SiH BSF Deposition of p a-SiH Surface Texturing Formation of random pyramids Oxide Removal HF/HCl Metal contact formation on both sides Sputtering of TCO on both sides ITO I-V characterization P type a-SiC window layer Different electron and hole selective layers Electron and Hole Selective Layers 2 TiOx/c-Si interface has a small conduction band offset allowing feasible electron transport from silicon to TiOx and a large valence band offset which blocks the holes transport from silicon to TiOx TiOx as electron selective layer MoOx as hole selective layer P MoOx Ec Ev n-type cSi Ef −− − − Δ - qVbi For an n-type substrate, holes are accummulated at the MoOx/Si interface and forms a hole rich layer there. They can tunnel to the contuction band of MoOx Standard Al BSF Solar cell with MoOx layer on the back side 156x156 mm2 large, 180 µm thick p-type CZ solar wafers 1-3 Ω.cm To our knowledge this is the best efficiency reported for a direct heterojunction between MoOx and p-type c-Si without passivation layers and First reported industrial scale p-type c-Si/MoOx solar cell H. Nasser et al., Under Preparation 20 SHJ Solar cell with MoOx instead of p-Si 21 SHJ MoOx Voc mV 650 641 Jsc mA/cm2 39.34 39.51 FF 75.24 73.05 Efficiency 19.85 18.50 EQE Results 22 SHJ with MoOx has significantly higher blue response than standard SHJ due to low absorpiton MoOx can be considered as an alternative to the doping in the HAC approach Electron Selective Passivating TiOx 3 High efficiency solar cells with electron selective TiOx S. Avatsthi et al, App. Physc. Lett. 102 2013 203901 J. Jhaveri, IEEE PVSC Proc. 2015 X. Yang et al., Sol. Energy Mater. Sol. Cells 150 2016 32 X. Yang et al., Adv. Mater. 28 2016 5891 X. Yang et al., Prog. Photovolt. Res. Appl. 2017 J. Bullock et Al., ACS Energy Lett. 3 2018 508 2013 2014 2015 2016 2017 2018 6 8 10 12 14 16 18 20 22 24 Efficiency Year Efficiency N-wafer Front Doped N-wafer PEDOTPSS MOCVDP-wafer single junction CVD Contact resistivity degrades with increasing TiOx thickness Better to deposit TiOx by ALD ALD deposition may also provide good passivation X. Yang et al., Adv. Mater. 28 2016 5891 Electron Selectivity of the TiOx/Si Junction 20 n-type Revealing the electron selectivity of TiOxBoth TiOx and c-Si exhibit n-type conductivity TiOx/n-type c-Si heterojunctions behave like quasi-standard p-n junction diodes When TiOx is deposited on n-type c-Si, only electrons can transport to TiOx contact while holes are repelled away band alignment This is a clear indication of electron selectivity of the junction D. Ahiboz et al., Semicond. Sci. Technol. 33 2018 045013 Passivation Capacity of ALD TiOx Effect of Interlayer 25 ALD at 230 °C was chosen lowest Dit, amorphous, no N contamination WCOx Wet Chemical Oxide SiO2 An impressive effective lifetime of 2.3 msec was obtained from 3.5 nm thick TiOx deposied on WCOx followed by forming gas annealing The results proves a promising passsivation ability of TiOx for solar cell aplications 2 2.5 3 3.5 0 500 1000 1500 2000 2500 t eff m S Thickness nm Ti Ox/ c- Si Ti Ox/ c- SiF GA Ti Ox/ WCOx/c- Si Ti Ox/ WCOx/c- SiF GA 2 2.5 3 3.5 0 100 200 300 400 500 600 700 800 iV oc mV Thickness nm Wet chemical oxide SiO2 was produced by nitric acid HNO3 oxidation Electroplating for Metalization 26 Wet Chemical Cleaning Processes RCA-1, RCA-2 and Piranha Deposition of i a-SiH on both sides Passivation layer Deposition of n a-SiH BSF Deposition of p a-SiH Surface Texturing Formation of random pyramids Oxide Removal HF/HCl Metal contact formation on both sides Sputtering of TCO on both sides ITO I-V characterization Screen printing vs. electroplating Electroplating for Metalization 27 The limitations of screen printed contacts Lower fill factors, Higher shading loss Cost the use of expensive Ag pastes There is a need for an alternate metallization scheme Solar Cell Results for Electroplating 28 Cell Parameters Ag-Screen Printed Ref. Cell Ni/Cu Plated Cell Cell eff. [] Cell eff. [] SunsVoc 17.69 18.13 17.32 17.82 Voc [mV] Voc [mV] SunsVoc 621 622 609 610 F.F. [] pF.F [] SunsVoc 77.06 81.84 76.00 78,64 Jsc [mA/cm2] 36.54 36.40 Series resistance Rs [Ω-cm2] 0.199 0.333 Metal contact width [µm] 90 40 ρc [mΩcm2] 5,47±1,3 3,17±0,35 Summary and Conclusions 25 SHJ solar cells are the best performing crystalline solar cells Gas ratio, passivating layers should be carefully optimized Alternative materials for p- and n-type amorphous layer can lower the cost. MoOx and TiOx are good candidates for hole and electron selective layers for dopant free solar cells. 17.6 2 2.5 3 3.5 0 500 1000 1500 2000 2500 t eff m S Thickness nm Ti Ox/ c- Si Ti Ox/ c- SiF GA Ti Ox/ WCOx/c- Si Ti Ox/ WCOx/c- SiF GA
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