Silicon Heterojunction Solar Cells Screen-printing: PECVD: intrinsic - PowerPoint PPT Presentation

W INDOW L AYERS FOR S ILICON H ETEROJUNCTION S OLAR C ELLS : Properties and Impact on Device Performance Johannes P. Seif UNSW, Sydney / SPREE Seminar – 01.05.2019

Silicon Heterojunction Solar Cells Screen-printing: PECVD: intrinsic Ag front electrode PECVD: p -doped a-Si:H passivation a-Si:H layers hole-collecting layer PVD: front and rear transparent c-Si conductive oxide Absorber: (TCO) n -type c-Si wafer, cleaned and textured Reported record efficiencies: PECVD: n -doped PVD: silver (Ag) Std. SHJ: 25.1% [Adachi] a-Si:H rear electrode IBC SHJ: 26.7% [Yoshikawa] electron-collecting layer Physical Vapor Deposition / Sputtering (PVD) Plasma Enhanced Chemical Vapor Deposition (PECVD) D. Adachi et al., APL, 2015, 107, 233506. 2 K. Yoshikawa, Nature Energy, 2017, 2(5), 17032.

Challenges: optical losses Ag front Parasitic absorption in the front layers: generation of charge carriers that recombine before being collected. TCO front −2.1 mA/cm 2 a-Si:H( p ) Lower current output of the c-Si( n ) c-Si( n ) a-Si:H( i ) device, i.e. reduced short-circuit current density ( J sc ) a-Si:H( n ) −0.5 mA/cm 2 TCO rear Free-carrier absorption in the highly doped TCO layer & plasmonic losses in Ag rear the metal rear reflector. 3 Z. C. Holman et al. IEEE J. PV, 2 (1) pp. 7-15, 2012.

Challenges: electrical losses Ag front Recombination TCO front - Radiative a-Si:H( p ) - Auger - Via defect states in the bulk or at the surface c-Si( n ) a-Si:H( i ) Lowers the maximal attainable voltage, i.e. the open-circuit voltage a-Si:H( n ) ( V oc ) and limits the voltage at the maximum power point (fill factor, FF ) TCO rear Ag rear 4

Challenges: electrical losses Ag front TCO front The Transparent Conductive Oxide (TCO) can have an influence on the a-Si:H layers a-Si:H( p ) (Schottky barriers) and the a-Si:H/c-Si interface (increased recombination). c-Si( n ) c-Si( n ) a-Si:H( i ) Lowers the maximal attainable voltage, i.e. the open-circuit voltage a-Si:H( n ) ( V oc ) and limits the voltage at the maximum power point (fill factor, FF ) TCO rear Ag rear 5 S. W. Glunz et al., EU-PVSEC, Milan, Italy, 2007. M. Bivour et al., IEEE J-PV, 4 , 2014.

Motivation Ag front Using alternative materials to mitigate losses TCO front ALD ZnO:Al Goals a-Si:H( p ) nc-Si:H( p ) • Development of PECVD processes for a-SiO x :H and nc-Si:H • Understand their impact ( material c-Si( n ) a-Si:H( i ) a-SiO x :H( i ) properties and processing ) on device performance ( J sc , V oc , FF ) • Reduction of optical and electrical losses a-Si:H( n ) nc-Si:H( n ) TCO rear ALD ZnO:Al Ag rear Atomic Layer Deposition (ALD) 6

Outline Temperature coefficients Temperature impact on lifetime and cell performance Alternative transparent electrodes atomic-layer-deposited ZnO:Al Wide-bandgap materials as protective layer vs. sputter-damage a-SiO x :H for passivation band offsets and transport barriers Organic overlayers spin-coated PVK for work function engineering Nanocrystalline layers Deposition strategies & device performance 7

Wide-bandgap materials: a-SiO x :H – optical gain versus transport

Motivation Variation of... i -layer thickness, [CO 2 ]/[SiH 4 ] and device structure. a -Si:H +a -SiO x :H(i) In this section: i -layer More transparent NOTE: Using a-SiO x :H only front side  sub-optimal passivation  J sc gain J. P. Seif et al., JAP, 115, 2014 9

a-SiO x :H: optical properties From spectroscopic ellipsometry Increasing [CO 2 ]/[SiH 4 ] ratio leads to a decrease of both: • refractive index n and • extinction coefficient k • ref. Bandgap: + 0.1 eV for [CO 2 ]/[SiH 4 ] = 2.5 Linked to incorporation of * hydrogen and oxygen * T. F. Schulze, L. Korte, F. Ruske, and B. Rech, J. P. Seif et al., JAP, 115, 2014 10 Physical Review B 83 , 165314 (2011).

a-SiO x :H: structural properties • Thermal desorption spectroscopy to analyse the layer structure UHV H 2 H 2 O ? O 2 … What we observe: What we learn: • • H 2 effusion spectrum varies Structure gets increasingly porous high- T peak decreases, low- T peak with increasing CO 2 /SiH 4 ratio increases with CO 2 /SiH 4 • The area under the curves • The amount of H 2 in the layer increases with CO 2 /SiH 4 increases with CO 2 /SiH 4 J. P. Seif et al., JAP, 115, 2014 11

a-SiO x :H: applied to hole contact Variation in a-SiO x :H thickness with various [CO 2 ]/[SiH 4 ] ratios 740 740 740 740 740 740 38 38 38 38 38 38 implied V oc (mV) implied V oc (mV) implied V oc (mV) implied V oc (mV) implied V oc (mV) implied V oc (mV) (a) (a) (a) (a) (a) (a) (b) (b) (b) (b) (b) (b) 2 ) 2 ) 2 ) 2 ) 2 ) 2 ) J sc (mA/cm J sc (mA/cm J sc (mA/cm J sc (mA/cm J sc (mA/cm J sc (mA/cm 720 720 720 720 720 720 a-Si:H a-Si:H a-Si:H a-Si:H a-Si:H a-Si:H 37 37 37 37 37 37 CO 2 /SiH 4 CO 2 /SiH 4 CO 2 /SiH 4 CO 2 /SiH 4 CO 2 /SiH 4 CO 2 /SiH 4 Good passivation: 700 700 700 700 700 700 0.4 0.4 0.4 0.4 0.4 0.4 Implied V oc s between 0.8 0.8 0.8 0.8 0.8 0.8 36 36 36 36 36 36 720 – 730 mV 680 680 680 680 680 680 2.5 2.5 2.5 2.5 2.5 2.5 100 100 100 100 100 100 (c) (c) (c) (c) (c) (c) (d) (d) (d) (d) (d) (d) Improved current: 20 20 20 20 20 20 efficiency (%) efficiency (%) efficiency (%) efficiency (%) efficiency (%) efficiency (%) 80 80 80 80 80 80 by up to 0.4 mA/cm 2 FF (%) FF (%) FF (%) FF (%) FF (%) FF (%) 15 15 15 15 15 15 (no clear trend with thickness 60 60 60 60 60 60 * due to reduced reflection). 10 10 10 10 10 10 40 40 40 40 40 40 5 5 5 5 5 5 20 20 20 20 20 20 BUT drop in fill factor: For a-SiO x :H cells, strongly 0 0 0 0 0 0 0 0 0 0 0 0 5 5 5 5 5 5 10 10 10 10 10 10 15 15 15 15 15 15 20 20 20 20 20 20 5 5 5 5 5 5 10 10 10 10 10 10 15 15 15 15 15 15 20 20 20 20 20 20 influenced by [CO 2 ]/[SiH 4 ] i -layer thickness (nm) i -layer thickness (nm) i -layer thickness (nm) i -layer thickness (nm) i -layer thickness (nm) i -layer thickness (nm) ratio and thickness. * Z. C. Holman et al., IEEE JPV, 2(1), 2012 12

a-SiO x :H permutations reference a-SiO x :H below p a-SiO x :H below n 13

Wide-bandgap a-SiO x :H: impact on FF 0 T (°C) Due to: 76 76 25 55 -10 J (mA/cm 2 ) Transport problem FF (%) FF (%) 35 65 72 72 associated with increased 45 85 -20 valence band offset 68 68 (confirmed by simulation) -30 reference 64 64 -40 20 40 60 80 20 40 60 80 0.00 0.25 0.50 0.75 temperature (°C) temperature (°C) V (V) 0 76 a-SiO x :H below p -10 J (mA/cm 2 ) FF (%) 72 -20 68 -30 64 -40 20 40 60 80 0.00 0.25 0.50 0.75 temperature (°C) V (V) J. P. Seif et al., JAP, 120, 2016 14 M. Liebhaber et al., APL, 106, 2015.

Wide-bandgap a-SiO x :H: impact on FF 76 76 FF (%) FF (%) 72 72 hole electron contact contact 68 68 reference a-SiO x :H below n 64 64 20 40 60 80 20 40 60 80 temperature (°C) temperature (°C) electron hole contact contact 76 a-SiO x :H below p Confirmation that FF (%) holes are the carriers 72 that are affected most by the 68 a-SiO x :H( i ) layer Potential gain in cells with hole collection at the rear 64 20 40 60 80 temperature (°C) J. P. Seif et al., JAP, 120, 2016 15

Temperature coefficients: Ambient-temperature impact on lifetime & performance

Motivation Typical temperature evolution of the cell parameters (SHJ solar cell) (a) (b) 725 2 ) 39 J sc (mA/cm 10 V oc (mV) 700 effective minority-carrier Auger 675 38 8 650 lifetime  (ms) 37 625 6 79 22 (c) (d) 4 78 FF (%)  (%) 20 77 2 76 18 1E14 1E15 1E16 effective minority-carrier 75 20 40 60 80 20 40 60 80 -3 ) density (cm temperature (°C) Temperature coefficients (TCs) important for operation in the field: cell T up to 90 °C J. P. Seif, et al, IEEE J.PV., 2015. 17 S. Kurtz, et al. Prog. PV Res. Appl. 2011

Lifetime( T ) measurements: passivated wafers Typical effective lifetime curve: Evolution with temperature 15 15 15 15 15 T (°C) T (°C) T (°C) T (°C) T (°C) 30 30 30 30 30 effective minority-carrier effective minority-carrier effective minority-carrier effective minority-carrier effective minority-carrier 60 60 60 60 60 Possibly explained by change in 90 90 90 90 90 recombination statistics / capture 10 10 10 10 10 lifetime (ms) lifetime (ms) lifetime (ms) lifetime (ms) lifetime (ms) 120 120 120 120 120 cross-sections with temperature 150 150 150 150 150 D. M. Goldie, American J. Material Science, 2013 5 5 5 5 5 a-Si:H( ip ) cell parameters ( J sc , V oc , FF ) c-Si( n ) a-Si:H( in ) 0 0 0 0 0 1E14 1E14 1E14 1E14 1E14 1E15 1E15 1E15 1E15 1E15 1E16 1E16 1E16 1E16 1E16 effective minority-carrier effective minority-carrier effective minority-carrier effective minority-carrier effective minority-carrier -3 ) -3 ) -3 ) -3 ) -3 ) density (cm density (cm density (cm density (cm density (cm J. P. Seif, et al, IEEE J.PV., 2015. 18