Highly Transparent and Highly Passivating Silicon Nitride for Solar - PowerPoint PPT Presentation

Highly Transparent and Highly Passivating Silicon Nitride for Solar Cells Yimao Wan The Australian National University (ANU) 23/10/2014

Outline • Motivation • Reviews of SiNx properties • Process development • Recombination studies – Planar – Texturing • Cell simulation and application 2

Motivation Success of SiNx on silicon solar cells PECVD SiN x is incorporated into most laboratory and industrial silicon solar cells, fulfilling three functions: i. it comprises the antireflection coating; ii. it provides surface and bulk passivation; and iii. it forms a chemical barrier to protect underlying interfaces from the degrading effects of moisture, humidity and sodium ions. 3

Motivation Success of SiNx on silicon solar cells PECVD SiN x is incorporated into most laboratory and industrial silicon solar cells, fulfilling three functions: i. it comprises the antireflection coating; ii. it provides surface and bulk passivation; and iii. it forms a chemical barrier to protect underlying interfaces from the degrading effects of moisture, humidity and sodium ions. 4

Motivation Challenge of SiNx on silicon solar cells Planar undiffused FZ p -Si • Classic trend: SRV decreases as n increases, irrespective of deposition techniques. High absorption associated with Si-rich SiN x 5

Review of SiNx properties Optics: refractive index 3.3 3.2 3.1 Refractive index n at 632 nm 3.0 Stoichiometric: Claassen et al. 1983 (LF direct, RBS) 2.9 Bustarret et al. 1988 (RF direct, ERD) N/Si = 4/3 2.8 Lenkeit et al. 2001 (  W remote, ERD) 2.7 2.6 Verlaan et al. 2009 (ERD) Hotwire 2.5  W remote 2.4 RF direct 2.3 LF direct 2.2 Si = 4 N 3.3 − 𝑜 632 𝑜 632 − 0.5 2.1 Calculated by Eq. (2.1) 3 2.0 (Bustarret et al. 1988) 1.9 1.8 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 N/Si ratio 6

Review of SiNx properties Optics: extinction coefficient 0 10 Stoichiometric: -1 ) Absorption coefficient  at 360 nm (cm 6 10 N/Si = 4/3 Extinctin coefficient k at 360 nm -1 10 5 10 4 10 -2 10 Doshi et al. 1997 (RF direct) van Erven et al. 2008 (ETP)  Duttagupta et al. 2012 3 10 (  W remote inline) This work (  W/RF dual-mode) -3 10 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 N/Si ratio 7

Review of SiNx properties Structures: [Si – N] 13 12 11 10 9 8 -3 ) 7 22 cm Stoichiometric: 6 N/Si = 4/3 [Si  N] (10 5 4 M ä ckel and L ü demann 2002 (RF direct) Cuevas et al. 2006 (  W remote) 3 Chen et al. 2007 (  W/RF dual-mode) van Erven et al. 2008 (ETP) This work (  W/RF dual-mode) 2 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 N/Si ratio 8

Review of SiNx properties Structures: [Si – H] and [N – H] 100 [Si  H] Lauinger et al. 1998 (  W remote) Chen et al. 2006 (  W/RF dual-mode) -3 ) 21 cm Cuevas et al. 2006 (  W remote) 10 Verlaan et al. 2007 (Hotwire) van Erven et al. 2008 (ETP) [Si  H] or [N  H] (10 This work (  W/RF dual-mode) [N  H] Lauinger et al. 1998 (  W remote) van Erven et al. 2008 (ETP) 1 Chen et al. 2006 (  W/RF dual-mode) Cuevas et al. 2006 (  W remote) Stoichiometric: Verlaan et al. 2007 (Hotwire) N/Si = 4/3 This work (  W/RF dual-mode) 0.1 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 N/Si ratio 9

Review of SiNx properties Structures: [Si – H] peak wavenumber H  Si  N 3 2220 -1 ) 2200 Si  H peak wavenumber (cm Increase of the back-bonded N atoms M ä ckel and L ü demann 2002 (RF direct) Cuevas et al. 2006 (  W remote) 2180 Verlaan et al. 2007 (Hotwire) H  Si  N 2 H 2160 Verlaan et al. 2009 Hotwire  W remote H  Si  N 2 Si 2140 RF direct LF direct Stoichiometric: 2120 N/Si = 4/3 This work (  W/RF dual-mode) H  Si  NSi 2 2100 H  Si  Si 3 2080 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 N/Si ratio 10

Review of SiNx properties Electronics: insulator charge density Q eff 13 10 Hezel et al. 1984 (LF direct) Bagnoli et al. 1991 (RF direct) Landheer et al. 1995 & 1998 (ECR) 12 10 Garcia et al. 1998 (ECR) Dauwe 2004 (  W remote) -2 ) De Wolf et al. 2006 (LF direct) Q eff (cm Lelievre et al. 2009 ( LF direct) Lamers et al. 2013 (  W remote) This work (  W/RF dual-mode) 11 10 Temperature NH 3 :SiH 4 ratio Stoichiometric: Pressure  W plasma power N/Si = 4/3 10 10 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 N/Si ratio 11

Review of SiNx properties Electronics: interface defect density D it 13 10 Hezel et al. 1984 (LF direct) 12 10 Landheer et al. 1998 (ECR) -2 ) Garcia et al. 1998 (ECR) -1 cm Lamers et al. 2013 (remote  W) D it (eV This work (  W/RF dual-mode) Temperature NH 3 :SiH 4 ratio 11 10 Pressure  W plasma power Stoichiometric: N/Si = 4/3 10 10 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 N/Si ratio 12

Review of SiNx properties Conclusion 1 Irrespective of deposition techniques, (i) the bulk structural and optical properties are universally correlated to the N/Si ratio; and (ii) the electronic properties ( Q eff and D it ) appear independent of the N/Si ratio.  Promoting an opportunity of decoupling SiN x surface passivation and optical transmission properties; and therefore  Circumventing the trade-off between the two properties. 13

Process development Methodology 1) Varying deposition parameters in Roth & Rau AK400: • NH 3 :SiH 4 ratio • Pressure • Temperature • Microwave plasma • RF plasma 2) Monitoring surface passivation  eff ( Δ n ) 3) Monitoring optical properties n & k 14

Process development Results – NH 3 :SiH 4 ratio p -type 0.85   cm 3.0 1600 2.8 1400 -3 (  s) 2.6 n at 632 nm 1200 15 cm 2.4  eff at  n = 10 1000 2.2 800 2.0 600 1.8 400 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 NH 3 /SiH 4 gas flow ratio NH 3 /SiH 4 gas flow ratio 15

Process development Results – Pressure and temperature 2.6 Refractive index n at 632 nm 2.5 -3 (  s) 3 10 2.4 15 cm 2.3 235 ºC  eff at  n = 10 290 ºC  cm 2.2 235 ºC  eff on p -0.85   cm 2 10 2.1 290 ºC  eff on p -0.85   cm 290 ºC  eff on n -0.47  2.0 1.9 1 1.8 10 0.0 0.1 0.2 0.3 0.4 0.5 0.0 0.1 0.2 0.3 0.4 0.5 Pressure (mbar) Pressure (mbar) 16

Process development Results – S eff,UL vs. n 17

Process development Surface passivation:  eff ( Δ n ) 18

Process development Optical transmission • OPAL 2 • Random textured Si surface • Si thickness: 180 µm Cell under air Cell encapsulated beneath glass/EVA n 632nm S eff,UL Ref. J inc t SiNx J gen J Abs J relf J Inc t SiNx J gen J Abs J relf (cm/s) (nm) (nm) (mA/cm 2 ) (mA/cm 2 ) 2.5 7.2 58 38.98 1.45 0.83 40 37.58 0.69 0.42 DuttaGupta et al. 41.27 38.69 1.87 1.6 80 40.38 0.21 0.67 88 38.24 0.11 0.34 This work at 290 ˚C Δ J 1.40 - 1.24 - 0.16 Δ J 0.66 - 0.58 - 0.08 19

Process development Thermal stability -3 (cm/s) 4 10 15 cm 3 10 As-deposited Post RTA S eff,UL at  n = 10 2 10 • Poor thermal stability 1 10 0 10 • Correlated to [Si – N] S eff,UL-Post RTA / S eff,UL-As deposited 3 10 • Further studies required 2 10 1 10 5 6 7 8 9 10 22 cm -3 ) [Si-N] (10 RTA 800 ° C for 5 seconds 20

Process development Conclusion 2 A single SiN x layer can provide both  high passivation of c-Si surface  high transmission at short wavelength 21

Recombination studies – planar Methodology 1) Extracting S eff,UL ( Δ n ) from measured 𝜐 eff ( Δ n ) 2) Probing electronic properties by C-V measurements: D it and Q eff 3) Modeling S eff,UL ( Δ n ) 22

Recombination studies – planar Extracting S eff,UL ( Δ n ) 3 10 𝑇 eff (∆𝑜) = 𝑥 1 1 Lauinger et al. - 0.7  cm (1996) 2 ( 𝜐 eff (∆𝑜) − 𝜐 bulk (∆𝑜)) Schmidt and Kerr - 1.0  cm (2001) Kerr and Cuevas - 1.0  cm (2002) 𝑥 : Si wafer thickness DuttaGupta et al. - 1.5  cm (2011) 𝜐 eff (∆𝑜) : effective minority carrier This work 0.85  cm - 235 ºC 2 10 lifetime This work 0.85  cm - 290 ºC -1 ) 𝜐 bulk (∆𝑜) : Si bulk lifetime S eff,UL (cm s 1 10 • Similar injection-level dependence FZ p -type {100} c-Si 0 10 • Lowest Auger-corrected S eff,UL 12 13 14 15 16 17 10 10 10 10 10 10 (1.6 cm/s) -3 ) Excess carrier density  n (cm 23

Recombination studies – planar Probing electronics: D it , Q eff and  n/p 3.0 × 10 11 D it-Midgap (cm -2 eV -1 ) 5.6 × 10 11 Q eff (cm -2 ) Extracted from quasi-static (QS) and high-frequency (HF) capacitance- voltage (C-V) measurements Schmidt et al. (1997) 24

Recombination studies – planar Modeling S eff,UL ( Δ n ) 5 10 Notes for the S eff,UL ( Δ n ) modeling: FZ p -type {100} c-Si • Adapting the latest Si intrinsic bulk Defect C lifetime model — Richter et al. (2012) 4 10 • Assuming a single defect at a single energy level (midgap) 3 10 -1 ) S eff,UL (cm s Assuming  n &  p saturate and • constant for the unmeasured gap 2 10 regions B • Defect A or B (or both) is likely to 1 10 dominate the recombination at the Si – A SiN x interface. 0 10 • Defect C is excluded. 12 13 14 15 16 17 10 10 10 10 10 10 -3 ) Excess carrier density  n (cm 25