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

• Motivation
• Reviews of SiNx properties
• Process development
• Recombination studies

– Planar – Texturing

• Cell simulation and application

2

Outline

3

PECVD SiNx 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.

Motivation

Success of SiNx on silicon solar cells

4

PECVD SiNx 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.

Motivation

Success of SiNx on silicon solar cells

• Classic trend:

SRV decreases as n increases, irrespective of deposition techniques.

5

High absorption associated with Si-rich SiNx

Planar undiffused FZ p-Si

Motivation

Challenge of SiNx on silicon solar cells

Optics: refractive index

6

Review of SiNx properties

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 Claassen et al. 1983 (LF direct, RBS) Bustarret et al. 1988 (RF direct, ERD) Lenkeit et al. 2001 (W remote, ERD) Verlaan et al. 2009 (ERD) Hotwire W remote RF direct LF direct Calculated by Eq. (2.1)

Refractive index n at 632 nm N/Si ratio

Stoichiometric: N/Si = 4/3

N Si = 4 3 3.3 − 𝑜632 𝑜632 − 0.5

(Bustarret et al. 1988)

Optics: extinction coefficient

7

Review of SiNx properties

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 10

• 3

10

• 2

10

• 1

10 Stoichiometric: N/Si = 4/3 Doshi et al. 1997 (RF direct) van Erven et al. 2008 (ETP)

Duttagupta et al. 2012

(W remote inline) This work (W/RF dual-mode)

Extinctin coefficient k at 360 nm N/Si ratio

10

3

10

4

10

5

10

6

Absorption coefficient  at 360 nm (cm

• 1)

Structures: [Si–N]

8

Review of SiNx properties

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 2 3 4 5 6 7 8 9 10 11 12 13 Stoichiometric: N/Si = 4/3

[SiN] (10

22 cm

• 3)

N/Si ratio

Mäckel and Lüdemann 2002 (RF direct) Cuevas et al. 2006 (W remote) Chen et al. 2007 (W/RF dual-mode) van Erven et al. 2008 (ETP) This work (W/RF dual-mode)

Structures: [Si–H] and [N–H]

9

Review of SiNx properties

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

[SiH] or [NH] (10

21 cm

• 3)

N/Si ratio

Structures: [Si–H] peak wavenumber

10

Review of SiNx properties

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 2080 2100 2120 2140 2160 2180 2200 2220 Increase of the back-bonded N atoms HSiSi3 Stoichiometric: N/Si = 4/3 HSiN3 HSiN2H HSiN2Si Mäckel and Lüdemann 2002 (RF direct) Cuevas et al. 2006 (W remote) Verlaan et al. 2007 (Hotwire) Verlaan et al. 2009 Hotwire W remote RF direct LF direct This work (W/RF dual-mode)

SiH peak wavenumber (cm

• 1)

N/Si ratio

HSiNSi2

Electronics: insulator charge density Qeff

11

Review of SiNx properties

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 10

10

10

11

10

12

10

13

Stoichiometric: N/Si = 4/3 Hezel et al. 1984 (LF direct) Bagnoli et al. 1991 (RF direct) Landheer et al. 1995 & 1998 (ECR) Garcia et al. 1998 (ECR) Dauwe 2004 (W remote) De Wolf et al. 2006 (LF direct) Lelievre et al. 2009(LF direct) Lamers et al. 2013 (W remote) This work (W/RF dual-mode) Temperature NH3:SiH4 ratio Pressure W plasma power

Qeff (cm

• 2)

N/Si ratio

Electronics: interface defect density Dit

12

Review of SiNx properties

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 10

10

10

11

10

12

10

13

Stoichiometric: N/Si = 4/3 Hezel et al. 1984 (LF direct) Landheer et al. 1998 (ECR) Garcia et al. 1998 (ECR) Lamers et al. 2013 (remote W) This work (W/RF dual-mode) Temperature NH3:SiH4 ratio Pressure W plasma power

Dit (eV

• 1cm
• 2)

N/Si ratio

Conclusion 1

13

Review of SiNx properties

Irrespective of deposition techniques, (i) the bulk structural and optical properties are universally correlated to the N/Si ratio; and (ii) the electronic properties (Qeff and Dit) appear independent of the N/Si ratio. Promoting an opportunity of decoupling SiNx surface passivation and optical transmission properties; and therefore Circumventing the trade-off between the two properties.

Methodology

14

Process development

1) Varying deposition parameters in Roth & Rau AK400:

• NH3:SiH4 ratio
• Pressure
• Temperature
• Microwave plasma
• RF plasma

2) Monitoring surface passivation eff(Δn) 3) Monitoring optical properties n & k

Results – NH3:SiH4 ratio

15

0.0 0.5 1.0 1.5 2.0 2.5 3.0

NH3/SiH4 gas flow ratio

1.8 2.0 2.2 2.4 2.6 2.8 3.0

n at 632 nm

0.0 0.5 1.0 1.5 2.0 2.5 3.0 400 600 800 1000 1200 1400 1600

p-type 0.85  cm eff at n = 10

15 cm

• 3 (s)

NH3/SiH4 gas flow ratio

Process development

Results – Pressure and temperature

16

0.0 0.1 0.2 0.3 0.4 0.5

Pressure (mbar)

1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 235 ºC 290 ºC

Refractive index n at 632 nm

0.0 0.1 0.2 0.3 0.4 0.5 10

1

10

2

10

3

235 ºC eff on p-0.85 

cm

290 ºC eff on p-0.85 

cm

290 ºC eff on n-0.47 

cm

eff at n = 10

15 cm

• 3 (s)

Pressure (mbar)

Process development

Results – Seff,UL vs. n

17

Process development

Surface passivation:eff(Δn)

18

Process development

19 Ref. n632nm Seff,UL (cm/s) Cell under air Cell encapsulated beneath glass/EVA Jinc tSiNx (nm) Jgen JAbs Jrelf JInc tSiNx (nm) Jgen JAbs Jrelf DuttaGupta et al. 2.5 7.2 41.27 58 38.98 1.45 0.83 38.69 40 37.58 0.69 0.42 This work at 290 ˚C 1.87 1.6 80 40.38 0.21 0.67 88 38.24 0.11 0.34 ΔJ 1.40

• 1.24 - 0.16

ΔJ 0.66

• 0.58 - 0.08
• OPAL 2
• Random textured Si surface
• Si thickness: 180 µm

(mA/cm2) (mA/cm2)

Optical transmission

Process development

Thermal stability

20

Process development

5 6 7 8 9 10 10

1

10

2

10

3

10 10

1

10

2

10

3

10

4

Seff,UL-Post RTA / Seff,UL-As deposited [Si-N] (10

22 cm

• 3)

As-deposited Post RTA

Seff,UL at n = 10

15 cm

• 3 (cm/s)
• Poor thermal stability
• Correlated to [Si–N]
• Further studies required

RTA 800 °C for 5 seconds

Conclusion 2

21

A single SiNx layer can provide both  high passivation of c-Si surface  high transmission at short wavelength

Process development

Methodology

22

1) Extracting Seff,UL(Δn) from measured 𝜐eff (Δn) 2) Probing electronic properties by C-V measurements: Dit and Qeff 3) Modeling Seff,UL(Δn)

Recombination studies – planar

Extracting Seff,UL(Δn)

23

• Similar injection-level

dependence

• Lowest Auger-corrected Seff,UL

(1.6 cm/s)

10

12

10

13

10

14

10

15

10

16

10

17

10 10

1

10

2

10

3

Lauinger et al. - 0.7 cm (1996) Schmidt and Kerr - 1.0 cm (2001) Kerr and Cuevas - 1.0 cm (2002) DuttaGupta et al. - 1.5 cm (2011) This work 0.85 cm - 235 ºC This work 0.85 cm - 290 ºC

FZ p-type {100} c-Si

Seff,UL (cm s

• 1)

Excess carrier density n (cm

• 3)

Recombination studies – planar

𝑇eff(∆𝑜) = 𝑥 2 ( 1 𝜐eff(∆𝑜) − 1 𝜐bulk(∆𝑜)) 𝑥 : Si wafer thickness 𝜐eff(∆𝑜) : effective minority carrier lifetime 𝜐bulk(∆𝑜) : Si bulk lifetime

Probing electronics: Dit, Qeff and n/p

24

Dit-Midgap (cm-2eV-1) 3.0 × 1011 Qeff (cm-2) 5.6 × 1011 Extracted from quasi-static (QS) and high-frequency (HF) capacitance- voltage (C-V) measurements Schmidt et al. (1997)

Recombination studies – planar

Modeling Seff,UL(Δn)

25

Notes for the Seff,UL(Δn) modeling:

• Adapting the latest Si intrinsic bulk

lifetime model—Richter et al. (2012)

• Assuming a single defect at a single

energy level (midgap)

• Assuming n & p saturate and

constant for the unmeasured gap regions

• Defect A or B (or both) is likely to

dominate the recombination at the Si– SiNx interface.

• Defect C is excluded.

10

12

10

13

10

14

10

15

10

16

10

17

10 10

1

10

2

10

3

10

4

10

5

FZ p-type {100} c-Si Defect C B A

Seff,UL (cm s

• 1)

Excess carrier density n (cm

• 3)

Recombination studies – planar

Conclusion 3

26

• We obtained a low Seff,UL = 1.6 cm/s on 0.8 Ωcm p-type FZ Si, with

Dit = 3.0 x 1011 eV–1 cm–2 at midgap, and Qeff = 5.6 x 1011 cm–2.

• Defect A or B (or both) is likely to dominate the recombination at the

Si–SiNx interface, and

• Defect C is excluded.

Recombination studies – planar

Methodology

27

1) Employing the optimum deposition condition and varying the NH3:SiH4 ratio to obtain a wide range of film refractive indices (n at 632nm = 1.83 – 4.1) 2) Monitoring Seff,UL as a function of n at 632nm on three types of Si surfaces: i. Planar {100} ii. Planar {111}

• iii. Texturing with random upright pyramids

3) Probing electronic properties at textured surfaces by by depositing corona charge

Recombination studies – texturing

28

Drivers of enhanced recombination

• 1. Enlarged surface area
• 2. Orientation of surface planes:

fO = {111}/{100}

• 3. Presence of concave and convex

surface features: fV = texture/{111}

Recombination studies – texturing

Results: surface area-corrected Seff,UL

1.8 2.0 2.2 2.4 2.6 2.8 4.0 4.2 10 10

1

10

2

NH3:SiH4 gas ratio {100}

Refractive index n at 632 nm Surface area-corrected Seff,UL at n = 10

15 cm

• 3 (cm s
• 1)

a-Si:H

29

1.8 2.0 2.2 2.4 2.6 2.8 4.0 4.2 10 10

1

10

2

NH3:SiH4 gas ratio {100} {111}

Refractive index n at 632 nm Surface area-corrected Seff,UL at n = 10

15 cm

• 3 (cm s
• 1)

a-Si:H 1.8 2.0 2.2 2.4 2.6 2.8 4.0 4.2 10 10

1

10

2

NH3:SiH4 gas ratio {100} {111} Rantex

Refractive index n at 632 nm Surface area-corrected Seff,UL at n = 10

15 cm

• 3 (cm s
• 1)

a-Si:H

Recombination studies – texturing

• Consistent trend: Seff,UL first

decreases and then saturates

• Two exceptions:

─ low Seff,UL ─ saturates at lower n

• Low and constant Seff,UL over a

range of

─ n (1.85 – 4.1 at 632 nm) ─ surface morphologies

Lauinger et al. 1998

Results: surface area-corrected Seff,UL ratio

30

1.8 2.0 2.2 2.4 2.6 2.8 4.0 4.2 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 fO: {111}/{100}

(b) Surface area-corrected Seff,UL ratio: fO, or fV Refractive index n at 632 nm

a-Si:H 1.8 2.0 2.2 2.4 2.6 2.8 4.0 4.2 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 fO: {111}/{100} fV: rantex/{111}

(b) Surface area-corrected Seff,UL ratio: fO, or fV Refractive index n at 632 nm

a-Si:H

• fO is constantly low (0.9-1.7) 

less orientation dependence

• fV relates strongly to n

─ high at low n  sensitive to morphology ─ close to 1.0 when n ≥ 2.3  negligible increase at texture, after area-correction

Recombination studies – texturing

Probing electronics by corona-lifetime technique

31

Recombination studies – texturing

• 6
• 5
• 4
• 3
• 2
• 1

1 2 3 10

1

10

2

n at 632 nm 1.83 1.93 2.50

Qeff

S0

Planar {100} n-type FZ Si

Sit Seff,UL at n = 10

15 cm

3 (cm/s)

Applied surface charge density Qs (10

12 cm

2)

Qeff

Sit S0

Results: Sit or Qeff versus S0

32

Recombination studies – texturing

10 10

1

10

2

10

1

10

2

(b)

{100} {111} rantex

Surface area-corrected Sit (cm/s) Surface area-corrected S0 (cm/s)

{100} {111} rantex

(a)

Surface area-corrected Qeff (10

12 cm

• 2)

n-type 1.1cm FZ Si 10 10

1

10

2

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Surface area-corrected S0 (cm/s)

Conclusion 4

34

Recombination studies – texturing

• A constant and low SRV is achieved on planar and textured Si surfaces
• ver a wide range of n (1.85-4.1),
• When passivated by N-rich SiNx, the increase in recombination at

textured surfaces is high, and it is

i. due to the presence of vertices/edges on pyramids rather than due to the {111} orientation (i.e., high fV and low fO), and ii. primarily caused by an increase in Dit rather than a reduction in Qeff.

• When passivated by Si-rich or a-Si:H, the increase in recombination at

textured surfaces is negligible after are-correction (i.e., fV and fO are close to unity).

Simulating impact of SiNx on cells

35

• SiNx functions as front surface passivation and ARC layer
• p-type PERC (n+-diffused front) and n-type IBC (undiffused front)
• Cell simulation using Quokka 2 [Fell 2012]
• Optical simulation using OPAL 2 [McIntosh et al. 2012]

─ Random upright pyramids ─ Spectrum AM 1.5g ─ Operating in air

200 400 600 800 1000 1200 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 200 300 400 500 600 700 800 10

• 4

10

• 3

10

• 2

10

• 1

10 2.0 2.1 2.3 2.5 2.8 2.9 1.83 1.85 1,87 1.89 1.91 1.93 1.83 1.85 1,87 1.89 1.91 1.93 2.0 2.1 2.3 2.5 2.8 2.9

Refractive index, n Wavelength (nm) (a) (b) Extinction coefficient, k Wavelength (nm)

Cell simulation and application

Simulation results (1/3): ARC

36

1.8 2.0 2.2 2.4 2.6 2.8 3.0 0.0 0.5 1.0 1.5 2.0 2.5 1.8 2.0 2.2 2.4 2.6 2.8 3.0 0.0 0.5 1.0 1.5 2.0 2.5

1.8 2.0 2.2 2.4 2.6 2.8 3.0 38 39 40 41 42 43

(b)

JRefl (mA/cm

2)

n at 632 nm (c) JAbs (mA/cm

2)

n at 632 nm JGen (mA/cm

2)

n at 632 nm (a)

1.8 2.0 2.2 2.4 2.6 2.8 3.0 30 40 50 60 70 80 90

Optimised SiNx thickness (nm)

Refractive index n at 632 nm

Cell simulation and application

1.8 2.0 2.2 2.4 2.6 2.8 3.0 645 650 655 660 665 670 675 1.8 2.0 2.2 2.4 2.6 2.8 3.0 36 37 38 39 1.8 2.0 2.2 2.4 2.6 2.8 3.0 80.2 80.4 80.6 80.8 81.0 81.2 1.8 2.0 2.2 2.4 2.6 2.8 3.0 19.0 19.5 20.0 20.5 21.0 21.5

VOC (mV)

Refractive index n at 632 nm

(b)

JSC (mA/cm

2)

Refractive index n at 632 nm

(c)

FF (%)

Refractive index n at 632 nm

(d)

 (%) Refractive index n at 632 nm

(a)

37

Simulation results (2/3): p-PERC cell

High absorption High recombination

Cell simulation and application

1.8 2.0 2.2 2.4 2.6 2.8 3.0

650 660 670 680 690 700 710 1.8 2.0 2.2 2.4 2.6 2.8 3.0 36 37 38 39 40 41 42 1.8 2.0 2.2 2.4 2.6 2.8 3.0 80.0 80.5 81.0 81.5 82.0 82.5 1.8 2.0 2.2 2.4 2.6 2.8 3.0 19 20 21 22 23 24 25 IBC PERC

VOC (mV) Refractive index n at 632 nm

(b)

IBC PERC

JSC (mA/cm

2)

Refractive index n at 632 nm

(c)

IBC PERC

FF (%) Refractive index n at 632 nm

(d)

IBC PERC

 (%)

Refractive index n at 632 nm

(a)

38

Simulation results (3/3): n-IBC cell

High absorption High recombination

Cell simulation and application

Application on n-type IBC

VOC (mV) JSC (mA/cm2) FF (%) η (%) 705 41.9 ± 0.7 82.7 24.4 ± 0.5 39

k

• r o ta ts

ta r th r a i 2 C i C i

Cell simulation and application

Under standard testing conditions (25C, AM1.5G spectrum) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 5 10 15 20 25 30 35 40 45 Measured Simulated

Current density J (mA/cm

2)

Voltage (V)

Conclusion 5

40

• The optimum SiNx for both p-type PERC and n-type IBC has n = 1.9,
• wing to:

─ high surface recombination when n < 1.9 ─ high film absorption when n > 1.9

• Application of the optimum SiNx onto the front textured undiffused

surface of an n-type IBC cell enabled a conversion efficiency of 24.4 ± 0.5%.

Cell simulation and application

41

• No universal correlation exists between surface

recombination and bulk structural/optical properties.

• A highly transparent and highly passivating SiNx is

attained, enabling a 24.4%-efficient n-type IBC cell.

• An increase in recombination of the textured surfaces is

i. related to the presence of vertices and/or edges of the pyramids rather than to the presence of {111}-orientated facets, and; ii. primarily attributable to an increase in Dit rather than a decrease in Qeff.

Summary

42

Thank you!