APCVD Oxide Films for c-Si Solar Cells. APCV… · APCVD Oxide Films for c-Si Solar Cells ... (co nfirme d b y F ra u nho fe r ISE CalLab ) ... G. Dingemans and W. M. M. Kessels,

APCVD Oxide Films for c-Si Solar Cells

Kristopher O. Davis, Kaiyun Jiang,

Carsten Demberger, Heiko Zunft, Dirk

Habermann and Winston V.

Schoenfeld

Overview

• Introduction to APCVD

– Overview of the APCVD platform

– Key advantages

• Applications for c-Si Solar Cells

– Passivation of p+ and p-type surfaces

– Multi-layer passivating ARC stacks

– Diffusion and co-diffusion using doped SiO2 films

• Conclusions and Future Work

In-line APCVD: • Continuous operation without loading or

pump down delays

• High throughput and low cost of ownership

• Gas controls run in steady-state no

transients for turn-on and turn-off

• Multiport injection to prevent mixing of the

reactant gases

• Flexible platform proven to work in high

volume production environments

Key Processes: • Dielectric layers: TiO2, SiO2, AlOx

• Solid dopant sources: PSG, BSG

• TCO layers also possible

In-line Belt APCVD

In-line Roller APCVD

3

SCHMID APCVD Platform

Low Cost + High Throughput = APCVD

• Suitable for large area deposition

• i.e. Five lanes of 156x156 mm wafers (up to 4000 wafers per hour)

• High deposition rates

• e.g. For AlOx, static >1000 nm∙min-1, dynamic up to 150 nm∙min-1

Applications Overview

Aluminum Oxide (AlOx)

Titanium Oxide (TiO2)

Silicon Oxide (SiO2)

Phosphosilicate Glass (PSG)

Borosilicate Glass (BSG)

Material Systems c-Si Cell Applications

Rear side passivation (p-type wafers)

Emitter passivation (n-type wafers)

Single layer ARC (p- or n-type wafers)

Double layer ARC (p- or n-type wafers)

PERC capping layer (p-type wafers)


Dopant source for emitter (p-type wafers)

Dopant source for BSF (n-type wafers)

Dopant source for emitter (n-type wafers)

Dopant source for BSF (p-type wafers)


















AlOx Passivated PERC Cells

[1] IMEC PVSEC 2012

[2] SCHMID Press Release 2012

(confirmed by Fraunhofer ISE CalLab)

Research Scale: η > 21.6% [1] Industrial Scale: η = 20.74% [2]

p-Si

ARC

Rear Al contact

Local p+ BSF

Passivating

dielectric layer

(e.g. AlOx)

n+

PERC Cross-Section

Front Ag fingers

Key advantages of the PERC concept:

• Drastically reduce SRVrear through improved chemical and field effect passivation

• Increase internal back reflectance at the rear side of the cell, which improves light

trapping (important for near bandgap photons, where c-Si has a low absorption

coefficient)

• Both advantages above are critical for enabling thin wafer formats – also PERC

concept can reduce/eliminate wafer bow

1.50

1.52

1.54

1.56

1.58

1.60

1.62

1.64

1.66

1.68

40

60

80

100

120

140

160

180

0 10 20 30 40 50 60 70 80 90 100

n

Dep

osit

ion

Rate

(n

m/m

in)

O2/TMA ratio

• High deposition rates (70-

150 nm∙min-1) for a wide

process window

• Excellent reaction efficiency

of TMA, based on XRR

measured density of 2.5

g∙cm-3

8

Deposition Rate and Refractive Index (n)

References [1] G. Dingemans and W. M. M. Kessels, Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 30, 040802 (2012).

[2] T.-T. Li and A. Cuevas, physica status solidi (RRL) - Rapid Research Letters, 3, 160 (2009).

[3] T.-T. A. Li and A. Cuevas, Progress in Photovoltaics: Research and Applications, 19, 320 (2011).

[4] T.-T. A. Li, S. Ruffell, M. Tucci, Y. Mansoulié, C. Samundsett, S. De Iullis, L. Serenelli, and A. Cuevas, Solar Energy Materials and Solar Cells, 95, 69 (2011).

[5] P. Poodt, A. Lankhorst, F. Roozeboom, K. Spee, D. Maas, and A. Vermeer, Advanced Materials, 22, 3564 (2010).

[6] B. Vermang, A. Rothschild, A. Racz, J. John, J. Poortmans, R. Mertens, P. Poodt, V. Tiba, and F. Roozeboom, Progress in Photovoltaics: Research and

Applications, 19, 733 (2011).

[7] S. Miyajima, J. Irikawa, A. Yamada, and M. Konagai, Applied Physics Express, 3, 012301 (2010).

[8] P. Saint-Cast, D. Kania, M. Hofmann, J. Benick, J. Rentsch, and R. Preu, Applied Physics Letters, 95, 151502 (2009).

[9] P. Saint-Cast, D. Kania, R. Heller, S. Kuehnhold, M. Hofmann, J. Rentsch, and R. Preu, Applied Surface Science (2012).

[10] P. Saint-Cast, J. Benick, D. Kania, L. Weiss, M. Hofmann, J. Rentsch, R. Preu, and S. W. Glunz, IEEE Electron Devices Letters, 31, 695 (2012).

0

20

40

60

80

100

120

140

160

ALD (Thermal, PA) [1] PVD (RF Sputtering)[2-4]

Spatial ALD [1,5,6] PECVD [7-10] APCVD (PresentWork)

Dep

ositio

n R

ate

(nm

/min

)

9

APCVD AlOx Deposition Rate vs. Literature

Effect Surface Recombination Velocities

As deposited 15-20nm Al2O3

As deposited 15-20nm Al2O3+ 70nm SiO2

Al2O3 after firing

Al2O3+ SiO2 after firing

1

10

100

20 40 70

O2/TMA ratio

Se

ff (

cm

/s)

• Low Seff is achieved (< 8 cm/s)

• Controllable by deposition

window

• Stacks of AlOx/SiO2 or AlOx/TiO2

have also yielded excellent Seff

• No post-deposition annealing

required with Tdep > 440°C

Film Composition (O/Al ratio)

• Determined using EDX with a

sapphire standard

• Film composition measured after

deposition and after firing

• Films feature over-stoichiometric

ratio suggesting possible

incorporation of OH groups [1,2]

1.45

1.50

1.55

1.60

1.65

1.70

1.75

30 50 70

O/A

l R

ati

o

O/Al ratio in stoichiometric Al2O3

O2/TMA ratio

[1] V. Verlaan, L. R. J. G. van den Elzen, G. Dingemans, M. C.

M. van de Sanden, and W. M. M. Kessels, physica status

solidi (c), NA (2010).

[2] V. Naumann, M. Otto, R. B. Wehrspohn, and C. Hagendorf,

Journal of Vacuum Science & Technology A: Vacuum,

Surfaces, and Films, 30, 04D106 (2012).

11

Interfacial layer

(likely an aluminum silicate layer)

Si AlOx

AlOx

Si

HR-TEM of c-Si/AlOx Interface


















Multi-Layer Passivating ARC Stacks

Thin passivation layer

c-Si

PECVD SiNx 78 nm

Provides passivation and single layer

ARC (SLARC) c-Si

First ARC layer

Second ARC layer



c-Si

PECVD SiNx 78 nm


ARC (SLARC) c-Si

First ARC layer

Second ARC layer

APCVD enables three films in one

process run by utilization of

sequential CVD injection chambers

Optical Properties/Microstructure of TiO2

Optical Coating

c-Si


TiO2

c-Si

Optical Coating

c-Si

• Effective Medium Approximation used by B. Richards to model TiO2

roughness using 0.5 as fraction of air/TiO2 [1]

• Very effective method to extract complex refractive index


[1] B. S. Richards, Solar Energy Materials and Solar

Cells 79 (3), 369 (2003)

TiO2

c-Si

EMA

• Effective Medium Approximation used by B. Richards to model TiO2

roughness using 0.5 as fraction of air/TiO2 [1]

• Very effective method to extract complex refractive index

• However, assumption of 0.5 fraction of air/TiO2 seems arbitrary

• We found using this fraction as a variable allowed for better fitting of

ellipsometry data


[1] B. S. Richards, Solar Energy Materials and Solar

Cells 79 (3), 369 (2003)

TiO2

c-Si

EMA


0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.00

0.10

0.20

0.30

0.40

0.50

0.60

250 300 350 400 450 500 550 600

Ra

tio

of R

ou

gh

/De

nse

La

ye

r T

hic

kn

ess

EM

A F

ractio

n

Deposition Temperature (°C)

Ratio of Rough/Dense Layer Thickness EMA Fraction

Films deposited at 250°C

didn’t show the characteristic

roughness layer and n

values suggest amorphous

TiO2 (confirmed by HR-TEM)


• TEM cross-sectional images verify this proposed

modification to the standard TiO2 optical model

250°C, EMA = 0

Amorphous 570°C, EMA = 0.57

Polycrystalline (Anatase)

400°C, EMA = 0.44

Polycrystalline (Anatase)

High and Low Index TiO2 Films



c-Si

SiNx 78 nm


ARC (SLARC) c-Si

First ARC layer

Second ARC layer


AlOx

20 nm c-Si SiNx

78 nm c-Si

SiO2

10 nm c-Si SiO2

20 nm c-Si

Amorphous TiO2 Anatase TiO2

(1) (2)

(3) (4)

AlOx

20 nm c-Si (5)

SiO2

10 nm c-Si (6)

Proprietary APCVD DLARC Stack


0

2

4

6

8

10

12

14

16

18R

(%

)

400 500 600 700 800 900 1000 1100

λ (nm)

Y SiNx 78 nm AlOx 20 nm - TiO2 20 nm - EMA 5 nm - a-TiO2 35 nm

(1) SiNx 78 nm

(2) AlOx 20 nm + TiO2 DLARC

(3) SiO2 20 nm + TiO2 DLARC

(4) SiO2 10 nm + TiO2 DLARC

(6) SiO2 10 nm + Proprietary DLARC

(5) AlOx 20 nm + Proprietary DLARC


















Emitter Formation for P-Type Wafers

• Good uniformity for film thickness and dopant

concentration

• Good control and repeatability of Rsheet and

dopant depth profile, including limiting surface

concentration of P, in the case of p-type wafers

(much improved over H3PO4 in-line doping)

• Low cost and high throughput!

p-Si

p-Si

PS

G

p-Si

PS

G

n+ p-Si n+

PSG Deposition (phosphosilicate glass)

Diffusion PSG Removal and

Edge Isolation


• Four groups of solar cells fabricated with ≈60 Ω/☐

emitters + one 80 Ω/☐ group added later [1]

• Standard screen-printed Al-BSF cell format

Edge isolation and PSG-removal

PECVD SiNx

Screen printing and co-firing of front and rear contacts

6 in. CZ Wafers (2 Ω-cm)

Alkaline texture

HCl/HF pre-cleaning

Group 1

PSG (13.0 wt.% P)

SiO2 cap

Diffusion (905°C)

59±2 Ω/☐

Group 2

PSG (14.5 wt.% P)

SiO2 cap

Diffusion (890°C)

58±2 Ω/☐

Group 3

PSG (15.7 wt.% P)

SiO2 cap

Diffusion (870°C)

58±2 Ω/☐

Group 4

PSG (16.7 wt.% P)

SiO2 cap

Diffusion (855°C)

64±1 Ω/☐

[1] K.O. Davis et al., physica status solidi (RRL) -

Rapid Research Letter, (2013).


1E+16

1E+17

1E+18

1E+19

1E+20

1E+21

0 100 200 300 400 500 600 700 800

P C

on

cen

tra

tion

(cm

-3)

Depth (nm)

Group 1 Group 2 Group 3 Group 4

1E+20

2E+20

3E+20

4E+20

5E+20

6E+20

7E+20

0 20 40 60 80

Group 1 Group 2 Group 3 Group 4




• Rsheet, depth profiles and solar cell I-V data

• Most recent solar cell run has improved

upon these results



Emitter Formation for N-Type Wafers

• Avoids issues associated with BBr3 doping

• Additionally, can be used to form BSF for p-type

wafers

• Simultaneous co-diffusion possible with PSG/BSG

“sandwich” to form both emitter and BSF with two

depositions and a single drive-in step

• Again, low cost and high throughput!

n-Si

n-Si

BS

G

n-Si

BS

G

p+ n-Si p+

BSG Deposition (borosilicate glass)

Diffusion BSG Removal and

Edge Isolation

Conclusions and Future Work

• Conclusions – Flexible platform with demonstrated ability to provide

passivation, optical coatings, capping layers and act as a solid dopant source for subsequent diffusion or co-diffusion

• Future Work – Improve fundamental understanding of short-range

atomic coordination in AlOx films and Qf

– Also, improve understanding of the influence of H content on Dit for AlOx passivation

– Continue development on BSG and simultaneous co-diffusion using PSG/BSG films (“sandwich”)

c-Si Metrology Challenges

• Near-Term (1-3 Years) – Improving methods of turning data into useful information

– Better methods of quantifying light trapping enhancements (not the same thing as reflectance)

– Decoupling surface and bulk recombination

– Correlating stress/strain to cracks

• Long-Term (4+ Years) – Quantifying recombination in novel wafer/cell formats

– Low cost methods of characterizing bulk and surface impurities/contaminants

– Predicting reliability/durability issues upstream by integrating metrology data with predictive models

Feedstock-Wafering Challenges

• Near-Term (1-3 Years) – Scaling up of n-type wafer production (to fully realize

benefits)

– Improving diamond wire saw technology to enable thinner and thinner wafers

– Recycling of kerf fines

• Long-Term (4+ Years) – Cell and module process integration for thin kerfless

wafers

– Adoption of lower cost and lower energy consumption synthesis routes for polysilicon

– Faster ways to validate new feedstock and wafering technologies to investors in terms of performance, reliability, etc.

Documents

APCVD Oxide Films for c-Si Solar Cells. APCV… · APCVD Oxide Films for c-Si Solar Cells ... (co nfirme d b y F ra u nho fe r ISE CalLab ) ... G. Dingemans and W. M. M. Kessels,