Understanding and implementing high quality contacts to advanced emitters

for high efficiency solar cells

Xjet Solar LTD, Israel

Aba Ebong, John Renshaw, Ian Cooper, Brian Rounsaville, Keith Tate & Ajeet Rohatgi

University Center of Excellence for Photovoltaics Research and Education School of Electrical and Computer Engineering

Georgia Institute of Technology

1

April 14, 2010

Ag

2-ways to improve current solar cell efficiency

Keep full Al back and improve front contact on high sheet resistance emitter

P-base : 180μm thick

n+ emitter : 100 ohms/sq

BSF

Ag

SP Al

Fine and tall finger

Use of high Sheet rho

emitterSiN AR

2

3

Importance of line width for high sheet resistance emitters (100 Ω/sq)

Strategies to Implement Contacts to Advanced Emitters

• Selective-Emitter formation

• Ag paste and firing optimization

PECVD SiNx

p-Silicon Substrate

p+ Al BSF

Gridlinen+ emitter 100 Ω /

PECVD SiNxn+ emitter 100 Ω /

n++

p-Silicon Substrate

Gridline

p+ Al BSF

≤40 Ω/

4

Factors influencing the efficiency of a solar cell

5

η (Voc, Jsc, FF)

Front Printing

Front Ag paste Co-firing

Ag particle size

% Glass frit & size

rheology

Screen opening

Peak temp

Back printing

Ramp up & down rates

uniformity

Finger width & height

Voc

Jsc

Joe, Voc

Emitter formation(POCl3, ILD, SP) Sheet resistance

(45, 60, 100-OPS)Ns

Jsc,

Voc

Jsc,

Voc

ρm

Junction depth

Rsh

Jo2

FF

Rsh, Rsρm

Ns Ag Crystallite density

ρm

FF

Additives

Glass layer thickness

Al thickness

Jsc,

Voc

Jsc,FF

FF

ρc

Print settings

Joe

Metal semiconductor contacts

6

↑ sheet resistance emitter with ↓Ns(Ns<1x1019 cm-3)

↓ sheet resistance emitter with ↑Ns(Ns≥1x1019 cm-3)

Ef

High ND - Field Emission

Depletion-type contacts to n-type substrate with decreasing doping concentration

Ohmic

Ef

Intermediate ND - Thermionic-field Emission

In between Schottky &Ohmic

Rapid contact co-firing profiles

7

Effect of sinter dwell time on contact resistance, glass layer thickness and Ag penetration

• Tailored Ag contacts through control of sinter dwell time• Thickness of glass at Ag/Si interface directly proportional to Ag specific contact resistance

Cooper, et al. (2010) IEEE Electron Device Letters [accepted for publication]

A. 5.2 second dwell time; B. 1.2 second dwell time • Arrows show trend of Ag and O (glass) penetration as dwell time shortens •Emitter penetration by Ag and glass frit at contact is less aggressive at shorter dwell times • Less aggressive etching of the emitter leads to lower, more controlled Ag contact resistance and less shunting

8

Effect of sinter dwell time on Al BSF, long wavelength IQE, Series resistance and FF of high sheet resistance

0102030405060708090

100

800 850 900 950 1000 1050 1100 1150 1200

Wavelength (nm)

IQE

(%)

5

4

3

12.27

1.03 0.99 0.84 0.69

0.243

0.7760.7660.759 0.763

0

2

4

6

8

10

12

14

1 2 3 4 5Ramp rate (oC/s)

Serie

s R

esis

tanc

e (o

hm-c

m2)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Fill

fact

orRs FF

9

Large Area 18.4%-Efficient Cell with Screen-Printed and Co-Fired Contacts on CZ Si

POCl3 Diffusion

PECVD SiNx AR

Al Screen-Printing

Ag Screen-Printing

Belt Co-Firing

Edge Isolation

18.4% Textured CZ cell I-V data

VOC (mV) 627

JSC(mA/cm2) 37.0

FF 0.797

Eff. (%) 18.4

p-Si

Al-BSFAl contact

n+ emitter

SiNx AR coatingAg grid

0

1

2

3

4

5

6

0 0.2 0.4 0.6

Voltage (V)

Cur

rent

(A)

TexturingSiO2

10

11

SP versus ink jetted finger

Jetted grid line width ~ 40 μm

SP grid line widths ~ 100 μm

Thinner grid lines increases light absorption

Requirements for high quality ink-jetted contacts to high sheet resistance emitters

Requirements for high-quality ink-jet contacts to high sheet emitters

• A regular distribution of many Ag crystallites of small depth ( ≤0.1 μm).

• intimate contacts between the seed layer and plated silver

• No excessive diffusion of Ag or other impurities in the p-n junction region.

Seed (40-μm) Seed + Ag LIP(64-μm width)& 25-μm tall

Number of fingers = 92

Thinner grid lines increases light absorption

12

How does LIP help sp/seeded contacts

Short time in LIP

13

As fired SP contact

Ef

Medium doping

Thermionic-field Emission

In between Schottky &Ohmic

Ef

High ND - Field EmissionOhmic

Long time plating

Large Area 18.7%-Efficient Cell with ink jetted fingers and Co-Fired Contacts on 95 Ω/sq emitter CZ Si

POCl3 Diffusion

PECVD SiNx AR

Al Screen-Printing

Ink Jetted fingers

Belt Co-Firing

Edge Isolation

I-V data

VOC (mV) 633

JSC(mA/cm2) 37.5

FF 0.787

Eff. (%) 18.7

p-Si

Al-BSFAl contact

n+ emitter

SiNx AR coatingInk jet + LIPAg grid

0

1

2

3

4

5

6

0 0.2 0.4 0.6

Voltage (V)

Cur

rent

(A)

Texturing

LIP

18.7% Textured CZ cellCell Area – 239 cm2

14

60 Ω Std SP Cell (18.39%)

15

95 Ω Xjet SP Cell (18.73%)

16

STD XJETfinger width 100 65

STD # fingers

74 82

Sheet Res 95.00 95.00FF: 0.784 0.785

Efficiency: 18.37 18.73Calc Rseries 1.001 0.783

Metal coverage

7.75% 6.11%

Jsc (mA/cm2) 37.04 37.70

16

Thank you