06 weeber ecn

Beyond 20% efficiency n-type solar cells

Arthur Weeber

www.ecn.nl

Thanks to:

Ingrid Romijn, Teun Burgers, Bart Geerligs, Anna Carr, Astrid Gutjahr, Desislava Saynova, Jan Bultman, Benoit Heurtault, Bas van Aken, Mark Jansen, Nicolas Guillevin, Loek Berkeveld, John Anker and Martien Koppes

Lang Fang, Xiong Jingfeng, Li Gaofei, Xu Zhuo, Wang Lang Fang, Xiong Jingfeng, Li Gaofei, Xu Zhuo, Wang Hongfang, Hu Zhiyang, Zhao Wenchao, Wang Jianming, Wang Ziqian, Chen Yingle, Shen Yanlong, Chen Jianhui, Yu Bo, Tian Shuquan

Peter Venema, Ard Vlooswijk

2 25-4-2012

Why n-type?

• Absence of boron-oxygen complexes- No light induced degradation in n-type Cz material

• Less sensitive to common impurities- Higher diffusion length in n-type

3 25-4-2012

Why n-type?

• Absence of boron-oxygen complexes- No light induced degradation in n-type Cz material

• Less sensitive to common impurities- Higher diffusion length in n-type

Potential for higher efficiencies - Advanced structures:

- IBC cells (Sunpower) - HIT cells (Sanyo)- MWT (different designs)

- More conventional structures: - Panda cells (Yingli)

4 25-4-2012

n-type versus p-type

n-type:Boron diffused emitterPhosphorous diffused BSFOpen rear side metallization & passivating coating

p-type:Phosphorous diffused emitterAluminum alloyed BSFFull aluminum rear metallization & solder pads

5 25-4-2012

n-type

n+ BSF

p+ emitter

Contact grid Passivating coating

p-type

p+ BSF

n+ emitter

Solder pads Full metal rear surface

ECN’s n-pasha cell concept• Open rear metallization

- Bifacial cell- Suitable for very thin wafers – no bowing- No additional solder pads required to interconnect cells

- Only 2 print steps necessary

6 25-4-2012

n-type

n+ BSF

p+ emitter

Contact grid Passivating coating

ECN’s n-pasha cell concept• Open rear metallization

- Bifacial cell- Suitable for very thin wafers – no bowing- No additional solder pads required to interconnect cells

- Only 2 print steps necessary

• Industrial process

7 25-4-2012

• Industrial process- Panda cells in production at Yingli Solar

Recent progress on n-pasha

Stable processing for different wafer suppliers

Reduced front side metallization

Improved rear surface passivation Improved rear surface passivation

8 25-4-2012

Stable processing for different wafer suppliers

9 25-4-2012

• Wafer material from different commercial suppliers• Base resistivity between 1.5 - 10 Ωcm

Reduced front side metallization

Screen print~ 100 microns wide

Stencil print~ 60 microns wide

Screen printed fingers stencil printed fingers

• Finer lines less shading: higher Jsc

• Less contact area reduced contact recombination: higher Voc

• Higher aspect ratio similar line resistance: similar FF• Efficiency gain: 0.3 – 0.4 % absolute

10 25-4-2012

100 microns

Improved rear surface passivation• Lighter doped BSF

- Reduced recombination- Reduced free carrier absorption

• Adjusted passivating layer- Improved effective surface passivation

11 25-4-2012

n-type Si wafer

n++ BSF

p+ emitter

Passivating coating

n-type Si wafer

n+ BSF

Passivating coating

p+ emitter

Improved rear surface passivationIm

plie

d V

oc [V

]

0.652

0.656

0.66

0.664

Lighter doped BSF

12 25-4-2012

Impl

ied

Voc

[V]

BSF - 1 BSF - 2 BSF - 30.64

0.644

0.648

BSF-3: • Less doping: higher Rsheet• Reduced recombination: higher Implied Voc

Increase Rsheet

Improved rear surface passivationIm

plie

d V

oc [V

]

0.652

0.656

0.66

0.664

0.15

0.2

0.25

0.3

0.35

Fro

nt

refl

ect

ion

BSF - 1

BSF - 2

BSF - 3

Lighter doped BSF BSF with less absorption

13 25-4-2012

Impl

ied

Voc

[V]

BSF - 1 BSF - 2 BSF - 30.64

0.644

0.648

0

0.05

0.1

0.4 0.6 0.8 1 1.2 1.4

Fro

nt

refl

ect

ion

Wavelength [microm]

BSF-3: • Less doping: higher Rsheet• Reduced recombination: higher Implied Voc• Reduced absorption: Higher escape reflection at λ > 1000 nm

Increase Rsheet

Improved rear surface passivation on cells

• Jsc and Voc values of cells with 2 different BSFs

• Different symbols: n-type Cz material from different

Rear side passivating layer is kept the same

14 25-4-2012

Cz material from different suppliers

• For all materials cells with improved BSF show a similar improvement of >1% in both Jsc and Voc

Cell resultsBalance all requirements:• Good passivation and low absorption: high Voc, Jsc

• Good contact resistance with rear metallization: high FF• Sufficient lateral conduction: high FF

15 25-4-2012

Cell resultsBalance all requirements:• Good passivation and low absorption: high Voc, Jsc

• Good contact resistance with rear metallization: high FF• Sufficient lateral conduction: high FFExperimental results: η gain of 0.4% absolute

Isc [A] Jsc Voc [V] FF [-] η [%]

16 25-4-2012

Isc [A] Jsc[mA/cm2]

Voc [V] FF [-] η [%]

Group 1 (standard BSF with reduced front metallization)

avg 9.26 38.7 0.640 0.784 19.4

max 9.27 38.8 0.640 0.787 19.5

Group 2 (improved BSF with reduced front metallization)

avg 9.38 39.2 0.648 0.78 19.8

max 9.40 39.3 0.649 0.783 20.0

• “Unit cell” design

- Easy optimization and up-scaling

- Up to 2.5% less shading

- Lower Rseries possible

ECN n-MWT cell technology

17 25-4-2012

• “Unit cell” design

- Easy optimization and up-scaling

- Up to 2.5% less shading

- Lower Rseries possible

ECN n-MWT cell technology

• Simple and industrial process steps

- Very similar to our industrial processfor n-pasha

- LASER drilling of via-holes

18 25-4-2012

• Same material

• Same ‘19.4%’ processing

n-MWT – front side n-MWT – rear side

n-MWT versus n-pasha – cell performance

• n-MWT grid design: “H-pattern look-

alike”

- Suitable for direct comparison

- Narrower busbars

- ≈2.5% less metal coverage

n-MWT – front side

n-pasha – rear siden-pasha – front side

n-MWT – rear side

19 25-4-2012

t = 200µm – area=239cm 2 MaterialJsc

[mA/cm 2]Voc

[mV]FF[%]

ηηηη[%]

Rse[Ω]

Average (4 cells)

n-pasha Cz-Si (ρ≈5 Ω.cm) 38.4 638 78.9 19.3* 4.6E-3

n-MWT Cz-Si (ρ≈5 Ω.cm) 39.5 644 77.1 19.6* 5.8E-3

Best cell efficiency


n-MWT Cz-Si (ρ≈5 Ω.cm) 39.6 644 77.2 19.7* 5.7E-3


* measured at ECN includes spectral mismatch correction

• 1%rel. Voc gain (+6 mV)- Reduced front metal coverage: less emitter contact recombination

20 25-4-2012


[mA/cm 2]Voc

[mV]FF[%]

ηηηη[%]

Rse[Ω]

Average (4 cells)


n-MWT Cz-Si (ρ≈5 Ω.cm) 39.5 644 77.1 19.6* 5.8E-3



n-MWT Cz-Si (ρ≈5 Ω.cm) 39.6 644 77.2 19.7* 5.7E-3




• 2.8% rel. Jsc gain (+1.1 mA/cm2)- Reduced front metal coverage: less shading loss

21 25-4-2012


[mA/cm 2]Voc

[mV]FF[%]

ηηηη[%]

Rse[Ω]

Average (4 cells)


n-MWT Cz-Si (ρ≈5 Ω.cm) 39.5 644 77.1 19.6* 5.8E-3



n-MWT Cz-Si (ρ≈5 Ω.cm) 39.6 644 77.2 19.7* 5.7E-3





• 1.8% abs. FF loss: Rseries increase

22 25-4-2012


[mA/cm 2]Voc

[mV]FF[%]

ηηηη[%]

Rse[Ω]

Average (4 cells)


n-MWT Cz-Si (ρ≈5 Ω.cm) 39.5 644 77.1 19.6* 5.8E-3



n-MWT Cz-Si (ρ≈5 Ω.cm) 39.6 644 77.2 19.7* 5.7E-3





• 1.8% abs. FF loss: Rseries increase• 0.3% absolute efficiency gain over n-pasha

23 25-4-2012

• Single step curing during lamination: CA & encapsulant

• Less wafer stress:no tab-around, no soldering

ECN MWT module technology (applied to n-type Si)

no tab-around, no soldering

• Higher packing density of cells

• Low FF loss

• But, no bifacial application possible (using foils)

Interconnection foilCA

EVA

EVA

Cell

CA: conductive adhesive

24 25-4-2012

Interconnection foilCA

Cell

Pmax [W]Cell-to-module

FF loss

n-MWT 273* 0.8%

n-pasha 265* 3%

* Measured at ECN

n-MWT versus n-pasha – Module performance

n-MWT module

First 60-cell n-MWT module made and compared to n-pasha module:• Very low cell-to-module FF loss • 3% more power

Class A multiflash tester (IEC60904-9 ) n-MWT module

25 25-4-2012

Summary and conclusions• Improvements in n-pasha cell process:

- Better process stability- Reduced front metallization (gain 0.3 – 0.4% absolute)- Improved BSF (gain 0.4% absolute)

26 25-4-2012



• 20% efficiency obtained•- Using both reduced front metallization and improved BSF- Fully industrial n-pasha process- Ready for industrial implementation

27 25-4-2012



• 20% efficiency obtained•- Using both reduced front metallization and improved BSF- Fully industrial n-pasha process- Ready for industrial implementation

• Beyond 20%- Improve emitter and passivation- Further reduce front and rear metallization- 0.3% absolute gain for MWT cell design (19.7% reached so far)

28 25-4-2012

Thank you for your attention !

Bifacial n-pasha cell process with• 20% efficiencyBeyond 20% possible• Improved junctions, passivation

and reduced metallization•

29 25-4-2012

and reduced metallization• MWT

www.npv-workshop.com

Presentations & Public Speaking

06 weeber ecn