wafer cleaning and etching in high efficiency n- type...

Wafer cleaning and etching in high efficiency n-

type solar cell processing

Dr. Florian Buchholz

[email protected]; +49 7531 36 183 553

Florian Buchholz, June 9th 2017

Motivation

2

From lab to fab: n-PERT solar cells (BiSoN)

Advantages:

high bificaility (>90%), eta potential > 21%, no degradation

Only standard equipment required (wet etching, P / B tube

diffusion (low pressure), PECVD, screen priting)


Motivation

3

From lab to fab: n-PERT solar cells (BiSoN)

Industrial feasible cleaning before diffusion:

Lab standard (POR):

Not feasible


Motivation

4

Industrialization of n-PERT solar cells (BiSoN)

Issues with rear side contact formation, increasing likelyhood


Motivation

5


Laser edge isolation causes 0.2 – 0.4%abs efficiency losses


Motivation

6


Open points

highly efficient industrially feasible cleaning methods required

problems with contact formation on rear side

huge losses due to laser edge isolation (0.2 – 0.4%abs)


Content

7

Cleaning before diffusions

Alkaline polishing of diamond wire sawed wafers

Wet chemical edge isolation

Summary and conclusion

Florian Buchholz, June 9th 2017 8






Model system for cleaning optimization

Thermal oxidation more sensitive than actual production

process

Cleaning before diffusion

9

Boron diffusion of n-type wafers

cleaning after texturing cleaning after texturing

Thermal oxidation of p-type wafers


Ozone based cleaning known from semiconductor fabrication


10

Pretest x=

H2O

HCl (0.1%)

HCl/HF (0.1%/0.05%)

HF (0.05%)

HCl/HF dip only


Results pre-test on KOH/IPA textured wafers


Simple HF-base system chosen for further testing


Optimization for different surfaces

Design of Experiment (DoE)

Diamond and slurry sawed wafers

(of different suppliers)

saw damage etching and texturing (RENA monoTEX)

Concentration range: 0.01 – 1%

Immersion time 2 – 10 min


12


Modelling outcome

Cleaning after SDE (wafers from different suppliers were used)


13

Optimum: c(HF) = 0.02 – 0.035%, longer times slightly favored


Modelling outcome

Cleaning after texturing (RENA monoTEX)


14

Optimum: c(HF) = 0.04 – 0.1%, longer times clearly favored


Application in cell processing

Cleaning after SDE and

after alkaline texturing

No significant losses

Best cell so far: 20.8% cleaned with HF/O3

Fraunhofer ISE certified


15


Problem: Contact formation on rear side fails

Rear side surface too flat? Doping profile variation?

Varied in the experiment:

Wafer supplier

Alkaline polishing (SDE) etching depth: 3 – 10 µm

45 and 82 Ohm/sq diffusion

Alkaline polishing of diamond wire

sawed wafers

17


FF


sawed wafers

18


Eta


sawed wafers

19


Surface roughness measurement


sawed wafers

20

State-of-the-art material only requires 3 µm saw damage removal

For high sheet resistance diffusion FF heavily relates to surface roughness


Optimized edge isolation for (n-)PERT cells

using standard equipment

slightly modified process


sawed wafers

22


Optimized edge isolation for (n-)PERT cells


sawed wafers

23

no more losses due to edge isolation

overall gain in eta has been observed


Content

25

Open points

highly efficient industrially feasible cleaning methods required

problems with contact formation on rear side

huge losses due to laser edge isolation

Solved:

low concentration HF/O3 has been optimized and tested

low rear side etching and lower sheet resistance on BS

wet edge isolation has been developed

Process transfer will start in Q4 2017

Thank you for your attention

and special thanks to the ISC Konstanz BiSoN team.

Dr. Florian Buchholz

[email protected]; +49 7531 36 183 553

wafer cleaning and etching in high efficiency n- type...

Documents