fv4

57Behind photovoltaics

c-Si industrial cell process: Whole value chain

Today’s c-Si cell process


Firma Elkem, Norwegen

- Elkem produced ca. 49% of the world volume 200 000 t per year for

- Aluminium-Industry- Chemistry (Silicone)- Semiconductor-Industry

- PV requiered 2008 ca. 50000 t per year

- SiO2 (s)+ 2C(s) Si(l) + 2CO

- Purity: 99%

Industrial process – MG-Si


Today’s c-Si industrial cell process: Whole value chain


Required purities:

- for Elektronic-Industry: 98 % 10-13 %- for Photovoltaic: 98 % 10-9 %

Process:- 1. Destillation of SiHCl3- 2. Deposition of Si from gas phase

1. Si(s) + 3HCl(g) SiHCl3(g)+ H2 (g) + heat2. SiHCl3(g) + H2(g) Si(s) + 3HCl(g)



Industrial process: Crystallization

Mono crystalline Multicrystalline

Crystallisation

Boron doped SiOxygen

Metal impurities e.g. Fei


240 kg

Industrial process : crystallization


Industrial process: sawing


- Max. Drahtgeschwindigkeit: 15 m/s - Drahtdurchmesser: 0,14 mm, - Länge: 800 km

Industrielle Maschine

Industrial process: Slicing


How is the costs division for material, cell and module?



Typical c-Si industrial solar cell

Properties

- P-type Si bulk (ca. 0.5 - 3 Ωcm and 200 µm)

- Texture (KOH+IPA or isotexture)

- Homogeneous P- emitter (40-60 Ω/sq)

- Al-BSF (depth 5 -10 µm)

- Front side PECVD SiNx ARC (73 nm/n=2.1)

- Screen printed front Ag-contact

- Full Al rear contact + AgAl pads

n+

p-type Si

Alp+

Ag

AgAl

Today’s c-Si industrial cell process


Texture and wafer cleaning

POCl3-diffusion

PECVD SiNx

Co-firing

Edge isolation

Screen printing

p-type mc-Si (ca. 1 Ωcm) 220 µm



210 µm


POCl3-diffusion

PECVD SiNx

Co-firing

Edge isolation

Screen printing

p

mc-Si: acidic textureCz-Si: alkaline texture



POCl3-diffusion

PECVD SiNx

Co-firing

Edge isolation

Screen printing

n+

p 55 Ω/sq. on both sides




PECVD SiNx

Co-firing

Edge isolation

Screen printing

n+

p

one sided 70 nm


POCl3-diffusion



Co-firing

Edge isolation

Screen printing

n+

p

Ag

Al

Ag on frontAl on rear side

AgAl


POCl3-diffusion

PECVD SiNx



Co-firing

Edge isolation

n+

pp+

Ag

Al

metal belt furnace

AgAl


POCl3-diffusion

PECVD SiNx

Screen printing



Edge isolation

n+

p

Ag

Alp+

− Laser− Wet chemical treatment− Dicing saw

AgAl


POCl3-diffusion

PECVD SiNx

Co-firing

Screen printing



Single processes


210 µm


POCl3-diffusion

PECVD SiNx

Co-firing

Edge isolation

Screen printing

p

mc-Si: acidic textureCz-Si: alkaline texture

Texture, saw damage etch

Three ways to remove saw damage at ISC?


Alkaline etching (NaOH, KOH)

Actec etching and cleaning bench 400 500 600 700 800 900 1000 11000

10

20

30

40

50 NaOH-etch - 36.0%

Ref

lect

ance

, %

Wavelength, nm


SEM picture of an isotropic textured surface with a screen printed finger

400 500 600 700 800 900 1000 11000

10

20

30

40

50 NaOH-etch - 36.0% acidic etching - 25,4%

Ref

lect

ance

, %

Wavelength, nm

reflectance curves of wafers after etching. Large reductions due to texturing.

Isotexture (HF, HNO3, H2O)

RENA inline acidic texturing tool

standard texture takes ca. 5µm Si from each side

Is KOH+IPA even more effective?

HNO3: oxidises the surfaceHF: etches the oxideonly on as-cut surfaces!!!


Alkaline texture (KOH+IPA)

Licht

- The reproducibility of the results often not satisfactory - IPA is expensive and organic- Texture quite slow to be in-line compatible >>>

but ...

- Reduces the reflection on (100) Si- surfaces from 36% to 11% (400-1100 nm);

- Simple and cost effective texturisation with NaOH-IPA or KOH-IPA on (100) surfaces only



0

10

20

30

40

50

60

70

300 400 500 600 700 800 900 1000 1100 1200

Wellenlänge [nm]

Ref

lexi

on [%

]

keine Textur UKONx-Textur-2 KOH-IPA

How can you reduce the reflection even more? What other texturisation do you know?



- Mechanical abrasion + damage etch!!

- Masking, photolithography, etching

- Remote Plasma Source (RPS) etch

- Reactive Ion Etching (RIE)


POCl3-diffusion

PECVD SiNx

Co-firing

Edge isolation

Screen printing

n+

p

55 Ω/sq. on both sides


Diffusion


BBr3-diffusion

principle of BBr3 diffusion and a loaded quartz boat

>> standard 60Ohm/sq diffusion is done at 940 °°°°C for 40min (2h 38 minutes total time)

N(x,t)=Ns erfc(x/2sqtr(Dt))0,0 0,2 0,4 0,6

1017

1018

1019

1020

B-emitter profiles (BBr3-diffusion)

2 step oxidation (90 Ohm/sq) in-situ oxidation (90 Ohm/sq)

carr

ier

conc

entr

atio

n p

[cm

-3]

depth [µm]

4BBr 3+ 3O2 →→→→ 2B2O3 + 6Br 2

2B2O3 + 3Si → 4B + 3 SiO2

What other possibilities for diffusion do you know?

ECV measurements of different B-emitters


BBr3-diffusion: other methods

Precursor deposition by: screen printing, roller, spin-on, spray on, APCVD…

Diffusion in: in-line furnace, tube furnace…

+


N2, O2, POCl3

ECV measurements of different P-emitters processed at different temperatures and times

Principle of POCl3 diffusion and a loaded quartz boat

>> standard 55Ohm/sq diffusion is done at 850 °°°°C for 20 minutes (1h 40 minutes total time)

POCl3-diffusion

Why is the P-profile so different from B-profile?


PECVD SiNx

Co-firing

Edge isolation

Screen printing

n+

p one sided 70 nm


POCl3-diffusion

PECVD SiNx-deposition



NH3

400 500 600 700 800 900 1000 11000

5

10

15

20

25

30

35

40

45

50 NaOH-etch - 36.0% acidic etching - 25,4% NaOH-etch with SiN

x - 9,8%

isotexture witch SiNx - 6.8%

Ref

lect

ance

, %

Wavelength, nm

Standard SiN x is about 70nm thick resulting in blue colour

Graphite boat for PECVD SiNx deposition

Schematic drawing of PECVD SiNx reactor

What is SiNx doing to increase the efficiency?



surface Ref. [%]

NaOH-etched 36*

NaOH+SiNx 9.8

isotexture 25.4

Iso+SiNx 6.8

KOH+IPA 11

KOH+IPA+SiNx 4

*reflectance between 400 and 1100nm

Anti Reflection Coating (ARC)


Co-firing

Edge isolation

Screen printing

n+

p

Ag

Al

Ag on frontAl on rear side

AgAl


POCl3-diffusion

PECVD SiNx

Screen printing


Screen printing

To ensure good fillfactors:- Geometry of standard screen printed finger:

- Height: 20 µm- Width110-140 µm

- Amount of paste ~ 150 mg for 156x156 mm2 wafers

Baccini screen printer with improved camera system principle of screen printing

standard screen printed fingerHow can the shadowing be reduced?


Screen printing: other metallisation

Si

P-emitterFront contact

SiNxARC

Standard screen printing

+ easy contact formation

+ good fill factors (78%)

- high shading losses

Fine-line screen printing

Buried contact (BC)

+ easy contact formation

+ low shading losses

- poor fill factors

- stability problems

+ very low shading losses

+ very good fill factors (>80%)

- additional process steps

Angled Buried Contact (ABC)

+ NO shading losses

+ very good fill factors (>80%)

- additional process steps


Screen printing: other metallisation

plated Ni/Cu

silicon nitride

platedNi/Cu

pp

+

n+ +

n+

texture hydrogen passivation

Voc [mV]

Jsc [mA/cm 2]

FF [%]

ηηηη [%]

alkaline no 593 31.0 77.2 14.2

alkaline yes 602 32.1 77.4 15.0

V-textured yes 628 36.3 76.8 17.5*

Roller-printing techniquefor finger metallisation

Buried-contact cell concept

Solar cell size: 10x10 cm2

Solar cell size: 12.5x12.5 cm2

* POLIX wafer; independently confirmed by FhG-ISE, Freiburg, Germany

Material: mc-Si (Baysix), as-cut thickness 330 µm

texture printing Voc [mV]

Jsc [mA/cm 2]

FF [%]

ηηηη [%]

V-textured screen 614 34.2 76.2 16.0 V-textured roller 612 35.4 75.4 16.3

M. McCann, B. Raabe, W. Jooss, R. Kopecek and P. Fath, 18.1% Efficiency for a Large Area, Multi-Crystalline Silicon Solar Cell, IEEE 4th WorldConference on Photovoltaic Energy Conversion, Hawaii, USA (2006)


Co-firing

Edge isolation

n+

pp+

Ag

Al

Metal belt furnace

AgAl


POCl3-diffusion

PECVD SiNx

Screen printing

Belt-furnace firing


Belt-furnace firing

6 zone belt furnace for fast firing

Placement of solar cell on belt

Temperature on the cell during process Firing temperature dependent on area and thickness of solar cell

How does the front contact look after firing?

What contains the Ag-metal paste?


Belt-furnace firing: Front contact


Belt-furnace firing: front contact

Cross section of a simple metal/Si interface (top) and a more realistic picture of glass frit containing metal Ag paste/Si interface for an industrial solar cell (bottom).

Ag glass

Ag crystals tunneling

− Separation of Ag-crystals by glass layer

− Contact of Ag to emitter only through few points

− Tunneling of carriers through glass

− Contact resistance?

How does the BSF look after firing?


Edge isolation

n+

p

Ag

Alp+

AgAl


POCl3-diffusion

PECVD SiNx

Co-firing

Screen printing

Edge isolation

How can the edges be isolated?

− Laser− Wet chemical treatment− Dicing saw

99What is behind a solar cell?, June 2013

Edge isolation

dicing saw:slow but very good isolation

laser: fast and good, used in the industry

sand paper: good isolation, low yield if not done properly

wet chemical:fast and good, used in the industry !has to be done directly after P-diffusion!

these methods are used the most at isc

fill factors exceeding 79% were already processed

What are we talking about? c-Si solar cells

Semiconductor device consisting of a p-n junction and metalized regions that directly converts sunlight into electricity .

area 15.6x15.6 cm2

thickness 180 µm Ag, Al contacts 17-20 % efficiency 4-5 Wp

area 90x160 cm2

60 cells in series 250-300 Wp

5/20

Processing


p-n junction formation (diffusion)

Plasma enhanced chemical vapour deposition (passivation)

Metallization (screen printing)

Firing

Laser edge isolation

Elkem (1904)Elkem (1904)Elkem (1904)Elkem (1904)

NorwayNorwayNorwayNorway

6/20

Reflexion:Reduced to 11% (400 nm–1100 nm)

Cell efficiency

Cell efficiency where

FF: fill factor (measure of quadrature of JV curve)JmaxPP and VmaxPP: maximum power point valuesJsc and Voc: short circuit current density and open circuit voltage

in

ocsc

P

VFFJ=ηocsc

PPPP

VJ

VJFF maxmax=

8/20

OpticalReflexion, shadingTransmission

Electrical: (1) recombination, (2) ohmic(1) Radiative, Auger & Rec. via trapsSurfaces, emitter, bulk & space charge region

(2) Resistance of bulk metals (Ag fingers, Al area), SiContact resistance: metal to Si (Ag-Si , Al-Si)

Loss mechanismsLoss mechanisms

Drawing from: H. Mäckel, 2004

9/20

Resulting contact

Ag finger (Ag bulk)

Ag bulkAg crystallites, glass, nano particles

11/20


Trend in PV technology


Thinner wafers

- 400 µm in 1990, 180 µm in 2007

- aim: 150 µm in 2010

Larger wafers

- 1990: 100 cm2

- today: 225 cm2

Use of SoG-Si

- chemical purification

- metallurgical purification


2002 2004 2006 2008 20100

100

200

300

400

?

waf

er th

ickn

ess

[µm

]

What happens with the efficiency when <d?


n+

p-type Si

Alp+

Ag

industrial solar cell

Thinner and larger solar cells

Main losses on the rear side

Improved passivation: +25-30 mV / ca.+2

mA/cm2

250 200 150 100 5014.0

14.5

15.0

15.5

16.0

16.5

sola

r ce

ll ef

ficie

ncy

solar cell thickness [µm]

standard process improved rear side passivation



bow

relaxationpassivationopen rear contact



front junction bifacial device with BSF

main advantages

- elimination of bow- improved rear side passivation- less paste and printing steps used- increased power output - applicable for p and n-type Si- simplified module interconnection



What is the advantage of bi-facial modules??



30°

white board

Conditions

- module embedded in black frame (1x0.5) m2

- placed on white board

- inclined by 30° and facing to the South

- measured in bifacial and monofacial (rear side

covered) arrangement

10 12 14 16 180,0

0,2

0,4

0,6

0,8

1,0

bifacial monofacial

Pm

pp [a

rbitr

ary

units

]

time during one day in August [h]10 12 14 16 18

20

30

40

50

60

70

80

average gain in P of 33%

time during one day in August [h]

gain

in P

[%]

Outdoors measurement on roof of PV lab at UKON

(southern Germany)

What efficiency can be reached with both sided cont acted cells?



Why other processes?

1 10 100 100013

14

15

16

17

18

19

20

effic

ienc

y [%

]

minority carrier lifetime [µs]

- standard cell

- standard rear

- selective emitter

- selective emitter- B-BSF rear side

Efficiencies >19% possible with

both sided contacted cells

To exceed 20% rear contact solar

cells needed

Do you know other trends?



rear contact solar cells

- MWT, MWA, EWT, IBC

improvement of standard process

- Selective emitters- Rear passivated cells

n-type solar cells

- Al-rear emitter- B-front emitter

Thin film solar cells

- Thin Si absorbers- CdTe, CIGS, GaAs

n



Improvement of standard

process: front

Improvement of standard

process: rear

0,0 0,2 0,4 0,61018

1019

1020

profile of selective emitter30 nm SiN

x

with barrier (80 Ω/sq) lasered regions (19 Ω/sq)

carr

ier

conc

entr

atio

n n

[cm

-3]

- Selective emitters

- Improved passivation

- Innovative metallisation

- Selective BSF

- Open rear contact

- Improved

passivation

- Innovative

metallisation

What selective emitter methods do you know?



n-type solar cells

What kind of rear contact cells do you know?

Rear Al-emitter solar cell Front B-emitter solar cel l

mc-Si: 15.0%FZ-Si: 17.4%

mc-Si: 16.4%Cz-Si: 18.5%

Similar to p-type solar cell process Easy implementation into existing process lines

High quality, lowly doped material needed

Low quality material acceptable Bifacial character

Passivation of p+ surfaces? B-diffusion with mc-Si?

np+ (boron-emitter)

n+ (phosphorous-BSF)

nn+ (phosphorous-FSF)

p+ (Al-emitter)

Isofoton BOSCH



MWA MWA MWT EWTVoc[mV] 611 614 612 591

Jsc[mA/cm2] 37.2 35.9 37.2 37.4FF [%] 77.2 75.5 75.8 75.1h [%] 17.5 16.6 17.2 16.6

Area [cm2] 5x5 10x10 5x5 5x5

Rear contact solar cells

Metallisation Wrap Through (MWT)

MetallisationWrap Around (MWA)

n+

p+

n+ +

n+ + n

+

n+ +

p+ n

+

p+ n

+ +

Emitter Wrap Through (EWT)

What is the large area cell with highest efficiency ?



Rear contact solar cells:

IBC cell

Efficiency of 23% was reached!!



What thin film technologies do you know?

Efficiency of 22% was

reached


HIT solar cell


Thin film solar cells:

- CdTe (First Solar)

- CIS, CIGS (Würth)

- a-Si

- micromorph Si

- crystalline Si

UMG - substrate


Behind photovoltaics

ISC building

120


ISC building

121


Well known research institutes


Well known research institutes

technlogy transfer highest efficiency


ZEBRA cell: history and roadmap

17

18

19

20

17.6%

21

22

2010 2011 2012 2013 2014 2015 2016

21%

23

24

ZEBRA gen1

ZEBRA gen2

22.5%

23.5%pilot line and technology transfer

diffusionsscreen printed contacts300Wp 60 cell modules

diffusionsscreen printed contacts 315Wp 60 cell modules

ion-implantationscreen printed contacts330Wp 60 cell modules

2012 EU project moderN-type: Eurotron modules2013 BMU project MetalTopp: p+ metallisation2013 EU project HERCULES: ion-implantation

19.7%


CoO of Module Technologies

(e.g. Sunpower IBC, Panasonic HIT, ..)

ZEBRA-IBC Module(expected CoO)

2013 Standard Module

High efficiency Module

MWT Module

Data from: Pvinsights, Photon, ITRPV 2013, Eurotron, own calculations

High efficiencyHigh cost

High efficiencyLow cost

Sta

ndar

d ef

ficie

ncy

Low cost

sales price range (Q1/2013)

avg. estimated CoO

ZEBRA

fv4

Documents

deposition of si

type si bulk

acidic textureczsi

industry pv

sihcl3g h2g sis

rear sideagaltexture

norwegen elkem

sq al