L ENEA e la ricerca di sistema elettrico: il fotovoltaico innovativo. Sviluppo di ossidi trasparenti e conduttivi mediante processo Sol-Gel

L’ENEA e la ricerca di sistema elettrico:

il fotovoltaico innovativo

UNIVERSITA’ NAPOLI FEDERICO II

Dipartimento di Ingegneria dei Materiali e della

Produzione

Sviluppo di ossidi trasparenti e conduttivi

mediante processo Sol-Gel

Thin film silicon solar cells

The front TCO layer influences

the performances of the device

Glass Glass ≈≈0.7mm0.7mm TCO TCO ≈≈0.5 0.5 µµmm p paa--SiSi≈≈10 10 nmnm i a i a--SiSi0.50.5µµmm + + ++ ++ + + ++ ++ ++ -- -- -- -- _-- - -E E n naa--SiSi≈≈15 15 nmnm Metal

Metal ContactContact

Glass Glass ≈≈0.7mm0.7mm Glass Glass ≈≈0.7mm0.7mm TCO TCO TCO ≈≈0.5 0.5 µµmm TCO ≈≈0.5 0.5 µµmm p paa--SiSi≈≈10 10 nmnm p paa--SiSi≈≈10 10 nmnm i a i a--SiSi0.50.5µµmm + + ++ ++ + + ++ ++ ++ -- -- -- -- _-- - -E E i a i a--SiSi0.50.5µµmm + + ++ ++ + + ++ ++ ++ -- -- -- -- _-- - -E E n nnaa--SiSi≈≈15 15 nmnm naa--SiSi≈≈15 15 nmnm Metal

MetalMetal ContactContact

Metal ContactContact

Until now, In

₂

O

₃

:Sn (ITO) and SnO

₂

:F were widely used

as front electrode thanks to their low resistivity.

However, indium is somewhat rare and really expensive,

while tin oxide shows low stability to the high

temperature. So, recently, doped zinc oxide (ZnO) has

been considered ideal alternative materials due to their

lower cost and wide availability.

ZnO via Sol-Gel

Low temperature process

Easy obtainment of films

Advantages:

High control of purity, composition and

microstructure of final product

Uniformity on large area (Dip-coating)

Low equipment cost

High Stoichiometry of ZnO

Drawbacks:

Difficulty to obtain films of suitable thickness

by a single dipping step

molecular precursors

solvent

wet gel

solvent

evaporation

solvent

extraction

aerogel

solvent

ev

apo

rat

ion

hydrolysis

po

ly

cond.

thin film xerogel

Sol-Gel technology

heat

heat

xerogel

ceramic

ceramic thin film

in acidic conditions (pH < 6), Zn

2+

(aq)

in neutral to slightly basic conditions, Zn(OH)

_2(s)

in basic conditions (pH>12), Zn(OH)

₄

2-(aq)

Main equilibria into aqueous solution of Zn

2+

2

(

)

2

(

)

(

)

(

)

2

2(

)

(

)

6

2(

)

2( )

2

-17

2( )

(

)

(

)

(

)

(

)

(

)

(

)

(

)

K=10

(

)

2

Kps=3.5 10

aq

aq

aq

aq

aq

aq

aq

s

s

aq

aq

Zn

H O

Zn OH

H

Zn OH

H O

Zn OH

H

Zn OH

Zn OH

Zn OH

Zn

OH

+

+

+

+

+

+

−

+

+

+

+

+

×

precursor

[Zn

_{] (M)}

_hydrolysis

solvent

deposition

film thickness for 1 step

(nm)

ZA anhy 0.3-1.3 stirring at 50°C for 60 min followed by filtration i-PrOH spin-coat. 84-437 MEA O’Brien et al. 2008 i-PrOH -dip-coat. dip-coat. -~40 ~20 five spins dip- or spin-coat. spin-coat.

36-247 (after multiple dipping)

ZAD 0.1 reflux at 80°C for 180 min

addition of LiOH⋅H₂O

ZAD 0.6 stirring at 60°C for 30 min

ZAD 0.05-1 reflux at 80°C for 120-180 min; stirring at 60°C for 120 min

ZAD 0.3-1.3 stirring at 50°C for 60 min

ZAD DEA 0.03-0.1 room temperature

Dutta et al. 2008

MEA Kim et al. 2005

MEA TEA Lactic acid Znaidi et al. 2003 MEA DEA Ohyama et al. 1996 -Spanhel and Anderson 1991

chelating

agent

Our approach

•

•

Precursor: Zinc Acetate Dihydrate:

Precursor:

Zn(CH

₃

COO)

₂

·2H

₂

O (ZAD)

•

•

Solvent: Ethanol Anhydrous: C

Solvent:

₂

H

₅

OH

(EtOH)

•

•

Additives: 1) Acetic Acid: CH

Additives:

₃

COOH (HAc)

•

2) Triethanolamine: N(C

₂

H

₄

OH)

₃

(TEA)

Whole process is at room temperature

(with TEA)

Our approach

Acetic Acid plays different roles: decreases the pH of the solution, influencing

both the hydrolysis and the solubility equilibria of ZAD; produces acetate ions

that act as complexing

agents for the Zn

_{ions influencing the}

hydrolysis/condensation equilibria of oxo-acetato species

Zn

O

O

CH

TEA acts as tetra dentate ligand for the Zn

_{ions stabilizing them in the}

solution, moreover it increases the viscosity of the starting solution.

N

OH

OH

OH

Starting  Solution Dip‐coating the solution  on glass substrates Drying the film at  100°C in air for 2 hours Annealing the film in  air or nitrogen  environment for 2  hours

ZAD in EtOH: 0.1 M

ZAD in EtOH with HAc:0.5 M

ZAD in EtOH with TEA:

0.5 M; 0.6 M; 0.75 M;

1.0 M ( Visc.= 6.5 cP); 1.2 M

Synthesis procedure

Corning 1737

500

550

600

½ ‐ 1 ‐ 2

½ ‐ 1 ‐ 2

½ ‐ 1 ‐ 2

T (°C) 

t (hours) 

Wettability of the substrate

0

5

10

15

20

25

30

35

40

I

H

G

F

E

D

C

B

Water

Contact Angle (

d

egr

ees)

Chemical etching

A

Cleaning in common

soap solution

Etching in NaOH

Washing in ethanol

Washing in water

(A) cleaning in common soap solution; (B) chemical etching in

Piranha solution for 2 minutes; (C) etching in Piranha solution

for 15 min.; (D) etching in 0.25M NaOH solution for 2 min.; (E)

etching in 0.25M NaOH solution for 15 min.; (F) etching in

0.5M NaOH solution for 2 min.; (G) etching in 0.5M NaOH

solution for 15 min.; (H) etching in 0.75 M NaOH solution for 2

30 40 50 60 70 (2 01 ) (1 12 ) (1 03 ) (1 10 ) (1 02 ) (1 01 ) (0 02 ) (1 00 ) 0.5 M TEA 0.12 M Intensity (a.u.) 2θ (°) 0.5 M HAc 0 10 20 30 40 50 60 70 80 M=0.5 TEA M=0.5 HAc Solution Thickness (nm)

Average Crystal size (nm)

M=0.12 10 11 12 13 14 15 6 8 10 Elec trical Re sistivity ( Ω∗ cm) x103 20 40 60 80 100 T ran sm it tan ce ( % ) M=0.15 M=0.5 TEA M=0.5 acetic acid Glass • Corning 1737 • 0.12 M • 0.5 M HAc • 0.5 M TEA

30

40

50

60

70

Intensity (

a.u.)

2

θ

(°)

Molar Concentration (mol/l) 0.5 Thicknes s ( n m) 0 5 10 15 20 25 30 1.2 1.0 0.75 0.6 0.5

Average Crystal Size (nm

)

Morphological characterization (AFM)

0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

Roughness

RMS (nm)

Molar Concentration (mol/l)

0.5 M

1.0 M

The roughness increases with the

molar concentration: the maximum is

achieved at 1.0 M and is about 7 nm,

higher than the rugosity obtained with

sputtering

Morphological characterization (SEM)

500 nm

_{SEM image of ZnO thin}

80 84 88 92 1.2 1.0 0.75 0.6 T ransmittance ( % )

Molar Concentration (mol/l)

0.5

Optical characterization

average transmittance in

the visible region

Optical Bandgap

versus Molar

Concentration

Molar

Molar

Concentration

Concentration

(mol/l)

Absorption coefficient versus

Photon Energy, the intercept of the

linear curve with the x-axis give the

value of the optical bandgapc

(mol/l)

Optical

Optical

Bandgap

Bandgap

(

(

eV

eV

)

)

0.5

3.282

0.6

3.275

0.75

3.272

1.0

3.249

1.2

3.243

3

3.1

3.2

3.3

3.4

0

2

4

6

8

10

12

14

x 10

Photon Energy (eV)

[α

*h

ν]

(cm

eV

)

Optical characterization

Electrical characterization

12

14

16

18

20

22

0

1

2

3

4

5

6

7

Electr

ical Resistivity (

Ω∗

cm)

Crystal size (nm)

Air

Nitrogen

x 10

10

20

30

40

50

60

70

80

2 h -600°C

1 h -600°C

Inte

nsity (a.u.)

2

degrees)

1/2 h -600°C

1.0 M

Time of annealing (hours) Thickness (nm)

Average Crystal Size (nm)

1/2

Annealing: time-temperature

Annealing Time (hours)

T= 500°C T= 550°C T= 600°C 1/2

Film

Film

Crystal Size

Crystal Size

(nm)

(nm)

600°C – ½ h

21

550°C – 1h

18.8

Main result

Dott. Maria Luisa Addonizio

Centro Ricerche ENEA di Portici

Prof. Santolo Daliento

Dip. Ing. Elettronica

Prof. Pasquale Pernice

Dott. Esther Fanelli

Dip. Ing. Materiali e Produzione

Ing. Orlando Tari

PhD student of Innovative Technologies for Materials,

Sensors and Imaging

(

TIMSI)

We have set a Sol-Gel procedure that allows to produce ZnO

thin films by dip-coating with a high uniformity

The properties of the best films obtained by a single dip

immersion are:

The electrical resistivity is 2•10

2

_Ω•cm;

The transmittance in the visible region is about 90% and

the absorbance about 1.3%;

The average crystal size is about 20 nm;

The thickness is 125 nm in a single dip immersion;

The roughness is about 7 nm;

The adhesion force of the film is 150 Kg/cm

2

_.

Perspectives

The high resistivity can be reduced with an appropriate

doping process with aluminum, boron and so on. Nowadays we

are developing a synthesis procedure to dope ZnO thin films.