Low – temperature heat treatment (80 oC) effect on the electrochemically synthesized CuInTe2 thin films for energy harvesting applications

4th International Conference on Materials Science & Engineering

(Materials Science 2015)

held at

Florida, USA

14th – 16th Sept. 2015

By

Manorama Gahininath Lakhe

Under the Supervision of

Dr. Nandu B. Chaure

DEPARTMENT OF PHYSICS, SAVITRIBAI PHULE PUNE UNIVERSITY , INDIA

Introduction

http://revolution-green.com/new-4g-solar-power/

Thin Film Technologi

es

Amorphous Silicon

Si:H13.6 %

CIGS(Concentrat

or)23.3 %

CIGS21.7 %

CdTe21.5 %

Importance of the CIT material

Direct bandgap semiconductor material.

The band gap varies from ~ 0.96 eV to 1.1 eV depending upon the processing.

Absorption coefficient = 105 cm-1 [1]

Highest reported efficiency for 6.92 % [2] by MBE technique.

References1. M.R. Ananthan, S. Kasiviswanathan, Solar Energy Materials & Solar Cells 93 (2009) 188 – 192

2. Takahiro Mise and Tokio Nakada, Prog. Photovolt: Res. Appl. 2013; 21:754–7593. R Diaz, M Cervera and F Rueda, Journal of Physics D: Applied Physics 45 (2012) 235101 – 235110

It has been showed that two single crystals with compositions close to CuIn2Te3.5, CuIn3Te5 and CuIn4Te6 polycrystal present a similar chalcopyrite structure with a different number of (2VCu + InCu) defect pairs [3].

Because of these defect pairs, the different number of structural vacant sites in the Cu sublattice permits an ion motion which make these compound new mixed ionic and electronic conductors MIEC .

4. M. Lakhe, N. B. Chaure: Solar Energy Materials & Solar Cells 123 (2014) 122–129

Precursors used : CuSO4, In2(SO4)3 ,TeO2 & Complexing agent: Citric acid [4]

pH of CIT solution: 4.0 by H2SO4 or NaOH.

Stirring rate = 150 rpm & Bath/deposition temperature = 75 oC

Experimental setup

0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2

1

0

-1

-2

-3

Anodic Scan

Cathodic Scan

Cur

rent

den

sity

J (m

A/c

m2 )

Voltage(V)

Cyclic voltammogram recorded at 75 C in an aqueous bath at pH 4 containing precursor ratios Cu/In = 0.25 and Cu/Te = 0.35 on CdS coated FTO substrate. Scan rate was 2 mV/sec.

Cyclic voltammetry Samples deposited on the small area

Backside interface

Samples deposited on the large area

The XRD pattern of (a) and (c) as deposited and (b) and (d) heat treated at 80 oC and deposited at 0.7 V & – 0.8 V respectively

100 150 200 250 3000

2

4

6 [A]

B3

1

A1

E & B2

100 125 150 175 2000

500

1000

1500

150128

Inte

nsity

Cou

nt

Raman Shift (cm-1)

(Int

ensi

ty) x

103 (c

ount

s)

Raman Shift (cm-1)

100 150 200 250 3000

2

4

6 [B]

100 150 200

400

800

1200109

150126

Inte

nsit

y C

ount

Raman Shift (cm-1)

E & B2

B3

1

A1

(Int

ensi

ty) x

103 (c

ount

s)

Raman Shift (cm -1)

20 30 40 50 600

1

2

(a)

2(degree)

0

1

2

3In

4Te 3

In4T

e 3

(b)

Inte

nsit

y x

103 (

coun

ts)

0

1

2

(FT

O)

(FT

O)

(FT

O)(F

TO

)

(c)

0

2

4

CIT

(312

/116

)

CIT

(204

/220

)

CIT

(112

)

(d)

Raman Spectra of as - deposited (black line) and heat treated FTO/CdS/CIT films (red line) at 80 oC deposited at [A] - 0.7 V and [B] - 0.8 V respectively. Inset shows the Lorentzian fitting for as deposited films for both potentials.

Results of heat treated samples at 80 oC

X ray diffraction Raman Spectroscopy

Mode Std. Mode frequencies (cm-1) Observed mode frequencies (cm-1) in present data

Literature data-0.7 V -0.8 V

As deposited Soft annealed As deposited Soft annealed

E 109 silent silent 109 silent

A1 123 silent 123 silent 123

E 128 128 silent 126 silent

B31 143 silent 142 silent 142

E 159 150 silent 150 silent

E5 and/ or B23 171 180 silent silent silent

E and B2 267 261 267 silent 266

Phonon mode frequencies

(b)

(112), d = 3.61 Ao

(204/220), d = 2.185 Ao(a)

(112), d = 3.57 Ao

(204/220), d = 2.19 Ao

(d)

(112), d = 3.605 Ao

(204/220), d = 2.188 Ao(c)

(112), d = 3.605 Ao

(204/220), d = 2.19 Ao

(A) HRTEM Images of the samples (a) as deposited and (b) heat treated sample, deposited at -0.7 V; (c) as deposited and (d) heat treated sample, deposited at – 0.8 V. The region defined by square in both the films (b) and (d) shows fringing pattern in the heat treated films.

(B) Diffraction pattern of CIT samples (a) as deposited, (b) heat treated; samples deposited at -0.7 V and (c) as deposited, (d) heat treated; samples deposited at -0.8 V

HR TEM results of as deposited and heat treated CIT thin films deposited on FTO/CdS

(d)

d = 0.360 nm

(b)

d = 0.186 nm

(a)

(c)

Deposition Potential (Volts)

SubstrateSample

Condition

Elemental Composition in At. %Cu/In ratio

Cu In Te

- 0.7

FTO/CdS

As deposited 19.57 23.05 57.38 0.85

Heat treated 18.98 25.29 55.73 0.75

- 0.8As deposited 17.81 29.56 52.62 0.60

Heat treated 22.28 26.28 51.44 0.84

EDAX

(a)

(c)

As deposited

(d)

(b)

Heat treated

- 0.7 V - 0.7 V

- 0.8 V - 0.8 V

FESEM

Plot of (αhν)2 Vs Eg (hν) for FTO/CdS/CIT films (a) & (b) shows the as deposited & heat treated film deposited at -0.7 V respectively and (c) & (d) shows as deposited and heat treated film deposited at -0.8 V respectively

Optical study

Optical band gap in eV

Deposition Potential (V) As deposited Heat treated

- 0.7 1.08 0.91- 0.8 1.11 1.02

0.9 1.0 1.1 1.2 1.3 1.40

1

2

3

(h

)2 eV

cm-2 x

1013

(a)

(b)[B]

Energy (h)0.9 1.0 1.1 1.2 1.3 1.4

0

2

5

7

10

Energy (h)

(h

)2 eV

cm-2 x

1013

(b)(a)

[A]

I-V characteristics of a FTO/CdS/CIT/Au of (a) – as deposited (black line) and (b) heat treated at 80 0C for 60 hour (red line) CIT films deposited at [A] - 0.7 and [B] - 0.8 V. (Inset shows both as deposited and soft annealed I-V plots on different scale

Typical 1/Cs2 Vs Voltage (V) plots for as-deposited (black line with square symbols) and heat treated (red line with circles) films deposited at – 0.7 V and – 0.8 V

I-V characteristic of as deposited and heat treated CIT thin films deposited on FTO/CdS

Capacitance – Voltage plot of as deposited and heat treated CIT thin films deposited on FTO/CdS

-2 -1 0 1 2-1

0

1

2

3

4

5

-2 -1 0 1 2

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0

1

2

3

4

5

Cur

rent

den

sity

J (

mA

/cm

2 )

Cur

rent

den

sity

J (

mA

/cm

2 )

Voltage (V)

baked

unbaked

[B]

(b)

(a)

Cur

rent

den

sity

J (

mA

cm

-2)

Voltage (V)

-2 -1 0 1 2

0

10

20

30

40

-2 -1 0 1 2-0.35

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0

10

20

30

40

Cur

rent

den

sity

J (m

A/c

m2 )

Cur

rent

den

sity

J (m

A/c

m2 )

baked

unbaked

Voltage (V)

Cur

rent

den

sity

J (

mA

cm

-2)

(b)

(a)

[A]

Voltage (V)

-0.75 -0.50 -0.25 0.00 0.25 0.500

1

2

3

4

0

2

4

6

(b)

(a)

Cs-2

x 1

08 (F

-2)

(III)

[B]

Cs-2

x 1

08 (F

-2)

Voltage(V)

(I)(I)

(II) (II)

-0.75 -0.50 -0.25 0.00 0.25 0.500

2

4

6

8

0

1

2

3

4

5

Cs-2

x 1

06 (F

-2)

(III)

[A]

Cs-2

x 1

07 (F

-2)

Voltage (V)

(b)

(a)

(II)

(II)

(I)(I)

-0.75 -0.50 -0.25 0.00 0.25 0.500

2

4

6

8

10

12

-0.75 -0.50 -0.25 0.00 0.250.0

0.2

0.4

0.6

0.8

1.0

1.2- 0.7 V

(a)

(b)

Car

rier

con

cen

trat

ion

x 10

19 (c

m-3)

Voltage (V)

[A]

(a)

(b)C

arri

er c

once

ntr

atio

n x 1

024 (

cm-3)

Voltage (V)

-0.75 -0.50 -0.25 0.00 0.25 0.500.0

0.5

1.0

1.5

2.0

2.5[B]

(b)

(a)Car

rier

con

cen

trat

ion x

1019

(cm

-3)

Voltage (V)

-1.0 -0.5 0.0 0.5 1.00.0

0.4

0.8

1.2

1.6

2.0(b)

(a)

[B]W

x 1

0-7(m

eter

s)

Voltage (V)-1.0 -0.5 0.0 0.5 1.00

2

4

6

8[A]

(b)

(a)

W x

10-

8 (m

eter

s)

Voltage (V)

Carrier concentration Vs Voltage

Depletion width (W) as a function of voltage (V)

Carrier concentration – voltage (V) of as deposited and heat treated CIT thin films

Change in depletion width as a function of applied potential (V) of as deposited and heat treated CIT thin films

Deposition Potential

(V)

Sample condition

Bulk Concentratio

n (cm-3)

Mobility (Cm2/Vs)

Conductivity(1/Ω cm)

Resistivity( Ω cm)

Sheet Concentratio

n (cm-2)

Average Hall

Coefficient cm3/C

-0.7As - deposited 7.55 x 1019 23.06 2.78 x 102 3.58 x 10-3 3.53 x 1016 8.27 x 10-2

Heat treated 6.48 x 1019 29.44 3.05 x 102 3.27 x 10-3 3.04 x 1016 9.64 x 10-2

-0.8As - deposited 8.36 x 1019 18.40 2.46 x 102 4.05 x 10-3 3.93 x 1016 7.46 x 10-2

Heat treated 7.32 x 1019 24.13 2.83 x 102 3.53 x 10-3 3.44 x 1016 8.52 x 10-2

Hall probe measurement

model ECOPIA HMS-3000 having area 0.5 cm x 1.0 cm under the constant magnetic field 0.54 T and probe current 10 mA

Development of CuInTe2 solar cells

-0.4 -0.2 0.0 0.2 0.4

-60

-40

-20

0

20

40

60

80

Cu

rren

t d

ensi

ty (

mA

cm-2)

Voltage (V)

Cell dark illuminated

Short circuit current density (Jsc) = 40.75 mA/cm2

Open circuit voltage (Voc) = 255 mVFill factor (FF) = 43 % and Efficiency = 4. 01 %

-1.0 -0.5 0.0 0.5 1.00

2

4

6

8

10

12

III

II

I

Cs-2

x 1

05 (F

-2 )

Voltage (V)

under dark 100 kHz

-1.0 -0.5 0.0 0.5 1.00

1

2

3

III

II

II

under illumination 100 kHz

Cs-2

x 1

04 (F

-2)

Voltage (V)

Capacitance – Voltage plot of FTO/CdS/CIT/Au structure after etching treatment

Conclusion

In conclusion thick films can be deposited by electrodeposition technique on CdS deposited FTO substrate for superstrate configuration of the solar cell.

Even low temperature heat treatment gives good structural, optical, morphological and transport properties.

The superstrate solar cell structure FTO/CdS/CIT/Au obtained for higher pH 4; capacitance-voltage profile reveals increase in diffusion capacitance after successive annealing whereas the carrier concentration was found to be ~ 1 x 1019 cm-3.

The superstrate solar cell structure (FTO/CdS/CIT/Au) obtained for lower pH 4 exhibits the short circuit current density (Jsc), 40.75 mA/cm2; open circuit voltage (Voc), 255 mV; fill factor (FF), 43 % and power conversion efficiency (η), 4.01 % with power intensity 100 mW/cm2.

This lower temperature annealing i.e. 80 oC could be very much useful for flexible solar cells devices.