“ZnO thin films as buffer and antireflective layer in glass/ITO/ZnO/CdS

structures”

E. Adrián Martín Tovar and R. Castro-Rodríguez

September 26, 2013

Centro de Investigación y de Estudios Avanzados Unidad

Mérida

Zinc Oxide (ZnO) is a II-VI semiconductor material with the following properties:

A 3.37 eV direct band gap Thin film resistivity in the range of 10-4-1013 -cm Refractive index of 2.03 High transparency in the visible region of the

electromagnetic spectrum Crystallizes in three different phases :

Hexagonal wurtzite. Zincblende Rocksalt

Introduction

To deposit ZnO thin films using PLD technique for their use as a buffer and anti-reflective layer in Glass/ITO/ZnO/CdS structures using two types of targets.

Goals

As an anti-reflective layer:

0

4 ZnO

dn

For a buffer layer high resistivities are required

102-103 -cm

Thickness of 80-150 nm for wavelenghts in a range of 650- 1200 nm

Experimental Procedures

Laser pulse Nd:YAG

=1064 nm~2 J/(cm2) per shot

5 Hz

Number of shots:3000

Vacuum of10-5 TorrWhat’s PLD?

Substrate heatingTs = 300° C

Target-substrate distance50 mm

Target (type A)

ZnO sintered powder, 99.99% Area 50.3 mm2 Force = 10 Ton T = 600° C (Heating) Time = 2 hours

Target (Type B)

Cured powder ZnO, 99.99% Mixed with cyanoacrylate (C5H5NO2) Proportion of 70:30 in weight (70% ZnO,

30% cyanoacrylate) Dried at room temperature for three days

Results and discussions

Thickness

Sample Thickn

ess

(nm)

Deposition time

(min)

Type A 111.5 10

Type B 101.1 10

0

4 ZnO

dn

80-150 nm for wavelenghts in a

range of 650 a1200 nm

Morphological Properties

200nm

200nm

Type A

Type B

Sample RMS

(nm)

Type A 17.8

Type B 6.50

2( )

1

nn

rms

h hR

n

36-1451 ICCD crystallographic letter associated with ZnO.

Hexagonal wurtzite structure.

Preferential growth along the c-axis (002)

Displacements of diffraction peaks (002) to the left

FWHM (2)

Type A

20 30 40 50 60 70 80 90

Inte

nsi

ty (u. a

.)

2 (°)

Structural Properties

36-1451 ICCD crystallographic letter associated with ZnO.

Preferential growth along (100) and (101) planes

Hexagonal wurtzite structure.

Displacements of diffraction peaks (002) to the right

FWHM (2)

Type B

20 30 40 50 60 70 80 90

Inte

nsi

ty (u. a

.)

2 (°)

Structural Properties

Structurales Properties

Sample e (%) D (nm)

Type A -2.03 9.60

Type B 0.213 38.95

Nano-estructured thin films

D for type A suggests a quantum confinement effect

A negative sign indicates that the unit cell of type A has a smaller volume than the standard unit cell

A positive sign indicates that the unit of type B has a greater volume than the standard unit cell

12 sincos D

Williamson Hall equation:

Structurales Properties

Sample a (Å) c (Å) Volume

(Å3)

Volume

difference

(%)

Unit cell

stress e

(%)

Standard

ZnO 3.2495 5.2069 47.615

0 0

Type A 3.1937 5.2384 46.272 -2.82 -2.03

Type B 3.2548 5.198 47.689 0.155 0.213

c

b

c

baa

Optical Properties

Transmitance 85%

Abortion edge isn’t defined

400 600 80060

70

80

90

100

Tra

nsm

itanc

e (%

)

(nm)

Type B

Type A

UV VIS IR

Band gap Values2 2 2

0 2 * *

1 1 1.8

2g ge h d

h eE E

D m m D

2 2 10( ) 75.885 (nm) 1.902 (nm)g gE eV E D D

Sample Experimental

Gap (eV)

Grain size

(nm)

Calculated gap

(eV)

Standard ZnO 3.37 ---------- --------

Type A 4.08 9.60 4.00

Type B 3.32 38.95 3.37

Photocurrent Calculations

400 600 80030

40

50

60

70

80

90

100

Glass/ITO/ZnOTypeB

/CdS

Glass/ITO/CdS

Glass

Tra

nsm

itanc

e (%

)

nm

Glass/ITO

Glass/ITO/ZnOType A

/CdS

StructureJsc total

(mA/cm2)

Loss

(mA/cm2)

Glass 23.23 2.49

Glass/ITO 21.80 3.92

Glass/ITO/CdS 18.57 7.15

Glass/ITO/ZnOtipoA/

CdS

19.04 6.67

Glass/ITO/ZnOtipoB/

CdS

19.09 6.63

Elimination of 7.52% of losses

max

min

SC SolarJ T J d

Maximum value of 25.72 mA/cm2

Resitivity Measures

Measuare at RT

For CdS/CdTe Solar cells: 102 -103 -cm

Sample Resistence (M) Resistivity (-cm)

Type A 10.52 + 0.300 1.17 x102

Type B 12.80 + 0.300 1.30 x102

ZnO thin films were obtained through PLD by using two types targets.

Eg for the films A and B were 4.08 and 3.32 eV respectively

Eg sample for type A is the result of quantum confinement effects.

Conclusions

Conclusions

The samples were suitable as anti-reflective coating, the presence of ZnO resulted in an increase of about 10% of transmittance in glass/ITO/ZnO/CdS structures.

Photocurrent was obtained to be 19.04 and 19.09 mA/cm2 for samples A and B respectively, with a 7.52% elimination of photocurrent losses.

The type B film has the best results for an application as buffer layer and anti-reflective.

Thank you!