Post on 01-Jan-2016
“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!