The document summarizes research on developing novel thin film cadmium telluride (CdTe) solar cells with an ultra-thin oxygenated cadmium sulfide (CdS:O) window layer deposited by magnetron sputtering. Key points discussed include: 1) Conventional CdS window layers have limitations like a mismatch with CdTe that increases defects, while CdS:O has a better lattice match and higher bandgap allowing more light to reach the CdTe layer. 2) Thin films of ZnO:Sn, CdS:O, and CdTe were deposited by sputtering and characterized, with the CdS:O showing nanocrystalline grains, bandgaps tun

1. Novel Structure, Ultra-Thin CdS:O/CdTe
Thin Film Solar Cells by Magnetron
Sputtering
Mohammad Aminul Islam
Ph.D Student
Supervisor: Prof. Dr. Nowshad Amin
Solar Energy Research Institute (SERI)
Universiti Kebangsaan Malaysia, Malaysia

2. *
 INTRODUCTION
 CdS/CdTe SOLAR CELLS: SHORT REVIEW
 ZnO:Sn THIN FILMS
 CdS:O THIN FILMS
 CdTe THIN FILMS
 CdS:O/CdTe SOLAR CELLS
 CONCLUSION
2

3. *
Cadmium Telluride Solar Cells
Glass Superstrate
Transparent Conducting Oxide
N-type CdS
P-type CdTe
Metal
Back Contact: Cathode
Front Contact: Anode
Window Layer
Absorber layer
Incident Light
3~8 um
0.1 um
0.05 um
~1000 um
• Direct bandgap, Eg=1.45eV
• Good efficiency (Record:17.3%, & 19.6%)
• High module production speed
• Long term stability (20 years)
• Process flexibility (PVD,CVD,CBD,CSS etc)
• Less material use (1µm CdTe absorbed light compared with around
10µm of Si).
3

4. 4
*

5. Place of CdTe as a Solar Cell Material Candidate
Bandgap (eV)
Efficiency(%)
Solar cell efficiency vs.
Bandgap
AbsorptionCoefficienta(cm-1)
Photon Energy (eV)
Absorption coefficient spectrum of
principal semiconductors for solar
cells
5
*

6. Place of CdTe Solar Cell as a commercial production and market share
6
*
Market Share by
Technology in 2013

7. Production Capacity of CdTe and Other Thin Film Solar Cell Until 2017 (MW)
7
 After the huge growth expectations of TF technologies some years ago, the competing market price
of c-Si has slowed the development of TF.
 The predominant c-Si technology is maintained its market share of around 80%, because of the
maturity of the technology and also because of the existing and growing capacity in China and
APAC countries, which favour wafer-based technologies.
 The TF technologies are expected to grow anyway at a lower rate, and therefore will stabilize
their market share over the next five years.
*

8. CdTe Solar Cell Market share Experience
8
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9. The polycrystalline CdS thin films are conventionally used as
n-type partner. Limitations of this layer are:
 Poly-CdS has a lower band gap of 2.42 eV,
 Absorbed the light of wavelength below 510 nm,
 responsible for lower Jsc & quantum efficiency (QE).
 Produces an unwanted layer of CdS1-xTex; band gap (1.30eV),
 Light is absorbed more than CdS layer by this layer, so, Jsc
decreases.
 Somewhere CdS diffused to CdTe completely and increases
pin hole effect.
Limitations of conventional CdS Thin Films
There are nearly 10% lattice mismatch between the poly-CdS and poly-CdTe.
 Increases defect density at the junction region.
 Increases recombination at the junction region.
 Some other materials like ZnS, ZnO, Ins, CuS are also tried by the researchers, but
it has been found that they have higher lattice mismatch with CdTe and other
absorber layers.
References
1. Xuanzhi Wu, High-efficiency polycrystalline CdTe thin-film solar cells, Solar Energy 77 (2004) 803–814.
2. X. Wu et all. High-Efficiency Polycrystalline CdTe Thin-Film Solar Cells with an Oxygenated Amorphous CdS (a-
CdS:O) Window Layer, 29th IEEE PV Specialists Conference New Orleans, Louisiana May 20-24, 2002.
3. Yan et al. The Effects of Oxygen on Junction Properties in CdS/CdTe Solar Cells, NCPV Program Review Meeting
Lakewood, Colorado, 14-17 October 2001. 9
Ref.1

10.  The oxygenated cadmium sulfide (CdS:O) window layer looks promising
and might overcome the problems of CdTe based solar cells mentioned
above.
 CdS:O window material have broad energy band gap from 2.42 eV to 3.1
eV.
 It has better lattice match with CdTe absorber layer.
 It permits light spectra below wavelength 510 nm to go to the CdTe
absorber layer, resulting a noticeable increase of Jsc.
 Due to CdS:O layer, CdS(1-x)Ox alloy nano-particles forms at the junction
of CdTe and CdS:O layers while increase crystallinity of CdTe.
Oxygenated CdS (a-CdS:O)
References
1. Xuanzhi Wu, High-efficiency polycrystalline CdTe thin-film solar cells, Solar Energy 77 (2004) 803–814.
2. Yan et al. The Effects of Oxygen on Junction Properties in CdS/CdTe Solar Cells, NCPV Program Review Meeting
Lakewood, Colorado, 14-17 October 2001.
3. Zhang et al. Raman Studies of Nanocrystalline CdS:O Film, DOE Solar Energy Technologies Program Review
Meeting October 25-28, 2004 Denver, Colorado.
10

11. Fig. Proposed CdS:O/CdTe Solar Cells
Proposed CdS:O/CdTe Solar Cells
11

12. Fig. : XRD, SEM and bandgap of ZnO:Sn thin film (RF power, ZnO: 3 watt/cm2 & Sn 0.5 watt/cm2)
 The film is polycrystalline hexagonal wurtzite structured, ZnO with most prominent peak along with
(002) plane confirmed by JCPDS no. 01-089-1397.
 The SEM image shows that the Sn doped ZnO nanoparticles are homogeneous in nature with
particle size of less than10 nm.
 The band gap of the film has been found as 3.49 eV.
 The carrier mobility 12.3 x 10-4 cm2/V-s was found for as-deposited film, it increased to 896.1 x 10-4
cm2/V-s with the carrier concentration and resistivity in the rang of 1018 cm-3 and kΩ-cm,
respectively.
The films are prepared at 300 oC, with a pressure of 14 mTorr by continuous Ar gas
flow of 10 SCCM.
12

13. Fig. EDX diffraction patterns of CdS:O thin films
prepared in argon-oxygen ambient
Sample
ID
RF power
(watt/cm2)
O2 partial
pressure
Cd
(at.%)
S
(at.%)
O
(at.%)
A1 1.50 (*XPS) 0.18m
Torr
29.55 28.80 41.65
B1 1.75 28.65 30.48 40.87
C1 2.00 34.31 34.92 30.77
D1 2.15 36.74 36.31 26.95
Composition of CdS:O thin films prepared in argon-
oxygen ambient
 The films prepared with low deposition
power incorporate the maximum amount of
oxygen.

14. Fig. XPS images of CdS:O thin films prepared in argon-oxygen ambient (deposition power 1.5 watt/cm2 )
14
Peak B.E, eV FWHM, eV Atomic conc. , %
Cd 3d 405.6 0.946 33.57
S 2p 161.2 0.731 28.38
O 1s 531.9 1.610 38.05
Quantification of the compositions
The oxygen are only contributed to
form SO3 or SO4 complexes.
As the oxygen content in the films
increases, the coordinate number of the
nearest S shell around Cd decreases, as a
result, local disorder increases and the
films loses its crystallinity.

15. CdS:O THIN FILMS FROM REACTIVE SPUTTERING
SEM Analysis
Fig. SEM images of CdS:O thin films prepared in argon-
oxygen ambient
 Nano structured grains are engaged
in the thin films with a compact and
rough surface.
 The average grain size of the film is
around 20 nm.
 The small-grains agglomerate to form
large grains are observed in films
prepared at 1.50 watt/cm2 and 1.75
watt/cm2
15

16. CdS:O THIN FILMS FROM REACTIVE SPUTTERING
XRD and UV-Vis Analysis
Fig. XRD diffraction patterns of CdS:O films
Fig. Absorption spectra and Bandgap evaluation
graph (inset) of CdS:O thin films
RF power
Watt/cm2
Peak
height of
(111)
(a.u)
FWHM
(radian)
Crystallite
size (D)
(nm)
Dislocation
density
ε (x 10 -3)
Strain
δ
(x 10 -3)
1.50 - - - - -
1.75 - - - - -
2.00 82 0.00872 17.58 4.35 3.23
2.15 215 0.00437 25.22 2.18 0.81
RF
power
Watt/cm2
Band
gap
(eV)
Resistivity
(x 102)(Ω-
cm)
Carrier
concentration
(x1014) /(cm-3)
Mobility
(cm2/V-s)
1.50 2.723 64.4 44.78 2.16
1.75 2.705 54.2 32.22 3.12
2.00 2.682 16.4 12.21 3.31
2.15 2.668 42.6 41.23 3.68
Table: Bandgap and electrical properties of CdS:O thin films
prepared in argon-oxygen ambient with different RF power
Table: FWHM, crystalline size, dislocation density and strain
of CdS:O thin films prepared in argon-oxygen ambient
O2 composition

17. Fig. Size dependence of bandgap for CdS:O
thin films calculated using Brus equation
The average particle size as a function of RF power
determined using the simplified Brus equation,
Egn – Egb = [ħ 2π2/2R2][ 1/me
*+1/mh
*] .
 Due to the typical quantum confinement effect, the particle size and absorbance reduces, while
the bandgap increases with the increase of O2 concentration and/or, with the decrease of RF
power.
17
 The calculated values for the CdS:O particles are
found 3.01 nm to 3.42 nm.
 R ≤ RB(8nm), indicating the strong quantum
confinement effect on the film.

18. The mass of CdS:O thin films in a unit volume could be considered as,
m = ρn(π/6)d3
= ρn(π/6)[2π2 ħ2{(1/me
*)+(1/mh
*)}(1/ΔEg)]3/2 α P
Fig. Curve of (1/ΔEg)3/2 with respect to the
deposition power for the prepared CdS:O
thin films
 Employ a simple effective mass approximation
(EMA) method by considering the particles are
monodisperse and spherical in shape.
 In this method, the particle diameter (d) related with the
change of bandgap (ΔEg) of the CdS:O films.
 The non-linear curve in the Fig. indicating that the
number density of particle is not constant in the
films.
 The results signpost that the growth of the films
proceeds through new nucleation and increases
the number particles in the films.
18

19. Fig. PL spectra for CdS:O thin films prepared
with different RF power
 At the low energy side, a broad band centered
around 2.2 eV, are attributed to the S vacancy (Vs).
 A broad high-energy band in the range 2.5 to 2.8 eV
related to cadmium atoms located at the interstitial
sites and corresponding to the band-to-band
transition.
 The quotient Ivs/Ibb increases as the O2 content
increases, which would mean that the oxygen
concentrations are influenced to increases the
impurity defects in the films.
 The increase of the Ivs/Ibb is caused by the
deterioration of the crystalline quality of the
CdS:O thin films.
19

20. 300 oC 10 mT
20
Sample ID Dep. power Cryst. Size, D (nm) I(111)/I(220) Micro strain, ε (x 10 -3) Dislocation density, δ
as-grown/treated 1.0 watt/cm2 50.99/104.86 12.42/0.94 3.27/1.59 3.59/0.97 (x1011 cm-2)
as-grown/treated 1.5 watt/cm2 52.26/101.04 16.11/0.30 3.19/1.65 3.59/0.92 (x1011 cm-2)
as-grown/treated 2.0 watt/cm2 42.86/93.66 22.22/0.31 3.89/1.78 5.31/1.05 (x1011 cm-2)
as-grown/treated 2.5 watt/cm2 56.71/127.3 37.55/0.57 2.94/1.31 3.01/0.67 (x1011 cm-2)
as-grown/treated 3.0 watt/cm2 55.76/114.98 36.74/16.25 2.99/1.45 3.15/1.82 (x1011 cm-2)
Table : Calculated value of the structural parameters of CdTe thin films prepared by sputtering technique with variation of RF power
RF power varies from 1.0 watt/cm2 to 3.0 watt/cm2

21. 21
Sample ID Dep. power Nature of Strain
06M13A1/
06M13A2
as-grown/
treated
1.0 watt/cm2 Tensile /
Tensil(strong)
06M13B1/
06M13B2
as-grown/
treated
1.5 watt/cm2 Tensile (strong)/
Tensile (weak)
06M13C1/
06M13C2
as-grown/
treated
2.0 watt/cm2 Compressive (weak)/
Tensile (weak)
06M13D1/
06M13D2
as-grown/
treated
2.5 watt/cm2 Tensile /
Tensil (strong)
06M13E1/
06M13E2
as-grown/
treated
3.0 watt/cm2 Compressive (weak)/
Tensile (strong)
Table : Strain in CdTe thin films prepared by sputtering technique with variation of RF power
W-H plot for CdTe thin films, (a) as-deposited, (b) CdCl2 treated

22. CdTe THIN FILMS
Film’s Morphology & Electrical Properties
Sample
ID
Deposition
power
(watt/cm2)
Resistivity x
104 (Ω-cm)
Carrier
concentration
x 1013 cm-3
06M13A1 1.0 3.41 1.45
06M13B1 1.5 1.51 4.52
06M13C1 2.0 9.19 0.12
06M13D1 2.5 1.06 0.78
06M13E1 3.0 2.21 0.49
06M13A2 1.0 2.68 2.24
06M13B2 1.5 4.43 10.43
06M13C2 2.0 1.24 13.45
06M13D2 2.5 1.59 7.30
06M13E2 3.0 1.61 68.69**
Values of the electrical parameters of
CdCl2 treated CdTe thin films
SEM images of CdTe thin films
22

23. The complete CdTe thin film solar cells has been
fabricated from FTO/ZnO:Sn/CdS:O/CdTe stack
with the following parameters:
Cu doped Carbon paste has been employed as a back contact of the solar cell and
finally Silver paste is used as a front and back electrode.
Layers Ambient Substrate
temperature
Working
pressure
RF power
(watt/cm2)
Deposition
time, min
Approximate
thickness
ZnO:Sn Ar 300 oC 14 mT 3.0:1.0 30 200 nm
CdS:O Ar:O2 (99:1) RT 14 mT 1.5 30 200 nm
CdTe Ar 300 oC 14 mT 1.0/2.0/2.5/3.0 100/60/50/40 1200 nm
23

24. *
Table : Solar cell performance with FTO/ZnO:Sn/CdS:O/CdTe/Cu:C/Ag configuration
RF power (CdTe) Voc (V) Jsc (mA/cm2) FF (%) Efficiency (%) Cell area (cm2)
1.0 watt/cm2 0.56 18.58 59 6.14
0.252.0 watt/cm2 0.72 20.11 65 9.41
2.5 watt/cm2 0.68 21.89 62 9.23
3.0 watt/cm2 0.67 22.55 68 10.27
Fig.: Cross sectional image & J-V curves of the ZnO:Sn/CdS:O/CdTe/Cu:C/Ag solar cells
24* High deposition rate is better for CdTe solar cells

25. *
 The conversion efficiency as high as 10.27% with performance parameters
of Voc = 0.67 Volt, Jsc = 22.55 mA/cm2; and FF = 0.68; was obtained in
the CdS:O/CdTe based solar cells.
 Further improvement in the efficiency is expected in near future by
optimizing back contact material Cu:C as well as other layers.
25

26. Thank You
Q & A

27. * DEPOSITION PARAMETERS OF
DIFFERENT LAYERS
Process Variables Characterization
ZnO:Sn Thin Film as HRT layer
ZnO & Sn co-
sputtering
Power: Sn: 10 watt & ZnO: 60 watt XRD, AFM, SEM, UV-Vis,
Hall-Effect
Complete Cell Structure
Pressure: 14mTorr at 300 oC
CdS:O Thin Films
Reactive
sputt.(Ar:O2)
Power: 20- 40 watt at RT
Pressure: 18mTorr at RT
CdTe preparation and optimization
Sputtering (1000 nm) Power: 40- 60 watt at 300 oC
Pressure: 8mTorr
CdCl2 Treatment
N2/O2 ambient, 15 min,
500 mTorr, 390 oC
27