<span class='text_page_counter'>(1)</span><div class='page_container' data-page=1>

Original Article

Photovoltaic device performance of electron beam evaporated

Glass/TCO/CdS/CdTe/Au heterostructure solar cells

K. Punitha

a

, R. Sivakumar

a,*

, C. Sanjeeviraja

b
a_{Department of Physics, Alagappa University, Karaikudi, 630 003, India}

b_{Department of Physics, Alagappa Chettiar College of Engineering and Technology, Karaikudi, 630 003, India}

a r t i c l e i n f o

Article history:
Received 7 August 2017
Received in revised form
21 November 2017
Accepted 6 December 2017
Available online 14 December 2017
Keywords:

CdTe

Grain boundary effect
Optical tailoring
Recombination losses
Ideality factor
Parasitic resistances
Conversion efficiency

a b s t r a c t

We report on substrate temperature and Cu addition induced changes in the photovoltaic device
perfor-mance of Glass/TCO/CdS/CdTe/Au heterostructures prepared by the electron beam evaporation technique.
Prior to the photovoltaic study, the structural and optical properties of CdTe, CdTe:Cu and CdS/CdTe, CdS/
CdTe:Cu layers were studied. X-ray diffraction (XRD) analysis showed that the depositedfilms belong to a
zinc blende structure. The existence of the Te peak in the XRD pattern revealed the presence of excess Te in
the depositedfilm structures, which confirmed the p-type conductive nature of the films. This was further
substantiated by the electrical study. The low resistivity of 1
103_U_{cm was obtained for 4 wt.% of the}
Cu-doped CdTefilm, which may be due to the substitutional incorporation of more efcient Cu2ỵ_(Cd2ỵ_{) into}
the CdTe lattice. The decrease in band gap with increasing Cu content may be attributed to the existence of
shallow acceptor level formed by the incorporation of Cu into the CdTe lattice. The efficiency of the cell was
increased with increasing Cu concentration and the cell prepared at room temperature with 4 wt.% of Cu
addition possessed the maximum conversion efficiency of 1.68%. Further, a good photoresponse of the
device is achieved as the Vocand Iscare increased with increase in the input power.

© 2017 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.
This is an open access article under the CC BY license ( />

1. Introduction

Nowadays, energy conversion is a critical challenge to get and
utilise the energy in an efficient, cost-effective and sustainable way,
due to limited nature of fossil fuel reserves within the earth's crust

[1,2]. In the effort tofight this serious threat for the survival of
mankind the solar energy is found as a perennial bountiful, safe,
clean, diversely convertible and sustainable source that offers an
inexhaustible supply. Solar energy can be harvested in many ways.
Among them, photovoltaic conversion of solar energy has paved a
promising way to meet the increasing energy demands. As the
solar cell converts light energy directly into electrical energy, its
performance and efficiency depends on the properties of the

ma-terial used. Solar cells started to emerge with Si as an absorber
material, which produces photogenerated carriers in the incident of
light. To outsmart the high material and processing cost of Si based
solar cells, chalcogenide based thin film solar cells have been
developed, which includes, Cu(In,Ga)Se2, CuInSe2, CdSe, HgTe and

CdTe. Among these materials, a significant focus is being given to
the CdTe-based solar cells in a renewed attention as an attractive
potential light absorbing layer with a high absorption coefficient
and a direct energy band gap of 1.45 eV which is an optimal match
with the solar spectrum, and thus facilitates its efficient utilization
(as it can absorb 90% of solar radiation with 1e2

m

m thickness of
CdTefilms, whereas, Si requires 20

m

m thickness offilm to absorb
the similar range of solar radiation) of solar light.

Normally, CdTe crystallizes in the zinc blende structure (as the
stable form). The zinc blende lattice has two types of surface
po-larities, namely (111)A and (111)B, and hence, there will be an
electrostatic attraction between these different planes[3]. Such an
attractive force makes it difficult to separate between these planes.
Also, if viewed perpendicular to the direction of the (111) plane, it
appears to consist of stacked planes of hexagonally packed
alter-nating Cd and Te layers[4]. In addition, its strong ionicity of 72% and
its chemical bond>5 eV makes it extremely stable (both chemically
and thermally). Besides, CdTe can be prepared with both n-type
conductivity and p-type conductivity, which makes it useful for
both homojunction and heterojunction based solar cell con
figura-tions. The extended and point defects in CdTe are electrically active
states. Therefore, they have a strong effect on the optical and
photoelectric properties of CdTe lms, which considerably

* Corresponding author. Fax: ỵ91 4565-225202.

E-mail address:(R. Sivakumar).
Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j s a m d

/>

2468-2179/© 2017 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license
( />

</div>
<span class='text_page_counter'>(2)</span><div class='page_container' data-page=2>

determine its solar cell efficiency. In addition, the optical and
electrical properties of CdTe can be tuned by suitably incorporating
an appropriate dopant into the CdTe matrix. Cu is an amphoteric
type of dopant (i.e., Cu acts as a donor in the interstitial sites (Cui)
and as an acceptor when substituting Cd (CuCd))[5]suitable for the
CdTe matrix. In order to realize the p-type conductivity with
optimal transport properties, the compensation of the CuCd
ac-ceptors by the Cuidonors should be avoided. The inherently nested
valence bands in Te enable the approximate hole packets of 4,
leading to a p-type semiconductor. Hence in this work, a controlled
level of Cu is doped into the CdTe matrix to maintain the p-type
conductivity and also to obtain the Te-rich CdTefilms.

The photovoltaic performance of a solar cell is basically depends
on the structural properties of the absorber layer, which in turn are

greatly controlled by the technique that is adopted for the film
deposition. Till now, various techniques have been employed to
de-posit CdTe thinfilms, viz. thermal evaporation [6], closed space
sublimation[7], liquid-phase deposition[8], pulsed laser deposition

[9], pulsed laser evaporation [10], molecular beam epitaxy [11],
chemical vapor deposition [12], and electrodeposition [13]. It is
worthwhile to mention here that the electron beam evaporation
(EBE), one of the physical vapor deposition methods, has been
considered largely for the preparation of device quality thinfilms
owing to the maximum possibility of direct transfer of energy to the
source. Though the production cost offilms by the EBE technique is
high (compared to the chemical methods), it imparts some feasible
devised-based qualities to the _{films, which are the key factors}
determining the suitable performance of thefilms for developing the
specialised devices. To date, very few reports are available on CdTe
thinfilms prepared by the EBE technique[14]. For instance, Murali
et al.[14]studied the effect of substrate temperature on the electrical
properties of CdTefilms deposited by EBE technique. However, no
attempt was made to understand the photovoltaic device
perfor-mance of electron beam evaporated CdTe thin films. Hence, the
present work focuses on the photovoltaic device performance of the
FTO/CdS/CdTe:Cu/Au structure prepared by the EBE technique. Prior
to the photovoltaic study, the structural and optical properties of
deposited layers were investigated. The effect of the substrate
tem-perature on the properties mentioned above was also studied.
2. Experimental

Thin_{films of CdTe, and Cu doped CdTe (CdTe:Cu; Cu ¼ 2, 3 and}
4 wt.%) were deposited onto thefluorine doped tin oxide (FTO)

coated glass substrates using the EBE technique (HINDHIVAC
vac-uum coating unit model 12A4D with the electron beam power
supply model EBG-PS-3K) under a chamber vacuum of better than
5
105_{mbar. CdTe powders (Sigma}_{eAldrich; 99.99% purity) were}
casted into pellets of 10 mm diameter with 4 mm thickness. The
pelletized CdTe ingots were placed in a graphite crucible (12 mm
outer diameter
10 mm inner diameter
6 mm depth) and kept
on water-cooled copper hearth of the electron gun, inside the
vacuum chamber. The distance between the substrate and the
target material wasfixed as 12 cm. The chamber was evacuated to a
high vacuum of better than 2
105mbar using rotary and
diffu-sion pumps and the chamber pressure was measured by pirani and
penning gauges. In the electron gun, the electrons extracted from a
dc-heated cathode of tungstenfilament, by the application of an
electric field, pass through an anode, and deflected through an
angle of about 180by the magneticfield to reach the target
ma-terial. The surface of the CdTe pellet on the graphite crucible was
scanned by the resultant and deflected electron beam with an
accelerating voltage of 5 kV and a power density of about
1.5 kW cm2. The ablated material was evaporated and the vapor
phase condensed and deposited as thin film on the precleaned

substrate. The homogeneous distribution of the evaporated CdTe
particles on the substrate was attained by continuously rotating
the substrate during deposition. The deposition time was 10 min
and the deposition rate was 0.1

m

m/min. The thickness of deposited
film was in the range of ~1.00 (±0.03)

m

m, measured by surface
profilometer (Mitutoyo, SJ-301). The films were deposited at
different substrate temperatures (Tsub) like room temperature (RT),
100C, 150C and 200C. Similarly, CdS/CdTe and CdS/CdTe:Cufilm

structures were deposited (without breaking the chamber
pres-sure) onto the FTO substrate with 100 nm thickness of CdS, which
would ever serve as the window layer. In order to improve the
crystallinity, the depositedfilms were annealed in air (post
depo-sition heat treatment) (Tannea) at 400C for 10 min.

Prior to the photovoltaic study, the structural and optical
properties were investigated for the CdTe, CdTe:Cu, CdS/CdTe and
CdS/CdTe:Cu thinfilms. The structural property of the films was
analyzed by X-ray diffraction (XRD; X'pert Pro PANalytical) using
Cu-K

a

radiation (

l

¼ 0.154 nm) over a 2

q

scan range of 10e80_{. The}
surface morphology of CdS/CdTe:Cu thinfilm was studied using
scanning electron microscopy (SEM; TESCAN VEGA 3). The optical
properties offilms were studied with a UV-Vis-NIR
spectropho-tometer (JASCO). The photoluminescence (PL) property of thefilms
was studied using a photoluminescence spectrometer (Cary eclipse
VARIAN), whereas a xenonflash lamp (15 W) and a photomultiplier
tube were used as the source of excitation and the detector,
respectively. In addition, the electrical properties of the CdTe and
CdTe:Cu _{films deposited on the glass substrate and annealed at}
400C were studied by the Van der Pauw configuration. Finally,
solar cell characteristics of the FTO/CdS/CdTe:Cu/Au structure was
studied using a solar simulator (4200 Keithley Semiconductor
Characterization System).

3. Results and discussion

3.1. Structural and surface morphological properties

</div>
<span class='text_page_counter'>(3)</span><div class='page_container' data-page=3>

Fig. 1. XRD patterns of CdTe and CdTe: Cu (2, 3 and 4 wt%) thinfilms deposited on FTO substrate at different substrate temperatures (Tsub¼ RT, 100, 150 and 200C) and annealed at

400C.

Fig. 2. XRD patterns of CdS/CdTe and CdS/CdTe: Cu (2, 3 and 4 wt%) structures deposited on FTO substrate at different substrate temperatures (Tsub¼ RT, 100, 150 and 200C) and
annealed at 400C.

</div>
<span class='text_page_counter'>(4)</span><div class='page_container' data-page=4>

The degree of crystallinity is high for the RT deposited annealed
film and the deterioration in the crystallinity is observed for the film
deposited at the substrate temperature of 200C and annealed at
400C. The observed higher degree of crystallinity for RT deposited
annealedfilm may be due to the primary nucleus formed on the
substrate surface at room temperature which can easily be driven
towards the location with lower surface free energy. This leads to
the effective growth of the material. On the other hand, the
dete-rioration of the crystallinity for thefilm deposited at the substrate
temperature of 200C and annealed at 400C may be due to the
re-evaporation of adatoms from the substrate surface. Also, at higher
substrate temperatures, there is a possibility for the dissociation and
desorption of adatoms that makes the films thermodynamically
unstable and deteriorates the crystallinity[19]. Besides, from the
XRD patterns of the CdS/CdTe and CdS/CdTe:Cu structures (Fig. 2),
the signature of CdS corresponding to the hexagonal phase (JCPDS
card No.: 02-0563) was identified from the small peaks of the (100),
(103) and (106) planes, which raised from the window layer.
Further, the signature of the Te peak revealed the presence of excess
Te in the depositedfilm for the solar cell structures.

The crystallite size of the CdTe and Cu doped CdTe films
deposited on the FTO substrate was calculated using the Scherrer's
formula and the result is given inTable 1. The crystallite size is
found to increase with the increase of the Cu content. However, one

can observe that the crystallite size in the 2 wt.% Cu doped CdTe
film is lower than in the undoped one. This may be due to the lattice
distortion induced by the incorporation of Cu into the CdTe matrix.
Since the ionic radius of Cu2ỵ(0.72 ) is lower than that of Cd2ỵ
(0.97 ), the decrease in the crystallite size may be attributed to the
substitutional incorporation of the Cu2ỵions instead of the Cd2ỵ
ions[9]. It is worthwhile to mention here that copper is the fast
migrating impurity in the CdTe compound. Cu migration in single
crystalline CdTe and in other II-VI compounds is characterized by
the two component diffusion. The fast diffusion component has
been assigned to the interstitial copper (Cuiỵ), while the slower one
has been assigned to the substitutional copper (CuCd) and the Cu
complexes, such as (CuiỵỵVCd2) and (CuỵeCuCd)[20]. Upon the
in-crease in the concentration of Cu (3 and 4 wt.%) together with the
annealing treatment, the Cuiỵ may diffuse fast and occupy the
substitutional Cd vacancy or the Cu complexes, which in turn
in-crease the crystallite size.

The surface morphology of the prepared FTO/CdS/CdTe:Cu
4 wt.% structure at the substrate temperatures of RT, 100, 150 and

200C and then annealed at 400C was studied using the scanning
electron microscopy and the obtained images are shown in

Fig. 3(aed). The influence of the substrate temperature on the
morphology is clearly seen from the micrographs. The film
deposited at RT and annealed at 400C (Fig. 3(a)) shows an uniform
morphology with netted surface. When the film deposited at RT
was subjected to the annealing treatment, the adatoms may gain
kinetic energy from the thermal energy and start to form clusters

from the nucleation sites. These nuclei grow large enough to touch
each other, coalescence takes place at the interface between them
which will minimize the surface free energy[16]. This results in the
growth of grains with netted-surface-like morphology. On the
other hand,Fig. 3(bed) show the SEM images of the films deposited
at the substrate temperatures of 100, 150 and 200C and
subse-quently annealed at 400C. The morphology of thefilm deposited
at 100C shows an uniform distribution of very small crystal grains,
whereas, thefilm deposited at 150_{C shows a different morphology}
with large sized grains grown outwards to form a netted feature.
The SEM image of thefilm deposited at 200_{C shows the}
deteri-oration in the grain growth, which is consistent with the result of
the X-ray diffraction study.

3.2. Optical property

When the light of sufficient energy is incident onto a material,
the electromagnetic radiation interacts with the discrete atomic
energy levels and induces the transition of electrons from occupied
states below the Fermi energy to unoccupied states above the Fermi
energy. A quantitative study of these transitions provides the
un-derstanding of the initial andfinal states involved in the transition
and hence knowledge of the band structure. Also, it is known that the
efficiency of any photovoltaic device depends on the amount of
photons absorbed by the material, which in turn is related with the
energy of the photon and the band gap of the material. In order to
evaluate the energy band gap of the CdTe, CdTe:Cu, CdS/CdTe and
CdS/CdTe:Cu thinfilms deposited on the FTO substrate, the optical
transmittance of thefilms were measured in a UV-Vis-NIR
spectro-photometer. During the optical transmission measurements, the

respective contributions from FTO and FTO/CdS have been nullified
by introducing them as the references and hence the information
corresponding to CdTe and CdTe:Cu only was observed. The
ab-sorption coefficient was calculated from the optical transmittance
using the formula,

a

ẳ lnTị

t (1)

where T is the transmittance and t is the thickness of thefilm. The
energy band gap is related to the absorption coefficient (

a

) through
the Tauc relation[21],

a

h

y

¼ B h

y

 Eg

n ₍₂₎

where B is a constant which arises from the Fermi's Golden rule of
fundamental electronic transition within the framework of the
parabolic approximation for the dispersion relation, Egis the energy
band gap and n takes the values depending upon the type of the
transition. CdTe is a direct band gap material and hence the Tauc
plot drawn between (

a

h

y

)2and photon energy (h

y

) is expected to
show a linear behavior in the higher energy region and the
extrapolation to the linear region at

a

¼ 0 gives the Egof thefilms
(graph not shown here). It is observed that the value of Egchanged
from 1.48 eV to 1.38 eV for the CdTe and CdTe:Cufilms deposited on
the FTO substrate and from 1.57 eV to 1.38 eV for the CdS/CdTe and
CdS/CdTe:Cu structures deposited also on the FTO substrate, as

Table 1

Crystallite size of CdTe and CdTe:Cu thin films deposited on FTO
substrate.

Sample conditions Crystallite size (nm)

Tsub¼ RT; Tannea¼ 400C

FTO/CdTe 61

FTO/CdTe:Cu 2 wt.% 41

FTO/CdTe:Cu 3 wt.% 49

FTO/CdTe:Cu 4 wt.% 80

Tsub¼ 100C;Tannea¼ 400C

FTO/CdTe 41

FTO/CdTe:Cu 2 wt.% 40

FTO/CdTe:Cu 3 wt.% 43

FTO/CdTe:Cu 4 wt.% 50

Tsub¼ 150C;Tannea¼ 400C

FTO/CdTe 54

FTO/CdTe:Cu 2 wt.% 39

FTO/CdTe:Cu 3 wt.% 47

FTO/CdTe:Cu 4 wt.% 53

Tsub¼ 200C;Tannea¼ 400C

FTO/CdTe 46

FTO/CdTe:Cu 2 wt.% 45

FTO/CdTe:Cu 3 wt.% 46

</div>
<span class='text_page_counter'>(5)</span><div class='page_container' data-page=5>

summarized inTables 2 and 3. This decreasing band gap may be
attributed to the existence of the shallow acceptor level formed by
the incorporation of the Cu dopant into the CdTe lattice. Since Cu is
an amphoteric nature of dopant, it acts as a donor when occupies
the interstitial sites and as shallow acceptors when substituting Cd
(CuCd), and also in forming the structure complexes with Cd
va-cancies such as (CuiỵVCd2) and (CuiỵeCuCd). It was reported that the
activation energy of the CuCdacceptor center is about 0.15 eV above

the valence band[22]. Further, thermal annealing creates Cd
va-cancies (VCd) to facilitate the substitution of the Cu atoms in the Cd
sublattices[5,20]. The observed Egvalues are in agreement with
those reported by Dharmadasa et al.[23]and Hu et al.[22]for CdTe
layers deposited on the FTO substrate for the fabrication of FTO/

CdS/CdTe heterostructure solar cells. The reduction in Egvalue with
the increasing Cu concentration revealed the dopant acting as a
substitutional impurity in the Cd vacancy, i.e. CuCd. This result is

Fig. 3. SEM images of CdS/CdTe: Cu 4 wt% structures deposited on FTO substrate at different substrate temperatures (Tsub¼ RT, 100, 150 and 200C) and annealed at 400C.

Table 2

Optical energy band gap values of pure and Cu doped CdTe
thinfilms deposited on FTO substrate.

Sample conditions Eg(eV)

Tsub¼ RT; Tannea¼ 400C

FTO/CdTe 1.44

FTO/CdTe:Cu 2 wt.% 1.42

FTO/CdTe:Cu 3 wt.% 1.41

FTO/CdTe:Cu 4 wt.% 1.38

Tsub¼ 100C;Tannea¼ 400C

FTO/CdTe 1.47

FTO/CdTe:Cu 2 wt.% 1.44

FTO/CdTe:Cu 3 wt.% 1.43

FTO/CdTe:Cu 4 wt.% 1.42

Tsub¼ 150C;Tannea¼ 400C

FTO/CdTe 1.45

FTO/CdTe:Cu 2 wt.% 1.43

FTO/CdTe:Cu 3 wt.% 1.42

FTO/CdTe:Cu 4 wt.% 1.40

Tsub¼ 200C;Tannea¼ 400C

FTO/CdTe 1.48

FTO/CdTe:Cu 2 wt.% 1.47

FTO/CdTe:Cu 3 wt.% 1.45

FTO/CdTe:Cu 4 wt.% 1.43

Table 3

Optical energy band gap values of FTO/CdS/CdTe
structures.

Sample conditions Eg(eV)

Tsub¼ RT; Tannea¼ 400C

CdTe 1.44

CdTe:Cu 2 wt.% 1.42

CdTe:Cu 3 wt.% 1.42

CdTe:Cu 4 wt.% 1.41

Tsub¼ 100C;Tannea¼ 400C

CdTe 1.45

CdTe:Cu 2 wt.% 1.42

CdTe:Cu 3 wt.% 1.40

CdTe:Cu 4 wt.% 1.38

Tsub¼ 150C;Tannea¼ 400C

CdTe 1.46

CdTe:Cu 2 wt.% 1.44

CdTe:Cu 3 wt.% 1.43

CdTe:Cu 4 wt.% 1.42

Tsub¼ 200C;Tannea¼ 400C

CdTe 1.44

CdTe:Cu 2 wt.% 1.57

CdTe:Cu 3 wt.% 1.56

CdTe:Cu 4 wt.% 1.48

</div>
<span class='text_page_counter'>(6)</span><div class='page_container' data-page=6>

consistent with the report of Ding et al.[24], where they have
observed the band gap narrowing for CdTe thinfilms deposited
with different substrate temperatures.

3.3. Photoluminescence study

The optical quality of the CdTe, CdTe:Cu, CdS/CdTe, and CdS/
CdTe:Cufilms deposited on FTO substrates was further studied by
photoluminescence spectroscopy with an excitation wavelength of
600 nm and the recorded spectra are shownFigs. 4 and 5. The broad
single emission peak localized around 822 nm corresponds to the
band to band radiative recombination of CdTe. The energy value
corresponding to the emission peak is found as 1.51 eV, which
approximately matches with the energy band gap obtained from the
optical measurement. Also, it is observed that the intensity of
emis-sion peaks are increased with the Cu content up to 3 wt.%. The
in-crease in the peak intensity may be due to the existence of the defect
traps which may lead to the emission of the large number of excitons
releasing large amount of emitted energy. However, a decrease in the
PL peak intensity is observed for the 4 wt.% of Cu doped CdTefilm.

This may be due to the complete saturation of defects and some of
these defects may trap two electrons or holes (doubly excited), which
increase the activation energy[25]. In addition, the dopant
concen-tration and substrate temperature induced variation in the intensity
of the PL emission peaks may be attributed to the change in surface
state density of thefilms. Further, the broadeningof the emission
peak may be due to the photo-assisted transition.

3.4. Electrical properties

The electrical properties such as resistivity, carrier mobility and
carrier concentration of the CdTe and the Cu-doped CdTe films
deposited on glass substrates at different substrate temperatures
and annealed at 400C were measured using the Van der Pauw
configuration. The conductive nature of the films was found using
the hot probe technique, where, the current was observed toflow
from hot to cold junction in all thefilms, which revealed the p-type
nature of conductivity. This observation is consistent with the result
of de Moure-Flores et al.[9], where the authors have observed the
p-type nature of conductivity for the Cu-doped CdTe_{films up to 5 wt.%,}
which was changed to n-type conductivity when the dopant
con-centration was increased to 10 wt.%. The substrate temperature and
Cu concentration induced changes in the electrical parameters of
thefilms are given inTable 4. The resistivity of thefilms is found to
decrease with the increasing Cu content. It may be mentioned that
de Moure-Flores et al.[9]have observed the lowest resistivity of
68.8
103

_U

_{cm for the 3 wt.% Cu doped CdTe sample and}
21
104

_U

_{cm for the 5 wt.% Cu doped CdTe samples prepared at the}
substrate temperature of 300 C. However, our result shows the
lowest resistivity of 1
103

_U

_{cm for the 4 wt.% Cu doped CdTe}_film.

This may be due to the substitutional incorporation of more efcient
Cu2ỵ(Cd2ỵ) in the CdTe lattice, which in turn leads to the increase in
the mobility and the free carrier concentration of thefilms.
3.5. Photovoltaic study

3.5.1. Construction of p-n heterostructure solar cells

The schematic sketch for the construction of a p-n
hetero-structure solar cell and the IeV graph of an ideal solar cell is shown in

Fig. 6(a) and (b). The transparent ordinary window glass (about 2
-3 mm thick) was used to protect the active layers from the
envi-ronment. The transparent conducting oxide of FTO acts as a front
contact of the device because of its high work function and larger
mechanical stability. A thin layer of n-CdS (about 100 nm) was
employed as the window layer of the device owing to its wide band

gap and transparent nature down to the wavelength of about
500 nm. The p-type CdTe (1

m

m thick) was used as an active absorber
layer. It is the effective region of the device, where the generation and
collection of carriers occur. The back contact provides a low
resis-tance electrical connection to the CdTe. A thin gold (Au) layer (few
tens of nm thick) was used as back contact on CdTe layer. The
cur-rentevoltage (IeV) characteristics of this cell structures were
measured using the solar simulator (4200 Keithley Semiconductor
Characterization System). The photocurrent was measured by
illu-minating the cell with the white light using a halogen lamp. The
conversion efficiency of the cell was measured with a power density
of 100 mW/cm2. The photoresponse of the solar cell was measured by
varying the power density (60, 80, and 100 mW/cm2).

3.5.2. IeV characteristics

The currentevoltage (IeV) characteristics of the cell (Glass/TCO/
CdS/CdTe:Cu/Au) structure, prepared at the substrate temperatures
of RT, 100, 150 and 200C and post heat treated at 400C are shown
inFig. 7. The span of the IeV curve ranges from the short circuit
current (Isc) at zero volts, to zero current at the open circuit voltage
(Voc) (Fig. 6(b)). The‘knee’ of the IeV curve is the maximum power
point (Imax, Vmax), i.e. the point at which the solar cell generates the
maximum electrical power. At voltages well below Vmax, theflow of
the photogenerated electrical charge to the external circuit is
relatively independent of the output voltage. Near the‘knee’ of the
curve, this behavior starts to change. As the voltage further
in-creases, an increasing percentage of the charges recombines within
the solar cells rather thanflowing out through the external circuit.
At Voc, all of the charges recombine internally. The maximum power
point, located at the knee of the curve, is the (I, V) point at which the
product of the current and the voltage reaches its maximum value.
The various solar cell parameters, such as the open circuit
voltage (Voc), the short circuit current (Isc), thefill factor (FF), the
efficiency (

h

), the series resistance (Rs) and shunt resistance (Rsh)
were evaluated from the IeV curve and the results are presented in

Table 5. It is observed that the Vocvaries between 290 and 643 mV
and the Iscchanges from 2.87 to 4.75 mA/cm2. It is also seen from

</div>
<span class='text_page_counter'>(7)</span><div class='page_container' data-page=7>

However, the lower Iscvalues are responsible for the low
con-version efficiency values. Bhandari et al.[27]fabricated CdTe solar
cells with CdCl2surface treatment and produces a solar conversion

efficiency of 8.7% with an Au back contact. The efficiency was

further improved to 11.4% when the Cu/Au back contact was used. It
was said that Cu in the Cu/Au back contact reduces the width of the
space charge region. Moreover, in the case of CdTe, the height of the
Schottky barrier (q

F

B), which is measured between the top of the

Fig. 4. Photoluminescence spectra (excited at 600 nm) of CdTe and CdTe: Cu (2, 3 and 4 wt%) thinfilms deposited on FTO substrate at Tsub¼ RT, 100C, 150C and 200C and
further annealed at 400C.

Fig. 5. Photoluminescence spectra (excited at 600 nm) of CdS/CdTe and CdS/CdTe: Cu (2, 3 and 4 wt%) structures deposited on FTO substrate at Tsub¼ RT, 100C, 150C and 200C
and further annealed at 400C.

</div>
<span class='text_page_counter'>(8)</span><div class='page_container' data-page=8>

semiconductor valence band and the Fermi level at the
metal-semiconductor interface, is given by,

qfBẳ qfM 4:3eV ỵ 1:5eVị ẳ qfM 5:8eV (3)

This equation implies that the barrier can only be reduced to
zero if a metal with a work function of at least 5.8 eV has to be
applied. However, due to its covalent nature, CdTe does not follow
the Schottky theory rigorously[28]. To balance the surface change,
the bands of CdTe bend towards its surface giving rise to a space
charge region. Also, it is reported that Paudel et al. have obtained
the efficiency of 0.5% for undoped CdTe solar cells[29]. However, in
the present work, Au was used as a the back contact which has a
high work function of 5.1 eV and this acts as a better contact and
produced measurable conversion efficiency without the step of
surface treatment for the CdTe surface.

The observed low value of the short circuit current may be
attributed to the surface recombination. This may be explained as
follows: the absorption coefficient (

a

) of CdTe steeply increases in a
narrow range at h

y

z Egand becomes higher than 104cm1at
h

y

_{> E}g. As a result, the penetration depth of photons (

a

1) is less
than ~ 1

m

m. When the electricfield in the space charge region is
not strong enough, these photogenerated electrons may recombine
before running through the external circuit which leads to the
insufficient collection of charge carriers and hence lowers the short
circuit current[26]. In addition, it was stated that the short circuit
current density will be lowered if a significant portion of radiation
is absorbed outside the space-charge region[26].

Further, the diffusion component of the short circuit current
depends on the thickness of the CdTe layer. The losses of the
diffusion component of the short-circuit current are 5, 9 and 19% for
10, 5 and 2

m

m of the CdTefilm layer, respectively[26]. The higher
the thickness, the lower the losses of the diffusion component. In
our case, the thickness of the CdTe layer is 1

m

m from which the
losses of the diffusion component of the short circuit current may
be expected more because of the higher absorption coefficient
(>104_cm1_{) and so the lower penetration depth (}_<1

_m

_m)_[26]_{. Thus,}
the carriers arisen outside the space-charge region diffuse into the
neutral part of the CdTe layer shall penetrate deeper into the
ma-terial. Carriers reaching the back surface of the layer will recombine
and do not contribute to the photocurrent. If the layer thickness is
low, recombination may take place even at the back surface which
annihilates the photogenerated electrons. Thus, thinning the CdTe

layer reduces the short circuit current density due to the

incom-plete charge collection in the neutral part of the CdTefilm. Besides,
if the space charge region is too wide, the electric_{field becomes}
weak and cannot keep the mobile charge carriers separated until
they run through the external circuit and hence the short circuit
current is reduced. Further, the secondary cell parameters like the
saturated current density (Jo) and the ideality factor (n) were also
calculated for the solar cell fabricated at Tsub¼ RT. These
parame-ters can be evaluated from the graph plotted between ln JDvs. the
anode voltage (V) which is shown inFig. 8. The intercept of the
linear portion gives the saturated current density (Jo) and the slope
gives the ideality factor (n). For an ideal pn-junction, the current is
carried by the thermionic emission of carriers over the junction
barrier[29]. For the purely thermionic emission, the ideality factor
is always 1. Any deviation in the value of n from 1 is attributed to
other current transport mechanisms like tunneling through the
barriers and/or to the presence of a the generation/recombination
current in the junction region[30]. In such cases, the IeV curve will
be less than square and the corresponding values of Imaxand Vmax
will be proportionally smaller. Thus, the process of the
recombi-nation of carriers in the depletion region is an important cause for
high values of the ideality factor. In the CdS/CdTe solar cell, the
lattice constant of CdTe is 6.48 Å and of CdS is 5.82 Å. This lattice
mismatch gives rise to large interfacial defect states which act as
recombination sites at the interface and deteriorate the device
performance. Besides, the disorders due to their amorphous nature
also cause the defect states as interstitials and impurities do. These
defects are distributed in energy in the band gap and act as
recombination centers. Deep defects, sometimes called mid-gap
defects, are located near the center of the band gap and usually
act as recombination centers. The calculated saturate current

density and ideality factor are 7.59
104 mA/cm2 and 3.20,
respectively. This clearly infers that there are recombination losses
which may occur either in the interface region and/or through the
deep defect states. Paudel et al.[29]have reported that the ideality
factor (n) and the reverse saturation current density (Jo) of CdTe
films vary from 2.7 to 1.7 and from 1.01
104_{to 1.38}₁₀7_mA/
cm2, respectively. It was stated that the back contact interface
recombination influences the parameters n and Jo.

However, observation of this measurable currentflow is due to
the grain growth attained by the post-deposition heat treatment of
the fabricated CdS/CdTe heterostructure. The heat treatment

Table 4

Electrical parameters of CdTe and CdTe:Cu thinfilms.

Samples Resistivity (r) (
103₎_U_cm _{Mobility (}_m_{) cm}2_/Vs _{Carrier concentration}

(N) (
1011_)/cm3
Tsub¼ RT; Tannea¼ 400C

Pure CdTe 55 39.5 29.7

CdTe:Cu 2 wt.% 49 48.9 25.6

CdTe:Cu 3 wt.% 24 53.8 48.6

CdTe:Cu 4 wt.% 01 89.0 403.2

Tsub¼ 100C;Tannea¼ 400C

Pure CdTe 59 41.1 25.4

CdTe:Cu 2 wt.% 45 56.1 24.9

CdTe:Cu 3 wt.% 44 68.2 20.8

CdTe:Cu 4 wt.% 58 81.2 131.7

Tsub¼ 150C;Tannea¼ 400C

Pure CdTe 48 48.1 26.6

CdTe:Cu 2 wt.% 26 63.4 37.6

CdTe:Cu 3 wt.% 06 71.4 130.9

CdTe:Cu 4 wt.% 05 75.8 154.9

Tsub¼ 200C;Tannea¼ 400C

Pure CdTe 53 46.9 25.0

CdTe:Cu 2 wt.% 49 49.7 25.1

CdTe:Cu 3 wt.% 47 60.8 21.4

</div>
<span class='text_page_counter'>(9)</span><div class='page_container' data-page=9>

reduces the defect density, grain boundaries (which act as
recom-bination centers in CdTe) and promotes the interdiffusion between

the CdTe and CdS layers that reduces the recombination rate to
some extent[31]. During operation, the efficiency of the solar cells
is reduced by the dissipation of power across the internal

resistances. These parasitic resistances can be termed as series
resistance (Rs) and parallel shunt resistance (Rsh). Series resistance
(Rs) is caused by the ohmic losses in the surface of the solar cell. The
parallel shunt resistance (Rsh) is caused by the losses due to the
leakage current which arises because of the non-idealities and

Fig. 6. (a) Schematic diagram of thinfilm solar cell with various active layers and metal contacts and (b) IeV graph of an ideal solar cell under dark and light conditions.
K. Punitha et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 86e98

</div>
<span class='text_page_counter'>(10)</span><div class='page_container' data-page=10>

impurities near the junction and causes the partial shorting of the
junction near the solar cell edges[32]. For an ideal cell, the series
resistance (Rs) should be zero, resulting in no further voltage drop
before the load, and the shunt resistance (Rsh) should be infinite
and would not provide an alternate path for the current toflow.
From our results, it is observed that both the ohmic losses through
the higher series resistance and the leakage current loss through
the lower shunt resistance may be the reason for the obtained solar
cell parameters and hence, the conversion efficiency. This can also
be observed from the IeV graph (Fig. 7). The effects of these
para-sitic resistances on the IeV characteristic of the solar cell fabricated

with the 4 wt.% Cu doped CdTe absorber layer are graphically
shown inFig. 9. For an ideal solar cell, the IeV graph near Iscwill be
flat. The slope of the IeV curve between Impand Iscis affected by the
amount of shunt resistances. Reduced shunt resistance results in a
steeper slope in the IeV curve near Iscand a reducedfill factor. This

decrease in the shunt resistance may be due to changes within the
device. Similarly, the slope of the IeV curve between Vmpand Vocis
affected by the amount of series resistances. Increased series
resistance reduces the steepness of the slope and also reduced the
fill factor. However, the decreasing trend of the series resistance
and the increasing trend of the shunt resistance with respect to the

Fig. 7. Effects of substrate temperature and Cu concentration on the change in I-V characteristics of the Glass/TCO/CdS/CdTe/Au heterostructure solar cell.

Table 5

Solar cell parameters of Glass/TCO/CdS/CdTe/Au structures.

Samples and conditions VocmV IscmA/cm2 VmaxmV ImaxmA/cm2 FF h(%) RsUcm RshUcm

Tsub¼ RT; Tannea¼ 400C

Pure CdTe 576 3.52 376 2.83 0.53 1.07 60.7 825

CdTe:Cu 2 wt.% 615 4.33 398 3.40 0.51 1.35 50.3 909

CdTe:Cu 3 wt.% 620 4.52 406 3.66 0.53 1.48 44.9 946

CdTe:Cu 4 wt.% 643 4.60 476 3.55 0.57 1.68 31.4 953

Tsub¼ 100C;Tannea¼ 400C

Pure CdTe 457 2.91 314 2.21 0.52 0.69 46.5 878

CdTe:Cu 2 wt.% 465 3.26 344 2.27 0.52 0.78 34.2 978

CdTe:Cu 3 wt.% 473 3.84 342 2.83 0.53 0.96 26.5 1120

CdTe:Cu 4 wt.% 482 4.14 378 3.02 0.57 1.21 13.4 1650

Tsub¼ 150C;Tannea¼ 400C

Pure CdTe 550 4.09 390 3.05 0.53 1.19 32.5 496

CdTe:Cu 2 wt.% 576 4.40 387 3.37 0.52 1.32 29.2 496

CdTe:Cu 3 wt.% 594 4.61 431 3.49 0.55 1.51 26.7 503

CdTe:Cu 4 wt.% 609 4.75 469 3.32 0.53 1.53 34.8 620

Tsub¼ 200C;Tannea¼ 400C

Pure CdTe 290 2.87 229 1.60 0.44 0.36 23.5 240

CdTe:Cu 2 wt.% 304 3.23 214 2.20 0.48 0.47 20.6 308

CdTe:Cu 3 wt.% 309 3.59 220 2.50 0.49 0.54 17.0 326

</div>
<span class='text_page_counter'>(11)</span><div class='page_container' data-page=11>

increasing Cu concentration lead to the increase in Iscand in the
ef_{ficiency (}Table 5). Paudel et al.[33]reported the similar range of
shunt resistance andfill factor values for the CdS/CdTe solar cells.
Madhu et al.[34]have reported the Vocof 209 mV, Iscof 2.3 mA/cm2,
FF of 0.3, and

h

of 1.88% for the electrochemically deposited CdS/
CdTe solar cells. Literature survey shows that the efficiency of CdTe
(1

m

m) based solar cells have been enhanced by improving the

crystallinity and the richness of Te through the CdCl2treatment
along with surface etching. Han et al.[35]observed the conversion
efficiency of 2.62%, with the open circuit voltage of 465 mV and the
fill factor of 37.6%. The improved collection of the short circuit
current may be due to the passivation of the grain boundaries
through the surface treatment. From the trend of our results, it is

well understood that the incorporation of the Cu dopant and the
temperature treatment facilitate the maintenance of the Te
rich-ness or the p-type conductivity. The increasing trend of the short
circuit current density infers that the grain boundary effect is
nullified to some extent with the improvement of the crystallinity
facilitated by the temperature treatment. Hence, it may be
mentioned that the obtained solar cell parameters may further be
improved by the above mentioned CdCl2heat treatment and
sur-face etching. However, the photoresponse of the device is good as
the Vocand Iscis increased with the increase in the input power as
observed fromFig. 10.

4. Conclusion

The structural and optical properties of undoped CdTe, Cu doped
(2, 3 and 4 wt.%) CdTe, CdS/CdTe, and CdS/CdTe:Cu layers deposited
on FTO substrate were studied. Thefilms exhibit a polycrystalline
state in the cubic zinc blende structure for the CdTe and in the
hexagonal structural phase for the CdS compounds. The crystallite
size was found higher for the samples deposited at RT and annealed
at 400C. The absence of the oxide and other elemental peaks
except Te in the XRD patterns inferred that the prepared materials
are single-phase of cell structure nature. The surface morphological

study showed the formation of netted surface features. The
pho-toluminescence study further confirmed the optical quality of the
prepared cell structures. The Cu-concentration-induced decrease in
band gap values of the samples was observed. The p-type
conductive nature of the preparedfilms was revealed by the hot
probe method. The low resistivity (1
103

_U

_{cm) of the CdTe:Cu}
(4 wt.%)film is due to the substitutional incorporation of the more
efcient Cu2ỵ_(Cd2ỵ_{) in the CdTe lattice as compared to other}_lms,
which increased the free carrier concentration. The study of the IeV
characteristics of the heterojunction solar cell structures has shown

Fig. 8. Plot of ln (JD) vs. anode voltage (V) of the Glass/TCO/CdS/CdTe: Cu (4 wt%)/Au
heterostructure solar cell prepared at RT and annealed at 400C.

Fig. 9. Change in current density and power with anode voltage of Glass/TCO/CdS/CdTe: Cu (4 wt%)/Au heterostructure solar cells prepared at different substrate temperatures
(Tsub¼ RT, 100, 150 and 200C) and annealed at 400C.

</div>
<span class='text_page_counter'>(12)</span><div class='page_container' data-page=12>

a low efficiency of the solar cells, which might be due to the nature
of the CdTe and CdS layers, the junction formation, and the grain
boundary effects. In addition, the low values of Iscwere observed,
which can be ascribed to the internal electricfield being not strong
enough to keep the liberated electrons and holes separated to pass
through the external circuit and also due to the low minority carrier
lifetime that leads to the recombination losses. Moreover, the
ef-ficiency is increased with increasing the Cu concentration. The
study reveals that the solar cells prepared at RT with 4 wt.% Cu
addition possess the maximum conversion efficiency of 1.68%.
Further, the device shows a good photoresponse as the Vocand Isc
are increased with increase in the input power.

References

[1] B.A. Chambers, B.I. MacDonald, M. Ionescu, A. Deslandes, J.S. Quinton,
J.J. Jasieniak, G.G. Andersson, Examining the role of ultra-thin atomic layer
deposited metal oxide barrier layers on CdTe/ITO interface stability during the
fabrication of solution processed nanocrystalline solar cells, Sol. Energy Mater.
Sol. Cell. 125 (2014) 164e169.

[2] Y. Sayad, Photovoltaic potential of III-nitride based tandem solar cells, J. Sci.:
Adv. Metab. Dev. 1 (2016) 379.

[3] Sadao Adachi, Properties of Group-IV, IIIeV and IIeVI Semiconductors, John
Wiley& Sons Ltd, 2005, p. 18.

[4] D. Bonnet, P. Meyers, Cadmium-telluride-materials for thinfilm solar cells,
J. Mater. Res. 13 (1998) 2740e2753.

[5] Z. Ma, K. Man Yu, L. Liu, L. Wang, D.L. Perry, W. Walukiewicz, P. Yu, S.S. Mao,
Copper- doped CdTefilms with improved hole mobility, Appl. Phys. Lett. 91
(2007), 092113e92121-92113-3.

[6] Z.R. Khan, M. Zulfequar, M. Shahid Khan, Structural, optical,
photo-luminescence, dielectric and electric studies of vacuum evaporated CdTe thin
films, Bull. Mater. Sci. 35 (2012) 169e174.

[7] J. Schaffner, M. Motzko, A. Tueschen, A. Swirschuk, H.-J. Schimper, A. Klein,
T. Modes, O. Zywitzki, W. Jaegemann, 12% efficient CdTe/CdS thin film solar
cells deposited by low- temperature close space sublimation, J. Appl. Phys. 110
(2011), 064508e64511-064508-6.

[8] B.M. Huang, L.P. Colletti, B.W. Gregory, J.L. Anderson, J.L. Stickney, Preliminary
studies of the use of an automatesflow-cell electrodeposition system for the
formation of CdTe thin films by electrochemical atomic layer epitaxy,
J. Electrochem. Soc. 142 (1995) 3007.

[9] F. de Moure-Flores, J.G. Qui~noones-Galvan, A. Guillen-Cervantes, J.S.
Arias-Ceron, G. Contreras-Puente, A. Hernandez- Hernandez, J. Santoyo-Salazar,
M. de la L. Olvera, M.A. Santana-Aranda, M. Zapata-Torres, J.G.
Mendoza-Alvarez, M. Melendez-Lira, Physical properties of CdTe:Cu films grown at low
temperature by pulsed laser deposition, J. Appl. Phys. 112 (2012),
113110e113112-113110-5.

[10] P. Bhattacharya, D.N. Bose, Pulsed laser deposition of CdTe thinfilms for
heterojunction on silicon, Semicond. Sci. Technol. 6 (1991) 384e387.
[11] R.F.C. Farrow, G.R. Jones, G.M. Williams, I.M. young, Molecular beam epitaxial

growth of high structural perfection, heteroepitaxial CdTefilms on InSb (001),
Appl. Phys. Lett. 39 (1981) 954e956.

[12] C. Gaire, S. Rao, M. Riley, L. Chen, A. Goyal, S. Lee, I. Bhat, T.-M. Lu, G.-C. Wang,
Epitaxial growth of CdTe thinfilm on cube-textures Ni by metal organic
chemical vapor deposition, Thin Solid Films 520 (2012) 1862e1865.
[13] M.P.R. Panicker, M. Knaster, F.A. Kroger, Cathodic deposition of CdTe from

aqueous electrolytes, J. Electrochem. Soc. 125 (1978) 566e572.

[14] K.R. Murali, I. Radhakrishna, K. Nagaraja Rao, V.K. Venkatesan, Properties of
CdTefilms deposited by electron beam evaporation, Surf. Coat. Technol. 41
(1990) 211e219. O.Caporalettei, M.R. Westcott, Fabrication of CdTe thin films
by electron beam evaporation, Can. J. Phys. 63(1985) 798e800.

[15] H. Kranenburg, C. Lodder, Tailoring growth and local composition by
oblique-incidence deposition: a review and new experimental data, Mater. Sci. Engg.
RII (1994) 295e354.

[16] A.I. Vovsi, L.P. Strakhov, O.A. Yakovuk, Mechanical strains in
vacuum-deposited CdTefilms Sov, Phys. Solid State 14 (1972) 1251e1254.
[17] R.W. Birkmire, E. Eser, Polycrystalline thinfilm solar cells: present status and

future potential, Annu. Rev. Mater. Sci. 27 (1997) 625e653.

[18] A.V. Kokate, M.R. Asabe, P.P. Hankare, B.K. Chougule, Effect of annealing on
properties of electrochemically deposited CdTe thinfilms, J. Phys. Chem. Solid
68 (2007) 53e58.

[19] M. Liu, X.Q. Wei, Z.G. Zhang, G. Sun, C.S. Chen, C.S. Xue, H.Z. Zhuang, B.Y. Man,
Effect of temperature on pulsed laser deposition of ZnOfilms, Appl. Surf. Sci.
252 (2006) 4321e4326.

[20] T.D. Dzhafarov, S.S. Yesilkaya, N. Yilmaz Canli, M. Caliskan, Diffusion and
in-fluence of Cu on propertied of CdTe thin films and CdTe/CdS cells, Sol. Energy
Mater. Sol. Cell. 85 (2005) 371e383.

</div>
<span class='text_page_counter'>(13)</span><div class='page_container' data-page=13>

[21] B. Pejova, The Urbach-Martienssen absorption tails in the optical spectra of
semiconducting variable-sized zinc selenide and cadmium selenide quantum
dots in thinfilm form, Mater. Chem. Phys. 119 (2010) 367e376.

[22] P. Hu, B. Li, L. Feng, J. Wu, H. Jiang, H. Yang, X. Xiao, , Effects of the substrate
temperature on the properties of CdTe thinfilms deposited by pulsed laser
deposition, Surf. Coating. Technol. 213 (2012) 84e89.

[23] I.M. Dharmadasa, P.A. Bingham, O.K. Echendu, H.I. Salim, T. Dryffel,
R. Dharmadasa, G.U. Sumanasekara, R.R. Dharmasena, M.B. Dergacheva, K.A. Mit,
K.A. Urazov, L. Bowen, M. Walls, A. Abbas, Fabrication of CdS/CdTe-based thin
film solar cells using an electrochemical technique, Coatings 4 (2014) 380e415.
[24] C. Ding, Z. ming, B. Li, L. Feng, J. Wu, Preparation and Characterization of
pulsed laser deposited CdTe thinlms at higher FTO substrate temperature
and in ArỵO2 atmosphere, Mater. Sci. Eng. B 178 (2013) 801e806.
[25] Caroline R. Corwine, Role of the Cu-O Defects in CdTe Solar Cells, Ph.D.

Des-sertation, Colorado State University, Colorado, 2006, pp. 30e31.

[26] L. Kosyachenko, Efficiency of Thin-Film CdS/CdTe Solar Cells, in: Radu
D. Rugescu (Ed.), Source: Solar Energy, Chernivtsi National University,
Ukraine, 2010, p. 432. ISBN 978-953-307-052-0.

[27] K.P. Bhandari, P. Koirala, N.R. Paudel, R.R. Khanal, A.B. Phillips, Y. Yan,
R.W. Collins, M.J. Heben, R.J. Ellingson, Iron pyrite nanocrystalfilm serves as a
copper-free back contact for polycrystalline CdTe thinfilm solar cells, Sol.
Energy Mater. Sol. Cell. 140 (2015) 108e114.

[28] C.A. Gretener, Back Contact, Doping and Stability of CdTe Thin Film Solar Cells
in Substrate Configuration, D.Sc. Dissertation, ETH Zurich, Switzerland, 2015.
[29] N.R. Paudel, K.A. Wieland, M. Young, A. Asher, A.D. Compaan, Stability of
sub-micron- thick CdTe solar cells, Progress in Photovoltaics: Research and
Ap-plications 22 (2014) 107.

[30] S.M. Sze, Physics of Semiconductor Devices. p-n junction diode, second ed.,
John Wiley and Sons, 1981, pp. 84e92 (Chapter 2).

[31] A.D. Compaan, J.R. Sites, R.W. Birkmire, C.S. Ferekides, and A.L.
Fahren-bruch, Critical issues and research needs for CdTe-based solar cells, Proc.
195th Meeting of the Electrochemical Society, PV 99-11, Seatle, WA, pp.
241e249.

[32] W.F. Mohammed, O. Daoud, M. Al-Tikriti, Power Conversion Enhancement
of CdS/CdTe solar cell interconnected with tunnel diode, Circ. Syst. 3
(2012) 230.

[33] N.R. Paudel, K.A. Wieland, A.D. Compaan, Ultrathin CdS/CdTe solar cells by
sputtering, Sol. Energy Mater. Sol. Cell. 10 (2012) 109e112.

[34] U. Madhu, N. Mukherjee, N.R. Bandyopadhyay, A. Mondal, Properties of CdS
and CdTe thinfilms deposited by an electrochemical technique, Indian J. Pure
Appl. Phys. 45 (2007) 226e230.

[35] J. Han, C. Liao, T. Jiang, C. Spanheimer, G. Haindl, G. Fu, V. Krishnakumar,
K. Zhao, A. Klein, W. Jaegermann, An optimized multilayer structure of CdS
layer for CdTe solar cells application, J. Alloy. Comp. 509 (2011) 5285e5289.
K. Punitha et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 86e98

</div>

<!--links-->
<a href=' /><a href=' />