energies
Article
Aluminum–Titanium Alloy Back Contact Reducing
Production Cost of Silicon Thin-Film Solar Cells
Hsin-Yu Wu 1 , Chia-Hsun Hsu 2 , Shui-Yang Lien 2, * and Yeu-Long Jiang 1
1
2
*
Graduate Institute of Optoelectronic Engineering and Department of Electrical Engineering,
National Chung Hsing University, Taichung 40227, Taiwan; [email protected] (H.-Y.W.);
[email protected] (Y.-L.J.)
Department of Materials Science and Engineering, Da-Yeh University, Changhua 51591, Taiwan;
[email protected]
Correspondence: [email protected]; Tel.: +886-404-851-1888 (ext. 1760)
Academic Editor: Senthilarasu Sundaram
Received: 12 August 2016; Accepted: 16 November 2016; Published: 22 November 2016
Abstract: In this study, metal films are fabricated by using an in-line reactive direct current magnetron
sputtering system. The aluminum–titanium (AlTi) back contacts are prepared by changing the
pressure from 10 mTorr to 25 mTorr. The optical, electrical and structural properties of the metal back
contacts are investigated. The solar cells with the AlTi had lower contact resistance than those with
the silver (Ag) back contact, resulting in a higher fill factor. The AlTi contact can achieve a solar cell
conversion efficiency as high as that obtained from the Ag contact. These findings encourage the
potential adoption of AlTi films as an alternative back contact to silver for silicon thin-film solar cells.
Keywords: silicon thin-film solar cells; aluminum–titanium (AlTi) alloy; contact resistance
1. Introduction
Silicon thin-film solar cells have received much attention, and appear to be promising
alternatives to crystalline silicon solar cells, which are dominant in photovoltaic power generation.
The main benefits of silicon thin-film solar cells over crystalline silicon solar cells are cost-effective
fabrication, compatibility with various flexible substrates, reliable large-area module encapsulation,
low temperature coefficient, very low weight per unit power and aesthetic design for building
integrated photovoltaic applications [1–3]. To achieve silicon thin-film solar cells with high short circuit
currents and conversion efficiencies, the incident light has to be efficiently scattered and diffracted.
Conventional solar cells normally achieve light trapping with three methods, a textured front surface,
a highly reflective rear reflector and an antireflection coating [4–6]. Silver (Ag) is the conventional metal
for back reflectors, but its drawbacks are high price, low adhesion and agglomeration behavior [7–9].
Therefore, researchers are searching for a cheaper alternative. Aluminum (Al) is a good candidate
owing to its low price and good adhesion [10], but it has lower reflectance and conductivity
than Ag. Metallic impurities such as titanium (Ti), chromium and vanadium can be added to the
Al films to improve the thermal stability, conductivity and ohmic contact [11–14]. For this purpose,
aluminum–titanium (AlTi) film, which is 1/100 of the cost of Ag, is proposed as a back contact for
p-i-n silicon thin-film solar cells. The AlTi thin films are prepared by a direct current (DC) magnetron
sputter system. The effects of sputtering pressure on the optical and electrical properties of AlTi films
are investigated. Finally, the performance of the solar cells with the AlTi back contact is compared to
the device with the traditional Ag contact.
Energies 2016, 9, 975; doi:10.3390/en9110975
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Energies 2016, 9, 975
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2. Experimental
2.
ExperimentalDetails
Details
The AlTi
AlTi and
and Ag
Ag films
films were
were prepared
prepared on
on glass
glass substrates
substrates by
by an
an in-line
in-line reactive
reactive DC
DC magnetron
magnetron
The
sputtering system
UHV Technology
Technology Co.,
Co., Ltd.,
Ltd., Hsinchu,
Hsinchu, Taiwan).
Taiwan). The
The composition
composition of
of the
the AlTi
AlTi
sputtering
system (LJ
(LJ UHV
−77 Torr;
−
target
was
99
wt%
Al
and
1
wt%
Ti.
The
base
pressure
of
the
deposition
chamber
was
5
×
10
target was 99 wt% Al and 1 wt% Ti. The base pressure of the deposition chamber was 5 × 10 Torr;
the DC
DC power
power was
was 200
200 W;
W; the
the argon
argon (Ar)
(Ar) gas
gas flow
flow rate
rate was
was 20
20 sccm,
sccm, and
and the
the substrate
substrate was
was at
at room
room
the
temperature. The
to 25
25 mTorr
on the
the
temperature.
The dependence
dependence of
of the
the sputtering
sputtering pressure
pressure changing
changing from
from 10
10 mTorr
mTorr to
mTorr on
properties
of
the
sputtered
AlTi
films
was
investigated.
The
structure
of
the
a-Si:H
thin-film
solar
properties of the sputtered AlTi films was investigated. The structure of the a-Si:H thin-film solar
cell was
wasglass/fluorine-doped
glass/fluorine-dopedtin
tindioxide
dioxide
(SnO
2:F)/p-a-Si:H/i-a-Si:H/n-a-Si:H/(gallium, Al)-doped
cell
(SnO
2 :F)/p-a-Si:H/i-a-Si:H/n-a-Si:H/(gallium, Al)-doped
zinc oxide
oxide(GAZO)/metal
(GAZO)/metal
back
contact.
The GAZO
was deposited
by radiofrequency
(RF)
zinc
back
contact.
The GAZO
was deposited
by radiofrequency
(RF) magnetron
magnetron
sputtering
with
a
thickness
of
80
nm.
The
p-,
iand
n-a-Si:H
layers,
with
a
total
thickness
sputtering with a thickness of 80 nm. The p-, i- and n-a-Si:H layers, with a total thickness of 288 nm,
of 288deposited
nm, were by
deposited
by a plasma-enhanced
chemical
vapor deposition.
Theresistance
contact resistance
were
a plasma-enhanced
chemical vapor
deposition.
The contact
between
between
back
contact
and
GAZO
was
measured
using
the
transmission
line
model
(TLM)
back contact and GAZO was measured using the transmission line model (TLM) method method
[15–17].
[15–17].
The
optical of
reflectance
of the
films with
was an
measured
with an spectrophotometer
ultraviolet-visible
The
optical
reflectance
the films was
measured
ultraviolet-visible
spectrophotometer
(UV-VIS,
Hong-Ming
Co., Ltd.,
Taipei, Taiwan).
The
refractive
(UV-VIS,
Hong-Ming
Technology
Co., Ltd.,Technology
Taipei, Taiwan).
The refractive
index was
determined
index
was
determined
by
a
spectroscopy
ellipsometry.
The
electrical
resistivity
was
measured
using
by a spectroscopy ellipsometry. The electrical resistivity was measured using a four-point probe
a
four-point
probe
method.
The
cross-section
microstructure
of
the
solar
cells
was
examined
by a
method. The cross-section microstructure of the solar cells was examined by a scanning electron
scanning electron
(SEM, JEOL
Tokyo,
Finally, the
microscopy
(SEM, microscopy
JEOL Ltd., Tokyo,
Japan).Ltd.,
Finally,
the Japan).
current-voltage
(I-V)current-voltage
characteristics (I-V)
and
characteristics
and
external
quantum
efficiency
(EQE)
of
the
devices
were
measured
by
a
solar
external quantum efficiency (EQE) of the devices were measured by a solar simulator under AM1.5G
2 ) conditions.
simulator
under
AM1.5G (100 mW/cm2) conditions.
(100
mW/cm
3. Results
Resultsand
andDiscussion
Discussion
of of
thethe
AlTi
films
prepared
at different
pressures.
The
Figure 1a
1a shows
showsthe
theoptical
opticalreflectance
reflectance
AlTi
films
prepared
at different
pressures.
reflectance
of the
filmsfilms
decreases
as the
increases
from from
10 mTorr
to 25to
mTorr.
The
The
reflectance
of AlTi
the AlTi
decreases
aspressure
the pressure
increases
10 mTorr
25 mTorr.
average
reflectance
at wavelengths
of 400–800
nmnm
at at
1010
mTorr
The
average
reflectance
at wavelengths
of 400–800
mTorrisis89.8%,
89.8%,which
whichisis about
about 7% lower
the Ag
Ag film.
film. The reflectance in the
the short
short wavelengths
wavelengths should have insignificant effects
than that of the
thesolar
solarcell
cellperformance,
performance,
since
be nearly
absorbed
reaching
rear
on the
since
the the
lightlight
will will
be nearly
absorbed
before before
reaching
the rearthe
contact.
contact.
long wavelengths
the AlTi
have a reflectance
around 85%–88%,
For
long For
wavelengths
(650–800 (650–800
nm), the nm),
AlTi films
havefilms
a reflectance
of aroundof
85%–88%,
while the
while
Ag
film has a reflectance
of around
97%.reflectance
The lowerof
reflectance
of the
films
at higher
Ag
filmthe
has
a reflectance
of around 97%.
The lower
the AlTi films
at AlTi
higher
pressure
can
pressure
can
be
explained
by
more
collision
among
sputtered
atoms
and
argon
ions,
lowering
be explained by more collision among sputtered atoms and argon ions, lowering the density of the
density of films
the sputtered
[18].
This is also
reflected
by the measurement
of the
refractive
index,
sputtered
[18]. Thisfilms
is also
reflected
by the
measurement
of the refractive
index,
as shown
in
as shown
Figure
1b. The
AlTi
filmhas
at 10
has
the highest
refractive
index value,
Figure
1b. in
The
AlTi film
at 10
mTorr
themTorr
highest
refractive
index
value, indicating
thatindicating
it has the
that it has
thestructure.
densest film structure.
densest
film
100
1.6
90
1.4
Refractive index
Reflection (%)
80
70
60
50
40
30
20
10
0
400
Pressure
(mTorr)
Average R
(%)
Ag
10
15
20
25
97.5
89.8
88.9
87
84.9
500
600
Ag
10
15
20
25
700
Wavelength (nm)
1.2
1.0
0.8
Ag
10
15
20
25
0.6
0.4
0.2
800
0.0
400
500
600
700
800
Wavelength (nm)
(a)
(b)
Figure 1. (a) Optical reflection and (b) refractive index of aluminum–titanium (AlTi) films prepared
Figure 1. (a) Optical reflection and (b) refractive index of aluminum–titanium (AlTi) films prepared
with different pressures.
with different pressures.
Figure 22 shows
films
prepared
with
different
pressures.
It can
that
showsthe
theresistivity
resistivityofofAlTi
AlTi
films
prepared
with
different
pressures.
It be
canseen
be seen
−6 Ω-cm −6
−6 Ω-cm −6
the
resistivity
of
the
AlTi
films
increases
from
7.94
×
10
to
11.8
×
10
with
the
pressure
that the resistivity of the AlTi films increases from 7.94 × 10 Ω-cm to 11.8 × 10 Ω-cm with the
pressure increasing from 10 mTorr to 25 mTorr. The AlTi film with a pressure of 10 mTorr has a
Energies 2016, 9, 975
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increasing from 10 mTorr to 25 mTorr. The AlTi film with a pressure of 10 mTorr has a minimum
minimum
electrical
resistivity,
can alsotobe
its dense structure.
In comparison,
the
electrical
resistivity,
and
this canand
alsothis
be linked
itslinked
densetostructure.
In comparison,
the resistivity
−6 -cm. Overall, the AlTi and Ag films have low resistivity at a
−
6
resistivity
of
the
Ag
film
is
4.13

10
of the Ag film is 4.13 × 10 Ω-cm. Overall, the AlTi and Ag films have low resistivity at a level
−6 Ω-cm,
level
of 10−6 so
Ω-cm,
their
bulk resistance
make
an insignificant
contribution
the device
total
of 10
theirso
bulk
resistance
shouldshould
make an
insignificant
contribution
to thetototal
device
series resistance.
series
resistance.
-6
Resistivity (10 ohm-cm)
15
12
9
6
3
0
Ag
10
20
15
25
Pressure (mtorr)
Figure
2.2.Resistivity
pressures.
Figure
ResistivityofofAlTi
AlTifilms
filmsprepared
prepared at
at different
different pressures.
contact
quality
between
the and
metal
and semiconductor
is important
when
TheThe
contact
quality
between
the metal
semiconductor
device isdevice
important
when considering
considering
an
electrode
material.
In
this
work,
the
back
electrode
is
covered
on
the
GAZO,
so
the
an electrode material. In this work, the back electrode is covered on the GAZO, so the contact resistance
contact resistance
c between
these two by
layers
is the
determined
by using
method
with the
Rc between
these twoRlayers
is determined
using
TLM method
withthe
theTLM
following
equation
[19]:
following equation [19]:
R ·d
(1)
Rt = 2Rc + SH ∙
(1)
=2 + W
where Rt denotes the total resistance between any two contacts (of width W) separated by a distance d,
denotes
total resistance
any two
contacts
(of widthfrom
W) separated
byinterception.
a distance
Rt the
andwhere
RSH is
sheetthe
resistance
of the between
GAZO film.
Rc can
be obtained
the y-axis
d,
and
R
SH
is
the
sheet
resistance
of
the
GAZO
film.
R
c
can
be
obtained
from
the
y-axis
interception.
Figure 3 shows Rc as a function of the sputtering pressure, where Rc increases with the increasing
−3 Ω) using
Figure The
3 shows
Rc as R
a cfunction
of the
sputtering
Rcthan
increases
with×the
pressure.
minimum
at 10 mTorr
is 3.0
× 10−3 pressure,
is lower
that (7.84
10increasing
Ω, which where
−3
−3
pressure. The
Rc at 10 mTorr
3.0 × 10 ,with
which
lowerfunction
than that(Φ)
(7.84
10 metal
Ω) using
Ag/GAZO.
The minimum
contact resistance
can beisassociated
theiswork
of×the
films
Ag/GAZO.
The
contact
resistance
can
be
associated
with
the
work
function
()
of
the
metal
films
andEnergies
the adhesion
at
the
contact
interface.
As
the
metal
films
are
responsible
for
collecting
electrons
2016, 9, x
3 of 6
the adhesion
at theacontact
interface.close
As the
films than
are responsible
foraffinity
collecting
electrons
in aand
device,
they require
work function
to metal
or smaller
the electron
of the
GAZO
in a device, they require a work function close to or smaller than the electron affinity of the GAZO
films
[19]. The
Ag (Φ resistivity,
= 4.26 eV) and
andthis
AlTican
(Φalso
= 4.28
eV) films
all available
work functions
minimum
electrical
be linked
to itsare
dense
structure.with
In comparison,
the
films [19]. The Ag ( = 4.26 eV) and−6 AlTi ( = 4.28 eV) films are all available with work functions
lower
than GAZO
4.45iseV)
to achieve
the n-type
ohmic
contact.
Consequently,
the resistivity
adhesion factor,
resistivity
of the(Φ
Ag=film
4.13
10 -cm.
Overall,
the AlTi
and Ag
films have low
at a
lower than−6GAZO ( = 4.45 eV) to achieve the n-type ohmic contact. Consequently, the adhesion
levelthan
of 10
so their bulk
resistance
should for
make
insignificant
contribution
to the
total
rather
the Ω-cm,
work function,
might
be the reason
thean
lower
contact resistance
of the
AlTi
film
factor, rather than the work function, might be the reason for the lower contact resistance of the
device series
compared
to theresistance.
Ag film.
AlTi film compared to the Ag film.
-6
-3
Resistivity
(10 (10
ohm-cm)
Contact
resistance
ohm)
15
7
12 6
9
5
4
6
3
32
1
0
Ag
0
Ag
10
10
20
15
25
25
Pressure15(mtorr) 20
Pressure (mtorr)
Figure 3. Contact
resistance
for metal/GAZO
functionpressures.
of sputtering pressure.
Figure
2. Resistivity
of AlTi filmsinterface
preparedasatadifferent
Figure 3. Contact resistance for metal/GAZO interface as a function of sputtering pressure.
The 4contact
betweenof the
metal cells
and with
semiconductor
device is
important
when
Figure
shows quality
the I-V curves
the solar
AlTi back contacts
prepared
at different
Figure 4 shows the I-V curves of the solar cells with AlTi back contacts prepared at different
considering
an
electrode
material.
In
this
work,
the
back
electrode
is
covered
on
the
GAZO,
so
the
pressures. Table 1 summarizes the photovoltaic parameters of the solar cells such as open-circuit
pressures. Table 1 summarizes the photovoltaic parameters of the solar cells such as open-circuit
contact
resistance
Rc between these
two layers is factor
determinedand
by conversion
using the TLM
method(η).
with
voltage
(V oc
efficiency
Thethe
oc
voltage
(V),ocshort-circuit
), short-circuitcurrent
currentdensity
density(J(Jscsc),), fill
fill factor (FF)
(FF) and conversion
efficiency
(η). The
VV
oc
following
values
do notequation
change[19]:
significantly for all the AlTi and Ag films. Jsc and FF both decrease with the
=2
+
∙
(1)
where Rt denotes the total resistance between any two contacts (of width W) separated by a distance
d, and RSH is the sheet resistance of the GAZO film. Rc can be obtained from the y-axis interception.
Energies 2016, 9, 975
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voltage (Voc), short-circuit current density (Jsc), fill factor (FF) and conversion efficiency (η). The Voc
values
not
change significantly
for all the
and Ag
films.
sc and FF both decrease with the
voltagedo
(Voc
), short-circuit
current density
(Jsc),AlTi
fill factor
(FF)
and Jconversion
efficiency (η). The Voc
increasing
pressure,
due
totothe
reduced
reflectance
and
increasedresistivity,
resistivity,respectively.
respectively.The
The
cell
increasing
pressure,
due
the
reduced
reflectance
and
increased
cell
values do not change significantly for all the AlTi and Ag films. Jsc and FF both decrease with
the
with
the
AlTi
deposited
at
10
mTorr
has
the
highest
conversion
efficiency
of
8.15%
with
V
=
0.84
with
the AlTi
deposited
mTorr
has the
highest conversion
efficiency
of 8.15%
with Voc oc
=The
0.84cell
V, V,
increasing
pressure,
dueatto10the
reduced
reflectance
and increased
resistivity,
respectively.
2
2 and
Jsc Jwith
=sc 13.7
mA/cm
== 10
0.71.
Thishas
conversion
efficiency
value
evenhigher
higher
than
that
of0.84
the
cell
= 13.7
mA/cm
and FF
FFat
This
conversion
efficiency
value
isiseven
than
that
cell
the AlTi
deposited
mTorr
the highest
conversion
efficiency
of 8.15%
with
Vocof=the
V,
with
the
Ag
film,
owing
to
the
greater
FF
compensating
for
the
lower
J
.
with
the
Ag
film,
owing
to
the
greater
FF
compensating
lower
J
sc
.
sc
Jsc = 13.7 mA/cm2 and FF = 0.71. This conversion efficiency value is even higher than that of the cell
with the Ag film, owing to the greater FF compensating for the lower Jsc.
2
2
Current
density
(mA/cm
Current
density
(mA/cm
) )
15
15
12
12
9
Ag
10
Ag
15
10
20
15
25
20
25
9
6
6
3
3
0
0.0
0
0.0
0.2
0.4
0.6
0.8
1.0
0.8
1.0
Voltage (V)
0.2
0.4
0.6
Voltage
Figure
4. 4.
I-V
AlTi
back(V)
contactsprepared
preparedatatdifferent
differentpressures.
pressures.
Figure
I-Vcurves
curvesofofp-i-n
p-i-nsolar
solar cells
cells with
with AlTi
back
contacts
Figure 4. I-V curves of p-i-n solar cells with AlTi back contacts prepared at different pressures.
Table
1. 1.Performance
at different
differentpressures.
pressures.FF:
FF:fill
fillfactor.
factor.
Table
Performanceofofp-i-n
p-i-nsolar
solar cells
cells prepared at
Table 1. Performance
of p-i-n Vsolar
cellsJscprepared
oc (V)
(mA/cm2at
) different
ηpressures.
(%) Roc FF:
(Ω) fill factor.
Pressure (mTorr)
FF
V oc (V)
Pressure (mTorr)
Jsc (mA/cm2 )
Ref. Ag
oc (V)
Pressure
(mTorr) 0.84
V0.84
Ag 10
0.84
Ref.
Ag
0.84
10
0.84
15
0.84
15 10
0.840.84
20
20 15
0.840.84
0.84
25 25
0.840.84
20
0.84
25
0.84
13.93 2)
Jsc
(mA/cm
13.93
13.70
13.93
13.70
13.31
13.70
13.31
13.05
13.05
13.31
12.68
12.68
13.05
12.68
FF
0.68
FF
0.68
0.71
0.68
0.71
0.71
0.71
0.71
0.71
0.71
0.71
0.69
0.69
0.71
0.69
η (%)
Roc (Ω)
η8.03
(%) R106.0
oc (Ω)
8.158.03 74.0 106.0
8.038.15 106.074.0
7.99
82.3
8.157.99 74.0 82.3
7.827.82 88.4 88.4
7.99
82.3
7.387.38 99.1 99.1
7.82
88.4
7.38
99.1
External
Quantum
Efficiency
External
Quantum
Efficiency
(%) (%)
Figure 5 shows the EQE curves of the p-i-n silicon thin-film solar cells with AlTi back contacts
Figure 5 shows the EQE curves of the p-i-n silicon thin-film solar cells with AlTi back contacts
prepared
at 5different
pressures.
The EQE
values
similar
in the solar
short-wavelength
region
Figure
shows the
EQE curves
of the
p-i-n are
thin-film
cells with AlTi
back(400–550
contacts
prepared
atalldifferent
pressures.
The EQE
values
areissilicon
similar
in
the short-wavelength
region
(400–550
nm)
nm)
in
cases,
as
the
short-wavelength
light
nearly
completely
absorbed
before
reaching
the
prepared at different pressures. The EQE values are similar in the short-wavelength region
(400–550
in back
all cases,
as the
light is nearly
completely absorbed
reaching
the back
contact.
Theshort-wavelength
in the long-wavelength
with
thebefore
increasing
deposition
nm) in
all cases,
asEQE
the short-wavelength
light isregion
nearly decreases
completely
absorbed
before reaching
the
contact.
The
EQE
in
the
long-wavelength
region
decreases
with
the
increasing
deposition
pressure.
pressure.
The
cell
with
the
Ag
back
contact
has
the
highest
long-wavelength
EQE
response,
due
to
back contact. The EQE in the long-wavelength region decreases with the increasing deposition
The
cell
withreflectance.
the Ag back
contact has
the
highest
long-wavelength
EQE
response,
due
to the
higher
the
higher
Comparing
the
best
AlTi
film
(at
10
mTorr)
to
the
Ag
film,
the
lower
EQE
of
pressure. The cell with the Ag back contact has the highest long-wavelength EQE response, due to
reflectance.
Comparing
best
AlTi
film
(at loss
10 mTorr)
to the Ag film,
the lower EQE of
back
the
AlTi back
contact the
only
leads
tothe
a 1.6%
in
Jsc.(at
Therefore,
into
combination
thethe
IVAlTi
result,
the higher
reflectance.
Comparing
best AlTi
film
10 mTorr)
the Ag film,with
the lower
EQE
of
contact
only
leads
to
a
1.6%
loss
in
J
.
Therefore,
in
combination
with
the
IV
result,
the
present
study
the
present
study
shows
that
the
reduction
in
the
contact
resistance
is
more
helpful
than
the
sc a 1.6% loss in Jsc. Therefore, in combination with the IV result,
the AlTi back contact only leads to
shows
that the study
reduction
in surface
the
resistance
is
more
helpful
thancell
the
improvement
in
the
back
improvement
in theshows
back
for
theresistance
solar
The AlTi
back
the present
thatcontact
thereflectance
reduction
in increasing
the
contact
isefficiency.
more helpful
than
the
contacts
demonstrate
high
potential
for
application
in
thin-film
solar
cells.
surface
reflectance
for
increasing
the
solar
cell
efficiency.
The
AlTi
back
contacts
demonstrate
high
improvement in the back surface reflectance for increasing the solar cell efficiency. The AlTi back
potential
for
application
in
thin-film
solar
cells.
contacts demonstrate high potential for application in thin-film solar cells.
80
70
80
60
70
50
60
40
50
30
40
20
30
10
20
0
10
400
0
400
Ag
10
Ag
15
10
20
15
25
20
25
500
600
Wavelength (nm)
500
600
700
800
700
800
Wavelength
Figure 5. External quantum efficiency (EQE)
curves of(nm)
p-i-n solar cells with Ag and AlTi films
prepared
at
different
pressures.
Figure
5. External
quantum
efficiency
(EQE)
curves
of p-i-n
cells Ag
with
Ag
andfilms
AlTiprepared
films
Figure
5. External
quantum
efficiency
(EQE)
curves
of p-i-n
solar solar
cells with
and
AlTi
prepared
at
different
pressures.
at different pressures.
Energies 2016, 9, 975
Energies 2016, 9, 975
5 of 6
5 of 6
Figure 6 illustrates the cross-section
cross-section SEM
SEM image
image of
of the
the solar
solar cells
cells with
with the
the AlTi
AlTi back
back contact.
contact.
The textured SnO
SnO22:F,
:F, in
in order
orderto
toscatter
scatterlight
lighttotoincrease
increasethe
thelight
lighttraveling
traveling
path
absorber,
path
in in
thethe
absorber,
is
is
observed.
The
thicknesses
therough
roughSnO
SnO
thickfilm
filmand
anda-Si:H
a-Si:Hdevice
deviceare
are800
800nm
nmand
and 280
280 nm,
nm,
observed.
The
thicknesses
ofofthe
2:F
thick
2 :F
respectively. The
The interfaces
interfaces in
in the
the solar
solar cells
cells closely
closely follow
follow the
the surface
surface profile
profile of
of the SnO22:F
:F layer.
layer.
The thin,
ofof
about
8080
nmnm
is clearly
seen
on on
the the
a-Si:H,
andand
the
thin, crystalline
crystallineGAZO
GAZOlayer
layerwith
witha athickness
thickness
about
is clearly
seen
a-Si:H,
AlTi
film film
is covered
at theattop.
the back
could lead
voids
cracks
the valleys
the AlTi
is covered
theSputtering
top. Sputtering
the contact
back contact
couldtolead
toor
voids
or at
cracks
at the
or
mountains
of the textured
butsurface,
those are
notthose
observable
the interface.
confirms
valleys
or mountains
of thesurface,
textured
but
are notat observable
at This
the result
interface.
This
that
good
adhesion
coverage
of and
the AlTi
film lead
to good
contact
properties
betweenproperties
AlTi and
result
confirms
thatand
good
adhesion
coverage
of the
AlTi film
lead
to good contact
GAZO
films.
between
AlTi and GAZO films.
Figure 6. Cross-section scanning electron microscopy (SEM) image of p-i-n silicon thin-film solar cell
with AlTi
AlTi back
back contact.
contact.
4. Conclusions
Conclusions
the back contact of p-i-n silicon thin-film solar cells are
In this study, low-cost AlTi films as the
prepared.
The
films
achieved
a
high
average
reflectance
90%
in the
wavelength
range
of 400–800
prepared. The films achieved a high average reflectance
of of
90%
in the
wavelength
range
of 400–800
nm,
−
6
−6
nm, and
a low
resistivity
of ×
7.910× 10 Ω-cm
Ω-cmcould
couldbebeobtained.
obtained.The
Theoptimal
optimal AlTi
AlTi back
back contact
contact has a
and
a low
resistivity
of 7.9
short-circuit current
current density
densityof
of13.7
13.7mA/cm
mA/cm22, which is slightly
slightly lower
lower than
than the
the13.93
13.93mA/cm
mA/cm22 obtained
short-circuit
the Ag
Agback
backcontact.
contact.However,
However,the
the
AlTi
back
contact
a factor
fill factor
of 0.71,
which
is higher
from the
AlTi
back
contact
hashas
a fill
of 0.71,
which
is higher
than
than0.68
thefor
0.68
Agcontact.
back contact.
theback
AlTicontact
back contact
performs
as back
the Ag
back
the
thefor
Agthe
back
Thus, Thus,
the AlTi
performs
as well as
as well
the Ag
contact.
contact.
In conclusion,
AlTi
metalare
alloys
are a promising
alternative
to Agasmetal
as contact
a back contact
for
In
conclusion,
AlTi metal
alloys
a promising
alternative
to Ag metal
a back
for silicon
silicon thin-film
solar
cells.
The
AlTi
back can
contact
can low
achieve
low high
cost and
high efficiency.
Future
thin-film
solar cells.
The
AlTi
back
contact
achieve
cost and
efficiency.
Future work
will
work willthe
examine
the use
AlTi for high-efficiency
Si-based
tandem
examine
use of AlTi
forof
high-efficiency
Si-based tandem
solar
cells. solar cells.
Acknowledgments:
Acknowledgments: This work is sponsored by the Ministry of Science and Technology of the Republic of China
under
under contract
contract No.
Nos.105-2632-E-212-001105-2632-E-212-001-and
and104-2221-E-212-002-MY3.
104-2221-E-212-002-MY3.
Author Contributions: Hsin-Yu Wu and Shui-Yang Lien designed the experiments; Hsin-Yu Wu performed the
Author Contributions:
Hsin-YuHsin-Yu
Wu andWu,
Shui-Yang
LienHsu,
designed
the experiments;
Hsin-Yu
Wu
performed
experiments
and measurements;
Chia-Hsun
Shui-Yang
Lien and Yeu-Long
Jiang
analyzed
the
the experiments
and
measurements;
Hsin-Yu
Wu,
Chia-Hsun Hsu, Shui-Yang Lien and Yeu-Long Jiang
results;
Hsin-Yu Wu
and
Chia-Hsun Hsu
wrote the
paper.
analyzed the results; Hsin-Yu Wu and Chia-Hsun Hsu wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
Conflicts of Interest: The authors declare no conflict of interest.
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