Developing of dual junction GaInP/GaAs solar cell devices: effects of different metal contacts

Abstract

DJ GaInP/GaAs SC structure was designed by using analytical solar cell model. The electrical parameters (Jsc, Voc, FF and η) were calculated by determining optimum conditions for improving the performance of the SCs. Considering the optimization conditions in design of SC, lattice and current matched DJ GaInP/GaAs SC structure was grown using MBE technique. Alloy composition and lattice constants of each layers in the structure were estimated from measured XRD data. To evaluate of effects on conversion efficiency of different metal contact materials, SC devices were fabricated by photolithographic technique. Two types of front-side electrodes, which included Au and Au/Ti metals, were separately fabricated on devices and denoted as S1 and S2, respectively. Performance of the S1 and S2 were determined using I–V measurements under the AM1.5 illuminations. S2 possesses 4.16% enhancement in conversion efficiency compared to that of S1. The better performance of the S2 can be attributed to having higher Isc and Voc due to higher conductivity of titanium as well as good adhesion on GaAs. In addition, Al2O3/TiO2 anti-reflective coating effect on performance of the S1 and S2 was also investigated. Sputtered anti-reflective layer increased the efficiencies from 14.65 to 15.72% and 15.26 to 16.90% for S1 and S2, respectively.

Full text

Vol.:(0123456789)
Optical and Quantum Electronics (2018) 50:277
https://doi.org/10.1007/s11082-018-1546-5
1 3
Developing ofdual junction GaInP/GaAs solar cell devices:
eects ofdierent metal contacts
TugceAtaser1 · NihanAkinSonmez1,2 · YunusOzen1,3· VeyselOzdemir4·
OrhanZeybek5· SuleymanOzcelik1,3
Received: 14 February 2018 / Accepted: 16 June 2018
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
DJ GaInP/GaAs SC structure was designed by using analytical solar cell model. The elec-
trical parameters (Jsc, Voc, FF and η) were calculated by determining optimum conditions
for improving the performance of the SCs. Considering the optimization conditions in
design of SC, lattice and current matched DJ GaInP/GaAs SC structure was grown using
MBE technique. Alloy composition and lattice constants of each layers in the structure
were estimated from measured XRD data. To evaluate of effects on conversion efficiency
of different metal contact materials, SC devices were fabricated by photolithographic tech-
nique. Two types of front-side electrodes, which included Au and Au/Ti metals, were sepa-
rately fabricated on devices and denoted as S1 and S2, respectively. Performance of the S1
and S2 were determined using I–V measurements under the AM1.5 illuminations. S2 pos-
sesses 4.16% enhancement in conversion efficiency compared to that of S1. The better per-
formance of the S2 can be attributed to having higher Isc and Voc due to higher conductivity
of titanium as well as good adhesion on GaAs. In addition, Al2O3/TiO2 anti-reflective coat-
ing effect on performance of the S1 and S2 was also investigated. Sputtered anti-reflective
layer increased the efficiencies from 14.65 to 15.72% and 15.26 to 16.90% for S1 and S2,
respectively.
Keywords III–V multi-junction solar cells· Modeling· MBE· Anti-reflective coating·
Al2O3/TiO2
* Tugce Ataser
tugceataser@gmail.com
1 Photonics Application andResearch Center, Gazi University, Ankara, Turkey
2 Department ofElectrics andEnergy, Technical Sciences VS, Gazi University, Ankara, Turkey
3 Department ofPhysics ofScience Faculty, Gazi University, Ankara, Turkey
4 Department ofIndustrial Design Engineering ofTechnology Faculty, Gazi University, Ankara,
Turkey
5 Department ofPhysics, Faculty ofArts andSciences, Balikesir University, Balikesir, Turkey

T.Ataser et al.
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1 Introduction
Nowadays, multi-junction solar cells have high device performance than that known other
solar cells. Both of achieving high conversion efficiency and of overcoming complexity,
high cost and time-consumption is based on well-designing lattice-matched these type
solar cells because the main physical phenomena and behaviors can be easily understood
with numerical modeling or simulation. The solar cell structures growing by different tech-
niques such as MBE (Paxman etal. 1993), MOCVD (Jani etal. 2006), MBE-MOCVD
(Campesato et al. 2017) and MOVPE (Wheeldon et al. 2010) considering the design
results, have high crystallinity and current match between the cells. The contribution of the
solar cell studies, which are presented both theoretically and experimentally, is valuable for
literature.
Basically, III–V compound multi-junction solar cells have a stack of two or more single
junction solar cells with different materials (GaAs, AlGaAs, GaInP, GaInAs, GaAsP etc.)
and band gap of the cells decrease from top to bottom cells. Thus, photons come from the
sun are converted to photo-currents and high optical efficiency can be achieved thanks to
a good device architecture leading to increase device performances. With this method, in
the 1970s, Bedair and Lamorte (1979) has been reported that AlGaAs/GaAs dual junc-
tion solar cell has an efficiency of 25.00% under one-sun illumination. At the same years,
GaInP/GaAs dual junction solar cell with an efficiency of 14.00% was developed by Olsen
etal. to provide lattice matching at the growth of the cell structures (Olsen etal. 1978). For
GaInP/GaAs cells, in the 1990s–2000s, there are also studies reported different conversion
efficiency such as 25.20 and 27.35% (Lei etal. 2009; Bertness etal. 1994). Recently, one-
sun efficiency of flexible thin film InGaP/GaAs solar cells described from NREL-verified
were 30.8% (Kayes etal. 2014). In addition, conversion efficiency of the double-junction
GaInP/GaAs solar cells was increased up to 33.80% by developing of double-sided GaInP/
GaAs/InGaAs triple-junction solar cells due to increasing absorption of more IR part of the
solar spectra, having inverted grown of InGaAs sub-cell on back-side of the GaAs (Geisz
etal. 2007).
The resistance at the interface or contact resistivity can be minimized with the opera-
tion of metal–semiconductor contacts. Most III–V compound multi-junction solar cells use
high doped GaAs as a cap layer to obtain good quasi-ohmic behavior between metal and
semiconductor (Cotal etal. 2009). It can be easily understood that the contacts contain-
ing thin Ti metal layer on n + GaAs semiconductor have important benefits since Ti will
be useful on the adhesion of the subsequent base metal layers. So, determination of metal
contact materials of the cells is an important parameter during multi-junction device fabri-
cation to achieve good conduction of the current.
Our aim here is to design of dual junction GaInP/GaAs solar cell (DJ GaInP/GaAs SC)
and growth of the structure using Molecular Beam Epitaxy (MBE) technique considering
the optimization conditions in the design of the cell. Alloy composition and lattice con-
stants of each layers in the structure were analyzed with High Resolution X-Ray Diffraction
(HRXRD) measurements to understand lattice matching between the subcells. With using
vacuum thermal evaporation technique, Au and Au/Ti front contacts and Au and AuZn
back contacts were coated on DJ GaInP/GaAs SCs grown on GaAs substrates for operation
at one-sun illumination. Then, the effect of the contact materials on the conversion effi-
ciency was evaluated by electrical measurements performed on the I–V measurement sys-
tem. In addition, it was also determined the effect of the Al2O3/TiO2 anti-reflection sput-
tered on the produced cell devices.

Developing ofdual junction GaInP/GaAs solar cell devices:…
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2 Cell structure
DJ GaInP/GaAs SC structure was designed to attain higher conversion efficiency with
optimizing the thickness and doping concentration of the cells. The modeling DJ GaInP/
GaAs SC structure was schematically represented in Fig.1. In the modeling SC struc-
ture, GaAs bottom and GaInP top cells were interconnected by a thin and ultra-high
doped tunnel junction to provide electrical conduction of the cell. The placed tunnel
junction provides low resistance and high current density in the SCs (Garcıa etal. 2012;
Özen etal. 2015; Nayak etal. 2015; Siyu and Xiaosheng 2011).
The thicknesses of the base (p-type) and emitter (n-type) layers of the GaAs bottom
cell were determined to be 1000 and 150nm and for the GaInP top cell, its base and
emitter thicknesses were 800 and 170nm, respectively. The dopants described as Si (n)
and Be (p) in the cells. The concentrations of acceptor (NA) and donor (ND) were deter-
mined for the GaAs bottom and GaInP top cells considering the cell thicknesses. The
doping concentrations of the base and emitter layers of the GaAs bottom and GaInP top
cells were optimized to be 2 × 1017–2 × 1018 cm−3 and 2 × 1017–7 × 1018 cm−3, respec-
tively. The short circuit current density (Jsc), the open circuit voltage (Voc), fill factor
(FF) and the conversion efficiency (η) of the modeling DJ GaInP/GaAs SC were calcu-
lated from the equations given in our previous work (Ataser etal. 2016) and using the
algorithm (Castafier and Silvestre 2002) given in Fig.2.
The J-V graph of the modeling DJ GaInP/GaAs SC were given in Fig.3. Jsc of the
GaInP top and GaAs bottom cells were calculated to be 15.75 and 16.02 mA/cm2,
respectively. According to this, it can be easily seen that the current density of the
GaInP top cell is slightly less than the current density of the GaAs bottom cell. There-
fore, it can be said that there is a current matching between the cells which are con-
nected to each other in series (Arzbin and Ghadimi 2017). The current density of the DJ
GaInP/GaAs SC was limited by the GaInP top cell as shown in Fig.3.
Cap n+GaAs 60 nm
Window n_ AlGaAs 50 nm
Emitter n_ GaInP 170 nm
Base p_GaInP 800 nm
Emitter n_GaAs 150 nm
Base p_ GaAs 1000 nm
Substrate p_GaAs 3000 nm
Tunnel Diode n++ AlGaAs 50 nm
++ AlGaAs 50 nm
Bottom Cell
Tunnel Junction
Top Cell
Tunnel Diode
p
Fig. 1 The device architecture of the DJ GaInP/GaAs SC

T.Ataser et al.
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277 Page 4 of 13
Theoretically calculated electrical parameters of the DJ GaInP/GaAs SC were given in
Table1. Voc = 2.39 (V), Jsc = 15.57 (mA/cm2), FF = 94.07 (%) and η = 25.62 (%) were calcu-
lated for the structure, under AM1.5 (one-sun) illumination. A high fill factor, 94.07%, was
achieved without BSF for the DJ GaInP/GaAs SC with band gaps of GaInP and GaAs lay-
ers having 1.93eV and GaAs 1.42 eV, respectively, compared to the best-known fill factor,
90.90%, reported by Abbasian and Sabbaghi-Nadooshan (2017).
3 Experimental method
DJ GaInP/GaAs SC structure was grown in the VG-80H solid-source MBE system. Prior to
the growth process, for removing of volatile impurities on (100) oriented p-GaAs substrate
was heated at 400°C for 2h in the preparation chamber, then, the surface oxide desorption
Wavelenght Absorption
Coefficient
Solar
Spectrum
α
( )
IR( )
Voltage Source
DC
Parameters
Fig. 2 Solar cell performance calculation algorithm

Developing ofdual junction GaInP/GaAs solar cell devices:…
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Page 5 of 13 277
was realized under the arsenic flux in deposition chamber at 670°C. After the cleaning pro-
cess, firstly, the 1000nm thick p-GaAs layer was grown on p-GaAs substrate at 650°C and
then the 150nm thick n-GaAs layer was grown on the p-GaAs layer at 640°C. Then the
50nm thick high doped n++AlGaAs and the 50nm thick p++AlGaAs tunnel diodes were
grown on GaAs bottom cell at 640°C. By reducing the surface temperature to 530°C, the
800nm thick p-GaInP layer was grown on AlGaAs tunnel junction and then the 170nm
thick n-GaInP layer was grown on p-GaInP. The solar cell structure completed by a growth
of a 50nm thick n-AlGaAs window layer on GaInP top cell and a 60nm thick n + GaAs
cap layer on n-AlGaAs window layer. Si and Be sources were used for n-type and p-type
dopants, respectively. The growth rate of the epi-layers in the structure was determined
using Reflection of High Energy Electron Diffractions (RHEED) as 1 ML per second. A
schematic diagram of the grown DJ GaInP/GaAs SC structure was same with designed
structure given in Fig.1. All growth conditions were listed in Table2.
X-ray diffraction patterns of the produced solar cell structure were determined by a D-8
Bruker high-resolution diffractometer by using CuKα1 (1.5406 Å) radiation and a 4-crys-
taled Ge (220) symmetric monochromator. The x alloy ratio of the Ga1−xInxP material in
the solar cell structure was determined by a simulation program called LEPTOS applied to
the HRXRD measurement results.
DJ GaInP/GaAs SC structure divided into two parts of 1 × 1cm2. Having fingers width
100 the distance of 400 each other, mesa structures on these two parts that schematically
shown in Fig.4, were performed by chemical etching process and standard photolithogra-
phy technique using Kalr-Suss MJB4 mask aligner system.
While, one of them has the Au (200nm) metal contacts both on the front and back side
and other one has the Au/Ti (100/100nm) metal contact on the front and AuZn (250nm)
Fig. 3 J-V characterization of the
modeling DJ GaInP/GaAs SC
0
2
4
6
8
10
12
14
16
18
0.00.5 1.01.5 2.02.5 3.
0
Current Density (mA/cm2)
Voltage(V)
GaAs
GaInP
GaInP/GaAs
Table 1 Theoretically calculated
electrical parameters of DJ
GaInP/GaAs SC
Jsc (mA/cm2) Voc (V) FF (%) η (%)
15.57 2.39 94.07 %25.62

T.Ataser et al.
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277 Page 6 of 13
alloy contact on the back side of the structure with using vacuum thermal evaporation tech-
nique under the 10−8 mbar pressure. To easily expressing, fabricated devices denoted as S1
and S2, respectively. As a last step, the S1 and S2 devices were annealed via rapid thermal
annealing during 50s at 380°C to form ohmic metallization. Output parameters (Jsc, Voc,
FF and η) of the devices were determined using an I–V measurement system with Keithley
4200 sourcemeter under the AM1.5 illumination from Oriel-Sol1A solar simulator. Al2O3/
TiO2 (10/15nm) thin films were deposited consecutively by sputtering method between
the thin metal grids formed on the front surfaces of the devices in order to increase effi-
ciency of the S1 and S2 devices. Before analyzing the anti-reflective coating effects on
the performance of the devices, the transmittance of totally 25nm Al2O3/TiO2 thin film
sputtered on glass substrate were characterized by optical transmittance measurements.
Then, an Al2O3/TiO2 anti-reflective coating effect was investigated. In addition, external
Table 2 Growth conditions of DJ GaInP/GaAs SC
Solar cell structure Thickness (nm) Substrate Temperature
(°C) Time (min)
n + GaAs Cap Layer 60 530 3.6
n-AlGaAs Window Layer 50 530 3
n-GaInP Layer 170 530 10.2
p-GaInP Layer 800 530 48
p++AlGaAs Tunel Layer 50 640 3
n++AlGaAs Tunel Layer 50 640 3
n-GaAs Layer 150 640 9
p-GaAs Layer 1000 650 60
Fig. 4 Fabricated DJ GaInP/
GaAs SC
3'' p_GaAs (100)
p_GaAs 1000nm
n_GaAs 150 nm
n++AlGaAs 50 nm
p++AlGaAs 50 nm
p_GaInP 800 nm
n_GaInP 170 nm
n_AlGaAs 50 nm

Developing ofdual junction GaInP/GaAs solar cell devices:…
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Page 7 of 13 277
quantum efficiency (EQE) of the cells was performed by a Newport QEPVSI-B spectrom-
eter with a spot size equal to the cell area calibrated Si reference cell in spectral range of
300–1600nm.
4 Results
Experimental and simulated HRXRD diffraction patterns of the MBE grown DJ GaInP/
GaAs SC structure were given in Fig.5. It was seen that there are interference peaks next
to GaAs and GaInP peaks on the SC structure. Indium alloy ratio of the Ga1−xInxP was
found to be 45.91%, using Vegard-based
(
x=
|
θGaInP−𝜃InP
|
|
θGaP−θInP
|)
equation due to these interfer-
ence peaks (Adachi 2009). Then, based on this x alloy ratio, the energy band gap of the
GaInP top cell was calculated to be 1.93eV using Vegard’s law. This band gap enables
photons in the ultraviolet region to be absorbed efficiently (Vurgaftman etal. 2001). Fur-
ther, GaAs absorbs most of the sunlight in visible range thanks to its energy band gap of
1.42eV (Hossain etal. 2016). In addition, lattice constants of the GaInP and GaAs were
calculated to be 5.655805 Å and 5.655045 Å, respectively. The lattice mismatch between
the GaInP and GaAs cells was found to be 0.013% using
(
αGaInP−αGaAs
αGaAs
×100
)
equation
(Ochoa-Martínez etal. 2018). As expected, the lattice mismatch will not occur as calcu-
lated by the fact that the mentioned two structures have the same zinc blende crystal struc-
ture. The obtained structural parameter values are consistent with the literature and show
that the growth DJ GaInP/GaAs SC structure has low strain and high crystal quality (Olson
etal. 2006).
Fig. 5 HR-XRD patterns of the
DJ GaInP/GaAs SC structure
65 65.5 66 66.5
67
Intensity (arb.u)
ω-2 (degree)
Experimental
Simulation
GaAs
GaInP

T.Ataser et al.
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277 Page 8 of 13
Electrical characterization of fabricated S1 and S2 solar cell devices was performed by
the current–voltage (I–V) characteristics in dark and under AM1.5 illumination at room
temperature and shown in Fig.6. The detailed device characteristics consisting of Jsc, Voc,
FF and η of the S1 and S2 devices were summarized in Table3.
As seen in Table3, the efficiency of the S2 device is higher than the efficiency of the
S1 device. This difference is thought to be due to the different metal contact materials. It is
known that electrical conduction is better when Ti coated on Au metal in the front contact
(Cotal etal. 2009). This is related with the higher Jsc and Voc values of the S2 device in our
study and so the higher efficiency. However, it was seen that the electrical performance
obtained for the S1 and S2 devices is lower than theoretically calculated. This negativity
can be attributed to trapping the electron and hole carriers from the defect levels in the
growing crystal and increasing the series resistance due to metallization. With it, obtained
structural characteristics showed that the quality of the epi-layers produced highly. Taking
this into consideration, it is thought that the most important carrier trapping that negatively
effects on cell efficiency is caused by the defects in the used substrate.
Generally, III–V compound multi-junction solar cells require anti-reflective coating to
prevent large losses of incident light due to surface reflection (more than 25%) (Homier
-15
-10
-5
0
5
10
-0.5 0.00.5 1.01.5 2.02.5
Current (mA)
Voltage (V)
S1
dark
illuminaton
-15
-10
-5
0
5
10
-0.5 0.00.5 1.01.5 2.
02
.5
Current (mA)
Voltage (V)
S2
dark
illuminaton
Fig. 6 I–V characteristic of S1 and S2 devices at room temperature
Table 3 Experimental output
parameters of S1 and S2 devices Solar Cells Jsc (mA/cm2) Voc (V) FF (%) η (%)
S1 11.38 1.82 72.52 14.65
S2 12.49 1.94 66.48 15.26

Developing ofdual junction GaInP/GaAs solar cell devices:…
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Page 9 of 13 277
etal. 2012; Saylan etal. 2015). In view of this known, Al2O3/TiO2 anti-reflective coating
was also studied in order to achieve an increase in the efficiency of the devices. First step,
the transmittance and reflectance of totally 25nm Al2O3/TiO2 thin film sputtered on glass
and GaAs substrates was determined with using UV–VIS spectrometry. Second step, the
effects of Al2O3/TiO2 anti-reflective coating on the performances of the fabricated S1 and
S2 devices was investigated with I–V measurements.
The transmittance spectrum of the Al2O3/TiO2/glass and bare glass was shown in Fig.7a
the spectral region of 200–1100nm. It was seen that the Al2O3/TiO2/glass structure has a
transmittance of about 80% at > 600 nm wavelength. A decrease in optical transmittance
was observed at UV region. This decrease arises due to the absorption caused from the
defect levels in the band energy edge and band gap energy of the TiO2 film. The obtained
results show that TiO2 is similar to the transmittance spectrum existing in the literature, the
target film deposition was achieved.
The reflection spectrum of the Al2O3/TiO2/GaAs and bare GaAs was shown in Fig.7b
the spectral region of 325–1100 nm. The reflectivity of the GaAs surface without anti-
reflective coating was about 16% at around 600nm and decreased to around 5% with using
anti-reflective coating. According to the obtained reflection data, it is predicted that the
amount of radiation entering the solar cell significantly increased. Thus, additional increase
in JSC can be expected by anti-reflective coating.
Electrical, I–V, characteristics of the devices were measured in dark and illumination
conditions at room temperature and given in Fig.8. It was easily seen that Al2O3/TiO2 anti-
reflective layer on the S1 and S2 devices increase the conversion efficiency of the devices
as comparing given in Fig.6.
Calculated electrical parameters of the devices such as Jsc, Voc, FF and η were also given
in Table4. An increase of short circuit current of the S1 and S2 devices with Al2O3/TiO2
anti-reflective coating was observed, as expected (Liu etal. 2014). The efficiencies of the
S1 and S2 devices were 15.72 and 16.90%, respectively, with Al2O3/TiO2 anti-reflective
coating. The anti-reflective coating increased the efficiency of the devices because it causes
more photon absorption than the sunlight (Homier etal. 2012). So, it was observed that
0
10
20
30
40
50
60
70
80
90
100
200400 600800 1000
Transmitance (%)
Wavelenght (nm)
(a)
bare glass
Al2O3/TiO2/glass 0
5
10
15
20
25
400600 8001000
Reflectivity (%)
Wavelenght (nm)
(b)
bare GaAs
Al2O3/TiO2/GaAs
Fig. 7 a Transmittance spectrum of the Al2O3/TiO2/glass and bare glass b Reflectivity spectrum of the
Al2O3/TiO2/GaAs and bare GaAs

T.Ataser et al.
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277 Page 10 of 13
the efficiency of the S1 and S2 devices with anti-reflective coating increased by 7.30 and
10.74%, respectively. It is thought that this difference caused by the metallization processes
despite using the same anti-reflective coating which absorb the same amount light.
Short circuit current density can be calculated from measured external quantum effi-
ciency of the cell by
J
cs =
𝜆2
∫
𝜆1
q𝜙(𝜆)QE(𝜆)d
𝜆
(Lei etal. 2009): where, q is the elementary
charge, QE(λ) and ϕ(λ) are measured quantum efficiency of the cell and the photon flux of
the standard spectrum of AM1.5, respectively.
EQE of the GaInP/GaAs dual-junction cell (S2) with and without ARC were meas-
ured at room temperature and given in Fig.9. The EQE of the GaAs cell with and without
ARC was around 90 and 78% at 800nm, respectively. For the GaInP cell with and without
ARC, it was reached a maximum value of 82 and 70% at 500nm, respectively and then
it decreased with increasing wavelength. The Jsc of the GaInP and GaAs cells with/with-
out ARC were calculated as 15.25/13.93 and 15.12/13.81mA/cm2 based on the EQE data
measured. The Jsc was almost the same as the measured value from the I–V characteristics.
The calculations show that currency of the dual-junction cell is limited by GaInP top cell
-16
-12
-8
-4
0
4
8
12
-0.5 0.00.5 1.01.5 2.02.5
Current (mA)
Voltage (V)
S1
dark
illuminaton
-16
-12
-8
-4
0
4
8
12
-0.5 0.00.5 1.01.5 2.
02
.5
Current (mA)
Voltage (V)
S2
dark
illuminaton
Fig. 8 I–V characteristics of the Al2O3/TiO2 anti-reflective coated S1 and S2 devices
Table 4 Experimental output
parameters of the Al2O3/TiO2
anti-reflective coated S1 and S2
devices
Solar cells Jsc (mA/cm2) Voc (V) FF (%) η (%)
S1 11.89 2.04 66.63 15.72
S2 15.01 2.01 57.59 16.90

Developing ofdual junction GaInP/GaAs solar cell devices:…
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Page 11 of 13 277
because of Jsc of top cell was slightly smaller than the bottom cell. In addition, Jsc obtained
from EQE data was slightly higher than calculated value from I–V measurement data. This
difference may be attributed to less recombination lose due to low irradiance in case of
EQE measurement (Zhang etal. 2018).
5 Conclusion
In this study, DJ GaInP/GaAs SC structure was designed by using analytical solar cell
model to use in AM1.5G. The electrical parameters (JSC, Voc, and η) of the modeling solar
cell were calculated. The theoretical efficiency of DJ GaInP/GaAs SC was found to be
25.62%. Then, the SC structure was grown with MBE technique considering the optimized
conditions of the designed the cell. According to HRXRD patterns, it was observed that
grown of lattice match SC structure was achieved. Alloy composition and energy band gap
of the structure were calculated from HRXRD measurement to be 45.91% and 1.93eV,
respectively.
S1 and S2 devices was fabricated using photolithography technique. Electrical charac-
terizations of the devices were performed under AM1.5G illumination using a solar simu-
lator. The efficiencies of the S1 and S2 devices were obtained 14.65 and 15.26%, respec-
tively. Au/Ti metal contacted on S2 device provided a 4.16% increase in cell efficiency
compared to Au metal contacted on S1 device. This increase is thought to be due to the
better adhesion of Au metal on the surface of Ti metal, as well as the prevention of metal
diffusion in the solar cell layer during the formation of ohmic contact. This leads to a
decrease in series resistance. Thereby, efficiency of the solar cell devices value increases
due to reducing the photo-current losses in the devices. In addition, the anti-reflective
layer, which was sputtered between the contacts on the front side of the DJ GaInP/GaAs
SC devices, increased the efficiencies from 14.65 to 15.72% and 15.26 to 16.90% for S1
and S2, respectively.
0
20
40
60
80
100
350450 550650 750850 950
EQE (%)
Wavelength (nm)
With ARC
Without ARC
Fig. 9 EQE of S2 DJ solar cell with and without antireflective coating

T.Ataser et al.
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277 Page 12 of 13
Acknowledgements This work was supported by Development Minister in Turkey under the Project Num-
ber 2016K121220.
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