materials
Article
Photovoltaic Performance Characterization of
Textured Silicon Solar Cells Using Luminescent
Down-Shifting Eu-Doped Phosphor Particles of
Various Dimensions
Wen-Jeng Ho *, Yu-Jie Deng, Jheng-Jie Liu, Sheng-Kai Feng and Jian-Cheng Lin
Department of Electro-Optical Engineering, National Taipei University of Technology, No. 1, Section 3,
Zhongxial East Road, Taipei 10608, Taiwan; [email protected] (Y.-J.D.); [email protected] (J.-J.L.);
[email protected] (S.-K.F.); [email protected] (J.-C.L.)
* Correspondence: [email protected]; Tel.: +886-2-2771-2171 (ext. 4639)
Academic Editors: Mady Elbahri, Shain Homaeigohar and Mehdi Keshavarz Hedayati
Received: 30 November 2016; Accepted: 26 December 2016; Published: 1 January 2017
Abstract: This paper reports on efforts to enhance the photovoltaic performance of textured silicon
solar cells through the application of a layer of Eu-doped silicate phosphor with particles of various
dimensions using the spin-on film technique. We examined the surface profile and dimensions of the
Eu-doped phosphors in the silicate layer using optical microscopy with J-image software. Optical
reflectance, photoluminescence, and external quantum efficiency were used to characterize the
luminescent downshifting (LDS) and light scattering of the Eu-doped silicate phosphor layer. Current
density-voltage curves under AM 1.5G simulation were used to confirm the contribution of LDS and
light scattering produced by phosphor particles of various dimensions. Experiment results reveal that
smaller phosphor particles have a more pronounced effect on LDS and a slight shading of incident
light. The application of small Eu-doped phosphor particles increased the conversion efficiency
by 9.2% (from 12.56% to 13.86%), far exceeding the 5.6% improvement (from 12.54% to 13.32%)
achieved by applying a 250 nm layer of SiO2 and the 4.5% improvement (from 12.37% to 12.98%)
observed in cells with large Eu-doped phosphor particles.
Keywords: Eu-doped phosphor; luminescent downshifting (LDS); textured silicon solar cells
1. Introduction
The conversion efficiency of solar cells is limited by optical absorption, carrier transport, and
carrier collection. The maximum theoretical efficiency of a single-junction crystalline-silicon (C-Si)
solar cell is 31% under AM 1.5G illumination [1]. This limitation can be attributed to losses associated
with the excess energy of above-bandgap photons, photon transparency below the band gap, and
radiative and Auger recombination. The efficiency of solar cells is also affected by the surface reflection
of incident solar radiation and/or the unproductive absorption of light at the back contact, losses due
to the recombination of photo-generated carriers, losses due to recombination, and losses at contacts.
A number of methods have been devised to reduce these losses. Researchers have achieved efficiency
of 25% in the laboratory using passivated emitter and rear locally diffused (PERL) solar cells [2];
however, the ultimate challenge is to engineer solutions that could be produced in high volumes at low
cost. Nanomaterials are currently being employed in photovoltaics to reduce the fundamental spectral
losses in single-junction silicon solar cells, which can reach 50% [3]. Modification of the spectrum using
down- and/or up-conversion or shifting is a relatively straightforward and cost-effective means of
enhancing the conversion efficiency of single-junction cells [4–15]. The conversion efficiency of C-Si
solar cells is relatively low, due to high reflectance and low spectral response at ultraviolet (UV) and
Materials 2017, 10, 21; doi:10.3390/ma10010021
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Materials 2017, 10, 21
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blue wavelengths (300–450 nm). Incident photons of higher energy (within UV-blue wavelengths) are
absorbed within a short distance from the surface, which results in high recombination loss. Many
researchers have sought to enhance the overall conversion efficiency by applying a down-conversion
(DC) layer or a down-shifting (DS) layer over the top surface of the C-Si in order to improve its spectral
response in the UV-blue region [16–18]. Other researchers have developed phosphor materials for
luminescent downshifting (LDS) with the aim of converting high-energy incident photons into lower
energy photons in photovoltaic devices [19–25].
This study characterized the coverage, LDS, and reflectance of a layer of Europium-doped
(Eu-doped) silicate phosphor particles of various dimensions containing 3 wt % Eu-doped silicate
phosphors, for use in coating textured-type C-Si solar cells. Optical reflectance, photoluminescence (PL),
and external quantum efficiency (EQE) measurements were used to characterize the LDS and optical
properties of the Eu-doped silicate phosphor layer. We also investigated the LDS, light scattering,
and light shading properties of a Eu-doped silicate phosphor layer deposited on bare textured C-Si
solar cells and the influence of phosphor particle dimensions. EQE response and photovoltaic current
density-voltage (J-V) characteristics under AM 1.5G simulation measurements were used to quantify
improvements in photovoltaic performance as a function of LDS and particle dimensions.
2. Experiment
2.1. Characterization of Eu-Doped Silicate Phosphor Layer
This study employed Eu-doped silicate phosphors ((Sr1−x Bax )2 SiO4 :Eu2+ F; O6040TM;
InteMatix Company, Fremont, CA, USA) with dimensions of 15–25 µm (referred hereafter as large
particles) to induce LDS. Phosphor particles of 5–15 µm (referred to as medium particles) and 2–5 µm
(referred to as small particles) were obtained by grinding larger silicate phosphor particles in an agate
mortar for approximately 30–60 min. Powders of 0.06 g Eu-doped silicate phosphor particles (large,
medium, and small phosphor particles) were respectively mixed with a 1.94 g silica film solution
(Emulsitone Company, Whippany, NJ, USA). Planar C-Si substrates were coated with these solutions
using the spin-on film technique at 3000 rpm for 60 s, before being baked at 200 ◦ C for 30 min
under an air atmosphere. This resulted in a layer that included 3 wt % Eu-doped silicate phosphors
particles on the planar C-Si substrates. For comparison, we also produced a planar C-Si substrates,
which was coated with a SiO2 layer (using the same Silicafilm solution), i.e., without any phosphor
particles. The surface morphology of the Eu-doped silicate phosphor layer was examined using SEM
(Hitachi S-4700, Hitachi High-Tech Fielding Corporation, Tokyo, Japan). The LDS and light scattering
and shading properties of Eu-doped silicate phosphor layer were characterized according to PL spectra
(Ramboss 500i Micro-PL Spectroscopy, Spectrolab, Newbury, UK) and optical reflectance using an
UV/VIS/NIR spectrophotometer (Lambda 35, PerkinElmer Inc., Waltham, MA, USA).
2.2. Fabrication and Characterization of Textured C-Si Solar Cells Coated with a Eu-Doped Phosphor Layer
Czochralski (CZ)-grown boron-doped C-Si wafers (525 µm-thick) with a resistivity of a 10 Ω·cm
(100) orientation were first cleaned using a standard RCA (Radio Corporation of America) cleaning
process. The surface of the C-Si substrate was then etched by being dipped in an anisotropic solution
of H2 O/KOH/IPA at 80 ◦ C for 20 min to produce a pyramidal surface structure. Top-view and
side-view SEM images of the randomly etched surface are presented in Figure 1a,b. The minimum and
maximum spacing between pyramids were respectively 3 µm and 7 µm, whereas the minimum and
maximum heights were 3 µm and 7 µm. An n+ -Si emitter layer (0.3 µm-thick) with a sheet resistance of
approximately 80 Ω/sq was applied to the textured C-Si substrates using a POCl3 diffusion process in
a tube diffusion chamber at 850 ◦ C over a period of 3 min. Any phosphorous silicate glass remaining
on the surface was removed using a buffered oxide etchant prior to deposition of the electrode films.
Aluminum (Al) film with a thickness of 500 nm was deposited on the rear surface using electron-beam
(E-beam) evaporation and annealed in an RTA chamber at 450 ◦ C for 5 min under ambient N2 /H2 to
Materials 2017, 10, 21
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3 of 10
3 of 10
form was
a back
electrode.
Finally,
top contact
comprising
a 20 nm and
titanium
(Ti) film
deposited
on the
rear surface
usinggrid-electrodes
electron-beam (E-beam)
evaporation
annealed
in anand a
was
deposited
on450
the°Crear
using
electron-beam
evaporation
and
annealed
an
RTA
chamber
at
for surface
5 min
under
ambient
N2/H2 to(E-beam)
form aand
back
electrode.
Finally,
top contact
300 nm
Al
film were
fabricated
using
lift-off
photolithography
E-beam
evaporation
toinproduce
RTA
chamber
at
450
°C
for
5
min
under
ambient
N
2/H2 to form a back electrode. Finally, top contact
comprising
20 nm solutions
titanium (Ti)
film and 3
a wt
300%
nm
Al film were
fabricated
using were
bare grid-electrodes
textured C-Si solar
cells. aSilicate
containing
Eu-doped
silicate
phosphors
grid-electrodes
comprisingand
a 20
nm titanium
(Ti) film
and a 300
nm
Al filmC-Si
weresolar
fabricated
using
lift-off
photolithography
E-beam
evaporation
to
produce
bare
textured
cells.
Silicate
deposited on the bare textured C-Si solar cells using the spin-on film deposition. Figure 2 presents
lift-off
photolithography
E-beam evaporation
to produce
textured
cells. Silicate
solutions
containing 3 wtand
% Eu-doped
silicate phosphors
werebare
deposited
onC-Si
the solar
bare textured
C-Si
a schematic
diagram of 3a wt
textured
C-Si solar
cell
coated with
layer of on
Eu-doped
phosphor
solutions
% Eu-doped
silicate
phosphors
were adeposited
the bare silicate
textured
C-Si
solar cellscontaining
using the spin-on
film deposition.
Figure
2 presents
a schematic
diagram
of a textured
particles.
For comparison,
we also
a textured
Si solara cell
that was
coated
a layer of
solar
cells
using
the spin-on
film produced
deposition.
presents
schematic
diagram
of with
a textured
C-Si solar
cell
coated
with a layer
of Eu-dopedFigure
silicate2 phosphor
particles.
For
comparison,
we also
SiO2 C-Si
(using
the
same
Silicafilm
solution),
i.e.,
without
any
phosphor
particles.
The
resulting
solar cell
coated with
a layer
Eu-doped
silicate
phosphor
particles.
For comparison,
we alsorough
produced
a textured
Si solar
cell of
that
was coated
with
a layer of
SiO2 (using
the same Silicafilm
surfaces
produced
by the
phosphor
particles
were
examined
Thetheoptical
reflectance of
produced
a textured
Si
solar
cell that
was coated
with
a rough
layerusing
of
SiOSEM.
2 (using
same
solution),
i.e.,
without
any
phosphor
particles.
The
resulting
surfaces
produced
by
the Silicafilm
phosphor
solution),
i.e.,
anyusing
phosphor
Thereflectance
resultingspectrophotometer.
rough
produced
the phosphor
all solar
cellswere
waswithout
characterized
using
an optical
UV/VIS/NIR
Thebyexternal
quantum
particles
examined
SEM.particles.
The
of all surfaces
solar cells
was characterized
using
particles
wereof
examined
reflectance
ofefficiency
allofsolar
cells
wasofcharacterized
efficiency
(EQE)
the
cellsusing
wasSEM.
also The
measured
over
a range
wavelengths
from
350was
tousing
1100 nm
an UV/VIS/NIR
spectrophotometer.
Theoptical
external
quantum
(EQE)
the
cells
also
an
UV/VIS/NIR
spectrophotometer.
The
external
quantum
efficiency
(EQE)
of
the
cells
was
also
measured
over
a
range
of
wavelengths
from
350
to
1100
nm
using
a
solar
cell
spectral
response
using a solar cell spectral response measurement system (EQE-RQE-R3015, Enli Technology Co., Ltd.,
measured
oversystem
a range(EQE-RQE-R3015,
of wavelengths from
to 1100 nm
using
a solar
cell spectral
response
measurement
Enli350
Technology
Co.,
Ltd.,
Kaohsiung,
Taiwan).
The were
Kaohsiung,
Taiwan).
The photovoltaic current-voltage
(I-V) characteristics
of the proposed
cells
measurement
system
(EQE-RQE-R3015,
Enli
Technology
Co.,
Ltd.,
Kaohsiung,
Taiwan).
The
photovoltaic
(I-V)
characteristics
of Electric
the proposed
cells were
measured
usingsource
a solar
measured
using acurrent-voltage
solar simulator
(XES-151S,
San-Ei
Co., Ltd.,
Osaka,
Japan) and
meter
photovoltaic
current-voltage
characteristics
the proposed
cellssource
were
measured
using a 2400,
solar
simulator (XES-151S,
San-Ei (I-V)
Electric
Co., Ltd., of
Osaka,
Japan) and
meter (Keithley
◦
(Keithley
2400,(XES-151S,
Keithley Instruments,
Cleveland,
OH, USA)
25 C.meter
The solar simulator
simulator
San-Ei
ElectricInc.,
Co.,
Ltd.,
Osaka,
andat
source
2400, was
Keithley Instruments,
Inc., Cleveland,
OH,
USA)
at 25Japan)
°C. The
solar
simulator (Keithley
was calibrated
calibrated
according
to
an
NREL-certified
crystalline
silicon
reference
cell
(PVM-894,
PV
Measurements
Keithley
Inc., Cleveland,
OH,silicon
USA)reference
at 25 °C.cell
The
solar simulator
was calibrated
accordingInstruments,
to an NREL-certified
crystalline
(PVM-894,
PV Measurements
Inc.,
Inc., according
Boulder,
CO,
USA)
before
obtaining
measurements.
to
an
NREL-certified
crystalline
silicon
reference
cell
(PVM-894,
PV
Measurements
Inc.,
Boulder, CO, USA) before obtaining measurements.
Boulder, CO, USA) before obtaining measurements.
Figure
1. SEM
images
textured surface
surface etched
using
an anisotropic
solution
of H2O/KOH/IPA
at 80 °C
Figure
1. SEM
images
ofoftextured
etched
using
an anisotropic
solution
of H2 O/KOH/IPA
at
Figure
1. SEM
images
of textured
surface etched using an anisotropic solution of H2O/KOH/IPA at 80 °C
for
20
min
in
(a)
side-view;
(b)
top-view.
◦
80 C for 20 min in (a) side-view; (b) top-view.
for 20 min in (a) side-view; (b) top-view.
Figure 2. Schematic diagram of textured C-Si solar cell coated with a layer of Eu-doped silicate
Figure
2. Schematic
solarcell
cellcoated
coated
with
a layer
of Eu-doped
silicate
Figure
2. Schematic
diagramofoftextured
textured C-Si
C-Si solar
with
a layer
of Eu-doped
silicate
phosphor
particles.diagram
phosphor
particles.
phosphor
particles.
3. Results and Discussion
3. Results and Discussion
3. Results
and Discussion
Figure
3 presents top-view optical microscope (OM) images (multiplicative: ×500) of planar C-Si
Figure
3 presents
top-view
optical
microscope
(OM)
images
(multiplicative:
×500)
of planar
C-Si
substrates
coated
with
an Eu-doped
silicate
phosphor
layer
comprising
(a) large; (b)
medium;
and
Figure 3 presents
top-view
optical
microscope
(OM)
images (multiplicative:
×500)
of(c)
planar
substrates
coated
with
an
Eu-doped
silicate
phosphor
layer
comprising
(a)
large;
(b)
medium;
and
(c)
small particles. The coverage and average dimensions are as follows: large particles (12.35% and 16.47
C-Si small
substrates
coated
with
an
Eu-doped
silicate
phosphor
layer
comprising
(a)
large;
(b)
medium;
particles. The coverage and average dimensions are as follows: large particles (12.35% and 16.47
and (c) small particles. The coverage and average dimensions are as follows: large particles
(12.35% and 16.47 µm), medium particles (16.18% and 8.49 µm), small particles (21.35% and 3.66 µm),
as calculated from OM images using Image J software. These results indicate that some of the particles
Materials
Materials
2017, 2017,
10, 2110, 21
4 of 104 of 10
μm), medium particles (16.18% and 8.49 μm), small particles (21.35% and 3.66 μm), as calculated
from 30
OMµm,
images using
Image J software. These results indicate that some of the particles exceededcould
exceeded
Materials
2017, 10,due
21 to the aggregation of some of the phosphor particles. Denser coverage
4 of 10
30 μm, due to the aggregation of some of the phosphor particles. Denser coverage could be expected
be expected to produce stronger LDS effects, and larger particles could be expected to increase light
to produce
stronger
LDS(16.18%
effects, and
and larger
particles
be expected
to
increase
lightas
shading.
We
μm),
medium
particles
8.49 μm),
smallcould
(21.35%
and
3.66 μm),
calculated
shading.
We therefore
sought to distribute
a layer
ofparticles
Eu-doped
silicate
phosphors
of
appropriate
therefore
sought
to
distribute
a
layer
of
Eu-doped
silicate
phosphors
of
appropriate
dimensions
at
an
from OM
using Image
J software.
These across
results the
indicate
that some
of the particles exceeded
dimensions
at images
andensity
appropriate
density
uniformly
textured
surface.
appropriate
uniformly
across
the
textured
surface.
30 μm, due to the aggregation of some of the phosphor particles. Denser coverage could be expected
to produce stronger LDS effects, and larger particles could be expected to increase light shading. We
therefore sought to distribute a layer of Eu-doped silicate phosphors of appropriate dimensions at an
appropriate density uniformly across the textured surface.
Figure 3. Optical microscope (OM) images (multiplicative: ×500) of planar C-Si substrates coated
Figure 3. Optical microscope (OM) images (multiplicative: ×500) of planar C-Si substrates coated with
with a layer of Eu-doped silicate phosphors: (a) large; (b) medium; (c) small particles.
a layer of Eu-doped silicate phosphors: (a) large; (b) medium; (c) small particles.
Figure 4a presents the PL excitation (PLE; i.e., absorption) and PL fluorescence (radiation or
Figure 3. Optical microscope (OM) images (multiplicative: ×500) of planar C-Si substrates coated
emission)
ofpresents
a layer comprising
3 wt % Eu-doped
silicate
phosphors deposited
on a C-Si substrate.
Figure
the PL
excitation
(PLE;
i.e.,
absorption)
fluorescence
(radiation or
with4a
a layer
of Eu-doped
silicate
phosphors:
(a) large;
(b)
medium; (c)and
smallPL
particles.
The
peak
PLE
of
Eu-doped
silicate
phosphor
was
measured
at
approximately
375on
nma with
full
emission) of a layer comprising 3 wt % Eu-doped silicate phosphors deposited
C-Si asubstrate.
width
at
half
maximum
(FWHM)
of
approximately
110
nm,
which
means
that
incident
photons
of
4aEu-doped
presents the
PL excitation
(PLE;
i.e.,measured
absorption)atand
PL fluorescence
The peakFigure
PLE of
silicate
phosphor
was
approximately
375(radiation
nm withor
a full
higher
energy
(within
wavelengths
of
260–480
nm)
were
absorbed
by
the
Eu-doped
silicate
of a layer comprising
% Eu-doped silicate
phosphors
on incident
a C-Si substrate.
widthemission)
at half maximum
(FWHM) 3ofwtapproximately
110 nm,
whichdeposited
means that
photons of
phosphor.
In contrast,
peak silicate
PL emissions
at approximately
610
with FWHM
of nm
approximately
The
peak PLE
of Eu-doped
phosphor
waswere
measured
at nm
approximately
375
with aphosphor.
full
higher
energy
(within
wavelengths
ofhigh-energy
260–480 nm)
absorbed
by
the
Eu-doped
silicate
100
nm
indicates
the
absorption
of
photon,
resulting
in
the
emission
of
visible
photons
width at half maximum (FWHM) of approximately 110 nm, which means that incident photons
of
In contrast,
peak PL emissions
at
approximately
610 nm with FWHM of
approximately
100
nm indicates
at wavelengths
of 525–725
nm.
Whenofa phosphor-material
absorbs
a photon,
it generally
higher
energy (within
wavelengths
260–480 nm) weresystem
absorbed
by the
Eu-doped
silicate
the absorption
of and
high-energy
photon,
resulting
in the
of visible photons attowavelengths
gains energy
enters
an PL
excited
state.
way
foremission
the610
phosphor-material
relax is
phosphor.
In contrast,
peak
emissions
atOne
approximately
nm with FWHMsystem
of approximately
of 525–725
nm.
When
a
phosphor-material
system
absorbs
a
photon,
it
generally
gains
energy
through
the
emission
of
a
photon
and
its
associated
energy.
When
the
energy
of
the
emitted
photon and
100 nm indicates the absorption of high-energy photon, resulting in the emission of visible photons
is an
less
than that
of
absorbed
photon,
the
difference in energy
is referred
toaas
the
Stokes
shift.
In
enters
excited
state.
One
way
the aphosphor-material
system
to relax
is
through
the
emission
at
wavelengths
of the
525–725
nm.for
When
phosphor-material
system
absorbs
photon,
it generally
this
study,
the
Stoke
shift
of
the
Eu-doped
silicate
phosphor
was
approximately
235
nm.
The
of a photon
and
its
associated
energy.
When
the
energy
of
the
emitted
photon
is
less
than
that
gains energy and enters an excited state. One way for the phosphor-material system to relax isof the
measured
emission
PLE
absorption
results to
demonstrate
that
the
3In
wtof
%the
Eu-doped
silicate
absorbed
photon,
the
difference
in energy
referred
as theWhen
Stokes
this
study,
thephoton
Stoke shift
through
thePLemission
of and
a photon
and itsisassociated
energy.
theshift.
energy
emitted
phosphor
layer
absorbed
UV
photons
and
converted
them
into
visible
photons,
as
indicated
by the
less than that
of thephosphor
absorbed photon,
the difference in
energy
is referred
to as the
shift.
In PLE
of theisEu-doped
silicate
was approximately
235
nm. The
measured
PL Stokes
emission
and
excellent
LDS
Figure
4b
presents PL intensity
as a function
phosphor particle
this
study,
thebehavior.
Stoke shift
of
the
silicate phosphor
wasofapproximately
235 coverage.
nm.
absorption
results
demonstrate
that
theEu-doped
3 wt % Eu-doped
silicate phosphor
layer absorbed
UVThe
photons
We obtained
PL intensities
from samples
withdemonstrate
denser particle
the same
3 wt
measured
PLhigher
emission
and PLE
absorption
results
thatcoverage.
the 3behavior.
wt Under
% Eu-doped
silicate
and converted
them
into
visible
photons,
as indicated
by that
the excellent
LDS
Figure
4b presents
%
condition,
the
number
of
small
particles
far
exceeds
of
large
particles,
thereby
providing
far
phosphor layer absorbed UV photons and converted them into visible photons, as indicated by the
PL intensity
as a function
of phosphor
particle
coverage.
We energy.
obtained
higher
PL intensities
from
more
Eu-doped
phosphors
to
absorb
incident
photons
of
higher
The
measured
PL
emission
excellent LDS behavior. Figure 4b presents PL intensity as a function of phosphor particle coverage.
samples
withrevealed
denser that
particle
Under
the same
3 wt % condition,
the number
of small
particles
results
wtcoverage.
% Eu-doped
silicate
phosphor
with
denser
coverage
smaller
We
obtained
higher PL3 intensities
from samples
with
denserlayers
particle
coverage.
Under
the of
same
3 wt
far exceeds
that
of
large
particles,
thereby
providing
far
more
Eu-doped
phosphors
to
absorb
incident
particles
achieved
impressive
LDS
performance
thanks
to
high
PL
intensity.
% condition, the number of small particles far exceeds that of large particles, thereby providing far
0.025
Small phosphor
0.020
(a)
PLE
0.030
0.015
PL
0.025
0.010
0.020
0.005
0.015
0.000
0.010
250
0.005
300
350
400
450
500
550
Wavelength (nm)
600
650
700
750
Intensity (arb. units)
Intensity (arb. units)
Intensity (arb. units)
Intensity (arb. units)
photons
higher energy.
Thetomeasured
PL emission
revealed
thatmeasured
3 wt % Eu-doped
silicate
moreofEu-doped
phosphors
absorb incident
photons results
of higher
energy. The
PL emission
(a)with
phosphor
denser
coverage
ofsilicate
smaller
particles
achieved
impressive
LDSofperformance
(b) layers
0.030
resultslayers
revealed
that
3 wt %
Eu-doped
phosphor
with denser
coverage
smaller
0.030
PLE
PL
Large phosphor
thanks
to high
PL intensity.
particles
achieved
impressive LDS performance thanks
to
high
PL
intensity.
Medium
phosphor
0.025
0.020
0.030
0.015
(b)
Large phosphor
Medium phosphor
Small phosphor
0.025
0.010
0.020
0.005
0.015
0.000
0.010
500
0.005
550
600
650
700
750
800
Wavelength (nm)
0.000 4. (a) PL excitation and PL fluorescence from a layer comprising 3 wt % Eu-doped silicate
Figure
0.000
phosphor
deposited
on 450
a C-Si
substrate;
(b)
PL
intensity
as500a function
of600
phosphor
particles
coverage.
250
300
350
400
500
550
600
650
700
750
550
650
700
750
800
Wavelength (nm)
Wavelength (nm)
Figure 4. (a) PL excitation and PL fluorescence from a layer comprising 3 wt % Eu-doped silicate
Figure 4. (a) PL excitation and PL fluorescence from a layer comprising 3 wt % Eu-doped silicate
phosphor deposited on a C-Si substrate; (b) PL intensity as a function of phosphor particles coverage.
phosphor deposited on a C-Si substrate; (b) PL intensity as a function of phosphor particles coverage.
Materials 2017, 10, 21
Materials 2017, 10, 21
5 of 10
5 of 10
Figure 5 illustrates the optical reflectance of planar C-Si substrates with the following
configurations: a bare Si substrate, a Si substrate with a SiO22 layer of 250 nm, and a Si substrate with
either large, medium, or small phosphor particles disseminated within a SiO22 layer. The reflectance
of the Si substrate was altered by the application of a SiO22 layer, and the lowest reflectance value
the effects
effects of
of destructive
destructive interference.
interference. Compared with the bare Si
was observed at 525 nm, due to the
2 layer
presented
typical
anti-refractive
properties.
In contrast,
the
substrate, the
the sample
samplewith
withthe
theSiO
SiO
presented
typical
anti-refractive
properties.
In contrast,
2 layer
sample
withwith
Eu-doped
silicate
phosphors
presented
a areduction
the
sample
Eu-doped
silicate
phosphors
presented
reductionininreflectance
reflectanceacross
across the
the entire
range of wavelengths. The
The reflectance
reflectance of the sample with Eu-doped silicate phosphors was lower
than that with only a SiO22 layer at wavelengths below 450 nm, due to the absorption of high-energy
the phosphor
phosphor particles.
particles. Furthermore,
photons by the
Furthermore, samples
samples with denser coverage presented higher
absorption values. Within
Within aa wavelength
wavelength range
range of
of 730–1200
730–1200 nm,
nm, the
the decrease in reflectance can be
attributed to
toforward
forwardlight
lightscattering
scattering
phosphor
particles.
higher
reflectance
of samples
attributed
byby
thethe
phosphor
particles.
TheThe
higher
reflectance
of samples
with
with large
phosphor
particles
(atnm)
525 can
nm)be
can
be attributed
to a reduction
in destructive
interference
large
phosphor
particles
(at 525
attributed
to a reduction
in destructive
interference
and
andslightly
the slightly
higher
reflective
properties
of larger
particles.
Therefore,
the optical
reflectance
the
higher
reflective
properties
of larger
particles.
Therefore,
the optical
reflectance
results
results examine
clearly
the
LDS, reflection,
and
light scattering
effects
on Si substrate
coated
with
examine
clearly the
LDS,
reflection,
and light
scattering
effects on
Si substrate
coated with
various
various
phosphor
particles
mixed
in
a
SiO
2
layer,
compared
with
the
Si
substrate
coated
with
a
SiO2
phosphor particles mixed in a SiO2 layer, compared with the Si substrate coated with a SiO2 layer.
layer. Furthermore,
the smaller
(denser coverage)
stronger
LDS scattering
and light
Furthermore,
the smaller
particlesparticles
(denser coverage)
producedproduced
stronger LDS
and light
scattering
used
the LDS,and
reflectivity,
and light
scattering
results
planar
C-Si
effects.
We effects.
used theWe
LDS,
reflectivity,
light scattering
results
from planar
C-Sifrom
substrates
coated
substrates
coated
with phosphor
particles
as to
a baseline
by which
to evaluate
the performance
of
with
phosphor
particles
as a baseline
by which
evaluate the
performance
of textured
C-Si solar cells
textured
solar cells in various configurations.
in
variousC-Si
configurations.
80
Bare planar Si-Substrate (Si)
Si with SiO2
70
Si with large phosphor
Si with medium phosphor
Si with small phosphor
Reflectance (%)
60
50
40
30
20
10
0
400
500
600
700
800
900
1000
Wavelength (nm)
Figure 5.
5. Optical
substrate with
with SiO
SiO2 layer,
and Si
Figure
Optical reflectance
reflectance of
of aa bare
bare planar
planar Si
Si substrate,
substrate, aa Si
Si substrate
2 layer, and Si
substrates with
with large,
large, medium,
medium, and
and small
small phosphor
phosphor particles
particles disseminated
disseminated within
within the
the SiO
SiO2 layer.
layer.
substrates
2
Figure 6 presents the optical reflectance of C-Si solar cells with the following configurations: a
Figure 6 presents the optical reflectance of C-Si solar cells with the following configurations: a
bare textured cell, a textured cell with a 250 nm layer of SiO2, and textured cells coated with
bare textured cell, a textured cell with a 250 nm layer of SiO2 , and textured cells coated with Eu-doped
Eu-doped silicate phosphor particles (large, medium, and small) applied over a SiO2 layer. The
silicate phosphor particles (large, medium, and small) applied over a SiO2 layer. The reflectance of
reflectance of the textured cell was reduced by the application of either a SiO2 layer or a layer of
the textured cell was reduced by the application of either a SiO2 layer or a layer of Eu-doped silicate
Eu-doped silicate phosphor particles. Across the entire range of wavelengths, the reflectance of the
phosphor particles. Across the entire range of wavelengths, the reflectance of the textured cell with
textured cell with large phosphor particles was higher than that of the textured cell with a SiO2
large phosphor particles was higher than that of the textured cell with a SiO2 layer. The large phosphor
layer. The large phosphor particles situated near the top of the pyramids (Figure 2) interfered with
particles situated near the top of the pyramids (Figure 2) interfered with the anti-reflective properties
the anti-reflective properties of the texturing. The surface of the large particles reflected some of the
of the texturing. The surface of the large particles reflected some of the incident light and shaded the
incident light and shaded the pyramidal structures beneath, thereby altering their anti-reflection
pyramidal structures beneath, thereby altering their anti-reflection and multi-reflective functionality.
and multi-reflective functionality. In contrast, the reflectance of the cell with small phosphor
In contrast, the reflectance of the cell with small phosphor particles was monolithically reduced across
particles was monolithically reduced across the entire range of wavelengths, compared to the
the entire range of wavelengths, compared to the textured cell with only a SiO2 layer. In this work, the
textured cell with only a SiO2 layer. In this work, the diameter of the small phosphor particles was
diameter of the small phosphor particles was less than that the spacing between pyramids (Figure 2).
less than that the spacing between pyramids (Figure 2). Thus, the small particles did not undermine
the anti-reflective or multiple-reflective benefits of the pyramidal structures, and the incident light
Materials 2017, 10, 21
6 of 10
Materials 2017, 10, 21
6 of 10
Thus, the small particles did not undermine the anti-reflective or multiple-reflective benefits of the
was
forward
scattered
into
cell by
thewas
small
phosphor
particles,
insmall
lower
optical
pyramidal
structures,
and
thethe
incident
light
forward
scattered
into theresulting
cell by the
phosphor
reflectance
from
350
to
1100
nm.
Furthermore,
the
sample
with
small
phosphor
particles
presented
particles, resulting in lower optical reflectance from 350 to 1100 nm. Furthermore, the sample with
the
lowest
reflectance
at 350–370
nm the
duelowest
to the absorption
high-energy
incident
photons
by the
small
phosphor
particles
presented
reflectance of
at 350–370
nm due
to the
absorption
of
small
Eu-doped
phosphor
particles.
high-energy
incident
photons
by the small Eu-doped phosphor particles.
35
Bare textured solar cell (SC)
SC with SiO2
SC with large phosphor
20
Reflectance (%)
30
Reflectance (%)
SC with medium phosphor
SC with small phosphor
25
20
19
18
17
16
15
350
15
360
370
Wavelength (nm)
380
10
400
500
600
700
800
900
1000
1100
Wavelength (nm)
Figure
Figure 6.
6. Optical
Opticalreflectance
reflectance of
of C-Si
C-Si solar
solar cells
cells with
with the
the following
following configurations:
configurations: aa bare
bare textured
textured cell,
cell,
aatextured
cell
with
a
SiO
2 layer, and textured cells with Eu-doped silicate phosphor (large, medium,
textured cell with a SiO2 layer, and textured cells with Eu-doped silicate phosphor (large, medium,
and
and small
small particles).
particles).
Figure 7 presents the EQE response of C-Si solar cells with the following configurations: a bare
Figure 7 presents the EQE response of C-Si solar cells with the following configurations: a bare
textured cell, a textured cell with a SiO2 layer of 250 nm, and textured cells with Eu-doped
textured cell, a textured cell with a SiO layer of 250 nm, and textured cells with Eu-doped phosphor
phosphor particles applied over a SiO22 layer (large, medium, and small). The EQE values of solar
particles applied over a SiO2 layer (large, medium, and small). The EQE values of solar cells coated
cells coated with Eu-doped phosphor particles were higher at wavelengths of 350–370 nm due to
with Eu-doped phosphor particles were higher at wavelengths of 350–370 nm due to the effects of
the effects of LDS. Furthermore, the EQE response of the cell with small phosphor particles was
LDS. Furthermore, the EQE response of the cell with small phosphor particles was superior to that of
superior to that of cells with medium and large phosphor particles. The LDS exhibited by the cell
cells with medium and large phosphor particles. The LDS exhibited by the cell with small phosphor
with small phosphor particles was due to the large number of the particles (high surface coverage),
particles was due to the large number of the particles (high surface coverage), far exceeding that of
far exceeding that of the cells with medium or large particles. The cell with small phosphor particles
the cells with medium or large particles. The cell with small phosphor particles also exhibited a more
also exhibited a more pronounced increase in EQE via forward light scattering. These findings are
pronounced increase in EQE via forward light scattering. These findings are in good agreement with
in good agreement with those of reflectance. We also compared the average weighted EQE (EQEW)
those of reflectance. We also compared the average weighted EQE (EQEW ) of a bare textured cell,
of a bare textured cell, a textured cell with a SiO2 layer of 250 nm, and textured
cells with Eu-doped
a textured cell with a SiO2 layer of 250 nm, and textured cells with Eu-doped phosphor particles (large,
phosphor particles (large,
medium, and small) at wavelengths (λ) from 350 to 1100 nm. EQEW was
medium, and small) at wavelengths (λ) from 350 to 1100 nm. EQEW was calculated using Equation (1).
calculated using Equation (1).
R λmax
max EQE ( λ ) × φ ( λ )
λ
    phph  

EQEW =  minEQE
(1)
R
λ
max
φ ph (λ)
EQEW  min λmin
(1)
max


 ph   
min
where φph (λ) is the photon flux of AM 1.5G solar
energy spectrum as a function of wavelength.
The
resulting
EQE
values
are
listed
in
Table
1.
These
results demonstrate
small Eu-doped
W
where φph(λ) is the photon
flux of AM 1.5G solar energy spectrum
as a functionthat
of wavelength.
The
phosphor
particles
on
a
textured
cell
can
enhance
EQE
at
UV-wavelengths
via
and at
resulting EQEW values are listed in Table 1. These results demonstrate that smallLDS
Eu-doped
long-wavelengths
scattering.
phosphor
particlesvia
onlight
a textured
cell can enhance EQE at UV-wavelengths via LDS and at
long-wavelengths via light scattering.
Materials 2017, 10, 21
Materials 2017, 10, 21
7 of 10
7 of 10
100
Bare textured solar cell (SC)
SC with SiO2
90
SC with medium phosphor
SC with small phosphor
SC with large phosphor
80
70
50
EQE (%)
EQE (%)
60
40
30
20
350
10
0
26
24
22
20
18
16
14
400
500
360
370
Wavelength (nm)
600
700
800
380
900
1000
1100
Wavelength (nm)
Figure 7. EQE response of C-Si solar cells with the following configurations: a bare textured cell, a
Figure 7. EQE response of C-Si solar cells with the following configurations: a bare textured cell,
textured cell with SiO2 layer, and textured cells with Eu-doped silicate phosphor particles (large,
a textured cell with SiO2 layer, and textured cells with Eu-doped silicate phosphor particles (large,
medium, and small).
medium, and small).
Table 1. Average weighted EQE (EQEW) of all evaluated solar cells.
Table 1. Average weighted EQE (EQEW ) of all evaluated solar cells.
Cell Type
Cell TypeCell (SC)
Bare Textured
Bare
Textured
Cell
(SC)
SC with
a SiO
2 Layer
SC
with
a
SiO
Layer
2
SC with Large Phosphor
Particles
SC with Large Phosphor Particles
SC
with
Medium
Phosphor
Particles
SC with Medium Phosphor Particles
SC
PhosphorParticles
Particles
SCwith
with Small
Small Phosphor
EQEW (%)
EQE
W (%)
67.31
67.31
68.83
68.83
68.53
68.53
69.41
69.41
70.91
70.91
Figure 8 presents the photovoltaic current density-voltage curves of C-Si solar cells with the
Figure 8 presents the photovoltaic current density-voltage curves of C-Si solar cells with the
following configurations: a bare textured cell, a textured cell with a SiO2 layer of 250 nm, and
following configurations: a bare textured cell, a textured cell with a SiO2 layer of 250 nm, and
textured cells with Eu-doped phosphor particles (large, medium, and small)
applied over a SiO2
textured cells with Eu-doped phosphor particles (large, medium, and small) applied over a SiO2 layer.
layer. The photovoltaic performance of all evaluated solar cells is summarized in Table 2. We
The photovoltaic performance of all evaluated solar cells is summarized in Table 2. We averaged
averaged the short-circuit current density (Jsc), open-circuit voltage (Voc), and conversion efficiency
the short-circuit current density (J ), open-circuit voltage (Voc ), and conversion
efficiency (η) of the
(η) of the four bare-textured cellsscused in this study as
30.76 mA/cm2, 558.37 mV, and 12.475%.
four bare-textured cells used in this study as 30.76 mA/cm2 , 558.37 mV, and 12.475%. These values were
These values were used in subsequent comparisons. The application of a 250 nm layer of SiO2
used in subsequent comparisons. The application of a 250 nm layer of SiO2 2 increased the Jsc of the cells
increased the Jsc of the cells by 5.6%
(from 30.98 to 32.73 mA/cm ) due to the anti-reflective
by 5.6% (from 30.98 to 32.73 mA/cm2 ) due to the anti-reflective properties of the SiO . The application
properties of the SiO2. The application of Eu-doped phosphor particles over a SiO22 layer led to the
of Eu-doped phosphor particles over a SiO2 layer led to the following improvements in Jsc : for 2small
following improvements in Jsc: for small particles,
a 9.2% increase (from 30.62 to 33.43 mA/cm ); for
particles, a 9.2% increase (from 30.62 to 33.43 mA/cm2 ); for2 medium particles, a 6.8% increase
medium particles, a 6.8% increase
(from
30.75
to
32.83
mA/cm ); for large particles, a 4.5% increase
(from 30.75 to 32.83 mA/cm22 ); for large particles, a 4.5% increase (from 30.67 to 32.05 mA/cm2 ). These
(from 30.67 to 32.05 mA/cm ). These improvements can be attributed to the combined contributions
improvements can be attributed to the combined contributions of anti-reflectivity and LDS. Note that
of anti-reflectivity and LDS. Note that the Jsc of these solar cells was strongly correlated with the
the Jsc of these solar cells was strongly correlated with the intensity of the PL signal, EQE response, and
intensity
of the PL signal, EQE response, and EQEW values. The small Eu-doped phosphor particles
EQEW values. The small Eu-doped phosphor particles had a greater effect on Jsc than did the large
had a greater effect on Jsc than did the large particles or the SiO2 layer due to their high EQE
particles or the SiO layer due to their high EQE response. Solar cell efficiency generally depends on
response. Solar cell2 efficiency generally depends on Jsc, Voc, and the fill factor (FF); however, we
Jsc , Voc , and the fill factor (FF); however, we observed variations of less than 2% in Voc and FF among
observed
variations of less than 2% in Voc and FF among the cells with different surface structure
the cells with different surface structure profiles. The factors that made the greatest contribution to η
profiles. The factors that made the greatest contribution to η were Jsc and EQE. The application of
were Jsc and EQE. The application of small Eu-doped phosphor particles on a textured C-Si solar cell
small Eu-doped phosphor particles on a textured C-Si solar cell led to a 1.30% increase in absolute
led to a 1.30% increase in absolute conversion efficiency (from 12.56% to 13.86%) compared to that of a
conversion efficiency (from 12.56% to 13.86%) compared to that of a 0.78% (from 12.54% to 13.32%)
0.78% (from 12.54% to 13.32%) for a bare textured cell with a SiO2 layer of 250 nm.
for a bare textured cell with a SiO2 layer of 250 nm.
Materials 2017, 10, 21
Materials 2017, 10, 21
8 of 10
8 of 10
35
25
Bare textured solar cell (SC)
SC with SiO2
SC with medium phosphor
SC with small phosphor
SC with large phosphor
34
20
2
Current density (mA/cm )
2
Current density (mA/cm )
30
15
10
5
0
0.0
33
32
31
30
0.00
0.1
0.01
0.2
0.02
0.03
Voltage (V)
0.04
0.3
0.05
0.4
0.5
0.6
Voltage (V)
Figure
8. Photovoltaic
following
Figure 8.
Photovoltaic current
current density-voltage
density-voltage curves
curves ofof C-Si
C-Si solar
solar cells
cells with
with the
the following
configurations:
a
bare
textured
cell,
a
textured
cell
with
SiO
2
layer
of
250
nm,
and
textured
cells with
configurations: a bare textured cell, a textured cell with SiO2 layer of 250 nm, and textured
cells
Eu-doped
phosphor
particles
(large,
medium,
and small)
applied
over over
a layer
of SiO
with Eu-doped
phosphor
particles
(large,
medium,
and small)
applied
a layer
of2.SiO2 .
Table
2. Photovoltaic
of all
all evaluated
evaluated solar
solar cells.
cells.
Table 2.
Photovoltaic performance
performance of
Cell Type
Jsc (mA/cm2) Voc (mV)
Voc (mV)
Jsc (mA/cm2 )
Bare Textured Cell (SC)
30.98
554.93
Bare
Textured
Cell
(SC)
30.98
554.93
SC with a SiO2 Layer
32.73
559.40
SC with a SiO2 Layer
32.73
559.40
Bare
Textured
Cell
(SC)
30.67
554.51
Bare Textured Cell (SC)
30.67
554.51
SC
with
Large
Phosphor
Particles
32.05
556.67
SC with Large Phosphor Particles
32.05
556.67
Bare Textured
(SC)
30.75
556.98
Bare
TexturedCell
Cell
(SC)
30.75
556.98
Cellwith
with Medium
Particles
32.83
559.34
Cell
MediumPhosphor
Phosphor
Particles
32.83
559.34
Bare Textured Cell (SC)
30.62
557.07
Bare Textured Cell (SC)
30.62
557.07
SC with Small Phosphor Particles
33.43
560.77
SC with Small Phosphor Particles
33.43
560.77
Cell Type
FF (%)
72.94
72.94
72.75
72.75
72.74
72.74
72.70
72.70
72.51
72.51
72.75
72.75
73.61
73.61
73.94
73.94
FF (%)
η (%)
12.54
12.54
13.32
13.32
12.37
12.37
12.98
12.98
12.43
12.43
13.36
13.36
12.56
12.56
13.86
13.86
η (%)
4. Conclusions
4. Conclusions
This paper reports on the fabrication and characterization of textured C-Si solar cells coated with
This paper reports on the fabrication and characterization of textured C-Si solar cells coated
a layer of LDS Eu-doped silicate phosphor particles of various dimensions. The electrical and optical
with a layer of LDS Eu-doped silicate phosphor particles of various dimensions. The electrical and
properties were shown to depend largely on the dimensions of phosphor particles. The application of
optical properties were shown to depend largely on the dimensions of phosphor particles. The
smaller Eu-doped phosphor particles to textured solar cells led to significant improvements in external
application of smaller Eu-doped phosphor particles to textured solar cells led to significant
quantum efficiency and short-circuits current density. The factors that made the greatest contribution
improvements in external quantum efficiency and short-circuits current density. The factors that
to η were Jsc and EQE. The application of small Eu-doped phosphor particles on a textured C-Si solar
made the greatest
contribution to η were Jsc and EQE. The application of small Eu-doped phosphor
cell led to a 1.30% increase in absolute conversion efficiency (from 12.56% to 13.86%) compared to that
particles on a textured C-Si solar cell led to a 1.30% increase in absolute conversion efficiency (from
of a 0.78% (from 12.54% to 13.32%) for a bare textured cell with a SiO2 layer of 250 nm.
12.56% to 13.86%) compared to that of a 0.78% (from 12.54% to 13.32%)
for a bare textured cell with a
SiO
2 layer of 250 nm.
Acknowledgments: The authors would like to thank the Ministry of Science and Technology of the Republic of
China for financial support under Grant MOST 103-2221-E-027-049-MY3.
Acknowledgments: The authors would like to thank the Ministry of Science and Technology of the Republic of
Author Contributions: All the authors conceived the experiments; Yu-Jie Deng fabricated the solar cells;
China
for financial
supportFeng,
under and
Grant
MOST 103-2221-E-027-049-MY3.
Jheng-Jie
Liu, Sheng-Kai
Jian-Cheng
Lin performed the device performance characterizations;
Wen-Jeng Ho analyzed and wrote the first draft of the paper.
Author Contributions: All the authors conceived the experiments; Yu-Jie Deng fabricated the solar cells; Jheng-Jie
Conflicts
of Interest:
declare
conflict ofthe
interest.
Liu,
Sheng-Kai
Feng, The
and authors
Jian-Cheng
Linno
performed
device performance characterizations; Wen-Jeng Ho
analyzed and wrote the first draft of the paper.
Conflicts of Interest: The authors declare no conflict of interest.
Materials 2017, 10, 21
9 of 10
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