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Near infrared up-conversion in organic photovoltaic devices using anefficient Yb3+:Ho3+ Co-doped Ln2BaZnO5 (Ln = Y, Gd) phosphorA. A. Damitha Adikaari, Isabelle Etchart, Paul-Henri Guéring, Mathieu Bérard, S. Ravi P. Silva et al. Citation: J. Appl. Phys. 111, 094502 (2012); doi: 10.1063/1.4704687 View online: http://dx.doi.org/10.1063/1.4704687 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v111/i9 Published by the American Institute of Physics. Related ArticlesSimultaneous radiation pressure induced heating and cooling of an opto-mechanical resonator Appl. Phys. Lett. 100, 111115 (2012) A novel and compact nanoindentation device for in situ nanoindentation tests inside the scanning electronmicroscope AIP Advances 2, 012104 (2012) Invited Review Article: Photopyroelectric calorimeter for the simultaneous thermal, optical, and structuralcharacterization of samples over phase transitions Rev. Sci. Instrum. 82, 121101 (2011) Nanometer optomechanical transistor based on nanometer cavity optomechanics with a single quantum dot J. Appl. Phys. 110, 114308 (2011) Breakover mechanism of GaAs photoconductive switch triggering spark gap for high power applications J. Appl. Phys. 110, 094507 (2011) Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors
Near infrared up-conversion in organic photovoltaic devices using anefficient Yb31:Ho31 Co-doped Ln2BaZnO5 (Ln 5 Y, Gd) phosphor
A. A. Damitha Adikaari,1 Isabelle Etchart,2,3 Paul-Henri Guering,2 Mathieu Berard,2
S. Ravi P. Silva,1 Anthony K. Cheetham,3 and Richard J. Curry1,a)
1Advanced Technology Institute, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom2Saint-Gobain Recherche, 39 quai Lucien Lefranc, 93303 Aubervilliers, France3Materials Science and Metallurgy Department, University of Cambridge, Cambridge Pembroke Street,CB2 3QZ, United Kingdom
(Received 13 December 2011; accepted 15 March 2012; published online 2 May 2012)
The first detailed study that combines the use of a new generation of high-efficiency Yb3þ:Ho3þ
co-doped Y2BaZnO5 near-infrared up-converting phosphors with organic photovoltaic devices
is reported. We show that it is possible to obtain a Jsc of 16 lA cm�2 under 986 nm illumination
(�390 mW cm�2 corresponding to �37 suns) leading to an up-conversion external quantum
efficiency (gUCEQE) of 0.0052%. Through modification of the organic photovoltaic devices to incorporate
transparent electrodes we show that gUCEQE could be increased to 0.031 %, matching that achieved in
amorphous-Si:H PV cells. Accounting for the full spectral range that may be absorbed by the phosphor
(�870–1030 nm) yields an up-conversion power conversion efficiency (gUCPCE) of 0.073% which again
could be improved to 0.45% using transparent electrodes. This technique for utilizing the near-infrared
spectral region may therefore offer a potential route to improving the performance of organic
photovoltaic devices as research into discovering high-efficiency up-converting phosphors continues
to provide improved materials. VC 2012 American Institute of Physics.
[http://dx.doi.org/10.1063/1.4704687]
I. INTRODUCTION
Progress in polymer and organic photovoltaic (OPV)
devices in the recent years has succeeded to increase the
power conversion efficiency (PCE) achieved by this class of
devices to between 6% and �8%.1–4 To further improve
their performance, a number of approaches are being pur-
sued, including the development of new materials with
improved electrical and optical properties,5–9 the use of
nanoscale architectures within the devices,10–18 and the for-
mation of hybrid organic-inorganic systems for improved
charge carrier collection and solar spectral coverage.19–23
This latter effort is of particular importance as developments
in device designs and existing material properties are now
well advanced, so further improvement in PCE is likely to
rely on increased utilization of the solar spectrum. To date,
accessing the near-infrared (NIR) spectral region, below the
absorption of organic materials, has been achieved via the
use of semiconductor nanocrystals (e.g., PbS) allowing utili-
zation of the spectrum up to 1.7 lm, well beyond the typical
cut off of most organic materials utilized (�0.8 lm).24–28
Advanced models of these hybrid devices indicate that they
can lead to significant improvements in device PCE with the-
oretical values of up to 50% when combined with appropri-
ate device geometry.29 Other developments have included
the use of down-conversion via impact ionization.30 An area
of increasing interest that provides an alternative approach
towards utilization of the NIR spectral region is the use of
materials in which up-conversion (UC) may take place.
The use of triplet-triplet annihilation as an UC mecha-
nism has been recently proposed as a means of improving PV
device performance.31 The use of this approach to utilize the
NIR is limited, however, by the inability of the organo (me-
tallic) complexes to absorb light at wavelengths lower than
that �800 nm. As a result alternative approaches, such as the
use of rare-earth (RE) UC phosphors, are required if the spec-
tral region beyond this is to be harnessed. Initial work in the
area of RE phosphors can be traced back to the study of
thulium-doped calcium tungstate as a potential UC material.32
The first application of a RE UC phosphor to PV utilized a
co-doped Yb3þ-Er3þ system placed at the rear of a GaAs de-
vice.33 The device external quantum efficiency (EQE) due to
UC was reported as 2.5% upon �890 nm excitation (1 W
with a device area of 0.039 cm2) though it is likely that due to
attenuation of the incident irradiation intensity prior to reach-
ing the phosphor this value was higher. Nonetheless, the
authors concluded that such values were too low for practica-
ble application. Reports on the application of UC phosphors
to crystalline Si-based devices (with the Er3þ-doped phosphor
placed at the rear) yielded improved performance with an
EQE of 3.4% reported under 2.4 W=cm2 irradiation at �1523
nm.34 In amorphous-Si (a-Si) devices, an EQE of 0.02% has
been reported using a co-doped Yb3þ-Er3þ system placed at
the rear under 980 nm (1.2 W=cm2) irradiation.35 This was
more recently followed spin casting a film of Yb3þ:Er3þ-doped
NaYF4 nanorods decorated with Au nanoparticles on the
front surface of an amorphous-Si device.36 Under 980 nm
(1100 mW) an EQE of 0.14% was reported for these devices.
Application of this concept to other thin film devices has only
recently been considered as their PCE has been improved. A
dye-sensitized solar cell (DSSC) in which Yb3þ and Er3þa)E-mail: [email protected]. Tel.: þ44 (0)1483 682713. Fax: þ44
(0)1483 689404.
0021-8979/2012/111(9)/094502/7/$30.00 VC 2012 American Institute of Physics111, 094502-1
JOURNAL OF APPLIED PHYSICS 111, 094502 (2012)
were directly incorporated within the device via doping of the
TiO2 nanocomposite (as opposed to at the rear) provides an
attractive approach, as the UC luminescence is emitted within
the device active region.37 Other reports in which the UC ma-
terial was placed at the rear of the DSSC device yielded a
response though the UC EQE was said to be negligible.38
Though a photocurrent (Isc ¼ 0.036 mA) was obtained under
980 nm excitation, the EQE was not reported. A preliminary
study reporting the use of UC in conjunction with OPV devi-
ces again used a co-doped Yb3þ-Er3þ system yielding an EQE
of 8� 10�4% under 980 nm illumination (25 mW=cm2).39
There has, therefore, been a recent growth in the interest of uti-
lizing rare-earth doped UC materials to enhance the perform-
ance of PV devices. The exploration of the use of UC
phosphor materials is particularly timely following the recent
development of a series of rare-earth ion doped materials
which include the highest reported (�5.2%) UC efficiencies
from the NIR to visible and the rapid developments within this
field.40–44
In this work, we describe the first detailed study that com-
bines the use of these newly available high-efficiency UC
phosphors with OPV devices that provide the information
required for the further development of this new approach. Up-
conversion external quantum efficiencies reaching 0.0052%
are demonstrated upon 986 nm excitation which could be
increased to 0.031% via the use of transparent electrodes
matching those achieved in a-Si:H photovoltaic devices.
II. EXPERIMENTAL
A. UC phosphor preparation
Full details of the synthesis and characterization of the
Yb3þ:Ho3þ co-doped Y2BaZnO5 phosphor are provided in
Ref. 42. The pellets for use in this study were created using a
press and placed as shown in Figs. 1(a) and 1(b). The aper-
ture in Fig. 1(a) was created by hand drilling using a 0.9 mm
diameter bit.
B. OPV device characterization
Current-voltage (I–V) measurements were performed
using a Keithley 2425 as the electronic load, exposing the
devices to simulated AM 1.5G light (obtained from 300 W
Xe Arc lamp ORIEL simulator calibrated to an intensity of
1000 W m�2) at room temperature through the use of a
0.385 cm2 aperture to eliminate stray effects. For the UC
measurements, the same electronic load was used, with a
diode laser emitting at 986 nm (Thorlabs L980P030).
FIG. 1. Schematic of the organic photovoltaic device with an up-conversion phosphor placed in front (a) and behind the device (b). (c) PCDTBT:PCBM and
Ho3þ:Yb3þ doped Y2BaZnO5 absorption and organic photovoltaic device transmission spectra. (d) AM1.5G spectrum and Ho3þ:Yb3þ co-doped Y2BaZnO5
up-conversion spectrum under 986 nm excitation. In (a) and (b) the separation of the phosphor from the device was �0.5 mm.
094502-2 Adikaari et al. J. Appl. Phys. 111, 094502 (2012)
C. OPV device preparation
Poly[N-90-hepta-decanyl-2,7-carbazole-alt-5,5(40,70-di-2-
thienyl-20,10,30-benzothiadiazole] (PCDTBT) (3.8 mg, Ossila
Limited, United Kingdom) and 6,6-pheynl C71-butyric acid
methyl ester (PCBM) (13.3 mg, Solenne, The Netherlands)
were added to anhydrous chloroform (1 ml, Sigma Aldrich)
in a clean glass vial, and the solution was stirred on a mag-
netic stirrer for 48 h. The process was carried out in a N2
filled MBRAUN glove box, with O2 and moisture kept below
10 ppm. The solution was then filtered with a 0.25 lm PTFE
syringe filter, before spin coating onto an indium tin oxide
(ITO) coated glass substrate which had been cleaned in an ul-
trasonic bath using acetone and methanol, subsequently
treated with an oxygen plasma for 5 min using an Emitech
Plasma Asher. The solution was spin coated at 3000 rpm for
20 s using a Laurell (PA, USA) spin coater and was allowed
to dry for a further 10 min in the glove box. The partially fab-
ricated devices were then transferred into a thermal evapora-
tor, where a hole blocking layer, bathocuproine (BCP, 5 nm)
was deposited before depositing the Al electrode (12, 15 and
90 nm). The deposition rates were pre-calibrated using an
alpha-step 200 profilometer with rates were kept at �6 nm
per minute, at an evaporation pressure of 2� 10�6 mbar
through a shadow mask yielding a device area �0.6 cm2.
D. Optical characterization
UV-VIS absorption of the PCDTBT:PCBM film was
performed with a Varian Cary 5000 spectrometer on an ITO
coated substrate deposited using the same recipe as for the
OPV devices. The transmission analysis of the semi-
transparent OPV devices was performed using the same ap-
paratus. For the reflection measurements, a Helma Analytics
TrayCell module was inserted into the Cary 5000, incorpo-
rating a calibrated reflection standard.
III. RESULTS
In considering the use of UC phosphors, two geometries
may be considered. In Figs. 1(a) and 1(b), two schemes are
presented for the use of an UC phosphor with OPV devices.
In the first configuration (Fig. 1(a)), the UC phosphor is
placed in front of the device in a similar manner to that
recently reported.39 This approach has obvious disadvan-
tages that include the inability of the phosphor to transmit
incident radiation to the OPV device due to its opaque na-
ture. We, therefore, modified the UC phosphor in this config-
uration via the milling of an aperture as shown to partially
mitigate this serious shortcoming, allowing a portion of the
incident irradiation to enter the device. Overfilling this aper-
ture allowed the UC phosphor to be excited and a portion of
the resulting UC emission to enter the device. In the second
configuration, the phosphor is placed at the rear of the device
(Fig. 1(b)), thus enabling the OPV device to utilize the full
solar spectrum. For UC to be a viable source of photocurrent,
we, therefore, must ensure that the rear electrode is transpar-
ent to both the NIR excitation and the visible UC emission.
In Fig. 1(c), the absorption spectrum of the active
PCDTBT:PCBM layer and of the Yb3þ:Ho3þ co-doped
Y2BaZnO5 UC phosphor is shown. As is typical for organic
materials, little absorption occurs above 700 nm, thus dem-
onstrating the need to explore alternative approaches for uti-
lization of the spectral region beyond. As can also be seen,
the UC phosphor exhibits the characteristic Yb3þ 2F7=2 !2F5=2 absorption in the 870–1030 nm region, complementing
that of the active organic layer. In this spectral region, the
AM1.5G spectrum provides an integrated power density of
10.5 mW cm�2. Fig. 1(d) shows the AM1.5G solar spectrum
and the UC emission obtained via excitation at 986 nm. Both
the peak of the AM1.5G spectrum and the main 545 nm UC
emission (Ho3þ 5S2,5F4 ! 5I8) are well matched to the
absorption of the photoactive PCDTBT:PCBM layer.
To provide a transparent rear electrode compatible with
device fabrication and with acceptable electrical properties,
we trialed two approaches: (1) deposition of a grid electrode
configuration; and (2) the use of very thin semi-transparent
electrodes. The first approach was found to result in poor de-
vice performance under AM1.5G conditions and therefore
not pursued. Fig. 2(a) shows the current density vs. voltage
(J-V) behavior of OPV devices under AM1.5G irradiation
with Al electrode thicknesses of 12, 15, and 90 nm. The
12 nm device can be seen to have a slightly decreased open
circuit voltage (Voc) and short circuit current density (Jsc)
with the power conversion efficiency (gPCE) being 3.3%. The
reduction in Voc is directly related to the thickness, which
for a conventional device (90 nm electrode thickness) is
found to be 0.9 V. Reducing the electrode thickness further
significantly reduced device performance and reproducibil-
ity, hence, 12 nm thick electrodes were adopted for use. The
reduction in Jsc between the devices with 12 and 15 nm elec-
trodes is due to decreased reflection of incident AM1.5G
light from the electrode surface. In comparison with the
standard devices, the increased sheet resistance resulting
from the use of a thin electrode is an additional factor in
reducing Jsc.
Fig. 2(b) shows the optical transmission and reflectivity
of the devices with 12 and 15 nm thick electrodes. Transmis-
sion is found to be similar with light normally incident on to
either the ITO substrate (solid lines) or on to the Al electrode
(dashed lines). Below 700 nm, the transmission is dominated
by the absorption of the photoactive layer (Fig. 1(c)) whilst
at wavelengths above this, the Al electrode is the dominating
factor. Reflection of light incident upon the rear of the Al
electrode, which when the phosphor is placed at the rear of
the PV device prevents UC emission from reaching the
active region, is found to be �84% for the device with a
12 nm thick electrode and thus will negatively impact on the
UC EQE and UC PCE (gUCEQE and gUC
PCE, respectively).
To characterize gUCEQE and gUC
PCE, J-V studies were per-
formed under a variety of conditions on the devices with
12 nm Al electrodes, with the UC phosphor placed as shown
in Figs. 1(a) and 1(b) (Fig. 2(c) and Table I). The OPV devices
were first characterized in the dark (solid line) and under
986 nm laser irradiation with no UC phosphor present (dashed
line). Despite the excitation energy being below the optical
band gap of the PCDTBT:PCBM blend, a small photocurrent
is still generated which we attribute to band tail-state absorp-
tion enhanced via weak phonon coupling or a strong coupling
094502-3 Adikaari et al. J. Appl. Phys. 111, 094502 (2012)
mechanism, as recently reported.45 Upon the insertion of the
UC phosphor at the device rear a further increase in both Jsc
and Voc is observed, clearly demonstrating the use of UC to
generate a photocurrent (dotted line). In this configuration, it
should be noted that the incident intensity upon the UC phos-
phor is significantly reduced from that incident on the OPV
from 29.5 to 3.07 mW, mainly due to the reflection at the
rear electrode (Fig. 2(b)). It was found possible to obtain fur-
ther increases in Jsc and Voc by moving the UC phosphor to
the front of the device (as shown in Fig. 1(a)), modified to
include a small aperture as discussed above (dashed-dotted
line). This does not, however, result in a higher gUCEQE or
gUCPCEð986 nmÞ as the incident laser intensity on the UC phos-
phor is significantly higher in this configuration (Table I).
To calculate the gUCEQE, we use the measured Jsc and inci-
dent 986 nm intensity on the UC phosphor (Pin)
FIG. 2. (a) J-V characteristics of OPV devices under AM1.5G irradiation with 12 nm (dashed line), 15 nm (solid line), and 90 nm (dotted line) thick Al elec-
trodes. (b) Transmission (solid and dashed lines) and reflectance (dotted lines) spectra of OPV devices with 12 nm and 15 nm thick electrodes. (c) J-V charac-
teristics of OPV device with 12 nm thick Al electrode in the dark (solid line) and under 986 nm laser illumination with no UC phosphor present (dashed line),
with the UC phosphor at the rear of the device (dotted line) and at the front of the device (dashed-dotted line). (d) J-V characteristics of OPV device in (a) at
varying laser intensities on the UC phosphor.
TABLE I. Performance parameters of OPV device with 12 nm Al electrode measured under various configurations and 986 nm laser power densities.
Device
description Ia (mW cm�2)
Equivalent
AM1.5Gb (suns)
Voc
(V)
Jsc
(lA cm�2) FF (%)
UC EQE
(� 10�3)c (%)
UC PCE
(� 10�3)d (%)
No phosphor 3756e 256 0.47 6.1 53.5 0.20 —
Phosphor at front 3756 256 0.56 82.2 65.0 2.23 0.63
Phosphor at rear 391 37.2 0.51 16.0 59.5 5.15 1.24
Phosphor at rear 322 30.6 0.49 11.2 58.0 4.36 0.98
Phosphor at rear 134 12.7 0.43 3.4 46.9 3.21 0.52
Phosphor at rear 107 10.2 0.40 2.4 42.6 2.81 0.38
Phosphor at rear 37.1 3.5 0.25 0.7 27.9 2.31 0.13
Phosphor at rear 13.2 1.3 0.11 0.3 26.5 2.47 0.06
aIncident 986 nm laser power density on UC phosphor.bCalculated using AM1.5G power density within Yb3þ absorption range taken as 870–1030 nm following previous convention.30,35
cgUCEQE.
dgUCPCE(986 nm).
eFigure given is incident power density on OPV device.
094502-4 Adikaari et al. J. Appl. Phys. 111, 094502 (2012)
gUCEQE ¼
Jsc=q
Pin=h�; (1)
where q is the elementary charge, h is the Plank’s constant,
and � is the frequency of incident light. This results in a
maximum measured gUCEQE of 5.2� 10�3% with the phosphor
placed at the rear of the device under a 986 nm illumination
power density of 391 mW cm�2. The EQE is significantly
reduced as discussed above by the poor transmission of the
semi-transparent electrode at 545 nm but is still an order of
magnitude greater than that calculated from the only other
UC OPV device reported (8.0� 10�4%).39 Should a trans-
parent electrode be used to enable all UC emission to be cap-
tured and then the gUCEQE could increase to 0.031% matching
that reported for a-Si:H OPV devices.35
Whilst gUCEQE is an important measure of the UC potential
and a direct reflection of the Jsc contribution due to UC, it
does not provide an indication of the contribution to the PCE
which is dependent on other device parameters. Using the
measured UC PCE under 986 nm excitation, gUCPCEð986 nmÞ,
with the phosphor at the rear of the device (1.24� 10�3%),
we are able to calculate the potential wavelength dependent
UC PCE (gUCPCEðkÞ) under AM1.5G irradiation of equivalent
power density. Fig. 3(a) shows the normalized line shape,
G(k), of the product of the AM1.5G spectrum, following
transmission through the OPV device, T(k), and the Yb3þ
2F7=2! 2F5=2 absorption, aYb3þðkÞ as given by
GðkÞ ¼ AM1:5ðkÞaYb3þðkÞTðkÞ: (2)
We have previously shown that the 545 nm UC emission in-
tensity is proportional to [G(k)]2. (Ref. 42) We may therefore
obtain gUCPCEðkÞ, shown in Fig. 3(a), via normalization of
[G(k)]2 to the measured gUCPCEð986 nmÞ
gUCPCEðkÞ ¼
½GðkÞ�2gUCPCEð986 nmÞ
½Gð986 nmÞ�2: (3)
The total potential UC PCE available is then obtained via
integration of Eq. (3)
gUCPCE ¼
ðgUC
PCEðkÞdk: (4)
As shown in Fig. 3(a), this yields a value of gUCPCE ¼ 0:073 %
for the device configuration used, assuming concentration of
the NIR AM1.5G illumination to match the 986 nm laser in-
tensity incident on the UC phosphor (�37 suns). Again, as
for gUCEQE, the use of a transparent rear electrode could signifi-
cantly improve the total potential to gUCPCE ¼ 0:45% (ignoring
additional expected increases in Voc and device fill-factor
(FF)), which represents a significant contribution to power
generation.
Given the relatively high equivalent AM1.5G excitation
intensities used above, we undertook a series of measure-
ments at lower light intensities as presented in Fig. 2(d) and
Table I. It is clear that the Jsc decreases as the excitation in-
tensity is reduced, as would be expected. There is also a cor-
responding decrease in the Voc and FF which impacts on
gUCPCE. At the lowest 986 nm incident power on the UC phos-
phor (equivalent to �1 sun), we obtained an gUCEQE of
2.5� 10�3%, which again the use of a transparent electrode
could improve yielding a value of �0.015%. However, the
reduction in Voc and FF results in a much lower
gUCPCE(986 nm) of 5.7� 10�5% being obtained.
In Fig. 3(b), we plot the Jsc as a function of the incident
intensity on the UC phosphor, IUC, in addition to the Jsc
obtained in the absence of the UC phosphor at the highest
excitation power used. Also, indicated in Fig. 3(b) is the
FIG. 3. (a) Normalized product of AM1.5G spectrum and Yb3þ 2F7=2 !2F5=2 absorption corrected for the OPV device transmission and calculated
gUCPCEðkÞ for and gUC
PCE under such irradiance. (b) Jsc as a function of incident
986 nm power density on the UC phosphor (squares) and in the absence of
the UC phosphor (circle). A quadratic fit to Jsc is shown (solid line) along
with the linear component of that fit (dashed line). (c) gUCEQE (squares), Voc
(circles), and FF (triangles) as a function of incident 986 nm power density
on the UC phosphor. Also shown is a linear fit to gUCEQE.
094502-5 Adikaari et al. J. Appl. Phys. 111, 094502 (2012)
equivalent AM1.5G solar concentration for incident 986 nm
intensities used. Given that the UC efficiency is proportional
to the square of the incident intensity, it would be expected
that a quadratic fit, including a linear term to account for the
directly generated photocurrent (Fig. 2(c)), should describe
the power dependency of the Jsc. As shown in Fig. 3(b), this
provides an excellent fit to Jsc measured with the phosphor in
place. Additionally, the linear term of this fit is also found to
accurately predict the Jsc, we measured in the absence of the
UC phosphor at the highest incident power used.
Given that the Jsc is proportional to ðIUCÞ2, it can be
seen that the dependence of gUCEQE on IUC should be linear via
substitution for Jsc in Eq. (1). At the lowest value of IUC used
the contribution to Jsc from direct absorption of the 986 nm
dominates (Fig. 3(b)); we thus exclude this in our analysis,
considering only higher values of IUC which may be fitted
using a linear expression, as expected (Fig. 3(c)). Also
shown in Fig. 3(c) is the dependence of Voc and FF on IUC,
which have been discussed elsewhere in the literature.46
IV. DISCUSSION
The study of data presented above clearly indicates that
the optical properties of the rear electrode have the largest
effect on determining both the gUCEQE and gUC
PCE. Recent advan-
ces in the development of transparent OPV devices have led
to devices exhibiting a device transmission of �75% at 545
nm with the loss being mainly due to absorption within the
active layer.47 Equally, within the NIR at the wavelengths of
interest (�980 nm), the transmission is found to �70%.
Such a device would significantly benefit from the use of an
UC phosphor and with the increased NIR transmission fur-
ther improvements in gUCEQE exceeding the value of 0.031%
calculated above could be achieved. Given that Voc and FF
also increase with IUC the increase in gUCPCE will be greater
than gUCEQE (which scales linearly with IUC). It is, therefore,
feasible that the calculated value of gUCPCE ¼ 0:45% is also
conservative in nature and would be further improved. With
additional light management strategies to reduce surface
reflection and increase light confinement within the devices,
the approach demonstrated here has a clear potential role to
play in future OPV technologies.
It is important to consider the practicality of using the
UC phosphor under non-coherent conditions such as under
AM1.5G irradiation. From the intensity dependent data pro-
vided in Figure 3(b), it is clear that concentration of the
AM1.5G spectrum would be required in order to obtain a
measurable contribution due to UC. We note that it is only
the spectral region above �850 nm that would require con-
centration though technologically this may be difficult to
achieve without concentration of the entire spectrum. This
was confirmed via our attempts to measure gUCEQEðkÞ at
AM1.5G intensity which yielded no measurable response.
Without the ability to perform similar measurements under
concentrated light intensities, we must rely on the single
wavelength measurements reported above. With the UC
phosphor placed at the rear of the device, the highest inten-
sities used (391 mW=cm2) are equivalent to �37 suns. This
is well below what is commonly achievable (>100 suns) via
the use of solar concentrators.48 It is also below the excita-
tion power densities previously used to demonstrate UC
EQE in GaAs, Si, amorphous-Si, and DSSC PV devices.32–37
We note that we have previously demonstrated an UC effi-
ciency of 2.6% under 980 nm excitation at power densities
of �3500 mW=cm2 which would correspond to a concentra-
tion factor of 333 suns; thus, the stability of the phosphor is
not a concern under such conditions.
The effect of using a non-coherent excitation source pro-
viding a broader spectral output (e.g., the solar spectrum
above 850 nm) as opposed to coherent 980 nm laser excita-
tion (at equivalent integrated power densities) used should
also be considered. There is no requirement for coherent ex-
citation in achieving UC emission which is dominated by
two successive energy transfer steps from Yb3þ to Ho3þ.42
As a result, the full spectral range of the Yb3þ 2F7=2 to 2F5=2
absorption shown in Figure 1(c) may be utilized. Therefore,
so long as sufficient power density is achieved within this
spectral region as discussed above the approach remains
valid.
V. CONCLUSIONS
In this paper, we have demonstrated the use of high effi-
ciency Yb3þ:Ho3þ co-doped Y2BaZnO5 phosphors with
OPV devices for the utilization of the NIR solar spectrum
and report for the first time measurements of the gUCEQE and
gUCPCE. Our approach allows the optimized performance of the
OPV device under normal operating conditions to be main-
tained with the UC phosphor being placed externally to the
device. We show that it is possible to obtain a Jsc of 16 lA
cm�2 under 986 nm illumination (�390 mW cm�2 corre-
sponding to �37 suns) leading to an gUCEQE of 0.0052%.
Through modification of the OPV devices to incorporate
transparent electrodes, we show that gUCEQE could be increased
to 0.031%, matching that achieved in a-Si:H PV cells. Like-
wise, the gUCPCE calculated by taking into account the full
spectral range of the UC phosphor (�870–1030 nm) could
be increased from 0.073% to 0.45%.
ACKNOWLEDGMENTS
A.K.C. and I.E. thank Saint-Gobain Recherche for
funding.
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