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: r.j.curry@surrey.ac.uk. 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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