Electroluminescence in polymer-fullerene photovoltaic cells
Heejoo Kim, Jin Young Kim, Sung Heum Park, Kwanghee Lee, Youngeup Jin, Jinwoo Kim, and Hongsuk Suh
Citation: Applied Physics Letters 86, 183502 (2005); doi: 10.1063/1.1924869
View online: http://dx.doi.org/10.1063/1.1924869
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Electroluminescence in polymer-fullerene photovoltaic cells
Heejoo Kim, Jin Young Kim, Sung Heum Park, and Kwanghee Lee
a兲
Department of Physics, Pusan National University, Pusan 609-735, South Korea
Youngeup Jin, Jinwoo Kim, and Hongsuk Suh
Department of Chemistry, Pusan National University, Pusan 609-735, South Korea
共Received 20 December 2004; accepted 22 March 2005; published online 28 April 2005兲
We report electroluminescence 共EL兲 in photovoltaic 共PV兲 cells based on semiconducting
polymer-fullerene composites. By applying a forward bias to the PV cells, the devices exhibited a
clear EL action with a peak around 1.5 eV. We ascribe this peak to an “electric field-assisted
exciplex” formed between the electrons in the fullerenes and the holes in the polymers, thereby
resulting in radiative recombination in the composites. This finding is totally unexpected because of
a strong photoluminescence quenching in the same materials. Since the same devices also showed
typical photovoltaic effects under illumination, our results demonstrate a dual functionality in one
device; polymer photovoltaic cells and polymer light-emitting diodes. © 2005 American Institute of
Physics. 关DOI: 10.1063/1.1924869兴
Semiconducting polymer-fullerene 共C
60
兲 composites
continue to be promising materials for organic photo-
voltaics.
1
Recent progress in these polymer photovoltaics re-
sults in power conversion efficiencies around 3%–4% under
AM1.5 共AM⫽air mass兲 irradiation.
2,3
This class of devices is
based on an interesting photophysical phenomenon, called
“ultrafast photoinduced charge transfer.” After photoexcita-
tion of the semiconducting polymers, the excited
electrons in the polymers transfer to C
60
within ⬃50 fs
timescale.
4
Since this process is faster than any other com-
peting radiative and nonradiative decay processes, signifi-
cant photoluminescence 共PL兲 quenching occurs in the
composites.
4
The inhibition of early time recombination of-
fers a way to efficient charge separation, which stimulates
the application of high-efficiency photodiodes and photovol-
taic 共PV兲 cells using these composites.
It is reported that the back transfer of the electrons from
C
60
to the polymer is remarkably slow in the order of
miliseconds.
5,6
Steady-state photoinduced absorption 共PIA兲
and nearly steady-state PIA experiments have confirmed that
these transferred electrons are relaxed in the lowest unoccu-
pied molecular orbital 共LUMO兲 of C
60
.
6–8
Previous PL spec-
troscopic studies suggest that the recombination between the
LUMO of C
60
and the
band of the polymers is a nonradi-
ative decay based on an absence of the PL signature in the
corresponding energy ranges.
6,7
This nonradiative nature
originates from the lattice relaxation of the transferred elec-
trons in C
60
with a Jahn–Teller-type distortion.
9,10
The relax-
ation process induces an interfacial barrier at the interfaces
between the semiconducting polymers and C
60
, thereby
forming metastable states in C
60
with ⬃
s lifetime.
8
How-
ever, the interfacial barriers arising from such an electron-
lattice coupling can be overcome by applying an additional
external force such as electric field or by thermal activation.
In such a case, it might be possible to induce radiative black
recombination in these systems by applying substantial elec-
tric field. In particular, since relatively strong electric field
can be applied in recent polymer devices, the possibility of
electroluminescence 共EL兲 in polymer-fullerene devices is
quite plausible. In this work, we report a clear EL action in
the polymer-fullerene PV cells under forward bias above 5 V.
Composite solutions of poly-关2-methoxy-5-共2
⬘
-ethyl-
hexyloxy兲-1,4-phenylene vinylene兴共MEH-PPV兲, and soluble
C
60
, 兵6其-1-关3-共methoxycarbonyl兲propyl兴-兵5其-1-phenyl 关5,6兴-
C
61
共PCBM兲 were prepared by blending parent solutions
共MEH-PPV in tetrahydrofuran and PCBM in chlorobenzene兲
with various weight percents 共wt %兲 of PCBM; 0, 5, 10, 20,
and 50 wt %. These composite solutions were stirred at room
temperature for 12 h. Each solution was spun on precleaned
UV-graded fused silica substrates for optical absorption and
PL spectra measurements. All of the cast films did not show
any phase segregation of MEH-PPV and PCBM. Optical ab-
sorption spectra were recorded by a Varian 5E UV-VIS-NIR
spectrophotometer, while Oriel Instaspec IV charge-coupled
device detection system in combination with a solid state
laser 共532 nm兲 was used for the PL spectra measurements.
Polymer-fullerene PV cells of various concentrations of
PCBM 共0, 5, 10, 20, and 50 wt %兲 were fabricated with a
typical device structure of ITO/PEDOT:PSS/MEH-PPV
⫹PCBM composite/Al. Poly共3,4-ethylenedioxylene
thiphene兲-polystyrene sulphonic acid 共PEDOT:PSS兲 was
used as a buffer layer, which acts as a hole collecting layer
and also decreases morphological roughness of the ITO sur-
faces. The thicknesses of the composite films were around
100 nm. Then, an aluminum 共Al兲 electrode was deposited by
using a thermal evaporation in a vacuum of about 5⫻10
−5
.
The current density-voltage 共J–V兲 characteristics of the de-
vices were measured under illumination and in the dark us-
ing a Keithley 236 source measure unit 共SMU兲. A solid state
laser was used as a monochromatic light source 共532 nm,
30 mW/cm
2
兲 for the photovoltaic measurements. The
luminescence-voltage 共L–V兲 characteristics of these devices
were obtained by a Keithley 236 SMU equipped with a cali-
brated photomultiplier tube.
PL quenching in donor-acceptor composites is a useful
indication for the efficient charge transfer between the two
components.
4
Figure 1共a兲 shows the PL spectra of the MEH-
PPV/PCBM composite films with various concentrations of
PCBM. The results exhibit a significant PL quenching of the
MEH-PPV emission in the composites. Even for the lowest
a兲
Electronic mail: kwhlee@pusan.ac.kr
APPLIED PHYSICS LETTERS 86, 183502 共2005兲
0003-6951/2005/86共18兲/183502/3/$22.50 © 2005 American Institute of Physics86, 183502-1
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PCBM concentration 共⬃5%兲 film, the PL is completely
quenched with almost no spectral feature over the entire
ranges. This observation obviously indicates that the charge
transfer from MEH-PPV to PCBM occurs efficiently, and the
back-transfer proceeds with the nonradiative recombination
process.
11,12
Utilizing those composites as an active layer, we
fabricated typical PV cells with a structure of
ITO/PEDOT:PSS/MEH-PPV+PCBM/Al. Figure 1共b兲
shows the J–V curves of the PV cells under monochromatic
illumination 共=532 nm,30 mW/cm
2
兲. The data clearly in-
dicate typical PV effects for all devices. The short circuit
current 共I
sc
兲 increases gradually with increasing PCBM con-
centration, while the open circuit voltage 共V
oc
兲 remains
around 0.8 V for all devices. From these PV parameters, we
evaluate the power conversion efficiencies 共
p
兲 as
p
=0.13%–0.8% for these devices. Using a higher PCBM ratio
共⬃75 wt %兲 composite, we achieved
p
=2.3% 共not shown
here兲 under monochromatic light excitation.
13
Using the same devices as above, we further applied the
forward bias up to ⬃10 V without any illumination. Then,
we surprisingly observed that these PV cells exhibit a typical
EL under forward bias as shown in Fig. 2. The light emission
starts above a turn voltage around 5 V and reaches the value
above ⬃20 cd/m
2
at 10 V for the lowest concentration de-
vice 共5% device兲. The luminous intensity decreases with in-
creasing PCBM concentration. Although the luminance of
the composite devices is weak as compared with that of the
typical polymer light-emitting diodes 共P-LED兲, this observa-
tion is quite substantial. We precisely duplicated the experi-
ments with consistent results both in the PV responses and
EL actions in this system. Moreover, we also observed iden-
tical effects using other electroluminescent polymer systems,
such as composites of poly 关2-methoxy-5-共3
⬘
,7
⬘
-dimethyl-
octyloxy兲-p-phenylene vinylene兴共OC
1
OC
10
-PPV兲 and
PCBM, and composites of polyalkyfluorene derivative with
C
60
pendant.
14
Therefore, we can safely conclude that this is
a general phenomenon for those systems of electrolumines-
cent polymer-fullerene composites.
Considering such a complete PL quenching in the same
materials 关see Fig. 1共a兲兴 even for the lowest concentration of
PCBM 共5%兲, this observation is somewhat unexpected. One
might consider that some nonideal states of the active layers,
such as phase segregation in the composites, would be re-
sponsible for this observation. However, obvious function as
photovoltaic cells for the same devices clearly rules out such
a possibility 关see Fig. 1共b兲兴. In general, the failure of the
formation of the interpenetrating networks in the composites
would lead to a poor photovoltaic performance.
In order to clarify the origin of light emission in the
composite PV cells, therefore, we have measured the EL
spectra of the devices as shown in Fig. 3共a兲. The low con-
centration devices 共5 and 10 wt %兲 show more or less similar
EL spectra with that of the pure MEH-PPV 共0%兲 with a peak
around 2.1 eV, which corresponds to the characteristic lumi-
nescent peak of MEH-PPV, and with a weak feature around
1.7 eV. However, the device of the 20 wt % PCBM exhibits
a dramatic change in the spectrum; the 1.7 eV feature grows
to the prominent peak and a new peak develops around 1.5
eV. For the 50 wt % device, the 1.5 eV peak dominates the
spectrum with completely suppressed features of the 2.1 eV
peak. Although previous PL studies on the C
60
thin films
unambiguously assign the 1.7 eV peak to the emission from
PCBM,
15
the origin of the 1.5 eV peak is unclear.
As shown in the energy level diagram of the devices in
Fig. 3共b兲, three kinds of recombination are possible in their
excited states; transitions I, II, and III. Transition I corre-
sponds to the
-
*
recombination of MEH-PPV 共2.1 eV兲,
while the highest occupied molecular orbital 共HOMO兲-
LUMO transition of PCBM 共transition II兲 yield the 1.7 eV
peak. Transition III is a direct recombination between the
two components, and generally characterized as an
“exciplex.”
16
We attribute this to the 1.5 eV feature.
Although we can easily assign those features in the EL
spectra as above, the questions arise how the EL spectra
evolve with the concentration of PCBM in the composites,
FIG. 1. 共a兲 PL spectra of the MEH-PPV+PCBM composite films with
various PCBM concentrations. The inset shows the photoinduced charge
transfer between MEH-PPV and PCBM. 共b兲 J–V characteristics of
the ITO/PEDOT:PSS/MEH-PPV+PCBM/Al device under 532 nm
illumination.
FIG. 2. Current density-voltage and luminance-voltage characteristics of the
typical ITO/PEDOT:PSS/MEH-PPV+PCBM/Al devices.
183502-2 Kim
et al.
Appl. Phys. Lett. 86, 183502 共2005兲
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On: Thu, 25 Sep 2014 07:37:56
and why transition III appears to be radiative in the EL spec-
tra in contrast with its nonradiative nature in the PL spectra.
The first question can be understood as follows. In such a
bulk heterojunction photovoltaic geometry, charge carriers
can be injected into both components, MEH-PPV and
PCBM. However, since the percolation threshold in this sys-
tem is around 20 wt % for PCBM,
17
the low concentration
devices 共5 and 10 wt %兲 are dominated by the charges in-
jected mainly into the MEH-PPV, thereby yielding only the
characteristic 2.1 eV peak of MEH-PPV. For the 20 wt %
device, the PCBM networks start to form and show the bulk
nature of PCBM.
18
In such a case, the charge carriers are
injected into both components and induce transitions in each
components; transition I at 2.1 eV for MEH-PPV, and tran-
sition II at 1.7 eV for PCBM. Since most of the LUMO
levels of PCBM might be occupied by directly injected elec-
trons from the Al electrode, the charge transfer from MEH-
PPV to C
60
would be unfavorable for the 20 wt % device. We
believe this would be also true even for the low concentra-
tion devices 共5 and 10 wt %兲. Although the low concentra-
tion devices are below the percolation threshold for PCBM,
substantial portion of the PCBM might be occupied by the
directly injected charges from the electrode, thereby prohib-
iting electron transfer from MEH-PPV into C
60
. This might
be the reason why the 2.1 eV peak is still dominant in the EL
spectra of the low concentration devices in contrast with its
complete quenching in the corresponding PL spectra.
When the concentration of PCBM increases to 50 wt %,
the network of PCBM would be uniformly distributed over
the MEH-PPV matrix with a closer distance for the exciton
dissociation.
18
In such a situation, the injected electrons in
MEH-PPV transfer to the LUMO levels of PCBM, and the
holes injected to the PCBM part move to the HOMO of
MEH-PPV. In particular, when a strong electric field is ap-
plied over thin active layers, the charges form an exciplex by
reducing the interfacial barrier at the heterojunction as simi-
lar to the case of the bilayer-type organic-LEDs.
19,20
These
exciplex recombine eventually with a light emission, corre-
sponding to the 1.5 eV feature.
In conclusion, a dual functionality in one device is dem-
onstrated by observing an EL action in the polymer-fullerene
photovoltaic cells under forward bias. Moreover, the EL
spectra show a systematic evolvement of a peak around 1.5
eV with increasing fullerene concentration in the composites.
We ascribe this peak to an “electric field-assisted exciplex”
formed between the electrons in the fullerenes and the holes
in the polymers. The strong electric field in such thin-film
devices reduces the barriers of the meta-stable electrons in
C
60
, thereby inducing radiative recombination in the
polymer-fullerene composites. We expect that such a dual
functionality will provide an opportunity to create a smart
display equipped with a self-energy supplying capability.
This work was supported by the MIC 共Ministry of Infor-
mation & Communication兲, Korea, under the ITRC 共Infor-
mation Technology Research Center兲 support program super-
vised by the IITA 共Institute of Information Technology
Assessment兲.
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FIG. 3. 共a兲 EL spectra of the devices using the MEH-PPV+PCBM compos-
ites with a sandwich structure of ITO/PEDOT:PSS/MEH-PPV
+PCBM/Al. 共b兲 Energy level diagram of the MEH-PPV+C
60
composites. I:
-
*
transition in MEH-PPV; II: HOMO-LUMO transition in C
60
; and III:
direct transition between the electrons in C
60
and holes in MEH-PPV.
183502-3 Kim
et al.
Appl. Phys. Lett. 86, 183502 共2005兲
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