Surface Plasmon-Induced Band Gap in the Photocurrent Response of Organic Solar Cells

Abstract

A 260 nm layer of organic bulk heterojunction blend of the polymer poly(3-hexylthiophene) (P3HT) and the fullerene [6,6]-phenyl C61-butyric (PCBM) was spin-coated in between aluminum and gold electrodes, respectively, on top of a laser inscribed azo polymer surface-relief diffraction grating. Angle-dependent surface plasmons (SPs) with a large band gap were observed in the normalized photocurrent by the P3HT-PCBM layer as a function of wavelength. The SP-induced photocurrents were also investigated as a function of the grating depth and spacing.

Full text

Hindawi Publishing Corporation
International Journal of Photoenergy
Volume 2010, Article ID 698718, 5pages
doi:10.1155/2010/698718
Research Article
Surface Plasmon-Induced Band Gap in the Photocurrent
Response of Organic Solar Cells
Ribal Georges Sabat,1Marcos Jose Leite Santos,2and Paul Rochon2
1Department of Physics, Royal Military College of Canada, P.O. Box 17000, STN Forces, Kingston, ON, Canada K7K 7B4
2Instituto de F´ısica de S˜ao Carlos, Universidade de S˜ao Paulo, CP 369, 13566-590 S˜ao Carlos, SP, Brazil
Correspondence should be addressed to Ribal Georges Sabat, sabat@rmc.ca
Received 14 September 2010; Revised 12 November 2010; Accepted 8 December 2010
Academic Editor: Mark van der Auweraer
Copyright © 2010 Ribal Georges Sabat et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
A 260 nm layer of organic bulk heterojunction blend of the polymer poly(3-hexylthiophene) (P3HT) and the fullerene [6,6]-
phenyl C61-butyric (PCBM) was spin-coated in between aluminum and gold electrodes, respectively, on top of a laser inscribed
azo polymer surface-relief diffraction grating. Angle-dependent surface plasmons (SPs) with a large band gap were observed in
the normalized photocurrent by the P3HT-PCBM layer as a function of wavelength. The SP-induced photocurrents were also
investigated as a function of the grating depth and spacing.
1. Introduction
In the recent years, research on photovoltaic cells has spread
beyond inorganic materials, especially in the advent of thin-
film solar cells. Despite their low efficiency, organic solar cells
have generated much research interest, mainly because of
their ease of fabrication and processability [1–3]. The bulk
heterojunction blend of the polymer poly(3-hexylthiophene)
and the fullerene [6,6]-phenyl C61-butyric (P3HT-PCBM)
is one of the most promising organic solar cell materials
[4]. The P3HT is a conducting polymer that produces the
photovoltaic effect via the excitation of the π-orbit electrons,
while the PCBM possesses a high hole mobility and acts
as an acceptor. The P3HT is the hole conductor while the
PCBM is the electron conductor [5]. Many papers have
reported on how the photovoltaic properties of the P3HT-
PCBM blend can be enhanced depending on the sample
preparation conditions, such as thermal annealing [6,7]
and its concentration in solvent [8]. Other papers have
also reported on how to further increase the efficiency and
the challenges present with these bulk heterojunction cells
[9–11].
Since thin-film solar cells have a thickness in the order
of a micron or less, wavelength-sized structures can be used
to trap light inside the photovoltaic film. The integration
of diffraction gratings either in the substrate or in the solar
cell material itself has been successfully studied in thin-film
silicon cells [12,13] and in organic solar cells [14,15].
Surface plasmons (SPs), which are electromagnetic waves
that propagate at the interface between a metal and a
dielectric, have also proved to increase light entrapment
and absorption in both silicon [16–18] and organic thin-
film solar cells [19–22]. Surface plasmons can be excited
by matching the momentum and energy of the incident
light beam to that of the plasmon along the direction of
propagation. This can be done using a corrugated relief
grating as well as metallic nanoparticles. In the absence of
a band gap, the surface plasmon wave number kSP for a flat
surface is given by the following dispersion relation:
kSP =kεmεd
εm+εd
,(1)
where kis the light wave number propagating in a dielectric
with refractive index n,wherek=(2π/λ0)n,andεmand εd
are the permittivities of the metal and the dielectric material
respectively. Since n=√εdand usuallyεm/(εm+n2)≈1.1,
for a quick estimate of the resonance wavelength, (1)canbe
approximated to
ksp ≈1.1nk. (2)

2International Journal of Photoenergy
Scan distance (4 μm)
Zdistance (46.89 nm)
X
YZ0
23.45
46.89
(nm)
Figure 1: AFM picture of a surface relief diffraction grating with
Λ=250 nm.
In order for the SP to be generated, its wave number must be
phase-matched to that of the incident light beam using, for
instance, a diffraction grating, such that along the horizontal
(x) direction on the sample surface
klight =


ksin θ±2πm
Λ


,(3)
where Λis the grating spacing, θis the incidence angle, and
mis an integer. A positive diffraction order indicates forward
coupling, while the negative order represents backward
coupling. On a surface-relief grating, only light polarized
in the direction of the grating vector has an electric
field component perpendicular to the metal surface and
therefore can couple to an SP mode. Hence, if a sample
is rotated around the vertical (y) axis, only horizontally
(x) polarized light (TM) will generate a surface plasmon.
For normal incidence and first order diffraction (m=1),
(3) becomes
ksp ≈1.1nk ≈



2π
Λ


.(4)
Therefore, the surface plasmon resonance wavelength should
be
λSP ≈1.1nΛ.(5)
Nonetheless, photonic band gaps have also been observed in
the propagation of SPs on periodic media, and their physical
origin was explained [23]. Therefore, theoretical predictions
of the surface plasmon wavelength can only be useful for
findingthecentreofthesurfaceplasmonbandgap.Theband
gap width has also been shown to depend on the gratings’
depth.
In this paper, we use surface-relief diffraction gratings
inscribed on poly(4-{[2-(acryloyloxy)thel]ethyl-amino}-4-
nitroazobenzene) (pDR1A), also called azo polymer [24],
to induce SPs at the interface between the aluminum
electrode and the P3HT-PCBM blend. The SPs were mea-
sured as a function of the incidence angle and the wave-
length of the incoming light. The effects of the gratings’
Grating
Electrodes
P3HT-PCBM
Al
Azo-polymer
BK7 glass slide
Au
Figure 2: A cut-away side view of the test sample.
depth and spacing on the SP photocurrent were also
investigated.
2. Experiment
The azo-polymer compound was diluted in dichloromethane
with a mix ratio of 3% weight/weight, and thoroughly
mixed. A 200 nm layer of this solution was spin-coated on
a BK7 glass slide. Surface-relief gratings were written on
the azo-polymer films by direct holographic exposure to the
interference pattern of two coherent light beams at λ=
458 nm. The grating spacing can easily be controlled by
changing the incidence angle of the interfering beams, while
the depth of the gratings was dependent on the exposure
time, as described elsewhere [25]. Figure 1 shows an atomic
force microscope picture of a diffraction grating written with
spacing, Λ=250 nm. An aluminum electrode with 100 nm
thickness was subsequently evaporated on the diffraction
grating. As for the photovoltaic blend, a (1 : 1) P3HT-PCBM
solution was diluted in chlorobenzene with a mix ratio of
5% weight/weight. This solution was thoroughly mixed using
an ultrasonic bath and a mechanical shaker. The P3HT-
PCBM blend was then spin-coated on top of the aluminum
electrode with a thickness of approximately 260 nm, and
annealed at 95◦C for 1 hour under N2atmosphere. Finally,
a very thin (10 nm) layer of gold was evaporated on top of
the P3HT-PCBM layer. A cut-away view of the experimental
sample is illustrated in Figure 2. The illuminated section of
theactivelayerwasapproximately2×4mm
2.
Since gold absorbs strongly below the wavelength of
λ=600 nm, it was found better to use the aluminum to
generate the SP. The aluminum and gold electrodes were
essential parts of harvesting the highest photovoltaic signal
from the P3HT-PCBM blend because of the energy band
compatibility of the structure.
As for the experimental setup seen in Figure 3,the
light from a spectrometer passed through a mechanical
chopper was collimated by a concave mirror, passed though
a polarizer, and was finally incident on the test sample,
which was located on a computer-controlled turn table.

International Journal of Photoenergy 3
Mechanical chopper
To lock-in amplifier and computer
Test sample
Turntable
Pola ri zer
Concave mirror
Spectrometer
Figure 3: The experimental setup.
The signal from the organic solar cell was amplified by a lock-
in amplifier and recorded on a computer.
3. Results and Analysis
A preliminary plot in Figure 4 shows the photocurrent
response of the test sample at normal incidence on an
area with no diffraction grating. It can clearly be seen that
TE and TM light polarizations have near identical curves.
As discussed in the previous section, we expect the SP to
be generated only with TM polarized light, which has its
electric field vector oscillating perpendicular to the grating’s
peaks and troughs. This is evident in Figure 5 where we
repeated the photocurrent measurement on a grating with
spacing Λ=250 nm and a depth of approximately 50 nm
at normal incidence. The TE photocurrent response curve
stays the same, while a large increase was measured in the
TM curve. Two peaks can be identified at λ=432 nm and
λ=569 nm these peaks are associated with a photonic
band gap in the SP dispersion curve. In order to confirm
that these peaks are SP-induced, the photocurrent response
of the P3HT-PCBM blend was measured on a grating with
spacing Λ=325 nm and a depth of approximately 15 nm
at normal incidence, as seen in Figure 6.Asimilarincrease
was found for the TM polarization, but the peaks has
clearly shifted and one of them is now located at λ=
585 nm.
According to Mou´
l and Meerholz [26], the refractive
index of 1 : 1 P3HT-PCBM is around 1.75 at 500 nm and 2
at 700 nm. Therefore, using (5), for diffraction gratings with
Λ=250 nm and Λ=325 nm, surface plasmons should
be centered around 480 nm and 715 nm, respectively. This is
seen in Figure 5 since the band gap centre is clearly located
at500nm.However,inFigure 6, only the lower wavelength
resonance peak can be seen. This is because the band gap
centre for a 325 nm grating should be at around 715 nm, and
the higher resonance peak is located above the absorption
range of the active layer.
As we change the incidence angle on the 250 nm grating
with 50 nm depth, seen in Figure 7, the SP-induced peaks
350 400 450 500 550 600 650 700 750 800
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
TM
Photocurrent (A/W)
Wavelength (nm)
TE
No grating; θ=0◦
Figure 4: The TE and TM photocurrent responses of the P3HT-
PCBM layer as a function of wavelength with no grating and at
normal incidence.
350 400 450 500 550 600 650 700 750 800
0
0.01
0.02
0.03
0.04
0.05
λ=432 nm
λ=569 nm
TM
TE
Photocurrent (A/W)
Wavelength (nm)
Λ=250 nm; grating depth ∼50 nm; θ=0◦
Figure 5: The TE and TM photocurrent responses of the P3HT-
PCBM layer as a function of wavelength with Λ=250 nm and at
normal incidence.
in the photocurrent for TM polarized light seem to shift
as a function of the angle. This confirms that this is in
fact a photonic band gap measured in the photocurrent
response of the bulk heterojunction blend. The lower
wavelength peaks are associated with forward coupling of
light, while the higher wavelength peaks are associated with
backward coupling in the active layer, in accordance with
(3), as explained elsewhere [27]. The dispersion relation was
subsequently plotted using (3)inFigure 8 for two different
gratings with 250 nm spacing and with grating depths of
50 and 28 nm. The band gap becomes easily distinguishable
and appears to decrease as the grating depth decreases.

4International Journal of Photoenergy
TM
TE
350 400 450 500 550 600 650 700 750 800
0
0.01
0.02
0.03
0.04
0.05
Photocurrent (A/W)
Wavelength (nm)
λ=585 nm
Λ=325 nm; grating depth ∼15 nm; θ=0◦
Figure 6: The TE and TM photocurrent responses of the P3HT-
PCBM layer as a function of wavelength with Λ=325 nm and at
normal incidence.
400 450 500 550 600 650
0
0.01
0.02
0.03
0.04
0.05
0.06
Photocurrent (A/W)
Wavelength (nm)
Λ=250 nm; grating depth ∼50 nm
TM at 8◦
TM at 16◦
TM at 24◦
TM at 32◦
TM at 24◦
TM at 32◦
TM at 8◦
TM at 16◦
TM at 0◦
TE at 0◦
Figure 7: The TE and TM photocurrent responses of the P3HT-
PCBM layer as a function of wavelength with Λ=250 nm and at
various incidence angles.
This result is inline with the previous publications [23].
Finally, Figure 9 shows the relative photocurrent response
(TM/TE) for gratings with 250 nm spacing and decreasing
depths. The highest increase in photocurrent was found
to be 2.72 at a wavelength of 618 nm for a grating depth
of 50 nm. It can also be seen that as the grating depth
decreases, so does the photocurrent for this particular grating
spacing.
Further research is currently being conducted in our
laboratory on the effects of cross-corrugated and paral-
lel super-imposed gratings with different spacing on the
SP-induced enhancements in the photocurrent response.
20000000 24000000 28000000 32000000
Λ=250 nm
ω(rad/s)
|kSP|(m−1)
6E+015
6.5E+015
7E+015
7.5E+015
8E+015
8.5E+015
9E+015
9.5E+015
Grating depth ∼50 nm
Grating depth ∼28 nm
Figure 8: The dispersion relation plot for gratings with Λ=250 nm
and various depths.
400 450 500 550 600 650 700 750
1
1.5
2
2.5
3
Λ=250 nm; θ=0◦, TM/TE
Relative photocurrent increase
Wavelength (nm)
Grating depth ∼11 nm
Grating depth ∼28 nm
Grating depth ∼50 nm
Figure 9: The relative photocurrent responses of P3HT-PCBM
layer as a function of wavelength at normal incidence for gratings
with Λ=250 nm and at various depths.
This will allow tailoring the photocurrent increase over a
larger wavelength range.
4. Conclusion
In this experiment, corrugated gratings were used to generate
surface plasmons within the range of 450 to 650 nm. These
angle-dependent SP resonances increased the local electro-
magnetic field at the boundary between the P3HT-PCBM
and aluminum layers, hence increasing the photocurrent
generated. A photonic band gap was also apparent in the
measurements and it seemed to depend on the grating depth.

International Journal of Photoenergy 5
Acknowledgment
The authors acknowledge the funding from the Natural
Sciences and Engineering Research Council of Canada
(NSERC).
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