Electrochemistry
The Electrochemical Society of Japan
ArticleReceived: October 27, 2016
Accepted: February 14, 2017
Published: May 5, 2017
http://dx.doi.org/10.5796/electrochemistry.85.272

Electrochemistry, 85(5), 272–275 (2017)

Dependence of the Device Performance of Polymer Solar Cells
on the Insertion of Metal Nanoparticle Layers
at the Electron-collecting Electrodes
Kazuhiro MARUMOTO,a,b,* Atsushi KOSUGA,a Dong LIU,aOsamu TAKEUCHI,c and Hidemi SHIGEKAWAcaDivision of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
Tsukuba Research Center for Interdisciplinary Materials Science (TIMS), University of Tsukuba,
Tsukuba, Ibaraki 305-8571, Japanc
Division of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japanb* Corresponding author: marumoto@ims.tsukuba.ac.jp
ABSTRACTWe report a study of the dependence of the device performance of polymer solar cells with the blend ﬁlms
of regioregular poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61-butyric acid methyl ester (PCBM) on theinsertion of a layer of metal (Pd, Ag, Cu or Au) nanoparticles (NPs) at the electron-collecting electrodes. The shortcircuit current density, the incident photo-to-current conversion eﬃciency, and the power conversion eﬃciency ofthe cells increased by the insertion of metal-NPs; the insertion of the Pd-NP layer has a large enhancement eﬀect forthe device performance. The light absorption was found to increase by forming the Pd-NP layer on the blend ﬁlm
without changing the light reﬂection. The formation of Au NPs on the blend ﬁlms was directly observed by scanningtunneling microscopy (STM). The possible mechanism for such improvement is discussed using the results of thesolar-cell parameters and the STM.
© The Electrochemical Society of Japan, All rights reserved.

Keywords : Polymer Solar Cells, Metal Nanoparticles

1. Introduction
Organic photovoltaic devices based on conjugated polymers area promising alternative source of electrical energy, and have been
investigated by many diﬀerent methods with the aim of improving
the performance of these devices.1–11 In particular, the devices
containing blend ﬁlms of regioregular poly(3-hexylthiophene)(P3HT) and (6,6)-phenyl C61-butyric acid methyl ester (PCBM)
have been widely studied, and have exhibited a high power
conversion eﬃciency (PCE), almost 7.0%.8 During the preparationof the P3HT:PCBM active layer, the mixed phase spontaneously
segregates into interpenetrated domains, creating bulk heterojunction (BHJ) devices.5 For organic solar cells, the enhancement of the
PCE by the incorporation of a nanotextured Au layer at the electroncollecting electrodes has been reported.12 The previous work hasreported that the PCE increases because of the improvement in the
short-circuit current density (Jsc).12 Such improvement has been
explained by localized surface plasmon resonance (LSPR) within
the Au nanoparticles (NPs).12 The excitation of the LSPR within
metal NPs is known to create strong near-ﬁeld electromagnetic
ﬁelds, which enhances the light absorption and the photocurrentof organic photovoltaic devices.13–19 However, the eﬀects of metal
NPs other than Au on the characteristics of organic solar cells
have not yet been fully investigated. The research is an interesting
issue because various metal NPs have the ability to enhance theperformance of organic solar cells.
Here we report the study of the dependence of the Jsc, the PCE,
and the incident photo-to-current conversion eﬃciency (IPCE) of
the P3HT:PCBM BHJ photovoltaic cells on the insertions of Pd-,
Ag- or Cu-NP layer in addition to the Au-NP layer at the electroncollecting electrodes. The absorption enhancement of the blend ﬁlms
without changing the light reﬂection has been demonstrated. Thisenhancement is thought to contribute to the enhancement of the Jsc
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and the IPCE of the P3HT:PCBM solar cells. The shapes of metal
NPs on the blend ﬁlms were observed with a scanning tunnelingmicroscopy (STM) technique, which showed metal NPs withvarious dimensions.
2. Experimental
Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:
PSS) ﬁlms of around 30 nm thickness were obtained by spin-coating
an aqueous solution (Baytron PVP AL 4083) on indium tin oxide(ITO)-coated glass substrates, followed by annealing at 135°C for
10 min under the ambient atmosphere to form the hole-collecting
layers. Solutions containing P3HT and PCBM dissolved in chlorobenzene (1.4 wt%) or dichlorobenzene (2.5 wt%) with a mass ratioof 1:0.8 were spin-coated at 1000 or 2000 rpm on the top of the
PEDOT:PSS ﬁlms to form the blend ﬁlms of around 80 nm thickness.
Then, metal NPs (Pd, Au, Ag or Cu), LiF, and Al layers were
thermally deposited onto the P3HT:PCBM blend ﬁlms to form theelectron-collecting electrodes. Finally, the devices were annealed at
135°C for 8 min or 25 min under an Ar gas atmosphere. The nominal
thickness of the metal-NP layers of 1.2 nm was adopted to follow
the reported ratio of the ﬁlm thickness of Au-NP to that of LiF.12
The fabricated device structure was ITO/PEDOT:PSS/P3HT:PCBM/
metal-NPs (1.2 nm)/LiF (0.4 nm)/Al. The active area was 0.04 cm2.
Current density–voltage (J-V ) curves were measured under simulated
solar irradiation (100 mW/cm2, AM 1.5 G).
3. Results and Discussion
The J-V characteristics of the P3HT:PCBM polymer solar cells
without and with the Pd-, Ag-, Cu- or Au-NP layer are shown in
Fig. 1, where an ITO layer is the hole-collecting electrode. An
increase in the Jsc was clearly observed for the polymer solar cells

Electrochemistry, 85(5), 272–275 (2017)

Figure 1. (Color online) J-V characteristics (main panel) and
IPCE characteristics (inset) of ITO/PEDOT:PSS/P3HT:PCBM/
metal-NPs/LiF/Al. (a) These devices were fabricated with chlorobenzene (1.4 wt%), 1000 rpm spin-coating, and an 8 min annealing
time. (b) These devices were fabricated with dichlorobenzene(2.5 wt%), 2000 rpm spin-coating, and a 25 min annealing time.
Table 1. Solar cell parameters of the devices without and with themetal-NP layer (Pd, Ag, Cu or Au). For the fabrication (a), thesedevices were fabricated with chlorobenzene (1.4 wt%), 1000 rpm
spin-coating, and an 8 min annealing time. For the fabrication (b),these devices were fabricated with chlorobenzene (2.5 wt%),
2000 rpm spin-coating, and a 25 min annealing time.
Metal
Jsc
Voc
NPs (mA/cm2) (mV)

FF

PCE
(%)

Improved
PCE

(a)
(a)
(a)
(a)
(a)

w/o
Cu
Ag
Au
Pd

4.14
4.47
5.40
5.67
5.80

640
680
630
650
670

0.57
0.57
0.59
0.59
0.59

1.51
1.73
1.99
2.16
2.30

«0%
0.22%
0.48%
0.65%
0.79%

(b)
(b)

w/o
Pd

6.44
7.05

670
660

0.58
0.60

2.49
2.78

«0%
0.29%

Fabrication

with the Pd-, Ag-, Cu- or Au-NP layer. Table 1 shows the solar cellparameters of the devices without and with the metal NP layer for
the Jsc, the open-circuit density (Voc), the ﬁll factor (FF), the PCE,
and the improved PCE. We conﬁrm that the device performance
improves by the insertion of the Au-NP layer, which is consistent
with that of the previous work.12 In the current study, in particular,
the Jsc largely increased for the polymer solar cells with the Pd-NP

Figure 2. (Color online) (a) Absorption spectra of quartz/
P3HT:PCBM/Au-NPs (1.2 nm) and quartz/P3HT:PCBM. The inset
shows the enhancement ratio of the absorption spectra of quartz/
P3HT:PCBM/Au-NPs (1.2 nm) to that of quartz/P3HT:PCBM.
(b) Absorption spectra of quartz/P3HT:PCBM/Pd-NPs (1.2 nm)
and quartz/P3HT:PCBM. The inset shows the reﬂection rate ofquartz/P3HT:PCBM/metal-NPs (Pd or Au 1.2 nm) and quartz/
P3HT:PCBM.
layer, which showed the PCE of 2.78%. The performance improvements using the metal NPs/LiF/Al structures under thermal
annealing conditions were observed for the polymer solar cellswith various metal-NP layers. The insets of Fig. 1(a) and 1(b) show
the IPCE characteristics of the polymer solar cells without and with
the Pd-, Ag-, Cu- or Au-NP layer. The IPCE characteristics of thecells with the metal-NP layer were improved compared with that of
the cells without the metal-NP layer.
To investigate the reason for the variation of the J-V and the IPCE
characteristics, we measured the light absorption of the BHJ blend
ﬁlms without and with the metal-NP layer. Since it is diﬃcult to
discuss the diﬀerence of the light absorption between the blend ﬁlms
due to the variation of the ﬁlm thickness, we used the same blend
ﬁlm to evaluate the diﬀerential absorption for studying the eﬀect ofthe metal NP layer. First, we measured the absorption of the P3HT:PCBM blend ﬁlm on quartz substrates. Then, we fabricated themetal-NP layers on the top of the same blend ﬁlm and measured thelight absorption. As a result, we were able to reduce the ambiguity of
the light absorption due to the diﬀerence in the ﬁlm thickness. As
shown in Fig. 2, the absorption of the blend ﬁlms was increased withthe metal-NP layer (Pd or Au). The result for the Au case is consistent
with that of the previous study.12 The inset of Fig. 2(a) shows theenhancement ratio of the absorption of the blend ﬁlms with the Pd- or
Au-NP layer. This enhancement ratio was obtained from the ratioof the absorption of the blend ﬁlms with the metal-NP layer to thatwithout the metal-NP layer. To examine the inﬂuence of the lightreﬂection on the absorption, we measured the reﬂection rate of theblend ﬁlms without and with the Pd- or Au-NP layer (see the inset
of Fig. 2(b)). This result conﬁrms almost no diﬀerence between thereﬂection rates of the blend ﬁlms without and with the metal-NP
layer. Thus, we demonstrate that the absorption enhancement is notcaused by the eﬀect of the reﬂection from the metal-NP layers.
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Electrochemistry, 85(5), 272–275 (2017)

Figure 3. (Color online) Dependence of the absorption of quartz/
Pd-NPs on the nominal thickness of the Pd-NP layer.
To study the absorption of the metal-NP layers themselves,
we fabricated the samples with the structure of quartz substrate/Pd(2–8 nm) and measured their absorption. As shown in Fig. 3, theabsorption increased as the nominal Pd thickness increased. Almost
no clear peak in the absorption spectra was observed. This result isinconsistent with that of the previous study using the Au-NP layer,
where the peak in the absorption spectra was observed and the
maximum peak position in the absorption spectra shifted toward
longer wavelengths with increasing the nominal Au thickness.12
It has been reported that both the shape and the size of metal NPs
are key factors to determine the intensity and the wavelength regionof LSPR.12,17,20 However, the surface morphology of metal NPs
on blend ﬁlms has not yet been fully investigated.12 Thus, it is aninteresting issue to observe the shape and the size of metal NPs; wemeasured them using a STM technique. The STM measurements
were performed under high vacuum conditions below 5 © 10¹8 Pa.
Figure 4(a) shows the height image of STM of the surface of theblend ﬁlm with the Au-NP layer on the ﬁlm under an applied voltage
V = 3 V at a current I = 100 pA. Since the aggregated clusters in theblend ﬁlm look like particles, we could not determine the sites ofmetal NPs using only the STM image. Thus, we performed scanningtunneling spectroscopy (STS) measurements. Figure 4(b) shows thecurrent image of STM of the same ﬁlm at V = ¹1.2 V, which wereobtained by measuring the STS characteristics (see Fig. 4(c)) for
each region. As shown in Fig. 4(c), the STS characteristics for theregion marked with the pink square in Figs. 4(a) and 4(b) showed
semiconducting characteristics. This behavior was only observed
using the blend ﬁlms. However, the STS characteristics for the region
marked with the blue square in Figs. 4(a) and 4(b) showed metallic
characteristics. This behavior could not be observed using only blend
ﬁlms. The regions with a red color in Fig. 4(b) show a high current at
V = ¹1.2 V. From the current image of STM, the existence of metal
NPs with a size of several to a few tens nanometers is observed.
Thus, this result conﬁrms that the surface of the blend ﬁlm is notcompletely covered with the Au metal. Further studies for the surface
morphology of other metal (Pd, Ag and Cu) NPs and for the depth
proﬁles of metal NPs penetrating into the blend ﬁlms are interesting
issues because the diﬀerence of the distribution states between metal
NPs may cause the diﬀerence of the solar-cell parameters mentioned
above, which will be reported in a separate paper.
We now turn to a discussion of the origin of the variation of thedevice performance using the metal-NP layers. As discussed in theprevious work,12 the LSPR eﬀect may possibly enhance the lightabsorption of the blend ﬁlms. The LSPR should have an eﬀect ata particular wavelength in the absorption and the IPCE spectra,
depending on the type of metal and the NP dimensions.17 However,
we did not observe such clear enhancement at a particular274

Figure 4. (Color online) (a) The height image of scanningtunneling microscopy (STM) and (b) the current image of STMof the sample surface of ITO/PEDOT:PSS/P3HT:PCBM/Au-NPs(1.2 nm). The regions with a red color show a high current.
(c) Scanning tunneling spectroscopy (STS) characteristics of theregions marked by pink and blue squares in Figs. 4(a) and 4(b).
wavelength for the cells with the Pd- or Au-NP layer in the present
study (see the insets of Fig. 1, the inset of Fig. 2(a), and Fig. 3).
This result means that the formation of the Au-NPs conﬁrmed by the
STM study do not cause the LSPR eﬀect. Thus, the origin of theenhancement is unlikely due to the LSPR eﬀects in the case with the
Pd-NP layer.
Another possible origin of the variation of the device performanceis due to the suppression of the formation of PCBM radical anions
at the interface of the electron-collecting electrode LiF/Al.21
The thermal annealing for polymer solar cells with a structure of
ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al after the deposition of Al
electrode (a post-annealing treatment) has an eﬀect to form the
PCBM¹Li+ charge-transfer complexes due to the electron transferfrom LiF/Al to PCBM through a reaction of 3PCBM + 3LiF +Al ¼ 3PCBM¹Li+ + AlF3.22,23 The formation has been directly
observed using electron spin resonance (ESR) spectroscopy.21 The
formation of the complexes causes the electron scattering at the
PCBM/LiF/Al interfaces, and decreases the Jsc.21 When the metalNP layer is inserted between the layers of P3HT:PCBM and LiF/Al,
the metal-NP layer may suppress the electron doping to PCBM
due to the electron transfer from Al to metal, not to PCBM. This
explanation is consistent with the observation of the highest device
performance using the Pd-NP layer because Pd has a large work
function (5.44 eV) and may attract electrons from Al.24 Also, thehighest device performance of the cells with the Pd-NP layer may be
explained by the large surface segregation of PCBM at the electroncollecting electrode due to the post-annealing treatment of the cells
because Pd has larger surface energy than that of other metals (Au,Ag, Cu) and electrode materials (Al, LiF);25–27 the surface energy ofmetals (Pd, Au, Ag, Cu) is larger than that of the electrode materials(Al, LiF).26,27 The suppression of the PCBM¹Li+ complexes and/or
the large surface energy of metals may enhance the Jsc due to thedecrease in the electron scattering and/or the large surface segregation of PCBM at the LiF/Al interfaces. The larger surface energy ofmetals (Pd, Au, Ag, Cu) than that of the electrode materials (Al, LiF)
causes the larger surface segregation of PCBM at the electroncollecting electrode, which may explain the slight increase in the FF
for the devices with the metal NPs due to the reduction of the charge-

Electrochemistry, 85(5), 272–275 (2017)carrier recombination. Further experiments using ESR spectroscopy
to clarify the mechanisms of the variation of the device performanceare currently in progress, and will be reported in a separate paper.
4. Conclusions
We studied the eﬀects of metal NPs (Pd, Au, Ag, Cu) on thedevice performance using the polymer solar cells of ITO/PEDOT:PSS/P3HT:PCBM/metal-NPs/LiF/Al, and found that themetal NPs vary the device performance. The polymer solar cellswith the Pd-NP layer performed better than those with other metalNP layers. The origin of the enhancement of the device performance
was discussed considering the eﬀects of LSPR, the formation of the
PCBM¹Li+ charge-transfer complexes, and the surface segregationof PCBM due to the diﬀerent surface energy of metals (Pd, Au,
Ag, Cu). The detailed mechanisms of the variation of the device
performance are interesting issues, which would be clariﬁed byfurther experiments using ESR spectroscopy from a microscopic
viewpoints.
Acknowledgments
The authors would like to thank Prof. K. Okamoto for valuable
discussions and comments. This work was supported by JSPS
KAKENHI Grant Numbers JP24560004 and JP15K13329, by JST,
PRESTO, and by SEI Group CSR Foundation.
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