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2016 J. Phys. D: Appl. Phys. 49 185106(http://iopscience.iop.org/0022-3727/49/18/185106)
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Journal of Physics D: Applied Physics
J. Phys. D: Appl. Phys. 49 (2016) 185106 (6pp)

doi:10.1088/0022-3727/49/18/185106

Indium-free organic thin-film solar cells
using a plasmonic electrode
Kentaro Takatori1,2,3, Takayuki Nishino1,4, Takayuki Okamoto1,2,5,
Hiroyuki Takei6, Koji Ishibashi1,5 and Ruggero Micheletto3
1

Advanced Device Laboratory, RIKEN, Hirosawa, Wako, Saitama 351-0198, Japan
Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology,
Yokohama 226-8502, Japan
3
Graduate School of Nanobioscience, Yokohama City University, Seto, Kanazawa-ku, Yokohama,
Kanagawa 236-0027, Japan
4
Sanwa Corporation, Izuka, Kumagaya, Saitama 360-0231, Japan
5
RIKEN Center for Emergent Matter Science, Hirosawa, Wako, Saitama 351-0198, Japan
6
Faculty of Life Science, Toyo University, Izumino, Itakura, Gunma 374-0913, Japan2

E-mail: okamoto@riken.jp
Received 20 November 2015, revised 26 February 2016
Accepted for publication 1 March 2016
Published 7 April 2016
Abstract

We propose a new kind of organic solar cell (OSC) that substitutes the standard indium
tin oxide (ITO) electrode with a silver layer with randomly arranged circular nanoholes(plasmonic electrode). The quasi-random structure in the silver layer efficiently converts
wideband incident light into surface plasmon polaritons propagating along the surface of thesilver film. In this way, the converted surface plasmon polaritons enhance light absorption in
the active layer. We describe in detail the fabrication process we used and we give a thoroughreport of the resulting optical characteristics and performances. Although the transmittance ofthe plasmonic electrode is approximately one-third of that of the ITO electrodes, the power
conversion efficiency of the OSCs with our plasmonic electrode is comparable to that ofconventional inverted solar cells using ITO electrodes. Moreover, the obtained incident photon
to current efficiency was better than that of the inverted solar cells in the wavelength regions
around 400 nm and over 620 nm.
Keywords: surface plasmon, photovoltaic, organic materials(Some figures may appear in colour only in the online journal)

1. Introduction

such as zinc oxide and fluorine tin oxide, are being studied
as the transparent electrode, their optical and electrical properties are still inferior to those of ITO [5, 8]. Carbon nanotube (CNT) and graphene are also studied as candidates forthe transparent electrodes [5] in OSCs. While the mobilityof CNT is high [9], the transmittance is low due to CNTs’high absorption and reflection in almost the entire visible andinfrared regions [5]. Reducing the film thickness increases thetransparency, but unfortunately decreases the electrical conductance and its homogeneity. Recently, graphene has drawn
much attention as a next-generation transparent electrode due
to its high mobility [10, 11] and transparency. The predicted
optical and electrical properties of graphene film [12, 13] intheory are comparable to those of ITO, but experimentally theOrganic thin-film solar cells (OSCs) are promising as renewable clean energy sources because of their potential for lowcost and high throughput processing [1–3]. In OSCs, indium
tin oxide (ITO) is usually used as the material for the transparent electrodes [4]. Indium tin oxide exhibits high transparency in the visible range and high electrical conductivity.
However, ITO has some drawbacks. It is expensive, sinceindium is a rare-metal and a strategic material [5]. Anotherproblem is its low flexibility. Since ITO is a ceramic, iteasily cracks and fractures with relatively low stress [6].
Microcracks drastically decrease the electrical conductanceof ITO films [7]. Although other conductive metal oxides,
0022-3727/16/185106+6$33.00

1© 2016 IOP Publishing Ltd

Printed in the UKK Takatori et al

J. Phys. D: Appl. Phys. 49 (2016) 185106

Figure 1. (a) AFM image of a 30 nm thick plasmonic electrode. Cross-sectional height distribution along the line shown in the image isalso shown at the bottom. (b) Spatial frequency spectrum of (a).

obtained sheet resistance is higher than that of ITO [5, 14].
This suggests that it is still difficult to use graphene as a goodtransparent electrode for solar cells. Metal films are also candidates for a transparent electrode, but even metal films a few
nanometers thick exhibit insufficient transparency [15, 16].
In addition, as the thickness of the metal film decreases,
electron scattering from the surface and grain boundary dramatically decrease the conductance [17, 18].
An alternative idea to improve the transmittance of metal
films is the perforation of nanoslits or nanoholes on them.
Organic solar cells employing a 20–40 nm thick metal filmwith a nanostructure such as the transparent electrode has
been proposed [19–24]. These structures carved on metal
are called plasmonic structures, because surface plasmon
polaritons (SPPs) are supported on such a metal surface.
Obviously these apertures or slits enhance the transparencyof metal films, while degrading their electric conductance[25, 26]. Since the transparency of patterned metal films is lower
than that of ITO films, SPPs have to play an important rolein OSCs. The plasmonic structure converts incident light into
SPPs that propagate along the metal-dielectric interface. The
converted surface plasmon polaritons are efficiently absorbedby the active layer of OSCs due to its long lifetime, resulting in
the enhancement of the conversion efficiency [27–29].
If the goal is to replace the ITO layer, a structure that uses
localised surface plasmons is not suitable, because localised
surface plasmons are generated by metallic nanoparticles thatexhibit no macroscopic conductivity unless they are aggregated. Thus, in this study we use instead a continuous metallic
surface that allows the propagation of surface plasmons. Manykinds of OSCs boosted with propagating surface plasmons
have been proposed so far [19–23]. In order to couple incident
light to surface plasmons propagating along a metallic surface,
one-dimensionally (1D) or two-dimensionally (2D) periodic
perforation [19–22] is often employed. However, since thesestructures have only one or two reciprocal lattice vectors, only a

few wavelength components of the incident light are converted
to SPPs when the angle of incidence is fixed. Additionally, 1D
periodic structures exhibit a strong polarization dependency,
resulting in low conversion efficiency. In order to eliminate
dependence on the incident angle and the polarisation for
the efficient excitation of SPPs, random structures are moresuitable. Reilly et al proposed silver thin-films with randomly
perforated circular nanoholes [23, 24]. Their proposed random
structure has no polarization dependence in the excitation of
SPPs. However, since all nanoholes are identical in shape andthey are isolated from each other, the spectral bandwidth oftheir structure is not so wide [24]. For broadening the spectral
response, Stec and Hatton [30] proposed thin metal films with
nanoholes with a random shape, for which structure they control the adhesion of the metal film to substrates. In this paper,
we propose an improved nanostructure, where a thin silver
film with circular nanoholes are arranged in non-constrained
random positions. In this way the resulting structure has holesthat have mixed shapes, single isolated circles and different
random shapes made by one or more circles merged with eachother. The obtained plasmonic electrode provides a wider
spectral response than those previously proposed electrodes[24], presents no polarisation dependence and is also suitablefor flexible devices. Hereafter we describe in detail the opticaland electrical properties of these novel plasmonic electrodes;
we also constructed conventional standard OSCs in which the
ITO electrode is replaced with this original plasmonic one andfully evaluate their characteristics.
2. Plasmonic electrodes and their optical andelectrical properties
Figure 1(a) shows an atomic force micrograph (AFM) of theplasmonic electrode we fabricated. The diameter of the holes
is 100 nm with a coverage of  ∼30%. Figure 1(b) shows thespatial frequency spectrum of the plasmonic electrode shown
2

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J. Phys. D: Appl. Phys. 49 (2016) 185106

3. Fabrication of plasmonic solar cells

Table 1. Characteristics of the plasmonic solar cells and theinverted ones.

Voc (mV)Jsc (mA cm−2) FF (%)

We fabricated inverted OSCs in which the ITO electrodeis replaced by the plasmonic electrode described above.
Hereafter, we call these solar cells plasmonic solar cells.
Figure 3 shows the layer structure of the plasmonic solar cellcompared with a conventional inverted solar cell with an ITO
electrode. Both solar cells consist of a glass substrate that holds
the following layer structure: a plasmonic electrode or an ITO
electrode / TiO2 / poly(3-hexylthiophene): [6, 6]-phenyl C61
butyric acid methyl ester (P3HT:PCBM) / MoO3 / silver. The
plasmonic electrode was made 30 nm thick after consideringa trade-off between the optical and electric properties. The
thickness of the TiO2 layer in the inverted solar cells was 7 nm,
which was optimal for power conversion efficiency (PCE) in
the conventional OSCs. On the other hand, the obtained shortcircuit current of the plasmonic solar cells with a 7 nm thick
TiO2 layer was very low. This may have been caused by thesurface roughness of the plasmonic electrode. A 7 nm thick
TiO2 layer may be too thin to cover the plasmonic electrodecompletely. We employed a 26 nm thick TiO2 layer, which
exhibited the best performance in the plasmonic solar cells.
The thicker TiO2 layer does not affect the transmission ofincident light because of the high transparency of TiO2 in thevisible range. The thicknesses of the other layers are identicalin both solar cells.
The fabrication procedure is as follows: first, TiO2 was
sputtered on the ITO electrode or on the plasmonic electrode.
Then we prepared the P3HT:PCBM solution by dissolving
15 mg P3HT and 15 mg PCBM into 1 mL 1,2-dichlorobenzene. The mixture was stirred at 100 °C for 24 h in a nitrogen
atmosphere. Then, it was spin-coated onto the TiO2 layer.
The film thickness of the P3HT:PCBM layer was  ∼150 nm.
The devices were not intentionally but naturally annealed.
Finally, a 10 nm thick MoO3 layer and a 100 nm thick silver
back electrode were deposited onto the devices with thermal
evaporation.

PCE (%)

Plasmonic
554 ± 4.88 7.26 ± 0.896 55.6 ± 3.25 2.23 ± 0.203
solar cell
Inverted-type 541 ± 20.8 9.20 ± 0.681 48.3 ± 2.41 2.41 ± 0.325
solar cellin figure 1(a). The spectrum is isotropic and has a wideband,
which contributes to converting wideband sunlight into SPPs
propagating along the surface of the plasmonic electrode. The
wide bandwidth of our proposed structure comes from thearrangement of the nanoholes. Unlike the structures proposed
by Reilly et al [23, 24], in our structure the nanoholes form
chain-like clusters and the shapes are no longer circular.
A colloidal lithography methodology was used to fabricate
the plasmonic electrodes. A 1 mm thick glass substrate was
immersed in an aqueous solution of 3-aminopropyltrimethoxysilane (1 wt%) for 1 min to promote the subsequent bindingof silica nanospheres onto the glass surface. After washingwith deionised water, the substrate was dried in an oven at
50 °C, and then immersed in a 2.5% aqueous suspension ofsilica nanospheres 100 nm in diameter (polysciences) for
1 min. After rinsing with water to remove the excess nanospheres, the substrate was dried in the oven at 50 °C. Thena silver film of 10, 20, 30 or 40 nm was evaporated onto thesubstrate. Finally, we removed the particles with scotch tape
to complete the fabrication process. The AFM image showsthat all of the particles are completely removed by the scotch
tape and no tape glue remains. The use of scotch tape forremoving particles provides good reproducibility. This fact
can be confirmed from table 1, as will be shown later. The
obtained standard deviations are comparable to those for thesolar cells with an ITO electrode.
Figure 2(a) shows the thickness dependence of the sheet
resistance of our plasmonic electrodes compared with planesilver thin films. For further comparison, the sheet resistanceof 100 nm thick ITO films is also shown. Although the sheet
resistance of the plasmonic electrodes was higher than thatof the plane silver films with the same thickness, the sheet
resist­ance of the 30 nm thick plasmonic electrodes resulted
in being comparable to that of the ITO films. The measured
transmittance, reflectance and absorptance spectra of a 30 nm
thick plasmonic electrode on a glass substrate and a 30 nm
thick plane silver film on the same glass substrate are shown infigures 2(b) and (c), respectively. The transmittance of the planesilver film at a 500 nm wavelength was only 23%, whereas thatof the plasmonic electrode was 35%. In addition to the enhancement of the transmittance, the absorptance was also enhanced
by introducing the above described nanoholes. In this way, theabsorptance of the plasmonic electrode increased from 19% to
51% at a 500 nm wavelength. The absorption enhancement iscaused by the excitation of long-lived SPPs by the silver film,
since there are no other absorption media. In actual solar cells,
however, the SPPs are absorbed mainly by the active materials,
because these have a high absorption coefficient and most of
the SPP field lies within the dielectric media [29, 31].

4. Results and discussion
Figure 4(a) shows the measured I–V characteristics of theplasmonic solar cells and the inverted solar cells under simulated AM 1.5 solar illumination (100 mW cm−2). Table 1
summarises the I–V characteristics of both cells. The obtained
PCE and short-circuit current density, Jsc, of the plasmonic
solar cells were lower than those of the inverted ones, whereas
the fill factor (FF) of the plasmonic solar cells was higher
than that of the inverted ones. The open-circuit voltage, Voc,of the plasmonic solar cells was almost the same as that of theinverted ones. Although the transmittance of the plasmonic
electrode is only one third of that of the ITO, the obtained
PCE of the plasmonic solar cells is comparable to that of theinverted ones. This result indicates that the SPPs excited in
the plasmonic electrodes give an important contribution to thecurrent generation. The higher FF has a role in the improvement of the PCE of our cells. The higher FF may have beencaused by the thicker TiO2 layer, which may increase the3

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J. Phys. D: Appl. Phys. 49 (2016) 185106

Figure 2. (a) The sheet resistance of the plasmonic electrodes and plane silver films against the film thickness. Transmission, reflection andabsorption spectra of (b) a 30 nm thick plasmonic electrode and (c) a 30 nm thick plane silver film. The transmission spectrum of a 100 nm
thick ITO electrode on a glass substrate is also shown in (b) for comparison.

Figure 3. (a) Layer structure of plasmonic solar cells and (b) that of inverted solar cells.

shunt resistance. In the inverted solar cells, however, thicker
TiO2 layers only degraded their performance.
Figure 4(b) shows the measured incident photon to cur­rent efficiency (IPCE) spectra. The IPCE of the plasmonic
solar cells, η P, was higher than that of the inverted ones,η I, around 400 nm and over 620 nm wavelength. The IPCEs
were calculated for Jsc of the OSCs. Since Jsc of the plasmonic solar cells was lower than that of the inverted ones, η P
was also lower than η I over most of the wavelength region.
Figure 4(c) shows the absorption spectra of both solar cells.
The absorption spectrum of the inverted solar cell is dominated by the absorption of P3HT:PCBM. The absorption of

the plasmonic solar cells is higher than that of the inverted
ones in the regions around 400 nm and over 610 nm, where
the absorption by P3HT:PCBM is intrinsically weak but
enhanced by the SPPs. In the other regions, the IPCE of theplasmonic solar cells was lower than that of the inverted
ones. Since the lifetime of the SPP depends on the imaginary part of the permittivity of not only metals but alsodi­electric materials, the lifetime of the SPP in the regions
where P3HT:PCBM exhibits high absorption is quite short.
Thus, the SPPs contribute less to the absorption enhancement and the high surface reflection reduces the absorption
in these regions.
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J. Phys. D: Appl. Phys. 49 (2016) 185106

Figure 4. (a) Typical I–V curves of the plasmonic solar cell and the inverted solar cell under 1-SUN irradiation. (b) Typical incident photon
to current efficiency (IPCE) spectra and (c) absorption spectra of the plasmonic solar cell and the inverted solar cell: η P, plasmonic solarcell, η I, inverted one, η T, plasmonic solar cell only with the contribution of transmitted light and η P − η T, plasmonic solar cell only with thecontribution of SPPs.

Hereafter, we estimate the contribution of the SPPs to thecurrent generation in OSCs. Without SPP effects, only thelight that passes through the plasmonic electrode contributes
to the current generation. The contribution of the transmitted
light on the IPCE of the plasmonic solar cells is estimated tobe η T = η I(TPL /TITO ) (see figure 4(b)), where TITO and TPL are
the transmittances of the ITO film and the plasmonic electrode, respectively. Since the transmittance of the plasmonic
electrode is much lower than that of ITO, η T is also much
lower than η I. The rest, i.e. η P − η T corresponds to the contrib­
ution of the SPPs (see figure 4(b)), which is comparable tothat of the transmitted light and very wideband, as shown infigure 4(b). However, in the wavelength region shorter than
400 nm it is quite low because no SPPs can be supported bysilver here.

in diameter. The perforation is done without any constraints,
so the holes are both circular or of random shapes madeby the connections of the holes. Such plasmonic electrodes
can replace ITO ones in OSCs, leading to the advantages offlexibility and non-use of indium. Although the transmittanceof the plasmonic electrode is 35%, much lower than that of
ITO, the obtained PCE with plasmonic solar cells was comparable to that of the inverted solar cells. This is caused mainlyby the contribution of SPPs and partly by the improvement of
FF. The obtained IPCE of the plasmonic solar cells was higher
than that of the standard ones around 400 nm and over 620 nm
wavelength. This region corresponds to where the absorptionof the active layer is weak. In this study, the diameter andthe density of the holes in the plasmonic electrodes have notbeen optimised yet. The diameter and the density of the holes
can be controlled by the diameter of the silica particles andtheir concentration in suspension, respectively. We expect that
optimising these parameters will further improve the performance of the plasmonic solar cells, paving the way for nextgeneration flexible devices.

5. Summary
We proposed a novel plasmonic electrode composed by asilver thin film with randomly perforated nanoholes 100 nm
5K Takatori et al

J. Phys. D: Appl. Phys. 49 (2016) 185106

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A part of this work was supported by the RIKEN JuniorResearch Associate Program.
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