The dual localized surface plasmonic effects of gold nanodots
and gold nanoparticles enhance the performance of bulk
heterojunction polymer solar cells
Chih-Ming Liu, Chia-Min Chen, Yu-Wei Su, Shu-Min Wang, Kung-Hwa Wei
⇑Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30049, Taiwan
a r t i c l e
i n f o
Article history: Received 6 March 2013
Received in revised form 17 June 2013 Accepted 18 June 2013
Available online 1 July 2013 Keywords:
Polymer solar cells
Localized surface plasmon resonance Gold nanoparticles
Gold nanodots
a b s t r a c t
In this study, we investigated the effects of plasmonic resonances induced by gold nanodots (Au NDs), thermally deposited on the active layer, and octahedral gold nanopar-ticles (Au NPs), incorporated within the hole transport layer, on the performance of bulk heterojunction polymer solar cells (PSCs) based on poly(3-hexyl thiophene) (P3HT) and [6,6]-phenyl-C61butyric acid methyl ester (PC61BM). Thermal deposition of 5.3-nm Au
NDs between the active layer and the cathode in a P3HT:PC61BM device resulted in the
power conversion efficiency (PCE) of 4.6%—that is 15% greater than that (4.0%) for the P3HT:PC61BM device without Au NDs. The Au NDs provided near-field enhancement
through excitation of the localized surface plasmon resonance (LSPR), thereby enhancing the degree of light absorption.
In addition to the thermally deposited Au-NDs, embedding Au NPs within the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) to form a dual metallic nanostructure can further enhance PCE to 4.8%—that is about 20% greater than that of the conventional P3HT:PC61BM cell. Thus, Au NPs and Au NDs appear to have great
poten-tial for the application in high-efficiency LSPR-enhanced PSCs.
Ó 2013 Published by Elsevier B.V.
1. Introduction
The development of conjugated polymers for use in or-ganic optoelectronic devices has been an area of extensive investigation because of their potential applications as cheap, large-area, flexible devices[1–3], bulk heterojunc-tion (BHJ) polymer solar cells (PSCs)[4], incorporating con-jugated polymers and fullerene derivatives as electron donors and acceptors, respectively, have been the most widely investigated PSC systems because of their efficient exciton dissociation, tunable energy bands and solubility of the materials involved and simple processing. The power conversion efficiencies (PCEs) of PSCs featuring P3HT and PC61BM [5] as the photoactive layer have
reached approximately 4% [6–11]. The signature optical
property of noble metal nanoparticles (NPs) is their local-ized surface plasmon resonance (LSPR)[12–18], a phenom-enon in which metal NPs excited by electromagnetic radiation exhibit collective oscillations of their conduction electrons. LSPR-enhanced absorption arises from the reso-nant electromagnetic behavior of the metal NPs—that is, it arises from the confinement of the conduction electrons within the small particle volume. For particles having a diameter (d) much smaller than the wavelength of the radiation (k), the electrons within the particle all move in phase upon plane-wave excitation, leading to the buildup of polarization charges on the particle surface. Thus, the resonantly enhanced field arises within the whole volume of a small particle, and produces a dipolar field outside the particle. This phenomenon leads to enhanced light absorp-tion and scattering cross secabsorp-tions for electromagnetic waves, as well as a strongly enhanced near field in the vicinity of the particle surface.
1566-1199/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.orgel.2013.06.012
⇑ Corresponding author. Tel.: +886 3 5712121x31871; fax: +886 3 5724727.
E-mail address:khwei@mail.nctu.edu.tw(K.-H. Wei).
Contents lists available atSciVerse ScienceDirect
Organic Electronics
The wavelength corresponding to the maximum extinc-tion, kmax, of the LSPR is highly dependent on the size,
shape, and dielectric properties of the metal NPs [19]. The primary consequences of LSPR are the appearance of intense surface plasmon absorption bands and an enhance-ment of the local electromagnetic fields. The frequency and intensity of the surface plasmon absorption bands are characteristic of the types of materials (typically Au, Ag, Cu or Pt), and highly sensitive to the size, size distribution, and shape of the nanostructures, as well as the surround-ing environments[17,20–23]. Recently, the improvements in PCEs have been demonstrated in BHJ PSCs and small-molecule solar cells, ascribed mainly to the light concen-trating effect caused by plasmonic scattering or near-field enhancement[24–28]. Surface plasmon resonance of me-tal NPs can also modify the intrinsic properties of nearby fluorophores[29], and cause the quench emissions of fluo-rophores in the vicinity of metal NPs[30].
To obtain high PCEs in BHJ PSCs, it would be desirable to increase the absorption of the active layer without increas-ing its thickness, because a thicker layer usually leads to a higher probability of carrier recombination. Metal nano-structures exhibit LSPRs that couple strongly to the inci-dent light have been used previously to enhance the performance of BHJ PSCs—for example, by introducing metallic NPs in the carrier transport layer[24–27,31–34], into the active layer of bulk junctions [28,35–39], or on top of the active layer[40,41], in most cases when used in combination with P3HT.
Several studies reported using embedded metal NPs in optical dielectric materials [42–44]. A common method for forming such monolayer films involves the organic syn-thesized colloidal particles with ligands[45–47]. Neverthe-less, such films often exhibit inconsistent properties due to the aggregation of particles. Light scattering from a small metal NP embedded in a homogeneous medium is nearly symmetric in the forward and reverse directions.
In addition, this colloidal solution process has difficulty on controlling the density and stability of the films. To solve these problems, we deposited gold nanodots (Au NDs) directly onto the active layer using thermal evapora-tion. Compared to the colloidal solution process, thermal evaporation process has high throughput to produce high density Au NDs[41].
The shapes of the Au NDs depend mainly on the surface energy of the material upon which they are to be depos-ited; Au NDs are formed in their lowest energy state when the Au surface and the surface tension are balanced in the structure. Au NDs are in the shape of hemi-spherical or spherical. It has been noted that Au NDs having the mini-mum surface energy will feature oblate or prolate struc-tures, with greater deviation from the spherical [48–50]. In this study, two types of metallic nanostructures—Au NPs embedded in the PEDOT:PSS layer and Au NDs ther-mally deposited on the active layer—were added in the conventional BHJ (P3HT:PC61BM) solar cells. We expected
the LSPR effect from dual Au structure (glass/ITO/ PEDOT:PSS:AuNPs/P3HT:PC61PM/Au NDs/Ca/Al) would
im-prove the PCE. According to optical and electrical analyses, the LSPR enhanced the light absorption efficiency through two separate pathways: the optical path length was
increased through scattering effects and a strong near field was induced to enhance the absorption of the active layer. Our experimental results suggest an approach toward opti-mizing plasmonic PSCs structures through the use of sim-ple processing methods.
2. Experimental
2.1. PSCs device fabrication
The PSCs devices were fabricated on an ITO-coated glass substrate. After a routine cleaning process, the sub-strate was dried and treated with UV ozone. The Au NPs solution was prepared using the previously described pro-cedures[51]. To prepare the anodic buffer layer, 2 mg Au NPs was blended into 1 mL 1.5 wt% PEDOT:PSS (Baytron 4071) solution. The blended solution was spin-coated onto an ITO-coated glass substrate, and then thermally annealed at 150 °C for 15 min, giving a 12 wt% Au NPs in the composite film of PEDOT:PSS and Au NPs. Extra PEDOT:PSS was spin-coated as a thinner capping layer to decrease the surface roughness of the anodic buffer layer. The thickness of the capping PEDOT:PSS layer is about 10 nm, which is determined using
a
-step measurement. Equal weight of P3HT (Rieke Metals) and PC61BM (SolenneBV) were mixed, and dissolved in the dichlorobenzene (DCB) (Sigma–Aldrich) in a concentration of 40 mg/mL. The P3HT:PC61BM solution was agitated at 80 °C until all
solutes dissolved completely. The mixture solution was spin-coated onto the anodic buffer layer to provide an ac-tive layer (ca. 150 nm). Au NDs were deposited onto the active layer through thermal evaporation at a pressure of 5 107torr and a deposition speed of 0.1 Å/s. The
re-sulted Au NDs average sizes of 2.7, 3.1, 4.5, and 5.3 nm were obtained by various deposition times. Top contacts of Ca (15 nm) and Al (100 nm) were sequentially ther-mally evaporated onto the Au NDs.
2.2. Characterisation
The current density (J)–voltage (V) characteristics of the finished devices were evaluated by using a Keithley Model 2400 source meter under illumination intensity of 100 mW/cm2from a solar simulator (Newport 66902) with
AM 1.5G filter. EQEs were measured by using a spectral re-sponse measurement set-up (Optosolar SR150). The active layer of a finished device was detached in DI water and transferred to a Cu foil grid for top-view TEM imaging operated at 120 keV (FEI Tecnai G2). Focused ion beam was used for cross-sectional TEM imaging. Steady state PL spectra were recorded under ambient condition in air using an F-7000 fluorescence spectrophotometer. Samples for the PL study were prepared in the same manner as the active layers used in the devices without PC61BM. The only
difference is the film was made on Silicon substrate instead of ITO-coated glass. Time-resolved PL spectra of the P3HT:PC61BM films were measured using a home-built
single photon counting system (Horiba Jobin Yvon). A GaN diode laser (k = 470 nm) with the pulse duration of 50 ps was used as the excitation source. The signals
collected at 650 nm were dispersed with a grating spec-trometer, detected by a high-speed photomultiplier tube, and then correlated using a single photon counting card. 3. Results and discussion
3.1. The performance of bulk heterojunction photovoltaic devices with Au NDs on the top of the active layer
Fig. 1displays a schematic diagram of a dual metallic plasmonic nanostructured PSC device with the structure of glass/ITO/PEDOT:PSS:50-nm Au NPs/P3HT:PC61BM
(150 nm)/Au NDs/Ca (15 nm)/Al (100 nm). The number in bracket represents the film thickness of each layer. The size of the thermally deposited Au NDs on P3HT:PC61BM active
layer could be tuned by adjusting the depositing parame-ters. We first studied on how the size of Au NDs affects the device performance. Then, Au NPs was incorporated into the device which has the optimized Au NDs for the highest PCE.
Fig. 2a displays the current density–voltage (J–V) char-acteristics of P3HT:PC61BM devices incorporating various
sizes of Au NDs. The detailed photovoltaic parameters are listed inTable 1. Compared to the reference device (with-out Au-NDs), the PCE was increased appreciably upon increasing the average size of Au NDs greater than 4.5 nm. The PCE was increased to 4.3% in the case of the 4.5-nm Au NDs from 4.0% of the reference device. Keep increasing the Au NDs to 5.3 nm, the PCE was increased further to 4.6% —an increase of 15% than the reference cell. The short-circuit current density (Jsc) was enhanced from
9.2 mA/cm2(reference) to 10.6 mA/cm2(5.3-nm Au NDs).
The open-circuit voltages (Voc) and fill factors (FFs) of these
devices remained similar. This 15% increase in Jscwas
pre-sumably caused by the greater degree of light absorption in the P3HT:PC61BM/Au NDs active layer than that in the pure
P3HT:PC61BM active layer, due to the plasmonic effect of
the deposited Au NDs.
Fig. 2b displays the external quantum efficiencies (EQEs) of these devices. The absorption range from 360 to 620 nm shows the larger Au NDs can result higher EQE. The inte-grated values of Jscdetermined from our EQE spectra for
the devices incorporating the 2.7-, 3.1-, 4.5-, and 5.3-nm Au NDs were 9.6, 9.6, 10.0 and 10.6 mA/cm2, respectively.
The differences between the integrated values of Jscfrom
the EQE curves and the measured values of Jscwere within
3%, indicating the high accurate measurements.
Fig. 2c presents the corresponding UV–Vis absorption curves for the Au NDs having average dimensions ranging
Fig. 1. Schematic representation of the dual plasmonic structured device. From bottom to top: ITO/PEDOT:PSS:50-nm Au NPs/P3HT:PC61BM (150 nm)/5.3-nm Au NDs/Ca (15 nm)/Al (100 nm).
Fig. 2. (a) J–V and (b) EQE curves of devices incorporating P3HT:PC61BM (ca. 150 nm) and thermally evaporated Au NDs. (c) Absorption spectra of P3HT:PC61BM films presenting Au NDs deposited through thermal evaporation.
from 2.7 to 5.3 nm on the P3HT:PC61BM films. The
absorp-tion peak at 334 nm and 512 nm was contributed to the PC61BM and P3HT, respectively. The deposited 2.7- and
3.1-nm Au NDs slightly enhanced the absorptions of the P3HT:PC61BM films from 300 to 512 nm, while the
depos-ited 4.5- and 5.3-nm Au NDs increased the absorptions of the P3HT:PC61BM films from 300 to 800 nm, with the
absorption of the P3HT:PC61BM film incorporating the
5.3-nm Au NDs being greater than that of the film incorpo-rating the 4.5-nm Au NDs. This trend is consistent with that for the light absorption of the Au NDs; the UV–Vis spectra of the Au NDs deposited through thermal evapora-tion onto the PEDOT:PSS/ITO substrate (Fig. 3) reveal that the absorption peaks for the plasmonic resonance become more pronounced and were slightly red-shifted upon increasing the average size of the Au NDs. Specifically, the LSPR peak shifted from being barely distinguishable at 552 nm for the 4.5-nm Au NDs to 576 nm for the 5.3-nm Au NDs. Because of the low absorption intensities of the Au NDs, we believe that the enhanced light absorption of the P3HT:PC61BM films resulted from the optical
near-field, which was generated by surface plasmons and was associated with the localized plasmons of the nanosized particles.
3.2. TEM results
Fig. 4a presents a top-view TEM image of the pure P3HT:PC61BM film, revealing its rather homogeneous
structure.Fig. 4b–e display the corresponding images of
the 2.7-, 3.1-, 4.5-, and 5.3-nm Au NDs, respectively, that had been deposited directly onto the active layer; here, we varied the average size of the Au NDs by tuning the deposition conditions. The inset figures are the size distri-bution charts of Au NDs.Fig. 4e reveals that the mean dis-tance between two neighboring 5.3-nm Au NDs was approximately 9.8 nm—much smaller that the wavelength of the incident light, suggesting that their interactions in the vicinity of the particle surface occurred through a near-field–dominated process [22]. Fig. 4f presents a cross-sectional TEM image of the 5.3-nm Au NDs on the P3HT:PC61BM active layer surface and embedded in the
Ca electrode. The reason that Au NDs can form a dense layer instead of protruding into P3HT:PC61BM active layer
surface was controlling the deposition condition in high vacuum (5107torr) and low deposition rate (0.1 Å/s).
This smooth active layer surface was an important aspect of the efficient operation of the corresponding device. 3.3. Polymer solar cell performance of dual metallic nanostructure (Au NDs plus Au NPs)
Fig. 5a displays the photovoltaic performances of the devices incorporating both the 50-nm octahedral Au NPs in the PEDOT:PSS layer and the 5.3-nm Au NDs on the ac-tive layer.Table 1lists the detailed photovoltaic parame-ters. We have compared the phovoltaic performances of the devices incorporating either 30- or 50-nm octahedral Au NPs for being blended in the PEDOT:PSS layer for LSPR effect (see Fig. S3 and Table S1), and decided to adopt 50-nm octahedral Au NPs for this study. Fig. S2 shows the UV–Vis spectra of pristine P3HT:PC61BM with 30- or
50-nm octahedral Au nanoparticles incorporating in the PEDOT:PSS layer, and the 50-nm Au NPs appears to have better absorption than that of 30-nm Au NPs case.
The PCE of the dual metallic nanostructured P3HT:PC
61-BM device incorporating both the 5.3-nm Au NDs and the 50-nm Au NPs increased to 4.8% from a value of 4.0% for the pristine P3HT:PC61BM device. We suspect that the dual
plasmon effects enhanced the absorption of incident light in the active layer and increased the effective optical path length; that is, when light entered the PEDOT:PSS:Au NPs layer, the light scattering and LSPR effects of the Au NPs presumably increased the effective optical path length and enhanced the optical absorption of the P3HT:PC61BM
film. The Au NDs also had the effect of trapping the inci-dent light in the active layer[25–28].Fig. 5b and c respec-tively display the EQE curve and UV–Vis spectra of the device with dual metallic nanostructure. The intensities
Table 1
Performance of PSCs featuring PEDOT:PSS layers prepared with or without thermally evaporated Au NDs or incorporated 50-nm Au NPs.
Device Voc(V) Jsc(mA/cm2) FF (%) PCE (%)
Reference 0.63 ± 0.02 9.21 ± 0.08 69.3 ± 0.2 4.02 ± 0.17 2.7-nm Au NDs 0.62 ± 0.01 9.60 ± 0.13 68.2 ± 0.3 4.06 ± 0.13 3.1-nm Au NDs 0.62 ± 0.01 9.62 ± 0.18 68.2 ± 0.2 4.07 ± 0.15 4.5-nm Au NDs 0.61 ± 0.01 10.03 ± 0.11 70.2 ± 0.1 4.30 ± 0.12 5.3-nm Au NDs 0.62 ± 0.01 10.58 ± 0.15 70.9 ± 0.2 4.65 ± 0.15 6.2-nm Au NDs 0.62 ± 0.02 9.76 ± 0.21 69.1 ± 0.2 4.18 ± 0.23 Au NPs (50 nm) + Au NDs (5.3 nm) 0.64 ± 0.01 11.20 ± 0.09 67.0 ± 0.2 4.80 ± 0.12
Fig. 3. UV–Vis spectra of PEDOT:PSS/ITO substrates presenting Au NDs deposited through thermal evaporation.
of the signals in both figures were enhanced as a result of the presence of the dual metallic nanostructure; this in-creased absorption contributed to the inin-creased photocur-rent, relative to that of the pristine P3HT:PC61BM, and
thereby contributed to the improvement in the value of Jsc.
By plotting together the absorption enhancement spec-tra of the Au NDs–only and combined Au NDs/Au NPs de-vices, we confirmed that the absorption enhancement provided by the combined 50-nm metallic NPs and 5.3-nm Au NDs contributed to the improvement in the val-ues of Jscand PCE of the PSCs featuring the dual plasmonic
nanostructure. In terms of the optical effects arising from the dual plasmonic structures, we observed similar trends in the increasing EQEs and absorptions of device
incorporating the two types of metal nanostructure (Fig. 5b and c, respectively). The integrated value of Jsc
determined from the EQE spectrum for the device featur-ing the 5.3-nm Au NDs and the 50-nm Au NPs in the PED-OT:PSS layer was 10.9 mA/cm2. The difference between integrated and measured values of Jscwas within 3%, again
indicating the high accuracy of our PSC measurements. The photo-current of the device increased significantly when the active layer incorporated thermally deposited Au NDs. This wavelength regime 360-620 nm coincides with the extinction range of the Au NDs, indicating that LSPR effects were responsible for the improved photo cur-rent. To study the LSPR effect on the active layer, we chose the octahedral Au NPs to concentrate the light through their
Fig. 4. Top-view TEM images of P3HT:PC61BM films without (a) and with 2.7-, 3.1-, 4.5-, and 5.3-nm (b–e) thermal evaporated Au NDs. (f) Cross-sectional TEM image of thermally evaporated 5.3-nm Au NDs on a P3HT:PC61BM film. (Inset: size distribution chart of Au NDs).
near-field effect. Chemically and lithographically fabricated metal NPs and nanostructures have been used widely in PSCs; fluorophores in close proximity to the metal NPs can result in excitation enhancement (increased light absorption), emission enhancement (increased radiative decay), or quenching (increased nonradiative decay)[30]. 3.4. Steady state and dynamic photoluminescence results
To further estimate the influence of the LSPRs on the exciton generation, we performed the steady state and
dynamic photoluminescence (PL) measurements and the corresponding results are shown inFig. 6. Because P3HT predominately contributes to the light absorption and exciton generation in the P3HT:PC61BM active layer, we
spin-coated the P3HT and P3HT:PC61BM films on
PED-OT:PSS layer with thermally evaporated 5.3-nm Au NDs and incorporating 50-nm Au NPs.
Fig. 6a and b displays steady state PL spectra obtained using 470 and 550 nm excitation wavelengths; 550 nm excitation wavelength corresponds to the resonance peak of the LSPRs induced by Au NDs, 576 nm, and by Au NPs, 568 nm quite well. The P3HT films on PEDOT:PSS layer, however, has an absorption peak of 510 nm, and we would like to investigate the exciton life time from P3HT films, therefore, we chose 470 nm excitation wavelength for the time-resolved PL study. The PL intensity of these films was enhanced in both P3HT films and P3HT:PC61BM films
that incorporate Au NDs and Au NPs when they were ex-cited with 470 and 550 nm light, suggesting the increase of light absorption and exciton generation in P3HT upon the excitation of LSPRs. As a result of the resonance fre-quency of the Au NDs and Au NPs was close to the absorp-tion band of P3HT, the PL intensity enhancement of P3HT can be attributed to the fact that excitation of the LSPRs increased the degree of light absorption and, thereby, en-hanced the light excitation rate. Fig. 7 presents the PL
Fig. 5. (a) J–V curves, (b) EQE curves and (c) UV–Vis spectra of devices featuring P3HT:PC61BM layers prepared with thermal evaporation of 5.3-nm Au NDs and incorporating 50-5.3-nm Au NPs.
Fig. 6. Steady-state fluorescence spectra of P3HT and P3HT:PC61BM films incorporating thermally evaporated 5.3-nm Au NDs and embedded 50-nm Au NPs in the PEDOT:PSS layer using excitation wavelengths of (a) 470 and (b) 550 nm.
intensity decay profiles, detected at the wavelength of 650 nm via a 470 nm laser of P3HT:PC61BM films with
thermally evaporated 5.3-nm Au NDs on the top of P3HT films and incorporating 50-nm Au NPs in the PEDOT:PSS layer (plasmonic sample) and P3HT:PC61BM films
(refer-ence sample), the wavelength 650 nm was selected inten-tionally due to the greatest PL intensity inFig. 6a.
We then performed a time-resolved photoluminescence (PL) study in which we determined the intensity of the PL, I(t), using the following multi-exponential function[52]: IðtÞ ¼X n i¼1 Aie T si ð1Þ
where Aiis the amplitude of the ith decay, n is the number
of decays involved, and
s
iis the ith exponential constant.Fig. 7presents the evolution of the intensity of the PL for various P3HT:PC61BM films; small changes in the exciton
lifetimes (
s
exciton) were evident for the samples preparedwith and without dual plasmonic structures, implying the absence of strong coupling between the plasmonic field and the excitonic state. We analyzed the evolution of the PL intensity, I(t), using Eq.(1)with two values for the lifetimes,
s
1ands
2, and corresponding amplitudes, A1and A2. We then determined the average exciton lifetime
(
s
exciton) using the following equation:s
exciton¼A1
s
21þ A2s
22 A1s
1þ A2s
2ð2Þ For the reference sample, the values of the two fitted exponential constants
s
1ands
2 were 26.37 and 1.00 ns,respectively; they provided a corresponding exciton life-time,
s
exciton, of 11.20 ns. For the sample with dualplas-monic structure, the two exponential constants were 5.89, and 0.94 ns, respectively, providing
s
exciton of1.69 ns. It suggests that the excitons were dissociated much faster at the presence of both Au NDs and Au NPs. This finding is consistent with previous reports of the plas-monic field strongly influencing photo induced charge sep-aration or recombination processes through plasmon– exciton coupling[24,53,54].
4. Conclusion
Au NPs and Au NDs interact strongly with light through excitation of their localized surface plasmons; Au NPs incorporating into the PEDOT:PSS layer induced local field enhancement, not only leading to increased light absorp-tion but also benefiting the photo induced charge separa-tion processes, and Au NDs enhanced the parasitic absorption of light and an elevated degree of exciton disso-ciation. Accordingly, we have used both Au NPs and Au NDs to tune the resonance wavelength in PSC devices. We observed that the PCE of a dual LSPR device incorporat-ing 5.3-nm Au NDs and 50-nm Au NPs blended in the PED-OT:PSS layer increased to 4.8% from a value of 4.0% for the corresponding P3HT:PC61BM cell lacking any Au particles.
In the device incorporating 5.3-nm Au NDs, absorption dominated over the scattering of light; accordingly, PSCs fabricated with 5.3-nm Au NDs embedded between the ab-sorber layer and the cathode exhibited the greatest enhancement in EQE among the tested Au ND systems. The Au NDs functioned as an effective medium layer that resulted in greater absorption of light by the P3HT:PC61BM
film, resulting in enhanced exciton generation; when these excitons reached the P3HT-to-PC61BM interface, the degree
of charge carrier dissociation also increased. Therefore, this specific type of Au NDs is suitable for the plasmonic enhancement of PSCs. We believe that such an approach, employing two types of localized surface plasmons, has the potential to result in PSC devices exhibiting even high-er efficiencies.
Acknowledgements
We thank the National Science Council for financial support (NSC101-3113P-009-005) and Yin-Kai Lin for doing PL measurement.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in the online version, athttp://dx.doi.org/10.1016/ j.orgel.2013.06.012.
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