Enhanced Photoelectrochemical Response of Polythiophene

Mar 30, 2015 - Kagoshima 890-0065, Japan. •S Supporting Information. ABSTRACT: In the present work silver nanoparticles (cubes and spheres) with siz...
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Enhanced Photoelectrochemical Response of Polythiophene Photoelectrodes with Controlled Arrays of Silver Nanocubes Kwati Leonard,*,† Jing You,† Yukina Takahashi,†,‡ Hiroaki Yonemura,†,‡ Junichi Kurawaki,§ and Sunao Yamada*,†,‡ †

Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan Center for Future Chemistry, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan § Department of Chemistry and Bioscience, Graduate School of Science and Engineering, Kagoshima University, 1-21-35 Korimoto, Kagoshima 890-0065, Japan ‡

S Supporting Information *

ABSTRACT: In the present work silver nanoparticles (cubes and spheres) with sizes of 40 ± 5 nm have been synthesized by a similar experimental method in order to optimize both the photoluminescence and photoelectric conversion enhancement factor of poly(3-hexylthiophene) photoelectrodes with thickness comparable to those used in organic solar cells. The silver nanoparticles were assembled into monolayers with various coverage densities using the Langmuir−Blodgett technique. The dependence of surface plasmon evanescent field and energy transfer has been investigated with the largest enhancement in photoluminescence observed at approximately 14% coverage density of silver nanocubes. We attribute the enhancement to a strong local electric field as well as scattering properties associated with particles of this geometry that translate to significant photocurrent enhancements. The mean photoluminescence lifetimes of the photoelectrode decreased with decreasing silver nanocube spacing and represent an increase in the total decay rate by a factor of 1.24 at 14% coverage of the nanoarrays. The origin of this increase is due to enhancement in either the radiative and/or nonradiative decay rates.

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INTRODUCTION Recently, the interaction of noble metal nanoparticles (NPs) with organic materials such as conjugated polymers and dyes has attracted significant attention due to their diverse applications in material, chemical, and biological science.1−6 The great usefulness of these NPs stems from the ability to fine-tune their physical and optical properties.4,5,7 For example, metal NPs can adopt a wide range of sizes and shapes (cubes, prisms, rods, cages, etc.) which give rise to a wider range of spectral properties. When excited by light of appropriate wavelength, they exhibit localized surface plasmon resonance (LSPR) due to collective oscillation of their conduction electrons, which in turn induces strong electromagnetic fields on and around the NP surface.3−7 This generally leads to an enhancement in the radiative and nonradiative electronic properties of the NPs. During the past decade, numerous experimental and theoretical results have revealed large enhancement in light absorption efficiency,8−12 fluorescence,12−17 surface-enhanced Raman scattering3,18−20 (SERS), and photocurrents1,3,6,21−34 due to the proximal effects of NPs on conjugated polymers and dye systems. The observed enhancements are generally attributed to resonant coupling between radiation and particle plasmons in NPs and the possibility of tuning this resonance to © XXXX American Chemical Society

coincide with the electronic transition levels of nearby molecules. The degree of enhancement depends on the oscillating electric field intensity in the vicinity of the metal NPs, whereas the electric field intensity depends on the metal type, size, shape, proximity of neighboring particles, and dielectric environment.4 Our group recently developed photoelectrodes using gold NPs with monolayers of organic dye molecules and demonstrated that photocurrents could be about 10−50 times enhanced as compared with the monolayer prepared on planar gold surfaces.21 We have also reported several different techniques to assemble plasmonic nanostructures on substrates, some of which include electrochemical deposition,21 electrostatic layer-by-layer adsorption of NPs,22,25,29 vacuum deposition, and deposition of NP films formed on a liquid−liquid interface.35 More recently, we extended these studies to polythiophene, one of the most widely used conjugated polymer semiconductors in bulk heterojunction solar cells.23,30,31 We observed appreciable enhancement of photocurrents in photoelectrodes consisting of gold NPs that were Received: November 14, 2014 Revised: March 14, 2015

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slowly. The particles were diluted with acetone (twice the volume of the vial content) and collected by centrifugation, washed with water and ethanol, and then suspended in 2 mL of chloroform. The NPs capped with PVP in chloroform were then added dropwise to the air−water interface of a LB trough. The chloroform colloidal solution quickly spread on the water surface and then evaporated to leave an isotropically distributed monolayer of particles floating at the surface. The floating monolayers were then forced together to higher density on the surface by using a computer-controlled barrier. We then transferred the NPs off the air−water interface by horizontal lifting onto the precleaned silicon, quartz, and ITO substrates at surface pressures of 0−7 mN/m, resulting in controlled surface coverage density of 5−29%. The P3HT photoactive layer, with a concentration of 15 mg/mL, was then spin-coated on the assembled silver NP monolayer inside a drybox under nitrogen atmosphere. The photoelectrodes were dried for 10 min on a hot stage at 80 °C. The structure of the prepared photoelectrode is denoted as ITO/silver NPs/P3HT. Measurements. Photocurrent measurements were carried out in an aqueous solution containing 0.1 M NaClO4 using a standard three-electrode photoelectrochemical cell consisting of Pt wire as counter, Ag/AgCl (saturated in KCl solution) as reference, and the modified P3HT photoelectrodes as working electrode. The area of the working electrode was adjusted to 0.2 cm2. Before measurements, oxygen bubbling (as an electron acceptor) was carried out for 30 min. Excitation light from a 150 W Xe lamp propagating through a monochromator irradiated the modified electrode (working electrode), and the resultant photocurrents were measured with a Huso HECS318C potentiostat. From the photocurrent spectra Jsc (shortcircuit current density λ) and lamp power spectrum Pin (incident light power density) of the monochromator, incident photon-to-current conversion efficiency (IPCE) spectra were calculated using the following expression

deposited on ITO substrates by electrostatic adsorption or electrodeposition and the polythiophene layer prepared by electropolymerization.30 In most of these techniques, however, controlling the deposition density and interparticle spacing still remains challenging. An alternative approach of fabricating NP assembly is the Langmuir−Blodgett (LB) technique.36−39 The advantage of this technique lies in its ability to assemble twodimensional monolayer structures of NPs with high precision on substrates. It also allows for continuous variation of particle density, spacing, and even arrangement which can be very suitable for small devices. Although LB monolayer films of silver nanocubes and gold nanocages have been previously used to enhance either the absorption or fluorescence of polymers37,39 a comprehensive study is still missing that combines the relationship between photocurrents and photoluminescence (PL) enhancements resulting from the rigorous plasmonic effects of metal NPs and the distance dependence between these NPs and the fluorescing polymer. In this study, we have fabricated large-area arrays of twodimensional silver NPs (cubes and spheres) on ITO substrates using the LB technique and integrated them into functional photoelectrodes with poly(3-hexylthiophene) as the active layer. We also investigated the dependence of photocurrent and PL on the particle density and interparticle spacing with different surface plasmon field strengths. It was observed that the PL as well as photocurrent intensity increased with increasing NP surface density and decreasing NP spacing. A further increase in the surface density resulted in a rapid decrease in PL, which in turn affected the photocurrents. These results are discussed in terms of the changes in the surface plasmon field strength of the assembled NPs. The PL lifetimes of silver nanocubes/P3HT samples also decreased with decreasing spacing and represented an increase in the total decay rate by a factor of 1.24 at ∼14% coverage. The origin of this increase is due to enhancement in either the radiative or nonradiative decay rates.

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IPCE =

EXPERIMENTAL SECTION Materials. Silver nitrate (AgNO3, 194-00832 Wako), dehydrated ethylene glycol (EG) (059-06315 Wako), sodium sulfide nonahydrate Na2S·9H2O (19703362 Wako), anhydrous sodium perchlorate (Wako), reagent grade chloroform (99.9% Wako), poly(vinylpyrrolidone) (PVP, Mw ≈ 55 000 or 10 000 Sigma-Aldrich), chlorobenezene (Sigma-Aldrich), and other chemicals were used as received. Regioregular poly(3hexylthiophene) (P3HT) was supplied by Sigma-Aldrich and was stored and protected from sunlight inside a glovebox. 6,6Phenyl-C61-butyric acid methyl ester (PCBM, nanom spectra E100) was supplied by Merck chemicals. Preparation of Electrode. Silver nanostructures were prepared by reported procedures40,41 with minor modifications. The synthesized NPs (spheres and cubes) with diameters of about 40 ± 5 nm were prepared by a similar experimental method with shape variation achieved by simply altering the solution stirring speed. Slow stirring (∼300 rpm) yielded cubes, while high stirring (∼900 rpm) generated spheres. In brief 6 mL of EG was preheated under stirring at 150 °C for 1 h in a glass vial. PVP was dissolved in 1.5 mL of EG and added to the glass vial with gentle stirring. When the temperature restabilized to 150 °C, Na2S·9H2O (80 μL, 3 mM) solution in EG was added, followed by the slow injection of AgNO3 (0.5 mL, 0.28 M) in EG into the reaction mixture. After 15 min the glass vial was removed from the heat and allowed to cool

1240Jsc (A/cm 2) Pin (W/cm 2)λ (nm)

× 100

IPCE enhancement factor was obtained as the ratio of ΔIPCE and IPCE at a specific excitation wavelength, where ΔIPCE = IPCEITO/AgNPs/P3HT − IPCEITO/P3HT or by IPCEITO/AgNPs/P3HT/ IPCEITO/P3HT. All photocurrents were measured at an applied voltage of E = 0 V versus Ag/AgCl. The extinction spectra of the films were measured with a UV−vis−NIR spectrophotometer (UV-3150, Shimadzu or V-670, JASCO). Surface morphologies of the samples were observed via field emission scanning electron microscopy (FE-SEM; SU8000, Hitachi High-Technologies) and atomic force microscopy (AFM; JSPM-5400, JEOL) after drying the samples under vacuum for over 6 h. Transmission electron microscopy (TEM) was performed on a JEOL JEM2010. Steady-state PL emissions were measured on a JASCO FP6500 spectrophotometer with a photomultiplier (R928 Hamamatsu Photonics Co., Ltd.). The PL emission of the sample films was measured using an attachment specifically designed for solid thin-film measurements (FDA-430 type; JASCO FP6000 series). A band-pass filter at 530−585 nm (Chroma Inc.) was used in the collection path, thus eliminating the scattered excitation light and collecting the PL from the polymer probes in the region of interest. PL lifetime was measured with a single photon counting system (Hamamatsu B

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The Journal of Physical Chemistry C Photonics C420) using a dye laser (USHO DL-50, 420 nm) (Exciton Stilbene 420 dye) and a N2 laser (USHO KEN-800). PL intensity decay curves were reconvoluted with the instrument response function and analyzed as a sum of exponential terms n

IPL(t ) =

∑ Ai exp(−t /τi) i=1

where IPL(t) is the PL intensity at time t; Ai is a pre-exponential factor representing the fractional contribution to the timeresolved decay of the component with a lifetime τi; and n is the number of decays involved. The photoluminescence enhancement factor (PE) was calculated using the following expression: PE = PL.AITO/AgNPs/P3HT/PL.AITO/P3HT where PL.A is the integrated photoluminescence area under the desired spectrum from 570 to 800 nm

Figure 2. FE-SEM images of PVP-capped Ag-Ncu monolayers assembled on the surface of silicon substrates at surface pressures of 1 (a), 2 (b), 3 (c), 4 (d), 6 (e), and 7 (f) mN/m, resulting in controlled surface coverage densities of 5, 10, 12, 14, 20, and 29%, respectively (scale bar = 500 nm).

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RESULTS AND DISCUSSION 3.1. Monolayers of Silver Nanocube Photoelectrodes. Silver nanocubes (Ag-Ncu) and spheres (Ag-Nsp) prepared in this study are capped with PVP. TEM images of the NPs reveal a uniform distribution in terms of both shape and size with an average diameter of 40 ± 5 nm (see Supporting Information, S1). Figure 1 shows the extinction spectra of the colloidal

the calculation of surface coverage density and the average interparticle separation distance. The coverage densities were calculated using scanning probe image processor (SPIP) analysis software (Image Metrology, Hørsholm, DK) and were found to be 5, 10, 12, 14, 20, 22, and 29%, respectively, whereas corresponding interparticle spacing obtained from nearest-neighbor particle measurements ranged from 165, 102, 95, 90, 76, 62, and 19 nm. The FE-SEM images also indicate that the particle density increases as the surface pressure increases, whereas interparticle separation distance decreases. Here we assumed that a similar coverage density of the NPs (both Ag-Ncu and Ag-Nsp) obtained from the same surface pressure should approximately result in similar quantities of NPs on the substrate surfaces. The representative surface pressure−area isotherms for the prepared LB monolayers (AgNcu and Ag-Nsp) are shown in Supporting Information S4. Both theoretical and experimental studies have shown that the extinction spectra of Ag-Ncu contain several peaks and exact peak positions depending on the size and local environment of the nanocubes.44,45 As shown in Figure 3(a) a couple of traits appear for both sets of extinction spectra. First, the overall shape of the extinction spectra are almost similar for all monolayer arrays transferred on an ITO electrode. Second the extinction of monolayer transferred on the electrodes clearly increases with increasing coverage of AgNcu. Typically, the extinction spectra of the Ag-Ncu electrode show peaks at ∼430 nm and an extended tail around 520−590 nm. The former peaks can be attributed to the extinction of isolated Ag-Ncu and the latter to coupling between neighboring Ag-Ncu in the arrays.45,50 A noticeable red-shift in the plasmon resonance of NP arrays on the electrodes is also observed possibly due to an increase in the refractive index of the medium (ITO; n = ∼2).49−51 3.2. Characterization of Poly(3-hexylthiophene) Silver Nanocube Photoelectrodes. In the present study, we aim to optimize the efficiency of exciton formation in P3HT with controlled arrays of Ag-Ncu monolayers on the electrodes. We expect that by increasing the amount of excitons within the P3HT layer more electrons will be produced, thereby contributing to the photocurrent output.52,53 Typical extinction spectra of the photoelectrodes are shown in Figure 3(b). The spectrum of P3HT without Ag-Ncu indicates a broad absorption feature arising from π−π* transition with three

Figure 1. Extinction spectra of Ag-Nps (blue line) and Ag-Ncu (red line), in chloroform together with the calculated discrete dipole approximation (DDA) extinction spectrum of Ag-Ncu (green line), in the same medium.

solution of Ag-Nsp and Ag-Ncu measured in chloroform together with the calculated discrete dipole approximation (DDA) extinction spectrum of Ag-Ncu in the same medium. The peak position at 430 nm agrees well with the calculated peak and can be attributed to the dipolar plasmon oscillation,42 while the other weak peak in the shorter-wavelength regions probably results from multipolar oscillations.42−46 Both peak positions (experimental and calculated) are red-shifted compared with that of Ag-Nsp (410 nm). This phenomenon has been observed for nanostructures with sharp corners.44,47−49 The as-prepared NPs were hydrophobic because of the PVP capping and were capable of forming a stable monolayer at the air/water interface. The monolayers were then transferred onto the surfaces of ITO electrodes and quartz plates. Alternatively, the particles were also assembled on the surface of a silicon wafer for FE-SEM imaging. Figure 2(a−f) shows the FE-SEM images of six samples assembled at surface pressures of 1, 2, 3, 4, 6, and 7 mN/m, respectively (see Supporting Information S2 for a larger area description of the monolayers). These images which produced a good description of the morphology of the monolayers with very limited aggregated particles allowed for C

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whose cross-sectional profiles were analyzed. The results are shown in the Supporting Information S5. P3HT films on the electrodes without the Ag-Ncu arrays showed smooth and flat surface morphology which is indicative of the amorphous nature of the films with an estimated layer thickness of ∼27 nm in Figure S5(a) (Supporting Information). On the other hand, P3HT photoelectrodes with Ag-Ncu arrays of ∼14% surface coverage density were estimated to be ∼57 nm. A schematic cross-section of the two schemes is shown in the Supporting Information S5(c) and (d), respectively. 3.3. Photoelectric Response of P3HT/Ag-Ncu Films on ITO Electrodes. The effects of Ag-Ncu arrays on the photoelectrochemical response of P3HT films were investigated. Photoirradiation was carried out from the rear side (the ITO plate side) of the photoelectrodes. The photocurrent action spectrum was recorded only at wavelengths >400 nm where background photocurrents from ITO were negligible. All the prepared photoelectrodes exhibited stable cathodic photocurrents at E = 0 V vs Ag/AgCl. Figure 4(a) shows typical

Figure 3. (a) UV−visible extinction spectra of PVP-capped Ag-Ncu monolayers assembled on ITO substrates with surface coverage densities of 5 (dark blue line), 10 (red line), 12 (green line), 14 (light blue line), 20 (orange line), and 29 (dark green line) %. (b) The UV− visible extinction spectra of P3HT films (black line) and P3HT films spin-casted on Ag-Ncu monolayers with surface coverage densities of 5 (dark blue line), 10 (red line), 12 (green line), 14 (light blue line), 20 (orange line), and 29 (dark green line) %. (d) Raman spectra of P3HT alone (black line) and P3HT/Ag-Ncu arrays (purple line) at 514 nm excitation.

peaks at 513, 563, and 610 nm, in good agreement with previous results.53,54 The extinction spectra of P3HT films with Ag-Ncu arrays seem not to be significantly modified even in the presence of the Ag-Ncu, except for the noticeable appearance of the characteristic extinction peak due to LSPR of Ag-Ncu at ∼428 nm with a coverage density of 10% and a continuous increase as the density of particles increases. Figure 3(c) shows Raman spectra of P3HT with and without Ag-Ncu array with 14% surface coverage, at 514 nm excitation wavelength. The bands observed at 1087.5, 1211.3, 1375, 1450, and 1516 cm−1 are characteristic peaks of P3HT in the SERS spectrum.55 It can also be seen that the addition of Ag-Ncu arrays leads to an increase in the Raman-active modes of P3HT and second that there are no changes in peak positions due to the presence of the Ag-Ncu arrays, suggesting the interactions of P3HT with these Ag-Ncu arrays are purely electromagnetic, with negligible chemical components.55,56 AFM measurements of the electrodes with and without AgNcu arrays were also investigated. Selected portions of the films on the electrodes were scratched up, leading to a sharp step between the deposited films and the surface of the electrode,

Figure 4. (a) IPCE spectra of ITO/P3HT (green line), ITO/P3HT/ Ag-Nsp (red line), and ITO/P3HT/Ag-Ncu (blue line). (b) Ag-Ncu coverage density dependence of enhancement factor on the photocurrent of P3HT electrode at 430 (blue line) and 530 (red line) nm excitation (IPCE enhancement was obtained by IPCEITO/Ag‑Ncu/P3HT/ IPCEITO/P3HT from an average of three photo electrodes). The relationship between the integrated peak intensity and the surface coverage of the Ag-Ncu (green line) is also shown.

incident photon-to-current conversion efficiency (IPCE) spectra of ITO/Ag-Ncu/P3HT, ITO/Ag-Nsp/P3HT, and ITO/P3HT. Obviously, the general features of the IPCE spectra match well with the extinction spectra of the corresponding photoelectrodes as shown in Figure 3(b), suggesting photoexcitation of P3HT on the photoelectrode is responsible for the generated photocurrents. The effects of AgNcu on the photoelectrochemical response of ITO/Ag-Ncu/ P3HT:PCBM, ITO/Ag-Nsp/P3HT PCBM, and ITO/ P3HT:PCBM films were also investigated. The results are shown in Supporting Information S6. The generation of D

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their LSPR property to direct a more concentrated flux of photons onto the P3HT layer. As described in Figure 4(b) the enhancement of photocurrents from the ITO/P3HT/Ag-Ncu increased with increasing coverage density up to about 12% and then decreased thereafter. It is worth pointing out that a similar tendency in photocurrent enhancement has been reported by our group.25−30 For example photoelectrodes consisting of AuNsp (18 nm) and polythiophene prepared by electropolymerization reached maximum enhancement of photocurrent with a coverage density of ∼14% and then decreased above it.30 Plasmon-assisted photocurrent enhancement in photoelectrodes consisting of Ag-Nsp and zinc tetraphenyl porphyrin reached maximum enhancement with ∼20% coverage and then decreased thereafter.25 In a nut shell, enhancement of photocurrents by metal NP monolayers seems to saturate rapidly at relatively low coverage density of NPs (11− 15%) and significantly depends on metal type, size, and shape. 3.4. Steady-State Photoluminescence of P3HT/AgNcu Thin Films on Quartz Substrates. A common technique to probe plasmon−exciton interactions is to investigate the effect of metal NPs on the PL of nearby organic molecules. Many studies have shown that in the presence of metal NPs both exciton generation and decay can be appreciably modified by the LSPR of metal NPs.63−65 In this study, PVP-capped Ag-Ncu arrays with surface coverage of 5, 10, 12, 14, and 22% were fabricated on quartz substrates, and P3HT films were spin-casted on them with roughly similar thicknesses. The obtained samples are denoted as quartz/AgNcu/P3HT. These substrates were placed on a slide holder, and a 532 nm laser was directed from the rear side of the quartz substrates to excite the Ag-Ncu before P3HT. This is to ensure that scattering effects from the Ag-Ncu arrays are directed inside the P3HT layer. Two samples were prepared for each coverage density of Ag-Ncu, and PL was measured at five locations across each sample to account for PL variance across the films. The excitation peak intensity of the P3HT layer on P3HT quartz-only substrates was examined and found to be approximately the same for all samples and did not vary across the surface as shown in Figure S7 (Supporting Information). The PL values across each quartz/Ag-Ncu/P3HT sample were averaged followed by the averaging of the two samples. Each spectrum was then integrated from 570 to 870 nm, and the area under the Ag-Ncu/P3HT spectrum was divided by the corresponding spectrum of the P3HT layer without Ag-Ncu to determine the values of relative enhancement factor. A 3D PL spectra representation of P3HT alone excited at 360−560 nm together with the corresponding enhancement factor of P3HT in the presence of Ag-Ncu (12%) arrays is shown in Supporting Information S8. Clearly the LSPR peak region of Ag-Ncu around 410−460 nm is visibly observed. Steady-state PL spectra of quartz/P3HT and quartz/Ag-Ncu/P3HT with different surface coverage densities of Ag-Ncu, excited at 532 nm, are shown in Figure 6(a). From these measurements, one can notice that the quartz/Ag-Ncu/P3HT sample exhibits a higher PL intensity than that of quartz/P3HT without Ag-Ncu, with the PL peak intensity increasing with increasing coverage density and then decreasing rapidly thereafter. This gradual increase up to 14% can be explained in terms of increasing surface plasmonic field strength of the Ag-Ncu arrays associated with increasing NP density. A similar trend is observed when the samples are excited at 430 nm. The integrated PL intensity reaches a maximum value at the coverage of 14%, with an

photocurrents in both cases (P3HT and P3HT:PCBM) has been attributed to exciton generation inside the P3HT phase which then transfers the electrons to electron acceptors O2 (E 0 O2/O 2•− = −0.50 V vs Ag/AgCl) in the electrolyte22,30,54,57−59 as schematically shown in Figure 5. We also

Figure 5. Schematic representations of the energy diagram and electron transfer processes for cathodic photocurrent generation in the as-prepared photoelectrode.

observed a significant enhancement in photocurrents for the electrodes incorporating Ag-Ncu arrays than those with Ag-Nsp arrays with approximately the same coverage. The calculated photocurrent enhancement factors at 530 nm excitation wavelengths were 50.43 and 25.68% for the Ag-Ncu and AgNsp arrays, respectively, with the Ag-Ncu arrays approximately twice as much enhanced as the Ag-Nsp arrays. Enhancement of photocurrents by Ag-NPs has been explained in terms of increased absorption and exciton generation of the photoactive layer induced by LSPR.23−28 The superior photocurrent enhancement effect for the Ag-Ncu arrays relative to Ag-Nsp arrays can be readily understood in terms of the higher scattering efficiency of NPs of this geometry and supported by the calculated electric field distribution on the particles as described in our previous report.60 Also the magnitudes of the enhanced field intensity distribution (indicated by the color scale) on the Ag-Ncu vicinity appear to be stronger than that of Ag-Nsp calculated at 530 nm (see Supporting Information S1(b)). This wavelength region happens to overlap properly with the absorption of P3HT. Hence enhanced electric fields assist the dissociation of excitons generated in the photoactive layer, thereby improving photocurrents.61,62 Figure S6(b) (Supporting Information) shows the IPCE spectral difference (ΔIPCE/IPCE) plotted for P3HT:PCBM photoelectrodes incorporating Ag-Ncu and Ag-Nsp at a surface particle coverage density of ∼12% for comparison. Enhancements in spectral response are clearly observed within the wavelength region 400−650 nm for both electrodes. It is worth emphasizing that the blend of P3HT:PCBM in all the electrodes was prepared with almost the same thickness and that cathodic photocurrents of the NP (Ag-Ncu and Ag-Nsp) electrodes without P3HT:PCBM were ignorable. Therefore, one can reasonably infer that the increment in photocurrents should be ascribed to enhanced light harvesting and the ability of the Ag-NPs through E

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of spectroscopic and fitting parameters of the PL decays of some representative samples are tabulated in Table 1. It can be Table 1. PL Lifetime Parameter of P3HT Photoelectrodes in the Presence and Absence of Ag-Ncua

a

sample/PL lifetime parameters

τ1(ns)

A1

A2

χ

τmean(ns)

Γt

P3HT P3HT/Ag-Ncu P3HT/Ag-Ncu P3HT/Ag-Ncu P3HT/Ag-Ncu

0.79 0.35 0.36 0.33 0.30

0.56 0.86 0.89 0.67 0.75

0.44 0.14 0.11 0.33 0.25

1.02 0.95 1.29 1.10 1.01

0.51 0.48 0.42 0.42 0.41

1 1.05 1.20 1.21 1.24

5% 10% 12% 14%

Where Γt = (Γ′r + Γ′nr).

seen that, with Ag-Ncu surface coverage of 5, 10, 12, and 14%, the obtained decay lifetimes were 0.483, 0.420, 0.415, and 0.407 ns, respectively. The changes in the average decay lifetime values τmean after incorporating Ag-Ncu arrays is evidence for the presence of strong coupling between the plasmonic field of the Ag-Ncu arrays and excitons in P3HT.68−72 Surprising, however, is the trend in total decay rate Γt (where Γt = (Γ′r + Γ′nr) with Γ′r and Γ′nr representing the radiative and nonradiative decay rates), which gradually increases with increasing surface coverage of Ag-Ncu arrays and represents an increase by a factor of ∼1.24 at 14% surface coverage and almost stabilizes thereafter. The origin of this increasing trend could be due to enhancement in either Γ′r or Γ′nr associated with increasing Ag-Ncu surface coverage and/or decreasing interparticle spacing. Γt is obtained as the approximate ratio of the average decay lifetime of P3HT with and without the NPs. Since the PL intensity of P3HT/Ag-Ncu arrays is enhanced with increasing coverage up to 14% (by a factor of ∼3.5) coverage as described earlier, it is likely that Γ′ r is predominantly enhanced. However, at much shorter interparticle spacings (coverage >14% density), strong dipole coupling between P3HT excitons and the plasmonic resonance provides an efficient nonradiative Forster-like energy transfer or charge transfer pathway from the molecular excited state of P3HT to the Ag-Ncu arrays that exceeds the molecular emission rate in which case a reduction of PL is observed.7,68,70 The maximum enhancement of photocurrent generation at the 12% coverage of Ag-Ncu can be explained in terms of contribution from the energy transfer process which is dependent on the coverage density of the Ag-Ncu arrays. Our findings show that despite an initial increase in exciton generation promoted by the presence of Ag-Ncu arrays resulting in significant photocurrent enhancement energy transfer processes, including plasmon−exciton coupling especially at shorter interparticle spacings and higher coverage densities (≥14%), however, contributes to reduction in photocurrent in the P3HT/Ag-Ncu electrodes. A plausible explanation to this is that the gradual increase in Ag-Ncu coverage probably resulted in an increasing polaron population up to 12% coverage; however, at the 14% coverage the polaron population stales as a result of fast trapping of polarons by the nearby Ag-Ncu arrays. This increased trapping of polarons leads to monomolecular recombination which reduces photocurrents, hence the reserved trend in photocurrents observed in Figure 4(b). Generally the use of metal NPs in solar cell devices involves a trade-off between absorption enhancement and the reduction of the exciton lifetime, which in turn leads to a shorter exciton diffusion length. According to our results,

Figure 6. (a) Steady-state PL spectra of P3HT films only and P3HT films spin-casted on PVP-capped Ag-Ncu monolayers assembled at different coverage densities. (b) PL lifetime decay profiles, obtained using a 420 nm excitation source of P3HT film only (red line) and P3HT/Ag-Ncu arrays with coverage densities of 5 (green line), 10 (purple line), 12 (blue line), and 14 (orange line) %.

enhancement factor of ∼3.5, whereas maximum photocurrents were observed at an approximately identical coverage of 12%. These results confirm that the observed increments in photocurrents can be attributed to P3HT excitation below 14% coverage, while above 14%, photocurrent reduction occurs probably because of annihilation effects.66,67 The increasing tendency of the enhancement factor according to surface coverage density is also consistent with recent reports on PL enhancement using metal NPs reported by our group.23−30,60 3.5. Photoluminescence Lifetime of P3HT and P3HT/ Ag-Ncu Arrays. Photoluminescence lifetime measurements of the samples with and without Ag-Ncu arrays were investigated in order to establish a correlation between PL and photocurrents. It is well established that PL enhancement of emitting molecules in the vicinity of metallic nanostructures is accompanied by a decrease in the lifetime of their excited states.7,15,16,68,69 Figure 6(b) shows the PL lifetime decays profiles, obtained using a 420 nm excitation source of P3HT/ Ag-Ncu arrays with coverage densities of 5, 10, 12, and 14%, together with the PL lifetime of P3HT film alone. The PL decay data were fitted using the multiexponential function shown in the Experimental Section, whereas the average lifetimes were obtained from the weighted average of each component, using τmean = [A1/(A1 + A2)]τ1 + [A2/(A1 + A2)]τ2.62 The analysis yields an averaged PL lifetime of P3HT film of 0.505 ns, which is consistent with literature values for the solid-state, room-temperature PL lifetime of plain P3HT thin films (0.480−0.800 ns depending on regioregularity, crystallinity, etc.).68,70−72 In contrast, we observed considerable reduction in PL lifetime when P3HT films were in direct contact with Ag-Ncu arrays. Clearly as depicted in Figure 6(b) the PL decay lifetimes of P3HT incorporating the Ag-Ncu arrays are faster than that of plain P3HT and decrease gradually with increasing surface coverage of the Ag-Ncu arrays. Details F

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significant absorption as well as PL enhancement can be reached with increasing surface coverage of Ag-Ncu; however, a strong reduction in the exciton lifetime was observed with higher coverage density suggesting an optimum Ag-Ncu coverage of ∼12% for maximum enhancement of photocurrents. We therefore conclude that the exploitation of plasmonic far-field light-scattering effects from Ag-Ncu should be more promising to enhance the efficiency of thin-film solar cells, compared to Ag-Nsp of the same size.

ASSOCIATED CONTENT

S Supporting Information *

Additional information and TEM images of Ag-Nsp, Ag-Ncu, and their FDTD simulation model (Figure S1). Extinction spectra of smooth edges of PVP-capped Ag−Np monolayers deposited on ITO substrates (Figure S2). Large-area FE-SEM images of PVP-capped Ag-Ncu monolayers assembled on the surface of silicon substrates (Figure S3). Surface-pressure isotherms of Ag-Ncu and Ag-Nsp monolayers (Figure S4). AFM images and selected cross-sectional profiles, with and without Ag-Ncu (Figure S5). Photocurrent action spectra of ITO/P3HT:PCBM, ITO/P3HT:PCBM/Ag-Nsp, and ITO/ P3HT:PCBM/Ag-Ncu (Figure S6). PL emission of P3HT at 470 nm excitation wavelength measured at five locations (Figure S7) and the 3D PL spectrum of plain P3HT with excitation wavelength from 360−560 nm (Figure S8). This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

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5. CONCLUSIONS In this study, we have fabricated large-area arrays of P3HT/AgNcu photoelectrodes using the Langmuir−Blodgett technique and examined the correlation between PL and photocurrent enhancements. Our results suggest that an optimum Ag-Ncu surface coverage of 12% and 14% is necessary for maximum enhancement of photocurrents and PL. The enhancements are attributed to strong local electric field as well as far field lightscattering properties of Ag-Ncu which appear stronger than AgNsp of the same size. PL lifetime decay results confirm plasmon−exciton interactions which are critical to photocurrent enhancement.

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AUTHOR INFORMATION

Corresponding Authors

*S.Y. Tel.: +81-92-802-2812. Fax: +81-92-802-2815. E-mail: [email protected]. *K. L. Tel.: +81-92-802-2817. Fax: +81-92-802-2815. E-mail: [email protected]. Notes

The authors declare no competing financial interest. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Center of Advanced Instrumental Analysis, Kyushu University, for SEM and TEM measurements. The present study was financially supported by JSPS KAKENHI (No. 12345678), Grants-in-aid for Scientific Research (No. 24651145 SY and 25870510 for YT) from JSPS, and Research grants from Asahi Glass foundation f (for YT). Nanotechnology Network Project (Kyushu Area Nanotechnology Network) G

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