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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 1166

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Effects of silver nanoparticles with different sizes on photochemical responses of polythiophene– fullerene thin films† Jing You,a Kwati Leonard,a Yukina Takahashi,ab Hiroaki Yonemuraab and Sunao Yamada*ab Effects of size and coverage density of silver nanoparticles (AgPs) on the fluorescence emission and fluorescence lifetime of poly(3-hexylthiophene-2,5-diyl) (P3HT) thin films were investigated. AgPs of 64 nm diameter showed greater effects on the fluorescence decay process of P3HT films as compared with 7 nm AgPs. The fluorescence lifetime (FL) of P3HT decreased from 0.61 to 0.22 ns in the presence of 64 nm AgPs, while no appreciable change (0.60 ns) was seen in the case of 7 nm AgPs. The results suggest that the 64 nm AgPs showed a greater effect on the enhancement of the decay rate of excited P3HT. The photoelectric conversion of thin films consisting of P3HT and phenyl-C61-butyric acid methyl ester (PCBM) was also

Received 6th August 2013, Accepted 11th November 2013

investigated. AgPs of 7 or 64 nm diameters were first deposited on indium-tin-oxide substrates with

DOI: 10.1039/c3cp53331j

controlled surface coverage densities from B1 to 40%. When the coverage densities of deposited AgPs were B20% for both 7 and 64 nm, the enhancement of photoelectric conversion efficiency reached

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maximum. The degree of enhancement in the case of 64 nm AgPs was larger than in the case of 7 nm AgPs.

Introduction The development of organic thin film solar cells (OSCs) is one of the most important fields because of their unique advantages such as simple fabrication with wet processing, inexpensive materials, light-weight and so on.1–5 At present, the most important issue in OSCs is to increase the photoelectric conversion efficiency.6 Recently, application of silver or gold nanoparticles (AgPs or AuPs) for improving the efficiency of photoelectric conversion has been widely studied in polythiophene–fullerene thin films.7–12 We previously reported that both AuPs and AgPs are capable of improving the photoelectric conversion in the organic thin films.12–17 It is well known that scattering from metal nanoparticles can improve the light absorption by trapping light in the active layer of OSCs.18–21 Also, the localized surface plasmon resonance (LSPR) property of metal nanoparticles (MNPs) can increase the excitation of molecules around nanoparticles.22–24 Light scattering properties and LSPR are substantially dependent on the size and shape of metal nanoparticles.25–28 The size effect of AgPs and AuPs on a

Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka, Japan. E-mail: [email protected]; Fax: +81-92-802-2815; Tel: +81-92-802-2812 b Center for Future Chemistry, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka, Japan † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cp53331j

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LSPR and optical properties have been widely investigated both experimentally and theoretically.29–31 The common characteristic is that larger MNPs show higher light scattering efficiency. This means that the larger MNPs are more beneficial for trapping incident light in the active layer than the smaller MNPs.28,30,31 Absorption and scattering efficiencies of nanospheres with different sizes have been simulated by the Mie theory in many reports, and no matter AuPs or AgPs, the ratio of scattering in extinction is directly proportional to the diameter of nanoparticles.29–34 This phenomenon has been proven by comparing the optical properties of AgPs with diameters from 29 to 135 nm.35 In the photoelectric conversion systems, the MNP with a diameter larger than 40 nm generally showed an effect on the improvement of photon harvesting, which is beneficial in improving the photoelectric conversion.28,36–38 In addition, it is known that, as compared with AgPs with diameters >60 nm, the scattering of AgPs with diameter o10 nm is negligible. However, the near-field of AgPs > 60 nm was only B3 times larger than AgPs o 10 nm. For AgPs > 60 nm, increasing the diameter results in the decrease of near-field enhancement.39 The large (>60 nm) and small (o10 nm) AgPs will show different behaviours based on far-field and near-field effects. Although the size effect on the optical property of AgPs has been reported, its effect on the photoelectric conversion of polythiophene–fullerene thin films has not been clarified. In this study, the AgPs were protected by citrate ions, the diameters of larger AgPs were distributed in a wide range. In order to avoid

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the overlap of diameter distribution of small and large AgPs, we synthesized two kinds of AgPs with average diameters of 64 and 7 nm; then investigated their effects on the fluorescence decay of polythiophene and the photoelectric conversion of polythiophene–fullerene thin films.

Experimental section Materials Silver nitrate (AgNO3, Wako), trisodium citrate dihydrate (Na3C6H5O72H2O, Wako), sodium borohydride (NaBH4, Wako), hydrogen peroxide (H2O2, 30%, Wako), aqueous ammonia (NH4OH, 28%, Wako), poly(ethyleneimine) (PEI, Mw = 5000–100 000, Wako), regioregular poly(3-hexylthiophene-2,5-diyl) (rr-P3HT, Aldrich), phenyl-C61-butyric acid methyl ester (PCBM, Nanom Spectra E100), and other agents were used as received. AgPs were synthesized as reported previously.14,16,40 In the case of 64 nm AgPs, 38 mg of AgNO3 was dissolved in 200 mL of ultrapure water. After refluxing under a N2 atmosphere, 4 mL of 1 wt% aqueous solution of Na3C6H5O72H2O was added and then the mixture was refluxed for 30 min giving the colloidal solution of AgPs with an average diameter of 64.1  1.4 nm (average  standard error, n = 40). In the case of 7 nm AgPs, on the other hand, 100 mL of an ice-cold aqueous solution of NaBH4 (0.08 mg mL 1) with 14.4 mg of Na3C6H5O72H2O was dropped into 100 mL of an ice-cold aqueous solution of AgNO3 (0.17 mg mL 1) and the mixture was stirred until it reached room temperature, giving the colloidal solution of AgPs with an average diameter of 7.4  0.3 nm (average  standard error, n = 40). The 7 nm colloidal solution of AgPs was diluted with ultrapure water in a ratio of 1 : 3, whereas the 64 nm AgPs colloids were used as prepared. Average diameters were determined using a transmission electron microscope (JEM-200 CX, JEOL) and images are shown in the inset of Fig. 1 (diameter distribution is shown in ESI† I). Preparation of samples for fluorescence measurements Quartz plates were washed by sonication in acetone and ethanol for 15 min each. The cleaned quartz plates were boiled in a mixture of H2O2 : NH4OH with the volume ratio of 1 : 1

Fig. 1 Extinction spectra of AgP colloidal solution with an average diameter of 64 nm (solid line) and 7 nm (dashed line); (inset) TEM images of AgPs.

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followed by treating with ozone (UV 263E, Filgen) for 15 min, to give a hydrophilic surface. After that, these plates were preserved in ultrapure water, and dried with N2 gas just before using. The hydrophilic quartz plates were immersed for 20 min in an aqueous solution of PEI with the concentration of 40 mg mL 1 containing 0.2 M NaCl at 30 1C, and then washed twice with ultrapure water followed by drying with N2 gas, giving the positively charged PEI-modified quartz plates. Next, the PEI-modified quartz plates were immersed into the colloidal solution of AgPs (64 or 7 nm) with different immersion times. The excess AgPs were removed by washing with ultrapure water. Finally, a chlorobenzene solution of P3HT (15 mg mL 1) was spin-coated on the top of the deposited AgPs at 2000 rpm for 30 s. Film thicknesses of P3HT layers were distributed in the range of 60–100 nm.41 Preparation of electrodes Indium-tin-oxide (ITO) coated glass plates (%7 O sq 1, Ra % 1 nm, Kuramoto Co. Ltd) were cleaned and preserved as described above. The positively charged surface of the ITO electrode was also obtained by modification with PEI as described above. Then, the 64 and 7 nm AgPs were deposited on ITO/PEI electrodes. Finally, a chlorobenzene solution of P3HT : PCBM (15 : 11 mg mL 1) was spin-coated on the AgP-deposited ITO electrode at 2000 rpm for 30 s to obtain P3HT:PCBM films with thickness in the range of 100–130 nm. The film thickness was estimated from the absorption spectra (see the ESI† II). Measurements Extinction spectra were measured using a UV-vis spectrophotometer (JASCO V-670). Surface morphologies of deposited AgPs were observed using a field emission scanning electron microscope (FE-SEM; Hitachi High-Technologies, SU8000) with an accelerating voltage of 1 kV. Fluorescence spectra were measured using a spectrofluorophotometer (JASCO FP-6600) at an excitation wavelength of 530 nm and a filter of 567 nm. The slit width for excitation and emission was 5 nm. The measurement area was 0.28 cm2 and the beam angle was 601. Scattering spectra were measured in synchronous scanning mode of the same equipment and under the same conditions without a filter. Fluorescence lifetime (FL) measurements were carried out using a single-photon counting system (Hamamatsu Photonics C4780) equipped with a dye laser (USHO DL-50, 530 nm, Coumarine 500), and pumped by N2 laser (USHO KEN-800, 337.1 nm) with the beam angle of 451. The coverage densities of deposited AgPs were 55% (64 nm) and 47% (7 nm), respectively. Both the fluorescence and fluorescence lifetime measurements were carried out from the quartz-plate side. Photocurrent measurements were carried out with a three-electrode photo electrochemical cell using a potentiostat (Huso. HECS 318). An aqueous solution of 0.1 M NaClO4 with saturated oxygen as an electron acceptor was used as an electrolyte. The modified ITO substrate, a Ag/AgCl (sat. KCl), and a platinum wire were used as working, reference and counter electrodes, respectively. Monochromatic light from a 150 W Xe lamp irradiated the modified ITO with the beam angle of 01 and the measurement

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area of 0.28 cm2. All the measurements were carried out at the applied potential of 0 V versus Ag/AgCl.

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Characterization of AgP-modified ITO electrodes The normalized extinction spectra of the colloidal solution of AgPs are shown in Fig. 1, in which the plasmon bands of AgPs shift from B398 nm to B418 nm upon increasing the average diameter from 7.4  0.3 to 64.1  1.4 nm determined from TEM images.42 The full width at half maximum of the plasmon band was also doubled. This might be because the diameter distribution of 64 nm AgPs is broader than the case of 7 nm AgPs (see ESI† I). The AgPs were deposited on the PEI-modified ITO electrodes with different immersion times from 2 to 10 h. The coverage densities of deposited AgPs were determined from SEM images, which were from 1 to 39% for 64 nm AgPs and from 4 to 42% for 7 nm AgPs, respectively. The extinction spectra of AgPs deposited on the ITO substrates are shown in Fig. 2. In the case of 64 nm AgPs (Fig. 2(A)), the plasmon band was composed of a set of extinction bands in the regions of

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380–450 and 600–800 nm. Isolated 64 nm AgPs showed a plasmon band at B400 nm. When 64 nm AgPs were deposited on the plate, the aggregation was ubiquitous. The interaction between neighbouring AgPs resulted in two plasmon bands, one at a shorter wavelength region corresponding to the bonding dimer plasmon model and a longer wavelength region corresponding to the charge transfer plasmon model.43 The degree of separation of plasmon bands depends on the distance between two particles. Upon increasing the coverage density, the average distance between two particles decreased, and the plasmon band of the longer wavelength region became broader.44 As shown in Fig. 2(A), the plasmon band in the region of 600–800 nm became broader with increasing immersion time. In the case of 7 nm, when the immersion time was 2 h, a sharp extinction band was observed at B400 nm (Fig. 2(B)). By increasing the immersion time, the plasmon band broadens and is red-shifted. Due to the same reason as described above, the coupled 7 nm AgPs also caused a separation of the plasmon band. Theoretical investigations showed that the smaller particles have a higher resonance energy level in the charge transfer plasmon model.45,46 Therefore, the plasmon band of coupled 7 nm AgPs appeared in the region of 500–600 nm. In addition, the formation of nanoplates and nanoprisms can be identified from SEM images. These nanoplates and nanoprisms also resulted in the observed plasmon band in the region of 500–700 nm.35,47 Since the plasmon band of isolated 7 nm AgPs, nanoplates and/or nanoprisms, and coupled 7 nm AgPs are distributed in the regions of 380–450, 450–700 and 500–600 nm, respectively, these plasmon bands overlapped with each other and formed a broad plasmon band in the region of 380–700 nm. As shown in Fig. 2(B), the plasmon band in the region of 500–600 nm increased with longer immersion time. Steady state fluorescence

Fig. 2 Extinction spectra of deposited AgP on ITO with immersion time varying from 2 to 10 h (64 nm (A), 7 nm (B)).

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In order to clarify the effect of AgPs on the behaviour of P3HT excitation, the fluorescence emissions of P3HT films in the presence of AgPs with different coverage densities were investigated. The scattering effect from AgPs was obtained by synchronous scanning measurements of deposited particles before coating P3HT. The coverage densities of AgPs on a quartz plate were determined from SEM images (Fig. 4). Film thicknesses of P3HT film were roughly the same for all samples as shown in ESI† III. The fluorescence emission spectra of P3HT in the presence of AgPs with different densities are shown in Fig. 3(A) and (B). The maximum enhancement of fluorescence in the presence of 64 nm AgPs was obtained at a density of B15%, and in the presence of 7 nm AgPs, it was B26%. Since the fluorescence emission was measured from the side of AgP/quartz, the exposed area of P3HT decreased with increasing density of AgPs. In spite of that, the presence of AgPs still showed an enhancement of fluorescence. The well-known effects of AgPs on fluorescence emission are as follows: the forward scattering (far-field) of AgPs can increase the utilization of photons; and the near-field enhancement of plasmon resonance can increase the excitation and emission of dye molecules within a certain distance.48 Obviously the 64 nm AgPs showed greater

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Fig. 3 Fluorescence emission spectra of P3HT film in the presence of 64 (A) and 7 (B) nm AgPs with an excitation wavelength of 530 nm; Rayleigh scattering spectra of deposited AgPs with diameters of 64 (C) and 7 (D) nm.

enhancement of fluorescence emission as compared to the 7 nm AgPs. As shown in Fig. 3(C) and (D), the scattering intensity of 64 nm AgPs was B6 times higher than 7 nm AgPs, which suggested that the 64 nm AgPs had higher far-field effect than 7 nm AgPs. However, the excitation wavelength (530 nm) is not located in the plasmon band region of 64 nm (Fig. 2(A)). It is known that the enhancement of near-field mainly occurs in the wavelength region of the plasmon band.49,50 On the other hand, the scattering intensity of 7 nm AgPs was very low (Fig. 3(D)). In addition, the deposited 7 nm AgPs has maximum plasmonic extinction almost at the wavelength of 530 nm. These results indicate that the 64 nm AgPs assisted fluorescence enhancement mainly due to the far-field effect, whereas the 7 nm AgPs due to the near-field effect. In the case of 64 nm AgPs, the longer immersion time increased the coverage density, despite that the shape of particles remained unchanged. In the case of 7 nm AgPs on the other hand, longer immersion time caused the formation of nanoplates and nanoprisms from 7 nm AgPs, possibly as a result of visible light illumination.51 The formation mechanisms of nanoplates and nanoprisms are shown in ESI† IV. We simulated the scattering efficiency of aggregated nanoparticles and nanoplates with the same volume by the DDA method as shown in ESI† V. The scattering efficiency of isolated nanoplates is much smaller than the aggregated nanoparticles with the same volume. In the present study, we immersed the PEI-modified ITO or quartz plate into the colloidal solution of AgPs from 2 to 10 h. Increase in the immersion time induced not only the increase of the amount of adsorbed AgPs but also the progress of coalescence. SEM images of deposited 64 nm AgPs are shown in Fig. 4(A)–(C). With longer immersion time, the coverage density increased from B11% to 32%. Correspondingly, the number of particles per mm2 increased from B40 to 78. The increase in coverage density is proportional to the increase in the number of particles per mm2. SEM images of deposited 7 nm AgPs are shown in Fig. 4(D)–(F)

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Fig. 4 SEM images of deposited 64 nm AgPs with different immersion times: (A) 4 h, (B) 6 h, (C) 8 h correspondingly. SEM images of deposited 7 nm AgPs with different immersion times: (D) 4 h, (E) 6 h, (F) 8 h.

(see large scale SEM images of (D)–(F) in the ESI† IV). As shown in Fig. 4(E), when the coverage density was B26%, the number of particles per mm2 was about 1900 whereas, the number of particles per mm2 was about 1600 for the sample with a density of B36% (Fig. 4(F)). This indicates that the aggregated AgPs resulted in the formation of isolated larger particles (plate and prism). In addition, the Ag nanospheres show a plasmon band at B400 nm, and Ag nanoplates show a plasmon band at B550 nm.51 The deposited 7 nm AgPs show a broad plasmon band in the region of 500–600 nm which indicated the existence of Ag nanoplates in Fig. 4(F). This result confirms that the far-field effect from the 64 nm AgPs is the primary factor for the enhancement of the fluorescence of P3HT films. Dynamic fluorescence The Fluorescence lifetimes (FL) of P3HT with and without AgPs were applied to investigate the size effect of AgPs on the exciton generation. The fluorescence lifetime was achieved by fitting the decay profile with a multiexponential function of second order (I = A1 exp(t/t1) + A2 exp(t/t2); where, I is the signal intensity, and A1 and A2 are the amplitudes of first and second components, respectively. Then, t is the time, and t1 and t2 are lifetimes of first and second components, respectively).10,52 The average lifetime (tmean) was calculated by the weighted average of each component as: tmean = [A1/(A1 + A2)]t1 + [A2/(A1 + A2)]t2 (as shown in Table 1). The results show that the average FL of P3HT film without deposited AgPs was about 0.61 ns. It is almost unchanged (0.60)

Table 1 Fluorescence lifetime of P3HT in the presence and absence of AgPs with diameters of 7 and 64 nm

Fluorescence lifetime (ns) Sample

t1

A1

t2

A2

tmean

P3HT 64 nm AgP/P3HT 7 nm AgP/P3HT

0.88 0.57 2.72

0.028 0.010 0.001

0.43 0.20 0.57

0.044 0.133 0.073

0.61 0.22 0.60

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Fig. 5 Illumination schematic of the AgP effect on the fluorescence emission process of P3HT molecules; E: excitation, Em: increase of excitation by LSPR, kr: radiative decay rate of P3HT without AgPs, krm: increase of the radiative decay rate of P3HT with AgPs, knr: non-radiative decay rate of P3HT without AgPs, knrm: increase of the non-radiative decay rate with AgPs. Fig. 6 Reduction of fluorescence lifetimes of P3HT films with 64 nm deposited AgPs as a function of particle density.

in the presence of 7 nm AgPs, but decreased substantially (0.22 ns) in the presence of 64 nm AgPs. Since the fluorescence lifetime of P3HT with deposited AgPs was also measured from the side of AgP/quartz at an excitation wavelength of 530 nm; the near-field effect for 64 nm AgPs should not be dominant. While, as shown in Table 1, the 64 nm AgPs showed a stronger effect on the increase in the decay rate of P3HT. In the case of 7 nm AgPs, the near-field effect should be dominant. First and second components of the fluorescence lifetime of P3HT slightly increased, and the ratio of the first component decreased. One possible reason is that the interaction of the near-field with excited P3HT molecules stabilized the excited state of P3HT. A similar phenomenon has been reported for the system of AgP and tetraphenylporphyrin.52 The schematic illumination for the above described effect of MNP is shown in Fig. 5, where E is the excitation efficiency of P3HT, Em the excitation efficiency caused by LSPR, kr the radiative decay rate of P3HT without AgPs, krm the radiative decay rate of P3HT with AgPs, knr the non-radiative decay rate of P3HT without AgPs, and knrm the non-radiative decay rate with AgPs. Effects of MNPs on the fluorescence behaviour of dye molecules are dependent on the distance between the molecule and MNP without considering the far-field effect.52–54 When the molecules are located close to the AgPs surface (o10 nm), the presence of AgPs mainly increase the non-radiative decay rate. When the molecules are located slightly further away from the AgPs surface (10–20 nm), the presence of AgPs increase both the radiative and non-radiative decay rates. When the molecules are far from the AgPs surface (>50 nm), the increase in excitation by AgPs was negligible.53–55 As to the 64 nm AgPs, the decrease of fluorescence lifetime might be mainly due to the increasing non-radiative decay rate. ¨rster resonance energy This is the underlying principle of Fo transfer (FRET) from the excited molecule to MNP.56,57 The extinction of deposited 64 nm AgPs with the plasmon band from 600 to 800 nm increased with higher coverage densities. This means the spectral overlap integral of the emission spectra of P3HT film and the extinction spectra of AgPs increased with higher coverage densities. As a result, the deposited AgPs with

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higher coverage density has a larger rate for the FRET. The dependence of the reduction of fluorescence lifetime on the density of AgPs is shown in Fig. 6. Here, the reduction of fluorescence lifetime was defined as the fluorescence lifetime of P3HT in the absence of AgPs subtracted from the lifetime of P3HT in the presence of AgPs. The degree of fluorescence reduction became larger for higher coverage densities of deposited AgPs. Accordingly, it is concluded that the larger FRET rate will reduce the fluorescence lifetime of excited P3HT. From the results of steady and dynamic fluorescence measurements, the LSPR assisted enhancement of fluorescence emission with 64 nm AgPs is mainly due to the far-field effect, while the effect of 7 nm AgPs is more inclined to near-field. The decreasing behaviour of the fluorescence lifetime of P3HT with deposited 64 nm AgPs was due to FRET. Discrete dipole approximation (DDA) simulation of deposited AgPs with density of 20% The percentages of scattering in extinction of ITO/AgP/P3HT with the coverage density of 20% of 64 and 7 nm AgPs were investigated by DDA simulation (the Simulation conditions are shown in the ESI† VI).58 We confirmed that the simulation result agreed well with the experimental results. The extinction and scattering efficiency spectra are shown in Fig. 7.

Fig. 7 Differential optical cross section of ITO/AgP/P3HT with deposited 64 nm AgPs (filled triangle: extinction; opened triangle: scattering) (A); and 7 nm AgPs (filled square: extinction; opened square: scattering) (B).

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In Fig. 7(A), high values of excitation efficiency around 500 nm were due to the absorption of ITO and P3HT films. High values of scattering efficiency around 400 nm were due to the presence of deposited AgPs. The 64 nm AgPs showed a higher scattering in the extinction ratio than the 7 nm ones at the same density (Fig. 7(B)). As described above, the stronger scattering and LSPR of 64 nm AgPs leads to a larger effect on the fluorescence of P3HT than the 7 nm ones. Because the light was irradiated from the side of deposited AgPs, the low coverage density of AgPs mainly increased the photon harvest and excitation of P3HT by LSPR, while the high coverage density of the AgPs blocked a part of incident light and reduced the fluorescence of P3HT by FRET. The presence of 64 nm AgPs showed stronger enhancement of fluorescence emission mainly because of the stronger scattering as compared with 7 nm AgPs. Photoelectrochemical measurements of P3HT:PCBM films in the presence and absence of AgPs As reported before, incorporation of AgPs between the surface of the ITO electrode and P3HT:PCBM film showed an improvement in photoelectric conversion.16,17 In this study, the effect of deposited AgPs with different diameters (7 and 64 nm) on the photocurrent of P3HT:PCBM films were investigated. The extinction and IPCE% spectra of P3HT:PCBM in the absence and presence of 64 and 7 nm AgPs are shown in Fig. 8(A)–(C), respectively. As shown in Fig. 8(A), there are two peaks in IPCE% spectra of P3HT:PCBM film, one is at B500 nm corresponding to the maximum absorption of P3HT, and the other is at B620 nm corresponding to the p–p stacking of thiophene rings.59 With deposited 64 nm AgPs, the extinction intensity increased in the region of 400–500 nm and the IPCE% also increased correspondingly (Fig. 8(B)). On the other hand, the increase in extinction intensity with the deposited 7 nm AgPs

Fig. 8 (A) IPCE% (left, filled circle) and extinction (right, solid line) spectra of ITO/P3HT:PCBM; (B) IPCE% (left, filled triangle) and extinction (right, dashed line) spectra of ITO/AgP/P3HT:PCBM with the AgP size of 64 nm; (C) IPCE% (left, filled square) and extinction (right, dotted line) spectra of ITO/AgP/P3HT:PCBM with the AgP size of 7 nm; (D) the enhancement factor as a function of wavelength (filled triangle for 64 nm AgP, filled square for 7 nm AgP).

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was slightly lower than in the case of 64 nm AgPs, and the IPCE% increased but lesser than in the latter case (Fig. 8(C)). The enhancement factor is defined as the ratio of the IPCE% of the P3HT:PCBM film with AgPs to that without AgPs. The IPCE% spectra and enhancement factor at each wavelength of P3HT:PCBM without AgPs and with deposited AgPs for an immersion time of 6 h are shown in Fig. 8(D). In both cases of 64 and 7 nm AgPs, IPCE% values increased in the presence of deposited AgPs. With deposited 64 nm AgPs, the IPCE% might be modified by several approaches: first the near-field effect might increase the IPCE% in the region 400–500 nm and longer than 700 nm; second the forward scattering might increase the IPCE% in the region of 400–700 nm and the back scattering might decrease the light absorption of P3HT; third the FRET might cause energy losses. In the case of 7 nm AgPs, the plasmon band of deposited AgPs coincides with the maximum excitation band of P3HT. This suggests that the near-field effect is dominant in the process of LSPR assisted photoelectric conversion. Although the far-field effect of 7 nm was insignificant as compared to 64 nm AgPs, a maximum enhancement factor of B2 was observed probably due to the near-field effect. The effect of particle density on the enhancement of plasmon assisted photoelectric conversion has been reported in other systems.12,32,33,60,61 In the system of AuP–polythiophene, the enhancement reached maximum when the density of AuP was B20%.12 In the system of AuP assisted dye-sensitized photocurrent, the enhancement factor increased with increase in the ratio of the particle area to the electrode area when the ratio was o0.3.32 In both cases, the average distance between two AuPs affect the plasmon assisted enhancement of photocurrent. The electric-field enhancement greatly increased when the distance of two AuPs decreased. This indicates that the enhancement of the electric-field is one of the most important aspects for plasmon enhancement of photoelectric conversion efficiency. However, the difference between AuP and AgP is that the AgP has a strong tendency to aggregate. In the system of AgP–zinc tetraphenyl porphyrin, there is maximum enhancement of photocurrent when the density of AgP was B20%. The possible reason was the quenching effect and competition of charge transfer between AgP and oxygen. In this work, the dependence of the enhancement factor on particle density was also investigated. Using the same procedure, the enhancement factors of IPCE% at a wavelength of 520 nm in the presence of 7 and 64 nm AgPs in the immersion times from 2 to 10 h were calculated, as shown in Fig. 9. It is clear that, the enhancement factor increased with increasing coverage density of deposited AgPs and reached maximum when the density was B15–25%, and then monotonically decreased in both cases. This is similar to results of other systems. The maximum enhancement factor in the presence of 64 nm AgPs was estimated to be B3 and that of 7 nm to be B2. As discussed in the section on dynamic fluorescence, the higher density of deposited 64 nm AgPs showed a higher FRET rate, probably causing more energy losses. However, the increase in coverage density also causes larger near-field and far-field

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the increase in the fluorescence decay rate of polythiophene film as compared with the case of 7 nm.

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Acknowledgements The authors thank the JSPS Research Fellowship for Young Scientists (for JY and KL). This work was partly supported by Grants-in-Aid for Scientific Research (No. 24651145 for SY and No. 25870510 for YT) form JSPS, Precious Metals Research Grant (for YT) of Tanaka Holdings Co., Ltd., research grants (for YT) from the Hayashi Memorial Foundation for Female Natural Scientists (for YT), the Asahi Glass Foundation (for YT), and from the Murata Science Foundation (for YT).

Fig. 9 Dependence of the enhancement factor of IPCE% on the particle density of AgPs at the wavelength of 500 nm (filled triangle for 64 nm AgP; filled square for 7 nm AgP).

effects. As a result, the enhancement factor showed saturation when the density was B20%. The case of 7 nm AgPs should be similar. The 64 nm AgPs show a greater effect on the enhancement of photoelectric conversion efficiency in the P3HT:PCBM film as compared with 7 nm AgPs. With comprehensive consideration of the fluorescence of P3HT in the presence of deposited AgPs, the enhancement of both the fluorescence emission of P3HT and the IPCE% of P3HT:PCBM with deposited 64 nm AgPs is mainly due to the far-field effect. In the case of 7 nm AgPs, on the other hand, the near-field effect dominates. Also, 64 nm AgPs showed a more obvious FRET effect, resulting in the decrease in the fluorescence lifetime of P3HT with deposited AgPs. These results suggest that the 64 nm AgPs show a better effect on photon harvesting and generation of excitons, including improvement in the photoelectric conversion of P3HT:PCBM.

Conclusions We investigated effects of LSPR on the enhancement of fluorescence of P3HT and IPCE% of P3HT:PCBM film in the presence of deposited AgPs with mean diameters of 64 and 7 nm. The enhancement of fluorescence emission by deposited 64 nm AgPs was B1.5 times higher than 7 nm AgPs. The fluorescence lifetime of P3HT film was reduced by deposited 64 nm AgPs from 0.61 ns to 0.22 ns at 55% coverage density. Whereas in the presence of 7 nm AgPs it remained almost unchanged. The enhancement factor of IPCE% was substantially dependent on the coverage density of deposited AgPs on the ITO electrode. Results of IPCE% indicated that the P3HT:PCBM film with 7 nm AgPs showed a maximum enhancement of B2 times when the density was B25%, while B3 times when the coverage density was B30% in the case of 64 nm. These results indicate that the 64 nm AgPs show greater effects on the improvement of photoelectric conversion of polythiophene–fullerene film and on

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Effects of silver nanoparticles with different sizes on photochemical responses of polythiophene-fullerene thin films.

Effects of size and coverage density of silver nanoparticles (AgPs) on the fluorescence emission and fluorescence lifetime of poly(3-hexylthiophene-2,...
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