January 13, 2011
C 2011 American Chemical Society
Surface Plasmonic Effects of Metallic
Nanoparticles on the Performance
of Polymer Bulk Heterojunction
Solar Cells
Jyh-Lih Wu,†Fang-Chung Chen,‡,* Yu-Sheng Hsiao,§Fan-Ching Chien,§Peilin Chen,§Chun-Hong Kuo,^ Michael H. Huang,^and Chain-Shu Hsu)
†Department of Photonics and Institute of Electro-optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan,‡Department of Photonics and
Display Institute, National Chiao Tung University, Hsinchu 30010, Taiwan,§Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan, ^Department of Chemistry, National Tsing Hua University, Hsinchu 3001, Taiwan, and )Department of Applied Chemistry, National Chiao Tung University,
Hsinchu 30010, Taiwan
O
rganic photovoltaic devices (OPVs) are promising alternative tools for harnessing renewable energy be-cause of their properties including light-weight, low-cost, low-temperature fabrica-tion, semi-transparency, and mechanical flexibility. To date, the most representative high-performance OPVs reported in the literature have been fabricated using the bulk heterojunction (BHJ) concept,1-6 where a light-absorbing polymer (donor) and a solu-ble fullerene (acceptor) form a three-dimen-sional matrix possessing a large-area phase-separated interface for efficient exciton dissociation.7,8The resultant power conver-sion efficiencies (PCEs) of these polymer/ful-lerene BHJ OPV devices have reached as high as 6-8%,1-4opening up the possibility fortheir practical use asflexible, low-cost, renew-able energy systems. As a result, increasing effort is being exerted in the quest for higher-performance OPVs.
The overall efficiency of an OPV device is governed by its internal quantum efficiency and absorption efficiency.9,10 The former, which is influenced by the diffusion and dissociation of photogenerated excitons and charge collection at the electrode con-tacts, can approach 100%.3 Therefore, ab-sorption efficiency of incoming light in these devices remains one of the major limitations toward realizing high external quantum efficiencies and PCEs. Typically, the optimum thickness of the active layer for an OPV device is on the order of 100-200 nm, or possibly less; such a thin layer can lead to low absorption of light. Therefore, increasing the thickness of the active layer is one possible approach toward
more efficient light absorption. A thicker layer, however, inevitably increases the de-vice resistance, due to the low carrier mo-bilities and short exciton diffusion lengths of organic materials.11,12This situation
im-poses a trade-off between light absorption and charge transport efficiencies in OPV devices, motivating the development of a variety of light-trapping techniques.
Efficient light trapping in the active layer enhances photon absorption without the need for a thickfilm. The introduction of an optical spacer that spatially redistributes the light intensity in the device can result in enhanced light absorption.13-16Changes in
*Address correspondence to fcchen@mail.nctu.edu.tw.
Received for review September 6, 2010 and accepted January 7, 2011. Published online
10.1021/nn102295p
ABSTRACT We have systematically explored how plasmonic effects influence the characteristics of polymer photovoltaic devices (OPVs) incorporating a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). We blended gold nanoparticles (Au NPs) into the
anodic buffer layer to trigger localized surface plasmon resonance (LSPR), which enhanced the performance of the OPVs without dramatically sacrificing their electrical properties. Steady state photoluminescence (PL) measurements revealed a significant increase in fluorescence intensity, which we attribute to the increased light absorption in P3HT induced by the LSPR. As a result, the rate of generation of excitons was enhanced significantly. Furthermore, dynamic PL measurements revealed that the LSPR notably reduced the lifetime of photogenerated excitons in the active blend, suggesting that interplay between the surface plasmons and excitons facilitated the charge transfer process. This phenomenon reduced the recombination level of geminate excitons and, thereby, increased the probability of exciton dissociation. Accordingly, both the photocurrents andfill factors of the OPV devices were enhanced significantly. The primary origin of this improved performance was local enhancement of the electromagneticfield surrounding the Au NPs. The power conversion efficiency of the OPV device incorporating the Au NPs improved to 4.24% from a value of 3.57% for the device fabricated without Au NPs.
KEYWORDS: polymer photovoltaics • gold nanoparticles • surface plasmon• photoluminescence • exciton lifetime
WU ET AL. VOL. 5 NO. 2 959–967 2011 960 the device symmetry and the implementation of
peri-odic nanostructures can also increase the optical path length in the active layer.17-20Moreover, microcavity and photonic crystal effects have been proposed to effectively trap light in OPV devices.21-23
Recently, the exploitation of surface plasmon reso-nance (SPR) effects, based on advantageous optical properties such as light concentration and/or scatter-ing, has attracted much attention as a means for increasing the photocurrents of OPVs.24-32 Two pri-mary schemes are commonly employed for excitation of the SPR, which is the coherent collective oscillation of conduction electrons surrounding the metallic sur-faces. In one, surface plasmon polaritons (SPPs) propa-gating along the metal-dielectric interface are triggered by incorporating metallic nanostructures, such as periodic arrays or gratings.24-26In the other, surface plasmons are localized by noble metallic nanoparticles (NPs);such as Cu, Ag, Pt, and Au; resulting in localized surface plasmon resonance (LSPR).27-32The excitation of LSPR can be achieved
when the frequency of the incident light matches its resonance peak, resulting in unique optical proper-ties;selective light extinction as well as local en-hancement of electromagneticfield near the surface of metallic NPs. The resonance peak of LSPR depends strongly on the size, shape, and the dielectric environ-ment of the metallic NPs. Although both approaches have been employed to increase the photocurrents of OPV devices, a comprehensive understanding of such plasmonic effects in OPVs remains rare. In this study, we systematically explored how plasmonic effects influence the device characteristics of OPVs incorpor-ating a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM).
Ex-citation of the LSPR;triggered by adding Au NPs into the anodic buffer layer;significantly enhanced the overall PCEs of the OPV devices. Electrical characteriza-tion results revealed that the presence of the metallic NPs had a negligible effect on the charge transport process, suggesting that the electrical properties were not sacrificed. Moreover, steady state and dyna-mic photoluminescence (PL) measurements provided strong evidence that the LSPR induced by the Au NPs not only increased the degree of light absorption but also enhanced the degree of exciton dissociation. As a result, the photocurrent and overall device efficiency were both improved considerably after exploiting the optical effects of the LSPR. Compared with previous approaches toward plasmonic-enhanced OPV devices, which involved the nanofabrication of plasmoic nanostructures,24-26the method we report herein is simpler (solution processable) and also suitable for the applications to the large-area processes.
RESULTS AND DISCUSSION
Photovoltaic Characteristics. We used solution-proces-sable Au NPs to trigger the LSPR; the average particle size was ca. 45( 5 nm, estimated from a scanning electron microscopy (SEM) image (inset of Figure 1). The extinction spectrum of the Au NPs, determined using UV-vis spectroscopy, is displayed in Figure 1. The resonance of the Au NPs in solution was located at ca. 550 nm. The plasmonic resonance regime of the Au NPs was close to the absorption peak of the P3HT/PCBM blends, thereby suggesting enhanced light-harvesting efficiency. Figure 2a displays the architecture of our plasmonic-enhanced OPV devices. The presence of Au NPs embedded in the anodic buffer layer was evident from the SEM images recorded for the modified (Figure 2c) and pristine (Figure 2b) poly(3,4-ethylene-dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) layers. Notably, the distribution of the Au NPs (white dots) in the modified PEDOT:PSS layer was uniform; we observed no apparent aggregation between the Au NPs in the film prepared using this simple method.
Figure 3a displays the representative current density -voltage (J-V) characteristics, recorded under
Figure 1. UV-vis extinction spectrum of the solution-processable Au NPs. Inset: Corresponding SEM image. Average Au NP size =ca. 45 ( 5 nm.
Figure 2. (a) Device architecture of the OPV incorporating Au NPs in the PEDOT:PSS layer. (b,c) SEM images of anodic buffer layers prepared with (b) pristine PEDOT:PSS and (c) PEDOT:PSS featuring embedded Au NPs. The Au NPs appear as white dots in (c).
100 mW cm-2illumination (AM 1.5G), of OPV devices prepared using buffer layers with and without Au NPs. The reference device possessing the structure ITO/ PEDOT:PSS/P3HT:PCBM/Ca/Al exhibited character-istics comparable to those of previously reported devices,20,29with an open-circuit voltage (Voc) of 0.59 V,
a short-circuit current (Jsc) of 9.16 mA cm-2, and afill
factor (FF) of 66.06%, resulting in a PCE of 3.57%. To take advantage of the unique optical properties of the LSPR, we blended the Au NPs into the anodic buffer layer (denoted herein as the “plasmonic device”). The value of Vocfor the plasmonic device remained
at 0.59 V, suggesting the unchanged nature of the electrode-organics interface. In contrast, the values of Jsc and FF both increased, to 10.22 mA cm-2 and
70.32%, respectively. Notably, the device resistance, extracted from the J-V curves in the dark, increased
slightly from 1.79 to 1.82Ω 3 cm2after incorporating the Au NPs. This potential drawback was, however, overwhelmed by the advantageous plasmonic effects. Overall, the plasmonic device's PCE was 4.24%.
Figure 3b presents incident photon-to-electron conversion efficiency (IPCE) curves for these devices. We also compared the curve of the increase in IPCE (ΔIPCE) after incorporating Au NPs with the extinction spectrum of the Au NPs (Figure 3c). The photocurrent within the wavelength range from 450 to 650 nm increased significantly after incorporating the Au NPs. This wavelength regime coincides with the extinction range of the Au NPs, indicating that LSPR effects did indeed improve the photocurrent.
Photocurrent Behavior. To further explore the effects of LSPR in OPV devices, we determined the maximum exciton generation rate (Gmax). The devices were
biased sweeping fromþ1 to -10 V. Figure 4a reveals the dependence of the photocurrent density (Jph) on the
effective voltage (Veff), recorded under illumination at 100
mW cm-2, for our reference and plasmonic devices. Here Jph ¼ JL- JD
where JLand JDare the current densities under
illumina-tion and in the dark, respectively, and Veff ¼ Vo- Va
where Vois the voltage when Jphequals zero (i.e., JL=
JD) and Va is the applied voltage. Apparently, Jph
Figure 3. (a)J-V characteristics, recorded under illumina-tion at 100 mW cm-2(AM 1.5G), of polymer solar cells with (plasmonic device) and without (reference device) Au NPs in the PEDOT:PSS layer. (b) Corresponding IPCE curves of these OPV devices. (c) Comparison between the curve of the increase in IPCE (ΔIPCE) after incorporating Au NPs and the extinction spectrum of the Au NPs.
Figure 4. (a) Photocurrent density (Jph) plotted with respect
to effective bias (Veff) for the reference and plasmonic
devices. (b) Exciton dissociation probability [P(E,T)] plotted with respect to effective bias (Veff) for these OPV devices.
The dissociation probabilities for the reference and plas-monic devices under short-circuit conditions were 78.2 and 84.2%, respectively.
WU ET AL. VOL. 5 NO. 2 959–967 2011 962 linearly increased with the voltage at a low value of Veff
and then saturated at a sufficiently high value of Veff.
From Figure 4a, we therefore determined the values of the saturation photocurrent density (Jsat), which is
independent of the bias and temperature. Further-more, assuming that all of the photogenerated exci-tons dissociated and contributed to the current in the saturated regime due to the sufficiently high electric field, we obtained the values of Gmaxusing the
equation
Jsat ¼ qGmaxL
where q is the electronic charge and L is the thickness of the active layer.5,33The values of Gmaxfor the reference
and plasmonic devices were 3.96 1027m-3s-1(Jsat=
114 A m-2) and 4.27 1027m-3s-1(Jsat= 123 A m-2),
respectively. Thus, a noticeable enhancement in Gmax
occurred after incorporating the Au NPs into the device. Because the value of Gmaxis a measure of the maximum
number of photons absorbed,5,33such an increase
sug-gests enhanced light absorption in the active layer of the plasmonic device.
Next, we compared the exciton dissociation prob-abilities [P(E,T)], which are related to the electricfield (E) and temperature (T), for our devices. For OPVs, when the excitons are photogenerated, only a portion of them can be dissociated into free carriers. As a result, Jphcan be expressed using the equation5,33
Jph ¼ qGmaxP(E, T)L
As a result, the value of P(E,T) at any bias can be obtained from the plot of the normalized photocurrent density (Jph/Jsat) with respect to Veff.34Figure 4b reveals
that the value of P(E,T) under the short-circuit condi-tions (Va= 0 V) increased from 79.2% for the reference
device to 84.4% for the plasmonic device, indicating that excitation of the LSPR also facilitated excitons to dissociate into free carriers. Thus, excitation of the LSPR increased both the exciton generation rate and the dissociation probability, thereby enhancing the photo-current of the OPVs.
Charge Transport Behavior. We measured the values of Jscof our OPV devices at various illumination intensities
to examine how the plasmonic effects influenced the charge transport process. Figure 5 displays the depen-dence of the incident light intensity (Plight) on the
photocurrent. In general, the value of Jscof an OPV
device follows a power-law dependence with respect to Plight(i.e., Jsc Plights).5,10From Figure 5, we
deter-mined exponential factors (s) for the reference and plasmonic devices of 0.984 and 0.980, respectively. The nearly linear dependence of Jsc on the incident light
intensity suggests the absence of bimolecular recom-bination and space-limited charges in either of these devices.5,10Therefore, we suspect that the
incorpora-tion of Au NPs into the buffer layer had a negligible influence on the nature of the charge transport process in the device. More importantly, however, the result-ing plasmonic effects resulted in unique optical prop-erties that improved the performance of the OPV devices.
Steady State Photoluminescence. To explore the effect of the LSPR on the exciton generation behavior, we performed steady state PL measurements. Because absorption in the donor phase (i.e., P3HT) contributed predominately to the exciton generation in the active blend, we recorded fluorescence spectra of the pristine P3HT films. The samples were prepared by spin-coat-ing pristine P3HT films onto the PEDOT:PSS buffer layers in the presence and absence of the Au NPs. Figure 6 presents the room temperature PL spec-tra obtained using different wavelength excitation sources (λexc = 470 and 532 nm) for the reference
and plasmonic samples. The integrating PL intensity of the plasmonic sample was enhanced by ca. 12 and 20% with respect to the reference sample when we excited the sample at 470 and 532 nm, respectively; these excitation wavelengths were selected intention-ally to be located within the resonance regime of the LSPR (see Figure 1). The fluorescence process is de-pendent on the light excitation rate and the quantum yield, which is dependent on competition between the radiative and nonradiative decaying mechanisms.35 Because the resonance frequency of the Au NPs was close to the absorption band of P3HT, we attribute the enhanced PL intensity to the fact that excitation of the LSPR increased the degree of light absorption and, thereby, enhanced the light excitation rate. This hy-pothesis is consistent with our measured device char-acteristics;namely, the greater value of Gmaxfor the
plasmonic device relative to that of the reference device. Consequently, the PL measurements sug-gested that, upon excitation of the LSPR, the enhanced excitation rate increased the density of photogener-ated excitons in P3HT, thereby enhancing the fluores-cence intensity.36,37Notably, another possible factor responsible for the enhanced PL intensity was related to the modification of excitonic states of P3HT due to
Figure 5. Values ofJscof the OPVs plotted with respect to
the light intensity. Straight lines werefitted using the expressionJsc Plights; the values ofs for the reference and
plasmonic devices were 0.984 and 0.980, respectively.
the presence of Au NPs.31This therefore accounted for the occurrence of the strong interactions between plasmonic field and excitons.
Our observation that LSPR enhanced the light absorption efficiency might result from two possible factors. First, the forward scattering lengthened the optical path in the active layer, thereby enhancing the degree of light absorption, as illustrated schematically in Figure 7a. Alternatively, excitation of the LSPR resulted in local enhancement of the electromagnetic field in the vicinity of the Au NPs (see Figure 7b); enhancement factors of up to 100 can be achieved.38,39 Therefore, the resulting local field enhancement in-creased the total number of excitons created in the active layer because the energy dissipation is propor-tional to the intensity of the electromagneticfield.40,41 For plasmonic metallic NPs, however, the particle size
plays an important role in dominating the extinction behavior (e.g., the sum of light absorption and scattering). Light scattering, which is beneficial for collecting light in inorganic solar cells,42,43becomes more dominant when using larger NPs. Typically, the light scattering efficiency is lower than the light ab-sorption efficiency for Au NPs having particle sizes of less than 100 nm.42,44Because the average particle size of the Au NPs used in this study was only 45 nm, they were presumably unable to induce efficient light scat-tering. Therefore, we conclude that the LSPR effects, induced by the presence of the Au NPs, enhanced light collection in the active layer of OPVs predominately through localfield enhancement.
Dynamic Photoluminescence. We employed time-resolved PL spectroscopy to acquire insight into the coupling process between the plasmonic field and the photogenerated excitons within the photoactive blend. Through measurements of exciton lifetimes (τexciton), the dynamic PL signal allowed us to probe
how the plasmonic effects influenced the photophysi-cal processes between the donor and acceptor phases.45-47Figure 8a displays the PL intensity decay profiles, obtained using a 470 nm excitation source, of P3HT/PCBM films that had been deposited on PEDOT:PSS layers in the presence and absence of Au NPs. The PL decay data were fitted using the following
Figure 6. PL spectra of the reference and plasmonic samples recorded using excitation source wavelengths (λexc) of (a) 470 and (b) 532 nm.
Figure 7. (a) Schematic representation of light trapping through forward scattering as a result of Au NP induced LSPR. The optical path length increased because light was trapped through multiple and high-angle scattering. (b) Schematic representation of the local enhancement of the electromagneticfield. The plasmonic field decays exponentially with respect to the distance from the surface of the Au NPs.
Figure 8. (a) PL decay profiles for the P3HT/PCBM blends in the reference and plasmonic samples. Excitation source: 470 nm pulsed laser. Inset: Schematic representation of the charge transfer process in the P3HT/PCBM blend. For the reference and plasmonic samples, the values ofτexcitonwere
0.36 and 0.27 ns, respectively. (b) Cartoon depiction of the interplay between the LSPR and the excitons. The resulting interactions enhanced the rate of exciton dissociation, thereby reducing exciton recombination.
WU ET AL. VOL. 5 NO. 2 959–967 2011 964 multiexponential function:48 IPL(t) ¼ Xn i¼ 1 Aiexp -t τi (1) where Ai is the amplitude of the ith decay, n is the
number of decays involved, andτiis the ith exponential
constant. For the reference sample, we used two exponential constants (τ1andτ2) to obtain a good fit
to the data; their values of 0.200 and 0.558 ns, respec-tively, provided a corresponding value ofτexcitonof 0.36 ns.
For the plasmonic sample, however, we required a third exponential constant (τ3) to obtain a better fit; the
values of these three exponential constants were 0.131, 0.324, and 0.015 ns, respectively, providing a resulting lower value ofτexcitonof 0.27 ns. Note that the
introduction ofτ3was necessary presumably because
of the presence of the LSPR, which interacted with photogenerated excitons.
The dramatic change in the value ofτexcitonafter
incorporating Au NPs also accounted for the pre-sence of the strong coupling between the plasmo-nic field and excitons, which has been reported previously.31,49,50 We speculated that the resulting plasmon-exciton coupling participated in the charge transfer process, thus facilitating exciton dissociation, as illustrated in Figure 8b. The photophysical process can be further interpreted by the concept of “hot excitons” which possess excess energy to overcome their initial Coulombic potential.31,49,50Therefore, the generation of hot excitons is beneficial for enhancing the probability of dissociation into free polarons.31,49,50 Previous reports have shown that the plasmonicfield
can strongly modify the dynamic properties of photo-generated excitons through the plasmon-exciton coupling.51,52This phenomenon presumably increased the amount of hot excitons for efficient dissociation.31 As a result, the enhanced degree of exciton dissocia-tion reduced the recombinadissocia-tion level of geminate excitons through radiative and/or nonradiative pro-cesses and, thereby, improved the FFs of OPVs.53,54 Note that the device resistance for the plasmonic device increased slightly relative to that of the refer-ence device; therefore, it was not responsible for the enhanced FF. Consequently, we assigned the in-creased FF observed in Figure 3a primarily to the increased value of P(E,T) resulting from the interplay between the plasmonicfield and the photogenerated excitons.
We performed exciton lifetime mapping by com-bining the time-resolved PL measurement system with confocal laser scanning microscopy to further investi-gate the plasmonic effects.45,47This technique enabled us to examine the distribution of the exciton lifetimes on any given z-axial planes (close to or away from the PEDOT:PSS layer, in which the Au NPs were embedd-ed). As a result, we could obtain the vertical evolution ofτexcitonin the P3HT/PCBMfilms. Figure 9 displays the
exciton lifetime images (15 μm 15 μm) for the reference and plasmonic samples (P3HT/PCBMfilms). The values ofτexciton, both close to or away from the
PEDOT:PSS layer, generally decreased after incorporat-ing the Au NPs. To quantify the average values and distributions of the exciton lifetimes, we constructed the corresponding statistical histograms for these
Figure 9. Exciton lifetime images for the reference (a,c) and plasmonic (b,d) samples. (a,b) Positions away from the PEDOT:PSS layer; (c,d) positions close to the PEDOT:PSS layer.
samples. For the reference sample, the average value ofτexcitondecreased from 0.48 ns close to the PEDOT:
PSS layer (Figure 10c) to 0.35 ns away from the PEDOT: PSS layer (Figure 10a), presumably because the differ-ence between the surface energies of P3HT and PCBM resulted in an inhomogeneous distribution of PCBM molecules in the P3HT matrix when the blend under-went solvent annealing.45The average values ofτexciton
decreased significantly (to 0.27 and 0.23 in panels b and d in Figure 10, respectively) after incorporating the Au NPs; notably, the average value ofτexcitonat the
position close to the PEDOT:PSS layer was even lower than that away from the PEDOT:PSS layer. Therefore, for the plasmonic sample, we observed a reverse trend in the evolution ofτexciton. This result is not surprising
because the plasmonicfield decays exponentially with respect to the distance from the surface of the metallic NPs.55,56 Therefore, stronger interplay between the plasmonic field and excitons occurred at positions close to the PEDOT:PSS layer, due to the localized enhanced nearfield in the proximity of the Au NPs. Notably, the time-resolved PL measurement data, in which LSPR changed the lifetime of the excitons in the active blend, further support the field enhancement
mechanism that we proposed above because we would not expect the scattering scheme to influence the exciton lifetime significantly.
CONCLUSIONS
Efficient light absorption in thin film solar cells is critical for their high performance;especially for those based on organic materials, where the thickness of the active layer is relatively thin. In this study, we improved the device PCEs of OPVs by incorporating Au NPs in the PEDOT:PSS buffer layer. The degree of light absorption in the plasmonic-enhanced OPV device increased significantly as a result of LSPR-induced local field enhancement. Moreover, interactions between the plasmons and the photogenerated excitons resulted in an enhanced degree of exciton dissociation, thereby reducing the level of exciton loss through geminate recombination. Without sacrificing the electrical proper-ties, the incorporation of the Au NPs allowed the unique optical properties of the LSPR to improve the perfor-mance of the OPV. We believe that the results of this study might pave the way toward higher-efficiency OPV devices, and that this approach might also be applicable to systems featuring other types of active materials.
METHODS
Device Fabrication. The OPV device was fabricated on an indium tin oxide (ITO)-coated glass substrate. After a routine cleaning process, the substrate was dried overnight in an oven
and treated with UV ozone prior to use. To prepare the composite buffer layer, a Au NP solution was blended into the PEDOT:PSS (Baytron 4071) solution; the doping concentration of Au NPs inside the PEDOT:PSS solution was ca. 2 1011cm-3.
Figure 10. Histograms corresponding to the exciton lifetime images in Figure 9. Average values ofτexciton: (a) 0.35, (b) 0.27, (c)
0.48, and (d) 0.23 ns.
WU ET AL. VOL. 5 NO. 2 959–967 2011 966
The Au NP solution was prepared using procedures described previously.57The anodic buffer layer was deposited through spin-coating onto a ITO-coated glass substrate and then ther-mally annealed at 120C for 1 h. To reduce the surface rough-ness of the composite buffer layer, an additional thinner layer of PEDOT:PSS was spin-coated as a capping layer. For comparison, the thickness of each buffer layer was set at ca. 50 nm. In a nitrogen-filled glovebox, a solution of P3HT (Rieke Metals) and PCBM (Solenne) dissolved (each at 17 mg mL-1) in 1,2-dichloro-benzene (DCB) was spin-coated on top of the buffer layer. The wet film was subjected to solvent annealing in a glass Petri dish,6followed by a thermal annealing at 110C for 15 min. The
thickness of the resulting photoactive layer was ca. 180 nm. To complete the device, a cathode (30 nm Ca and 100 nm Al) was deposited through thermal evaporation under a vacuum of ca. 6 10-6Torr. The device area, defined through a shadow mask, was 0.12 cm2. The completed device was encapsulated with a cover glass and sealed with a UV-curing epoxy resin prior to device testing.
Characterization. The J-V characteristics of the devices were measured using a Keithley 2400 source measure unit. The photocurrent response was obtained under illumination from a 150 W Thermal Oriel solar simulator (AM 1.5G). The illumina-tion intensity was calibrated using a standard Si photodiode equipped with a KG-5 filter (Hamamatsu).58The IPCE
mea-surement system (Enli Technology) comprised a quartz-tungsten-halogen (QTH) lamp as the light source, a mono-chromator, an optical chopper, a lock-in amplifier, and a cali-brated silicon photodetector. The light intensity dependence study was conducted using neutral density filters to tune the light intensity. The dimensions of the Au NPs were measured using a JEOL JSM-7000F scanning electron microscope. Absorp-tion spectra were measured using an UV/vis/near-IR spectro-meter (PerkinElmer Lambda 950). The steady state PL spectra were recorded in the transmission mode under air ambient conditions; continuous-wave (CW) lasers (470 and 532 nm) were used as the excitation sources. The fluorescence emission signal was measured using a high-resolution spectrometer (HR4000, Ocean Optics). For the time-resolved PL measurements, the samples were excited using a 470 nm pulsed laser; the dynamic signal was recorded using a time-correlated single photon counting (TCSPC) spectrometer (LDH-P-C-470, Picoquant). For mapping of exciton lifetime images, the confocal laser scanning microscope (FV300, Olympus Corporation) was equipped with a single photon avalanche diode detector (PDM series, Picoquant).
Acknowledgment. We thank the National Science Council of Taiwan (NSC 98-3114-E-009-005 and NSC 99-2221-E-009-181) and the Ministry of Education of Taiwan (through the ATU program) forfinancial support.
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