Shape-Dependent Light Harvesting of 3D Gold Nanocrystals ... · BM-based active layers. Both...

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Shape-Dependent Light Harvesting of 3D Gold Nanocrystals on Bulk Heterojunction Solar Cells: Plasmonic or Optical Scattering Eect? Wei-Hsuan Tseng, ,Chun-Ya Chiu, § Shang-Wei Chou, Hsieh-Chih Chen, Meng-Lin Tsai, Ya-Ching Kuo, Der-Hsien Lien, Yu-Chi Tsao, § Kuo-You Huang, Chih-Ting Yeh, Jr-Hau He, Chih-I Wu,* ,Michael H. Huang,* ,§ and Pi-Tai Chou* ,Department of Chemistry, National Taiwan University, Taipei 106, Taiwan Department of Electrical Engineering and Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 106, Taiwan § Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan * S Supporting Information ABSTRACT: In the work, mechanisms behind various 3D nanocrystals enhanced performance of bulk heterojunction solar cells were studied comprehensively. Four types of gold nanoparticles (NPs) with distinctly dierent shapes and great uniformity were designed and synthesized, including cubes, rhombic dodecahedra (RD), edge- and corner-truncated octahedra (ECTO), and triangular plates, to systematically probe their inuences on photovoltaics. RD and triangular plates show a higher growth rate, while slower growth favors cubes and ECTO formation by controlling the reduction agent and capping ion amount. NPs with increasing corners and proper size of cross- section induce stronger near-eld coupling and far-eld scattering in P3HT:PC 61 BM-based active layers. Both nite-di erence time-domain simulation and UVvisible absorption spectra rmly support that RD exhibit the strongest localized surface plasmon resonance and optical scattering. With optimized conditions, a high power conversion eciency exceeding 4% was reproducibly achieved. 1. INTRODUCTION Over the last few decades, bulk-heterojunction (BHJ) solar cells composed of interpenetrating networks of polymer electron donor and fullerene acceptor have attracted tremendous attention due to their light weight, exibility, and potential for low-cost and large-area roll-to-roll manufacturing process. 16 The solar power conversion processes in the active layer include light absorption, exciton generation, dissociation, charge trans- port, and collection. 79 To improve sunlight harvesting, which is the principal step of conversion processes, a possible approach is to increase the thickness of the active layers. 10 However, a thicker active layer exceeding the typical value (100200 nm) encounters a serious problem in that the carrier extraction competes with charge recombination due to the low mobility and short exciton diusion lengths of organic materials. 1114 Therefore, to enhance light trapping without increasing photoharvesting layer thickness, more ecient photon absorption in the active layer should be developed. Several approaches have been reported to improve light harvesting such as the utilization of low-band-gap polymers, 15,16 the introduc- tion of an optical spacer between the active layer and metal electrode, 17,18 the use of periodic nanostructures for light trapping to further increase the optical path length, 19,20 and the development of tandem architectures. 21,22 In recent years, localized surface plasmon resonance (LSPR) in metal is of great interest in various kinds of photoelectronic applications due to the signicant electromagnetic eld enhancement near the metal surface. 2326 Introduction of metal nanoparticles (NPs), such as Cu, Au, and Ag that have free-electron-like behavior and could eectively support the LSPR eect, into solution- processed solar cells is a facile way to boost device performance. This is mainly due to the fact that NPs can be employed as subwavelength antennas to enhance the eective absorption cross-section through coupling the plasmonic near-eld to the photoactive layer. 27 Many researchers introduced NPs into organic photovoltaic (OPV) outside the active layer because they may worry that the NPs become charge recombination centers, leading to exciton quenching and nonradiative energy transfer between organic photoactive layers and metal NPs. 2733 Nevertheless, blending metal NPs directly into an active photoharvesting layer with proper concentration is still considered to be an ecient choice to promote device performance. 3438 The incident light could be reected and scattered directly within the active lm, Received: December 8, 2014 Revised: February 18, 2015 Published: March 18, 2015 Article pubs.acs.org/JPCC © 2015 American Chemical Society 7554 DOI: 10.1021/jp512192e J. Phys. Chem. C 2015, 119, 75547564

Transcript of Shape-Dependent Light Harvesting of 3D Gold Nanocrystals ... · BM-based active layers. Both...

Shape-Dependent Light Harvesting of 3D Gold Nanocrystals on BulkHeterojunction Solar Cells: Plasmonic or Optical Scattering Effect?Wei-Hsuan Tseng,†,‡ Chun-Ya Chiu,§ Shang-Wei Chou,† Hsieh-Chih Chen,† Meng-Lin Tsai,‡

Ya-Ching Kuo,‡ Der-Hsien Lien,‡ Yu-Chi Tsao,§ Kuo-You Huang,‡ Chih-Ting Yeh,‡ Jr-Hau He,‡

Chih-I Wu,*,‡ Michael H. Huang,*,§ and Pi-Tai Chou*,†

†Department of Chemistry, National Taiwan University, Taipei 106, Taiwan‡Department of Electrical Engineering and Graduate Institute of Photonics and Optoelectronics, National Taiwan University,Taipei 106, Taiwan§Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan

*S Supporting Information

ABSTRACT: In the work, mechanisms behind various 3D nanocrystalsenhanced performance of bulk heterojunction solar cells were studiedcomprehensively. Four types of gold nanoparticles (NPs) with distinctlydifferent shapes and great uniformity were designed and synthesized, includingcubes, rhombic dodecahedra (RD), edge- and corner-truncated octahedra(ECTO), and triangular plates, to systematically probe their influences onphotovoltaics. RD and triangular plates show a higher growth rate, while slowergrowth favors cubes and ECTO formation by controlling the reduction agentand capping ion amount. NPs with increasing corners and proper size of cross-section induce stronger near-field coupling and far-field scattering inP3HT:PC61BM-based active layers. Both finite-difference time-domainsimulation and UV−visible absorption spectra firmly support that RD exhibitthe strongest localized surface plasmon resonance and optical scattering. Withoptimized conditions, a high power conversion efficiency exceeding 4% wasreproducibly achieved.

1. INTRODUCTION

Over the last few decades, bulk-heterojunction (BHJ) solar cellscomposed of interpenetrating networks of polymer electrondonor and fullerene acceptor have attracted tremendousattention due to their light weight, flexibility, and potential forlow-cost and large-area roll-to-roll manufacturing process.1−6

The solar power conversion processes in the active layer includelight absorption, exciton generation, dissociation, charge trans-port, and collection.7−9 To improve sunlight harvesting, which isthe principal step of conversion processes, a possible approachis to increase the thickness of the active layers.10 However, athicker active layer exceeding the typical value (100−200 nm)encounters a serious problem in that the carrier extractioncompetes with charge recombination due to the low mobilityand short exciton diffusion lengths of organic materials.11−14

Therefore, to enhance light trapping without increasingphotoharvesting layer thickness, more efficient photonabsorption in the active layer should be developed. Severalapproaches have been reported to improve light harvesting suchas the utilization of low-band-gap polymers,15,16 the introduc-tion of an optical spacer between the active layer and metalelectrode,17,18 the use of periodic nanostructures for lighttrapping to further increase the optical path length,19,20 and thedevelopment of tandem architectures.21,22 In recent years,

localized surface plasmon resonance (LSPR) in metal is of greatinterest in various kinds of photoelectronic applications dueto the significant electromagnetic field enhancement near themetal surface.23−26 Introduction of metal nanoparticles (NPs),such as Cu, Au, and Ag that have free-electron-like behaviorand could effectively support the LSPR effect, into solution-processed solar cells is a facile way to boost device performance.This is mainly due to the fact that NPs can be employed assubwavelength antennas to enhance the effective absorptioncross-section through coupling the plasmonic near-field to thephotoactive layer.27

Many researchers introduced NPs into organic photovoltaic(OPV) outside the active layer because they may worry that theNPs become charge recombination centers, leading to excitonquenching and nonradiative energy transfer between organicphotoactive layers and metal NPs.27−33 Nevertheless, blendingmetal NPs directly into an active photoharvesting layer withproper concentration is still considered to be an efficientchoice to promote device performance.34−38 The incident lightcould be reflected and scattered directly within the active film,

Received: December 8, 2014Revised: February 18, 2015Published: March 18, 2015

Article

pubs.acs.org/JPCC

© 2015 American Chemical Society 7554 DOI: 10.1021/jp512192eJ. Phys. Chem. C 2015, 119, 7554−7564

thus increasing the optical path length in the wholespectrum range, which is stronger than passing throughpoly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS) or other buffer layers with enhancements onlyin specific frequency. The absorption of metal NPs stronglydepends on the size, shape, and dielectric environment of themetallic NPs.39−41 Among these, the shape of NPs is a criticalfactor since it directly governs the plasmonic resonance andscattering of incident light, but the relevant OPV literature isvery limited. Most reports titled “shape-dependent” are actuallythe comparison of merely sphere and rod42,43 or prism.44,45

Accordingly, a systematic study of shape-dependent polyhedralmetal NPs might open up a new avenue effectively in-corporating nanostructures for electronic applications as wellas gaining insight into both optical and electrical properties ofnanocrystals.In this regard, we have designed and synthesized four distinct

polyhedral gold NPs with different shapes, including cubes,rhombic dodecahedra (RD), edge- and corner-truncatedoctahedra (ECTO), and triangle plates, to precisely investigatetheir LSPR and optical effects on poly(3-hexylthiophene)(P3HT):phenyl-C61-butyric acidmethyl ester (PC61BM)-basedOPVs. Au NPs were chosen because the work function of goldis close to the highest occupied molecular orbital (HOMO)level of P3HT, which may further assist hole transporting inorganic film.37 Large NPs with diameters around 70 nm weredesigned, of which the scattering coefficient should dominate theextinction coefficient.46 The shape-dependent LSPR effects arecarefully analyzed by UV−visible absorption spectra and finite-difference time-domain (FDTD) simulation. Both experimentaland theoretical results are mutually in agreement that RD NPsexhibit the strongest LSPR and optical scattering. With theoptimized blend ratios, a consistent and prominent enhancementof power conversion efficiency (PCE) is demonstrated, from3.46% for the control device to 4.14% for the plasmon-enhanceddevice with NPs included. The comprehensive analyses andrationalization are elaborated as follows.

2. EXPERIMENTAL SECTION

2.1. Materials. Hydrogen tetrachloroaurate trihydrate(HAuCl4·3H2O, 99.9%, Aldrich), cetyltrimethylammoniumchloride (CTAC, 95%, TCI), sodium borohydride (NaBH4,98%, Aldrich), ascorbic acid (AA, 99.7%, Riedel-de-Haen),sodium bromide (NaBr, UCW), and potassium iodide (KI, J. T.Baker) were used as received. Ultrapure distilled and deionizedwater was used for all solution preparations. Polyvinylpyrrolidone(PVP, Aldrich), hydrochloric acid (HCl, Osaka, Japan), andsodium hydroxide (NaOH, Osaka, Japan) were used for ligandexchange. Zinc acetate dehydrate (Zn(CH3COO)2·2H2O,Aldrich, 99.999%), monoethanolamine (MEA, Aldrich, 99.7%),regioregular poly(3-hexylthiophene) (P3HT, Rieke Metals), and[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM, Nano-C)were used for device fabrication.2.2. Synthesis of Gold Seeds. A volume of 10 mL

aqueous solution containing 2.5 × 10−4 M HAuCl4 and 0.10 MCTAC was prepared. Concurrently, 10 mL of 0.02 M ice-coldNaBH4 solution was made. To the HAuCl4 solution was added0.45 mL of the NaBH4 solution with stirring. The resultingsolution turned brown immediately, indicating the formation ofgold particles. The seed solution was aged for 1 h at 30 °C untilexcess borohydride is decomposed. The seed particles haveaverage sizes of 3−5 nm.

2.3. Synthesis of Cubic, Rhombic Dodecahedral, ECTO,and Triangle Gold Nanocrystals. The synthetic proceduresfollow our previous reports.47,48 Two vials were labeled A andB. A growth solution was prepared in each of the two vials.First, a stock aqueous solution containing 0.1 M CTACsurfactant was prepared. Depending on the morphology of goldnanocrystals to be synthesized, slightly different volumes ofdeionized water were added to each vial. The vials were thenkept in a water bath set at 30 °C. To both vials were added250 μL of 0.01 M HAuCl4 solution and NaBr/KI solution(10 μL of 0.01 M NaBr for cubes and RD, 30 μL of 0.001 M KIfor ETCO, and 10 μL of 0.01 M KI for trianglar plates). Finally90 to 220 μL of 0.04 M ascorbic acid was introduced. Forexample, in the synthesis of gold nanocubes, 90 μL of ascorbicacid was used, whereas 150 μL of ascorbic acid was added forthe growth of rhombic dodecahedra. Total solution volume ineach vial was 10 mL. The solution color turned colorless afterthe addition of ascorbic acid, indicating the reduction of Au3+ toAu+ species. Next, 25 or 40 μL of the seed solution was addedto the solution in vial A with shaking until the solution colorturned light pink (∼5 s). Then 25 or 40 μL of the solution invial A was transferred to vial B with thorough mixing for ∼10 s.The solution in vial B was left undisturbed for 15 min forparticle growth and centrifuged at 6000 rpm for 10 min.

2.4. Ligand Exchange of Au Nanostructures. Forremodification of PVP on the surface of Au nanostructures,acid treatment was performed to remove CTAC from theAu nanostructures. In a typical procedure of acid treatment,HCl solution (0.1 M) was added into the solution of Aunanostructures by the volume ratio of 20%. The above solutionwas sonicated for 5 min and then neutralized with NaOH. Theneutralized Au nanostructures were separated by centrifugationat 10 000 rpm for 5 min. Then the supernatant was removed,and the residue was redispersed in water. The above procedureof separation was repeated twice. In the remodification of PVP(29 000), the PVP solution was mixed with the solution ofneutralized Au nanoparticles and then sonicated for 4−5 h. ThePVP-modified Au nanostructures were collected by centrifuga-tion at 10 000 rpm for 5 min. The aforementioned procedureof separation was repeated twice to remove excess PVP. Finally,the PVP-modified Au nanostructures were obtained and dispersedin dichlorobenzene.

2.5. Measurement and Characterization. TM-AFMimaging was carried out in air using a NanoScopeIIIa controller(Veeco Metrology Group/Digital Instruments, Santa Barbara,CA) with built-in software (version V6.13R1) to capture images.The UV−vis absorption spectra were recorded with a HitachiU-4100 spectrophotometer. Scanning electron microscopy(SEM) images of the samples were obtained using a JEOLJSM-7000F electron microscope.

2.6. Solar Cell Device Fabrication and Characterization.PSCs were fabricated with the configuration of glass/ITO/ZnO/P3HT:PC61BM/MoOx/Ag. The ITO-coated glass sub-strates (Kintec Company, 15 Ω per square) were first cleanedby ultrasonication in water with 1% neutral detergent, thendeionized water, followed by acetone, and finally 2-propanolfor 20 min each. The ZnO precursor was prepared usingZn(CH3COO)2·2H2O as starting material and isopropylalcohol and MEA as the nontoxic solvent and stabilizer undercontinuous stirring for 12 h for the hydrolysis reaction. Theresulting solutions were spin-coated on ITO at 5000 rpm for40 s and annealed at 280 °C for 10 min in ambient conditionsto remove residual organic materials. The P3HT:PC61BM

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blend solutions were prepared in 1,2-dichlorobenzene (DCB)in a weight ratio of 1:1 with a P3HT concentration of 25 mgmL−1 incorporating with 0−5% polyhedral Au NPs. The blendsolutions were heated under 60 °C and stirred for more than12 h for device fabrication. The P3HT:PC61BM films embeddedwith various kinds of NPs were then cast on top of ZnO layersin a nitrogen-filled glovebox. The thickness of the active layer isabout 230 nm, which is accurately measured by cross-sectionSEM (shown in Figure S6, Supporting Information). After slowdrying of active films for about 2 h, the samples were annealedat 140 °C for 10 min before MoOx and Ag anode deposition.Subsequently, thin MoOx films (≈1.5 nm) were thermallyevaporated (0.03 nm s−1) on the top of active layers. Finally, thesilver anodes (≈85 nm) were deposited (0.1 nm s−1) under abase pressure of 7 × 10 −6 Torr. The J−V curves were measuredwith a Newport−Oriel (Sol3A Class AAA Solar Simulators) AM1.5G light source operating at 100 mW cm−2 and independentlycross-checked using a 300 W AM 1.5G source operating at100 mW cm−2 for verification. The light intensity was determinedby a monosilicon detector (with KG-5 visible color filter)calibrated by the National Renewable Energy Laboratory (NREL)to minimize spectral mismatch. A 3 mm × 3 mm shadow maskwas used to cover the active area for avoiding interference fromboth scattering light and adjacent current leakage. The IPCEspectra were measured using a lock-in amplifier with a currentpreamplifier under short-circuit conditions. The devices wereilluminated by monochromatic light from a xenon lamp passingthrough a monochromator with a typical intensity of 30 μW.A calibrated monosilicon diode with known spectral response wasused as a reference.

3. RESULTS AND DISCUSSION

3.1. Characterization of Au Nanoparticles. The study ofshape-dependent plasmonic effect mainly relies on the homo-geneity of the as-prepared gold nanocrystals. We synthesizedgold nanocrystals with distinct shapes in aqueous solution via aprecise seed-mediated approach that was modified from ourprevious reports.47,48 A schematic illustration of the syntheticprocedure used for the preparation of Au nanocrystals withvarious shapes is shown in Scheme S1 (Supporting Information).The seed-mediated method involved the preparation of 3−5 nmAu seed solutions, followed by the addition of nanoparticle seedsinto two growth solutions consisting of gold salt, cetyltrimethy-lammonium chloride (CTAC), KI/NaBr solution, and ascorbicacid. The associated growing mechanisms can be rationalized bythe following equations.49

→ +− +HAuCl AuCl H4 4 (1)

→ +

+ −

+ −

CH (CH ) (CH ) N Cl

CH (CH ) (CH ) N Cl3 2 15 3 3

3 2 15 3 3 (2)

+

→ ‐

+ −

CH (CH ) (CH ) N AuCl

CH (CH ) (CH ) N AuCl (Orange yellow)3 2 15 3 3 4

3 2 15 3 3 4(3)

+ −

→ − + +

Radical AuCl CTA

AuCl CTA HCl Dehydroascorbic acid3

2 (5)

− +

→ + + +

AuCl CTA Ascorbic acid

Au radical HCl CTACl2

0 (6)

The formation of a stable ion-pair complex (CTA−AuCl4−),showing an orange-yellow color, provides direct evidence tosupport the proposed mechanism (eq 3). The ascorbic acidtransferred a proton to the AuCl4−CTA complex in a one-step,one-electron oxidation reduction (eq 4), followed by the radicalreduction of the AuCl3−CTA complex (eq 5) leading to theformation of stable oxidation (dehydroascorbic acid) and reduc-tion products (AuCl2−CTA complex), respectively. Under anexcess amount of ascorbic acid, the AuCl2−CTA complex couldfurther undergo one-step one-electron transfer to permit theformation of Au0 (eq 6). In this step, ascorbic acid serves asa weak reducing agent to generate Au atoms (Au+ → Au0) onthe gold seed.The realization of the above fundamental mechanisms

makes it feasible to prepare various shapes of nanocrystals bymodulating the reduction rate of the gold source.50,51 For thesynthesis of RD and cubes, different amounts of ascorbic acidserving as the reducing agent were added. More importantly,tuning of the reduction rate by slight adjustment of the amountof reducing agent could effectively control the particle shapewith uniform distribution (see Scheme S1, SupportingInformation). In this study, by introducing bromide and iodineions through the addition of NaBr and KI, ion concentration inthe growth solution could be precisely controlled withoutchanging the surfactant concentration. The underlying mecha-nism is that iodide ions with strong affinity toward metal ionscould lower the redox potential of the complexes formed.Accordingly, for the synthesis of ECTO and triangular plates,different amounts of iodide ion were added to alter thereduction potentials and hence fine tune the growth rate.Figure S1 (Supporting Information) provides photographs ofthe nanocrystal solutions at different time of check pointsamid the growth of Au ECTO nanocrystals and triagular plates.The solution turned to a dark wine solution in about 8 min inthe formation of ECTO, while the solution reached dark purplewith more red shift of SPR at the same time in the growthof triangular plates. The extent of nanocrystal growth can befollowed by simply monitoring the solution color changes,which clearly demonstrated that triagular plates were synthe-sized under a higher growth rate than ECTO. In sum, Scheme 1depicts the comparative synthetic routes of Au nanocrystals withdifferent reaction rates by varying the amount of ascorbic acid inthe presence of CTAC aqueous solution and addition of NaBror KI. RD and triangular plates were obtained at a higher growthrate, while slower growth rate favors cube and ECTO formationby controlling the reduction agent and halide ion amount.Since the growth rate can be precisely tuned, distinctly differentshapes of gold nanocrystals can thus be synthesized withexcellent uniformity.Figure 1a−d shows the scanning electron microscopy (SEM)

images of these four polyhedral as-synthesized Au NPs in thisstudy, which clearly demonstrate the well-defined edges andcorners of NPs in each kind of shape. All these four kinds ofNPs are well-dispersed in a uniform order in dilute hexane, andno obvious aggregation between NPs is observed. These NPs aregood in monodispersity with average diameters of 75 ± 2 nm for

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cubes (Figure 1a), 75 ± 3 nm for RD (Figure 1b), 65 ± 3 nmfor ECTO (Figure 1c), and 80 ± 6 nm for triangular plates(Figure 1d).UV−vis absorption spectra of these four polyhedral NPs

were also taken and shown in Figure 1e. All four NPs exhibitapparent surface plasmon resonance absorption bands centeredin the range of 550−600 nm. The specific peaks for cubes andRD are located at 590 nm, followed by ECTO at 558 nm andtriangular plates at 564 nm. The relatively narrow and blue-shifted main peak of ECTO compared with RD and cube is dueto the fact that the particle size of ECTO (∼65 nm) is a little

smaller than the size of the RD and cube (∼75 nm). Accordingto Mie theory, the larger the NPs become, the more significantthe higher-order plasmon modes at lower energies are. Thisleads to the red shift and broadening of the plasmon band inRD and cubes.40 Moreover, the small and broad LSPR peaksof triangular plates account for weak plasmonic absorption inthese two-dimensional NPs. It is worth noting that there arevery few literatures44 that directly compare the 3D sphere with2D triangular plates. In this work, we took an unprecedentedapproach by making a direct comparison between 3D and 2Dnanoparticles. This comparative experiment has been repeatednumerous times and proved to be reproducible, which is alsofirmly supported by the simulation results (see section 3.5).

3.2. Performance of Plasmonic Photovoltaics. Thephotophysical properties of P3HT:PC61BM films and deviceperformance of plasmonic BHJ solar cells with introductionof four kinds of polyhedral Au NPs are presented in Figure 2.The optical properties of NP-doped photoactive films are firststudied. Figure 2a shows the UV−vis absorption spectra ofP3HT:PC61BM blend films with NPs embedded. The controlfilm of plain P3HT:PC61BM blend without NPs added is alsoshown in Figure 2a, which illustrates three obvious absorptionpeaks at 513, 550, and 600 nm and is well consistent with theprevious report.52 As compared to the pristine P3HT:PC61BMfilm, the absorption spectra of the active layers doped with fourkinds of NPs do not show any clear change from the controlfilm, indicating that the crystalline π-stacking structure of P3HTremains unchanged after NP doping. However, the absorptionintensity increases considerably when NPs are incorporated intothe active layer. The degrees of absorbance enhancements withthe shapes of NPs are in the following order: RD, triangularplates, ECTO, and cubes in the visible region, revealing theeffectiveness of solar light harvesting. The absorbance of RDNPs is much better than the other three types of Au NPs. Onthe other hand, although triangular plates present the weakest

Scheme 1. Schematic Illustration of the Seed-MediatedMethod for the Growth of Au Nanocrystals Related to theReaction Rate

Figure 1. SEM images of (a) cubes, (b) rhombic dodecahedra (RD), (c) edge- and corner-truncated octahedra (ECTO), and (d) triangular plates ofgold NPs. (e) Absorption spectra of cubes, RD, ECTO, and triangular plate solutions with localized surface plasmon resonance (LSPR) peaks at 590,590, 558, and 564 nm, respectively.

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plasmonic absorption among all titled NPs (Figure 1e) it shouldbe noticed that they still exhibit comparable absorbance uponblending into active film (vide infra). The result implies that theabsorption of active layers doped with nanoplates involves notonly LSPR absorption but also a light-scattering term, and thelatter optical scattering effect contributes a dominant portion ofP3HT:PC61BM film absorption.The NPs incorporating active blends were then introduced

into photovoltaics to study the influences of device perform-ances. We applied inverted cell geometry with devicearchitecture of ITO/ZnO/P3HT:PC61BM:NPs/MoO3/Ag(Figure 2b) in this study. Even though the photon conversionefficiency (PCE) of inverted devices might be slightly lowerthan a conventional structure cell, the high work function metalanode and metal oxide-based carrier transporting layers provide agreater opportunity for developing OPVs with prominentstability. Figure 2c shows the current density−voltage (J−V)curves of P3HT:PC61BM solar cells with various types ofNPs (0.2%) under AM 1.5G illumination at 100 mW cm−2.Key parameters are summarized in Table 1. The reference cell,device A, exhibits an open-circuit voltage (Voc) of 0.60 V,a short-circuit current (Jsc) of 9.23 mA cm−2, and a PCE value of3.46%, respectively, which are comparable to the reported valuesfrom other literature using the inverted cell structure.53,54 All solarcells incorporated with Au NPs show a clear enhancement inPCE. Among them, the device with NPs in RD shape exhibits thelargest increase in PCE from 3.46% to 4.14% with an outstandingfill factor (FF) of 65.3%, which is among the highest forP3HT:PC61BM-based OPVs with metallic plasmonic substancesintroduced into active layers.30,55−57 This improvement couldbe mainly attributed to the enhancement of Jsc, from 9.23 to10.39 mA cm−2. Devices containing cubes and ECTO NPs

illustrate a similar increase of PCE, and the device incorporatingtriangular plates presents the least improvement. Although theECTO particles are slightly smaller than cubes and RDs, webelieve this slight difference is not critical since the device withRD particles outperforms those with ECTO particles. On theother hand, the performance of ECTO-doped devices mightsurpass those with the cubes if the ECTO particle size is increasedto 75 nm since they originally have comparable performance.The incident photon-to-current efficiency (IPCE) spectra of

the solar cells added with four shapes of Au NPs were furtherinvestigated, and the results are shown in Figure 2d. The IPCE

Figure 2. (a) UV−vis spectra of P3HT:PC61BM (1:1 by weight) blend films with 0.2% cube, RD, ECTO, and triangle plate embedded. (b) Aschematic of the device structure. (c) J−V characteristics of P3HT:PC61BM devices with 0.2% cube, RD, ECTO, and triangle plate incorporated. (d)IPCE spectra of P3HT:PC61BM-based solar cells composed of different Au NPs of 0.2% doping ratio.

Table 1. Performance of OPVs with Various LSPR-AssistedConditionsa

devicetype ofNPs

NP ratio[wt %]

Voc[V]

Jsc[mA cm−2]

FF[%]

PCEmax(PCEave)[%]

D1 - 0 0.60 9.23 62.5 3.46 (3.39)D2 cube 0.2 0.60 9.95 62.8 3.75 (3.66)D3 cube 1 0.60 9.84 57.2 3.38 (3.31)D4 cube 5 0.58 9.77 51.3 2.91 (2.81)D5 RD 0.2 0.61 10.39 65.3 4.14 (4.07)D6 RD 1 0.60 10.23 62.4 3.82 (3.74)D7 RD 5 0.58 10.15 53.0 3.12 (3.05)D8 ECTO 0.2 0.60 9.94 63.0 3.76 (3.68)D9 ECTO 1 0.60 8.97 63.4 3.41 (3.35)D10 ECTO 5 0.60 8.77 58.0 3.05 (2.98)D11 triangle 0.2 0.60 9.57 63.1 3.62 (3.58)D12 triangle 1 0.59 9.60 55.9 3.17 (3.11)D13 triangle 5 0.56 1.09 30.9 0.19 (0.12)

aThe table shows the optimized cell performance for each condition.Only the optimized recipes were considered for the estimation of theaverage PCE; data have been averaged over eight devices.

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spectra correlate well with the trend obtained from Jsc and PCEof these devices. Moreover, when NPs are incorporated into theactive layer, the spectral responses of the active layer increasedover a wide wavelength range, indicating that light scatteringinduced by NPs played a critical role in the improvementof solar harvesting. Nevertheless, the slight differences in thefeature of spectra, such as the apparent absorption maxima at600 nm of devices with RD and cubes as well as the relativelyhigher shoulders around 560 nm of ECTO- and triangularplate-embedded devices, correlate well with the LSPRresponses of pristine NPs shown in Figure 1e, suggesting thatthe absorption enhancement is also attributed to LSPRs in theAu NPs.The optimal doping ratio of each kind of NP was further

finely tuned to study the influences of NP-incorporated devices,and the results are summarized in Table 1. In general, cellswith 0.2% of NPs added present the most improvement in bothPCE and Jsc, and a further increase in concentrations (higherthan 1%) has a negative effect on devices. The obviousreductions of Voc and FF result in poor efficiencies of deviceswith high NP doping ratios.3.3. Optimized Conditions. Among the four NP-enhanced

OPVs, the devices with RD show the best performance, andtherefore their J−V characteristics, IPCE, and UV−vis spectrawith various blend ratios are shown in Figure 3 for furtherdetailed discussion. With 0.5% RD NPs introduced inP3HT:PC61BM (Device D5), the PCE substantially increasesto 4.14%, which corresponds to a 19.6% improvement ascompared to the control device. Accordingly, as shown inFigure 3a, the potent boost in Jsc of more than 1 mA cm−2 withlow RD concentration clearly suggests enhanced light trappingwithin the photoactive layer. The IPCE spectra shown inFigure 3b also confirm this result. The external quantumefficiency increases remarkably upon the introduction of RDNPs, which complies with the enhanced Jsc observed withina reasonable error.58 Compared to the pristine cell, the IPCEof the device with RD NPs rises in a broad spectral range(400−700 nm), indicating that the increase of light scatteringefficiently boosts the panchromatic solar harvesting. Further-more, the IPCE of the reference device maximizes at 560 nm,whereas the IPCE of the device with RD doped maximizes at600 nm with a best efficiency of 74.97% (RD 0.2% by weight),which nearly coincides with the spectral range of LSPR responsein RD and might be due to the contribution of near-fieldcoupling. It should be mentioned that RD NPs with a dopingratio lower than 0.2% (e.g., 0.1%) have also been applied todevices, but the performances are close to the reference devices,with no apparent enhancement being observed. On the otherhand, the devices with higher doping ratio present poor Voc andFF, which lead to inferior PCE values. The excessive nano-clusters might break the interpenetrating networks of P3HT andPCBM domains, resulting in a rough surface morphology andlower carrier mobility.56 Although the UV−vis spectra shown inFigure 3c suggest that the higher the doping concentration is,the stronger the absorption exhibits, the total extinction maystill include certain fractions that have no benefit for the lighttrapping but dissipate into heat or scatter out to the cell.In addition, the absorption enhancement of the best dopingcondition (RD 0.2%) is shown in Figure S5 (SupportingInformation). The percentage of absorption enhancement iscounted via the formula of [(absorbance of RD 0.2%) −(absorbance of undoped P3HT:PCBM)]/(absorbance ofundoped P3HT:PCBM). The absorption enhancement exhibits

an obvious peak maximized at around 610 nm, which isconsistent with the LSPR response of RD shown in Figure 1with a reasonable red-shift caused by the difference of opticalconstant between diluted n-hexane and the P3HT:PCBMenvironment. For detailed information, the J−V curves andUV−vis spectra of the other three types of Au NPs are alsoshown in Figure S2 and Figure S3 (Supporting Information).The electrical and optical properties of the other threeNP-added devices have a trend similar to that of RD-includeddevices.Since the electrical effects also account for the performance

improvement when incorporating metallic NPs,30,35,57 theseries and shunt resistances (Rs and Rsh) of these devices areanalyzed and summarized in Figure 4. With 0.2% of Aunanocrystals included, all four plasmonic devices show clearreductions of Rs in comparison to the reference cell, suggesting

Figure 3. (a) J−V characteristics of P3HT:PC61BM (1:1 by weight)devices with RD Au nanocrystal concentrations varying from 0% to5%. (b) IPCE spectra of P3HT:PC61BM-based solar cells composed ofdifferent amount of RD NPs. (c) UV−vis spectra of P3HT:PC61BMblend films with various concentrations of RD NPs introduced.

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that the electrical conductivities are effectively improved, whichis in a good agreement with the previous report.35 However,devices with higher doping concentration of 1% or 5% displaythe incremental Rs values regardless of the shape of NPsas shown in Figure 4b. This may be due to the fact that theligands surrounding NPs are insulators, which induce thebarriers for carrier transport and decrease electric conductivity.

Moreover, Rsh of RD shown in Figure 4a gradually diminisheswith the ascending doping ratio of NPs, implying theoccurrence of regional shunt paths due to the NP aggregatesthat directly bridge the ITO and Ag anode. Consequently,tapping-mode atomic force microscopy (TM-AFM) studieswere further conducted to assist the investigation of the surfacemorphology of the NP-incorporated active layers.

3.4. Morphology Studies. The surface images of plainP3HT:PC61BM (1:1, w/w) film and the active layers with RDNPs blended with weight ratios of 0.2−5% were revealed inFigure 5. The AFM images of the active layer with the otherthree NPs introduced were also investigated, and the results areshown in Figure S4 (Supporting Information). As presented inFigure 5a, the pristine P3HT:PC61BM film displays a smoothsurface morphology with a slight fiber nanostructure, indicatingthe good interpenetrating network for efficient carrier genera-tion and transportation.9,59,60 As shown in Figure 5b, the activelayer with 0.2% of RD embedded presents an even morehomogeneous surface than the pristine one with the surfaceroot-mean-square (RMS) roughness of only 3.1 nm. Such aprominent morphology is beneficial to exciton dissociation andcharge transportation and hence provides a better contactcondition for more carrier extractions in the plasmon-enhancedOPV. Subsequently, as the doping ratio of Au clusters increasesto 1%, the RMS roughness rises to 5.6 nm with the appearanceof a certain porous domain at the film surface. However, theslightly rougher surface is still good enough for photocurrentgeneration and dissociation, so that the device D6 shown inTable 1 displays a comparable Jsc with D5 with only a slightdecrease in FF. Finally, upon adding 5 wt % Au nanocrystalsinto the active film, as shown in Figure 5d, the aggregation ofNPs leads to the uneven surface morphology of the blend(RMS roughness of 8.4 nm). Large clusters distributed at thefilm surface may interrupt the exciton dissociation interfacesand break the bicontinuous charge transport channels, resultingin the reduction of Jsc in device D7. Moreover, it is interestingto mention that we could roughly estimate the distance amongneighboring NPs inside the active film. Using the density ofgold of 19.32 g cm−3, together with the concentration of the

Figure 4. (a) Series and shunt resistances of devices with RD NPsembedded concentration varying from 0% to 5%. (b) The seriesresistances of devices with various concentrations of cubes, ECTO,and triangular plates embedded.

Figure 5. Morphology characterization of (a−d) TM-AFM topography and (e−h) phase images of P3HT:PC61BM blend films with variouscompositions of RD NPs added: (panels a and e) 0%, (panels b and f) 0.2%, (panels c and g) 1%, and (panels d and h) 5%. The imaging size is2 μm × 2 μm for each panel.

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initial NP solution and particle size, the number of NPs permilliliter is calculated to be about 4.5 × 1011.Assuming that NPs have an equal size of 75 nm and

are uniformly dispersed into a volume of blended film with100 nm × 2 cm × 2 cm and that no aggregation takes place, thisdistance is calculated to be on the order of 300 nm for NPs with aweight ratio of 1%. The spacing between NPs becomes evensmaller when the doping ratio increases to 5%. The distance amongneighboring NPs in a 5% Au cluster embedded P3HT:PC61BM iscalculated to be around 160 nm, which is on the same order ofthe size of NPs. This value is expected to induce the aggregation ofNPs and reduce the device performance due to the formation ofrecombination centers and the shunt paths penetrating through theactive layer. The spacing between NPs might become even shorter,while the initial solution is not homogeneously dispersed.Accordingly, upon doping with high concentration (5% dopingratio), the corresponding film surfaces of all four types of NPspresent poor morphologies, accompanied by large clusterformations as shown in Figure S4 (Supporting Information).In particular, the device incorporated with a 5% triangle platedemonstrates a destructive roughness (Figure S4c, SupportingInformation), resulting in an inferior Jsc of 1.08 mA cm−2, alongwith a FF of only 30.9%, and hence a poor PCE of 0.19%.3.5. FDTD Simulations. To gain further insight into the

aforementioned enhancement, the titled NPs were modeled

with 3-dimensional FDTD calculations based on the solutionsof Maxwell’s equations, which provide the distribution ofelectromagnetic field around the particle surfaces. Accordingly,four types of Au NPs with accurate shape and size like thoseshown in Figure 1 have been built up with the environmentalparameters of P3HT:PC61BM.61 The electric-field profilesof the transverse electric (TE)-polarized light are shown inFigure 6 with the excitation wavelength of 550 nm, which isaround the peak of the IPCE spectrum of P3HT:PC61BM.It can be clearly observed that the near-field resonances aredistributed along the edges or corners of 3-dimensional NPs(cube, RD, ECTO), indicating that the LSPRs are significantlyinduced around the three sphere-like gold clusters under solarillumination. Among them, RD presents the strongest electric-field response, followed by ECTO and cube, which correlateswell with the improvements of photocurrent in devices andprovides evidence that LSPRs certainly contribute to theincreased light harvesting. According to Mie theory, the LSPReffect is generated from the resonance between the incidentelectromagnetic field and the free electrons at the metal particlesurface. Therefore, it is easy to imagine that the density of surfaceelectrons along the edges or corners is larger than a uniform area.This is the main reason why the near-field resonances principallydistribute along the edges or corners of 3-dimensional NPs. Onthe other hand, there is nearly no intensity of electric resonance

Figure 6. Time-averaged and normalized transverse electric field distribution, |Ey|, simulated by FDTD analysis with different polyhedral gold NPs:(a) cube, (b) RD, (c) ECTO, and (d) triangle plate. Each panel size is 100 nm × 100 nm.

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being observed at the surface of the planar triangular plate asshown in Figure 6d. These numerical results illustrate tworemarks: (1) planar triangular plates have almost no LSPR butstill exhibit a comparable device performance with cubes andECTO upon incorporation into the active layer, suggesting themechanism of enhancement is dominated by the NP scatteringand (2) LSPR absorption plays a secondary role and takes placewhile NPs have proper edges and corners distributed spatially,which differentiates the overall performance of each kind ofNP-cooperated device. Different from most of the previousstudies,29,31−34,62 which introduced small NPs (<40 nm) intoP3HT:PC61BM or PEDOT:PSS to increase photocurrent mainlydue to LSPR effects, our work alternatively uses large NPs(∼70 nm) synthesized by a systematic seed-mediated methodto enhance device performance via a combination of plasmoniccoupling and effective light scattering. This has been firmlyverified from the above experimental and theoretical approaches.The near-field plasmonic resonances together with strong far-field scattering effects of polyhedral Au NPs efficiently boost thelight harvesting inside the active layer, rendering more than 20%improvement in PCE as compared to the control device.

4. CONCLUSION

In conclusion we report on the distinct shape-dependent effectsof plasmon-enhanced polymer bulk heterojunction solar cellswith polyhedral gold nanoparticles (NPs). Differing fromnumerous previous literatures comparing merely conventionalsphere, rod, or prism, in this paper, we study a series of3-dimensional (3D) NPs with different spatial shapes in detail.We designed and synthesized four types of gold NPs withdifferent 3D shapes, including cubes, rhombic dodecahedra,edge- and corner-truncated octahedra, and triangular plates.The shape control technologies of nanocrystal synthesis rendereach 3D NP with a very homogeneous distribution for a faircomparison. We then for the first time incorporated them forphotovoltaic applications in this work. Through careful andprecise comparison, we conclude that NPs with more cornersand proper size of cross-section induce more near-fieldcoupling and far-field scattering in P3HT:PC61BM-based activelayers, which increases the optical path length and is therebyresponsible for the enhancement of the light harvesting. Bothfinite-difference time-domain simulation and device performaceindicate that rhombic dodecahedra exhibit the strongestlocalized surface plasmonic resonance and optical scattering.Thus, fine-tuning the shape of NPs combines LSPR responsesand a wide range of multiple light scatterings. The net effect isto efficiently boost light harvesting within the P3HT:PC61BM,enhancing PCE in a consistent and reproducible way.

■ ASSOCIATED CONTENT

*S Supporting InformationFigures S1−S5 and Scheme S1. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Authors*P.-T. Chou. E-mail: [email protected].*C.-I. Wu. E-mail: [email protected].*M. H. Huang. E-mail: [email protected].

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was financially supported by National ScienceCouncil of the Taiwan (NSC 101-2113-M-007-018- MY3, 103-2811-M-002-177).

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