Plasmonic nanostructures for organic photovoltaic devicesecee.colorado.edu/~wpark/papers/J. Opt. 2016 Ahn.pdf · 2016. 2. 16. · Keywords: organic photovoltaics, plasmonics, nanostructure,

Topical Review

Plasmonic nanostructures for organicphotovoltaic devices

Sungmo Ahn1, Devin Rourke1 and Wounjhang Park1,2

1Department of Electrical, Computer, and Energy Engineering, University of Colorado, Boulder, CO80309-0425, USA2Materials Science and Engineering Program, University of Colorado, Boulder, CO 80309-0425, USA

E-mail: [email protected]

Received 21 August 2015, revised 9 November 2015Accepted for publication 16 November 2015Published 9 February 2016

AbstractDue to the increasing demand on high-efficiency organic photovoltaic (OPV) devices, lightmanagement technique has become an active research subject. Especially, plasmonic approachwas proven to be suitable for application in OPV and has shown lots of successful results. In thisreview, we summarize recent studies on plasmonic nanostructures for OPV with their underlyingenhancement mechanisms. Optical absorption enhancement by the resonant scattering and thestrong plasmonic near field will be discussed for various implementation geometries includingmetal nanoparticles, patterned electrodes, and plasmonic metamaterials. In addition, we will alsolook into the electrical effects originating from plasmonic nanostructures, which inevitably affectthe device’s efficiency. Future research directions will be also discussed.

Keywords: organic photovoltaics, plasmonics, nanostructure, solar cell, organic electronics

(Some figures may appear in colour only in the online journal)

1. Introduction

Organic photovoltaic (OPV) devices have become one of themajor technologies in the field of solar energy harvesting. Asthe silicon or other inorganic semiconductor-based solar cellsare under increasing pressure for lower cost [1, 2], OPV offersa promising alternative. Moreover, OPV is compatible withflexible plastic substrates which may open an array of newapplications such as foldable and portable devices that can bedirectly integrated on textile [3], curved surfaces [4–6] ormoving parts of robots and organs [7–9]. Semi-transparencyis also attractive for a variety of applications including power-generating and color-decorative windows in buildings andautomobiles [10–13].

The history of photovoltaic technology based on organicsemiconductors stretches back to the early 20th century whenthe photoconductivity was first observed from an organicmolecule, anthracene [14–16]. Since then, small moleculesand polymers have been investigated to realize photovoltaicdevices. Unfortunately, the power conversion efficiency

(PCE) from these organic materials could not exceed 0.1%until the bi-layer structure made of donor and acceptormolecules was demonstrated by Tang in 1986 [17], leading toa dramatic increase of PCE to 1%. The ensuing research onthe reason for this improvement revealed that the chargeseparation from an exciton in an organic semiconductor is farmore efficient at the interface of the heterojunction [17–19].Later, bulk heterojunction of interpenetrating donor–acceptornetwork was introduced to overcome the short exciton dif-fusion length which sets a limit in the active layer thicknessand consequently in the optical absorption [20]. Bi-layer andbulk heterojunction solar cells are schematically shown infigure 1.

In the past decade, polymer:fullerene-based solar cell wasextensively studied and showed a great efficiency improve-ment. The famous Poly(3-hexylthiophene) (P3HT):Phenyl-C61-butyric acid methyl ester (PCBM) device typicallyshows 4∼5% PCE [22–24]. In addition, poly(N-9-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-ben-zothiadiazole) (PCDTBT) with deep HOMO level was

Journal of Optics

J. Opt. 18 (2016) 033001 (24pp) doi:10.1088/2040-8978/18/3/033001

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developed to show PCEs of 6∼7% [25–27]. More recently,narrower bandgap polymers such as polythieno [3,4-b]-thio-phene/benzodithiophene (PTB7) and PTB7-Th were used toimprove the PCE by absorbing lower-energy photons. Almost10% PCE were obtained from a single stacked polymer:full-erene solar cell by using PTB7 as donor material [28]. Smallmolecule-based solar cell has been also improved to achieve aPCE over 9% from single junction recently [29].

Though lots of improvements in material and devicestructure have been accomplished so far [22, 25, 27, 30–33],the PCE of OPV devices still remains low compared withtheir inorganic counterparts. According to the efficiency chartreported by National Renewable Energy Laboratory [34], thehighest efficiency of OPV marked 11.5% while other inor-ganic technologies show efficiencies higher than 20%. Asmentioned earlier, the active layer thickness in an OPV deviceis limited up to a few hundred nanometers because of theshort exciton diffusion length of organic materials, whichinevitably limits optical absorption [35, 36]. This trade-offbetween the optical absorption and the charge carrier collec-tion makes it difficult to achieve higher PCE in OPV devices.

For this reason, light management has emerged as animportant area of research. Since the optical absorption

depends on the optical path length and the electromagneticfield strength, it is possible to increase total absorptionwithout increasing the active layer thickness by incorporatingphotonic structures. For example, a microcavity formed in theactive layer resulted in higher interaction between the opticalfield and the absorbing material, yielding enhanced PCE [37].To increase the effective absorption length while maintainingsmall active layer thickness, various nano-photonic approa-ches have been investigated including photonic crystals [38–41], quasi-periodic or disordered nano-patterns [42–44],nanoparticles (NPs) [45], and plasmonic nanostructures [46].

In this review, we will focus on the plasmonic nanos-tructures used in organic solar cells. During the past decade,surface plasmon has become an important technique in thefield of solar cells [47, 48]. Surface plasmon is a collectiveoscillation of surface charges, which can interact stronglywith external electromagnetic waves [49]. Due to the near-field nature of surface plasmons [50], they are intrinsicallysuitable for thin film devices. Photonic crystals, which haveshown great successes in inorganic solar cells, have been alsoapplied in OPV devices [41, 51, 52]. However, this approachwill be limited by the relatively low refractive indices oforganic materials since the strong photonic crystal effect can

Figure 1. Schematics of (a) bilayer and (b) bulk heterojunction organic photovoltaic (OPV) device. (c) Device operating principle of OPVdevice from light absorption to charge collection. Reprinted from [21]. Copyright 2014 MDPI.

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J. Opt. 18 (2016) 033001 Topical Review

be seen with high index contrast and also with a large enoughthickness to support the guided modes [53, 54]. Therefore,plasmonic nanostructures are considered better suited forapplication in OPV devices.

Metal NP and patterned metal electrode are two mostpopular plasmonic nanostructures used in OPV devices.Metal NPs can impact OPV performance by enhanced scat-tering and local field enhancement resulting from the excita-tion of localized surface plasmon resonances (LSPR).Patterned electrodes may improve OPV through local fieldenhancement and increased optical path length by couplinginto the surface plasmon polaritons (SPPs). These effects willbe first discussed and the relevant research works will besummarized. Also, the recent studies on plasmonic metama-terials will be highlighted with their future applications inOPV technology. In the second part of this review, theelectrical effects of plasmonic nanostructures incorporated inOPV devices will be discussed, including morphologicalchange of bulk heterojunction solar cell after the metal NPincorporation and the effects on the charge collection effi-ciencies of non-uniform plasmonic fields across the activelayer. It is noted that the various plasmonic effects describedin this paper are not mutually exclusive and a structure mayexhibit more than one plasmonic effect simultaneously. Infact, designing a plasmonic structure making use of multipleplasmonic effects to achieve maximum enhancement is one ofthe main future research directions. It is also noted that therewere recent reviews [55–57] on the plasmonic enhancementof OPV and this paper attempts to provide more up-to-dateadvances and insights in this field.

2. Optical effects

In this section, the optical effects from surface plasmonresonance in OPV devices will be discussed. When plasmonicstructures are incorporated into photovoltaic devices,absorption enhancement can be achieved by two differentmechanisms. One is the scattering effect, known to bestronger in metal NPs which support LSPRs. Enhancedscattering at the front surface of a photovoltaic device reducesthe Fresnel reflection while the scattering inside the absorbinglayer makes the effective optical path length much longer thanthe physical thickness of the absorber layer [47, 58, 59]. Theother mechanism is the local field enhancement effect ofsurface plasmons. Surface plasmons are known to be highlyeffective in concentrating electric field at the interfacebetween the metal and the dielectric material. When thisconcentrated field is present within the active material, opticalabsorption will be increased proportionally to the square ofthe field enhancement factor.

The contribution from each mechanism on the absorptionenhancement depends on the type of plasmonic structures andtheir different geometries as schematically shown in figure 2.Therefore, in the following subsections, we will discuss theplasmonic optical effects in OPV devices according to theirgeometries: the metal NPs and the patterned metal electrodes.

2.1. Metal nanoparticles

Metal NPs are one of the most intensively studied plasmonicstructures due to the LSPRs which may be controlled by theirsize and shape [49, 60]. As opposed to the patterned metalelectrode we will discuss in the next section, the relativeeasiness of fabrication and the solution processibility leadmany researchers to use metal NPs in various applicationssuch as light-emitting devices [61–63], solar energy harvest-ing [48, 64, 65], bio-imaging and medical therapies [66], etc.OPV is also an important application area where the metalnanoparticles can improve the absorption efficiency throughtheir plasmonic resonances.

When the metal NPs are incorporated in solar cell, boththe enhanced scattering and the strong near field of surfaceplasmons can contribute to the overall efficiency enhance-ment. Light scattering by small particles has been a subject ofinterest in optics for a long time. When the NP size is muchsmaller than the wavelength of light, the scattering (Csc) andthe absorption (Cabs) cross sections from a spherical NP canbe expressed as

C a Q Q ka,8

3

1

2,sc

2sc sc

4 d

d

2

( )p ee

= =-+

C a Q Q ka, 4 Im1

2,abs

2abs abs

d

d( )

⎡⎣⎢

⎤⎦⎥p

ee

= =-+

where k is the wave number, a is the particle radius, andεd=εp/εm is the relative permittivity given by the ratiobetween the permittivity of the particle (εp) and thesurrounding medium (εm) [67]. One can notice from theabove equations that these cross sections are enhanced at theFröhlich condition of Re[εd]=−2, which represents theexcitation of LSPR. The general solution for the lightscattering by a spherical NP is given by the Mie theory,which contains all of the higher order terms that become non-negligible for large particle sizes [68]. On the other hand,when the absorbing active materials are in close proximity tothe metal surface, the exciton generation rate can also beenhanced as proportional to the square of the local fieldenhancement factor as follows

G P E1

2,2e wµ =

where G is the exciton generation rate, P is the electro-magnetic energy dissipation rate, e is the imaginary part ofthe complex permittivity of the active material, w is theangular frequency of the electromagnetic wave, and E is thelocal electric field.

For the past decade, there has been a substantial amountof theoretical and experimental studies about the plasmonicenhancement of OPV device performance using metal NPs.Polymer bulk heterojunction solar cells as well as smallmolecule based devices have shown a successful efficiencyenhancement from plasmonic metal NP inclusion [69–72].The experimental studies so far have shown the PCEenhancement of a few tens of percent using various types ofmetal NPs. The origin of the enhancement is believed to be a

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combination of the enhanced scattering and the local fieldenhancement as we will discuss in detail below. The con-tributions from these two different mechanisms dependstrongly on the exact geometry of metal NP-embedded in theOPV device. As shown in the equations for the scattering andthe absorption cross sections, the scattering increases muchmore rapidly than the absorption as the particle size isincreased. Therefore, scattering enhancement is dominant forlarge size NPs while the local field enhancement effect isprevalent for NPs with small diameters. A comprehensive listof theoretical and experimental studies is given in table 1.

Metal NP’s position is an important factor that affects theenhancement mechanisms of plasmonic OPV devices. Inmany cases, metal NPs are placed inside the anodic bufferlayer such as poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as shown in figure 3(a). Variousfabrication methods like electrodeposition [74], thin filmevaporation [73] and chemical synthesis [100] were used toincorporate metal NPs into OPV devices. The PCEenhancement ranging from 5%∼69% has been reportedexperimentally. Morfa et al reported 69% efficiencyimprovement of P3HT:PCBM device from vapor-phasedeposited Ag nanoparticles on ITO, but the overall efficiencyremained low at 2.23% [73]. Generally, when the originalefficiency is low, it becomes easier to achieve largeenhancement. With moderate original efficiencies, it seemsthat ∼20% efficiency enhancement is the best experimentallyobserved enhancement so far [74–77, 90, 91, 96]. To identifythe origin of this PCE enhancement, the external quantumefficiency (EQE) spectrum is often measured and comparedwith the metal NPs’ extinction spectrum as shown in figure 4.The EQE also called incident photon to current conversionefficiency (IPCE), is defined as the ratio between the numberof electrons collected and the number of photons incident onthe device. It can be expressed as the product of the absorp-tion efficiency and the charge collection efficiency. Assumingthat the charge collection is not affected by the plasmonicstructure or at least it does not have wavelength dependence,the spectral overlap between the metal NP’s extinction andthe EQE enhancement is an indication that the opticalabsorption enhancement by the metal NP LSPR is the mainreason for the PCE enhancement.

Since the metal NPs embedded in buffer layer are not indirect contact with the active layer, it is believed that theforward scattering of the incident sunlight is mainly respon-sible for the absorption enhancement. Lee et al pointed outthat the spatial extent of the absorption enhancement aroundmetal NP is so small that it exponentially decays away andreaches below 10% at distances of only 10∼20 nm from themetal surface [214]. It is also theoretically shown that theplasmonically enhanced local field of metal NPs embedded inbuffer layer does not extend into the active layer [80]. Thesestudies all support the conclusion that the forward scatteringis the main mechanism of the efficiency enhancement frommetal NPs embedded in buffer layer. However, in manyexperimental studies, the metal NP’s size is comparable to thethickness of PEDOT:PSS layer or even larger than that. Inthis case, even the highly concentrated plasmonic field aroundthe metal NP can contribute to the absorption enhancement ofthe active layer. Several studies have shown that the metalNPs positioned at the interface between the active and thebuffer layer lead to higher enhancement [120–122]. From Quet al’s theoretical study, Ag nanospheres embedded inbetween P3HT:PCBM and PEDOT:PSS layer could result inabsorption enhancement as high as 108% [121].

Incorporating metal NPs in the buffer layer seems to be agood way to get enhancement from plasmonic effect withoutdeteriorating the interfacial morphology of blended activelayer or quenching of photogenerated excitons [102], whichcould adversely affect the device performance. But it has alsobeen reported that the PCE can be enhanced from the metalNPs directly embedded in the active layer as shown infigure 3(b). Since the metal NP is in direct contact with theabsorbing material in this case, one can expect higher nearfield effect than the metal NPs embedded in buffer layer. Upto about 40% efficiency enhancement was achieved so far[107, 109]. Wang et al reported ∼32% PCE enhancement inpolymer solar cell with Au nanoparticles incorporated in theactive layer [108]. Numerically solving Maxwell’s equations,the near field profile at LSPR was revealed to show verystrong field strength laterally distributed along the active layeras in figure 5(b). An absorption enhancement of over 100% atLSPR wavelength was experimentally achieved and matchedwell with the theoretical expectation as shown in figure 5(a).

Figure 2. Plasmonic light trapping geometries in thin film solar cell. Light trapping (a) by scattering from metal nanoparticles (NPs) whichincreases the effective light path length, (b) by the excitation of localized surface plasmons creating strong near field around the metal NPs,(c) by the excitation of surface plasmon polaritons propagating laterally along the metal-dielectric interface. Reprinted with permission from[47]. Copyright 2010 Nature Publishing Group.

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Table 1. List of theoretical and experimental studies about plasmonic OPV devices utilizing metal nanoparticles.

Donor:Acceptor Type of metal nanoparticle (NP) Position PCE (%) Enhancement factor Reference

P3HT:PCBM Ag nanoisland Buffer (PEDOT:PSS) 2.2 (1.3) 1.69 [73]P3HT:PCBM Ag NP Buffer (PEDOT:PSS) 3.69 (3.05) 1.21 [74]P3HT:PCBM Au nanosphere Buffer (PEDOT:PSS) 4.19 (3.48) 1.20 [75]P3HT:PCBM Au nanodots Buffer (PEDOT:PSS) 3.65 (3.04) 1.20 [76]P3HT:PCBM Au nanosphere Buffer (PEDOT:PSS) 4.24 (3.57) 1.19 [77]P3HT:PCBM Au-Cu alloy NP Buffer (PEDOT:PSS) 3.35 (2.90) 1.16 [78]MEH-PPV:PCBM Au nanosphere Buffer (PEDOT:PSS) 2.36 (1.99) 1.19 [79]P3HT:PCBM Au nanosphere Buffer (PEDOT:PSS) 3.51 (3.1) 1.13 [80]P3HT:PCBM Au NP Buffer (Cs2CO3) 3.54 (3.12) 1.13 [81]P3HT:PCBM Au NP-graphene oxide Buffer (PEDOT:PSS) 3.55 (3.23) 1.10 [82]P3HT:PCBM Au nanosphere, nanorod Buffer (PEDOT:PSS) 4.28 (3.46) 1.24 [83]P3HT:PCBM Au nanowire Buffer (PEDOT:PSS) 2.72 (2.44) 1.11 [84]P3HT:PCBM Ag NP Buffer (PEDOT:PSS) 2.82 (2.41) 1.17 [85]P3HT:PCBM Ag nanosphere Buffer (PEDOT:PSS) 34% absorption enhancement,

theoretical[86]

P3HT:PCBM Ag nanoprism Buffer (PEDOT:PSS) 5.21 (4.58) 1.14 [87]P3HT:PCBM Au NP Buffer (MoO3) 4.20 (3.68) 1.14 [88]P3HT:ICBA Ag NP Buffer (MoO3) 7.21 (6.26) 1.15 [89]PCDTBT:PCBM Ag nanosphere Buffer (PEDOT:PSS) 7.6 (6.4) 1.19 [90]PTB7:PCBM Ag nanosphere Buffer (PEDOT:PSS) 8.6 (7.9) 1.09PTB7:PCBM Au, Ag nanosphere Buffer (PEDOT:PSS) 8.67 (7.25) 1.20 [91]P3HT:PCBM Pt NP Buffer (PEDOT:PSS) 2.57 (2.29) 1.12 [92]P3HT:PCBM Ag NP, nanodot Buffer 4.80 (4.02) 1.19 [93]P3HT:PCBM Au NP-graphene oxide Buffer 3.98 (3.26) 1.22 [94]P3HT:ICBA Au NP-graphene oxide Buffer 5.05 (4.02) 1.26PCDTBT:PCBM Ag NP Buffer (MoO3) 5.87 (5.07) 1.16 [95]PCDTBT:PCBM Au@Ag core–shell nanocube Buffer (PEDOT:PSS) 6.3 (5.3) 1.19 [96]PTB7:PCBM Au@Ag core–shell nanocube Buffer (PEDOT:PSS) 9.2 (8.0) 1.15PCDTBT:PCBM Ag nanosphere Buffer (PEDOT:PSS) 7.35 (6.50) 1.13PCDTBT:PCBM Au nanosphere Buffer (PEDOT:PSS) 6.01 (5.29) 1.14PTB7:PCBM Au nanosphere Buffer (PEDOT:PSS) 8.31 (7.95) 1.05PTB7:PCBM Au NP Buffer (MoS2) 7.25 (6.18) 1.17 [97]P3HT:PCBM Au NP Buffer (ZnO) 3.6 (2.3) 1.57 [98]P3HT:PCBM Cu NP Buffer (PEDOT:PSS) 3.96 (3.58) 1.11 [99]PTB7:PCBM Cu NP Buffer (PEDOT:PSS) 7.43 (6.79) 1.09PTB7:PCBM Au nanocube Buffer (PEDOT:PSS) 8.2 (7.5) 1.09 [100]ZnPc:C60 Ag NP Buffer (BF-DPB:F6-

TCNNQ)2.65 (1.93) 1.37 [72]

F4-ZnPc:C60 Ag NP Buffer (BF-DPB:F6-TCNNQ)

3.4 (2.7) 1.26

P3OT:C60 Ag NP Active 1.9 (1.1) 1.73 [101]P3HT:PCBM Au nanosphere Active

Authors also pointed out the electrical effect of Au nano-particles, which sometimes can counteract the opticalenhancement. But at the optimized concentration of Aunanoparticles, they could get positive effects both fromoptical and electrical aspect. Wang et al also reported a PCEenhancement from Au nanoparticles directly embedded in theactive layer [103]. They studied various donor–acceptorsystems with Au nanoparticles and confirmed consistentefficiency enhancement by the Au nanoparticles. At theoptimized Au nanoparticle ratio, the PCE increased from3.54% to 4.36% (P3HT:PCBM), from 5.77% to 6.45%(PCDTBT:PCBM), and from 3.92% to 4.54% (Si-PCPDTBT:PCBM). The enhancement factors were 23%, 12%, and 16%,respectively.

In the plasmonics literature, Au and Ag are almostequally used as plasmonic material. Au exhibits higherabsorption loss in the visible than Ag but has superior che-mical stability. The two materials also exhibit different

plasmon resonance wavelengths. When designing the plas-monic system for OPV devices, one of the most importantthings to consider is the spectral matching between the par-ticles’ LSPR and the active materials’ absorption band. Asshown in figure 6(a), Ag nanoparticle has LSPR at shorterwavelength around 400∼500 nm than that of Au at around500∼600 nm. Baek et al pointed out that the proper choiceof metal NPs to compensate weak absorption region of adevice is of prime importance in designing plasmonic OPVdevice [96]. As noted earlier, Au nanoparticles show higherlevel of self-absorption than Ag. Even though both Au andAg nanoparticles show enhanced scattering and absorptionefficiencies, Au nanoparticles have greater absorption powerthan scattering as shown in figure 6(a). One can notice thescattering-to-absorption power ratio of Au nanoparticles issignificantly smaller than that of Ag nanoparticles infigure 6(b). Therefore, it can be said that Ag nanoparticles aremore effective than Au for scattering enhancement. In Baek

Table 1. (Continued.)

Donor:Acceptor Type of metal nanoparticle (NP) Position PCE (%) Enhancement factor Reference

P3HT:PCBM Au@SiO2 core–shell nanosphere Active 3.77 (3.67) 1.03PBDTT-DPP:PCBM Au@SiO2 core–shell sphere Active 5.38 (4.93) 1.09PCDTBT:PCBM Au, Al NP Active 6.12 (5.33) 1.15 [114]P3HT:PCBM Ag@oxide nanoprism Active 4.15 (3.10) 1.34 [115]P3HT:PCBM Rhombic dodecahedra Au NP Active 4.14 (3.46) 1.20 [116]p-DTS(FBTTh2)2:PCBM

Au@SiO2 nanorod Active 8.2 (6.5) 1.26 [117]

P3HT:PCBM Ag@SiO2 core–shell nanosphere Active 3.94 (3.69) 1.07 [118]P3HT:PCBM Ag nanosphere Interface 1.2 (2.2) 0.55 [119]PCDTBT:PCBM Ag nanosphere Interface 90% absorption enhancement,

theoretical[120]

P3HT:PCBM Ag nanosphere Interface 106% absorption enhancement,theoretical

[121]

PTB7:PCBM Ag@SiO2 core–shell nanosphere Interface 8.49 (7.26) 1.17 [122]P3HT:PCBM Ag nanowire Interface 3.59 (3.06) 1.17 [123]PTB7:PCBM Au nanoparticle cluster Interface 9.48 (8.29) 1.14 [124]P3HT:PCBM Ag nanowire Interface 3.72 (3.16) 1.18 [125]P3HT:PCBM Au nanosphere All 3.85 (3.16) 1.22 [126]P3HT:PCBM Au@SiO2 core–shell NP All 3.80 (3.29) 1.16 [127]

Figure 3. Schematic drawings of plasmonic OPV devices with metal nanoparticles incorporated in the buffer layer (a) and in the active layer(b). Reprinted with permission from [122]. Copyright 2013 American Chemical Society.

6


et al’s comparative study [96], Ag and Au nanoparticles of50 nm size were embedded in PEDOT:PSS layer ofPCDTBT:PCBM devices to result in 13% and 14% PCEenhancement, respectively. Although the PCE enhancements

were about the same, the short circuit current densityincreased a lot more in the Ag-embedded device which is anindication of improved optical absorption due to the enhancedscattering. On the other hand, Au-embedded device has its

Figure 4. Schematic drawing of P3HT:PCBM device with Au metal nanoparticles (NPs) embedded in PEDOT:PSS layer (top left). Scanningelectron microscopy (SEM) image of Au metal NPs and their extinction spectrum (top right). Measured external quantum efficiency (EQE)spectra of P3HT:PCBM device with and without Au NPs (bottom left). Comparison between the EQE enhancement factor and the extinctionspectrum of Au NP (bottom right). Reprinted with permission from [77]. Copyright 2011 American Chemical Society.

Figure 5. (a) Experimental and theoretical (inset) absorption enhancement factor of the active layer with different amounts of Aunanoparticles. (b) Theoretical near field distribution around Au nanoparticle embedded in the active layer. Reprinted with permission from[108]. Copyright 2011 Royal Society of Chemistry.

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origin of efficiency enhancement in the increased fill factor(FF) rather than the short circuit current density, which meansthat the charge collection process was improved. In addition,it is well known that Au nanoparticles are more durable andeasier to synthesize than Ag nanoparticles [128, 129]. How-ever, not much works have been done about the long-termstability of metal NP-embedded OPV devices so far [130].Future studies on this aspect will be useful for identifying theproper plasmonic material for the OPV applications.

Though spherical metal NPs have shown successfulresults, more efforts to further increase the efficiency are inprogress. Engineering the metal NP’s shape is one way toachieve a broadband absorption enhancement[100, 116, 123, 125]. For example, Au nanorod possessestransverse and longitudinal LSP modes according to theaspect ratio as shown in figures 7(a) and (b). In theirextinction spectra, a resonance peak at around 520 nm resultsfrom electron oscillations along the short axis while thelonger axis generates a distinct one at 600∼900 nm,depending on the aspect ratio. From Au nanorods embeddedin PBDTT:PCBM active layer, the EQE enhancement wasalso measured simultaneously at both the transverse and thelongitudinal LSPRs [113]. Mixing two different shapes ofmetal NPs was also proven to be efficient in broadening thespectral absorption enhancement [111]. Spherical Ag nano-particles and Ag nanoprisms shown in figure 7(c) were mixedtogether to result in an effective spectral broadening of theabsorption enhancement. From the measured absorptionspectra, the mixture was found to have both resonances fromspherical nanoparticles and nanoprisms.

Another approach to further enhance the efficiency is toincorporate multiple metal species simultaneously. Recently,Baek et al reported an enhanced scattering by Au@Ag core–shell nanocubes compared to the spherical Au nanoparticles[96]. Due to the dual nature of Au@Ag core–shell nanocubes,they showed 2.2-fold higher EQE enhancement than Aunanoparticles. Lu et al also reported a cooperative plasmoniceffect by simultaneously embedding Ag and Au nanoparticlesin the PEDOT:PSS layer [91]. These studies also showed aneffective spectral broadening of plasmonic absorption

enhancement from all species consisting the plasmonicstructures.

Many theoretical and experimental reports have shownthat metal NPs can play an important role in improving theOPV efficiency. The plasmonic effects on the scattering crosssection and the local field strength make it possible toenhance the optical absorption without increasing the physicalthickness of active layer. More research efforts are directed toachieving further improvement of the plasmonic effect viaprecise engineering of nanoparticle’s shape and material.However, it is not well understood yet about the non-opticaleffects of metal NP inclusion and their contribution to thePCE enhancement. In addition, the maximum enhancementwe can get from metal NPs is still unknown and the optimizedgeometry of plasmonic OPV is undetermined. More theor-etical and experimental studies could provide a betterunderstanding and new route to even further improve theplasmonic OPV’s performance. New plasmonic materialssuch as Al or Cu might also provide an interesting researchdirection since they are less expensive than Ag and Au[131–134].

2.2. Patterned metal electrodes

Although the metal NPs provide an easy way to incorporate aplasmonic structure into the solution-based fabrication pro-cess of OPV device, a patterned metal electrode offers apromising alternative with better control. A patterned metalelectrode, which is usually periodic, acts as an efficient cou-pler of vertically incident solar radiation into laterally pro-pagating SPP modes or waveguide modes in the organicactive layer [131, 135]. This makes the optical path lengthmuch longer and thus increases absorption significantly.Moreover, the launching of SPP at the interface between theactive layer and the metal electrode results in a highly con-centrated electromagnetic field inside the active layer that alsoleads to higher absorption. The grating-coupled SPP usuallyshows sharper resonance than the LSP in metal NP, which isnot desirable in solar cell applications. But the couplingefficiency is much higher than that of metal NPs and the

Figure 6. (a) The absorption and scattering power spectra of Ag and Au nanoparticles of 50 nm size. (b) The scattering-to-absorption powerratio of Ag and Au nanoparticles with different sizes. Reprinted with permission from [96]. Copyright 2014 American Chemical Society.

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resonance wavelength can be easily tuned from visible tonear-infrared region by changing the grating parameters. Inaddition, it is often the case that the SPP mode extends furtherthan LSP and is more beneficial to obtain larger absorptionenhancement throughout the active layer.

Some early studies showed photocurrent enhancement ofpolymer materials in a prism configuration [136, 137]. Here,SPP is launched when total internal reflection occurs. Inconventional solar cell structures, one- or two-dimensionalgrating structures are fabricated lithographically on either thefront or back electrode instead of the bulky prism config-uration. The fabrication of these nanopatterns can be done inseveral different ways [135, 138–145]. The most straightfor-ward method is a lithographic patterning of resist layer fol-lowed by the metal evaporation and lift-off. Laser interferencelithography [144], nanosphere lithography [140], andnanoimprint lithography [139, 145] are widely used to definesub-micron features efficiently over a large area. Another wayto fabricate nanopatterned metal electrode is direct imprintingof active layer. Several groups have imprinted the organicactive layer using pre-patterned mold and then depositedmetals on top of the corrugated active layer to form a metalgrating [135, 138, 143]. These fabrication techniques areschematically shown in figure 8.

Since the ITO is normally used as the transparent frontelectrode, plasmonic nanopattern is typically made on thebackside metal electrode. Similar to the metal NP studies,various donor–acceptor systems have been studied [135, 138]and PCE enhancement of up to 25% was reported [146]. Theplasmonic effect on the efficiency improvement can beobserved in the EQE spectrum. Li et al showed the PCEenhancement of 19% using Ag grating electrode in P3HT:PCBM device [135]. Figures 9(a) and (b) show the gratingprofile measured by atomic force microscopy and aphotograph of the fabricated device, respectively. From theEQE spectrum shown in figure 9(c), strong enhancementpeaks were observed at 400 nm and 715 nm. Numericalsimulations revealed that these two peaks are due to theexcitation of the waveguide mode and the SPP mode,respectively. Furthermore, they can also be observed in thereflection spectrum shown in figure 9(d) that confirms theEQE enhancement is due to the enhanced absorption at theseresonant mode excitations.

It should be noted that the waveguide mode resonanceand the SPP resonance independently contributed to theoverall efficiency enhancement. Simultaneous excitation ofdifferent modes lead to the spectral broadening of efficiencyenhancement, which is essentially the same approach as thecooperative effect of Au and Ag nanoparticles discussed in

Figure 7. TEM images of Au nanoparticles with different aspect ratio (a) and their extinction spectra. (b) (reprinted with permission from[113], copyright 2013 American Chemical Society.) SEM image of P3HT:PCBM film incorporated with mixed Ag nanoparticles andnanoprisms (c) and the corresponding absorption spectra (d) (reprinted with permission from [111], copyright 2015 Wiley.)

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the previous section [91]. Lu et al also studied the interactionof the LSP and the SPP modes supported by a one-dimen-sional grating Ag electrode [147]. The authors observed theanti-crossing behavior between the two modes indicative ofstrong interaction between them. To utilize the LSP and SPPmodes simultaneously offers a promising way of furtherenhancing the solar cell’s efficiency. Dual metallic nanos-tructure in poly((4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]-alt-[2-(2′-ethyl-hex-anoyl)-thieno[3,4-b]thiophene-4,6-diyl (PBDTTT-C-T):PCBM device was reported to get PCE enhancement from7.59% to 8.79% [148]. The dual metallic nanostructures werecomposed of Au nanoparticles embedded in the active layerand silver nanograting as a back electrode. Due to thesimultaneous excitation of LSP and SPP modes at the metalNPs and the patterned electrode, respectively, a broadabsorption enhancement was successfully achieved.

In Li et al’s work [135], it is noteworthy that the SPPresonance generally resulted in much higher enhancementfactor than the waveguide mode resonance. This can be

attributed to the strong near field enhancement of SPP whichcannot be obtained from waveguide modes. However, whencomparing the EQE spectra of flat and patterned devices infigure 9(c), one can notice that the increment from the SPPpeak at 715 nm is much smaller than those from the visiblerange. Because the SPP resonance is positioned in sub-bandgap region, its contribution to the total PCE enhancementis believed to be small. Instead, a much larger total absorptionis expected when the SPP resonance is positioned where thereis a fair amount of intrinsic absorption to begin with.

In collaboration with researchers at National RenewableEnergy Laboratory, we proposed a new route to obtain agreater plasmonic enhancement from bulk heterojunctionOPV device using an infrared sensitizer material and thepatterned Ag back metal electrode [149]. Recently, lowbandgap polymers such as poly[(4,40-bis(2-ethylhexyl)dithieno[3,2-d:2′,3′-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5′-diyl] (Si-PCPDTBT) have beenproposed as an infrared sensitizer that can extends theabsorption band into the near infrared region [150–154].

Figure 8. Schematic diagrams of fabrication methods; direct imprinting of active layer (top) [143] (reprinted with permission, copyright 2012American Physical Sociey) and laser interference lithography and metal lift-off process (bottom) [144]. Reprinted with permission from[144]. Copyright 2014 Optical Society of America.

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Figure 9. (a) Atomic force microscopy image of P3HT:PCBM grating formed by soft nanoimprint approach. (b) Photograph of a patterneddevice showing a diffraction color from grating structure. (c) Incident photon-to-current conversion efficiency (IPCE) spectra and (d) thezeroth-order reflection spectra (bottom right) of the devices with and without grating [135]. Reprinted with permission from [135]. Copyright2012 American chemical Society.

Figure 10. Chemical structure of polymer materials used in ternary blend and its energy diagram (left). External quantum efficiency spectra ofdifferent ratio of infrared sensitizer (right) [152]. Reprinted with permission from [152]. Copyright 2012 John Wiley and Sons.

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Figure 10 shows the chemical structure of Si-PCPDTBT andthe energy diagram of ternary system of P3HT:Si-PCPDTBT:PCBM. From the EQE spectra with different amount of Si-PCPDTBT, one can see the dramatic increase of the infraredabsorption due to this sensitizer. However, excessive additionof sensitizer into the host donor:acceptor matrix is known todeteriorate the device performance. For example, P3HT:Si-PCPDTBT:indene-C60-bisadduct (ICBA) device showed thebest performance at 0.8:0.2:1 weight ratio and the efficiencydecreased at higher sensitizer concentrations [153]. Becauseof this limited sensitizer amount, the optical absorption in thenear infrared region cannot be sufficiently enhanced. Plasmonenhancement offers an excellent solution to this problem. Weproposed to implement a silver grating electrode supporting aSPP mode at the absorption band of infrared sensitizer so thatthe infrared absorption can be sufficiently enhanced withminimum sensitizer amount that does not hurt the visibleabsorption significantly. From numerical simulations usingthe finite element method, we demonstrated that the small

infrared absorption from Si-PCPDTBT sensitizer could beenhanced up to almost the same level of the visible absorp-tion. As a result, about 40% increase in the short circuitcurrent was obtained as shown in figure 11.

Instead of making gratings on the back electrode, somereported patterned front electrodes as well. Ag nanoholearrays and gratings are used as a transparent front electrode[155, 156]. In these studies, the transparency and the con-ductivity of patterned metal electrodes are important indetermining the device performance. In terms of plasmonicenhancement, these patterned front metal electrodes canenhance the forward scattering efficiency and the local fieldstrength simultaneously. Transparent front metal electrode isnot only beneficial for the plasmonic enhancement but alsoprovides a chance to replace ITO, which is brittle and alsosuffering from its limited supply. For example, chemicallysynthesized Ag nanowires [157] or patterned silver electrode[158] have been used as transparent electrodes with hightransparency comparable to ITO. Figure 12 shows a

Figure 11. Schematic of inverted device structure of ternary blend with silver grating bottom electrode and the simulated surface plasmonpolariton mode (left). Calculated photocurrent density with and without the silver grating (right) [149].

Figure 12. SEM image of transparent metal electrode fabricated by nanoimprint (left) and optical transmittance of Au, Cu, Ag metalelectrodes and ITO (right) [158]. Reprinted with permission from [158]. Copyright 2008 John Wiley and Sons.

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transparent metal electrode fabricated by nanoimprint litho-graphy. The transmittance of these metal electrodes is com-parable to that of ITO film across 400–800 nm wavelengthrange.

In summary, the patterned metal electrode approach wasproven to be efficient in enhancing the optical absorption ofOPV device. Front or back metal electrodes are successfullypatterned by direct imprinting of soft active material or bydoing lithography and metal lift-off. Measuring the EQE orthe absorption spectrum revealed that the coupling of incidentsunlight to the SPP, LSP, and waveguide modes leads toenhanced optical absorption and eventually the PCEenhancement. Especially, SPP mode was found to show highpeak enhancement factor due to its strong plasmonic fieldpresent in the active layer. Ternary blend system with lowband gap sensitizer was also studied to achieve a greatamount of infrared absorption enhancement from SPP reso-nance. Top metal electrode with high level of transparency isan active research subject. Because of their better spectralcontrol throughout visible to near-infrared region as well ashigh enhancement factor, SPP modes supported by differenttypes of patterned metal electrode have a great potential torealize high efficiency OPV devices. More theoretical studiesincluding numerical simulations and experimental works withstate-of-the-art fabrication techniques should prove beneficialin the future.

2.3. Plasmonic metamaterials

Recently, more complex plasmonic structures instead ofsimple nanoparticle or periodic structure were studied in orderto get higher enhancement from solar cells. When the plas-monic resonance overlaps the solar spectrum better, theoverall enhancement becomes higher. Thus it is more desir-able to have a broader plasmonic resonance which can extendthroughout the visible and the near infrared region. In addi-tion, it is beneficial to make the coupling insensitive to theincident angle since the incident angle of the direct sun lightchanges over time while the diffuse sun light is incident at allangles. The SPP mode from periodic patterned electrode has asharp resonance and usually designed for coupling at normalincidence. The LSP mode in metal NPs exhibit a broaderresonance due mainly to inhomogeneous broadening by theirpolydispersity and thus the enhancement factor is low ingeneral. In recent years, plasmonic metamaterial has emergedas a fascinating optical structure due to their exotic propertiessuch as perfect absorption [159, 160], optical phase mod-ulation [161], broadband and omni-directional absorption[145]. Even though the plasmonic metamaterials are notalways aimed at the application of OPV devices, it still is ofgreat interest as well as importance since it provides aninsight to the future of plasmonic OPV technology.

Since the SPP-supporting patterned electrode showed ahigh enhancement factor at the resonance wavelength, it isdesirable to make it broader. Ostfeld et al reported a broad-band, omnidirectional, and polarization-insensitive absorptionenhancement of P3HT:PCBM from two-dimensional periodicand quasi-periodic silver hole arrays which are shown in

figures 13(a) and (b) [162]. They patterned 300 nm thicksilver films with holes of 80 nm in diameter and 70 nm deeparranged periodically and quasi-periodically. Upon introduc-tion of 24 nm thick P3HT:PCBM film on top of the hepta-deca-grid patterns, total absorption enhancement greater than100% was observed over a broad spectral range from 450 nmto 800 nm with peak enhancement of up to 600% at 700 nm.From the Fourier transformed images, one can notice that thequasi-periodicity enables omnidirectional and broadbandcoupling. The relative advantage of random or quasi-periodicstructures over periodic ones is still a controversial subject[163–165]. More theoretical studies will be necessary to fullyaddress the effects of random structures.

Wide-angle plasmonic perfect absorber [159] was alsoreported based on metallic hole array and experimentallyapplied to OPV devices [145]. Chou et al reported broadbandand omni-acceptance plasmonic enhancement from P3HT:PCBM device showing 4.4% PCE, which is 52% higher thanthe reference ITO device [145]. This seems to be the largestPCE enhancement ever reported for bulk heterojunction solarcell with moderate efficiency. Here, the authors suggested asub-wavelength metallic hole array act as a transparent frontelectrode and to form a plasmonic cavity in conjunction withthe flat bottom metal electrode. This plasmonic cavity wasfound to be extremely efficient in coupling incident light intothe absorbing layer placed in between two metal layers. TheSEM image of their hole array structure and the corresp-onding EQE spectrum are shown in figures 13(c) and (d). Abroadband polarization-independent super-absorber based onthe crossed trapezoidal array is also shown in figures 13(e)and (f) [160].

It is noted that the reported plasmonic perfect absorbersare mostly based on metal-insulator-metal sandwich structure.In this case, LSPR can be established and a strong electro-magnetic field is formed in between the two metal layers.According to the theoretical study by Wu et al [166], criticalcoupling occurs when a leaky eigenmode of the structure hasequal radiative and resistive losses. Then, the incomingenergy is transformed into resistive loss without generatingreflection, which means perfect absorption. Detailed studieson this plasmonic perfect absorber are beyond the scope ofthis review and can be found elsewhere [167–170].

As already proven in many theoretical and experimentalstudies, novel plasmonic structure including perfect absorbercan provide a greater improvement in OPV devices thansimple metal NPs or periodic patterns. However, more studiesneed to be done to practically incorporate these novel struc-tures into OPV devices and successfully achieve high effi-ciency enhancement. Their complex structure demands highcost fabrication processes and precise control of geometricalparameters. To alleviate this problem, simpler plasmonicmetamaterial structures were also reported. Instead of usingpatterned structure, a perfect plasmonic absorber in a stack ofmetal film, SiO2 spacer, and a percolated film of nano-composite (Au/SiO2) was reported to show a broadbandabsorption over 90% and was also fairly angle-insensitive[171]. Compared to the patterned metal/insulator/metalstructure, the fabrication is easy and cost-effective which will

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be important for wide deployment. Mattiucci et al demon-strated an efficient absorber consisting of periodic metallo-dielectric multilayer by exploiting broadband, inherently non-resonant, surface impedance matching mechanism [172]. It ispromising that the metamaterial in this work requires nosurface features or nanopatterning. Using this metamaterialstructure, it became possible to achieve perfect absorptionwith an alternating SiO2–Ag–SiO2 structure from 350 nm to750 nm wavelength. An organic tandem cell might be a sui-table application of this type of plasmonic absorber, which iscomposed of several thin organic layers and the intermediatemetal layer.

One should note that most plasmonic perfect absorberconsists of metal and non-absorbing insulating layer. Atperfect absorption condition, the incident energy is absorbedby the metal through ohmic loss and transformed to heat. Inorder to obtain useful optical absorption for photovoltaicapplications, it is desirable to minimize the ohmic loss andenhance the active layer absorption instead. Inserting theabsorbing semiconductor layer into this plasmonic perfectabsorber results in a partial exchange of undesired ohmic losswith the useful absorption [173]. However, more studies willbe necessary to fully understand the effect of absorbing layeron the perfect absorption condition and also to apply it to thepractical OPV devices.

3. Electrical effects

Besides the optical effects of the plasmon resonance, theplasmon effects on the electrical properties of OPV deviceshave also been studied. Since the charge transport and

collection processes are also important as well as the chargegeneration in determining the device efficiency, any smallchanges in the electrical properties after plasmonic nanos-tructure incorporation need to be understood well in order toachieve the best performance enhancement. Here, the effectson the organic layer morphology and the charge carriermobilities will be discussed as they are directly influenced bythe incorporation of plasmonic nanostructures into the organiclayers and also have a huge impact on the OPV efficiency.Also, the plasmonic electrical effect regarding the non-uni-form charge carrier generation caused by the strong plas-monic field will be also discussed.

3.1. Effects of plasmon on morphology and carrier mobility

Research in the OPV field is very active, particularly as itrelates to the nanoscale electrical behavior of devices.Ongoing research deals with molecular engineering of bandgap [174], charge carrier mobilities [175], molecular morph-ology [30, 176–180], etc. This multidisciplinary researchrelies on the fields of organic chemistry, materials science,electrical engineering, and physics. Since the nanoscaleelectrical properties of organic materials are so important indetermining the OPV device efficiency, they should be alsoconsidered when the plasmonic nanostructures are to beincorporated in OPV devices.

Metal NPs in organic matrix was proven to have variouselectrical effects in addition to the optical effects previouslydiscussed. It is not difficult to imagine that the addition ofmetal NPs into the active layer of the OPV device can easilycause marked changes to the morphology of the active mat-erial, which in turn affects the electrical behavior of the

Figure 13. (a) Periodic and (b) quasi-periodic Ag hole array. Inset shows Fourier transformed power spectra of each image [162] (reprintedwith permission, copyright 2011 AIP Publishing). (c) Transparent top electrode of Au hole array fabricated by nanoimprint technique and (d)the corresponding external quantum efficiency spectra [145] (reprinted with permission, copyright 2012 OSA). (e) Broadband, polarization-independent plasmonic super-absorber’s SEM image and (f) the extinction spectra plotted as a function of angle of incidence [160] (reprintedwith permission, copyright 2011 Nature Publishing Group).

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device. Once electrons and holes are generated in the OPVactive layer, these carriers must travel through the active bulkof the device in order to be collected at the electrodes. Theease with which the carriers move through the device ismeasured by their mobility in units of (cm2 V−1 s−1). Mor-phological changes, on both the nano and the macro scale,tend to have drastic effects on carrier mobilities (both positiveand negative), and thus represent a significant portion ofcurrent research in the field. A positive effect of increasedconductivity of organic layers has been reported[70, 101, 103, 105, 108, 117, 124]. Metal NP incorporation atlow concentrations has been shown to introduce dopant stateswithin the bandgap of the OPV polymer, which can providehopping sites for holes and thus enhance the mobility[101, 181]. The increased conductivity leads to the higherchance of exciton dissociation and, hence, usually results inenhanced FF as well as short-circuit current density (Jsc). Onthe other hand, high nanoparticle concentration can adverselyaffect root-mean-square (rms) roughness and layer uniformityduring deposition, which degrades carrier mobilities[108, 117, 119]. Figures 14(a) and (b) show atomic forcemicroscopy (AFM) images of the active layer with andwithout Au NPs respectively, obviously indicating anincrease of rms roughness with Au NPs. These two factors areknown to compete with each other and determine the optimalconcentration of metal NPs to obtain the best efficiencyenhancement. As shown in figures 14(c) and (d), the carriermobilities keep increasing up to Au NP concentration of

2 wt%. But the exciton dissociation probability reaches themaximum at lower concentration. Here, the degradation ofexciton dissociation efficiency at higher Au NP concentra-tions can be attributed to the morphology change of the activelayer, which overwhelms the positive effect of the increasedcarrier mobilities.

Other studies have also shown that embedded metal NPscan improve the structural and morphological stability of theactive layer, resulting in slower device degradation underprolonged illumination [130, 182, 183]. In addition, it hasbeen also reported that more efficient hole collection arisingfrom morphological change is partially responsible for theefficiency enhancement [126]. When Au NPs are incorpo-rated in the anodic buffer layer of PEDOT:PSS, the rough-ened surface of PEDOT:PSS layer leads to an increase insurface roughness which naturally increases the interfacialcontact area between the active layer and the hole transportlayer. This allows more efficient hole collection at the anodeand resulted in the improvement of Jsc and FF. It is stillcontroversial how the metal NPs can affect the carriermobilities and the nanoscale morphology of organic materialssince both positive and negative results have been reportedsimultaneously.

In addition to the effects on mobilities and morphology,another negative effect of metal NPs in OPV devices has beenreported. It has been proposed and experimentally proven thatthe exciton quenching at metal NP sites can be a pathway toefficiency decrease in OPV devices [102]. The time resolved

Figure 14.AFM images of the active layer (PFSDCN:PCBM): (a) the active layer film without Au nanoparticles (NPs), rms=0.617 nm and(b) with Au NPs: 6 wt%, rms=8.062 nm. (c) The hole and electron mobilities with different Au NP concentrations in the active layer. (d)The decay rate of bound electron-hole pair and the exciton dissociation probability under short circuit condition at different Au NPconcentrations [108]. Reprinted with permission from [108]. Copyright 2011 Royal Society of Chemistry.

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photoluminescence study of P3HT:PCBM blend film revealedthat Au NPs can enhance the exciton quenching process viathe coupling between the strong plasmonic field and thephotogenerated excitons [98]. However, to fully understandthe exciton dynamics including the exciton quenching anddissociation, more dedicated studies such as ultra-fastspectroscopy should be performed.

Since the effect on the nanoscale electrical propertiesincluding carrier mobilities, organic layers’ morphology, andthe exciton quenching induced by metal NP incorporation ismanifold in both positive and negative way, a careful designof plasmonic nanostructure is important to obtain the bestefficiency enhancement. The metal NP concentration is one ofthe most critical factors determining the balance between theincreased charge carrier mobilities and the detrimental mor-phological change. Other effects such as the improved long-term stability, faster exciton quenching are also important andneed to be investigated further.

3.2. Non-uniform carrier generation

Other than the effect on the nanoscale electrical propertiesarising from metal NP incorporation, there also have beensome studies about how the localized plasmonic field and thecorresponding local enhancement of charge carrier generationrate can directly affect the carrier transport and collection.This non-uniform charge carrier generation along with theunbalanced carrier mobilities results in a new plasmonicelectrical effect on the performance of OPV devices. Thisaspect will be presented below.

The space charge limit is a fundamental electrical prop-erty which originates from unbalanced carrier mobilities andthe corresponding accumulation of space charges. Thesespace charges will limit the ultimate efficiency due to thebuild-up of a potential difference and the higher recombina-tion rate in the active layer [184–187]. In most organicsemiconductor devices, the mobility of holes is lower thanthat of electrons which results in hole accumulation in the

device [22, 188, 189]. This will eventually reduce Jsc and FFof the OPV device. However, it is recently reported that anovel plasmonic-electrical concept can break this spacecharge limit effect [190, 191]. The space charge limit effectwith and without the plasmonic resonance was studied usingthe metallic grating electrode as a plasmonic structure. Theapproach is schematically drawn in figure 15.

When charge carriers are generated upon photonabsorption in the active layer, the external photocurrent isdetermined by the mean electron and hole drift length. In theinverted device, the Sun light will first penetrate through thetransparent cathode and the charge carriers are mostly gen-erated close to the cathode. In this case, since the mean holedrift length is smaller than the mean electron drift length andalso the active layer thickness, the holes are accumulated andlimit the external photocurrent. In this regime the photo-current has a square-root dependence on the effective appliedvoltage [185]. While this space charge limit regime wasobserved in the Ag-planar-inverted devices, it was found thatthis space charge limit characteristics are eliminated when theAg nanograting is incorporated in the hole-collecting elec-trode [190]. By solving the Maxwell’s equations, it wasshown that the exciton generation rate near the Ag gratingelectrode is highly increased due to the localized surfaceplasmon field as shown in figure 16. Thanks to this plasmon-induced redistribution of holes, the highly-shortened transportpaths of holes are expected to increase the collection by theanode, which eventually eliminates the hole accumulationeffect.

Because one can now expect improvements from theplasmonic nanostructures in the electrical properties as well asthe optical absorption enhancement, it is necessary to considerboth electrical and optical effects in designing a plasmon-enhanced OPV device. This requires a combined simulationof the optical behavior derived from Maxwell’s equation andthe electrical behavior described by the kinetic model incor-porating the Poisson equation, continuity equation, and drift/

Figure 15. Schematic drawings of (a) inverted organic solar cell with a planar metallic anode and (b) with a grating metallic anode [190].Reprinted with permission from [190]. Copyright 2014 Nature Publishing Group.

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diffusion equations [191–195]. Rourke et al found that theexciton generation rate is highly dependent on the positionand the photon wavelength when the plasmonic resonance atAg grating electrode is incorporated into the device as shownin figure 17. From the calculated exciton generation map inthe active layer, the charge carriers are mostly generated at thetop portion of the active layer under illumination of visiblephotons. In contrast, the infrared sub-bandgap photons whichare coupled to the surface plasmons generate most chargecarriers near the Ag-grating bottom electrode. This highlyposition-dependent carrier generation leads to a substantialdifference in photocurrent when the mobility ratio betweenthe different charge species changes. When the hole mobilityis decreased relative to the electron mobility, a substantialdecrease in the photocurrent was observed since holes aresubject to more chance of recombination during their path to

the electrode. In addition, this photocurrent decrease wasproven to be also highly dependent on the device structure,namely the standard and the inverted structures. In the stan-dard structure, holes are collected at the transparent frontelectrodes while electrons are collected at the front electrodesin the inverted structure. In the standard structure the photo-current decrease originating from the unbalanced mobilities isclearly observed in the infrared region, but not in the visibleregion. This is explained well by the non-uniform excitongeneration rate mentioned above. Because holes are collectedat the front electrode in the standard device structure, thosegenerated near the back electrode will suffer more severelywhen the hole mobility is decreased. The inverted devicestructure behaves in the opposite way since in this case holesare collected at the back electrode and those generated nearthe front electrode will suffer more severely.

Figure 16. Photocarrier generation rate, Gopt,n,p, within the active layer at surface plasmon resonance for planar (a) and grating (b) electrodeplotted in a log scale. (c) log10Gopt,n,p for various horizontal slices through the active layer at positions indicated by the red dotted, dashed andsolid lines in (a) and (b) [191]. Reprinted with permission from [191]. Copyright 2014 AIP Publishing LLC.

Figure 17. (a) AM0 spectral irradiance as a function of wavelength. Spectral short circuit current density for standard (b) and the inverted (c)device structure. Three different mobility ratios are shown, where the hole mobility, μp, is smaller than the electron mobility, μn, by a factorof zero, one, and two orders of magnitude. Plots of electric field strength are shown as insets at visible and near-infrared wavelength [191].Reprinted with permission from [191]. Copyright 2014 AIP Publishing LLC.

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From the above experimental and theoretical studies, theplasmonic electrical effect was shown to greatly influence theOPV’s performance. Due to the unbalanced carrier mobilitiesand the strongly redistributed photocarrier generation fromlocalized plasmonic field, it was found that there is a clearplasmonic electrical effect on the charge transport and col-lection processes beyond the simple optical absorptionenhancement. A space charge limit arising from the low holemobility and the consequent hole accumulation effect in theinverted structure can be eliminated by forming a plasmonicstructure at the metal anodic electrode. This plasmonic elec-trical effect will be a critical factor in determining the ultimateefficiency of plasmonic OPV devices. Also, the developmentof methodology to simulate the electrical as well as the opticalbehavior of OPV will become more important to design andoptimize the plasmonic nanostructures for significantlyimproved efficiency.

4. Summary and outlook

OPV technology shows a bright prospect for cost-effectivesolar energy harvesting. Their unique properties such asmechanical flexibility, light weight, semi-transparency alsopresent a great promise with a variety of applications andmake OSCs as an interesting research subject. However, theirrelatively low efficiency originating from the large bandgapand short exciton diffusion length calls for an efficient lightmanagement technique that can enhance the optical absorp-tion effectively without negatively impacting the electricalproperties. As shown in this review, plasmonic nanostructuressuch as metal NPs, patterned metal electrodes, and plasmonicmetamaterials were successfully demonstrated to improve theefficiency through the resonant scattering and/or the stronglocal field enhancement. In addition, we also looked into theplasmonic effects on the electrical properties of OPV devicessuch as the morphology and the mobility changes after metalnanoparticle inclusion and also the non-uniform carrier gen-eration rate originated from local plasmonic field.

Looking ahead, there are some new directions beingexplored. The tandem organic solar cell composed of a stackof two or more sub-cells with different bandgaps provides apromising way to realize high efficiency OPV devices. Byvirtue of the complementary absorption from each sub-cells,the thermalization loss of photonic energy can be reduced. Inaddition, the open circuit voltage can be maximized viaproper voltage matching. Plasmonic structure is expected toplay a positive role in the tandem cells as has been demon-strated in the single junction devices. Some pioneering studiesalready have shown that Ag clusters [196] or Au nano-particles [197] implemented at the interconnecting layer canincrease the efficiency of tandem organic solar cells from theplasmonic near field enhancement. However, more dedicatedresearch efforts are needed in order to fully exploit the fulleffects of plasmonic nanostructures. For example, the role ofmetal nanoparticles on the current and voltage matchingneeds to be better understood since these are critical to thetandem device performance [198, 199]. Patterned metal

electrode is yet to be applied in tandem cells, which mayproduce higher enhancement. The combined optical andelectrical modeling [191, 200] will also play an important rolein optimizing the plasmonic tandem cell geometries.

Another big opportunity can be found in the perovskitesolar cell. The inorganic-organic hybrid perovskite solar cellwas first reported in 2009 [201] and has shown a rapidincrease of their PCE [202–205]. Plasmonic enhancement ofperovskite solar cell is currently being researched [206–210].Au, Ag, and Au–Ag alloy nanoparticles have been incorpo-rated into the mesoporous scaffold of hole-blocking TiO2 orAl2O3 matrix to enhance the optical absorption via plasmonicresonances. Thanks to the inherently high efficiency of per-ovskite devices, even a small fractional enhancement canrepresent a large absolute efficiency enhancement. Mesopor-ous scaffold is also a very interesting platform where one canachieve enhancements by both photonic crystal and plas-monic effects [211–213].

By reviewing various plasmonic nanostructures andclearly elucidating the underlying physics governing differenteffects by them, this review is aimed at helping to realizerational design of optimized device architecture. Ongoingresearches are expected to unveil the plasmonic enhancementmechanisms more clearly on both the optical and the elec-trical properties. Further investigations on plasmonic meta-materials, tandem cells, and perovskite solar cells areexpected to pave the way to the realization of ultimate OPVdevices of higher-than-20% efficiency.

Acknowledgments

This work is supported in part by the ARPA-E grant DE-AR0000289 and NSF grant CHE-1125935.

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Plasmonic nanostructures for organic photovoltaic devicesecee.colorado.edu/~wpark/papers/J. Opt. 2016 Ahn.pdf · 2016. 2. 16. · Keywords: organic photovoltaics, plasmonics, nanostructure,