polymers

Review Recent Advances of Plasmonic Organic Solar Cells: Photophysical Investigations

Lin Feng 1, Mengsi Niu 1, Zhenchuan Wen 1 and Xiaotao Hao 1,2,* ID

1 School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, Shandong, China; [email protected] (L.F.); [email protected] (M.N.); [email protected] (Z.W.) 2 ARC Center of Excellence in Exciton Sciences, School of Chemistry, the University of Melbourne, Parkville, Victoria 3010, Australia * Correspondence: [email protected]; Tel.: +86-531-5878-3287

Received: 29 December 2017; Accepted: 21 January 2018; Published: 26 January 2018

Abstract: The surface plasmon resonance (SPR) of metallic nanomaterials, such as gold (Au) and silver (Ag), has been extensively exploited to improve the optical absorption, the charge carrier transport, and the ultimate device performances in organic photovoltaic cells (OPV). With the incorporation of diverse metallic nanostructures in active layers, buffer layers, electrodes, or between adjacent layers of OPVs, multiple plasmonic mechanisms may occur and need to be distinguished to better understand plasmonic enhancement. Steady-state photophysics is a powerful tool for unraveling the plasmonic nature and revealing plasmonic mechanisms such as the localized surface plasmon resonance (LSPR), the propagating plasmon-polariton (SPP), and the plasmon-gap mode. Furthermore, the charge transfer dynamics in the organic semiconductor materials can be elucidated from the transient photophysical investigations. In this review article, the basics of the plasmonic mechanisms and the related metallic nanostructures are briefly introduced. We then outline the recent advances of the plasmonic applications in OPVs emphasizing the linkage between the photophysical properties, the nanometallic geometries, and the photovoltaic performance of the OPV devices.

Keywords: photophysics; organic photovoltaics; plasmonics

1. Introduction In the photovoltaic industry nowadays, the inorganic solar cells based on poly- and mono-crystalline silicon account for more than half of the market share. The efficiencies of the silicon solar cells are typically 15–18%, and the thickness is in the range of 160–240 µm [1]. However, the inorganic solar cells are expensive as a result of the high-cost and complicated processing of Si. Therefore, new generations of solar cells are in great demand. In recent years, organic photovoltaic cells (OPVs) have garnered much research interest, because they are promising candidates for a next-generation renewable power source due to their advantages such as being low-cost, light-weight, and having mechanical flexibility. Up to date, the power conversion efficiency (PCE) of OPV has been as high as 13–14% [2–4], demonstrating its potential for final commercial productions. In OPVs, owing to their relatively low carrier mobility (on the order of 10−4 cm2/V·s) and short diffusion length in most of the organic semiconductor materials [5], the photoactive layer is usually required to be quite thin, i.e., around 100 nm or less, to facilitate the charge carrier diffusion and extraction [6]. It results in poor absorption of the incident light, while much photon energy that could have been converted to the electrical energy is lost. Hence, intensive research has been stimulated to further enhance the light absorption of OPVs. To date, many studies presented that without varying the thickness of the photoactive layer, the surface plasmon resonance (SPR) effect can be introduced via the involvement of metal

Polymers 2018, 10, 123; doi:10.3390/polym10020123 www.mdpi.com/journal/polymers Polymers 2018, 10, 123 2 of 33 nanostructures to boost the light harvesting in the photoactive layer, thereby enhancing the photovoltaic performances [7,8]. The shapes, sizes, and compositions of the metallic nanoparticles (NPs) can be chemically tailored to manipulate the plasmonic enhancement of the optical absorption [9–11]. Between the structural configurations of the plasmonic NPs and the ultimate device performance, there is an important intermediate link, which is the photophysics of the system [12–14]. By monitoring the photophysical properties by means of steady-state and transient absorption or photoluminescence spectroscopy, the charge transfer dynamics and detailed plasmonic processes can be investigated. This article gives an overview of the advances in plasmonic-enhanced OPV solar cells in recent years and emphasizes the involved photophysics. Firstly, the mechanisms of the surface plasmon resonance (SPR) effect are introduced. Then, the configuration and location of the diverse plasmonic nanostructures are summarized in association with relevant device performance enhancement. We show that the steady-state and transient photophysical characterizations have been widely used to clarify the plasmonic processes and operating principles in boosting the OPV performances. Finally, conclusion and outlooks based on plasmonic OPV are listed out.

2. Mechanism of Plasmonic Enhancement Effect in Opvs Adequate light absorption is prerequisite to the OPV devices for the improvement of PCE. The light trapping in the organic solar cells can be intensified by incorporating metallic NPs. At the metallic-dielectric interfaces in multiple plasmonic solar cell configurations, the electrons of the nanometals can be excited collectively, which amplifies the electromagnetic field in the photoactive layer of the device (Figure1). This is called SPR effect and can be elucidated by the Mie theory [ 15,16].

Figure 1. Architecture of OPV solar cells with plasmonic metallic NPs. (a) NPs in the active layer for LSPR enhancement; (b) NPs in the charge carrier transport layer (CTL) as light trapping centers due to the forward scattering effect; (c) NPs in CTL that induced the enhancement of the electromagnetic field in the photoactive layer due to the LSPR effect; (d) light trapping by the excitation of surface plasmon polaritons (SPP) at the metal/semiconductor interface. The center graph is a mechanism illustration of localized surface plasmon in metallic NPs.

The plasmonic enhancement can be achieved through several different SPR mechanisms, e.g., the far-field scattering effect (Section 2.1), the near-field localized or propagating SPR modes (Section 2.2), and some other plasmonic modes like waveguide mode and plasmon-cavity mode [17–21] (see Figure1). The enhancement of optical absorption can be achieved by far-field scattering effect due to the increase of the optical path of incident photons and the decrease of the reflection at illuminated surfaces. In cases of near-field SPR effect, the number of absorption events is increased, because Polymers 2018, 10, 123 3 of 33 the metallic nanostructures can confine electromagnetic waves at the metal-dielectric interface and produce intense near field.

2.1. Far-Field Scattering Effect Upon the far-field scattering, the optical path of the incident sunlight is lengthened, which may promote the absorption of the photons. The size of the metallic NPs that best fits the forward scattering effect usually falls in the regime of 30–60 nm [22]. For plasmonic nanostructures embedded in front buffer layer (i.e., HTL in conventional type or ETL in inverted type solar cells), the scattering in the forward direction is requisite. The geometry of the plasmonic nanostructures is optimized to minimize the reflection at the surfaces. The optical paths of the incident sunlight in the absorbing medium and their interaction time are thus increased, resulting in enhanced absorption efficiency of the photoactive layer [23]. For the NPs in back buffer layers further away from the incident photons, backward scattering should be manipulated to the maximum for efficient light absorption. The scattering direction can be tuned by controlling the size and geometry of the plasmonic nanostructures. From NPs with a diameter of c.a. 50 nm, photons are scattered forward and backward in comparable quantities [24]. As the particle size increases, more radiation is scattered in the backward orientation. NPs with sizes smaller than abovementioned intermediate range present weak scattering, which induces insufficient optical absorption. Moreover, the conductivity of the device is detrimentally affected because more charge trapping sites are formed at the NP sites due to their large surface-to-volume ratio. For plasmonic nanostructures embedded outside the active layer, such losses overwhelm the possible advantages that small NPs (e.g., ~20 nm) may exaggerate the coupling of the near plasmonic field to the active layer [25]. Likewise, overly large nanostructures (e.g., hundreds of nanometers) are also harmful in view of the inferior layer morphology, strong absorption inside the metal, and shunting problems [26].

2.2. Near-Field Localized and Propagating Plasmonic Effects

2.2.1. Localized Surface Plasmon Resonance (Lspr) The localized surface plasmon resonance (LSPR) is non-propagating excitation of electrons confined in a metallic NP. When LSPR occurs, the electrons inside metallic NPs collectively oscillate with the incident radiation [27], leading to greatly intensified electric fields near the NPs surfaces. LSPR often takes place when the surfaces of the plasmonic nanometals are curved or kinked [28]. The optical absorption reaches its maximum at the plasmon resonance frequency. The resonance frequency is geometry-dependent for the metal nanostructures. Therefore, tailoring appropriately the size and morphology of the NPs is an effective approach to control the plasmonic enhancement to meet specific demands. For noble metal NPs, the resonance usually occurs in the visible and near infrared wavelength region [29,30]. Normally, the size of the metal NPs should be smaller than the wavelength of the incident light for plasmon excitation. The NPs with sizes smaller than 30 nm act as subwavelength antennas upon the LSPR excitation. In such cases, the absorption enhancement is dominantly from the LSPR effect rather than the scattering effect [31]. Metallic NPs, which are smaller than 30 nm, behave as local field enhancers instead of scattering centers. The plasmonic near-field induced in the photoactive layer increases the absorption cross-section and thus effectively enhances the photon absorption. If these kinds of NPs are placed outside the active layer, e.g., in the buffer layer (see Figure2), the enhanced electric field may also be coupled to the absorbing layer, increasing its effective absorption cross section [32,33]. Polymers 2018, 10, 123 4 of 33 Polymers 2018, 10, x FOR PEER REVIEW 4 of 33

2.2.2.2.2.2. PropagatingPropagating SurfaceSurface PlasmonPlasmon PolaritonPolariton (Spp)(Spp) InIn contrastcontrast toto LSPR,LSPR, surfacesurface plasmonplasmon polaritonpolariton (SPP)(SPP) oftenoften occursoccurs onon oror inin vicinityvicinity ofof planarplanar metalmetal films.films. SPP SPP is is an an electromagnetic electromagnetic wave wave at at infrared infrared or or visible visible frequencies frequencies that that propagates propagates in in a awave wave-like-like fashion fashion along along the the planar planar metal metal-dielectric-dielectric interface interface [34 [34––36].36]. The The charge motions of SPP SPP involveinvolve contributions contributions from from both both the the metal meta andl and the dielectric. the dielectric. The wavelengths The wavelengths of SPP of waves SPP are waves shorter are inshorter comparison in comparison with the incidentwith the irradiation, incident irradiation, resulting in resulting more intense in more magnetic intense field. magnetic The amplitude field. The of theamplitude SPP magnetic of the waves SPP magnetic decays exponentially waves decays with exponentially increasing distance with increasing into each distance medium into from each the interface.medium fro Perpendicularm the interface. to the Perpendicular interface, the to spatial the interface, confinement the ofspatial SPP is confinement in a subwavelength-scale of SPP is in a range.subwavelength An SPP- canscale propagate range. An alongSPP can the propagate interface along for 10–100 the interfaceµm until for its 10~100 energy μm is until consumed its energy by theis consumed absorption by or the scattering. absorption This or scattering. increased opticalThis increased path with optical the lateral path with propagation the lateral promotes propagation the absorptionpromotes the significantly. absorption significantly.

FigureFigure 2.2. ((a)) W Waveguideaveguide model model at at the the wavelength wavelength of of 400 400 nm nm [35]; [35 ];(b ()b device) device material material pro profile;file; and and (c) (electricc) electric and and (d ()d magnetic) magnetic field field intensity intensity pro profilesfiles on on Au Au NPs NPs at at 560 560 nm nm [37]. [37]. Repr Reprintedinted with permissionpermission fromfrom Ref.Ref. [[35]35] andand Ref.Ref. [[37]37]..

2.3.2.3. SomeSome OtherOther ModesModes forfor AbsorptionAbsorption EnhancementEnhancement

2.3.1.2.3.1. PhotonicPhotonic WaveguideWaveguide ModeMode BesidesBesides SPP SPP mode, mode, another another kind of kind waveguide of waveguide mode may mode be excited may at be the excited planar metal/dielectric at the planar interfacesmetal/dielectric upon the interfaces incident upon irradiation, the incident which irradiation, also contributes which to also the absorption contributes enhancement. to the absorption It is calledenhancement. “waveguide It is mode”called “waveguide because it resembles mode” because the propagation it resembles pattern the in propagation slab dielectric pattern waveguides in slab (Figuredielectric2). waveguides Along its lateral (Figure propagation, 2). Along part its lateral of the waveguidepropagation, mode part canof the be absorbedwaveguide by mode the organic can be semiconductors,absorbed by the inorganic which semiconductors, excitons are generated in which [38 excitons,39]. The are simulation generated of [38,39]. the waveguide The simulation mode can of bethe achieved waveguide by solvingmode can Maxwell’s be achieved equations by solving governing Maxwell optical’s equations properties governing of solar cellsoptical by properties using the finite-differenceof solar cells by frequency-domainusing the finite-difference (FDFD) frequency method. -domain (FDFD) method. TheThe excitation excitation of of the the waveguide waveguide mode mode is is independent independent on on the the polarization polarization of of the the incident incident irradiation.irradiation. InIn anotheranother word,word, therethere cancan bebe waveguidewaveguide modesmodes regardlessregardless ofof whetherwhether thethe polarizationpolarization directiondirection ofof thethe excitationexcitation isis parallelparallel withwith oror orthogonalorthogonal toto thethe metalmetal nanopatternnanopatterndirections directions [ 40[40].].

2.3.2.2.3.2. Plasmon-CavityPlasmon-Cavity ModeMode WhenWhen specificspecific metal-dielectric metal-dielectric configurations configurations are are constructed, constructed, the the spectral spectral wavelength wavelength ranges ranges for absorptionfor absorption enhancement enhancement can be can further be further expanded expanded due to thedueplasmonic to the plasmonic coupling coupling modes.For modes. instance, For ains metal-dielectric-metaltance, a metal-dielectric (MDM)-metal structure (MDM) is obtained structure when is obtainedthe photoactive when layer the isphotoactive sandwiched layer by the is metalsandwiched electrode by and the metallic metal electrodeNPs layer. and Byappropriately metallic NPs selecting layer. By the appropriately NPs geometries, selecting standing the waves NPs ofgeometries, the surface standing plasma canwaves be generated of the surface as a result plasma of can constructive be generated interferences as a result in the of finite constructive cavity, whichinterferences decreases in the the light finite leakage cavity, from which the decreases surface plasmon the light generation leakage in from lateral the gaps surface between plasmon the NPs.generation Thus, thein lateral plasmon-cavity gaps between mode the is NPs. excited Thus, in MDM the plasmon and strong-cavity electromagnetic mode is excited field in isMDM confined and strong electromagnetic field is confined into the light absorbing layer. It is worth noting that the field Polymers 2018, 10, 123 5 of 33 into the light absorbing layer. It is worth noting that the field enhancement from the plasmon-cavity plasmonicsPolymers 2018 is independent, 10, x FOR PEER REVIEW of the polarization or incidence angles of the incoming light [415]. of 33

3. Materialenhancement Nanostructures from the plasmon for Plasmonic-cavity plasmonics Enhancement is independent of the polarization or incidence angles of the incoming light [41]. Up to date, the metals most often adopted for plasmonic enhancement in bulk heterojunction (BHJ)3. OPVMaterial solar Nanostructures cells are gold for and Plasmonic silver, because Enhancement the plasmonic resonance is readily observed in Au and Ag due to their delocalized valence electrons [42]. However, in consideration of the high-cost of Up to date, the metals most often adopted for plasmonic enhancement in bulk heterojunction Au and(BHJ) Ag, OPV itis solar also cells important are gold toand use silver, inexpensive because the plasmonic plasmonic materials resonance inis readily organic observed solar cells, in Au since a majorand advantage Ag due to of their organic delocalized photoelectric valence electrons conversion [42]. system However, is the in consid low cost.eration The of utilization the high-cost of of copper and aluminumAu and Ag, forit is plasmonicsalso important have to use also inexpensive been reported plasmonic in the materials literature in [organic43]. solar cells, since a majorThe spectral advantage position of organic of the photoelectric LSPR peak conversion can be controlled system is bythe tailoringlow cost. The the utilization shape, size, of andcopper spacing of theand nanometals aluminum for and plasmo the dielectricnics have also environment. been reported The in shapesthe literature of the [43]. particles can be fabricated to be spheres,The ellipsoids, spectral position hemispheres, of the LSPR prisms, peak can cylinders, be controlled and by anisotropic tailoring the structures. shape, size, When and spacing metals are usedof to the construct nanometals the and electrode, the dielectric they canenvironment. be patterned The shapes to gratings, of the nanoholeparticles can arrays, be fabricated and nanomeshes to be (Figurespheres,3)[ 41 ellipsoids,,42,44,45]. hemispheres, The LSPR peaks prisms, of cylinders, the metallic and NPs anisotropic arise from structures. multiple When orders metals of dipoleare used modes to construct the electrode, they can be patterned to gratings, nanohole arrays, and nanomeshes and usually have narrow bandwidths for plasmonic absorption enhancement. In spherical NPs with (Figure 3) [41,42,44,45]. The LSPR peaks of the metallic NPs arise from multiple orders of dipole identicalmodes sizes, and Auusually NPs have whose narrow permittivity bandwidths is largerfor plasmonic exhibit LSPRabsorption peaks enhancement. at longer wavelengths In spherical with respectNPs to with Ag NPs identical [46]. sizes, Au NPs whose permittivity is larger exhibit LSPR peaks at longer wavelengthsIn order to expandwith respect the spectralto Ag NPs region [46]. of the plasmonic absorption enhancement to wide visible or even toIn infrared order to regime,expand the one spectral needs region to incorporate of the plasmonic multiple absorption metals or enhancement metals combined to wide with visible organic moleculesor even into to infrared the OPV regime, devices one [needs47–49 to]. incorporate For instance, multiple broadband metals or SPR metals enhancement combined with can organic be achieved by mixingmolecules Au andinto the Ag OPV NPs devices in one layer[47–49]. or For stacking instance, multiple broadband metallic SPR NPsenhancement layers. The can nano-biobe achieved hybrids withby Ag mixing nanoprisms Au and andAg NPs natural in one sensitizer layer or stacking molecules multiple allow metallic the harvest NPs layers. of photons The nano in- abio broad hybrids spectral bandwith due Ag to thenanoprisms plasmonic and light natural trapping sensitizer from molecules Ag nanoprisms allow the andharvest the of energy photons transfer in a broad from spectral the natural band due to the plasmonic light trapping from Ag nanoprisms and the energy transfer from the solar energy absorbing protein. natural solar energy absorbing protein. AnotherAnother alternative alternative approach approach is is the the employmentemployment of of nanometals nanometals with with complex complex and andasymmetric asymmetric geometriesgeometries [9,50 [9,50].]. In suchIn such anisotropic anisotropic nanostructures, nanostructures, these these exist exist in inan aninherent inherent coupling coupling between between the the centercenter parts parts and and the surroundingthe surrounding branches, branches, which which generates generates concentrated concentrated fieldfield with high high intensity intensity and remarkableand remarkable scatterings. scatterings. This novel This novel methodology methodology enables enables broadband broadband plasmonic plasmonic resonancesresonances and and has greathas potential great potential to enhance to enhance the photovoltaic the photovoltaic performance performance of OPVof OPV devices. devices.

Figure 3. Some metallic nanostructures for plasmonic enhancement, including (a) NPs, (b) Nanorods Figure 3. Some metallic nanostructures for plasmonic enhancement, including (a) NPs, (b) Nanorods (NRs), (c) nanoarrows, (d) grating, (e) multidiffractive grating, (f) nanohole array, (g) nanomeshes, c d e f g (NRs),(h) (bimetallic) nanoarrows, NPs, (i) ( asymmetric) grating, nanostars, ( ) multidiffractive (j) nano-bio grating,hybrid, (k ( )) Ag nanohole nanowire array, networks, ( ) nanomeshes, and (l) h i j k ( ) bimetallicNPs linked NPs,with 2D ( ) material asymmetric WS2. Reprinted nanostars, with ( ) permission nano-bio hybrid,from Refs. ( )[51 Ag–60] nanowire. networks, and (l) NPs linked with 2D material WS2. Reprinted with permission from Refs. [51–60]. Polymers 2018, 10, 123 6 of 33

Metallic NPs in the active layer may serve as combination centers for the electrons and holes and dampen the OPV performances. A solution is to encapsulate the nanometals with non-absorbing shells like TiO2 or SiO2. Theoretical simulations have shown that even very thin layers may attenuate the plasmonic intensity of the metals [61]. Hence, there is a tradeoff between the losses in the electrical current and the plasmonic effect attenuation. In recent years, two-dimensional (2D) atom-thick materials have attracted attention due to their fantastic thermal, optical, and electric properties [62–64]. 2D materials have been adopted in OPV devices to work concomitantly with metallic nanostructures to advance the stability and solar energy harvest. Lee et al. transferred graphene both on top and at the bottom of Ag NRs networks to protect them against thermal degradation. Constructive interference of the incident light in grapheme-Ag-graphene produced better transmittance than pure Ag NRs. It is of great importance to point out the plasmonic effect in 2D materials. The SPR frequency of grapheme resides in the range of 1012–1014 Hz. It has been reported that grapheme plasmons can be excited at visible wavelength [65]. However, even they were involved in the solar cells, the plasmonics from the 2D materials have scarcely been discussed. Viable approaches still need to be developed to unambiguously discriminate the plasmonics of non-metal materials from those of the metals.

4. Device Architectures for Plasmonic Enhancement The metallic NPs in OPV devices play different roles depending on their spatial arrangement across the device cross-section [66,67]. Nominally, the nanometals distribute in the active layer, and/or in the charge carrier transport layer, in the electrodes, or between the above mentioned layers. The configuration of the nanostructures, the relevant plasmonic mechanisms, and the performance of the corresponding organic solar cells are summarized in Table1. Polymers 2018, 10, 123 7 of 33

Table 1. Performance of OPV solar cells with various nanostructures at different locations.

Short-Circuit Open-Circuit Fill Factor System Configuration Mechanism PCE Enhancement Current Jsc Voltage (Voc) (FF) P3HT: PCBM: control 8.67 0.60 61.4 3.19 LSPR ~23% in PCE Ag NPs [68] In active layer 10.41 0.60 62.8 3.92 control 9.49 0.63 63.47 3.77 Ag NPs 10.60 0.63 63.61 4.21 P3HT:PCBM:Ag NPs/NRs in active layer [51] Ag NRs LSPR 10.99 0.63 63.64 4.37 ~28% in PCE Au NPs 11.17 0.63 63.70 4.44 Au NRs 12.21 0.63 63.75 4.85 PCDTBT:PC BM: control 10.6 0.89 60.2 5.6 71 LSPR ~13% in PCE WS2-Au [52] In active layer 12.3 0.89 58.4 6.3 PBDT-TS1:PC BM control asymmetric 18.37 0.81 67.00 9.97 71 ~5% in PCE Au nanostars in active and PEDOT [53] In active and HTL modes 19.24 0.81 67.70 10.50 control 16.5 0.74 0.60 7.52 PEDOT/Au NR@ SiO /PTB7:PC BM [69] between CTL and active Scattering, LSPR ~28% in PCE 2 71 21.2 0.74 0.60 9.55 layer control Backward 10.87 0.89 61.7 5.96 PCDTBT:PC71BM/Au NRs in TiOx Au NRs in back ETL scattering 12.03 0.89 62.9 6.75 ~13% in PCE PTB7:PC71BM/Au NRs in TiOx [70] control Backward 16.27 0.76 60.1 7.43 ~8% in PCE Au NRs in back ETL scattering 17.17 0.76 61.4 8.01 control Forward 16.18 0.717 60.7 7.0 PTB7:PC BM/ ZnO@CNT-Au (ETL) [71] ~13% in PCE 71 ZnO@CNT-Au as ETL scattering 16.81 0.721 64.7 7.9 control Forward 14.70 0.85 49.1 6.14 PCDTBT:PC BM/ZnO (ETL) [72] ~27% in PCE 71 Au arrows in ETL scattering, LSPR 17.40 0.85 52.9 7.82 control Forward 14.49 0.75 61.7 6.67 PBDTTT-CF:PC BM/ZnO (ETL) [73] ~18% in PCE 71 Au NPs in ETL scattering, LSPR 15.81 0.75 66.2 7.86 control Backward 8.71 0.87 61.4 4.65 P3HT:ICBA/WO (HTL) [62,74] ~37% in PCE 3 Cu NPs in rear HTL scattering 11.79 0.87 62.2 6.38 control Backward 7.91 0.87 64.6 4.57 P3HT:ICBA/ WO (HTL) [54] Ag-Au bimetallic NPs in ~43% in PCE 3 scattering 11.01 0.87 67.6 6.55 rear HTL control P3HT:PC BM/Ag grating in PEDOT [75] LSPR 12.1 — — — ~47% in Jsc 61 tapered Ag grating in HTL Polymers 2018, 10, 123 8 of 33

Table 1. Cont.

Short-Circuit Open-Circuit Fill Factor System Configuration Mechanism PCE Enhancement Current Jsc Voltage (Voc) (FF) PTB7:PCBM/PEDOT/Ag nanodot array/ control LSPR, forward 17.43 0.75 0.60 7.70 ~39% in PCE ITO [76] between HTL and anode scattering 23.26 0.73 0.61 10.72 hybridization of control Ag networks /ZnO/PCDTBT:PCBM/ LSPR and 9.32 0.882 63.5 5.22 Ag oblate NPs array ~15% in PCE MoO /Ag oblate NPs/anode [77] plasmonic gap 11.37 0.881 60.0 6.01 3 between HTL and anode mode control PTB7:PC BM/ZnO/Au NPs/ 15.53 0.72 60.70 6.75 71 Au NPs between ETL and MDM absorber ITO [37] 15.69 0.72 63.99 7.27 cathode control PTB7:PC BM/nano-bio hybrid/ZnO/ 16.01 0.79 0.72 9.03 71 Ag prisms-LHCII between LSPR ~17% in PCE ITO [55] 17.99 0.80 0.73 10.57 the active layer and ETL control PBDTTT-C:PC BM/Au NPs/PEDOT/ 10.62 0.73 59.9 4.78 60 Au NPs ~15 nm between LSPR ~15% in PCE ITO [78] 11.74 0.73 63.4 5.52 the active and HTL reference Ag NWs between cathode 8.13 0.60 0.62 3.10 ITO/ZnO/P3HT:PC61BM/MoO3/Al [79] and ETL LSPR 9.87 0.61 0.63 4.05 ~31% in PCE Ag NWs between ETL and 8.92 0.60 0.62 3.31 active layer control Higher 9.75 0.58 57 3.16 P3HT:PC BM/ITO (or ZAZ) [80] ZnO/Ag NWs/ZnO as tranmission ~12% in PCE 61 11.6 0.54 57 3.53 transparent electrode above 450 nm P3HT:PC BM/PEDOT/ control SPP, photonic 6.13 0.63 0.59 3.03 61 ~16% in PCE Au (flat or grating) [81] Au grating as rear electrode waveguide mod 6.83 0.63 0.62 3.53 SPP, photonic control ITO/ZnO/P3HT:PCBM/Ag grating [56] waveguide factor of ca. 5 enhancement of the EQE Ag grating as rear electrode mode control Multidiffractive Al grating coupling of SPP P3HT:PCBM/Al grating [57] absorption in NIR range under light absorbing modes medium Polymers 2018, 10, 123 9 of 33

4.1. In the Active Layer In BHJ solar cells, plasmonic NRs/NPs can be blended with the organic semiconductor donor and acceptor in the active layers. When the sizes of the nanostructures are smaller than ~30 nm, the LSPR effect plays the main role in achieving better absorbance. Concurrently, the incident irradiation may also be scattered by the NRs/NPs to increase the propagation length that leads to the absorption enhancement. Theoretical calculations demonstrate that, as the NPs increase from 10 to 80 nm in size, the corresponding LSPR wavelength changes from 413 to 446 nm for Ag NPs embedded in P3HT:PCBM. For Au NPs in the same environment, the resonant wavelength falls in the range of 626–641 nm [82]. From the comparison of the plasmonic effect of silver and gold NRs/NPs with average sizes of 50 nm [51], one can see that the optical absorption and the charge carrier transport can be improved by all of the Au/Ag NPs/NRs embedded in the active layer. Finite difference time domain (FDTD) calculations based on Mie scattering suggest that the enhancement arises from LSPR and the efficient light scattering from the Ag and Au nanostructures. The materials and the nanostructures indeed influenced the exact extent of the performance boost. Au NRs/NPs outperformed their counterpart geometries of silver. For a fixed type of metal, NRs are superior to NPs, because the LSPR spectrum covers a broader wavelength range for NRs due to their synergistic transverse and longitudinal resonances [83]. When the thickness of the active layer is large enough, the NP ensemble possibly distributes in certain sub-layers (Figure4). It may locate near the cathode/anode, or in the middle. In such cases, the location of photoinduced excitons and thereby the transport path of charge carriers may be optimized in consideration of electrical properties of the charge carriers [66]. Sha et al. investigated the impact of the sub-layer dispersion on the photovoltaic performance of small-molecule organic solar cells. Since the hole mobility of CuPc:C60 is about four orders of magnitude smaller compared to the electron mobility, the optimal charge transport path was obtained with Ag NPs in the active layer near the anode. While free electrons and holes are generated in the exciton region near the anode, the transport path of the low mobility hole to the anode is shortened [84]. Despite the fact that Au NRs serve as a good selection for plasmonic enhancement, some studies pointed out that bare metal NPs/NRs surfaces may act as recombination centers for holes and electrons, which may deteriorate their photovoltaic characteristics [85]. A proposed solution is to cap the metal NPs/NRs with inorganic materials to form core-shell structures [86]. Yamada et al. demonstrated that Ag NPs with diameters of 46–60 nm that were encapsulated with a 2 nm thick TiO2 shell resulted in the shift of plasmonic resonant peak from 430 to 438 nm, because of the relatively high refractive index of TiO2 (>2.4). The external quantum efficiency (EQE, which equals to photon conversion efficiency (IPCE)) of the solar cells presented the maximum enhancement at wavelengths near the LSPR peak of Ag NPs@TiO2. Apart from being placed in the active layer alone, a novel strategy is to first link the metallic NPs onto 2D materials before the incorporation. 2D laminar materials have drawn considerable attention due to their extraordinary properties and great potential for various applications [87]. Polymers 2018, 10, 123 10 of 33 Polymers 2017, 9, x FOR PEER REVIEW 10 of 33

Figure 4. 4. (a(a)) Schematic Schematic illustration illustration of the of sandwich-typethe sandwich-type BHJ solar BHJ cell; solar (b cell;) Ag NPs (b) Ag located NPs in located sublayers in sublayersof the active of the layer active [84 layer]; (c) [84]; Up: (c Dipolar) Up: Dipolar oscillations oscillations in spherical in spherical Au Au NPs NPs and and Au Au NRs. NRs. Bottom: Bottom: normalized UVUV-Vis-NIR-Vis-NIR absorptiabsorptionon spectra of of Au Au NPs NPs of of various various sizes sizes and and shapes shapes in in water water [83]. [83]. Reprinted with permission from Refs. [[83,84].83,84].

When the Ag NPs are in the photoactive layer, the PCE enhancement is usually correlated with When the Ag NPs are in the photoactive layer, the PCE enhancement is usually correlated with the Jsc increment, which principally arises from the near-field LSPR effect. While the Ag NPs are the J increment, which principally arises from the near-field LSPR effect. While the Ag NPs are placed placedsc in the electron transport layer, the increase of PCE may come from two aspects. One is in the electron transport layer, the increase of PCE may come from two aspects. One is analogously the analogously the Jsc increment due to the plasmonic effect. The other is the increment of fill factor (FF) Jsc increment due to the plasmonic effect. The other is the increment of fill factor (FF) and open-circuit and open-circuit (Voc), which is caused by the electrical resistance reduction due to the Ag NPs. (V ), which is caused by the electrical resistance reduction due to the Ag NPs. oc The utilization of asymmetric metallic nanostructures, e.g., nanostars, is a simple and promising The utilization of asymmetric metallic nanostructures, e.g., nanostars, is a simple and promising approach to achieve a broadband absorption enhancement (Figure 5). The plasmonic asymmetric approach to achieve a broadband absorption enhancement (Figure5). The plasmonic asymmetric modes that arise from the core-branch coupling significantly increase the local field intensity, thereby modes that arise from the core-branch coupling significantly increase the local field intensity, thereby enhancing the absorption in the active layer. The Au nanostars also help transfer the optical power enhancing the absorption in the active layer. The Au nanostars also help transfer the optical power in in the charge transport layer (which otherwise will be a waste) to the light absorbing medium and the charge transport layer (which otherwise will be a waste) to the light absorbing medium and thus thus improve the absorption. The absorption peaks from the high-order asymmetric plasmonic improve the absorption. The absorption peaks from the high-order asymmetric plasmonic modes are modes are spectrally consistent with the EQE enhancement, pushing the PCE up to 10.5% [53]. spectrally consistent with the EQE enhancement, pushing the PCE up to 10.5% [53].

Polymers 2018, 10, 123 11 of 33 Polymers 2017, 9, x FOR PEER REVIEW 11 of 33

FigureFigure 5. 5. ((aa)) The The schematic schematic illustration ofof thethe structurestructure of of Au Au nanostars nanostars (NSs) (NSs) incorporated incorporated OSC; OSC; (b )( theb) theabsorptive absorptive power power distributions distributions of of the the control control OSC OSC (without (without Au Au NSs).NSs). The absorptive power power in in Ca Ca cannotcannot con contributetribute to to the the carrier carrier generation generation and and would would be be waste wasted;d; (c (c) )the the variation variation of of absorptive absorptive power power distributiondistribution due due to to the the incorporation incorporation of of Au Au NSs NSs compared compared to to the the control control OSC OSC [53] [53].. Repr Reprintedinted with with permissionpermission from from Ref. Ref. [53] [53]..

4.2.4.2. In In Charge Charge Transport Transport Layer Layer DespiteDespite the the fact fact that that positive positive effects effects have have been been reported reported for for the the OPV OPV devices devices with with the the plasmonic plasmonic nanostructurenanostructuredd embedded embedded in in the the photoactive photoactive layer, layer, one one should should note note that that some some negative negative side side effects effects maymay be be introduced introduced by by them them into into the the photovoltaic photovoltaic performances. performances. Firstly, Firstly, the the surfaces surfaces of of nanometals nanometals maymay function function as as the the recombination recombination centers centers for for charge charge carriers, carriers, decreasing decreasing the the number number of of free free charges charges towardstowards the the collection collection at at the the electrode electrode [73]. [73]. Secondly, Secondly, the the nanostructures nanostructures do do not not form form continuous continuous networknetwork and and the the charge charge transport transport between between the the nanometals nanometals and and the the acceptor acceptor semiconductor semiconductor could could be be worseworse than than in in the the continuous continuous netwo networkrk of of the the electron electron acceptor acceptor material material [88]. [88]. ToTo avoid avoid those those disadvantages, disadvantages, the the plasmonic plasmonic nanostructures nanostructures have have been been placed placed in in the the charge charge transporttransport layers layers (CTL), (CTL), namely, namely, the the electron electron transport transport layer layer (ETL, (ETL, i.e., i.e., cathode cathode buffer buffer layer) layer) or or hole hole transporttransport layer layer (HTL (HTL,, i.e., i.e., anode anode buffer buffer layer). layer). The The scattering scattering may may be be forward forward or or backward backward depending depending thethe specific specific location location of of the the nanostructures. nanostructures. When When the the nanometals nanometals are are embedded embedded in in the the front front CTL CTL (Figure(Figure 6a), the forward scattering effect is mainly utilized to increase the op opticaltical path length and the absorptionabsorption in in the the active active layer. layer. For For nanostructures nanostructures located located in in the the rear rear CTL CTL (Figure (Figure 6b), the incident irradiationirradiation is is back back scattered scattered by by them them into into the the active layer to lengthen lengthen the the optical optical path. path. NPs/NRs located in the rear CTL of OPV devices may suppress the energy losses by the absorption of the metals and by the back-scattering of light, which would be unavoidable when they are in front CTL [70].

Polymers 2018, 10, 123 12 of 33 located in the rear CTL of OPV devices may suppress the energy losses by the absorption of the metals andPolymers by the2017 back-scattering, 9, x FOR PEER REVIEW of light, which would be unavoidable when they are in front CTL [7012]. of 33

Figure 6. 6.Forward Forward (a (,ba,)b and) and backward backward (c,d ()c scattering,d) scattering when when Au NPs/NRs Au NPs/NRs are embedded are embedded in ETL in[70 ETL,73]; ([70,73e) LSPR]; (e mode,) LSPR which mode, is which generated is generated in vicinity in vicinity of metallic of metallic NPs in NPs CTL in and CTL scattered and scattered into the into active the layeractive [89 layer]. (f) [89] Bimetallic. (f) Bimetallic NPs embedded NPs embedded in HTL [54 in]. ReprintedHTL [54]. withReprinted permission with frompermission Refs. [54 from,70,73 Refs.,89]. [54,70,73,89].

Au NRs/NPs with sizes around 40 nm doped in ETL like TiOx or ZnO can significantly enhance Au NRs/NPs with sizes around 40 nm doped in ETL like TiOx or ZnO can significantly enhance PCE by ~20% to about 7.86% relative to the control device [70]. The PCE enhancement is attributed to PCE by ~20% to about 7.86% relative to the control device [70]. The PCE enhancement is attributed light trapping mainly caused by the plasmonic far-field scattering. Furthermore, simulations revealed to light trapping mainly caused by the plasmonic far-field scattering. Furthermore, simulations that the electromagnetic field can be enhanced in the active layer even when the nanometals totally revealed that the electromagnetic field can be enhanced in the active layer even when the nanometals locate in CTL [89]. totally locate in CTL [89]. In addition to the conventional NPs/NRs, other types of nanostructures have been explored, In addition to the conventional NPs/NRs, other types of nanostructures have been explored, such as nanoarrows, or NPs hybridized with carbon nanotubes (CNT). Incorporating Au nanoarrows such as nanoarrows, or NPs hybridized with carbon nanotubes (CNT). Incorporating Au nanoarrows into the ZnO ETL can result in lower resistance and improved Jsc and EQE. The optimized doping into the ZnO ETL can result in lower resistance and improved Jsc and EQE. The optimized doping concentration of 1.5 wt % led to a 27.3% increase of PCE [72]. ZnO@CNT-Au, the hybrid of ZnO NPs concentration of 1.5 wt % led to a 27.3% increase of PCE [72]. ZnO@CNT-Au, the hybrid of ZnO NPs and Au NP-decorated carboxylic CNTs, has been prepared as ETL. Au NPs induce the surface plasmon effect to increase light absorption. CNT provides a template for the in situ growth of ZnO NPs, forming a homogeneous film with fewer defects and higher conductivity. By employing ZnO@CNT-Au as cathode buffer layer, the charge recombination was decreased and electron

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Polymersand Au 2017 NP-decorated, 9, x FOR PEER carboxylic REVIEW CNTs, has been prepared as ETL. Au NPs induce the surface plasmon13 of 33 effect to increase light absorption. CNT provides a template for the in situ growth of ZnO NPs, forming collectiona homogeneous was increased. film with Consequently,fewer defects and the higher device conductivity. with ZnO@CNT By employing-Au showed ZnO@CNT-Au a significant as enhancementcathode buffer of layer,PCE up the to charge 7.9%, which recombination provides wasa promising decreased method and electron to improv collectione the performance was increased. of PSCsConsequently, [59,71]. the device with ZnO@CNT-Au showed a significant enhancement of PCE up to 7.9%, whichOn provides the other a hand, promising metallic method NPs tohave improve been doped the performance in HTL such of PSCsas PEDOT:PSS, [59,71]. WO3, or MoO3 [54]. ForOn the instance, other hand, dual metallic Au-Ag NPs NPs have in the been WO doped3 HTL in HTL extended such as the PEDOT:PSS, wavelength WO range3, or MoO of light3 [54]. absorptionFor instance, due dual to the Au-Ag backward NPs inscattering the WO 3andHTL LSPR extended effect. the wavelength range of light absorption dueWhen to the backwardthe nanometals scattering are placed and LSPR in the effect. front CTL, i.e., HTL of the conventional type or ETL of the invertedWhen thetype nanometals solar cells, areand placedwhen the in theback front is the CTL, metal i.e., electrode, HTL of thethe conventionalMDM configurations type or ETLare constructedof the inverted in which type solarthe electric cells, and field when can thebe s backtrongly is the confined metal electrode,in the middle the MDM dielectric configurations materials (Figureare constructed 7). Such ina structure which the is electric beneficial field for can the be efficient strongly absorption confined in of the sunlight middle in dielectric the photoactive materials layer(Figure with7). possible Such a structure plasmonic is beneficial mechanisms for involved the efficient such absorption as forward of scattering,sunlight in LSPR, the photoactive plasmon- cavitylayer withmode, possible and plasmon plasmonic-gap mechanisms mode [75,77]. involved The nanometals such as forward are dispersed scattering, comparatively LSPR, plasmon-cavity randomly inmode, CTL and if they plasmon-gap are first mode blended [75 ,77 in ]. the The precursor nanometals solution are dispersed for CTL comparatively before the spin randomly-coating. in Alternatively,CTL if they are periodic first blended metallic in nanostructures the precursor solution can be first for CTLfabricated before on the ITO spin-coating. before the Alternatively,formation of CTLperiodic [55]. metallic nanostructures can be first fabricated on ITO before the formation of CTL [55].

FigureFigure 7. 7. PlasmonicPlasmonic effect effect of of MDM MDM structure structures.s. (a (a) )Schematic Schematic of of organic organic photovoltaic photovoltaic device device structure structure withwith Au Au NPs NPs in in front front ETL ETL;; (b (b) )a absorptionbsorption density density profiles profiles in in OPV OPV devices devices with with and and without without Ag Ag NDs NDs at the resonance wavelength (λ = 573 nm) [76]; (c) reflection spectra from PTB7:PC71BM layer [37]. at the resonance wavelength (λ = 573 nm) [76]; (c) reflection spectra from PTB7:PC71BM layer [37]. ReprintedReprinted with with permission permission from from Refs. Refs. [37,76]. [37,76].

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4.3. Between the Active Layer and CTL Apart from inside the photoactive layer and CTL, metallic nanostructures can also be placed between two adjacent layers (Figure8). For instance, the plasmonic nanostructures have been sandwiched between the active layer and CTL. Photons in a broad wavelength band can be harvested by the hybridization of extinction-tailorable Ag nanoprisms with natural photosensitizer protein molecules. The photocurrent is increased by the incorporation of the nano-bio hybrid as an interlayer between the active layer and ETL, which can attributed to LSPR of Ag nanoprisms and the energy transfer from LHCII [55]. Additionally, Au NRs with silica shell thickness of ~3 nm sandwiched between the active layer and HTL improved the Jsc and PCE of the organic solar cells, primarily due to the strong local electromagnetic field. The attenuation of the LSPR field is mitigated by the SiO2 shell, leading to a high local field intensity in the active layer and enhanced absorption of PTB7 [69].

Figure 8. Schematic illustration of the OPV devices incorporating the interlayer of nano-bio hybrids between ZnO and the active layer. The insets show the crystal structure of LHCII, and chemical structures of polymers PIDTT-DFBT, PBDT-DTNT, and PBT7-Th [55]. Reprinted with permission from Ref. [55].

4.4. As Electrode ITO is current widely used as transparent electrode, but it has some disadvantages such as brittleness and indium scarcity [58]. As a promising candidate for the transparent electrode to replace ITO, the metallic nanostructures exhibit several features beneficial to the OPV devices, like increased flexibity and enhanced near infrared (NIR) absorption due to the plasmonic effect. Ag NRs with excellent optical and electrical properties are promising candidates for future transparent electrodes. However, pure NRs possess low melting point and fail to work at c.a. 100 ◦C. The Ag NRs networks easily agglomerate at the intersection under thermal stress. In last years, multilayer structures have been developed with the metallic nanostructures sandwiched in between. The multilayer structure may block the migration and thermal-dynamic movement of the metal Polymers 2018, 10, 123 15 of 33

NRs for aggregation. Therefore, the optical and electrical properties, and thermal stability, may Polymersbe improved. 2017, 9, x FOR Furthermore, PEER REVIEW the two layers on top and beneath the metal layer may help present15 of 33 constructive interference for the electromagnetic field, boosting the plasmonic effect for the device. ForFor instance, instance, Chalh Chalh et et al. fabricated fabricated ZnO NP/Ag NR/ZnO NR/ZnO NP NP (ZAZ) (ZAZ) electrodes, electrodes, which which are are fully fully solutionsolution processable processable [80]. [80]. Lee Lee et et al. al. transferred transferred graphene graphene both both on on top top and and at at the the bottom bottom of of Ag Ag NR NR networksnetworks to to protect protect the the nanostructures nanostructures against against thermal thermal degradation degradation [63]. [63]. The The transmittance transmittance of of ZAZ ZAZ surpassessurpasses that of ITO atat wavelengthswavelengths larger larger than than 470 470 nm nm (see (see Figure Figure9). 9 Constructive). Constructive interference interference of the of theincident incident light light in ZAZ in ZAZ produced produced better better transmittance transmittance than than pure pure Ag Ag NRs. NRs. Whereas Whereas the the ITO ITO presents presents low lowtransmittance transmittance in the in UV the region, UV region, Ag NR-based Ag NR-based flexible flexible electrodes electrodes show ashow uniform a uniform transmittance transmittance of more ofthan more 90% than in all90% wavelength in all wave regions.length regions. When the When top ZnOthe top layer ZnO is thinlayer enough is thin enough (≤11 nm), (≤11 the nm), electric the electricfield in field the active in the layer active can layer be intensified can be intensified by the plasmonic by the effect.plasmonic The mechanicaleffect. The tensilemechanical strength tensile and strengththe thermal and stability the thermal are significantly stability are promoted significantly in the promoted graphene-Ag-graphene in the graphene (GAG)-Ag-graph structureene [(GAG)90–92]. structureIts sheet resistance[90–92]. Its remains sheet resistance stable when remains the devicestable when is annealed the device at 170 is ◦annealedC for several at 170 hundred °C for several hours, hundredwhile pure hours, Ag NWs while degraded pure Ag onlyNWs atdegraded 150 ◦C for only 1 h at [61 150]. °C for 1 h [61]. AnotherAnother alternative alternative substitute substitute for for ITO ITO may may be be metallic metallic nanomeshes, nanomeshes, nam namely,ely, periodic periodic nanohole nanohole arraysarrays due due to to the the combining combining features features of fare of optical fare optical transmittance transmittance and the and surface the plasmonic surface plasmonic properties properties(Figure 10 ).(Figure However, 10). itHowever, is still controversial it is still controversial whether the whether plasmonic the nanomeshplasmonic electrodesnanomesh can electrodes surpass canthe surpass conventional the conventional ITO in OPV ITO efficiencies in OPV [efficiencies59]. [59].

Figure 9. (a) Measured transmission spectra of ZAZ, AgNWs, AgNWs/ZnO NPs, and ITO electrodes. Figure 9. (a) Measured transmission spectra of ZAZ, AgNWs, AgNWs/ZnO NPs, and ITO electrodes. TheThe inset inset is is the the SEM SEM image image of the of ZnO-Ag-ZnO the ZnO-Ag electrode;-ZnO electrode; (b) calculated (b) calculated absorption insideabsorption P3HT:PCBM inside P3HT:PCBMof organic solar of organic cells integrating solar cells ZAZ integrating and ITO ZAZ electrodes and ITO [80 electrodes]; (c) UV-vis [80] transmittance; (c) UV-vis transmittance of several Ag of several Ag NWs electrodes system of /G, /Ag/G, and G/Ag/G films on Si (100)/SiO2 substrate, NWs electrodes system of /G, /Ag/G, and G/Ag/G films on Si (100)/SiO2 substrate, respectively. respectively.The inset is schematic The inset illustrationis schematic of illustration each electrode of each stack electrode of Graphene/Ag/Graphene; stack of Graphene/Ag/Graphene; (d) change in ( thed) changerelative in resistance the relative of resistance electrode materialsof electrode according materials to according the number to the of bendingnumber of cycles bending [63]. cycles Reprinted [63]. Reprintedwith permission with permission from Refs. from [63,80 Refs.]. [63,80].

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Figure 10. (a) Absorption spectra of PTB7:PC71BM solar cells with gold nanohole electrodes for different Figure 10. (a) Absorption spectra of PTB7:PC BM solar cells with gold nanohole electrodes for periodicities (solid lines), of the same solar cell 71without the nanomesh (dash-dotted line), and of the differentPTB7:PC periodicities71BM reference (solid lines),device on of ITO the (dashed same solar line); cell (b) withoutcontribution the of nanomesh the nanohole (dash-dotted electrode to the line), and of the PTB7:PCdevice absorption.71BM reference The nanomesh device absorption on ITO (dashedis obtained line); by subtracting (b) contribution the absorption of the (A nanohole= 1 − R) of the electrode to the devicesame solar absorption. cell device The without nanomesh a nanomesh absorption electrode is (Rblank, obtained dash by-dotted subtracting line) from the the absorption device (A = 1 − R) ofabsorption the same with solar nanomesh cell device electrode; without (c) current a nanomesh-density—Voltage electrode characteristics (Rblank, under dash-dotted illumination line) of from the devicesolar absorption cells built on withITO (dashed nanomesh line) and electrode; gold nanohole (c) current-density—Voltage array transparent conducting characteristics electrodes with under different periodicities [59]. Reprinted with permission from Ref. [59]. illumination of solar cells built on ITO (dashed line) and gold nanohole array transparent conducting electrodes5. Photophysical with different Investigations periodicities of the [59 Plasmonic]. Reprinted Effect with permission from Ref. [59]. Photophysics is an important link between the structural properties of the diverse layers in 5. Photophysical Investigations of the Plasmonic Effect organic solar cells and the ultimate device performance. Usually, the photophysical properties are Photophysicsinvestigated by is steady an important-state and transient link between optical spectroscopies. the structural The properties steady-state ofphotophysics the diverse, such layers in organicas solar the absorption cells and and the reflection, ultimate are device key criteria performance. to judge whether Usually, the the plasmonic photophysical effects take properties place are [51,76]. Normally, the enhancement in the optical absorption and the decrease in reflection are investigatedindicators by of steady-state effective plasmonics and transient due to the optical far-field spectroscopies. scattering and near The-field steady-state LSPR. It is important photophysics, such asto the point absorption out at the outset and reflection, of this part arethat keythese criteria optical footprints to judge should whether be analyzed the plasmonic in association effects take place [51with,76 ].the Normally, electrical properties the enhancement (e.g., EQE) to in unambiguously the optical absorption determine the and involvement the decrease of plasmonics in reflection are indicatorsfor ofboosting effective the OPV plasmonics performance. due to the far-field scattering and near-field LSPR. It is important to point out atThe the outset steady-state of this optical part that spectroscopies these optical have footprints been performed should to be reveal analyzed the in fundamental association with plasmonic mechanisms including various modes such as the scattering, LSPR, SPP, waveguide, and the electrical properties (e.g., EQE) to unambiguously determine the involvement of plasmonics for the plasmon-gap mode. In organic solar cells, characteristics like the exciton generation/dissociation boostingand the the OPV charge performance. carrier transport/collection, which are crucial to the device performance, can be Theaddressed steady-state based optical on the spectroscopies transient photophysical have been properties. performed The to charge reveal transfer the fundamental and transport plasmonic mechanismsproperties including have been various studied via modes monitoring such the as relaxation the scattering, kinetics of LSPR, excitons SPP, in excited waveguide, states with and the plasmon-gapthe time mode.-resolved In absorption organic solar and fluorescence cells, characteristics measurements. like the exciton generation/dissociation and the charge carrierOverall, transport/collection, controversial findings whichhave been are reported crucial to on the incorporation device performance, effect of the can plasmonic be addressed metallic nanostructures. Despite the fact that many studies demonstrated the improvement of the based onphotovolatic the transient performance photophysical thanks to properties. the introduction The charge of the nanometals, transfer and the transport device performance properties have been studieddeterioration via monitoring shown in some the relaxation other works kinetics should ofnot excitons be ignored. in excited Hence, statesit is urgently with the needed time-resolved to absorptionobtain and physical fluorescence insights into measurements. the plasmonics from the investigation of the photophysical processes for Overall, controversial findings have been reported on incorporation effect of the plasmonic metallic nanostructures. Despite the fact that many studies demonstrated the improvement of the photovolatic performance thanks to the introduction of the nanometals, the device performance deterioration shown in some other works should not be ignored. Hence, it is urgently needed to obtain physical insights into the plasmonics from the investigation of the photophysical processes for exciton dissociation into free charge carriers and charge recombination/transport/collection [93–95]. In this Polymers 2018, 10, 123 17 of 33 part, we focus on the photophysical properties in plasmonic photovoltaic structures in correlation with the devicePolymers configurations 2017, 9, x FOR PEER and REVIEW performances. 17 of 33

5.1. Steady-Stateexciton dissociation Characterizations into free charge of Absorption carriers and and charge Photoluminescence recombination/transport/collection [93–95]. In this part, we focus on the photophysical properties in plasmonic photovoltaic structures in 5.1.1. Tocorrelation Confirm with Plasmatic the device Effects configurations Rendered and by performances. the Incorporation of Nanometals

When5.1. Steady the metallic-State Characterizations nanostructures of Absorption are embedded and Photoluminescence inside the active layer as shown in Figure 11, the device performance is improved due to the increased absorption, which may arise from the near-field5.1.1. LSPR To Confirm or/and Plasmatic the far-field Effects Rendered scattering. by the For Incorporation these plasmonic of Nanometals nanostructures, it is generally believed thatWhen the the LSPR metallic at the nanostructures metal/dielectric are embedded interfaces inside is a primarythe active andlayer significant as shown in contribution Figure 11, to the enhancementthe device of performance electric field, is improved and hence due the to the absorption increased inabsorption, the light which absorbing may arise medium. from the The near near- field plasmaticfield effect LSPR canor/and be th validatede far-field byscattering. the FDTD For these calculation plasmonic results nanostructures, and can be it is experimentally generally believed supported by the largethat the deviations LSPR at the between metal/dielectric the curves interfaces of cross is sections a primary for and the significant scattering contribution and the total to the absorption enhancement of electric field, and hence the absorption in the light absorbing medium. The near field (Figureplasmatic 11e,f) [51 effect]. can be validated by the FDTD calculation results and can be experimentally Comparedsupported by with the LSPR,large deviations the cross between section the of curves scattering of cross issections not trivial for the for scattering the Au/Ag and the NPs/NRs total in the wavelengthabsorption range(Figure of 11e,f) 450–650 [51]. nm. The nanometals could possess a scattering cross-section much higher thanCompared the geometric with LSPR, cross the section cross section value. of In scattering this case, is thenot trivial absorption for the isAu/Ag enhanced NPs/NRs as ain result the of the increasedwavelength optical rangepath length of 450~650 in the nm. active The nanometals layer caused could by possess the light a scattering reemitted cross in-section different much directions higher than the geometric cross section value. In this case, the absorption is enhanced as a result of within the device [96]. The angular distribution of the scattered radiations is depicted in Figure 11c,d, the increased optical path length in the active layer caused by the light reemitted in different in whichdirections the Y-axis within denotes the device the direction[96]. The angular of forward distribution scattering. of the The scattered light radiations scattering is prefers depicted the in forward direction,Figure because 11c,d, in the wh diametersich the Y-axis of thedenotes Au/Ag the direction NPs are of c.a. forward 40 nm scattering. and the The NRs light are scattering 50 nm in length and 10prefers nm in the width, forward which direction, is consistent because the with diameters the criteria of the describedAu/Ag NPs inare Ref. c.a. [4022 nm] for and effective the NRs forward scattering,are 50 i.e., nm inin length 30–60 and nm 10 range. nm in width, The NPs which and is consistent NRs in the with active the criteria layer described function in asRef. subwavelength [22] for scatteringeffective elements forward and scattering, trap the i.e., freely in 30~60 propagating nm range. The light NPs waves. and NRs NRs in the present active layer large function absorption as cross subwavelength scattering elements and trap the freely propagating light waves. NRs present large sections and better Jsc and EQE than NPs despite the fact that their geometrical sizes are comparable, absorption cross sections and better Jsc and EQE than NPs despite the fact that their geometrical sizes probablyare becausecomparable NRs, probably possess because longitudinal NRs possess and longitudinal transverse and plasmatic transverse modes. plasmatic modes.

Figure 11. (a) Device architecture of ITO/rGO (0.4 wt %) ZnO/P3HT: PCBM: 20 wt % of Ag/Au NPs Figure 11. (a) Device architecture of ITO/rGO (0.4 wt %) ZnO/P3HT: PCBM: 20 wt % of Ag/Au or NRs; (b) EQE spectra of ITO/rGO (0.4 wt %) ZnO/P3HT:PCBM: 20 wt % of Ag/Au NPs or NRs NPs orphotovoltaic NRs; (b) EQEdevices spectra (Ag NPs of 71%, ITO/rGO Ag NRs (0.4 73%, wt 74% %) for ZnO/P3HT:PCBM: Au NPs and 76% for Au 20 wtNRs; % (c of,d). Ag/Au The NPs or NRsangular photovoltaic distribution devices of scattered (Ag NPs radiation 71%, inAg XY, NRs YZ, and 73%, XZ74% planes. for The Au Absorption NPs and (Qabs) 76% forand Au NRs; (c,d). Thescattering angular (Qscat) distribution cross sections of scattered of (e) Ag radiation NPs (or) ( inf) NRs XY, YZ,at various and XZ wavelengths planes. The of the Absorption incident (Qabs) and scatteringspectrum (Qscat)[51]. Reprinted cross sectionswith permission of (e) Agfrom NPs Ref. (or)[51]. ( f) NRs at various wavelengths of the incident spectrum [51]. Reprinted with permission from Ref. [51]. PolymersPolymers 20172018,, 910, x, 123FOR PEER REVIEW 1818 of of 33 33

Upon the incorporation of Ag nanodots (NDs) in the charge transport layers as shown in Figure 12, theUpon optical the path incorporation and hence of the Ag nanodotsEQE of OPV (NDs) solar in the cells charge are increased. transport layersThe wavelength as shown in bands Figure for 12 , thethe absorption optical path and and EQE hence enhancement the EQE of OPV match solar the cells extinction are increased. spectrum The of wavelength Ag NDs. In bands view for of the the locationabsorption of and Ag EQE NDs, enhancement which is outside match the the extinction active layer, spectrum the far of- Agfield NDs. scattering In view usuallyof the location takes of a remarkableAg NDs, which role isin outside the absorption the active enhancement. layer, the far-field It was scattering experimentally usually takes validated a remarkable in the spectra role in theof scatteredabsorption light enhancement. measured withIt was the experimentally integrated sphere validated system. in the Conspicuous spectra of scattered scattered light light measured can be observedwith the integratedfrom 460 to sphere 730 nm. system. Effective Conspicuous forward scattering scattered rendered light can by be the observed Ag NDs from in the 460 front to 730charge nm. transportEffective forwardlayer increases scattering the renderedlength of bythe the optical Ag NDs path in in the the front active charge layer, transport leading to layer promoted increases light the trappinglength of and the opticalincreased path Jsc. in the active layer, leading to promoted light trapping and increased Jsc.

FigureFigure 12. 12. (a()a Schematic) Schematic diagram diagram of ofan anOPV OPV device device with with Ag AgNDs; NDs; (b) measured (b) measured absorption absorption of the of PTB7:PC70BMthe PTB7:PC70BM layer layer coated coated on di onfferent different substrates. substrates. Absorption Absorption of PTB7:PC70BM of PTB7:PC70BM coated coated on the on glass/ITOthe glass/ITO substrate substrate is obtained is obtained by subtracting by subtracting the absorption the absorption of the of glass/ITOthe glass/ITO substrate substrate from from the glass/ITO/PTB7:PC70BM substrate (black line). Absorption of PTB7:PC70BM coated on the the glass/ITO/PTB7:PC70BM substrate (black line). Absorption of PTB7:PC70BM coated on the glass/ITO/Agglass/ITO/Ag ND ND substrate substrate is is obtained obtained by by subtracting subtracting the the absorption of the the glass/ITO/Ag glass/ITO/Ag NDs NDs substratesubstrate from from the glass/ITO/Ag glass/ITO/Ag NDs/PTB7:PC70BM NDs/PTB7:PC70BM substrate substrate (blue (blue line); line); (c ()c ) measured measured light light scatteringscattering factor factor using an integrated spheresphere system;system; (d(d)) EQE EQE enhancement enhancement after after embedding embedding Ag Ag NDs NDs in inthe the OPV OPV device device and extinction and extinction spectra spectra of 91 nm of Ag 91 NDs. nm Ag The NDs. wavelength The wavelength range of EQE range enhancement of EQE enhancementcorresponds withcorresponds the extinction with spectrumthe extinction [76]. spectrum Reprinted [76]. with Reprinted permission with from permission Ref. [76]. from Ref. [76]. One needs to determine whether (and if yes how much) the LSPR mechanism contributes to the One needs to determine whether (and if yes how much) the LSPR mechanism contributes to the absorption enhancement when the nanostructures are away from the active layer. FDTD simulations absorption enhancement when the nanostructures are away from the active layer. FDTD simulations show that a strong electric field enhancement can be generated in the charge transport layer and show that a strong electric field enhancement can be generated in the charge transport layer and can can diffuse efficiently into the active layer, confirming the involvement of the LSPR effect in the diffuse efficiently into the active layer, confirming the involvement of the LSPR effect in the plasmonic processes. plasmonic processes. The weight ratio of the plasmonic contributions from the near-field LSPR and far-field scattering The weight ratio of the plasmonic contributions from the near-field LSPR and far-field scattering effect may vary with parameters such as the locations and sizes of the nanostructures. In brief, it is effect may vary with parameters such as the locations and sizes of the nanostructures. In brief, it is generally accepted that LSPR is the main plasmonic mechanism for nanometals hybrided in the active generally accepted that LSPR is the main plasmonic mechanism for nanometals hybrided in the active layer, whereas the scattering is the first plasmonic mode to be considered for nanostructures embedded layer, whereas the scattering is the first plasmonic mode to be considered for nanostructures in buffer layers. As calculated in Ref. [70], the EQE enhancement was principally aroused by the embedded in buffer layers. As calculated in Ref. [70], the EQE enhancement was principally aroused far-field scattering, while the LSPR merely contributed to ~0.7% of the enhancement, when Au NRs by the far-field scattering, while the LSPR merely contributed to ~0.7% of the enhancement, when Au (30 nm long and 10 nm wide) were dispersed in the rear ETL as a back reflector. NRs (30 nm long and 10 nm wide) were dispersed in the rear ETL as a back reflector.

Polymers 2017, 9, x FOR PEER REVIEW 19 of 33 Polymers 2018, 10, 123 19 of 33

5.1.2. To Elucidate Detailed Plasmonic Mechanisms 5.1.2.In To addition Elucidate to Detailed the aforementioned Plasmonic Mechanisms LSPR and scattering effects, there are other plasmonic processesIn addition that can to be the discriminated aforementioned from LSPR the and photophysical scattering effects, characterizations. there are other plasmonic processes that canIn order be discriminated to assess the from SPP themode, photophysical the spectral characterizations. reflectivity and absorption have been calculated with Intheoretical order to simulations assess the SPP based mode, on the the structure spectral reflectivityof P3HT:PCBM and absorptionthin film on have the metallic been calculated grating aswith electrode. theoretical In inverted simulations solar based cells onwith the configurations structure of P3HT:PCBM of ITO/ZnO/P thin3HT:PCBM//P3HT:sorbitol/Ag film on the metallic grating as grating/PETelectrode. In [72]inverted and solar ITO/TiO cells2/P3HT:PCBM/PEDOT:PSS/Au with configurations of ITO/ZnO/P3HT:PCBM//P3HT:sorbitol/Ag grating [56], both SPP and photonic waveguidegrating/PET modes [72] and have ITO/TiO been observed.2/P3HT:PCBM/PEDOT:PSS/Au The occurrence of the SPP grating or waveguide [56], both mode SPP andis essentially photonic affectedwaveguide by modesthe relative have orientation been observed. of the The incident occurrence light ofpolarization the SPP or to waveguide the grating mode lines. is Only essentially with theaffected TM (i.e., by the inrelative p-polarization, orientation perpendicular of the incident with light the polarization grating lines) to the excitation grating lines.can SPP Only modes with thebe excited.TM (i.e., Inin p-polarization, contrast, the waveguide perpendicular mode with can the be grating detected lines) under excitation both can TM SPP and modes TE (i.e., be excited. in s- polarizIn contrast,ation, the parallel waveguide to the mode grating can lines) be detected incident under irradiations. both TM Waveguide and TE (i.e., modes in s-polarization, are not as parallelreadily observedto the grating as SPP, lines) because incident they irradiations. may be easily Waveguide deteriorated modes by are the not small as readily thickness observed of the as organic SPP, because solar cellsthey [56]. may beThe easily quantity deteriorated of the waveguide by the small modes thickness increases of the w organicith the solarthickness, cells [whereas56]. The quantitythe SPP mode of the doeswaveguide not [81] modes (see Figure increases 13). with the thickness, whereas the SPP mode does not [81] (see Figure 13).

Figure 13. (a) Simulated absorbance spectra as a function of the incident light polarization relative to Figure 13. (a) Simulated absorbance spectra as a function of the incident light polarization relative the grating period, for the active layer in a complete device stack incorporating the same Ag grating, to the grating period, for the active layer in a complete device stack incorporating the same Ag with the corresponding absorbance for a device with a flat Ag electrode for comparison [56]; (b) the grating, with the corresponding absorbance for a device with a flat Ag electrode for comparison [56]; predicted polarization-dependent absorbance enhancement as a result of incorporating the (b) the predicted polarization-dependent absorbance enhancement as a result of incorporating the nanostructured Ag Ag grating electrode; electrode; ( (cc)) d deviceevice EQE enhancement factor factor (see (see text text for definition) definition) for normal incidence with with polarization parallel parallel (TE; (TE; left) left) and and perpendicular to to the the period period of of the grating (TM; right). right). Reflectivity Reflectivity measurements measurements of of inverted inverted OSCs OSCs by by illumination illumination with with TE TE(s-pol) (s-pol) (d) (ord) TM or TM(p- pol)(p-pol) [81] [81. Reprinted]. Reprinted with with permission permission from from Refs. Refs. [56,81]. [56,81 ].

With appropriate selection of the grating pitch, the SPP mode can be tuned to the NIR spectral With appropriate selection of the grating pitch, the SPP mode can be tuned to the NIR spectral region. Hence, the wavelength range of the absorption is expanded in the active layer due to the region. Hence, the wavelength range of the absorption is expanded in the active layer due to the nanostructured electrodes. The light trapping and the optical field in the active layer are effectively nanostructured electrodes. The light trapping and the optical field in the active layer are effectively intensified by the SPP mode issued by the Ag grating electrode in the OPV device. The spectral intensified by the SPP mode issued by the Ag grating electrode in the OPV device. The spectral position position of the simulated SPP mode agrees well with the measured EQE enhancement, demonstrating of the simulated SPP mode agrees well with the measured EQE enhancement, demonstrating that the that the charge transfer states are photoinduced by the plasmatic trapping and can generate free carriers for charge collection when the nanograting is used as electrode [56].

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Polymerscharge 2017 transfer, 9, x FOR states PEER are REVIEW photoinduced by the plasmatic trapping and can generate free carriers20 of for33 charge collection when the nanograting is used as electrode [56]. ApartApart from from the the theoretical theoretical simulation, simulation, the the SPP SPP field field can can also also be be monitored monitored in in experimental experimental ways. ways. DueDue to thethe development development of of scanning scanning probe probe techniques, techniques the, theSPP SPP field field canbe can directly be directly and locally and locallyprobed probedon a surface, on a surface,with a resolution with a resolution in the nanometer in the nanometer range. The range. scanning The near-field scanning optical near-field microscopy optical microscopy(SNOM) apparatus (SNOM) is illustratedapparatus inis Figureillustrated 14. The in incidenceFigure 14. angle The ofincidence the laser angle can be of tuned the laser to excite can SPP be tuneat desiredd to excite surfaces, SPP andat desired the evanescent surfaces, field and of the SPP evanescent is detected field by the of SPP fiber is tip detected of SNOM. by Bythe scanning fiber tip theof SNOM.fiber tip By over scanning some areas, the the fiber sample tip over topography some areas, and the the SPP sample field distribution topography can and be simultaneously the SPP field distributionrecorded with can a SNOMbe simultaneously [97,98]. From recorded the topographical with a SNOM image, [97,98]. one can From see thethe smoothtopographical interfaces image, with oneroughness can see of the about smooth 10 nm interfaces except forwith the roughness bumps with of heightabout 10 of ~100nm except nm. The for near-fieldthe bumps optical with imagesheight ofexhibit ~100 nm. typical The pronounced near-field optical interference images patterns, exhibit typical which arepronounced produced interference by the interference patterns, between which are the producedscattered SPP by theand interference the SPP on the between surface. the The scattered signal intensity SPP and decreases the SPP to on 5% the if the surface. fiber tip The moves signal by intensity1 µm away decreases from the to surface, 5% if the indicating fiber tip the moves SPP substantiallyby 1 μm away propagates from the insurface, the lateral indicating direction the along SPP substantiallythe interfaces. propagates in the lateral direction along the interfaces.

FigureFigure 14. 14. (a()a Sc) Schematichematic of ofa SNOM a SNOM experimental experimental set-up set-up for studying for studying surface surface plasmon plasmon polaritons polaritons (SPP) [97].(SPP) Topography [97]. Topography (b), near (b),- near-field,field, (c) and (c) and far-field far-field (d) ( intensityd) intensity distributions distributions over over the the smooth smooth gold gold surfacesurface [98]. [98]. Reprinted Reprinted with with permission permission from from Ref. Ref. [97,98]. [97,98].

InIn organic organic solar solar cells, cells, there there may may exist exist some some additional additional modes modes like like mirror mirror charge charge mode mode or or gap gap modemode when when periodic periodic nanostructure nanostructure array array are are fabricated fabricated on on or or in in the the back back metallic metallic electrode electrode [87 [87].]. TransmittanceTransmittance measurements measurements showed showed the the redshift redshift of of the the spectral spectral position position of of th thee transmittance transmittance minimumminimum with the decreasingdecreasing distancedistance between between the the NP NP pairs pairs as a longs long as as they the arey are not not directly directly linked linked (see (seeFigure Figure 15). 15). There There is an is abrupt an abrupt spectral spectral blueshift blueshift once once the NP the pairsNP pairs are in are contact. in contact. This This is associated is associated with withthe competition the competition between between the dipolar the d andipolar quadrupolar and quadrupol contributionsar contribution for thes absorptive for the absorptive cross section cross in sectionseparated in separated Ag NPs. The Ag redNPs. shift The is red as large shift asis ~200as large nm as when ~200 the nm edge-to-edge when the edge distance-to-edge of the distance NP pairs of the NP pairs is 10 nm. Therefore, taking advantage of the gap mode, one can tune the local electromagnetic field by tailoring the spatial arrangement of the periodic arrays [99]. In devices illustrated in Figure 15c, plasmonic resonances can be generated between the Ag oblate NPs and the active layer. In addition to LSPR, plasmonic gap mode is formed in the gaps between the adjacent

Polymers 2018, 10, 123 21 of 33 is 10 nm. Therefore, taking advantage of the gap mode, one can tune the local electromagnetic field by tailoring the spatial arrangement of the periodic arrays [99]. In devices illustrated in Figure 15c, plasmonicPolymers 2017 resonances, 9, x FOR PEER can REVIEW be generated between the Ag oblate NPs and the active layer. In addition21 of to33 LSPR, plasmonic gap mode is formed in the gaps between the adjacent NPs and between NPs and NPsanode and [77 between]. The efficient NPs and hybridization anode [77]. ofThe the efficient LSPR andhybridization the gap modes of the can LSPR provide and the multiplicative gap modes canenhancement provide multiplicative of absorption overenhancement a broad band of absorption that tolerates over a wide a broad selection band of that grating tolerates period, a wide e.g., selectionfrom 190 of to grating 400 nm perio [100].d, e.g., from 190 to 400 nm [100].

Figure 15. (a(a) ) Polarized Polarized transmission transmission spectra spectra in in periodic periodic arrays arrays of of pairwise interacting gold nanoparticles. The lattice constant is 800 nm in parallel direction to the pair axis, 400 nm perpendicular, perpendicular, and the dot height is 30 nm; (b) pictorial illustration of charge distributions for dipole, quadrupole, quadrupole, and multipole modes for a single spherical particle [[99].99]. Reprinted with permission from Ref. [[99].99].

5.2. Transient Absorption and Photoluminescence SpectroscopiesSpectroscopies Paci et al. i incorporatedncorporated Au Au NPs NPs into into the the active active layer layer P3HT:PCBM P3HT:PCBM of of the the organic organic solar solar cells, cells, achieving an enhancement of PCE by 42% 42% and and improvement improvement in in stability. stability. From From the the PL PL decay decay profiles, profiles, one can see the fluorescence fluorescence decay of the polymer is is retarded retarded by by the the addition addition of of Au Au NPs NPs (Figure (Figure 16).16). The authors authors attributed attributed the increased the increased fluorescence fluorescence lifetime tolifetime the interaction to the between interaction the comparatively between the comparativelylong-lived triplet long excitons-lived triplet of P3HT excitons as the of donor P3HT and as the the donor Au NPs and as the the Au acceptor. NPs as the Such acceptor. interactions Such interactionscan strongly can quench strongly the quench triplet state the triplet and ameliorate state and ameliorate the photo-oxidation the photo- ofoxidation the organic of the materials. organic Thematerials. mitigation The mechanismmitigation mechanism for the device for degradation the device as degradation unveiled by as the unveiled photophysical by the characterization photophysical characterizationis supported by the is supported time resolved by energythe time dispersive resolved X-rayenergy reflectometry dispersive X (EDXR)-ray reflectometry study [101]. (EDXR) study [101].

Polymers 2018, 10, 123 22 of 33 Polymers 2017, 9, x FOR PEER REVIEW 22 of 33

Figure 16. (a) PL decay measurements: comparison of the pristine (black) (black) and and NP NP-doped-doped blend (red). TheThe insetinset isis thethe schematicschematic devicedevice architecturearchitecture ofof thethe solarsolar cell.cell. ((bb)) SchematicSchematic ofof thethe photo-oxidationphoto-oxidation processprocess inin thethe polymer:polymer: Au NPs active layer. Energy from thethe polymerpolymer triplettriplet excitonsexcitons excites singlet oxygen, which reacts withwith thethe polymerpolymer chainschains toto formform excitonexciton traptrap states.states. The Au NPs embedded into thethe blendblend actact asas quenchersquenchers ofof thethe triplettriplet excitons,excitons, andand inin thisthis wayway thethe photooxidationphotooxidation processprocess cancan bebe impeded.impeded. (1) Absorption; (2) luminescence; (3) system intercrossing; (4) triplet state; (5a,(5a, 5b)5b) triplettriplet quenching;quenching; (6a, 6b)6b) excitonexciton recombination via trap states. This process is limited in the presence of a triplettriplet quencherquencher [[101].101]. Reprinted with permission from Ref.Ref. [[101].101].

Since the metallic NPs embedded in the active layer may increase the trapping of polarons and Since the metallic NPs embedded in the active layer may increase the trapping of polarons and recombination of excitons, limiting the amount of free charge carriers, ultrathin layer of oxide can be recombination of excitons, limiting the amount of free charge carriers, ultrathin layer of oxide can be added to encapsulate the NPs. The femtosecond transient absorption results of P3HT:PCBM and their added to encapsulate the NPs. The femtosecond transient absorption results of P3HT:PCBM and their blends with Ag@TiO2 at 200 fs and 1 ps delay are displayed in Figure 17a,b. The films exhibited three blends with Ag@TiO2 at 200 fs and 1 ps delay are displayed in Figure 17a,b. The films exhibited three vibronic peaks in the wavelength range from 520 to 610 nm, which arise from ground-state bleaching vibronic peaks in the wavelength range from 520 to 610 nm, which arise from ground-state bleaching (GSB). The negative peak centered at 660 nm originates from the photoinduced absorption due to (GSB). The negative peak centered at 660 nm originates from the photoinduced absorption due to polaron pairs according to Korovyanko et al. [102]. The increase of this peak for the 200 fs and 1 ps polaron pairs according to Korovyanko et al. [102]. The increase of this peak for the 200 fs and 1 ps delay suggests that polaron pair generation is more efficient in the hybrid system with the capped delay suggests that polaron pair generation is more efficient in the hybrid system with the capped Ag Ag NPs than in P3HT:PCBM film. NPs than in P3HT:PCBM film. Consistently with the above Au NPs case, the relaxation time of the photoinduced polarons is Consistently with the above Au NPs case, the relaxation time of the photoinduced polarons also prolonged when Ag@oxides NPs are hybridized in the active layer. This is caused by the is also prolonged when Ag@oxides NPs are hybridized in the active layer. This is caused by the enhanced absorption due to LSPR, which is beneficial for the generations of charge carriers. Hence, enhanced absorption due to LSPR, which is beneficial for the generations of charge carriers. Hence, the P3HT:PCBM-based organic solar cells with Ag@TiO2-ODA nanoprisms in the active layer the P3HT:PCBM-based organic solar cells with Ag@TiO -ODA nanoprisms in the active layer achieved achieved a dramatically improved PCE of 4.03% with2 enhanced absorption over a broad spectral a dramatically improved PCE of 4.03% with enhanced absorption over a broad spectral range of range of 400–620 nm [93]. 400–620 nm [93]. PEDOT:PSS is a popular hole transport layer in OPV devices. Metallic nanostructures doped in PEDOT:PSS is a popular hole transport layer in OPV devices. Metallic nanostructures doped the HTL or inserted at the HTL-anode interface may contribute plasmonic improvement for the in the HTL or inserted at the HTL-anode interface may contribute plasmonic improvement for the photovoltaic devices. The photophysics of P3HT has been explored in two configurations where Ag photovoltaic devices. The photophysics of P3HT has been explored in two configurations where Ag NRs were mixed with PEDOT:PSS or covered with PEDOT:PSS prior to the deposition of the P3HT NRs were mixed with PEDOT:PSS or covered with PEDOT:PSS prior to the deposition of the P3HT layer. As shown in Figure 18, the P3HT fluorescence intensity is larger on Ag NRs than that off the layer. As shown in Figure 18, the P3HT fluorescence intensity is larger on Ag NRs than that off the NRs. The fluorescence emission is intensified due to the enhanced absorption of P3HT aided by the NRs. The fluorescence emission is intensified due to the enhanced absorption of P3HT aided by plasmonic interaction between Ag NRs and P3HT. The PL intensity changes with the varying the plasmonic interaction between Ag NRs and P3HT. The PL intensity changes with the varying distance between them. Compared with the case where Ag NRs are inside the HTL, the P3HT-Ag distance between them. Compared with the case where Ag NRs are inside the HTL, the P3HT-Ag NRs distance is more homogeneous but larger when Ag NRs were covered under the HTL. Therefore, NRs distance is more homogeneous but larger when Ag NRs were covered under the HTL. Therefore, the PL intensity and thus the absorption are comparatively more homogeneous but smaller when the the PL intensity and thus the absorption are comparatively more homogeneous but smaller when the nanometals are between the HTL and anode. Hence, there is a compromise between the homogeneity nanometals are between the HTL and anode. Hence, there is a compromise between the homogeneity and strength of the metal-polymer plasmonic interactions. This can serve as experimental evidence and strength of the metal-polymer plasmonic interactions. This can serve as experimental evidence to support the theoretical simulation conclusion that the LSPR may diffuse into the active layer for to support the theoretical simulation conclusion that the LSPR may diffuse into the active layer for absorption improvement even when the metallic nanostructures are placed outside the active layer. absorption improvement even when the metallic nanostructures are placed outside the active layer. If the enhancement of the optical absorption had only been caused by the far-field scattering effect of the Ag NRs, the amplitude of fluorescence intensity for the two geometries would have been quite similar [94].

Polymers 2018, 10, 123 23 of 33

If the enhancement of the optical absorption had only been caused by the far-field scattering effect of the Ag NRs, the amplitude of fluorescence intensity for the two geometries would have been quite Polymerssimilar 2017 [94, ].9, x FOR PEER REVIEW 23 of 33

FigureFigure 17. 17. RelativeRelative differe differentialntial transmission transmissionss of of P3HT:PCBM P3HT:PCBM film filmss with with delay delay times times of of (a ()a )200 200 fs fs and and (b(b) )1 1ps ps after after excitation excitation with with 400 400 nm nm laser laser beam. beam. (c ()c )Transient Transient decays decays of of P3HT P3HT film, film, P3HT P3HT blended blended withwith Ag Ag NPs, NPs, and and P3HT P3HT blended blended with with Ag@oxides Ag@oxides nanoprisms. nanoprisms. PIA PIA decay decay monitored monitored at at660 660 nm nm [93]. [93 ]. ReprintedReprinted with with permission permission from from Ref. Ref. [93]. [93].

The transient fluorescence decay profiles are displayed in Figure 18c,d. There is no clearly visible The transient fluorescence decay profiles are displayed in Figure 18c,d. There is no clearly visible variation to the fluorescence decay times for P3HT on and off the Ag NRs, suggesting there is no variation to the fluorescence decay times for P3HT on and off the Ag NRs, suggesting there is no direct charge transfer between the polymer and the Ag NRs. It is different from the case when the nanometals are blended in the P3HT:PCBM active layer for which the charge transfer between the polymer and the metal nanostructures gives rise to distinct change of PL decays.

Polymers 2018, 10, 123 24 of 33

direct charge transfer between the polymer and the Ag NRs. It is different from the case when the nanometals are blended in the P3HT:PCBM active layer for which the charge transfer between the

Polymerspolymer 2017 and, 9, x the FOR metal PEER nanostructuresREVIEW gives rise to distinct change of PL decays. 24 of 33

Figure 18. (a) Normalized fluorescence transients for samples where AgNWs were (a) mixed with Figure 18. (a) Normalized fluorescence transients for samples where AgNWs were (a) mixed with PEDOT:PSS and (b) covered with PEDOT:PSS prior to deposition of the P3HT layer. Red curves PEDOT:PSS and (b) covered with PEDOT:PSS prior to deposition of the P3HT layer. Red curves correspond to data measured off the nanowires, while black curves represent data obtained with a correspond to data measured off the nanowires, while black curves represent data obtained with a laser laser spot placed at the nanowire. The excitation energy of 485 nm was used; (c) fluorescence spot placed at the nanowire. The excitation energy of 485 nm was used; (c) fluorescence intensities of intensities of P3HT on and off AgNW for the sample where AgNWs were mixed with PEDOT:PSS P3HT on and off AgNW for the sample where AgNWs were mixed with PEDOT:PSS prior deposition prior deposition of P3HT; (d) fluorescence intensities of P3HT on and off AgNW for the sample where of P3HT; (d) fluorescence intensities of P3HT on and off AgNW for the sample where PEDOT:PSS PEDOT:PSSwas deposited was on deposited AgNWs on prior AgNWs deposition prior deposition of P3HT. In of bothP3HT. cases, In both the cases, results the obtained results forobtained lasers forexcitations lasers excitations of 405 nm of (blue 405 bars)nm (blue and bars) 485 nm and (red 485 bars) nm are(red displayed bars) are [displayed94]. Reprinted [94]. withReprinted permission with permissionfrom Ref. [ 94from]. Ref. [94].

However, it is possible to induce charge/energy transfer between the donor in the active layer However, it is possible to induce charge/energy transfer between the donor in the active layer and the nanometals in CTL. Chi et al. reported the photophysical process for metallic NPs in ETL as and the nanometals in CTL. Chi et al. reported the photophysical process for metallic NPs in ETL as shown in Figure 19. The size of the NPs was manipulated to vary from 16 to 72 nm. The absorption shown in Figure 19. The size of the NPs was manipulated to vary from 16 to 72 nm. The absorption of the plasmonic structures is enhanced due to the scattering and LSPR effects. Although the optical of the plasmonic structures is enhanced due to the scattering and LSPR effects. Although the optical absorption of the whole solar cell reached its maximum with Au NPs of 16 and 25 nm, the device absorption of the whole solar cell reached its maximum with Au NPs of 16 and 25 nm, the device PCE had the highest value, i.e., 7.86%, when Au NPs with diameters of 41 nm were doped in the ZnO PCE had the highest value, i.e., 7.86%, when Au NPs with diameters of 41 nm were doped in the ETL. Apart from the absorption, the efficiency of exciton dissociation into free charge carriers is ZnO ETL. Apart from the absorption, the efficiency of exciton dissociation into free charge carriers is another vital influencing factor to the device performance, which can be explored by time-resolved another vital influencing factor to the device performance, which can be explored by time-resolved PL PL decay characterizations. The PL from the plasmonic samples decays faster than the reference decay characterizations. The PL from the plasmonic samples decays faster than the reference structure, structure, indicating Au NPs improve the exciton dissociation into free holes and electrons. The indicating Au NPs improve the exciton dissociation into free holes and electrons. The fluorescence fluorescence lifetimes have been attained by a bi-exponential fitting, whose dependence on the lifetimes have been attained by a bi-exponential fitting, whose dependence on the particle size is particle size is presented as the inset of Figure 19c. Therefore, Au NPs with appropriate sizes are able presented as the inset of Figure 19c. Therefore, Au NPs with appropriate sizes are able to increase the to increase the absorption and exciton separation, giving rise to improved Jsc and PCE. Too small NPs absorption and exciton separation, giving rise to improved J and PCE. Too small NPs cannot convert cannot convert the absorbed photons efficiently into free chargesc carriers. Too large NPs may induce the absorbed photons efficiently into free charge carriers. Too large NPs may induce a high roughness a high roughness of the layer morphology, deteriorating the OPV devices. of the layer morphology, deteriorating the OPV devices.

Polymers 2018, 10, 123 25 of 33 Polymers 2017, 9, x FOR PEER REVIEW 25 of 33

Figure 19. ((aa)) J J-V-V curves curves of the devices devices with various Au particle sizes. The inset is the schematic of the device structure. (b) AbsorbanceAbsorbance spectraspectra ofof thethe structurestructure of of ITO/Au ITO/Au NPs:ZnO/PBDTTT-CF:PC70BMNPs:ZnO/PBDTTT-CF:PC70BM with various various Au Au particle particle sizes. sizes. The inset The is insetEQE spectra is EQE of spectrathe device of without the device Au NPs without and the Au device NPs withand the 41 nm device Au with NPs. 41 (c) nm Transient Au NPs. PL decays (c) Transient of the structuresPL decays of of ITO/Au the structures NPs:ZnO/PBDTTT of ITO/Au-

CF:PCNPs:ZnO/PBDTTT-CF:PC71BM with different 71sizesBM of with Au differentNPs. The sizes inset of gives Au NPs. the dependence The inset gives of the the lifetime dependence on the diametersof the lifetime of Au on NPs. the diameters(d) The dependence of Au NPs. of( thed) The maximum dependence exciton of generation the maximum rate excitonGmax an generationd exciton dissociationrate Gmax and probability exciton dissociation P(E, T) on the probability diameters P(E, of T)Au on NPs the [73] diameters. Reprinted of Au with NPs permission [73]. Reprinted from withRef. permission[73]. from Ref. [73].

In addition, metallic nanostructures can be located between neighboring layers, and the photophysical results can well explain the device performance enhancement. Yang et al. analyzed the steadysteady-state-state and time-resolvedtime-resolved PLPL curvescurves ofof P3HTP3HT forfor samplessamples withwith Ag Ag NRs NRs at at ITO/ETL, ITO/ETL, ETL/activeETL/active layer, andand activeactive layer/HTL layer/HTL interfaces,interfaces, respectively, respectively, as as exhibited exhibited in in Figure Figure 20 .20 From. From the the steady-state steady-state PL PL spectra in Figure 20b, the Huang-Rhys factor (S = I0–1/I0–0) is obtained as an indicator of the spectra in Figure 20b, the Huang-Rhys factor (S = I0–1/I0–0) is obtained as an indicator of the conjugation conjugationlength of polymers. length of The polymers. S value The is the S smallestvalue is the when smallest Ag NRs when are Ag between NRs are ITO between and ETL. ITO The and optical ETL. Theabsorption optical isabsorption amplified is to amplified the greatest to the extent greatest with extent the same with plasmonic the same plasmonic architecture. architecture. The transient The transientPL profiles PL presentedprofiles presented faster decays faster fordecays plasmonic for plasmonic samples samples compared compared with thewith reference the reference one (seeone (seeFigure Figure 20c), demonstrating20c), demonstrating strong coupling strong coupling of the excitons of the and excitons plasmonic and field. plasmonic The photophysical field. The photophysicalcharacterizations characterizations manifest that the m excitonanifest dissociation that the exciton efficiency, dissociation the electron efficiency, delocalization, the electron and the delocalization,absorption are improved and the absorption by the Ag NRs, are improved in particular by when the Ag they NRs, are in at ITO/ETLparticular interface. when they With are this at ITO/ETL interface. With this optimized plasmonic structure, Jsc is increased and Voc and FF remain optimized plasmonic structure, Jsc is increased and Voc and FF remain almost unchanged. The PCE of almost unchanged. The PCE of P3HT:PCBM solar cells is increased from 3.10% for the control device P3HT:PCBM solar cells is increased from 3.10% for the control device to 4.05% in the plasmonic device to 4.05% in the plasmonic device with Ag NRs inserted at the ETL/cathode interface [79]. with Ag NRs inserted at the ETL/cathode interface [79]. Additionally, organic-inorganic hybrid nanostructures comprised of polymer semiconductors Additionally, organic-inorganic hybrid nanostructures comprised of polymer semiconductors and plasmonic metal have been newly developed for future photocatalytic and photovoltaic and plasmonic metal have been newly developed for future photocatalytic and photovoltaic applications [95,103]. The nanostructures hold the polymer and metal in close proximity, facilitating applications [95,103]. The nanostructures hold the polymer and metal in close proximity, facilitating the charge/energy transfer and the exciton dissociations that are beneficial to charge collection in OPV the charge/energy transfer and the exciton dissociations that are beneficial to charge collection in OPV devices. For instance, Au@P3HT nanocomposites and Au-P3HT NRs have been fabricated, and the devices. For instance, Au@P3HT nanocomposites and Au-P3HT NRs have been fabricated, and the time-resolved PL profiles are illustrated in Figure 21. The mean decay time of Au@P3HT composite time-resolved PL profiles are illustrated in Figure 21. The mean decay time of Au@P3HT composite is is shorter than that of pristine P3HT. Analogously, Au-P3HT NRs possess notably decreased fluorescence lifetimes compared to neat P3HT NRs [85]. The increased fluorescence decay rate of

Polymers 2018, 10, 123 26 of 33

Polymersshorter 2017 than, 9 that, x FOR of PEER pristine REVIEW P3HT. Analogously, Au-P3HT NRs possess notably decreased fluorescence26 of 33 Polymers 2017, 9, x FOR PEER REVIEW 26 of 33 lifetimes compared to neat P3HT NRs [85]. The increased fluorescence decay rate of semiconductor polymers in the presence of plasmonic metals is associated with the energy transfer from the excited semiconductor polymers in the presence of plasmonic metals is associated with the energy transfer state of P3HT to the SPR state of gold NPs [103]. from the excited state of P3HT to the SPR state of gold NPs [103].

FigureFigure 20. ((aa)) Device Device architecture architecture of of the the reference reference inverted inverted PSCs PSCs (without (without Ag Ag NWs) NWs) or or the the plasmonic plasmonic invertedinverted PSCs PSCs (with (with Ag Ag N NRs);Rs); ( (bb)) PL PL spectra spectra of of the the reference reference and and plasmonic samples samples recorded recorded using using excitationexcitation source source wavelengths wavelengths of of 500 500 nm; nm; ( (cc)) PL PL decay decay profiles profiles for for the the P3HT:PCBM P3HT:PCBM blends in in the referencereference and plasmonic plasmonic samples. Excitation Excitation source: source: 500 500 nm nm pulsed pulsed laser; laser; ( (dd)) extinction extinction spectra spectra of of the the referencereference and and plasmonic s samples.amples. Inset Inset shows shows extinction extinction spectrum spectrum of of Ag Ag NWs [79] [79]... Reprinted with permission from Ref. [79]. [79].

Figure 21. (a) Picosecond time-resolved fluorescence kinetic profiles of P3HT and Au@P3HT in FigureFigure 21. ((aa)) Picosecond Picosecond time time-resolved-resolved fluorescence fluorescence kinetic kinetic profiles profiles of of P3HT P3HT and and Au@P3HT in in chloroform. Samples were excited at 355 nm and monitored at 580 nm, and solid lines are the best- chloroform.chloroform. Samples Samples were were excited excited at at355 355 nm nm and and monitored monitored at 580 at 580nm, nm,and solid and solidlines linesare the are best the- fitted curves to extract kinetic constants. The inset is the schematic structure of Au@P3HT fittedbest-fitted curves curves to extract to extract kinetic kinetic constants. constants. The The inset inset is is the the schematic schematic structure structure of of Au@P3HT nanocomposite [103]. (b) PL lifetime decays of a P3HT thin film (black), a single P3HT nanowire (blue), nanocompositenanocomposite [103]. [103]. (b ()b PL) PL lifetime lifetime decays decays of a of P3HT a P3HT thin thinfilm (black), film (black), a single a single P3HT P3HTnanowire nanowire (blue), and a gold-P3HT nanowire (red) along with the instrument response function (IRF) of the PL lifetime and(blue), a gold and-P3HT a gold-P3HT nanowire nanowire (red) along (red) with along the with instrument the instrument response response function function (IRF) of the (IRF) PL of lifetime the PL system (grey) [95]. Reprinted with permission from ref [97]. Reprinted with permission from Refs. systemlifetime (grey) system [95]. (grey) Reprinted [95]. Reprinted with permission with permission from ref from [97]. ref Reprinted [97]. Reprinted with permission with permission from Refs. from [95,103]. [95,103].Refs. [95 ,103].

As illustrated in Figure 22, the transient absorption spectra from P3HT on a flat Ag film and on Ag gratings were compared. Whereas the GSB signals distribute from 500 to 620 nm, the positive peak around 660 nm is attributed to the polaron pair absorption. From the comparison of the peak at 660 nm, one can see that much more polaron pairs are generated via E-E annihilation on samples

Polymers 2018, 10, 123 27 of 33

As illustrated in Figure 22, the transient absorption spectra from P3HT on a flat Ag film and on Ag gratings were compared. Whereas the GSB signals distribute from 500 to 620 nm, the positive peak around 660 nm is attributed to the polaron pair absorption. From the comparison of the peak Polymers 2017, 9, x FOR PEER REVIEW 27 of 33 at 660 nm, one can see that much more polaron pairs are generated via E-E annihilation on samples withwith Ag Ag gratings, gratings, which which may may be be correlated correlated to to the the enhanced enhanced density density of of the the singlet singlet excitons. excitons. OwingOwing toto thethe involvement involvement of of periodic periodic Ag Ag gratings, gratings, the the absorption absorption in in P3HT P3HT is is enhanced,enhanced, and and hencehence moremore singletsinglet excitonsexcitons are are generated. generated. InIn FigureFigure 22 22d,d, thethe normalizednormalized bleachingbleaching recoveryrecovery of of P3HTP3HT decaysdecays fasterfaster whenwhen itit is is on on Ag Ag grating, grating, suggesting suggesting that that the the E-E E-E annihilation annihilation is is enhanced enhanced due due to to the the improved improved absorption absorption in P3HTin P3HT [104 [104].].

FigureFigure 22. 22.( a(a)) Schematic Schematic of of the the fabricated fabricated samples samples with with the the structure structure of glass.of glass. TA TA spectra spectra recorded recorded at 0.4, at 5,0.4, 20, 5, 100, 20, and100, 1400 and ps,1400 respectively, ps, respectively, of P3HT of on P3HT a (b) on flat a Ag (b) film flat andAg (filmc) Ag and grating. (c) Ag (d grating.) TA dynamics (d) TA atdynamics one peak at of one the peak photobleaching of the photobleaching spectrums [ 104spectrums]. Reprinted [104]. with Reprinted permission with from permissio Ref. [104n from]. Ref. [104]. 6. Conclusions and Outlook 6. Conclusions and Outlook In summary, we reviewed in this work the recent advances of plasmonic OPVs and relevant In summary, we reviewed in this work the recent advances of plasmonic OPVs and relevant photophysical investigations, which reveal the underlying physics involved in the photovoltaic photophysical investigations, which reveal the underlying physics involved in the photovoltaic processes. Important information can be provided by the photophysics to unveil the plasmonic processes. Important information can be provided by the photophysics to unveil the plasmonic mechanisms and the charge transfer dynamics. It is worth noting that the electrical parameters should mechanisms and the charge transfer dynamics. It is worth noting that the electrical parameters should be monitored in parallel to the photophysics to make sure the plasmonic absorption enhancement in be monitored in parallel to the photophysics to make sure the plasmonic absorption enhancement in deed contributes to the charge generation and device performance improvement. deed contributes to the charge generation and device performance improvement. The increased optical absorption, extended wavelength range, and efficient exciton dissociation The increased optical absorption, extended wavelength range, and efficient exciton dissociation can be achieved by the plasmonic effect when metallic nanostructures are embedded within the OPV can be achieved by the plasmonic effect when metallic nanostructures are embedded within the OPV devices. The plasmonic effect is strongly dependent on the distance of the nanostructures to the active devices. The plasmonic effect is strongly dependent on the distance of the nanostructures to the active layer. The effects are distinct for nanostructures within or between diverse layers in OPV devices. layer. The effects are distinct for nanostructures within or between diverse layers in OPV devices. In In thick organic solar cells that are swiftly developing for the future large-scale production of printable thick organic solar cells that are swiftly developing for the future large-scale production of printable photovoltaic devices, the position of the nanostructures can be finely manipulated in certain sublayers photovoltaic devices, the position of the nanostructures can be finely manipulated in certain of the light absorbing medium. sublayers of the light absorbing medium. The detailed plasmonic mechanisms are determined by the device configuration and the location The detailed plasmonic mechanisms are determined by the device configuration and the location of nanometals in organic solar cells. In addition to the aforementioned plasmonic modes like the of nanometals in organic solar cells. In addition to the aforementioned plasmonic modes like the scattering, LSPR, SPP, waveguide, plasmonic-cavity, and plasmonic-gap mode, the couplings of the scattering, LSPR, SPP, waveguide, plasmonic-cavity, and plasmonic-gap mode, the couplings of the plasmonics with upconversion, or fluorescence resonance energy transfer (FRET), are also potentially useful routes to boost OPV performances [60,105]. A broadband absorption may be obtained in various ways such as the incorporation of multiple categories of nanometals or hybrid metallic nanostructures with non-metal functional molecules. The utilization of novel asymmetric nanostructures has appeared to be a simple and efficient approach to get a broadband absorber. These nanostructures are usually placed in CTL or between the active layer

Polymers 2018, 10, 123 28 of 33 plasmonics with upconversion, or fluorescence resonance energy transfer (FRET), are also potentially useful routes to boost OPV performances [60,105]. A broadband absorption may be obtained in various ways such as the incorporation of multiple categories of nanometals or hybrid metallic nanostructures with non-metal functional molecules. The utilization of novel asymmetric nanostructures has appeared to be a simple and efficient approach to get a broadband absorber. These nanostructures are usually placed in CTL or between the active layer and CTL. In addition, periodic 2D nanoarrays or gratings in CTL or electrodes may also extend the absorption to the NIR range. Increasing attention should be paid to the application of 2D materials in OPV devices. Graphene and its oxide (GO), and transition metal dichalcogenides (TMDCs), are among the common 2D materials that can be considered in the photovoltaic engineering. Nanosheets of TMDC materials like MoS2, WSe2, TiTe2, and boron nitride (BN) possess a laminar structure, and their electrical properties can be tuned from semiconductors to metals by controlling the number of layers in the sheet. The plasmonics from non-metal materials have emerged as a subject of increased attention in recent years to overcome the drawbacks of the coinage metals such as the optical loss due to the interband transitions and the high cost due to the scarce supply of Au, Ag, and Cu. For instance, the SPR frequency of graphene is in the infrared and THz range, and the excitation of graphene plasmons by visible light has been reported. Nevertheless, the graphene plasmonics have scarcely been employed in organic solar cells. Although rGO was incorporated in the CTL of the solar cells, the plasmonic effect was not discussed in correlation with rGO. Viable approaches still need to be developed to discriminate unambiguously between the plasmonics from metals and non-metal materials.

Acknowledgments: This work was supported by the National Natural Science Foundation of China (11404190, 11574181, 61631166001), the Fundamental Research Funds of Shandong University (2015JC047), Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (2015014), and the “National Young 1000 Talents” Program of China. Conflicts of Interest: The authors declare no conflict of interest.

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