Aperiodic Porous Metasurface-Mediated Organic Semiconductor

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Aperiodic Porous Metasurface-Mediated Organic Semiconductor Fluorescence Zeqing Shen, Kun Zhu, and Deirdre M. O'Carroll ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00722 • Publication Date (Web): 03 Feb 2018 Downloaded from http://pubs.acs.org on February 4, 2018

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Aperiodic Porous Metasurface-Mediated Organic Semiconductor Fluorescence Zeqing Shen†, Kun Zhu† and Deirdre M. O’Carroll†,‡,* †

Department of Chemistry and Chemical Biology, Rutgers University, 610 Taylor Road,

Piscataway, NJ 08854, USA ‡

Department of Materials Science and Engineering, Rutgers University, 607 Taylor Road,

Piscataway, NJ 08854, USA

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ABSTRACT: Plasmonic metasurfaces exhibit localized electromagnetic properties that are highly relevant for light management in thin-film organic optoelectronic devices, such as organic light-emitting diodes and organic solar cells. However, the effectiveness of metasurfaces for light management in these devices is complicated by molecular orientation effects. Here, we study how the fluorescence emission from organic semiconducting polymers is modified by aperiodic porous metasurfaces. In particular, we identify the role that molecular orientation plays in enhancing or diminishing the light management ability of the metasurface by comparing localand large-area fluorescence intensity and fluorescence quantum efficiency enhancements, and by varying viewing angle for three different semiconducting polymers. We find that the porous metasurfaces improve both the local and large-area quantum efficiency of semiconducting polymer films with more out-of-plane molecular chains by up to 414% and 53%, respectively, compared to planar metal surfaces, by extracting plasmonic surface waves and the Purcell Effect. However, there are almost no enhancements to the fluorescence of semiconducting polymer films with predominantly in-plane chains. In fact, in this case, certain porous metasurfaces reduce local- and large-area quantum efficiency by up to 77% and 15%, respectively, due to enhanced ohmic losses. Experimental observations and supporting electromagnetic simulations show that fluorescence modification is also highly sensitive to viewing angle because the emission pattern of each semiconducting polymer is affected differently by the porous metasurfaces. Further, a local excitation enhancement factor of up to 65 is experimentally observed in the case of semiconducting polymer films with more out-of-plane chains and low extinction coefficient on the porous metasurfaces. KEYWORDS: conjugated polymer, plasmonic, molecular orientation, nanohole, radiative decay rate, emission pattern


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Plasmonic nanostructures are often used to mediate thin-film optoelectronic device performance because of their extraordinary light management properties.1-3 Many types of plasmonic nanostructures have been applied to the different layers of optoelectronic devices to benefit their performance; for example, embedding metallic nanoparticles into the active layers.4-6 Among the various plasmonic structures introduced in devices, plasmonic metasurface electrodes exhibit both unique optical, electrical and morphological manipulation properties that could potentially benefit optoelectronic device performance.7 For example, organic polymer thin-film solar cells with plasmonic periodic and quasiperiodic nanohole metasurface back electrodes have exhibited optical absorption enhancement factors of up to 2.9 and more than 6, respectively, at particular wavelengths compared to planar metal electrodes.8-9 For light-emitting applications, it is well known that metallic surfaces can partially quench luminescence due to ohmic losses.10 Ohmic losses are attributed to interband electronic absorption and electron scattering by the metal.10 Furthermore, theoretical and experimental studies on the light-extraction efficiency of organic light-emitting diodes (LEDs) have shown that surface plasmons diminish light out-coupling because planar metal electrodes can trap emission in surface plasmon polaritons modes (SPPs) and lossy surface waves (LSWs).11-12 Both SPPs and LSWs actually have similar origins, although different terminologies and classification methods have been used to describe them by many researchers.10, 13-14 Due to the mismatch between the in-plane wavevectors of SPPs and LSWs of planar metal films with that of light traveling in adjacent semiconductor media, the electric fields of SPPs an LSWs are confined close to the semiconductor/metal interface and cannot be efficiently emitted to the far field.15 SPP electric fields can exist at distances between ten to several tens of nanometers away from the semiconductor/metal interface.10, 14 LSWs can exists between approximately one to ten


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nanometers away from the semiconductor/metal interface. In the case of organic semiconductors, SPPs and LSWs are considered to cause energy transfer from excitons in the organic material to electron-hole pairs in the metal (dipole-dipole interactions).10, 14, 16 Application of plasmonic metasurfaces to out-couple emission from SPPs and LSWs has been widely reported to mitigate this problem.11, 15, 17-18 For example, Frischeisen et al. applied periodic grating structures into the back metal electrodes of organic small molecule LEDs and successfully observed the extraction of emission coupled to SPPs as direct emission.15 By modifying the wavevector of SPPs through Bragg scattering, the conservation of momentum rule can be realized. Therefore, SPPs can be radiated to the far field.14-15 Additionally, it is believed that the localized surface plasmon resonances (LSPRs) of plasmonic nanostructures on electrodes can cause the luminescence from nearby emitting materials to be enhanced through three aspects: resonantly enhancing the local electric fields (LEFs) at the excitation wavelength (λex)19-23, changing the emission direction19, 21, 24-25, and modifying the spontaneous emission decay rate19-20, 26-28 (i.e., the Purcell Effect29). Particularly, nanoporous metasurfaces have been shown to enhance the luminescence from various light-emitting materials including organic small dye molecules18, 30-32, organic conjugated polymers33 and inorganic quantum dots or Si nanocrystals34-35. Ding et al. reported that organic small molecule LEDs with periodic nanohole metasurface top electrodes exhibited an external-quantum-efficiency enhancement factor of 1.57 compared to devices with an indium tin oxide electrode.36 Liu et al. reported that organic conjugated polymer LEDs with periodic nanohole metasurface top electrodes exhibited an electroluminescence efficiency enhancement of up to 7 compared to planar metal electrodes.33 However, on porous metasurfaces, the origin of the hole plasmon resonance (HPR) is believed to be the film SPPs instead of localized-surface 4 ACS Paragon Plus Environment

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plasmons that are responsible for LSPRs in other kind of nanostructures.37 A comprehensive understanding of the underlying mechanisms of the observed emission enhancements by porous metasurfaces is still not available.18, 36 When the emission enhancement mechanisms are discussed, either SPP out-coupling by the periodic porous structures,32-33 or the faster radiative decay rate due to the enhanced LEFs,28, 30, 34-35 are attributed as the reasons for the observed emission enhancements. The importance of periodicity of the pores also remains debatable.31 Moreover, the impacts of different molecular orientations on the effectiveness of metasurfaces for light management in organic optoelectronic devices are not accounted for. It should be noted that the luminescence enhancement or reduction of an emitter by an adjacent plasmonic metasurfaces is strongly dependent on the emission dipole orientation. Many theoretical studies indicate that the emission from molecules with different dipole orientations can be modified very differently when they are placed in close proximity to metallic surfaces that support low-loss surface plasmons.10, 14, 19, 38-41 With preferential in-plane orientation of molecular emission dipoles in the emitter layer, less emission can be coupled to and trapped in SPP modes42,43. The modification of the spontaneous emission lifetime of emission dipoles by planar metallic surfaces10, 39-40 and metallic nanostructures26, 44 can be significant for both out-ofplane and in-plane emission dipoles. Strong spontaneous lifetime reductions were reported for out-of-plane dipoles near metallic surfaces in multiple theoretical studies26, 39, 44. Semiconducting polymers (i.e., conjugated polymers) are versatile organic optoelectronic materials that are beginning to displace more traditional semiconductors for thin-film optoelectronics applications. However, semiconducting polymers can exhibit various different molecular orientations that can affect their electrical and optical properties45,46. For example, both the conductivity and optical constants have different values along the in-plane and out-of5 ACS Paragon Plus Environment

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plane direction for regioregular poly(3-hexylthiophene-2,5-diyl) thin films.47 Their structural disorder or anisotropic optical transition dipole orientations make it challenging to understand and control mechanisms of their fluorescence mediation by metasurfaces. Here, we investigate how the fluorescence of conjugated polymers with different molecular chain orientations can be affected by porous metasurfaces. The conjugated polymer fluorescence mediation mechanisms by aperiodic porous Ag metasurfaces (NPAg) were deconvoluted in detail and correlated with molecular chain orientation for the first time. We found the effectiveness of using plasmonic metasurfaces to enhance the fluorescence from conjugated polymer thin films is strongly dependent on the morphology of the polymer. This study provides knowledge and experimental method guidelines that are relevant to many thinfilm organic optoelectronic research areas and applications that incorporate plasmonic nanomaterials. In optimizing the performance of organic-polymer-based light-emitting devices, one can decide if adding plasmonic nanostructures into electrodes is practicable and cost-worthy based on the morphology of the polymer.


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RESULTS AND DISCUSSION A thermally-assisted substrate dewetting method was used to fabricate aperiodic porous Ag metasurfaces (see Supporting Information for details). This dewetting method allows the average pore size and pore density of NPAg to be tuned over large areas (see Supporting Information, Figure S1).48 The pores of NPAg with small average pore size of 0.12 ± 0.06 µm (sNPAg) and large average pore size of 1.30 ± 0.49 µm (lNPAg) have lower reflection, and higher transmission and backscattering compared to the adjacent planar Ag regions (Figure 1a-c). In general, stronger backscattering occurs from locations near where pores are indicated by transmission images (Figure 1b,c). At some locations, strong backscattering is also observed from rough Ag regions. Conjugated polymer films with thicknesses of ~80 nm were coated on bare glass, 100-nm-thick planar Ag (plAg) and NPAg samples by spin coating from polymer in chlorobenzene solutions (see Supporting Information for details); Figure 1d. A layer of polyvinyl alcohol (PVA) or a glass coverslip bonded by optical adhesive was used to passivate the sample. Three different semiconducting conjugated polymers were employed in this study: regiorandom poly(3-hexylthiophene-2,5-diyl) (RRa-P3HT), poly[2-methoxy-5-(2ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and regioregular poly(3-hexylthiophene2,5-diyl) (RR-P3HT). Although a substantial fraction of polymer molecular segments (i.e., chains) are aligned along the substrate plane in each sample due to centrifugal forces from the spin coating process, the polymers’ intrinsic chain conformation still plays a large role and results in internal film morphologies that are different from each other. RRa-P3HT is a wellknown amorphous polymer with its chains randomly entangled and randomly aligned (Figure 1e)49,50. RR-P3HT is a highly-crystalline polymer whose chains are predominately aligned in the


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in-plane direction (edge-on orientation) (Figure 1g)49,50. MEH-PPV is expected to exhibit a degree of order in between those of RRa-P3HT and RR-P3HT, with a large fraction of amorphous chains preferentially aligned in the in-plane direction and some small, highly-ordered domains embedded within the amorphous matrix (Figure 1f)51,52. The more amorphous polymer chain packing of RRa-P3HT is expected to result in mainly intra-chain emission dipoles oriented along the randomly aligned chains. Therefore, there are more out-of-plane-oriented emission dipoles in RRa-P3HT thin films compared to MEH-PPV and RR-P3HT (both have significant inplane-oriented emission dipoles due to the in-plane polymer chain alignment and in-plane ordering). Although the three polymers have different chain organization, they exhibit similar absorption and emission spectra (Figure 1h-i). Large-Area PL Enhancement Within a Narrow Collection Cone. Figure 2 shows the largearea, excitation-source-power-corrected photoluminescence (PL) enhancement (EPL) spectra of the three polymers on NPAg relative to plAg. These spectra were obtained using a large-area (~1200 µm2), transmission-mode PL setup, in which the PL was collected by a 20× microscope objective with a numerical aperture (N.A.) of 0.4 (i.e., collection cone half-angle of 23.6o) (Figure 2a inset; Methods). The calculated EPL minimizes the effect of excitation-source-power difference on PL intensity under transmission-mode PL measurements by normalizing the excitation power using an excitation-source-power correction factor, ETbare, which is the ratio of the transmittance of each bare NPAg relative to the transmittance of bare plAg. (Methods, Equation 1-3). Accounting for the higher refractive index of the epoxy/glass passivation than air, this setup collects polymer emission within a 15.3o collection cone half-angle (Figure 2a, inset). Since lNPAg and sNPAg have similar pore densities (2.5 × 107/cm2 for sNPAg and 1.5 × 107/cm2 for lNPAg), the much higher large-area EPL on sNPAg than those on lNPAg for all 8 ACS Paragon Plus Environment

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polymers (Figure 2a,b) suggested that NPAg with smaller pores enhanced the measured PL to a greater extent within the near-out-of-plane collection cone. Additionally, the RRa-P3HT on both sNPAg and lNPAg showed the highest large-area EPL among three polymers, followed by MEHPPV. Moreover, for polymers on NPAg with the same average diameter (0.38 ± 0.11 µm) (mNPAg) but different pore densities (~2.2 × 107/cm2 for mNPAg-hi and ~4 × 106/cm2 for mNPAg-lo) (Figure 2c,d), higher pore density resulted in larger large-area EPL for MEH-PPV, but slightly reduced large-area EPL in the RR-P3HT case. Therefore, the pores clearly played a role in enhancing the large-area EPL of MEH-PPV within a near-out-of-plane collection cone but did not affect RR-P3HT significantly. From the large-area EPL data it is apparent that the three polymers were affected very differently by NPAg with the same pore size and pore density even though the polymers absorb and emit in similar wavelength ranges. Large-Area, PL Quantum Efficiency and Relationship to Large-Area PL Enhancement. Table 1 shows the large-area, absolute PL quantum efficiency (η) enhancement factors for the various conjugated polymer films on sNPAg and lNPAg substrates compared to films on both glass and plAg substrates measured using an integrating-sphere system (Supporting Information). The measured η values of the passivated conjugated polymer on glass samples are higher than the literature values53-56. This is attributed to increased radiative decay rate (ΓR) resulting from an increase of the local density of optical states (LDOS; proportional to LEF intensity) due to guided photonic mode (GPMs; which occur as a result of the planar sample structure and the refractive index differences between polymer, glass/epoxy and air). The reduced η for most polymer films on any metal-containing substrate compared to glass is attributed to reduced ΓR resulting from a reduction of LDOS due to the disturbance of the symmetric GPM by the metal film, as well as increased non-radiative decay rate (ΓNR) arising 9 ACS Paragon Plus Environment

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Γ

from coupling to SPPs, LSWs and ohmic losses in the metal14, 39-40, 57. Note, η = Γ R = Γ tot

ΓR , R +ΓNR

where Γtot is the total PL decay rate of the polymer. The inceased η for polymers on NPAg compared to plAg (except RR-P3HT on sNPAg) indicates that the photophysics of the polymers are modified by an increased LDOS produced by the NPAg and results in either ΓR enhancements that are greater than ΓNR enhancements or minimizes additional ΓNR caused by the metal. RRa-P3HT on NPAg exhibited the highest enhancement of η (Eη; up to 53% enhancement) relative to the polymer on plAg, and RR-P3HT showed the lowest Eη of the three polymers. The lNPAg increased η to a greater extent than the sNPAg for all polymers. Since the large-area η measurements correct for excitation enhancement and collect all emission even from the sample edges, the higher apparent large-area EPL from polymer-coated sNPAg and lNPAg (Figure 2a,b; i.e., within a narrow collection cone) compared to Eη, particularly for sNPAg, suggests that there are excitation enhancements or photonic or plasmonic effects that affect the emission angle distribution of the polymer thin films. To understand the effects of pores and Ag regions on NPAg on EPL as well as the roles of the polymers’ film morphology on EPL, spatially-resolved PL studies of the conjugated polymer/NPAg composites were carried out. Spatially-Resolved Excitation and Emission Enhancement. The spatially-resolved excitation enhancement (Eex) factors of the polymers on pores (NPAg(p)) and on Ag regions of the NPAg (NPAg(Ag)) relative to the polymers on plAg were calculated based on an analysis of multiple transmission images (Methods, Equation 4) obtained using a 100X, 0.80 N.A. air objective (collection cone half-angle of 53.1o) (Figure 3a inset), and were plotted versus the local ETbare (Methods, Equation 3) of corresponding regions compared with plAg before coating with the 10 ACS Paragon Plus Environment

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polymer (Figure 3a). ETbare of a pore is used as a measure of pore size - the larger the pore is, the larger ETbare will be. Conjugated polymer on pore and Ag regions of NPAg show similar Eex. While there is no clear dependence of Eex on pore size, Eex of RRa-P3HT is enhanced to a larger extent by NPAg compared to MEH-PPV and RR-P3HT, with the highest measured Eex reaches 65 (Figure S4B). This is attributed to a greater fraction of out-of-plane optical transition dipoles that couple better to out-of-plane-polarized LEFs associated with pore and Ag structures at λex (~532 nm) (Figure S7B) and the smaller extinction coefficient of RRa-P3HT (Figure S5) 47, 58-59. Figure 3b shows the dependence of spatially-resolved emission enhancement (Eem) (within a collection cone with a half-angle of 31.7o in glass) on ETbare. Here, = / , such that Eem is corrected for excitation enhancements arising from the nanostructures (see Methods). The strongly-reduced values of Eem compared with EPL from the earlier-mentioned, larger-area PL study without Eex correction (Figure 2) for RRa-P3HT-coated NPAg samples suggest that Eex of RRa-P3HT on both NPAg(p) and NPAg(Ag) significantly contributed to the large-area EPL within the narrow emission cone. It is apparent that the spatially-resolved Eem generally increases with decreasing pore size for all polymers and the NPAg(Ag) showed similar Eem to the very small pores. The non-unity Eem values are attributed to modifications to the local quantum efficiency (ηloc) of the conjugated polymers, and differences in the emission redirection ability of different regions of NPAg. Spatially-Resolved Quantum Efficiency and Emission Angle Modifications. To better distinguish between the contributions to Eem from ηloc modification (i.e., by modifications to ΓR and/or ΓNR) and from the emission redirection ability of both NPAg(p) and NPAg(Ag), τPL from the polymer side and total excitation corrected PL intensity (Itotal for NPAg and IplAg for plAg)


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from the polymer side (and Ag side for pore regions) were obtained using a 100X, 1.4 N.A. oilimmersion objective (collection cone half-angle of 67.1o; Figure 4a) in reflection-mode (see Methods). We assume that by using a high-N.A. oil-immersion objective most of the emission of polymer thin films on Ag-containing samples is collected since GPMs are disturbed by the metal and the glass/air interface is eliminated; therefore, ηloc/ηplAg ≈ Itotal/IplAg, where ηplAg is the η of polymer on plAg (see Methods).

τPL differences can be observed between polymer in the pores and on the Ag regions for all three polymers on both lNPAg and sNPAg (Figure 4b-d, Figure S6a). To quantify τPL of the polymer at different locations, spatially-resolved PL decay curves were fit using exponential functions (Figure 4e, Figure S6b). All three polymers on plAg surfaces exhibit a notable decrease in τPL compared to that on glass (with τPL of RRa-P3HT, MEH-PPV and RR-P3HT decreasing by 4.4%, 14.5%, and 18.0%, respectively). Given that the absolute η of RRa-P3HT and MEHPPV drop by 43% and 30%, respectively, on plAg (Table 1), the decreases in τPL are primarily attributed to increases in ΓNR (emission coupling to SPPs, LSWs and ohmic losses) and reduced

ΓR (less emission coupled to a GPMs). Since the absolute η of RR-P3HT remains almost unchanged on plAg compared to glass (1% decrease), the large decrease in τPL is likely due to increases in both ΓR and ΓNR. The different origins of the decreasing τPL in the three polymers might be partially due to their different chain packing on the plAg and their different intrinsic ΓNR. The higher fraction of out-of-plane emission dipoles in RRa-P3HT and MEH-PPV compared to RR-P3HT results in better coupling of emission to SPPs and LSWs; resulting in greater increases in ΓNR 10. The dominant edge-on chain configuration in RR-P3HT leads to most of the emission dipoles 12 ACS Paragon Plus Environment

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oriented in-plane. This dipole orientation is known to show less vulnerability to non-radiative energy transfer to the metal than out-of-plane dipoles when placed near planar metal surfaces40. Furthermore, the predominant intrinsic non-radiative decay in RR-P3HT suggests that coupling of emission to LSWs and ohmic losses is unlikely16. For almost all polymer on NPAg cases, τPL is reduced compared to the polymer on glass cases, due to the enhanced ΓR and ΓNR introduced by the metallic surfaces. However, the τPL of polymer on NPAg can be either longer or shorter than that on plAg and different trends are observed for the three polymers. This is consistent with the fact that the PL on NPAg can be either enhanced, reduced or unchanged compared with that on plAg (Figure 2, Figure 3, Table 1). Figure 4f is the summary of the spatially-resolved ηloc, ΓR and ΓNR enhancement of RRaP3HT, MEH-PPV and RR-P3HT on different local surfaces relative to those on plAg (see Methods). It is apparent from these data that the main origin of the increased large-area η of RRa-P3HT on NPAg compared to plAg (Table 1) was the polymer on NPAg(Ag), which showed strongly-enhanced ηloc (up to 414% enhancement from lNPAg(Ag), primarily due to increased

ΓR). The large increase in ΓR indicates that there is reduced SPP coupling57 and enhanced LEFs from LSPRs, i.e., a Purcell Effect29, due to the roughness of NPAg(Ag). The stronger ΓR enhancement of RRa-P3HT in sNPAg(p) compared to that on lNPAg(p) indicates stronger LEF enhancement arising from hole plasmon resonances (HPRs)37. The stronger ΓR enhancement of RRa-P3HT on lNPAg(Ag) than that on sNPAg(Ag) might be due to rougher Ag surfaces on lNPAg than on sNPAg. MEH-PPV on pores and Ag regions of lNPAg showed highly enhanced ηloc. On lNPAg(p), ηloc increases because ΓNR is reduced, due to reduced coupling of emission to SPPs, 13 ACS Paragon Plus Environment

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LSWs and ohmic losses (i.e., the absence of Ag in the pores), and ΓR increases likely due to HPR and hole waveguiding that better enhance and direct emission to the out-of-plane direction19, 24. The enhanced ηloc of MEH-PPV on lNPAg(Ag) was due to enhanced ΓR by SPP out-coupling and LSPR enhancement of rougher Ag. However, unlike the RRa-P3HT case, the MEH-PPV on lNPAg(p) showed higher ηloc than those on lNPAg(Ag), indicating that the measured large-area Eη of lNPAg mainly comes from the pore regions. However, MEH-PPV on pore and Ag regions of sNPAg showed reduced ηloc compared with that on plAg. The noticeable enhancement of ΓNR for MEH-PPV on pores and Ag regions of sNPAg are likely due to increased coupling of emission to ohmic losses and LSWs as a result of increased metallic surface area and stronger LEFs. The slight enhancement of ΓR from MEH-PPV on sNPAg(p) suggests SPP out-coupling and HPR enhancement. The large-area η of MEH-PPV on sNPAg was higher than plAg while the localized measurement showed the ηloc of MEH-PPV on sNPAg was smaller than plAg. This could be due to the emission with large emission angle from sNPAg(Ag) or coupled to GPMs, which has higher η but is not collected by the objective. RR-P3HT on NPAg surfaces exhibits reduced ηloc in all cases compared to plAg. This is likely due to the much weaker GPMs in NPAg cases than in plAg cases, as GPMs are strongly disturbed by nanostructures. For sNPAg, the reduced ηloc was attributed to reduced ΓR due to weaker coupling to GPM and increased ΓNR caused by increased ohmic losses. For lNPAg reduced ηloc was attributed to reduced ΓR from weaker GPM coupling and increased emission with large emission angle that could not be collected by the oil-immersion objective. When comparing Figure 3b with Table 1, we noted that the Eem values of both RRa-P3HT on lNPAg(p) and lNPAg(Ag) are almost all below unity, although RRa-P3HT on lNPAg showed


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larger than unity Eη. This is likely due to a reduction in the fraction of emission collected by the microscope objective. However, the general agreement between Eη (Table 1) and the average local Eem (Figure 3b) on lNPAg(Ag) and lNPAg(p) for MEH-PPV and RR-P3HT suggests that redirection was not significant in the large area measurement (for both pore regions and Ag regions). By comparing Figure 3b with Figure 4f, we can distinguish the influence of different regions of NPAg on ηloc and the emission pattern of the different conjugated polymers. For polymer on sNPAg(p), although all three polymers exhibit very high Eem (especially for RRaP3HT and MEH-PPV, many pores exhibited an Eem larger than 2) on very small pores, the less than unity ηloc/ηplAg values for MEH-PPV and RR-P3HT indicates that the ability of very small pores to redirect emission to smaller emission angles at the polymer side of the sample is far more significant that their ability to modify ηloc for MEH-PPV and RR-P3HT. The larger Eem value than ηloc/ηplAg for RRa-P3HT on very small pores indicates very small pores both effectively redirect and enhance ηloc of RRa-P3HT. For polymer on lNPAg(p), the Eem is smaller than ηloc/ηplAg for MEH-PPV and RRaP3HT, indicating that although the lNPAg(p) can increase the quantum efficiency of these two polymers, they can also increase the fraction of emission at large angles or to the Ag side of the samples (i.e., angles greater than 31.7o). This might contribute to the small large-area EPL of lNPAg than sNPAg shown in Figure 2. Additionally, since ηloc/ηplAg is much larger compared to Eem for RRa-P3HT on NPAg(Ag), emission from these parts of the samples is expected to have a significant fraction of emission at large angles (i.e., angles greater than 31.7o). In contrast, the


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larger Eem than for MEH-PPV and RR-P3HT on NPAg(Ag) indicates the NPAg(Ag) can increase the fraction of light within a collection cone of 31.7o for polymers with more in-plane dipoles. Electromagnetic simulations were conducted to help understand the influence of substrate materials and structures on the emission patterns from emission dipoles with different orientations (see Methods, Figure S8-S10). These simulations aid in confirming redirection of emission apparent from the experimental results, above, and in fully deconvoluting differences in emission direction due to emission dipole orientation. Figure 5 shows a representative simulated emission pattern of an in-plane-oriented emission dipole in RR-P3HT, and the emission pattern of an out-of-plane-oriented emission dipole in RRa-P3HT on different substrates. Clear differences in the emission pattern are observed from emission dipoles on different substrates. For both in-plane- and out-of-plane- oriented dipoles on a glass substrate, very small fractions of radiative emission goes to out-of-plane directions (i.e., small angles) at the polymer side due to GPM. In comparison, plAg can increase the fraction of radiative emission within relatively small angles at the polymer side (Table S4-S6). Compared to dipoles on plAg, a single nanohole on the Ag film can noticeably increase the fraction of radiative emission from an out-of-plane-oriented emission dipole within relative small emission angles (e.g., 15.3o and 31.7o) at the polymer side and can slightly reduce the fraction of radiative emission from an in-plane-oriented emission dipole within relative small emission angles. In particular, smaller pores show larger enhancement of emission within small angles for out-of-plane-oriented dipoles (See Supporting Information page S23 and Table S4-S6 plAg, NPAg1a and NPAg2). This trend is consistent with the experimental data, i.e., smaller pore size causes a larger fraction of emission to occur within relatively narrow emission cones at the polymer side compared to that on plAg.


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Summary: Mechanisms of Aperiodic Porous Ag Metasurface-Enhanced Conjugated Polymer Fluorescence. Based on our experimental observations and supporting theoretical simulations, we conclude that there are three general mechanisms of PL mediation in different samples: (1) excitation enhancement; (2) redirection/emission pattern modification; (3) decay rate modification. These mechanisms vary in importance depending on polymer chain morphology, presence or absence of pores, and polymer extinction coefficient. Figure 6 summarizes the mechanisms that can enhance the fluorescence of conjugated polymer thin films on various substrates. Excitation Enhancement. Based on Figure 3a and Figure S5, the extent of Eex in the polymer (Figure 6a) that is due to modified LEF polarization and improved electric field localization (i.e., plasmonic “hot spots”) at λex by both NPAg(p) and NPAg(Ag), primarily depends on the chain alignment and extinction coefficient of the polymer at λex. This is further supported by the simulation results shown in Figure S7B, in which the excitation local fields polarizations were modified by the addition of pores in the silver thin films. Redirection/Emission Pattern Modification. The radiative emission pattern at the polymer-side of dipoles with different orientation on glass and plAg, depicted in Figure 6b, were mainly based on the electromagnetic simulation results (Figure S8-S10 and Table S4-S6). Moreover, the higher absolute PL quantum efficiency of the polymer on glass samples than the literature value (Table 1) indicates there are extra GPMs in the sample which lead to emission from the edge of samples. Compared with the emission pattern of a polymer thin film on glass60, a layer of optically-thick plAg always 17 ACS Paragon Plus Environment

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increases the fraction of emission with small emission angles at the polymer (Figure 6b), probably as a result of constructive and destructive interference of the emitter’s direct emission and reflected emission by the metal surface (interference effects)10, 39. There can be subtle differences in these trends due to differences in the refractive indices of the polymers (see Figure S8-S10). The Ag regions and pores of NPAg can further modify the emission pattern at the polymer side by their plasmonic properties and scattering ability. Based on the fact that there were much larger Eem enhancements within small collection cone angles (Figure 3b) than ηloc enhancements (Figure 4f) for polymers on extremely small pores, we conclude that very small pores have the ability to direct the emission to very small emission angles. This is mainly due to emission pattern modification of out-of-plane-oriented dipoles based on our simulation results (Figure S8-S10 and Table S4-S6), which arises from stronger coupling to the local electric fields of the pores. Decay Rate Modifications. The η of the polymer can also be modified (i.e., by modifying ΓR and ΓNR) by the enhanced electric fields associated with plasmonic and photonic modes (Figure 6c). Besides the intrinsic ΓR and ΓNR of the polymer (i.e., Γ R0 and Γ N0R ), the excited-state energy can couple to GPMs in the epoxy-passivated polymer on glass samples (Figure 6c, A). Only when the emission coupled to GPMs can be collected (i.e., measurement of absolute η; Table 1), can the GPMs be considered as a radiative decay channel. For most other situations (e.g., microscope measurements, thin-film LEDs), GPMs are effectively a non-radiative decay channel. Three other decay channels, i.e., SPPs, LSWs and ohmic losses exist in the passivated polymer on plAg samples (Figure 6c, B), and GPMs are largely disturbed by the asymmetric 18 ACS Paragon Plus Environment

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sample structure in this case. Emission dipoles oriented out-of-plane can couple better to these channels10, 39 (Table 1, Figure S8-S10). Emission coupling to SPPs and LSWs is usually considered as a “non-radiative” decay channel because it cannot be observed directly in most situations, but it has radiative intrinsic character and can be out-coupled when certain optical conditions are achieved (e.g., wavevector matching)10, 14, 39. In contrast, ohmic losses result in purely non-radiative decay of emission10, 57. Both the GPM, SPPs and LSWs can be reduced compared with plAg in passivated polymer on NPAg samples (Figure 6c, C), due to the pores and rough Ag regions of the NPAg. However, the ohmic losses can be either enhanced or reduced based on whether the polymer/metal interfacial area is increased or reduced, or they can be enhanced due to enhanced LEFs. The pores and roughness of the Ag regions can also increase ΓR by increasing the LEF intensity by HPR or LSPR. For polymers with more emission dipoles oriented out-of-plane, their

η can be enhanced more by the rough Ag surfaces, mainly due to out-coupled SPPs (Figure 4f). CONCLUSIONS In conclusion, molecular orientation is a critical parameter that can affect the effectiveness of organic polymer semiconductor fluorescence mediation by aperiodic porous metasurfaces. Emission from out-of-plane molecular dipoles ordinarily trapped in guided photonic or non-radiative propagating plasmonic modes of planar metal films can be partially extracted by both small and large pores and by rough metallic surfaces in the aperiodic porous metasurface. This results in an increase in the absolute quantum efficiency of the polymers compared to that on planar metal films. Enhancements of up to 53% in the large-area absolute quantum efficiency were observed for polymers with greater fractions of out-of-plane polymer


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chains. For in-plane molecular dipoles, emission does not couple well to non-radiative propagating plasmonic modes (i.e., absolute quantum efficiency was not reduced by the metal substrates compared to glass). Almost no enhancement, 11% increase or 15% reduction, in the large-area absolute quantum efficiency were observed for a polymer with predominate in-plane chains. The conjugated polymer thin film emission angles can also be modification by aperiodic porous metasurfaces. Small nanopores, in particular, increase emission within a narrow collection cone by up to almost 5 times, which is primarily attributed to the ability of small pores to redirect emission from out-of-plane-oriented dipoles (which can couple more strongly to the local electric fields of the pores).The excitation of conjugated polymers thin films with more outof-plane chains and low extinction coefficients was also notably enhanced by the pores and rough metal surface on aperiodic porous metasurfaces due to resonantly-enhanced local electrical fields. METHODS Materials and Reagents. Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV) (number-average molecular weight (Mn) of 40,000-70,000 Da; Sigma-Aldrich, Inc., PA, USA); poly(3-hexylthiophene-2,5-diyl), regioregular (RR-P3HT) (weight-average molecular weight (Mw) of 69,000 Da, polydispersity index (PDI) of 2.3, regioregularity of 96%; Rieke Metals, Inc., NE, USA); poly(3-hexylthiophene-2,5-diyl), regiorandom (RRa-P3HT) (Mw = 57,000 Da, PDI = 2.9; Rieke Metals, Inc.); polyvinyl alcohol (PVA) (Mw ~61,000 Da; Mowiol®10-98, Sigma-Aldrich, Inc.); silver shot (99.9%; Strem Chemicals, Inc., MA, USA); chlorobenzene (ACS reagent, Sigma-Aldrich, Inc.); micro cover glass (18 × 18 mm; VWR No.2); cover glass (25 × 25 mm; FisherbrandTM No. 1); optical adhesive (Norland 65, Norland Products Inc., NJ, USA). 20 ACS Paragon Plus Environment

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Steady-State Transmission and Photoluminescence Imaging and Spectroscopy. A Nikon Optiphot 66 optical microscope coupled with a pixeLINK CMOS camera and an Andor Shamrock SR 303i imaging spectrometer were employed for steady-state microscopy imaging and spectroscopy. Transmission and photoluminescence steady-state images and spectra were acquired with the microscope working in transmission mode in which the samples were illuminated/excited from the NPAg or glass side. The single pore excitation enhancement (Eex), emission enhancement (Eem) and excitation-source-power correction factor (ETbare) were obtained by analyzing 600 nm to 700 nm wavelength PL images (a 600 nm to 700 nm bandpass filter was applied between the sample and the spectrometer) and 510 nm to 560 nm wavelength transmission images (a 510 nm to 560 nm bandpass filter was applied between the sample and the spectrometer) (100X, 0.80 N.A. air objective) of sNPAg, lNPAg and plAg before and after polymer coating using Andor Solis software (see Figure S4). We define the PL enhancement (EPL) of polymer on NPAg compared with polymer on plAg after excitation-source-power correction as: ∗ = /

(1)

in which the PL enhancement without excitation-source-power correction is:

∗ =

(2)

and the excitation-source-power correction factor (transmittance enhancement) is:

=

=

(3)


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where

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!"

and are the measured PL intensity of polymer on NPAg and plAg,

!"

respectively. and are the transmitted excitation light intensity of bare NPAg and plAg, respectively, the ratio between whom equals to the ratio between the transmittance of bare

!"

NPAg ($ ) and bare plAg ($ ) at λex. To calculate the large-area EPL spectra, the PL intensity at the wavelength (λ) where the PL of polymer coated NPAg reached its maxima were !"

extracted from the large-area polymer coated plAg PL spectra, and used as , together with ∗ the PL intensities of polymer on NPAg at different wavelengths to calculate the spectra

based on Equation (2). The transmittance of bare NPAg and plAg at a wavelength of 532 nm were extracted from their large-area transmittance spectra collected by the same 20X objective (Figure S3) as the PL spectra to calculate the ETbare with Equation (3). It should be noted that due to the high refractive index of the three conjugated polymers at the λex (532 nm), the transparency of NPAg and plAg at 532 nm will be slightly enhanced when they were coated with polymer compared to the bare NPAg and plAg. The transparency enhancement factor due to polymer coating is higher for the Ag regions than the pore regions (glass substrate) (Table S2). Therefore, the excitation power correction factor calculated using transmittance of bare NPAg and plAg and Equation (3) could be slightly overestimated, especially for NPAg with higher porosity (larger pore/glass area). The calculated large-area EPL is then slightly smaller than their actual values. The higher the porosity, the larger the differences between the calculated largearea EPL values with the actual values. But the difference should be very small and the actual values are not expected to be larger than 1.5 times the calculated values. In the calculation of single pore Eex, Eem and ETbare, the total counts in an area of interest (AOI) containing a pore or a Ag region were extracted from the corresponding PL or transmission


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images acquired from two or three different regions of an NPAg sample before and after coated with polymer (Figure S4). The total counts in the correlated position were extracted from the average PL or transmission images of polymer-coated or bare plAg averaged from six different regions. The Eex factor is:

=

×' ()*+, ×' ()*+,

/

(4)

The new parameters - and -!" are the refractive index correction factors used to correct the enhanced excitation transmittance of substrates after polymer coating due to the higher

!"

refractive index of polymers than air (Table S2). ./01 and ./01 are the transmitted excitation light intensity of polymer coated NPAg and plAg, respectively. Here, the ETbare was

calculated using equation (3) and Eem was calculated using equation (1) to (4), in which or

!"

!"

./01 and or ./01 are the total transmitted photon counts in the wavelength range 510 nm to 560 nm from the corresponding AOI of bare or polymer-coated NPAg and plAg transmission images, respectively. The AOI on the NPAg image is either a pore or a Ag region

and have different - values (Table S2).

!"

and

are the total PL photon counts in the

wavelength range 600 nm to 700 nm from the above mentioned AOIs of polymer-coated NPAg and plAg transmission-mode PL images (Figure S4). The higher ETbare of lNPAg(Ag) than the sNPAg(p) was attributed to the rougher Ag surface on lNPAg, and/or light leakage due to imperfect focusing when acquiring the transmittance images. Spatially-Resolved Photoluminescence Lifetime and Quantum Efficiency Measurements. A confocal fluorescence lifetime imaging microscope61-62 was employed to study the PL intensity and transient PL lifetime decay behaviors of conjugated polymers on plAg, glass, sNPAg and 23 ACS Paragon Plus Environment

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lNPAg. In this study, a 40 MHz, pulsed, 532 nm diode laser (PicoQuant) with minimum pulse width 78 ps was used as the excitation source. A 15 µm pinhole was employed for confocal imaging and a 100X, 1.4 N.A. oil-immersion objective lens was used for illumination and collection to improve the spatial resolution. A 532 nm dichroic mirror and a 532 nm longpass filter were used to remove the excitation laser light from the collection path. To measure the spatially-localized PL lifetimes and intensities, a white light transmission image of a fresh region of the NPAg sample were first scanned to help localize pores and Ag regions. The decay histograms of PL from each polymer film in 10 pores, on 10 Ag regions of NPAg, on 10 regions of plAg and on 10 regions on glass were then acquired by point measurements for 10 s (MEHPPV) or 20 s (RRa-P3HT and RR-P3HT) to minimize photo bleaching. The collected data were then fitted and processed using SymPho Time 64 (PicoQuant) by single exponential (RR-P3HT) or double exponential decays (MEH-PPV and RRa-P3HT) with χ2 close to 1. The radiative decay rate (ΓR) and non-radiative decay rate (ΓNR) of the PL for an emitter can be expressed as:

Γ' = Γ0/0 × η' Γ' = Γ0/0 × (1 − η' )

(5) (6)

In which η' is the quantum efficiency of the emitter and the total decay rate is:

Γtot = 8

7

(7)

The modifications of PL lifetime (τPL) are indications of total decay rate changes.


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The local quantum efficiency (ηloc) of polymer on different surfaces can be calculated using the following equation:

ηloc =

Itotal I0

×η0

(8)

Where η0 is the intrinsic PL quantum efficiency of the polymer. I0 and Itotal are the total PL intensities from the same amount of polymer in free space and within the medium under study, respectively, after absorbing the same amount of excitation light. However, it is not straight-forward to experimentally measure the intrinsic PL quantum efficiencies of the polymer, even the η values of the conjugated polymer on glass samples measured using an absolute quantum efficiency measurement system were likely to have been modified from the true intrinsic η0 because of GPMs due to the sample structure. Therefore, instead of calculating the absolute values for the ηloc and decay rates, we calculated the enhancement factors of local nanostructures compared to plAg: η*) η

Γ< Γ

(13)

?,

where Fex,p is the excitation correction factor for pores when exciting the sample from the Ag side and equals to: C A .! = × A ,

(14)

C in which is the averaged Eex calculated using equation (4) from previous mentioned single

pore excitation enhancement study. Electromagnetic Simulations. Commercial simulation software based on the finite-difference time-domain method was used to perform the electromagnetic simulations (Lumerical FDTD Solutions, Lumerical Inc.). In the electromagnetic simulations of emission patterns, the x-y crosssectional area of the FDTD simulation region was set to be 1800 nm × 1800 nm. To mimic sample structures (80-nm-thick polymer layer, 80-nm-thick polymer layer on 100-nm-thick plAg, or 80-nm-thick polymer layer on 100-nm-thick NPAg with a hole in the silver layer - 120 nm or 240 nm hole diameter) were embedded in an environment with refractive index of 1.52 (Figure S7A). A point electric dipole source with monochromatic emission (601 nm for RRaP3HT, 594 nm for MEH-PPV and 664 nm for RR-P3HT) were used to mimic the conjugated polymer emission dipoles. In all the simulations, the x and y positions of the dipole are always set to be zero. The modification of the relative position of the dipole compared to the pore were achieved by modifying the x and y positions of the pore. Perfectly-matched layer (PML) boundaries were applied in the x, y and z directions to understand the influence of a single pore on a single dipole. Two frequency-domain electric field monitors spread over the whole simulation x-y plane (i.e., in-plane direction) were placed 110 nm below or 90 nm above the


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polymer/metal interface, respectively to record the emission pattern at both sides of each sample. A mesh-override region covered all sample structures and monitors, and a 2 nm × 2 nm × 2 nm mesh size was used in all of the simulations. Such large monitors and small mesh sizes were chosen to obtain the highest precision that the available computing capacity allowed. The farfield emission pattern of the dipole on different surfaces was calculated using the far-field projection function. The emission power in certain emission cones/angles were analyzed using the ‘farfield3dintegrate’ function in the softwared (Figure 5). It should be noted that emission pattern modification by the pore has a strong spatial dependence. Dipoles located at different positions in the pore exhibit different emission patterns (see Figure S8-S10 and Table S4-S6). In experiments, we measured PL from ensembles of polymer transition dipoles (with various orientations) located at throughout the polymer thin films. In contrast, in the electromagnetic simulations, we monitored the emission pattern from a transition dipole (either in-plane or out-of-plane oriented) located at a specific location. Also, in the simulations, when dipole emission monitored by a near-field monitor is projected to the farfield in FDTD Solutions, ideally the simulation area and monitor size need to be infinite to get precise results. However, this is not possible in practical simulation setups due to limitations in computational power. Moreover, the roughness of the silver regions and the irregular pore shapes in the real samples, which will lead to different local electric fields than smooth films and regular round pores, are not considered in the simulations. Therefore, comparisons between the experimental results and simulations were qualitative rather than quantitative.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Aperiodic porous Ag metasurface fabrication detailed procedure; Table S1, table for aperiodic porous Ag metasurface fabrication conditions; Figure S1, SEM images of aperiodic porous Ag metasurfaces; Detailed procedures for preparation and passivation of polymer films; Integrating sphere photoluminescence efficiency measurements; Additional information for steady-state large-area and spatially-resolved transmission and photoluminescence imaging and spectroscopy; Figure S2, transmission and PL images of polymer coated aperiodic porous Ag metasurfaces; Figure S3, large-area transmittance spectra of different substrates; Figure S4, supporting figure for spatially-resolved excitation enhancement study; Figure S5, optical constants; Table 2, table of transmittance enhancement due to high refractive index coating; Table S3, table of excitation enhancements; Additional information and discussion for spatially-resolved photoluminescence lifetime and quantum efficiency measurements; Figure S6, supporting figure for spatiallyresolved PL lifetime study; Detailed about simulations setup; Figure S7, supporting figure for excitation enhancement simulation; Figure S8, simulated polar figure of MEH-PPV dipole on different substrate; Figure S9, simulated polar figure of RRa-P3HT dipole on different substrate; Figure S10, simulated polar figure of RR-P3HT dipole on different substrate; Quantification of the fraction of emission power in an emission cone with a certain half-angle; Table S4. Fraction of light emitted by a dipole in an emission cone with a half-angle of 15.3o; Table S5. Fraction of light emitted by a dipole in an emission cone with a half-angle of 31.7o; Table S6. Fraction of light emitted by a dipole in an emission cone with a half-angle of 67.1o; Supporting Information references. 29 ACS Paragon Plus Environment

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AUTHOR INFORMATION Corresponding Author *Email: [email protected] ORCID: Deirdre O’Carroll: 0000-0001-7209-4278 Zeqing Shen: 0000-0001-6583-8574 Notes: The authors declare no conflict of interest. ACKNOWLEDGMENT This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DESC0012704. This work was also supported in part through funding provided by National Science Foundation (Grant No. DMR-1309459 and Grant No. CHE-1415881). The authors thank Dr. Mircea Cotlet for fluorescence lifetime imaging training and discussions. The authors thank O’Carroll group members for useful discussions.


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REFERENCES (1) Atwater, H. A.; Polman, A., Plasmonics for improved photovoltaic devices. Nat. Mater. 2010, 9, 205-213. (2) Lozano, G.; Rodriguez, S. R. K.; Verschuuren, M. A.; Rivas, J. G., Metallic nanostructures for efficient LED lighting. Light: Sci. Appl. 2016, 5. (3) Ozbay, E., Plasmonics: Merging photonics and electronics at nanoscale dimensions. Science 2006, 311, 189-193. (4) Tan, H. R.; Santbergen, R.; Smets, A. H. M.; Zeman, M., Plasmonic Light Trapping in Thin-film Silicon Solar Cells with Improved Self-Assembled Silver Nanoparticles. Nano Lett. 2012, 12, 4070-4076. (5) Kumar, A.; Srivastava, R.; Mehta, D. S.; Kamalasanan, M. N., Surface plasmon enhanced blue organic light emitting diode with nearly 100% fluorescence efficiency. Org. Electron. 2012, 13, 1750-1755. (6) Fujiki, A.; Uemura, T.; Zettsu, N.; Akai-Kasaya, M.; Saito, A.; Kuwahara, Y., Enhanced fluorescence by surface plasmon coupling of Au nanoparticles in an organic electroluminescence diode. Appl. Phys. Lett. 2010, 96. (7) Petoukhoff, C. E.; Shen, Z. Q.; Jain, M.; Chang, A. M.; O'Carroll, D. M., Plasmonic electrodes for bulk-heterojunction organic photovoltaics: a review. J. Photonics Energy 2015, 5. (8) Bai, W. L.; Gan, Q. Q.; Song, G. F.; Chen, L. H.; Kafafi, Z.; Bartoli, F., Broadband shortrange surface plasmon structures for absorption enhancement in organic photovoltaics. Opt. Express 2010, 18, A620-A630. (9) Flanigan, P. W.; Ostfeld, A. E.; Serrino, N. G.; Ye, Z.; Pacifici, D., A generalized "cut and projection" algorithm for the generation of quasiperiodic plasmonic concentrators for high efficiency ultra-thin film photovoltaics. Opt. Express 2013, 21, 2757-2776. (10) Ford, G. W.; Weber, W. H., Electromagnetic interactions of molecules with metal surfaces. Phys. Rep. 1984, 113, 195-287. (11) Hobson, P. A.; Wedge, S.; Wasey, J. A. E.; Sage, I.; Barnes, W. L., Surface plasmon mediated emission from organic light-emitting diodes. Adv. Mater. 2002, 14, 1393-1396. (12) Smith, L. H. W., J. A. E.; Samuel, I. D. W.; Barnes, W. L.,, Light out-coupling efficiencies of organic light-emitting diode structures and the effect of photoluminescence quantum yield. Adv. Funct. Mater. 2005, 15, 1839-1844. (13) Barnes, W. L., Light-emitting devices - Turning the tables on surface plasmons. Nat. Mater. 2004, 3, 588-589. (14) Lakowicz, J. R., Radiative decay engineering 5: metal-enhanced fluorescence and plasmon emission. Anal. Biochem. 2005, 337, 171-194. (15) Frischeisen, J.; Niu, Q.; Abdellah, A.; Kinzel, J. B.; Gehlhaar, R.; Scarpa, G.; Adachi, C.; Lugli, P.; Brütting, W., Light extraction from surface plasmons and waveguide modes in an organic light-emitting layer by nanoimprinted gratings. Opt. Express 2011, 19, A7-A19. (16) Markov, D. E.; Blom, P. W. M., Migration-assisted energy transfer at conjugated polymer/metal interfaces. Phys. Rev. B 2005, 72. (17) Leong, K.; Zin, M. T.; Ma, H.; Sarikaya, M.; Huang, F.; Jen, A. K. Y., Surface Plasmon Enhanced Fluorescence of Cationic Conjugated Polymer on Periodic Nanoarrays. ACS Appl. Mater. Interfaces 2010, 2, 3153-3159. (18) Liu, Y.; Blair, S., Fluorescence enhancement from an array of subwavelength metal apertures. Opt. Lett. 2003, 28, 507-509. 31 ACS Paragon Plus Environment

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TABLES AND FIGURES

Figure 1. Materials and sample configurations. (a) Bright-field reflection optical images of sNPAg (left; 0.001 s exposure time), lNPAg (right; 0.001 s exposure time) and plAg (inset; 0.001 s exposure time). (b) Bright-field transmission optical images of sNPAg (left; 2 s exposure time), lNPAg (right; 0.5 s exposure time) and plAg (inset; 2 s exposure time). (c) Reflectionmode dark-field optical images of sNPAg (left; 0.3 s exposure time), lNPAg (right; 0.3 s exposure time) and plAg (inset; 0.3 s exposure time). The brightness and contrast were enhanced by 50% and 30% for (b) and its inset; the brightness and contrast were enhanced by 30% and 30% for (c) and its inset. All images have the same scale (scale bar shown in (c)). (d) Schematic of sample configuration. (e)-(g) Molecular structures (top) and polymer chain alignments schematics (bottom; red indicates polymer, blue is the substrate) of RRa-P3HT, MEH-PPV, and RR-P3HT, respectively. (h)-(f) Large-area absorbance and normalized PL spectra (λex = 532 nm, green dotted line in (h)), respectively, of RRa-P3HT, MEH-PPV, RR-P3HT, on glass (20X, 0.4 N.A., air).


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Figure 2. Large-area PL enhancement within a near-out-of-plane collection cone. (a)(b) Largearea EPL spectra (λex = 532 nm) of RRa-P3HT, MEH-PPV and RR-P3HT on sNPAg (a) and lNPAg (b). Inset in (a) is the schematic of the measurement configuration. (c)(d) Large-area EPL spectra (λex = 532 nm) of MEH-PPV (c) and RR-P3HT (d) on mNPAg with different pore densities. Insets in each figure are bright-field transmission images of polymer-coated NPAg (top: mNPAg-hi (500 ms exposure time); bottom: mNPAg-lo (2000 ms exposure time)). The brightness and contrast were enhanced by 50% and 30% for insets in (d)) to more clearly distinguish the pore regions.


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Table 1. Large-area absolute PL quantum efficiency, η, of different polymer thin films on various substrates. The PL quantum efficiency enhancement (Εη) relative to polymer thin films on plAg are shown in parenthesis for polymer on NPAg.

η (Εη) Polymer

Literature

Glass

plAg

lNPAg

sNPAg

RRaP3HT

8%,53 10%54

19.4%

11.1%

17.0% (1.53)

15.2% (1.37)

MEHPPV

10%,55 15%54

21.4%

15%

19.2% (1.28)

17.9% (1.19)

RRP3HT

Aperiodic Porous Metasurface-Mediated Organic Semiconductor

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