Nanostructures for Plasmonic Effects in Solar Cells and LEDs

CHAPTE R 13

Nanostructures for Plasmonic Effects in Solar Cells and LEDs Ruipeng Xu, Yanqing Li, Jianxin Tang Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou, China

13.1 Introduction With the rapid development of material science and device structure design, solar cell and LED technologies have made significant progress in recent years. The solar cells made from crystalline silicon with thickness between 150 and 300 μm demonstrate high efficiencies, but they also face severe obstacles with high material usage and costs. Thin-film solar cells based on hydrogenated amorphous silicon (a-Si:H), GaAs, CdTe, CuInSe2, and organic semiconductors, as well as mesoporous materials, have emerged to address the cost issue and to improve the cells’ performance (Atwater and Polman, 2010). However, thin photovoltaic absorbers will limit the light absorption greatly, which leads to insufficient carrier generation and collection. This trade-off between the thickness and cell efficiency requires effective photon management to balance the mismatch and realize high-performance cells at large scale and low cost. Similarly, semiconductor-based LEDs are notorious for their low lightemission efficiencies (Ozbay, 2006). Inorganic LEDs fabricated by Group III-nitrides such as GaN, InN, InGaN have been commercialized but their internal quantum efficiencies (IQEs) decrease significantly due to nonradiative recombination processes, threading dislocations, and charge separation from the polarization fields (Cho et al., 2010). Moreover, theoretical and experimental results show that nearly 80% of the emitted light in a conventional multilayer OLED device is confined in the substrate (substrate mode), the emitting layer, and the transparent electrode (waveguide mode), or lost by coupling to surface plasmon polaritons of the metal electrode (SP mode). Of all these loss channels, the fraction of light quenched to SP modes can be as high as 40% (Hobson et al., 2002). This high loss has impeded progress toward the realization of high-efficiency LED devices and brought the issue of compensating for this loss to the forefront of research. Plasmonic nanostructures, which belongs to the general field of plasmonics, enables the precise manipulation of the flow of photons and are considered as promising candidates for light-trapping or extraction in optoelectronics. Plasmonics is a flourishing new field of science Advanced Nanomaterials for Solar Cells and Light-Emitting Diodes. https://doi.org/10.1016/B978-0-12-813647-8.00013-8 © 2019 Elsevier Inc. All rights reserved.

477

478  Chapter 13 and technology that explores light manipulation by metallic nanostructures at nanometer length scales. It originates from the discovery of collective oscillations of free electrons localized at metallic interfaces or in small metallic nanostructures, known as surface plasmons (SPs). In the early 20th century, Sommerfeld (1899) and Zenneck (1907) discovered electromagnetic waves propagating along the surface of a metal at radio frequencies and Wood (1902) observed an anomalous intensity decrease of light reflected off metallic gratings for visible frequencies. Several years later, in 1952, Pines and Bohm (Pines, 1956) began to use the term plasmon to describe the density fluctuations of the electrons in a dense electron gas. Then, Ritchie (1957) used the term surface plasmon for the first time in 1957, when explaining the interaction of the plasma oscillations with the surfaces of metal foils. Finally, Otto (1968) and Kretschmann and Raether (1968) achieved the excitation of surface waves with visible light using prism coupling in 1968, and thus a unified description of all these phenomena in the form of SPPs was established. At present, it is well known that SPPs are electromagnetic waves coupled to SPs propagating at planar metal-dielectric surfaces, which could lead to an enhanced optical near-field of subwavelength dimensions. For properly designed metallic nanostructures or small metallic particles, localized surface plasmon resonance (LSPR) can occur when the confined free electrons oscillate with the same frequency as the incident electromagnetic waves, resulting in efficient light scattering and concentration. On the other hand, the excitation of SPs by nanostructures also provides ways to convert the nonradiative SP modes into radiative light. Because the type of metal, size, shape, and dielectric environment of plasmonic nanostructures are easy to control, this suggests that plasmonic nanostructures are suitable for broadband light tailoring in various optoelectronics such as sensors, photodetectors, solar cells, and LEDs. This chapter starts with the fundamentals of SPs, either propagating at planar metallic surfaces (SPPs) or localized at metallic nanoparticles (LSPR), and introduces the optical techniques for SPP excitation, as well as the resonance conditions for LSPR. After a brief look at the fabrication and simulation methods of plasmonic nanostructures, various nanostructures to achieve plasmonic effects for the purpose of enhancing light-trapping or light emission in solar cells and LEDs are discussed. The chapter closes with a look at the prospects for the future development of plasmonic nanostructures based on the strategy of light manipulation.

13.2  Fundamentals of SP Effects 13.2.1  Surface Plasmon Polaritons The SP effects arise from the interaction processes between electromagnetic waves and free electrons at metallic interfaces or in small metallic nanostructures. To explain the fundamental interactions, the free electron gases in metals are often approximated to a plasma model over a wide range of frequencies. This model simply assumes that some aspects of the band structure are incorporated into the effective optical mass (m) of each electron. Considering

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   479 that the electrons oscillate in response to the applied electromagnetic field and are dampened by collisions, the optical response of metals can be described by the Drude model:

ε (ω ) = 1 −

ω p2 ω 2 + iγω

(13.1)

where ε(ω) is the permittivity of the metal; ω is the angular frequency; ωp2 = ne2/ε0m is the plasma frequency of the metal, which describes the natural frequency of a free oscillation of the electron sea; ωp is in the ultraviolet regime, which is of the order of 5–15 eV for most metals; and γ is the damping rate due to electron-electron and electron-phonon scattering. For simplicity, the complex dielectric function can be recast as ε(ω) = ε′(ω) + iε″(ω), in which ε′(ω) and ε″(ω) are the real and imaginary components, yielding:

ε ′ (ω ) = 1 −

ω p2 ω2 + γ 2

ε ′′ (ω ) =

ω p2γ

ω (ω 2 + γ 2 )

(13.2) (13.3)

This description is valid at low frequencies up into the visible light and near-infrared (NIR) regions that are most important for solar cell and LED applications. In particular, when ω = ωp and the damping is negligible, then ε′(ω) = 0. In this case, the dispersion of the free electron gas in the form of ω2 = ωp2 + K2c2 will result in K = 0, where K is the wave vector of the travelling plane wave. This means that the conducting electron gas is oscillating in a longitudinal mode. The quanta of these free electron oscillations are called volume plasmons. Now that the optical properties of the metals have been shown, we now consider the SP oscillations at a flat, semi-infinite interface between a metal and a dielectric, which are accompanied by an electromagnetic field. For the simplest, one-dimensional (1D) geometry, the interface is in the x-y plane, and it is assumed that the waves propagate along the x-direction of a Cartesian coordinate system (see Fig. 13.1). We simply assume these two media are linear, isotropic and nonmagnetic in the following, so that the dielectric profile

Fig. 13.1 SP oscillations at a flat, semi-infinite interface between a metal and a dielectric.

480  Chapter 13 ε = ε(r) over distance is negligible and the relative permeability will be μ = 1. The dielectric in z>0 half-space has a positive, real dielectric constant εd, and the metal in the z<0 half-space has a dielectric profile εm(ω). To describe the confined propagating waves between the dielectric and the metal, we need to solve Maxwell’s equations in the absence of external charge densities. By applying the harmonic, time-dependent, plane-wave electric field E(r, t) = E(r)e−iωt to Maxwell’s equations, we get the well-known Helmholtz equation: ∇ 2 E + k02ε E = 0,

(13.4)

where k0 = ω/c is the wave vector of the propagating wave in a vacuum. The detailed derivation process can be found in most textbooks on electromagnetics. Because the wave propagation is confined to the interface and along the x-direction, the field equation can be rewritten as E ( x,y,z ) = Ae kz z e ikx x ( z < 0 )

(13.5a)

E ( x,y,z ) = Ae − kz z e ikx x ( z > 0 )

(13.5b)

where kz is a positive real number denoted as the component of the wave vector perpendicular to the interface in the two media, and the complex parameter kx is the wave vector in the direction of propagation, which is defined as the propagation constant. Inserting Eq. (13.5) into Eq. (13.4), we obtain a wave equation in the form of

(k ε + k 2 0

2 z

)

− k x2 E = 0.

(13.6)

It should be noted that the magnetic field H has a similar wave equation. Then, by combining these two wave functions with the following Maxwell’s equations ∂H ∂t ∂E , ∇ × H = −ε 0ε ∂t ∇ × E = − µ0

(13.7) (13.8)

explicit expressions of E and H for both transverse electric modes (TE modes) and transverse magnetic modes (TM modes) in each half-space are calculated to be  kz ,m H y ,m = iωε 0ε m E x ,m  ( z < 0)  ik x H y ,m = −iωε 0ε m Ez ,m  kz ,m E x ,m − ik x Ez ,m = iωµ0 H y ,m  − kz ,d H y ,d = iωε 0ε d E x ,d  ( z > 0)  ik x H y ,d = −iωε 0ε d Ez ,d  − kz ,d E x ,d − ik x Ez ,d = iωµ0 H y ,d

(13.9)

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   481 for TM modes and  kz ,m E y ,m = −iωµ0 H x ,m  ik x E y ,m = iωµ0 H z ,m   kz ,m H x ,m − ik x H z ,m = −iωε 0ε m E y ,m  − kz ,d E y ,d = −iωµ0 H x ,d  ik x E y ,d = iωµ0 H z ,d   −kz ,d H x ,d − ik x H z ,d = −iωε 0ε d E y ,d

( z < 0) (13.10)

( z > 0)

for TE modes. In these expressions, ε0 and μ0 are the electric permittivity and magnetic permeability of vacuum, and the footnotes m and d represent metal and dielectric, respectively. Next, we look at the boundary conditions for TM modes in which continuity at the interface requires Ex, m = Ex, d and Hy, m = Hy, d. These yield k εm = − z ,m . k z ,d εd

(13.11)

Eq. (13.11) means that SPs exist only at interfaces where the real part of their dielectric permitivities have opposite signs. Given that in the visible light and NIR regions ω < ωp, we can see from Eq. (13.2) that the real part of the metal permittivity fulfills ε′(ω) < 0 where a metal is strongly absorbing. Then SPs demand that the dielectric medium should have a positive dielectric constant. On the other hand, the boundary conditions Ey, m = Ey, d and Hx, m = Hx, d for TE modes arrive at E y ( kz ,m + kz ,d ) = 0.

(13.12)

While the confined surface waves satisfy kz, m > 0 and kz, d > 0, meaning that Ey has to be 0. This suggests that the TE modes of SPPs do not exist—that is, SPPs can only have TM polarization. Most important, the dispersion relation of SPPs propagating at the interface between the two half-spaces can be calculated from Eqs. (13.6) and (13.11) as follows: kspp = k x =

ω ε mε d . c εm + εd

(13.13)

This complex SP wave vector also takes the form kspp = kspp′ + ikspp″, where the imaginary part kspp″ denotes the magnitude of the loss along the direction of propagation. The gradually attenuated SPPs on a flat metal surface arise from absorption in the metal, and its propagation length is thus described by 3

δ spp

c  ε ′ + ε d  2 ( ε ′m ) 1 = =  m .  2 k "spp ω  ε ′m ε d  ε "m 2

(13.14)

482  Chapter 13 Its value is typically between 10 and 100 μm in the visible regime. The silver/air interface, for example, which has the lowest loss in metals, has a propagation length of 20 μm at 500 nm. For a relatively absorbing metal such as aluminum, the propagation length is 2 μm at the same wavelength (Barnes et al., 2003). In addition, the SPP fields in the dielectric fall off as e− ∣ kz‖z∣, such that the evanescent decay length of the propagating waves perpendicular to the interface is defined by δ = 1/|kz| and represents the distance at which the intensity of the SPP electromagnetic field decreases by a factor of 1/e. Note that kz = k x2 − k02ε , and therefore the decay lengths in the metal and dielectric can be expressed separately as  c εm + εd δ m = Re  ω ε d2 

  

(13.15)

 c εm + εd δ d = Re  ω ε m2 

  

(13.16)

The decay length of the dielectric material is usually significantly larger than that of the metal. As shown in Fig. 13.2, for the most commonly used metals (Ag and Al), the decay lengths of the dielectric materials are on the order of half the wavelength of the light, while the decay lengths of the metals are between one and two orders of magnitude less than the light’s wavelength (Barnes et al., 2003). Finally, a clear picture for the physical significance of SPPs can be drawn. The SPPs are bound electromagnetic surface waves that propagate at the interface between a metal and a dielectric with maximum field intensity at the interface and decay exponentially into both adjacent media. In particular, for an ideal metal without damping effects, its dielectric function reduces to ε(ω) = 1 − ωp2/ω2. Inserting this expression into Eq. (13.12), the SP frequency can be estimated as

ωsp =

ωp 1+ εd

.

(13.17)

Fig. 13.2 SP length scales. The combinations chosen give an indication of the range from poor (Al at 0.5 mm) to good (Ag at 1.5 mm) SP performance. Reprinted with permission from Barnes, W.L., Dereux, A., Ebbesen, T.W., 2003. Surface plasmon subwavelength optics. Nature 424, 824–30. Copyright 2003, Nature Publishing Group.

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   483 At this limiting frequency of ωsp, εm″(ω) = 0, εm + εd = 0, and kx → ∞, and so the SPPs become nonpropagating, quasi-static electromagnetic waves, which are known as SPs. Fig. 13.3 shows the typical dispersion relation (momentum kx and energy ω) of SPPs for a lossless Drude metal/dielectric interface. The light-line for the incident photon in air, ω = ck, is shown by dashed lines. The region located on the left side of the light-line allows radiative light to propagate, while the right side is the nonradiative region. If the light is incident on the dielectric contact with the metal surface, the light-line will rotate to the right of the lightline in air according to ω = ck / ε d , as indicated by the straight solid line. In the small wave vector region corresponding to low frequency, the SPP dispersion curve approaches the lightline within the dielectric. With increasing wave vectors, the SPP dispersion curve will hit an asymptotic limit where the frequency approaches ωsp. An important thing to notice is that the dispersion curve is lying to the right of the light-line, which means that the SPP wavelength is shorter than the wavelength of a propagating electromagnetic wave at the same frequency. This phase mismatch means that the SPs at the planar metal/dielectric interface cannot be excited by an electromagnetic wave that propagates in the dielectric medium.

13.2.2  SPP Excitation As discussed previously, the wave vector of a nonradiative SP at the planar metal/dielectric interface is always larger than that of a photon of the same frequency propagating within the dielectric, such that SPPs are unable to be excited directly by light from the dielectric side due to the momentum mismatch. To overcome this difficulty, an additional wave vector in the plane of the surface must be provided to the incident light, which requires special coupling techniques. Since the discovery of SPP excitation by fast electrons from the measurement

Fig. 13.3 Typical dispersion relation (momentum kx and energy ω) of SPPs for a lossless Drude metal/ dielectric interface.

484  Chapter 13 of electron energy loss spectra, several optical excitation techniques have been developed, including prism coupling, diffraction gratings, highly focused beams, and near-field illumination with subwavelength apertures. The prism-coupling techniques are achieved inherently by attenuated total reflection, which makes use of evanescent waves tunneling to the metal surface. Two geometries for prism coupling were proposed in the 20th century, which are the Kretschmann configuration and the Otto configuration. In the Kretschmann configuration, a thin metal film is deposited on top of a high-index prism, and attenuated total internal reflection will take place when light is incident through the prism at appropriate angles. The evanescent waves with larger wave vectors tunnel through the metal film, thus exciting the SPPs at the metal/dielectric interface. Note that the tunneling requires that the metal film cannot be thick, so the metal film thickness must be strictly adjusted. The Otto configuration is analogous to the Kretschmann configuration, but a low-index dielectric gap is located between the prism and the metal surface. Here, the attenuated total reflection happens at the prism/dielectric interface and tunneling through the dielectric gap to excite SPPs. In this case, the SPP excitation strongly depends on the thickness of the dielectric spacer, and so this configuration also has limitations for practical use. Other ways to excite SPPs are using highly focused beams and near-field optical microscopy techniques; the former is a modification to the Kretschmann configuration, and the latter is an efficient method for local excitation of SPPs. However, the optical excitation schemes described here are not suitable for plasmonic applications in optoelectronic devices because of their difficulty in miniaturization, integration, and high-bandwidth coupling. A possible SPP excitation scheme for optoelectronic devices is diffraction gratings, which can be introduced to devices simply by patterning them with periodic grooves or holes on their metal surfaces. Here, instead of discussing the other SPP excitation schemes in detail, we would like to give an emphasis to the grating technique. To develop a general understanding of SPP excitation by grating diffraction, we investigate the simplest situation, where a monochromatic light shines onto the metal/dielectric interface from the dielectric side at an angle θ to the surface normal. The light is a plane wave propagating in the x-z plane, and the component of the wave vector of the incident light parallel to the interface is ω kin, x = kin sinθ = ε d sin θ (13.18) c Of course, as depicted in Fig. 13.4A, on a planar surface, the light-line of the incident light ω = ckin / ε d lies on the left side of the SPP dispersion curve. The in-plane component of the incident waves given by ω = ckin / ε d sin θ thus exhibits a smaller wave vector at the same frequency, implying that it is not sufficient to excite SPPs at the interface, even at the grazing

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   485

Fig. 13.4 (A) SPP excitation by grating diffraction when a monochromatic light shines onto the metal/ dielectric interface from the dielectric side at an angle θ to the surface normal. (B) Coupling and decoupling of a SPP wave using a 1D periodic grating: An extra momentum ∆k to increase the incident light momentum kx is needed for light coupling to SPPs, while light decoupling from SPPs require losing an extra momentum ∆k.

incidence. Although the SPP excitation is forbidden at the planar interface, a metal surface with corrugated grating can facilitate the coupling by providing an additional wave vector. Fig. 13.4B shows the light interactions at the metal/dielectric interface with 1D periodic grating. Integer multiples of the grating vector can be added or subtracted to the wave vector of the photon due to the grating’s diffraction effects, the grating’s unit in-plane vector orthogonal to the grooves is 2π K= , (13.19) Λ where Λ is the grating pitch. These orders of diffracted light will conserve momentum and excite SPPs when kspp = k x ,in + ∆k ∆k = mK = m

2π , Λ

(13.20) (13.21)

where m is an arbitrary integer and represents the mth order of diffracted light. Taking these details into account, Eq. (13.20) can be rewritten as

ω ε mε d ω 2π = ε d sinθ + m . c εm + εd c Λ

(13.22)

486  Chapter 13 This equation for wave vector matching reveals that the SPP excitation is decided by the frequency or wavelength of the light in the dielectric, the angle of the incident light, the dielectric environment at the metal/dielectric interface, and the grating pitch. Apart from SPP excitation, SPPs propagating along a grating also can reduce their wave vector by ∆k dropping back to the light cone and emitting as radiative light (as shown in the vector diagram of the decoupling process in Fig. 13.4B). This mutual reverse process allows grating to be an efficient technique for trapping or extracting light in organic solar cells (OSCs) and LEDs. In fact, Eq. (13.20) should take the form of vector superposition, as SPPs can propagate in a number of directions along the surface. Simply consider the SPPs propagating in both the forward and reverse directions along the x-axis, the SPP dispersion relations will have both positive and negative wave vectors. As can be seen in Fig. 13.5, the SPP dispersion relation is periodically distributed in various orders of Brillouin zones, where the interval between mth and m−1th orders is the unit grating vector. Clearly, many branches of the dispersion curves are folded into the light cone, transforming into light. The intersection points of the curves with the light-lines fulfill the SPP excitation condition, demonstrating that SPPs are excited by the incident light. It should be noted that when the 1D grating grooves are relatively deep, localized modes inside the grooves will lead to photonic band gaps at the Brillouin zone boundaries. For two-dimensional (2D) gratings, SPP propagation in all in-plane directions can be blocked, leading to a full band gap for SPPs. At the band edges, the density of SPP states is high, and there is a significant increase in the associated field enhancement (Barnes et al., 2003). The metal surfaces of actual devices are generally not smooth enough, but SPP excitation on rough surfaces can be achieved without additional alteration. The momentum conservation

Fig. 13.5 Dispersion of SPPs and photons propagating in both the forward and reverse directions along the grating interface. The intersections between the modes are the efficient excitation of shortwavelength SPPs.

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   487 is fulfilled by the scattering or diffraction of the light via random roughness. The excited SPPs are strongly scattered into different directions and move with increasing disorder. It can also decouple into photons emitting into the air space via roughness, leading to SPP loss. Moreover, scatterers including periodic or quasi-periodic nanoholes, nanowires, nanocones, and nanodomes are widely used to excite SPPs because they can be regarded as the superposition of gratings.

13.2.3  Localized SP Resonance We now turn to the localized SPs in metal nanostructures. In contrast to SPPs, localized SPs are nonpropagating oscillations of the free electrons in confined nanostructures with geometries that are smaller than the wavelength of the incident light. The study of localized SPs can be traced to the early 20th century, when Gustav Mie established a general electromagnetic theory for plane-wave scattering and absorption by a spherical particle of arbitrary size (Mie, 1908). Mie theory provides analytical solutions by solving Maxwell’s equations using a multipolar expansion where the electromagnetic fields are expanded in multipole contributions. However, the higher-order terms are not required when the particle is sufficiently small; thus, for simplicity, we consider only the first dipolar term. In this term, the radius of the metal sphere is much smaller than the wavelength of light (R ≪ λ), so the interaction of the particle with light can be treated using quasi-static approximation. Here, we assume that the phase of the harmonically oscillating electromagnetic field is practically constant over the particle volume and the electric field E is parallel to the z-direction. The isotropic metal sphere with dielectric function εm(ω) is fixed at the origin in a dielectric medium whose dielectric constant is εd (Fig. 13.6A). To find the dipole moment (p) of the sphere under the applied electric field, we first need to solve the Laplace equation: ∇ 2 Φ = 0.

(13.23)

Then, the electric field will be able to be calculated by E =  − 𝛻Φ. Details about the calculation process, with the boundary condition that both the tangential component of the electric field and the normal component of the displacement field are continuous have been utilized, may be found in most electromagnetic textbooks. Thereby, the potentials inside and outside the sphere can be calculated as Φ in = −

Φ out = − Er cosθ +

3ε d Er cosθ ε m + 2ε d

εm − εd cosθ p·r ER 3 2 = − Er cosθ + ε m + 2ε d r 4πε 0ε d r 3

(13.24)

(13.25)

488  Chapter 13

Fig. 13.6 (A) Sketch of a homogeneous nanosphere placed in an electrostatic field. (B) Schematic of LSPR. Coherent oscillations in the free electrons of the material are induced by the oscillating electric field of the incident light.

where θ is the angle between the position vector r at point P and the z-axis, and the dipole ε − εd moment p is introduced as p = 4πε 0ε d R 3 m E . This expression is usually written as ε m + 2ε d p = ε0εdαE so that a complex number called polarizability is obtained:

α = 4π R 3

εm − εd ε m + 2ε d

(13.26)

It is obvious from this equation that when Re ( ε m ) = −2ε d

(13.27)

polarizability is at a maximum. In this case, the dipolar LSPR of the metal nanoparticle will occur, resulting in a dramatically enhanced electric field outside the metal particle. For a lossless Drude metal particle (Im(εm) = 0), the corresponding SP resonance frequency is estimated as follows: ωp ωlsp = (13.28) 1 + 2ε d At this point, it is easy to understand that the electric field of the incident light will lead to polarization charges on the particles’ surface, which in turn exert a restoring force. For a plane-wave time-varying field with E(r, t) = E(r)e−iωt, the oscillating electromagnetic field drives the conducting electrons to oscillate collectively (see Fig. 13.6B). When the light arrives at the particular frequency ωlsp, this oscillation will be in resonance with the incident light, leading to remarkable field amplification outside the metal particle in the near-field.

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   489 In the far-field, the subwavelength metal nanoparticles act as light absorbers and scattering sites in optoelectronic devices. Especially when the LSPR takes place, the absorption and scattering efficiency simultaneously see a considerable enhancement. The effectiveness of an individual nanoparticle to absorb or scatter light can be quantified in terms of absorption and scattering cross sections (Brongersma et al., 2014). In the quasi-static limit, the cross sections for absorption (σabs) and scattering (σsca) determined by the Poynting-vector S = E×H are as follows:

σ abs =

Wabs = kIm [α ] ∝ R 3 S

(13.29)

σ sca =

Wsca k 4 2 α ∝ R6 = 6π S

(13.30)

where Wabs and Wsca are the power absorbed or scattered, respectively; and |S| is the power flow per unit area for the incident plane-wave. The scattering in the quasi-static limit is often referred to as Rayleigh scattering. Certainly, the guiding criterion for the light management of solar cell and LEDs is to keep metallic losses low, which requires σabs ≪ σsca. The effective area from which light is captured and subsequently absorbed or rescattered is analogously defined by an extinction cross-section: W σ ext = ext = σ sca + σ abs , (13.31) S where Wext is the total power removed from the incident light wave by both scattering and absorption. As shown in these equations, both absorption and scattering are enhanced significantly when the resonance condition is satisfied. The absorption cross section scale with the polarizability by a factor of R3, whereas the scattering cross section is increased more dramatically by a factor of R6. The near-field enhancement and far-field scattering magnification properties allow the metal particles (Fig. 13.7) to realize many outstanding applications in various optical devices. Here, it is noteworthy to point out that all equations in this section are also valid for dielectric particles. The only difference lies in εm and the real part of dielectric particle permittivity is larger than zero (Re[εd] > 0); thus, they do not exhibit resonant behavior (Ferry et al., 2010). A critical consideration for light management using LSPR is to establish the effects of various influence factors, including the dielectric environment, the particle size, shape, and material, as well as their ensemble behaviors. While SPP excitation requires that both the frequency and wave vector of the excitation light match the SPP frequency and wave vector, LSPR only needs the incident light with the appropriate frequency due to the curved surface of the metal nanoparticles. In other words, the localized SPs in nanoparticles can be directly excited by light illumination, regardless of the photon’s wave vector. But the excitation of an LSPR is

490  Chapter 13

Fig. 13.7 LSPR-induced near-field enhancement and far-field scattering magnification properties of a particle.

highly sensitive to the dielectric environment because the resonance frequency depends on the dielectric constant of the local medium, as shown in Eq. (13.28). When εd increases, the resonance position shows a red shift. As the size of the nanoparticle approaches λ, the Drude model in quasi-static approximation is not accurate anymore because interband transition and retardation effects will be invoked (Zayats and Smolyaninov, 2003). In this case, the plasmon resonance frequency moves toward lower energies due to the larger charge distance-induced and smaller restoring force. Size variation also plays an important role in light absorption and scattering. For sufficiently small nanoparticles, absorption is the more dominant phenomenon, whereas for large particles that are comparable to or larger than λ, the scattering becomes significant. But it must be mentioned that multipolar modes (quadrupole, octopole, hexadecapole, etc.) excited beyond certain limits will decrease the resonance line widths and scattering cross sections. The shapes of the plasmonic nanoparticles also determine the resonant frequencies of the localized SP modes that give rise to wide-range tunability of the optical response. This is because the nanoparticle’s shape determines the charge accumulation and electron density at various positions on the surface, which further determines the restoring force. A wide variety of nanostructure shapes, such as spheroids, nanorods, nanovoids, nanoprisms, nanoshells, triangles, and cubes, have been explored for light manipulation in solar cells and LEDs. For example, a spheroid or nanorod (Fig. 13.8A) will support two separated plasmon resonance modes that correspond to longitudinal modes and transverse modes along their major and minor axes. High aspect ratios bring about the concentration of fields at sharp points, thus lowering the restoring forces and resulting in significant red shifts in the resonance frequencies. Fig. 13.8B demonstrates the SP absorption spectra of gold (Au) nanorods with various aspect ratios, showing clear red-shifts in the plasmon absorption maximum with increasing aspect ratios of the nanorods (Huang et al., 2006). This suggests that plasmon resonances can be widely tuned by changing the aspect ratios of the nanorods

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   491 ++

+ +

a

– b –

– – –

–

+

–

+

–

+

–

+ – –

b

Aspect ratio = a/b

0.8 Optical density

+ + +

2.4 3.1 3.9 4.8 5.6

1.0

a

0.6 0.4 0.2 0.0 –0.2 300

(B)

(A) Antisymmetric – + – + – + – + + – – + – + – +

400

500

600 700 800 900 1000 1100 1200 Wavelength (nm)

15 nm 10 nm 7 nm

5 nm

Extinction

20 nm

Cavity 400

Sphere

(C)

–

+

600

800

1000

Wavelength (nm)

– + – + – – + + – + + – – + Symmetric

Decreasing shell thickness

(D) Fig. 13.8 (A) Schematic of a spheroid and a nanorod supporting two separate plasmon resonance modes. (B) The SP absorption spectra of Au nanorods with different aspect ratios. Reprinted with permission from Huang et al. (2006). Copyright 2006, American Chemical Society. (C) Schematic of the plasmon hybridization in metal nanoshells resulting from the interaction between the sphere and cavity plasmons. The charge distribution forming symmetric and antisymmetric plasmon resonances is also shown. (D) Plasmon resonances of a 120-nm-diameter SiO2 core coated with varying thicknesses of Au shell. Note the blue shift in the plasmon resonance as the size of the Au layer increases from 5 to 20 nm. Reprinted with permission from Lal et al. (2007). Copyright 2007, Nature Publishing Group.

or the spheroidal nanoparticles. Nanoshells also support two hybridized plasmon modes. As presented in Fig. 13.8C, the plasmon resonance of a nanoshell can be seen as an interaction or a hybridization of the nanosphere and nanocavity plasmons. As a result, the plasmon resonance splits into a higher-energy antisymmetric plasmon and a lower-energy symmetric plasmon (Prodan et al., 2003).

492  Chapter 13 Experiments revealed that the strength of the interaction depends on the thickness of the metal shell layer; thus, the plasmon resonances are highly sensitive to the inner and outer radii of the metallic shell. Fig. 13.8D shows the plasmon extinction spectra of a 120-nmdiameter silica (SiO2) core coated in gold shells of varying thicknesses (Lal et al., 2007). A significant red shift in the plasmon resonance is observed as the thickness of the gold layer decreases from 20 to 5 nm. This illustrates that nanoshells can provide a wide variety of optical responses, spanning from the visible range to the NIR range. Moreover, it was found that when the corners of nanocubes are truncated so that they become polyhedral shapes or even spheres, the main resonance is always blue-shifted and the width of the main SPRs increased (Noguez, 2007). All these results indicate that tuning the size and shape of the nanostructures affords new levels of control over the plasmon resonances, and this wide tunability has been summarized in Fig. 13.9. Note that the shape of these nanostructures can affect the angular and polarization distributions of the scattered light, as well as the efficiency of the chargetransporting process, to a remarkable extent. Optimizing the shape of nanostructures to match a given optical frequency and have appropriate field and charge behavior is of great importance for their use in solar-cell and LED devices. In contrast to the nanoparticle size and shape, the material is a critical factor for the plasmonic effects. Noble metals such as Au, Ag, and Cu are of interest to solar-cell and LED devices due to their plasmon resonance at the visible and NIR regions of the electromagnetic spectrum (Lindquist et al., 2012). As has been depicted in Fig. 13.9, the plasmon resonance range for Ag nanospheres spans from the UV region to the visible region, and for Au nanospheres, the plasmon resonance range likewise lies in the visible region.

Fig. 13.9 Plasmon resonances for a variety of particle morphologies. Reprinted with permission from Lal, S., Link, S., Halas, N.J., 2007. Nano-optics from sensing to waveguiding. Nat. Photonics 1, 641–648. Copyright 2007, Nature Publishing Group.

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   493 Table 13.1: Comparison of the suitability of different metals for plasmonic applications Nanostructure Formation

Cost (per Ounce) $0.049

Easy oxidation

Very few nanostructures; used in lithographic patterning Very few nanostructures

Very stable

Many nanostructures

$950

Stable

Many nanostructures

$265

Stable

Many nanostructures

$1207

Oxidation

Many nanostructures

$13.40

Metal

Plasmonic Ability

Chemical

Al

Good in UV region

Stable after surface passivation

Cu

Interband transitions below 600 nm Interband transitions below 500 nm; high quality factor Low quality factor; not suitable for plasmonics Low quality factor; not suitable for plasmonics Highest in quality factor

Au

Palladium (Pd) Pt Ag

$14.80

Reprinted with permission from Rycenga, M., Cobley, C.M., Zeng, J., Li, W., Moran, C.H., Zhang, Q., Qin, D., Xia, Y., 2011. Controlling the synthesis and assembly of silver nanostructures for plasmonic applications. Chem. Rev. 111, 3669–712. Copyright 2011, American Chemical Society.

In fact, studies show that Ag is the metal with the lowest absorption and highest overall radiation efficiency (i.e., ratio of the scattering to the extinction), which makes it the most popular choice for plasmonic applications (Uddin and Yang, 2014). Al supports SP resonances in the UV and blue regions, and so it is a viable candidate for optoelectronics, but it also has problems regarding oxidation and large losses in the visible region. For comparison, the suitability of different metals for plasmonic applications is given in Table 13.1, where more noble metals such as Pb, Pt are compared in different aspects (Rycenga et al., 2011). Metal alloys have shown great potential for tuning the resonance of pure metals and enhancing their plasmonic performance. Finally, it is also noteworthy that particle ensembles can lead to shifts in the spectral position of the plasmon resonance compared to the case of an isolated particle. The interactions among neighboring nanoparticles could generate hot spots in the gap between the particles, leading to further levels of field enhancement.

13.3  Plasmonic Nanostructure Fabrication and Simulation Techniques 13.3.1  Nanostructure Fabrication Techniques Nanostructures offer effective light management at the subwavelength scale via plasmonic effects for optoelectronic devices, but appropriate nanofabrication techniques are required for nanopatterning of the structures. An ideal technique for fabricating nanostructures should be low-cost, high-speed, scalable, and controllable, but each of these has its advantages and disadvantages.

494  Chapter 13 One can prepare metal nanoparticles on a substrate simply by electron beam or thermal evaporation of a thin metal film, followed by an annealing process that takes place at a moderate temperature. The heating causes agglomeration by surface tension of the metal film into a random array of nanoparticles. This method is cheap and fast, but it lacks control. The nanoparticle size, aspect ratio, and density may be controlled in part by depositing the metal through a porous alumina template and the subsequent thermal annealing (Atwater and Polman, 2010). However, the evaporation of metal through the template gradually clogs the apertures, leading to severe defects of the patterns. More control of the patterning resolution relies on precise fabrication techniques. Over the past several decades, sophisticated equipment has been developed in the semiconductor industry for the nanofabrication of transistors. The nanoscale patterning achieved using photolithography and plasma etching enables transistors to be manufactured with sub-10-nm gate lengths. These well-developed manufacturing techniques also may be theoretically applied to the patterning of the plasmonic nanostructures. By using an optical source to expose the spin-coated photoresist film through a photomask, the chemical structure of the photoresist changes upon light exposure. This allows the developers to remove either the exposed areas (positive photoresist) or the unexposed areas (negative photoresist) in preparation for the introduction of the nanostructure. The plasmonic nanostructure then can be created precisely by plasma etching the unprotected areas. Electron-beam lithography (EBL) and focused ion-beam milling (FIB) techniques are similar to extreme-ultraviolet (EUV) photolithography, but they use a focused beam of electrons and ions, respectively, to form patterns on the substrate. This provides EBL and FIB techniques with more flexibility and superb patterning resolution compared to what can be achieved with photolithography. Specifically, EBL can directly overwrite a layer of electron-sensitive material (resist) without a mask to form the desired pattern, and FIB can even be used to deposit metals directly as patterned nanostructures through milling or decomposing suitable precursors. Features as fine as about 2 nm have been demonstrated in a hydrogen silsesquioxane resist using EBL (Manfrinato et al., 2013), and structures with a size of about 6 nm have been fabricated by FIB utilizing a 50-kV Ga+ two-lens system (Xia and Halas, 2005). However, in practice, enormous challenges exist for the processing of noble metals due to the materials’ properties and equipment issues (Lindquist et al., 2012). Metals such as Au and Ag are not amenable to direct plasma etching because their plasma reaction by-products are nonvolatile. Feasible methods to pattern these noble metals thus can only rely on other techniques, such as physical ion milling, lift-off, electroplating, or shadow evaporation, which unfortunately may lead to complex processes and rough edges. Photolithography is not effective at creating shapes on flexible substrates; hence, its future application in flexible electronics would be severely limited. The serial processes of EBL and FBI are time-consuming, and it is possible to fabricate structures only over small-area substrates with these techniques. Another primary drawback is that all three techniques have the same issue: high cost.

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   495 Research is continuing on improving conventional lithography techniques for the purpose of realizing scalable manufacturing of plasmonic nanostructures. For example, the multibeam EBL technique has been developed as a variation of EBL to afford high resolution while overcoming the low-throughput rate. Some emerging techniques, such as block-copolymer directed-self-assembly lithography (DSA) (Stoykovich et al., 2007; Park, 1997; Cheng et al., 2006) and nanosphere lithography (also known as colloidal lithography or natural lithography) (Colson et al., 2013; Jensen et al., 2000), are attempting to merge bottom-up with top-down processes. Such techniques utilize self-assembled films as photolithographic masks for further pattern transfer, providing not only the capability of high-throughput fabrication of nanoscale patterns, but also compatibility with existing manufacturing techniques. Long-range arrays of dots, rings, wires, and rods have been precisely fabricated by these two techniques on both rigid and flexible surfaces (Xi et al., 2015; Kosiorek et al., 2005). However, self-assembly has its own problems with regularity and repeatability, and the use of close-packed spheres severely restricts the range of lattices that may be produced (Campbell et al., 2000). Several novel patterning strategies that do not use photomasks are also being developed. Laser direct writing (LDW), a maskless stereolithography technique, shows enormous potential for complex nanostructure patterning. Besides directly creating patterns on a photoresist layer, LDW can structure a surface via precursor deposition, ablation, or thermal processes. Nevertheless, the resolution of conventional, single-photon LDW is mainly confined by the optical diffraction limit; thus, its achievable feature size is restricted to the range of a few hundred nanometers to a few micrometers. As such, LDW based on nonlinear light-matter interactions, which is known as two- or multi-photon absorption (TPA and MPA), was developed to overcome the resolution problem. The typically used light source is an NIR laser beam with an extremely large transient power density. This enables the laser to be focused on a very narrow and local region inside the focal plane, which can initiate the transparent photoresist polymerization. Sub-100-nm structures have been obtained by choosing special agents such as a radical quencher (Lee et al., 2008), but the relatively long writing time and the high price of the procedure restricted to the range of the delicate equipment greatly hinder its widespread use. Interestingly, SPs can also be used to fabricate high-resolution plasmonic nanostructures beyond the diffraction limit due to their near-field optical properties. It is reported that maskless plasmonic lithography at 22-nm resolution is possible through the coupling of propagating SPs and localized SPs (Pan et al., 2011). As most plasmonic structures are simple periodic or quasiperiodic structures that do not require multiple aligned exposures for fabrication, a convenient lithography technique is more favorable. Interference lithography (IL), also referred to as holographic lithography or optical interferometric lithography, offers a high-throughput, inexpensive nanopatterning technique to fabricate large-area plasmonic structures based on multibeam interference. The coherent

496  Chapter 13 optical beams incident from different directions will generate interference patterns such as periodic arrays of nanoparticles, apertures, and gratings, which are directly recorded in the photoresist film and then transferred to the substrate. The pattern shape could be manipulated by the phase retardation, polarization, and incident angle of each coherent optical beam. When combined with an immersion medium with a higher refractive index or a shorterwavelength light source, resolutions as low as tens of nanometers can be achieved for the nanostructures. For example, gratings with a 22-nm half-pitch have been fabricated by immersion lithography employing a 157-nm light source (Bloomstein et al., 2006). More promisingly, IL is a maskless nanofabrication technique capable of patterning 1D, 2D, and 3D plasmonic nanostructures via multiexposure or multibeam interference. When it is extended to dot-matrix holography where a diffraction grating (beam splitter) is used to split the beams, and then the split beams are redirected onto the substrate using a focusing lens, the location, size, orientation, and spacing of the dot-matrix nanostructures can achieve precise control (Wan et al., 2015). The aforementioned lithographic techniques more or less require the assistance of a complex projection system or a clean-room facility, while soft lithography and nanoimprint lithography (NIL) can simply be carried out in an ambient laboratory environment, and they are not subject to the limitations set by optical diffraction and optical transparency. Soft lithography encompasses a series of techniques, such as microcontact printing (μCP), microtransfer molding (μTM), micromolding in capillaries (MIMIC), replica molding (REM), and solventassisted micromolding (SAMIM), where in each case, an elastomeric stamp or mold (usually poly(dimethylsiloxane), PDMS, or perfluoropolyether, PFPE) with patterned relief structures on its surface is used to generate patterns and structures (Xia and Whitesides, 1998). In soft lithography, a key step is the creation of the elastomeric stamp by replica molding from a structured Si master. Contact-printed self-assembled monolayers (SAMs) of molecules or embossed polymer films by elastomeric stamp are commonly used as etch masks, followed by the use of etching, selective deposition, or lift-off to form nanostructures on the substrate. Apart from the delicate master fabrication by EBL, LDW, MPA, or a combination of such techniques, structures that are smaller than 100 nm are achievable without the need for advanced lithographic techniques. On the other hand, NIL uses a hard mold made of Si, nickel (Ni), or silica (SiO2) to replicate the patterns via mechanical deformation of a soft polymer resist. The resist could be thermoplastic or photocurable polymers, and thus would be able to be cured by either using heat (thermal NIL) or UV light (UV NIL). The hard imprint mold enables higher spatial resolution (<10 nm) for NIL, but it also requires a lot of pressure to achieve conformal contact with substrates. This will create long-range, nonuniform distortion over the large areas of contact (Li et al., 2009), and can often be contaminated from the liquid resist or air bubble entrapment between the mold and substrate. More recently, soft-mold NIL and UV-curable hybrid-mold NIL have been developed to combine the advantages of low-cost, large-area patterning of nanostructure and

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   497 simultaneously overcome the challenges faced by conventional NIL. It utilizes a soft mold and a UV-curable hybrid-mold to achieve defect control, while the molds replicated from the master help to reserve the pattern integrity of the master for repeated use. Furthermore, they have been used for large-area patterning, such as low-pressure, roller-based NIL (roll-to-roll, roll-to-plate NIL) techniques, which not only greatly improve the throughput, but also provide capability for flexible device nanopatterning. However, both soft lithography and NIL also have drawbacks. The elastomeric mold distortion, contamination, contact uniformity between pattern areas, pattern transfer repeatability, and other issues have to be managed before these two techniques can be established for mass fabrication for practical use. Fig. 13.10 compares the various micromanufacturing and nanomanufacturing techniques in terms of resolution, cost, and throughput, which are the three most important parameters for scalable structure fabrication. A negative correlation between resolution and cost can be observed in this graph (Fig. 13.10A), while the throughput generally increases with the decrease in the resolution (Fig. 13.10B) (Qiao et al., 2016). Besides the lithographic methods generating plasmonic nanostructures by top-down strategies, bottom-up methods based on the techniques of chemical synthesis may be applied as a simple and versatile approach to the preparation of nanostructures. Today, various metal nanoparticles can be chemically synthesized with well-controlled sizes and shapes. The synthetic approaches generally involve reduction processes to generate elemental metal from a metal salt precursor in the presence of a colloidal stabilizer. The reduced atomic metal will grow into small clusters and eventually form the desired nanoparticles under appropriate experimental parameters. For example, Ag and Au nanodots and nanospheres are usually synthesized from the AgNO3 and HAuCl4 precursors. For more complex shapes such as nanorods, nanowires, nanoshells, and prisms, extra experimental conditions like seeds, light, and templates have to be used in order to let the nanostructure grow in the preferential directions. Solutionprocessed metal nanostructures indeed provide an inexpensive approach to produce a variety of high-quality, 3D metal nanoparticles with high throughput. But precisely controlling the particles with homogeneous configurations in solution is still challenging. The accidental production of a given nanoparticle shape must be overcome before many of the most exciting applications of these techniques can be realized, and this will require quantitative assessments of the assembled structures and understanding of kinetic factors, phase diagrams, and thermodynamic parameters.

13.3.2  Simulation Techniques Manipulating light at the nanoscale by plasmonic structures requires not only advanced fabrication methods, but also highly developed simulation techniques. To determine the plasmonic properties of the fabricated nanostructures, numerical approaches are needed to provide analytical solutions for Maxwell’s equations. The first numerical approaches for

498  Chapter 13

Fig. 13.10 Capital equipment cost versus resolution (A) and throughput versus resolution (B) of micro- and nanomanufacturing methods. Multibeam EBL: multiple-beamlets electron-beam lithography; NIL, nanoimprint lithography; IL, interference lithography; LDW, laser direct writing; MPA lithography, multiphoton absorption lithography; FIB, focused-ion-beam lithography; DSA, directed-selfassembly lithography. Reprinted with permission from Qiao, W., Huang, W.B., Liu, Y.H., Li, X.M., Chen, L.S., Tang, J.X., 2016. Toward scalable flexible nanomanufacturing for photonic structures and devices. Adv. Mater. 28, 10353–10380. Copyright 2016, Wiley-VCH.

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   499 plasmonic simulation were perhaps established by Mie, who attempted to explain the color of colloidal metal by computing the scattering of an incident plane wave. In Mie’s theory, however, it is impossible to estimate a substrate interaction, and Maxwell’s equations can be solved for only simple geometries such as spherical and ellipsoidal particles and concentric and conformal nanoshells (Parsons et al., 2010). Although obtaining an exact solution of Maxwell’s equations for arbitrary geometries is usually very difficult, a number of approximation computational approaches have been developed to solve this issue. The most commonly used methods that apply to particles of arbitrary shapes are the boundary element method (BEM), the multiple multipole (MMP) method, the finite integration technique (FIT), the finite element method (FEM), the discrete dipole approximation (DDA), and the finite-difference time domain (FDTD) method. The discussion given here does not attempt to give a comprehensive explanation of all these methods. Instead, it focuses on the methods that are most widely used for light simulations in LED and solar cell devices: namely, DDA, FEM, and FDTD. Discrete dipole approximation (DDA). This method solves Maxwell’s equations in the frequency domain by discretization of the volume-integral. Because dielectric materials are composed of polarizable atoms, the full nanostructure then can be considered as a finite array of polarizable point dipoles with polarizability α. Each dipole is polarized by the incident driving fields (Einc, i), as well as the fields of all the other dipoles. The local field at dipole i can be expressed as Elocal ,i (ω ) = Einc ,i (ω ) + ∑Gij (ω )α j (ω ) Einc , j (ω ) ,

(13.32)

j ≠i

where Gij is the free-space Green’s tensor, which propagates a field from j to i. Therefore, the calculation of scattering and absorption properties of an arbitrarily shaped nanostructure is approximately equivalent to computing the electric field associated with a grid of discrete, mutually interacting dipoles. The local fields of all dipoles will be written into a matrix equation and solved, and from these results, the electric field at any point outside the dipole array, as well as the absorption and scattering cross sections, can be obtained. An opensource, Fortran-based code called DDSCAT is now used to compute the optical response of both metallic and dielectric nanostructures. Finite element method (FEM). FEM is another available frequency-domain numerical method to simulate the plasmonic properties in optoelectronics. It is essentially a discretization strategy for solving partial differential equations. For plasmonic nanostructures, the field distribution is calculated by discretizing the Helmholtz equation in space and then solving numerically to find fields that satisfy the boundary conditions. The computational domain is discretized to finite-element tetrahedral meshes wherein the fields inside each grid mesh are described by simple equations that locally approximate the original complex equations. The coupling between the elements is now done in a straightforward way by applying the

500  Chapter 13 boundary conditions to the face nodes between neighboring elements. In practice, an accurate numerical solution can be obtained by comparing the results from a sequence of successively refined meshes, and a commercial FEM modeling software known as COMSOL has been widely used in the field of plasmonics. Finite-difference time domain (FDTD) method. Unlike the DDA and the FEM, the FDTD is a time-domain numerical method that directly solves Maxwell’s equations. A most commonly used scheme is proposed by Yee, in which Maxwell’s curl equations are discretized within staggered Cartesian grids in both time and space using central-difference approximations. The time discretization is performed with a time step ∆t in a leapfrog manner, and the space discretization uses a Cartesian volume element of the sides ∆x, ∆y, and ∆z. The FDTD algorithm employs an explicit time-marching method to solve the electric-field vector components in the grid at a given instant in time, and then the magnetic-field vector components in the same grid are solved the next instant. This process continues iteratively until the desired transient or steady-state electromagnetic field behavior is fully evolved. Both the near-field and far-field solutions can be computed simply from the FDTD method. Even though the staircasing effects of FDTD implementations may lead to inaccuracy for the analysis of metallic nanostructures, it has become the most popular and widely used technique for simulating arbitrary plasmonic nanostructures due to its simplicity. More than 40 software packages, such as Lumerical FDTD Solutions and Remcom XFdtd, are available nowadays to implement this method.

13.4  Various Nanostructures for Plasmonic Solar Cells and LEDs 13.4.1  Plasmonic Solar Cells Conventional PV absorbers, such as wafer-based crystalline Si (c-Si), must be optically thick to allow sufficient light absorption and photocarrier collection. The thicknesses of the c-Si active layers are typically in the range of 150–300 μm. As such, a direct consequence is high material usage, which thus leads to very high fabrication costs. For example, statistics show that around half the cost of c-Si modules is from the silicon wafers. The price of solar-generated electrical power so far is still significantly higher than that of generated by conventional energy sources. Moreover, minority-carrier diffusion lengths of effective solar devices require the thickness of the absorber layer must be thin enough for all photocarriers to be collected before recombining. To reduce the production cost of modules and minimize the probability for unwanted intrinsic electron-hole pair recombination, thin-film cells from a variety of semiconductor materials, including hydrogenated amorphous Si (a-Si:H), GaAs, CdTe, CuInSe2, and organic semiconductors, as well as mesoporous materials have been developed. As the thickness of the PV absorber is reduced, however, light absorption would inevitably decrease near the band gap of the semiconductor, resulting in insufficient carrier generation

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   501 and collection. The mismatch between the necessary thickness for complete optical absorption and the requisite electronic quality for long minority-carrier diffusion lengths constitutes a serious challenge for the construction of effective thin-film solar cells. This implies that one of the most crucial factors determining the efficiency of thin-film cells is light-trapping. Effective light-trapping schemes are thus needed to maximize light absorption across the solar spectrum. Traditionally, macroscopic pyramidal surface textures are commonly applied on thick c-Si cells to scatter light into large angles, thereby enhancing the effective path length in the cell and minimizing the reflection losses. But such geometries are not feasible for thin-film cells because the active layer thickness in an ultrathin inorganic cell (1–2 μm), an organic film (a few hundred nanometers), or a mesoporous film (about 300 nm) is comparable to or smaller than the macroscopic textures. Plasmonic metal nanostructures have been proposed as advanced methods to confine and guide incident sunlight into wavelength-scale or subwavelength-thickness absorber layer volumes. Due to their insensitivity to the angle of incidence and large scattering cross sections under plasmon resonance conditions, the incident light can be concentrated into the active layer effectively. Both localized SPs supported by metal nanoparticles and SPPs propagating at the metal/ semiconductor interface are viable to achieve light-trapping in thin-film solar cells. The plasmonic mechanisms for enhancing absorption in solar cells are associated with three light-trapping geometries for thin-film solar cells (Atwater and Polman, 2010), as schematically shown in Fig. 13.11. In the first geometry, random or periodic arrays of metal nanoparticles placed at the front surface of the solar cell can be engineered to have large

Fig. 13.11 Plasmonic light-trapping geometries for thin-film solar cells. (A) Light-trapping by scattering from metal nanoparticles at the surface of the solar cell. (B) Light-trapping by the excitation of localized SPs in metal nanoparticles embedded in the semiconductor. The excited particles’ near-field causes the creation of electron-hole pairs in the semiconductor. (C) Light-trapping by the excitation of SPPs at the metal/semiconductor interface. A corrugated metal back surface couples light to SPP or photonic modes that propagate in the plane of the semiconductor layer. Reprinted with permission from Atwater, H.A., Polman, A., 2010. Plasmonics for improved photovoltaic devices. Nat. Mater., 9, 205–13. Copyright 2010, Nature Publishing Group.

502  Chapter 13 scattering cross sections at the localized SP resonance wavelength. They scatter the incident light with forward anisotropy into the dielectric which has larger permittivity. The light scattered into a distribution of angles will acquire an increased optical path length, trapping the propagating light within the absorbing layer (Fig. 13.11A). Note that small particles suffer from significant ohmic losses, it is advantageous to use larger particles to increase the scattering rate. In the second geometry, metallic nanoparticles are incorporated into the active layer of the cell. It takes advantage of the localized SP resonance of the particle in which high near-field intensity is localized at the particle surface and coupled to the surrounding semiconductor. The nanoparticles act as subwavelength antennas for the incident light that store and couple energy to the absorbing layer, effectively increasing the optical absorption of the device. In this case, small particles with 5–20-nm diameter work especially well because of their low albedo (Fig. 13.11B). Third, a corrugated metal back contact is used to couple sunlight into SPP modes supported at the metal/semiconductor interface and guided modes in the semiconductor slab. The evanescent electromagnetic SPP fields are trapped and guided horizontally in the semiconductor layer near the plasmon resonance frequency, exciting electron-hole pairs and enhancing the device efficiency (Fig. 13.11C). Various plasmonic nanostructures have been explored extensively for inorganic solar cells. Properly engineered nanostructures are generally embedded at the front or rear of an inorganic solar cell to achieve efficient light-trapping. One of the most widely used scheme is placing metal nanoparticles on the top surface of solar cells. For example, Fig. 13.12A illustrates a thin-film a-Si:H solar cell with an array of Ag nanoparticles deposited on the top surface (Akimov et al., 2009). Ag and Au nanoparticles with diameters in the range of 50–350 nm are commonly used because of their large scattering cross section and potential for low absorption in the visible and NIR spectrum. The metal nanoparticles can be deposited by island annealing, colloidal spin coating, and porous anodized Al oxide templates assisted evaporation. In this case, nanoparticle array parameters such as size and coverage should be carefully optimized so that parasitic absorption in the particle could be minimized, and the light could be scattered into the cell. An optically thin GaAs solar cell (Fig. 13.12B) decorated with size-controlled (110 nm) Ag hemispheres has been observed to have an 8% increase in the short-circuit current density (Nakayama et al., 2008). This development is attributed to the interacting SPs in densely formed, high-aspect-ratio nanoparticles, which can effectively increase the optical path of the incident light. For bare metal nanoparticles, however, the plasmonic enhancement effect for increasing the conversion efficiency of PV devices may still be limited because of their poor tunability. Plasmonic metal-dielectric core-shell structure is thus developed to offer better plasmonic performance. As shown in Fig. 13.12C, a

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   503

Fig. 13.12 (A) Sketch of a thin-film a-Si:H solar cell structure with an array of Ag nanoparticles on top of an ITO layer. Reprinted with permission from Akimov et al. (2009) Copyright 2009, Optical Society of America. (B) An optically thin GaAs solar cell decorated with size-controlled (110 nm) Ag hemispheres. Reprinted with permission from Nakayama et al. (2008). Copyright 2008, American Institute of Physics. (C) The simulation result of enhancement ratio R with and without Au-citrate core-shell nanoparticles as a function of wavelength. The inset shows the schematic diagram of the simulation geometry. Reprinted with permission from Qu et al. (2011). Copyright 2011, American Institute of Physics. (D) Schematic of a proposed plasmonenhanced cell structure consisting of a periodic array of Ag strips on a SiO2-coated thin Si film supported by a SiO2 substrate. Reprinted with permission from Pala et al. (2009). Copyright 2009, Wiley-VCH. Schematics of the plasmonic solar cell using 2D periodic Ag nanodiscs (E) and Al cuboids (F) on top of the cells. Reprinted with permission from Rockstuhl and Lederer (2009). Copyright 2008, American Institute of Physics. Reprinted with permission from Fan et al. (2013). Copyright 2013, American Physical Society.

504  Chapter 13 metal-dielectric core-shell nanoparticle is located on the Si layer, where the diameter of the gold core D is variable and the thickness of the citrate shell is fixed at 10 nm. Simulation results demonstrate an obvious enhancement ratio (R) in the longer wavelength region and extensive tunability (Qu et al., 2011). But SP excitations have a drawback that useful absorption enhancements can be realized only at certain resonant scattering wavelengths due to the plasmonic resonance mechanisms. To optimize the net overall absorption of a thin-film solar cell over the entire solar spectrum, Brongersma et al. proposed a model system consisting of a periodic array of Ag strips on an SiO2-coated thin Si film supported by an SiO2 substrate (see Fig. 13.12D). By simultaneously taking advantage of high near-fields surrounding the nanostructures and coupling to waveguide modes, up to 43% enhancements in photocurrent is obtained for a thin (50-nm) Si film solar cell with a period of 295-nm Ag stripes (Pala et al., 2009). Here, 2D gratings are preferred to one-dimensional ones because 1D gratings can achieve ultrabroadband transmission only for transverse-magnetic (TM) polarization, while transverse-electric (TE) polarization is almost completely reflected. For this reason, 2D periodic metallic nanostructures such as nanodiscs (Rockstuhl and Lederer, 2009) (Fig. 13.12E) and cuboids (Fan et al., 2013) (Fig. 13.12F) have been used on top of the active layers of inorganic solar cells to achieve very efficient polarization-insensitive light-trapping. Plasmonic nanostructures on the front of the cell may lead to optical losses at wavelengths below the SP resonance; this occurrence can be suppressed by placing them on the rear side, so that they interact only with the long-wavelength photons, which are not absorbed during the first pass through the cell absorber material. Therefore, the nanostructures are preferentially located between the active layer and the back reflector (BR), forming a socalled plasmonic back reflector (PBR) (Mendes et al., 2014). One scheme is to incorporate metal nanoparticles on the back side of the cell, but using such a design to maximize the performance over either a broad spectrum or a large scale will remain a challenge. The nanoparticles also could increase recombination in the bulk and surface of the active layers, thereby deteriorating the electrical performance of the cells. An alternative strategy of placing a metallic nanograting at the bottom of the active layer will achieve a broadband and polarization-insensitive absorption enhancement by taking advantage of effective coupling to the SPPs’ resonance and planar waveguide modes. As shown in Fig. 13.13A and 13.13B, the a-Si solar cell employs an Ag nanograting filled with ITO as the bottom electrode. The absorption of the cell with a period of 350 nm and a thickness of 50 nm over the solar spectrum shows an up to ∼30% broadband absorption enhancement when comparing to bare thin-film cells. Through FEM simulation, normalized and time-averaged TM polarization field distribution across the cell structure is obtained, showing large enhancements associated with SPPs (Wang et al., 2010).

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   505

Fig. 13.13 (A) The device structure of the a-Si solar cell employs an Ag nanograting filled with ITO as the bottom electrode. (B) Normalized and time-averaged TM polarization field distribution across the cell structure by FEM simulation at a wavelength of 860 nm. The Ag nanograting has a period of 450 nm and a thickness of 50 nm. Reprinted with permission from Wang et al. (2010). Copyright 2010, American Chemical Society. (C) Schematic and (D) SEM cross section (at a 5-degree angle) of the cells conformally deposited over the patterned substrate. Reprinted with permission from Ferry et al. (2011). Copyright 2011, American Chemical Society. (E) Tilted view (at 45 degrees) of a nanodent array-patterned Al substrate. Reprinted with permission from Huang et al. (2013). Copyright 2013, The Royal Society of Chemistry. (F) SEM image of a cross section of a CIGSe solar cell with a periodic array of nanoparticles at the Mo/CIGSe interface. Reprinted with permission from Van Lare et al. (2015). Copyright 2015, American Chemical Society.

506  Chapter 13 Another strategy is to pattern the substrate so that the additional layers of the solar cell conformally coat the substrate and preserve its patterns. This can provide efficient light scattering on both sides of the cell. As illustrated in Fig. 13.13C and D, pseudorandom arrays of nanostructures have been introduced into the substrate of the thin-film a-Si:H solar cell. The integrated cell design consists of a patterned plasmonic back reflector and a nanostructured semiconductor top interface, which gives broadband and isotropic photocurrent enhancement (Ferry et al., 2011). Furthermore, patterned plasmonic Ag back reflectors on Al foil for enhanced optical absorption of a-Si:H solar cells were proved to be an efficient method to improve the light harvesting (Fig. 13.13E). Due to the coupling of waveguide modes and SP resonances with the aid of nanodent arrays, the optimized device configuration delivered 5–30 times absorption enhancement near the band edge (Huang et al., 2013). Presently, it has not been established which method of lighttrapping will be most successful for ultrathin cells, but attentions has been paid more to highrefractive-index dielectric and semiconductor nanostructures (Brongersma et al., 2014). These structures feature similar scattering albedo as metallic nanostructures, while the parasitic optical losses are greatly suppressed. Fig. 13.13F gives an example of an ultrathin Cu(In,Ga) Se2 solar cell using arrays of SiO2 dielectric-scattering patterns on top of the back contact. This leads to efficient light-trapping and does not deteriorate the carrier recombination properties of the device, resulting in a significant cell efficiency increase from 11.1% to 12.3% (Van Lare et al., 2015). OSCs are usually processed beginning with a transparent electrode on the substrate. Therefore, the plasmonic nanostructures in OSCs are incorporated into the front of the cell at the anode/hole-transporting layer (HTL) interface, or on top of the cell at the rear electrode, increasing light-trapping through SP effects. Furthermore, OSCs offer the additional possibility of blending metal nanoparticles into the active layer, which most efficiently exploits near-field enhancement. As can be seen in Fig. 13.14A, the commonly used approach to introduce plasmonic effects into polymer solar cells is to embed metallic NPs such as Au and Ag into the poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) hole-transporting interlayer. In this case, light-trapping can be achieved by LSPR-induced local field enhancement and scattering incident light at wide angles into the active layer. Material, shape, and size parameters play significant roles in enhanced light absorption with tailorable wavelengths. It has been reported that combining Au and Ag to form Au@ Ag core-shell structures could obtain broad LSPR region and high scattering efficiency at a long-wavelength range (Baek et al., 2014). Superior plasmonic properties also have been demonstrated using Ag nanoprisms (see Fig. 13.14B). They are expected to generate strong local fields due to their sharp edges. When they are further encapsulated with thin SiO2 shells by chemical means, the charge recombination effects can be prevented, and their electrical properties improved (Wu et al., 2012; Salvador et al., 2012).

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   507

Fig. 13.14 (A) The device structure of plasmonic OSCs embedded with metallic NPs inside the PEDOT:PSS hole-transporting interlayer. Reprinted with permission from Baek et al. (2014). Copyright 2014, American Chemical Society. Schematics of OSC that incorporate Ag nanoprisms (B) and Au metallic-mesh electrode (C) as plasmonic structures on the front of the cells. Reprinted with permission from Salvador et al. (2012) Chou and Ding (2013). Copyright 2012, American Chemical Society and Optical Society of America. Schemes of inverted PSCs with a nanostructured scattering rear electrode, where Ag NP arrays (D) and gratings (E) are used to form the plasmonic nanostructure. Reprinted with permission from Cheng et al. (2013) You et al. (2012). Copyright 2013, Elsevier B.V. and 2012, Wiley-VCH.

A subwavelength hole array is another plasmonic nanostructure widely used for light concentration in OSCs. It has been found that the transmittance enhancement from SPPs can be many times greater than the aperture ratio. This is because the scattering and interference of SPPs with Bloch waves in apertures create light that tunnels and propagates away from the structure. When the metal film is thin enough (i.e., about 50 nm), this tunneling may become resonant because the SP modes on the two surfaces can overlap and interact via the holes. Thus, perforated metal layers have shown superior ability as transparent electrodes.

508  Chapter 13 As shown in Fig. 13.14C, a 30-nm-thick front metal-mesh electrode with subwavelength hole array incorporated into a solar cell can achieve a broadband absorptance level as high as 96% (Chou and Ding, 2013). Note that the metal mesh also can be used in OLEDs, which will be discussed further later in this chapter. Similar to the situation with inorganic solar cells, plasmonic nanostructures on the front of the cell suffer from drawbacks such as the influence on the active layer morphology, and the aggregation of metallic NPs in the polymer host, which induce the possibility of charge trapping and exciton quenching. It may also give rise to destructive Fano interference and scatter the incident light in the reverse direction, leading to the inevitable loss of light into the active layer. Hence, a strategy that places a nanostructure on top of the cell at the rear electrode has been developed. Fig. 13.14D and E demonstrate two nanostructured plasmonic metal back-contact electrodes (Cheng et al., 2013; You et al., 2012). By embedding Ag NP arrays into the MoO3 holeextraction layer or imprinting the active layer, the plasmonic structures were introduced into the back electrodes of these two cells separately, resulting in significant improvements in device performance. Solution-processed metal NPs can mix directly in an active layer, enhancing light harvesting by either the formation of scattered waves at the large-diameter NPs or the excitation of LSPR modes at the smaller-diameter NPs. The enhancement effect extends to wavelengths longer than the plasmon resonance wavelength, for which the losses in the metallic NPs become insignificant (Stratakis and Kymakis, 2013). Au NPs (Wang et al., 2011a), Ag NPs (Wang et al., 2011b), and AU-SiO2 core-shell nanorods (Xu et al., 2013) have been incorporated into the active layer of OSCs, resulting in enhanced device efficiency. On the other hand, negative effects of imbedding NPs into active layers also exist. The exciton recombination at the NP surface in turn distracts from the plasmonic enhancement effects. For example, Fig. 13.15A and B demonstrate the experimental and theoretical (inset) absorbance-enhancement factors of active layers with different amounts of Au NPs. Continuously enhanced light absorption is observed with the increase of Au NP concentration due to stronger LSPR, while low Au NP concentration in fact benefits cell performance. When Au NPs concentration is greater than 0.5 wt%, deterioration of device performance occurs, which is unexpected from the viewpoint of LSPR effects. This means that electrical properties can counteract the optical enhancement from LSPR and thus reduce the overall amount of performance improvement. It is important to study both optical and electrical properties and optimize them simultaneously in order to achieve improved power conversion efficiency (PCE; Wang et al., 2012). Improved light-trapping also has been demonstrated by using a combination of plasmonic nanostructures. Dual metallic nanostructures (Li et al., 2012) combine the plasmon resonances of the two nanostructures, thus leading to broadband absorption enhancement. For example, carefully manipulated plasmonic resonances introduced by the embedded

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   509

Fig. 13.15 (A) Experimental and theoretical (inset) absorbance enhancement factors of active layers with different amounts of Au NPs. (B) Theoretical near-field distribution around an Au NP in the active layer. Reprinted with permission from Wang, C.C.D., Choy, W.C.H., Duan, C., Fung, D.D.S., Sha, W.E.I., Xie, F.-X., Huang, F., Cao, Y., 2012. Optical and electrical effects of gold nanoparticles in the active layer of polymer solar cells. J. Mater. Chem. 22, 1206–1211. Copyright 2012, The Royal Society of Chemistry.

Ag nanoprisms at both sides of the active layers with different LSPR peaks have shown complementary light-harvesting with microcavity resonance, leading to broad absorption and external quantum efficiency (EQE) enhancement (see Fig. 13.16A and B) (Yao et al., 2014). Selecting Ag grating with a 600-nm period as an anode and introducing metal NPs into the electron-transporting layer (ETL) allow the device to obtain broadband absorption enhancement in the range of 350–800 nm due to multiple plasmonic effects. As a result, a maximum PCE of 9.62% has been achieved from the device (see Fig. 13.16C and D) (Li et al., 2015). Recently, quasi-crystalline or random structures across the device have been used to offer broadband, broad-angle light-tapping. Quasi-random structures allow the modes guided in all azimuthal directions to be trapped over broadband and all incident angles and polarizations due to their broad distribution in wavenumber and random directionality. As Fig. 13.17 shows, moth’s-eye nanostructures (Zhou et al., 2014) (Fig. 13.17A–C) and deterministic aperiodic nanostructures (DANs; Chen et al., 2015) (Fig. 13.17D and E) were introduced to the surface of a carrier-transporting layer on the substrates by soft nanoimprinting lithography, which were well duplicated throughout the subsequently deposited organic layer and metal electrode. Comparisons of FDTD simulation results show that the light-trapping in the active layer is greatly enhanced due to the synergy effect of backscattering and SP resonance caused by patterned Al.

510  Chapter 13

Fig. 13.16 (A) Schematic illustration of the mechanism for the optical enhancement in a dual-plasmonic device by embedding Ag nanoprisms at both sides of the active layer. (B) EQE spectra of plasmonic devices in blend- and dual-type structures, compared with the reference device. Reprinted with permission from Yao et al. (2014). Copyright 2014, Wiley-VCH. (C), (D) A broadband absorption enhancement has been achieved for the dual-type device that combines NPs and grating. Reprinted with permission from Li et al. (2015). Copyright 2015, Wiley-VCH.

13.4.2  Plasmonic LEDs Both inorganic and organic LEDs have been improved and commercially used in solidstate lighting and full-color display panels, but their overall EQEs are still strongly limited. The EQEs of LEDs depend on the IQE and the outcoupling efficiency (ηout,the ratio of the total number of photons outcouple in the forward direction to the number of injected electrons), and given that the IQE is determined by the ratio of the radiative (krad) and nonradiative (knon) recombination rates of excitons, the mathematical expression of EQE can be given by EQE = IQE ×ηout =

krad ×ηout krad + knon

(13.33)

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Fig. 13.17 (A) Device structure of a plasmonic OSC integrated with dual-side, moth’s-eye nanostructures. (B), (C) Normalized crosssection magnetic field profiles of TM polarized light at 720 nm for cells without (B) and with (C) plasmonic nanostructures. Reprinted with permission from Zhou et al. (2014). Copyright 2014, Nature Publishing Group. (D) Device structure of a PSC containing dual-sided, nanoimprinted DANs. (E), (F) The normalized cross-section, near-field profiles and photon flux diagrams of TM polarized light at 810 nm for (E) reference and (F) patterned PSCs. Reprinted with permission from Chen et al. (2015). Copyright 2014, Wiley-VCH.

512  Chapter 13 Therefore, three strategies are possible to enhance the EQE values: (1) increase ηout, (2) decrease knon, and (3) increase krad (Okamoto and Kawakami, 2009). For inorganic LEDs based on the Group III-nitrides, ηout is easy to improve by using textured ITO window layers (Horng et al., 2005) or patterned sapphire substrates and mesh electrodes (Yamada et al., 2002). But their IQE decreases significantly due to nonradiative recombination processes, threading dislocations, and charge separation from the polarization fields, which are associated with the carrier behaviors and are difficult to control. On the other hand, the utilization phosphorescent and TADF materials as organic emitters have boosted the IQE of OLEDs from 25% to nearly 100%. The ηout of OLEDs, however, is typically limited to values around 20% due to the multiple loss channels, such as the excitation of substrate mode, WG mode, SP mode, and metallic electrode absorption (Xu et al., 2016). Fig. 13.18 gives the simulation results for the distribution of the various energy loss channels in a planar bottom-emission OLED (Meerheim et al., 2010). It shows that SP modes cause losses of typically up to 40% in conventional OLEDs, giving rise to enormous potential to increase the device efficiency. Hence, the focus of inorganic LED improvements is on increasing IQE, while for OLEDs, it is on increasing ηout. Plasmonic nanostructures have become promising candidates to overcome the efficiency problem by offering unique abilities to either decrease the nonradiative process in inorganic LEDs or recover the SP modes as light in OLEDs.

Fig. 13.18 Simulation results for the distribution of energy loss channels in a planar bottom-emission OLED. Reprinted with permission from Meerheim, R., Furno, M., Hofmann, S., Lussem, B., Leo, K., 2010. Quantification of energy loss mechanisms in organic light-emitting diodes. Appl. Phys. Lett. 97. Copyright 2010, American Institute of Physics.

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   513 Rough metal films and metallic nanoparticles are used to achieve SP coupling and LSP coupling, respectively, in inorganic InGaN/GaN quantum well (QW) LEDs, providing great enhancement of IQEs. As shown in Fig. 13.19A and B, when Ag or Al layers were deposited 10 nm above an InGaN light-emitting layer, the resulting SPs can increase the density of states and the spontaneous emission rate in the semiconductor. As a result, SP-QW coupling takes place, producing an additional recombination path that effectively suppresses the nonradiative recombination channel and increases the IQE value. Thus, a 14-fold PL enhancement and a 6.8-fold IQE enhancement are observed (Okamoto et al., 2004). Note that a perfectly flat metal/semiconductor interface may not extract light from the SP mode, but submicrometer-scale roughness and imperfections on the metal surface are able to scatter SPs as

Fig. 13.19 (A) Structure of InGaN/GaN QWs with plasmonic rough metal layers (10 nm) deposited above the InGaN light-emitting layer. (B) PL spectra of InGaN/GaN QWs coated with Ag, Al, and Au. (C) SEM image of the uncoated GaN surface. (D) SEM image of 50-nm Ag film evaporated on GaN. (E) SEM image of a grating structure with a 33% duty cycle, fabricated within a 50-nm-thick Ag layer on GaN. Reprinted with permission from Okamoto, K., Niki, I., Shvartser, A., Narukawa, Y., Mukai, T., Scherer, A., 2004. Surface-plasmon-enhanced light emitters based on InGaN quantum wells. Nat. Mater. 3, 601–5. Copyright 2004, Nature Publishing Group.

514  Chapter 13 light in an efficient manner. Such roughness has been verified by the SEM images of the uncoated GaN surface (Fig. 13.19C) and the Ag-coated surface (Fig. 13.19D). Moreover, metal grating structures of Ag on InGaN QWs (Fig. 13.19E) with 133-nm-wide stripes and a 400-nm period also demonstrates an enhancement of the emissions, whereas such an emission increase is not observed from 200-nm-wide Ag stripes in a 600-nm period grating. This suggests that patterned metal films can be used to optimize the light-emitting efficiency, but this depends on the grating geometry. Building on the fact that the verification of SPs can significantly enhance the quantum efficiency of InGaN emitters, plasmonic nanostructures in GaN-based LEDs were extensively studied. Amid all the various plasmonic nanostructures, metallic nanoparticles (Au, Ag, or Pt NPs) will undoubtedly take up the first position. This is because LSP-enhanced LEDs using metallic nanoparticles is an effective strategy to overcome a well-known contradiction. That is, efficient SP-QW coupling requires the distance between the metal surface and the InGaN QW must be very close (around 10–20 nm); whereas maintaining the p-n junction in LEDs need the p-type GaN layer to be thick enough. Fig. 13.20 depicts the schematic of a typical LSP-enhanced InGaN/GaN QW LED with metallic nanoparticles. The nanoparticle layers are usually fabricated by e-beam evaporation (or sputtering) and annealing processes, or by the sol-gel method. A major advantage is that the nanoparticles can be embedded a few nanometers below or above the multiquantum walls (MQWs) into n-GaN, p-GaN, or ITO layers. For example, Kwon et al. inserted an Ag nanoparticle layer between the n-GaN layer and the MQW layer by e-beam evaporation, obtaining a 32.2% enhancement of emissions for the plasmonic LED (Kwon et al., 2008). The SP-enhanced LEDs using Ag NPs embedded into p-GaN were demonstrated later, and the light emission efficiency was enhanced by 38% at an injection current of 20 mA due to the LSP-QW coupling enhanced ηout (Cho et al., 2010). Okada et al. utilized a 20-nm-thick p+GaN layer and embedded Ag NPs into ITO, the optimal blue LED exhibited an EQE that was approximately double than that of LEDs without the localized Ag NPs (Okada et al., 2017).

Fig. 13.20 Schematic of a typical LSP-enhanced, InGaN/GaN, QW LED with metallic nanoparticles. Note that the nanoparticles can be placed at different positions.

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   515 Similarly, by simply sputtering and annealing a 2-nm Au NPs arrayed layer on the ITO outer surface, a 1.8-fold enhancement in electroluminescent (EL) intensity with 20-mA injection current was observed, which is attributed to the LSPR effect and scattering by a rough surface (Sung et al., 2009). Further, sol-gel-synthesized core-shell Ag/SiO2 NPs were also used. The electric field distribution simulated from the FDTD method (Fig. 13.21) shows that the field penetration is deeper beyond the InGaN MQW region. This suggests that the Ag/SiO2 NPs have great potential for LSP enhancement in MQW LEDs. Due to an easier agglomeration and oxidation of solution-processed Ag NPs, core-shell Ag/SiO2 NPs may provide a more predictable and stable performance (Jang et al., 2012). Similar to the case of inorganic LEDs, incorporating plasmonic nanoparticles in OLEDs also could enhance light emissions. The enhancement is caused by the overlap of the local electromagnetic field of SPs and excitons in the emissive layer (LSP-exciton coupling), in which the exciton energy is transferred to the SP modes. Thus, the nanoparticles provide additional states for exciton recombination. As the coupling process is much faster than the spontaneous recombination of excitons, the enhanced density of states for exciton recombination can significantly increase the recombination rate. Then the nanoparticles scatter the high-momentum SPs, causing them to lose their momentum and radiate out as light. Furthermore, the significant scattering of the nanoparticles when SPR takes place can also improve the outcoupling of other modes trapped inside the OLEDs. Various kinds of nanoparticles such as Au NPs, Cu NPs, Au nanorods, and alloy NPs have been incorporated into different parts of OLED devices to improve their emission efficiency.

Fig. 13.21 The electric field distribution of LSP mode simulated by the FDTD method, where Ag (30 nm)/SiO2 (20 nm) core-shell NPs are placed on top of a 10-nm-thick GaN spacer at 470 nm. Reprinted with permission from Jang, L.W., Jeon, D.W., Kim, M., Jeon, J.W., Polyakov, A.Y., Ju, J.W., Lee, S.J., Baek, J.H., Yang, J.K., Lee, I.H., 2012. Investigation of optical and structural stability of localized surface plasmon mediated light-emitting diodes by Ag and Ag/SiO2 nanoparticles. Adv. Funct. Mater. 22, 2728–2734. Copyright 2012, WILEY-VCH.

516  Chapter 13 Fujiki et al. developed thin-film green OLEDs containing a single layer of 12-nm-diameter Au nanospheres or 50–60-nm Au nanorods along the longitudinal axis, with an aspect ratio of about 2.5–3 embedded in the hole-transporting layer to verify the improvement in emission efficiency through LSPR. They demonstrated 3-fold and 20-fold emission intensity enhancement by nanorods and nanospheres, respectively, due to LSPR (Fujiki et al., 2010; Tanaka et al., 2011). Fig. 13.22A shows an LSP-enhanced OLED created by embedding chemically synthesized, 40–50-nm Au NPs in PEDOT:PSS as a hole-injection layer (HIL). Its EL intensity demonstrates about 25% enhancement due to the near-field enhancement effect (Xiao et al., 2012). Heo et al. prepared arrays of 25-nm Cu NPs (Fig. 13.22B) on ITO using a block copolymer template and observed a enhancement of more than 20% in the subsequent deposited OLED device (Heo et al., 2011). As for alloy nanoparticles, Pt3Co NPs on ITO were proposed to fabricate OLED devices and achieved a double-enhancement in light extraction due to the coupling and scattering effect. The electromagnetic field distribution around isolated Pt3Co atrial natriuretic peptides (ANPs) with a diameter of 15 nm (as shown in Fig. 13.22C) presents an intuitive picture of an LSP-enhanced near-field in the x-direction and the scattered field in the y-direction (Gu et al., 2013). Ag-enhanced gold nanostars coated with a thin SiO2 shell in an OLED active layer (Fig. 13.22D and E) has resulted in up to 50% improvement in device performance due to the plasmon-mediated increase in the radiative recombination of excitons and the improved outcoupling of light otherwise trapped inside the OLEDs (Munkhbat et al., 2016). Obviously, LSP enhancement is more effective in an OLED with low IQE because of its LSPexciton coupling mechanism. It also has a disadvantage—namely, that the spectral overlap of the emitter band and the SPR band of the metal nanoparticles, as well as the distance between the nanoparticle and the emitter, should be carefully adjusted. To overcome this drawback, random SiO2 nanoparticles might be an alternative. As shown in Fig. 13.22F, the SiO2 nanoparticles can cause corrugation in OLED, thus extracting the SP mode at metal electrode surface through Bragg diffraction (Kim et al., 2014). This is closely analogous to recovering SP loss by grating coupling, which we will discuss next. According to the aforementioned principle, grating coupling allows Bragg scattering of SPs to increase or reduce their wave vectors. Because the SP modes at metal/organic interface have a well-defined wave vector, it is possible to shift the SP dispersion partly into the air or a glass light cone through carefully designed grating parameters. Thus, SPs can be coupled to far-field radiation for a range of frequencies such that some of the power lost to SPs can be recovered. The early studied were 1D or 2D periodic line gratings that were typically introduced by nanopatterning a spin-coated photoresist and subsequently building the device on top of this layer. Hobson et al. first introduced a periodic corrugation with a 290-nm period and a 30-nm

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   517

Fig. 13.22 (A) Device structure of an OLED incorporating Au NPs in a PEDOT:PSS layer. Reprinted with permission from Xiao et al. (2012). Copyright 2012, American Institute of Physics. (B) Height-mode atomic force microscopy (AFM) images of 25-nm Cu NP arrays on ITO. Reprinted with permission from Heo et al. (2011). Copyright 2012, WILEY-VCH. (C) The electromagnetic field distribution around the isolated Pt3Co ANPs with a diameter of 15 nm. Reprinted with permission from Gu et al. (2013). Copyright 2013, The Royal Society of Chemistry. (D) Device structure of an OLED with plasmonic nanostars. (E) TEM images of Au-Ag nanostars, with a scale bar of 100 nm. Reprinted with permission from Munkhbat et al. (2016). Copyright 2016, WILEY-VCH. (F) Outcoupled light from losses in a corrugated nanostructured OLED with an SiO2 NPs inside the PEDOT:PSS layer. Reprinted with permission from Kim et al. (2014). Copyright 2014, Elsevier B.V.

518  Chapter 13 depth into the Al cathode of an OLED by this method, coupled out the SPs into free-space radiation, and improved the efficiency of the OLED (Hobson et al., 2002). In a similar way, a 250-nm periodic corrugation in wavelength scale was introduced into bottom-emitting OLEDs and exhibited a 30% enhancement in efficiency. Furthermore, the introduction of the periodic 1D and 2D gratings into top-emitting OLEDs (as shown in Fig. 13.23A–D) has validated the enhancement in both EL efficiency and device stability. A simulated dispersion map of transverse-magnetic (TM) polarization for the corrugated device indicates that grating-induced cross-coupling between the SPPs associated with the top interface of the cathode and the microcavity modes within the device cavity is responsible for the enhanced light transmission through thick Ag cathodes (Jin et al., 2012). SPPs enable high-transmittance enhancement through subwavelength hole arrays in optically thick metallic films. A transparent OLED incorporating a periodically perforated WO3 layer by colloidal lithography is proposed (as illustrated in Fig. 13.24A). Fig. 13.24B shows the calculation results for the grating period required to extract the SP mode (TM 0) as a function of the wavelength, which indicate that small periods are needed so that first-order scattered SPs lie within the air light cone. The hexagonal grating structure with a period of 330 nm is

Fig. 13.23 AFM images of 1D (A) and 2D (B) gratings. (C) Schematic structures of OLEDs with 2D corrugation. (D) Calculated dispersion relation for the wavelength versus the incident angle of the corrugated OLEDs for TM polarization. Reprinted with permission from Jin, Y., Feng, J., Zhang, X.L., Bi, Y.G., Bai, Y., Chen, L., Lan, T., Liu, Y.F., Chen, Q.D., Sun, H.B., 2012. Solving efficiency-stability tradeoff in top-emitting organic light-emitting devices by employing periodically corrugated metallic cathode. Adv. Mater. 24, 1187–91. Copyright 2012, WILEY-VCH.

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   519

Fig. 13.24 (A) Device structure of a transparent OLED incorporating a periodically perforated WO3 layer. (B) The calculation results for the grating period required to extract the SP mode (TM 0) as a function of the wavelength. Reprinted with permission from Choi et al. (2013). Copyright 2013, WILEY-VCH. (C) Structure schematic of an OLED with a subwavelength hole-array plasmonic cavity (D) Electric field intensity distributions of the plasmonic OLED (with an 80-nm-thick active layer) excited by a plane-wave light source outside. Reprinted with permission from Ding et al. (2014). Copyright 2014, WILEY-VCH.

expected to extract the SP modes at wavelengths of 650 nm. As a result, the perforated metal layer with a thickness of 25 nm efficiently increased the EQE and the power efficiency due to enhanced outcoupling of the waveguide modes and the SP modes (Choi et al., 2013). Ding et al. demonstrated a 15-nm-thick Au mesh with a 200-nm period hole array with a 180-nm hole diameter and an AuOx atomic layer on its surface. A plasmonic nanocavity constructed by combining this mesh and an Al cathode in an OLED is shown in Fig. 13.24C; this leads to a 1.57-fold higher EQE. FDTD simulation results (Fig. 13.24D) show that the plasmonic cavity with a subwavelength hole-array is an excellent optical antenna, which efficiently radiates the light inside the cavity to the outside (Ding et al., 2014). Additionally, a plasmonic nanomesh electrode made from Ag on PET has been reported to exhibit not only excellent outcoupling, but also high flexibility (Lee et al., 2015).

520  Chapter 13 In spite of the benefits that periodic structures hold, there are also disadvantages. Periodic coupling can only enhance a narrow band of spectral emissions, and it causes dispersive emissions and color shifts. These drawbacks make periodic structures unsuitable for the full-color-enhancement applications, especially for white OLEDs, while quasi-random nanostructures can allow broadband light extraction. For example, a spontaneously formed buckling structure has been investigated with the goal of converting the dissipated energy of SPs and waveguide modes into useful light, as shown in Fig. 13.25A and B. The quasiperiodic buckling structure is imprinted on UV-curable resin by using a PDMS replica, whose structure is prefabricated through cooling a heated PDMS substrate with thermally deposited Al, utilizing their various thermal expansions (Fig. 13.25B). The broad periodicity distribution and the randomly oriented wave vectors of the buckles provide an invaluable advantage: possible outcoupling of the light propagating along any direction with a wide spectral range (Koo et al., 2010). Other types of quasi-random structures used in OLEDs include moth-eye deterministic aperiodic nanostructures (Ou et al., 2014), and nanofunnel arrays (NFAs) (Zhou et al., 2015). As shown in Fig. 13.25C, NFAs are introduced into a PEDOT:PSS HIL by soft nanoimprinting lithography, resulting in conformal structures in the following layers of the OLED. As a consequence, the SP modes supported by the nanostructured Al electrode shift to the air cone and radiate to the free space. This feature can be clearly seen from the dispersion diagrams for TM-polarized lights as a function of the frequency and in-plane wave vector Kx in the first Brillouin zone of white OLEDs with NFAs (Fig. 13.25D). Combining the same nanostructured UV-curable resin film on a glass surface as the outcoupling layer, OLEDs with double NFAs (dNFAs) demonstrate a substantial increase in efficiency—more than twice as high as a conventional device with a flat architecture. The angular dependence of the EL intensities of the devices (both with and without dNFAs, as plotted in Fig. 13.25E) shows a stronger side emission for the nanostructured device, implying the broadband and quasi-omnidirectional lightdiffraction capability of NFAs.

13.5  Conclusions and Outlook Efficiency has been an irreplaceable index to use to evaluate whether an optoelectronic device can be put into commerce. Despite the rapid development of highly efficient materials in solar cells and LEDs, an unsatisfactory uptake of light remains a challenge due to multiple light loss channels in sandwich-like device structures, especially SP modes. However, SPs can contribute to a better device performance in reverse by appropriate manipulation. With either excitation of SPPs at planar metallic surface or generation of LSPR at metallic nanostructures in certain conditions, a dramatic variation in SP intensity can be realized for light manipulation.

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   521

Fig. 13.25 (A) Schematic of an OLED with buckles. The buckled PDMS was transferred to UV-curable resin by an imprinting technique. (B) Buckled structures formed by the deposition of a 10-nm-thick Al layer three times. Reprinted with permission from Koo et al. (2010). Copyright 2010, Macmillan Publishers Limited. (C) Device structure of the white OLEDs with dNFAs. (D) Simulated dispersion diagrams of TM-polarized light as a function of frequency and the in-plane wave vector Kx in the first Brillouin zone of the nanostructured OLED. (E) Normalized angular dependence of light intensity for white OLEDs with and without dNFAs. Reprinted with permission from Zhou et al. (2015). Copyright 2015, WILEY-VCH.

To fabricate such plasmonic nanostructures, multiple techniques ranging from photolithography to nanoimprinting have been developed for much subtler patterns. Meanwhile, theoretical proof of light manipulation in SPs also has been confirmed with simulation support. Different models are established to solve Maxwell’s equations for a visual description on light distribution in optoelectronic devices. Thus, with the aid of developed

522  Chapter 13 fabrication methods and progressive theory models, SPs stimulated by nanostructures can function as a powerful light modulator in solar cells and LEDs. In terms of PV devices, thinfilm solar cells have dominated over current studies because of material saving and diffusion length shortening. Due to independent incident angles and large scattering cross sections brought by nanostructures, SPs are considered to be competent light-absorbing assistants to break the light-trapping limit in thin-film cells. As for LEDs, gratings and metallic nanoparticles are two key elements for IQE enhancement through achieving SP and LSP coupling, respectively, in inorganic devices. Similarly, nanoparticles introduced in charge-transporting layers are also good light-scatterers in organic ones to achieve higher EQE. Moreover, stronger light over a wide range of wavelengths can be outcoupled by both periodic and random nanopatterns. Even though great contributions have been made toward boosting the performance of both solar cells and LEDs, SPs remain popular in traditional devices. New devices made from updated materials like quantum dots and perovskites still need to be studied. One important reason for this is the rigorous surface conditions for high-quality film formed by such surfacesensitive materials. Uneven surfaces introduced by nanostructures may cause excess shortcircuit current and charge accumulation at the interface. Another issue that must be tackled is the improvement of nanopatterns and their manufacturing technologies. Although a variety of nanopatterns have been fabricated successfully, more kinds of structures whose size and shape match well with device constructions are to be explored. The manufacture process also should be improved in response to large-scale production and market demands. With the progress that has been made recently in research into SPs, charge collection at the quantum level may come to pass, and optoelectronic devices with extraordinary performance can serve humanity well in the near future.

References Akimov, Y.A., Koh, W.S., Ostrikov, K., 2009. Enhancement of optical absorption in thin-film solar cells through the excitation of higher-order nanoparticle plasmon modes. Opt. Express 17, 10195–10205. Atwater, H.A., Polman, A., 2010. Plasmonics for improved photovoltaic devices. Nat. Mater. 9, 205–213. Baek, S.W., Park, G., Noh, J., Cho, C., Lee, C.H., Seo, M.K., Song, H., Lee, J.Y., 2014. Au@Ag core-shell nanocubes for efficient plasmonic light scattering effect in low bandgap organic solar cells. ACS Nano 8, 3302–3312. Barnes, W.L., Dereux, A., Ebbesen, T.W., 2003. Surface plasmon subwavelength optics. Nature 424, 824–830. Bloomstein, T.M., Marchant, M.F., Deneault, S., Hardy, D.E., Rothschild, M., 2006. 22-nm immersion interference lithography. Opt. Express 14, 6434–6443. Brongersma, M.L., Cui, Y., Fan, S.H., 2014. Light management for photovoltaics using high-index nanostructures. Nat. Mater. 13, 451–460. Campbell, M., Sharp, D.N., Harrison, M.T., Denning, R.G., Turberfield, A.J., 2000. Fabrication of photonic crystals for the visible spectrum by holographic lithography. Nature 404, 53–56.

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   523 Chen, J.D., Cui, C., Li, Y.Q., Zhou, L., Ou, Q.D., Li, C., Li, Y., Tang, J.X., 2015. Single-junction polymer solar cells exceeding 10% power conversion efficiency. Adv. Mater. 27, 1035–1041. Cheng, J.Y., Ross, C.A., Smith, H.I., Thomas, E.L., 2006. Templated self-assembly of block copolymers: topdown helps bottom-up. Adv. Mater. 18, 2505–2521. Cheng, P.P., Zhou, L., Li, J.A., Li, Y.Q., Lee, S.T., Tang, J.X., 2013. Light trapping enhancement of inverted polymer solar cells with a nanostructured scattering rear electrode. Org. Electron. 14, 2158–2163. Cho, C.Y., Kwon, M.K., Lee, S.J., Han, S.H., Kang, J.W., Kang, S.E., Lee, D.Y., Park, S.J., 2010. Surface plasmon-enhanced light-emitting diodes using silver nanoparticles embedded in p-GaN. Nanotechnology 21, 205201. Choi, C.S., Kim, D.Y., Lee, S.M., Lim, M.S., Choi, K.C., Cho, H., Koh, T.W., Yoo, S., 2013. Blur-free outcoupling enhancement in transparent organic light emitting diodes: a nanostructure extracting surface plasmon modes. Adv. Opt. Mater. 1, 687–691. Chou, S.Y., Ding, W., 2013. Ultrathin, high-efficiency, broad-band, omni-acceptance, organic solar cells enhanced by plasmonic cavity with subwavelength hole array. Opt. Express 21 (Suppl 1), A60–A76. Colson, P., Henrist, C., Cloots, R., 2013. Nanosphere lithography: a powerful method for the controlled manufacturing of nanomaterials. J. Nanomater. 2013, 1–19. Ding, W., Wang, Y.X., Chen, H., Chou, S.Y., 2014. Plasmonic nanocavity organic light-emitting diode with significantly enhanced light extraction, contrast, viewing angle, brightness, and low-glare. Adv. Funct. Mater. 24, 6329–6339. Fan, R.H., Zhu, L.H., Peng, R.W., Huang, X.R., Qi, D.X., Ren, X.P., Hu, Q., Wang, M., 2013. Broadband antireflection and light-trapping enhancement of plasmonic solar cells. Phys. Rev. B 87, 195444. Ferry, V.E., Munday, J.N., Atwater, H.A., 2010. Design considerations for plasmonic photovoltaics. Adv. Mater. 22, 4794–4808. Ferry, V.E., Verschuuren, M.A., Lare, M.C., Schropp, R.E.I., Atwater, H.A., Polman, A., 2011. Optimized spatial correlations for broadband light trapping nanopatterns in high efficiency ultrathin film a-Si:H solar cells. Nano Lett. 11, 4239–4245. Fujiki, A., Uemura, T., Zettsu, N., Akai-Kasaya, M., Saito, A., Kuwahara, Y., 2010. Enhanced fluorescence by surface plasmon coupling of Au nanoparticles in an organic electroluminescence diode. Appl. Phys. Lett. 96, 043307. Gu, Y., Zhang, D.D., Ou, Q.D., Deng, Y.H., Zhu, J.J., Cheng, L., Liu, Z., Lee, S.T., Li, Y.Q., Tang, J.X., 2013. Light extraction enhancement in organic light-emitting diodes based on localized surface plasmon and light scattering double-effect. J. Mater. Chem. C 1, 4319–4326. Heo, M., Cho, H., Jung, J.W., Jeong, J.R., Park, S., Kim, J.Y., 2011. High-performance organic optoelectronic devices enhanced by surface plasmon resonance. Adv. Mater. 23, 5689–5693. Hobson, P.A., Wedge, S., Wasey, J.A.E., Sage, I., Barnes, W.L., 2002. Surface plasmon mediated emission from organic light-emitting diodes. Adv. Mater. 14, 1393–1396. Horng, R.H., Yang, C.C., Wu, J.Y., Huang, S.H., Lee, C.E., Wuu, D.S., 2005. GaN-based light-emitting diodes with indium tin oxide texturing window layers using natural lithography. Appl. Phys. Lett. 86, 221101. Huang, X.H., El-Sayed, I.H., Qian, W., El-Sayed, M.A., 2006. Cancer cell imaging and photothermal therapy in the near-infrared region by using gold nanorods. J. Am. Chem. Soc. 128, 2115–2120. Huang, H.T., Lu, L.F., Wang, J., Yang, J., Leung, S.F., Wang, Y.Q., Chen, D., Chen, X.Y., Shen, G.Z., Li, D.D., Fan, Z.Y., 2013. Performance enhancement of thin-film amorphous silicon solar cells with low cost nanodent plasmonic substrates. Energy Environ. Sci. 6, 2965–2971. Jang, L.W., Jeon, D.W., Kim, M., Jeon, J.W., Polyakov, A.Y., Ju, J.W., Lee, S.J., Baek, J.H., Yang, J.K., Lee, I.H., 2012. Investigation of optical and structural stability of localized surface plasmon mediated light-emitting diodes by Ag and Ag/SiO2 nanoparticles. Adv. Funct. Mater. 22, 2728–2734. Jensen, T.R., Malinsky, M.D., Haynes, C.L., Van Duyne, R.P., 2000. Nanosphere lithography: tunable localized surface plasmon resonance spectra of silver nanoparticles. J. Phys. Chem. B 104, 10549–10556. Jin, Y., Feng, J., Zhang, X.L., Bi, Y.G., Bai, Y., Chen, L., Lan, T., Liu, Y.F., Chen, Q.D., Sun, H.B., 2012. Solving efficiency-stability tradeoff in top-emitting organic light-emitting devices by employing periodically corrugated metallic cathode. Adv. Mater. 24, 1187–1191.

524  Chapter 13 Kim, D.H., Kim, J.Y., Kim, D.Y., Han, J.H., Choi, K.C., 2014. Solution-based nanostructure to reduce waveguide and surface plasmon losses in organic light-emitting diodes. Org. Electron. 15, 3183–3190. Koo, W.H., Jeong, S.M., Araoka, F., Ishikawa, K., Nishimura, S., Toyooka, T., Takezoe, H., 2010. Light extraction from organic light-emitting diodes enhanced by spontaneously formed buckles. Nat. Photonics 4, 222–226. Kosiorek, A., Kandulski, W., Glaczynska, H., Giersig, M., 2005. Fabrication of nanoscale rings, dots, and rods by combining shadow nanosphere lithography and annealed polystyrene nanosphere masks. Small 1, 439–444. Kretschmann, E., Raether, H., 1968. Notizen: radiative decay of non radiative surface plasmons excited by light. Z. Naturforsch. A 23, 2135–2136. Kwon, M.K., Kim, J.Y., Kim, B.H., Park, I.K., Cho, C.Y., Byeon, C.C., Park, S.J., 2008. Surface-plasmonenhanced light-emitting diodes. Adv. Mater. 20, 1253–1257. Lal, S., Link, S., Halas, N.J., 2007. Nano-optics from sensing to waveguiding. Nat. Photonics 1, 641–648. Lee, K.S., Kim, R.H., Yang, D.Y., Park, S.H., 2008. Advances in 3D nano/microfabrication using two-photon initiated polymerization. Prog. Polym. Sci. 33, 631–681. Lee, S.M., Cho, Y., Kim, D.Y., Chae, J.S., Choi, K.C., 2015. Enhanced light extraction from mechanically flexible, nanostructured organic light-emitting diodes with plasmonic nanomesh electrodes. Adv. Opt. Mater. 3, 1240–1247. Li, Z.W., Gu, Y.N., Wang, L., Ge, H.X., Wu, W., Xia, Q.F., Yuan, C.S., Chen, Y.F., Cui, B., Williams, R.S., 2009. Hybrid nanoimprint-soft lithography with sub-15 nm resolution. Nano Lett. 9, 2306–2310. Li, X.H., Choy, W.C.H., Huo, L.J., Xie, F.X., Sha, W.E.I., Ding, B.F., Guo, X., Li, Y.F., Hou, J.H., You, J.B., Yang, Y., 2012. Dual plasmonic nanostructures for high performance inverted organic solar cells. Adv. Mater. 24, 3046–3052. Li, X.H., Ren, X.X., Xie, F.X., Zhang, Y.X., Xu, T.T., Wei, B.Q., Choy, W.C.H., 2015. High-performance organic solar cells with broadband absorption enhancement and reliable reproducibility enabled by collective plasmonic effects. Adv. Opt. Mater. 3, 1220–1231. Lindquist, N.C., Nagpal, P., Mcpeak, K.M., Norris, D.J., Oh, S.H., 2012. Engineering metallic nanostructures for plasmonics and nanophotonics. Rep. Prog. Phys. 75, 036501. Manfrinato, V.R., Zhang, L.H., Su, D., Duan, H.G., Hobbs, R.G., Stach, E.A., Berggren, K.K., 2013. Resolution limits of electron-beam lithography toward the atomic scale. Nano Lett. 13, 1555–1558. Meerheim, R., Furno, M., Hofmann, S., Lussem, B., Leo, K., 2010. Quantification of energy loss mechanisms in organic light-emitting diodes. Appl. Phys. Lett. 97, 253305. Mendes, M.J., Morawiec, S., Simone, F., Priolo, F., Crupi, I., 2014. Colloidal plasmonic back reflectors for light trapping in solar cells. Nanoscale 6, 4796–4805. Mie, G., 1908. Beiträge zur Optik trüber Medien, speziell kolloidaler Metallösungen. Ann. Phys. 330, 377–445. Munkhbat, B., Pöhl, H., Denk, P., Klar, T.A., Scharber, M.C., Hrelescu, C., 2016. Performance boost of organic light-emitting diodes with plasmonic nanostars. Adv. Opt. Mater. 4, 772–781. Nakayama, K., Tanabe, K., Atwater, H.A., 2008. Plasmonic nanoparticle enhanced light absorption in GaAs solar cells. Appl. Phys. Lett. 93, 121904. Noguez, C., 2007. Surface plasmons on metal nanoparticles: the influence of shape and physical environment. J. Phys. Chem. C 111, 3806–3819. Okada, N., Morishita, N., Mori, A., Tsukada, T., Tateishi, K., Okamoto, K., Tadatomo, K., 2017. Fabrication and evaluation of plasmonic light-emitting diodes with thin p-type layer and localized Ag particles embedded by ITO. J. Appl. Phys. 121, 153102. Okamoto, K., Kawakami, Y., 2009. High-efficiency InGaN/GaN light emitters based on nanophotonics and plasmonics. IEEE J. Sel. Topics Quant. Electron. 15, 1199–1209. Okamoto, K., Niki, I., Shvartser, A., Narukawa, Y., Mukai, T., Scherer, A., 2004. Surface-plasmon-enhanced light emitters based on InGaN quantum wells. Nat. Mater. 3, 601–605. Otto, A., 1968. Excitation of nonradiative surface plasma waves in silver by the method of frustrated total reflection. Z. Phys. 216, 398–410. Ou, Q.D., Zhou, L., Li, Y.Q., Shen, S., Chen, J.D., Li, C., Wang, Q.K., Lee, S.T., Tang, J.X., 2014. Extremely efficient white organic light-emitting diodes for general lighting. Adv. Funct. Mater. 24, 7249–7256.

Nanostructures for Plasmonic Effects in Solar Cells and LEDs   525 Ozbay, E., 2006. Plasmonics: merging photonics and electronics at nanoscale dimensions. Science 311, 189–193. Pala, R.A., White, J., Barnard, E., Liu, J., Brongersma, M.L., 2009. Design of plasmonic thin-film solar cells with broadband absorption enhancements. Adv. Mater. 21, 3504–3509. Pan, L., Park, Y., Xiong, Y., Ulin-Avila, E., Wang, Y., Zeng, L., Xiong, S.M., Rho, J., Sun, C., Bogy, D.B., Zhang, X., 2011. Maskless plasmonic lithography at 22 nm resolution. Sci. Rep. 1, 175. Park, M., 1997. Block copolymer lithography: periodic arrays of ~1011 holes in 1 square centimeter. Science 276, 1401–1404. Parsons, J., Burrows, C.P., Sambles, J.R., Barnes, W.L., 2010. A comparison of techniques used to simulate the scattering of electromagnetic radiation by metallic nanostructures. J. Mod. Opt. 57, 356–365. Pines, D., 1956. Collective energy losses in solids. Rev. Mod. Phys. 28, 184–198. Prodan, E., Radloff, C., Halas, N.J., Nordlander, P., 2003. A hybridization model for the plasmon response of complex nanostructures. Science 302, 419–422. Qiao, W., Huang, W.B., Liu, Y.H., Li, X.M., Chen, L.S., Tang, J.X., 2016. Toward scalable flexible nanomanufacturing for photonic structures and devices. Adv. Mater. 28, 10353–10380. Qu, D., Lu, F., Yu, J.F., Xie, W.L., Xu, Q., Li, X.D., Huang, Y.D., 2011. Plasmonic core-shell gold nanoparticle enhanced optical absorption in photovoltaic devices. Appl. Phys. Lett. 98, 113119. Ritchie, R.H., 1957. Plasma losses by fast electrons in thin films. Phys. Rev. 106, 874–881. Rockstuhl, C., Lederer, F., 2009. Photon management by metallic nanodiscs in thin film solar cells. Appl. Phys. Lett. 94, 213102. Rycenga, M., Cobley, C.M., Zeng, J., Li, W.Y., Moran, C.H., Zhang, Q., Qin, D., Xia, Y.N., 2011. Controlling the synthesis and assembly of silver nanostructures for plasmonic applications. Chem. Rev. 111, 3669–3712. Salvador, M., Macleod, B.A., Hess, A., Kulkarni, A.P., Munechika, K., Chen, J.I.L., Ginger, D.S., 2012. Electron accumulation on metal nanoparticles in plasmon-enhanced organic solar cells. ACS Nano 6, 10024–10032. Sommerfeld, A., 1899. Ueber die Fortpflanzung elektrodynamischer Wellen längs eines Drahtes. Ann. Phys. Chem. 303, 233–290. Stoykovich, M.P., Kang, H.M., Daoulas, K.C., Liu, G.L., Liu, C.C., De Pablo, J.J., Muller, M., Nealey, P.F., 2007. Directed self-assembly of block copolymers for nanolithography: fabrication of isolated features and essential integrated circuit geometries. ACS Nano 1, 168–175. Stratakis, E., Kymakis, E., 2013. Nanoparticle-based plasmonic organic photovoltaic devices. Mater. Today 16, 133–146. Sung, J.H., Kim, B.S., Choi, C.H., Lee, M.W., Lee, S.G., Park, S.G., Lee, E.H., Beom-Hoan, O., 2009. Enhanced luminescence of GaN-based light-emitting diode with a localized surface plasmon resonance. Microelectron. Eng. 86, 1120–1123. Tanaka, T., Totoki, Y., Fujiki, A., Zettsu, N., Miyake, Y., Akai-Kasaya, M., Saito, A., Ogawa, T., Kuwahara, Y., 2011. Enhanced red-light emission by local plasmon coupling of Au nanorods in an organic light-emitting diode. Appl. Phys. Express 4, 032105. Uddin, A., Yang, X.H., 2014. Surface plasmonic effects on organic solar cells. J. Nanosci. Nanotechnol. 14, 1099–1119. Van Lare, C., Yin, G.C., Polman, A., Schmid, M., 2015. Light coupling and trapping in ultrathin Cu(In,Ga)Se2 solar cells using dielectric scattering patterns. ACS Nano 9, 9603–9613. Wan, W.Q., Huang, W.B., Pu, D.L., Qiao, W., Ye, Y., Wei, G.J., Fang, Z.B., Zhou, X.H., Chen, L.S., 2015. High performance organic distributed Bragg reflector lasers fabricated by dot matrix holography. Opt. Express 23, 31926–31935. Wang, W., Wu, S.M., Reinhardt, K., Lu, Y.L., Chen, S.C., 2010. Broadband light absorption enhancement in thinfilm silicon solar cells. Nano Lett. 10, 2012–2018. Wang, D.H., Kim, D.Y., Choi, K.W., Seo, J.H., Im, S.H., Park, J.H., Park, O.O., Heeger, A.J., 2011a. Enhancement of donor-acceptor polymer bulk heterojunction solar cell power conversion efficiencies by addition of Au nanoparticles. Angew. Chem. Int. Ed. Eng. 50, 5519–5523. Wang, D.H., Park, K.H., Seo, J.H., Seifter, J., Jeon, J.H., Kim, J.K., Park, J.H., Park, O.O., Heeger, A.J., 2011b. Enhanced power conversion efficiency in PCDTBT/PC70BM bulk heterojunction photovoltaic devices with embedded silver nanoparticle clusters. Adv. Energy Mater. 1, 766–770.

526  Chapter 13 Wang, C.C.D., Choy, W.C.H., Duan, C., Fung, D.D.S., Sha, W.E.I., Xie, F.X., Huang, F., Cao, Y., 2012. Optical and electrical effects of gold nanoparticles in the active layer of polymer solar cells. J. Mater. Chem. 22, 1206–1211. Wood XLII, R.W., 1902. On a remarkable case of uneven distribution of light in a diffraction grating spectrum. Philos. Mag. Ser. 6 (4), 396–402. Wu, B., Oo, T.Z., Li, X.L., Liu, X.F., Wu, X.Y., Yeow, E.K.L., Fan, H.J., Mathews, N., Sum, T.C., 2012. Efficiency enhancement in bulk-heterojunction solar cells integrated with large-area Ag nanotriangle arrays. J. Phys. Chem. C 116, 14820–14825. Xi, C.X., Marina, P.F., Xia, H.B., Wang, D.Y., 2015. Directed self-assembly of gold nanoparticles into plasmonic chains. Soft Matter 11, 4562–4571. Xia, Y.N., Halas, N.J., 2005. Shape-controlled synthesis and surface plasmonic properties of metallic nanostructures. MRS Bull. 30, 338–344. Xia, Y.N., Whitesides, G.M., 1998. Soft lithography. Annu. Rev. Mater. Sci. 28, 153–184. Xiao, Y., Yang, J.P., Cheng, P.P., Zhu, J.J., Xu, Z.Q., Deng, Y.H., Lee, S.T., Li, Y.Q., Tang, J.X., 2012. Surface plasmon-enhanced electroluminescence in organic light-emitting diodes incorporating Au nanoparticles. Appl. Phys. Lett. 100, 013308. Xu, X.X., Kyaw, A.K.K., Peng, B., Zhao, D.W., Wong, T.K.S., Xiong, Q.H., Sun, X.W., Heeger, A.J., 2013. A plasmonically enhanced polymer solar cell with gold–silica core–shell nanorods. Org. Electron. 14, 2360–2368. Xu, R.P., Li, Y.Q., Tang, J.X., 2016. Recent advances in flexible organic light-emitting diodes. J. Mater. Chem. C 4, 9116–9142. Yamada, M., Mitani, T., Narukawa, Y., Shioji, S., Niki, I., Sonobe, S., Deguchi, K., Sano, M., Mukai, T., 2002. InGaN-based near-ultraviolet and blue-light-emitting diodes with high external quantum efficiency using a patterned sapphire substrate and a mesh electrode. Jpn. J. Appl. Phys. 41, L1431–L1433. Yao, K., Salvador, M., Chueh, C.C., Xin, X.K., Xu, Y.X., deQuilettes, D.W., Hu, T., Chen, Y.W., Ginger, D.S., Jen, A.K.Y., 2014. A general route to enhance polymer solar cell performance using plasmonic nanoprisms. Adv. Energy Mater. 4, 1400206. You, J.B., Li, X.H., Xie, F.X., Sha, W.E.I., Kwong, J.H.W., Li, G., Choy, W.C.H., Yang, Y., 2012. Surface plasmon and scattering-enhanced low-bandgap polymer solar cell by a metal grating back electrode. Adv. Energy Mater. 2, 1203–1207. Zayats, A.V., Smolyaninov, I.I., 2003. Near-field photonics: surface plasmon polaritons and localized surface plasmons. J. Opt. A Pure Appl. Opt. 5, S16–S50. Zenneck, J., 1907. Über die Fortpflanzung ebener elektromagnetischer Wellen längs einer ebenen Leiterfläche und ihre Beziehung zur drahtlosen Telegraphie. Ann. Phys. 328, 846–866. Zhou, L., Ou, Q.D., Chen, J.D., Shen, S., Tang, J.X., Li, Y.Q., Lee, S.T., 2014. Light manipulation for organic optoelectronics using bio-inspired moth’s eye nanostructures. Sci. Rep. 4, 4040. Zhou, L., Ou, Q.D., Li, Y.Q., Xiang, H.Y., Xu, L.H., Chen, J.D., Li, C., Shen, S., Lee, S.T., Tang, J.X., 2015. Efficiently releasing the trapped energy flow in white organic light-emitting diodes with multifunctional nanofunnel arrays. Adv. Funct. Mater. 25, 2660–2668.