Plasmonic Solar Cells: From Rational Design to Mechanism Overview

Review pubs.acs.org/CR

Plasmonic Solar Cells: From Rational Design to Mechanism Overview Yoon Hee Jang,†,‡,⊥ Yu Jin Jang,†,⊥ Seokhyoung Kim,†,§ Li Na Quan,†,∥ Kyungwha Chung,† and Dong Ha Kim*,† †

Department of Chemistry and Nano Science, School of Natural Sciences, Ewha Womans University, 52, Ewhayeodae-gil, Seodaemun-gu, Seoul 03760, Republic of Korea ABSTRACT: Plasmonic effects have been proposed as a solution to overcome the limited light absorption in thin-film photovoltaic devices, and various types of plasmonic solar cells have been developed. This review provides a comprehensive overview of the state-of-the-art progress on the design and fabrication of plasmonic solar cells and their enhancement mechanism. The working principle is first addressed in terms of the combined effects of plasmon decay, scattering, near-field enhancement, and plasmonic energy transfer, including direct hot electron transfer and resonant energy transfer. Then, we summarize recent developments for various types of plasmonic solar cells based on silicon, dye-sensitized, organic photovoltaic, and other types of solar cells, including quantum dot and perovskite variants. We also address several issues regarding the limitations of plasmonic nanostructures, including their electrical, chemical, and physical stability, charge recombination, narrowband absorption, and high cost. Next, we propose a few potentially useful approaches that can improve the performance of plasmonic cells, such as the inclusion of graphene plasmonics, plasmon-upconversion coupling, and coupling between fluorescence resonance energy transfer and plasmon resonance energy transfer. This review is concluded with remarks on future prospects for plasmonic solar cell use.

CONTENTS 1. Introduction 2. Plasmonic Enhancement Mechanisms of Photovoltaic Cells 2.1. Plasmon Decay 2.2. Scattering Effect 2.3. Near-Field Enhancement 2.3.1. Near-Field Enhancement by SPP 2.3.2. Near-Field Enhancement by LSPR 2.4. Transfer Mechanisms 2.4.1. Hot Carrier Generation and Transfer 2.4.2. Resonant Energy Transfer 3. Plasmonic Effects in Various Photovoltaic Devices 3.1. Plasmonic Effects in Si Solar Cells 3.1.1. Theoretical Efforts 3.1.2. Experimental Efforts 3.2. Plasmonic Effects in DSSCs 3.3. Plasmonic Effects in Organic Photovoltaics (OPVs) 3.3.1. General Concept of OPVs 3.3.2. Principal Consideration in Designing High-Performance Plasmonic OPVs 3.3.3. Progress and Research Trends in Plasmonic OPVs 3.4. Plasmonic Effects in Other Types of Solar Cells 4. Critical Issues in Plasmonic Photovoltaics 4.1. Electrical and Chemical Stability 4.2. Balanced Light Absorption 5. Prospective Approaches © 2016 American Chemical Society

5.1. Non-Noble Metal Based Plasmonics 5.2. Graphene Plasmonics 5.3. Plasmon-Upconversion Coupling 5.4. Plasmon-Enhanced FRET Solar Cells 6. Concluding Remarks and Outlook Author Information Corresponding Author ORCID Present Addresses Author Contributions Notes Biographies Acknowledgments References

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1. INTRODUCTION Surface plasmons (SPs) were first defined nearly 60 years ago, and researchers are making use of recent advances in nanoscience and nanotechnology to develop applications that take advantage of the unexplored uses for plasmonics. As such, leading scientists have extensively discussed the fundamental principles and general concepts of plasmonics in numerous articles1−10 and in special issues in top-tier journals.11−14 To mention only a few representative applications, plasmonics have been demonstrated for use in the fields of sensing,15−20 light-emitting materials and devices,21−24 photo-

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Received: May 11, 2016 Published: December 7, 2016 14982

DOI: 10.1021/acs.chemrev.6b00302 Chem. Rev. 2016, 116, 14982−15034

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detectors,25−27 plasmofluidics,28 photocatalysis, including plasmonic water splitting,29−32 and photovoltaics. However, the latent applicability of plasmonics is not limited to these typical areas, and one can find recent reviews on their applicability in more cases over a wider range of subject areas.33−37 In particular, plasmonics can contribute to further developments in solar-to-electric energy conversion technology (i.e., plasmonic solar cells), and recent reviews have addressed the stateof-the-art progress of this topic.32,38−45 However, none of the previous reviews have provided a comprehensive overview of the various types of plasmonic photovoltaic devices, supporting mechanisms to improve device performance, inherent limitations that are yet to be resolved, and future prospects to produce viable and stable cells−all in one single article. To this end, we first briefly categorize the representative plasmonic effects and review current achievements for different types of photovoltaic devices, ranging from conventional silicon (Si) solar cells to dye-sensitized solar cells (DSSCs), organic photovoltaics (OPVs), and perovskite solar cells. We then address critical issues that must be resolved to obtain viable, reproducible high-performance plasmonic solar cells, and we finally suggest a few alternatives to maximize their efficiency.

read some articles focusing on this issue.30,32,53−56 The representative interpretations and elements for plasmonic effects have been addressed in numerous articles, and we discuss the key features of the relevant topics. 2.1. Plasmon Decay

There are seemingly inconsistent perspectives on the role of plasmonic materials in solar cells. Previous attempts to improve the performance of photovoltaics using plasmonic nanostructures have resulted in a decrease in the efficiency of thin-film silicon solar cells, and in some cases, nonradiative plasmon damping has been determined to be a form of parasitic absorption of light and as that has to be mitigated.57,58 On the other hand, several other efforts have exploited nonradiative damping to generate plasmonic hot carriers that are directly used for charge collection.31,59−61 To obtain a consistent and comprehensive understanding of the plasmonic effect, it is important to pay attention to the dynamics and decay processes of plasmons. Once SPs are excited through resonance with incident light, plasmon decay (plasmon damping or plasmon loss largely represent the same phenomenon) arises either radiatively by emitting photons (farfield scattering/radiative damping) or nonradiatively (nearfield/absorption/exciton generation).62 The size, composition, resonant frequency, and location in the device of the plasmonic nanostructures can be readily designed by considering two factors, the dephasing time (T2) and decay modes, depending on the purpose for which the plasmonic effect has been expected in the given photovoltaic devices. Conversely, the enhancement mechanism for fabricated plasmonic structures can also be anticipated. T2 is the time scale for the oscillation and decay of an electron cloud dipole to the decoherence,

2. PLASMONIC ENHANCEMENT MECHANISMS OF PHOTOVOLTAIC CELLS The photovoltaic community has extensively studied the lightmatter interaction in metallic nanostructures in order to improve photon absorption and thus improve the photon-toelectron conversion efficiency.4,39,46 Surface plasmons are collective oscillations of free electrons in metallic materials at the interface between a metal and its surrounding dielectric medium. Plasmonic enhancement is achieved through various mechanisms falling under the scope of far-field scattering (section 2.2), near-field enhancement (section 2.3), and charge carrier or resonant energy transfer (section 2.4). Far-field scattering increases the absorption efficiency by decreasing reflections at the illuminated surface and by increasing the length of the optical paths of the incident photons in the absorbing medium.47,48 Plasmonic nanomaterials can also confine electromagnetic waves at the metal-dielectric interface and produce high-intensity near-fields, resulting in an increase in the number of absorption events compared to their bare counterparts.4 In addition, plasmonic absorption generates free carriers in the plasmonic nanostructures,49 and these free carriers can be either injected directly into nearby semiconductors50,51 or transfer their energy via resonant energy transfer.52 In this section, each of these plasmonic enhancement mechanisms is individually discussed. Interplay of these mechanisms depends mainly on the size and shape of the plasmonic nanostructures as well as on the way in which they are incorporated into solar cell devices. Although careful scrutiny of each of these effects gives us a better understanding and an ability to build devices with more creative designs, it should be noted that more than one of these effects take place at the same time rather than occurring alone, which in turn makes it harder to interpret experimental observations. The plasmonic enhancement in photovoltaic devices is realized via the cooperative interaction between metallic nanostructures and the active layers of a semiconductor. Thus, it is crucial to understand the nature and dynamics of metal−semiconductor interfaces, and it is recommended to

T2 = 2ℏ/Γhom

(1)

where Γhom is the homogeneous line width of the SP resonance, and the relation between the plasmon lifetime τsp and T2 is T2 = 2τsp.63,64 Thus, a shorter T2 corresponds to faster plasmon decay and a higher homogeneous line width. For surface plasmon polaritons (SPPs), only nonradiative damping can occur while both radiative and nonradiative decay competitively occur for nanostructures with localized surface plasmon resonance (LSPR) properties. Such behavior can be described using eq 2,

Γhom = Γrad + Γnonrad

(2)

where Γrad and Γnonrad are the line width contributed from the radiative and nonradiative decay, respectively.65,66 The radiative decay rate is closely related to the number of electrons in the particles (i.e., metal particles with a larger size result in more light scattering). The optical properties of the materials and the upper limit of the line width from radiative decay can be calculated using eq 3,65 Γrad ≤

2 4π 2V ((ϵ1 − 1) − ϵ2) Γnonrad 3 ϵ2 3λ

(3)

where V is the volume of the structure, λ is the wavelength of the light, ϵ1 and ϵ2 are the real and imaginary parts of the complex dielectric constant of the metal. The nonradiative decay is dependent only on the optical properties of the materials, as described with eq 4,62,65 14983


Chemical Reviews Γnonrad ≈

2ϵ2 ϵ1′

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(4)

where ϵ1′ is the first derivative of the real part of the dielectric constant of metal. Nonradiatively dissipated plasmon energy can create hot carriers via Landau damping (intra- and interband transition of electrons in metal due to the electric field induced by the plasmon resonance) within a time scale of a femtosecond (1 to 100 fs) with a higher energy than that obtained through thermal excitation. This issue will be further discussed in section 2.4.1 along with related applications in photovoltaics and the corresponding transfer process into semiconducting materials.67 After Landau damping, hot carriers can be further relaxed close to the thermal energy with a continuous distribution in the energy level via electron−electron scattering on a time scale from 100 fs to 1 ps. The hot carriers then nearly reach the Fermi level via electron−phonon collisions, emitting heat into their surroundings in the time scale ranging from 1 ps to 10 ns. Although the heat emission has potential applications in the biomedical field, such as in photothermal therapy for cancer treatment or targeted drug delivery,68−70 such behavior is not desirable in photovoltaic applications. Instead, hot carrier transfer should precede thermal relaxation in order to minimize the loss of photon energy into thermal dissipation. Furthermore, if the given plasmonic nanostructures are to be embedded in a material with an energy level that does not match that of the metal nanostructures, such that cascade energy transfer becomes impossible, plasmon-induced hot carriers will eventually be damped to heat, so the device will have to be designed to maximize the scattering of incident light. In any case, either radiative or nonradiative plasmon damping is an inevitable consequence, by which, photons can either travel longer distances in solar cell devices or can be converted into useful species such as hot electrons/holes and used to induce a near-field.

Figure 1. (A) Light trapping effect between the plasmonic nanostructures and (B) simulated electric field profile for scattering using 642.4 nm-wavelength incident light. Reprinted with permission from ref 80. Copyright 2013 The Optical Society of America.

the scattering direction, multiple scattering events are expected to improve the apparent quantum efficiency. In a particular case, by using scattering from both sides of the active layer, unabsorbed photons can be reflected back and forth repeatedly within the active layer until they are absorbed.77,78 Due to the nature of absorption enhancement through scattering, the scattering effect is often referred to as “light trapping” or as a “far-field effect”.79 Special care is necessary to consider the use of plasmonic nanoparticles for the scattering effect. In practice, the plasmonic resonance modes of a nanoparticle are often evaluated according to their optical transmittance in solution. In general, a sudden drop in transmittance is considered as an indication of either scattering or parasitic absorption by the nanoparticles, and the latter does not contribute to light trapping81 but rather leads to an optical loss because light is absorbed by the particle. One should make sure that plasmonic nanoparticles mainly scatter photons but absorb little light to maximize the light trapping effect. 2.3. Near-Field Enhancement

An electromagnetic field with a more intense energy than the incident photon energy and confined at the surface of plasmonic structures is generally referred to as a “near-field”. It can improve the photon-to-electron conversion efficiency by increasing the light absorption and exciton-generation. 2.3.1. Near-Field Enhancement by SPP. On a planar metal film, the collective oscillation of surface electrons creates a strong, oscillating electric near-field that propagates along the surface (Figure 2A). These propagating surface electromagnetic waves are referred to as surface plasmon polaritons (SPPs) or as propagating surface plasmons (PSPs).82 The concentrated field extends into both metal and dielectric layers where the dielectric portion produces the photovoltaic enhancement. The efficient excitation of SPP modes has been achieved by implementing three-dimensional structures, such as grooves,82 holes,83 islands,72,84 or periodic grating85−88 on a planar film due to the mismatch of SPP momentum with that of light (Figure 2C). The absorption is improved through the strong propagation of the near-fields at the surface, and this is particularly useful for thin-film solar cells, for which complete absorption is hardly expected. Since a contribution from the nanostructure-induced scattering is likely to accompany as described in section 2.1, a careful effort for understanding the origins of these different contributions is required, and computational methods often provide useful insight into these different behaviors. For instance, Brongersma and colleagues studied the improvement in absorption by implementing a metal strip array on a Si solar cell and

2.2. Scattering Effect

As was described in the previous section, plasmonic nanoparticles efficiently scatter incident photons, and the scattering characteristics largely depend on the particle geometry,3,71 as has been readily observed for nanoparticles with a relatively large size greater than 50 nm. The idea of using scattering effects to improve the photovoltaic efficiency stems from the trade-off between maximizing the incident power density by illuminating light normal to the surface and the consequently insufficient absorption due to the loss as transmission especially in thin-film devices.72 Once scattered on the front surface, photons travel a longer distance than the thickness of the active layer, which the photons would otherwise penetrate (Figure 1). In the case of spherical nanoparticles, relatively smaller particles are known to result in more scattering in the forward direction while larger particles scatter more in the backward direction (as a reflection).73 One could purposefully determine the scattering direction by changing the size of the plasmonic nanoparticles to correspond with their intended location in the device. For example, small nanoparticles can be placed on top of the active layer to kink the optical path of the incoming photons by an acute angle (50 nm) • SiO2 shell: Jsc enhancement by plasmonic effect (near-field) • TiO2 shell: Voc enhancement by electron charging effect • synergic effects of enhancing the light absorption and accelerating the energy transfer from one dye to another • plasmon-induced charge excitation and separation

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• computational modeling of the systematic design of plasmon-enhanced DSSCs (plasmon-enhanced light absorption)

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effects • enhanced optical absorption by Ag plasmon resonance effect • acting as the Schottky barrier reducing the back transfer of electrons • enhanced dye charge carrier generation rate by polarization-dependent resonance frequency • optical absorption of visible light due to SPR • injection of electrons from Au NP into ZnO and simultaneous tunneling of electrons from dye into ZnO • blocked reverse reaction by interfacial ZnO-Au Schottky barrier • photoexcited electron transfer from Au (or Ag) to TiO2 • obstructed charge recombination by Schottky barrier established in the TiO2/Au (or Ag) • smaller dark current for TiO2/Au than TiO2/Ag due to bigger barrier of Schottky • examination of distance dependence between dye and Ag • pronounced plasmon-enhanced extinction of the dye in the sample with the thinnest layer of amorphous TiO2 (2.0 nm) • requirement of a pinhole-free TiO2 layer with minimum thickness 7.7 nm for Ag protection from corrosion • enhancement in photoabsorption by increase in electromagnetic field strength

performance

increased photoresponse in the visible region enhanced PCE from 0.26% up to 0.95% for water-based DSSC LSPR-enhanced photoconductivity (current) improved PCE from 0.7% to 1.2% for hybrid Au NP-coated ZnO NR DSSCs

plasmonic structures

Ag-island film Au nanoparticle (NP) elliptical Au disk Au NP

Table 1. Summary of Plasmonic DSSCs

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4-fold enhancement in cell efficiency for DSSC with Au-TiO2 NW as photoanode

increased photoelectric conversion of Au NP layers as the intensity of the SPR increases 40.3% increase of PCE for virus-templated Au@TiO2 DSSC

enhancement of 97 mV in Voc, 63% in Jsc, and 84% in PCE

13% enhancement in PCE by doping photoanode with different sizes of Au NP

36% improvement in PCE (mostly due to the increase in Jsc) for quasi-ss-DSSC

increased PCE by 28.87% with 0.8 wt% of plasmonic component

53% PCE improvement caused by a 57% Jsc enhancement for DSSC with syzygium-Ag-TiO2 photoanode PCE of DSSC with Au NP, 6.77%; short Au NR, 7.08%; long Au NR, 7.29%; and control, 6.21%

Au-TiO2 NW

layer-by-layer Au NP

Au NP inlaid TiO2

Au NP

Au@SiO2 NP

Ag@TiO2 nanocomposite

biomass coated Ag-TiO2 composite Au NP or Au NR

14993

enhancement of 23% in photocurrent after adding Au NP (0.3 wt%) in ss-DSSC using perovskite CsSnI3 as the hole transporter optimized conversion efficiency (5.85%) and Jsc (13.04 mA/cm2) at high concentration of Ag-ion implantations

Au NP

higher PCE of 6.9% for Nb-doped TiO2/Ag than neat TiO2 (4.7%) and Nbdoped TiO2 (5.4%) enhanced efficiency 4.86% for DSSC assembled with the Au/TiO2-photoanode compared to bare TiO2 (2.57%) photoanode

14% and 10% increase in PCE for DSSC with 0.5 wt% of SiO2@TiO2@Au and SiO2@Au@TiO2 in photoanode, respectively

Nb-doped TiO2/Ag ternary nanostructures Ag/TiO2 nanocomposite

SiO2@TiO2@Au or SiO2@Au@TiO2 sphere

Ag-ion implantation

11% improvement of PCE and 17% improvement in Jsc

PCE of 8.33% for ionic liquid-based DSSC with synergistic configuration of Ag NP

increase in efficiency from 2.81% to 5.52%

Au NP

aggregated Au@SiO2@TiO2 Ag NP

PCE enhancement by 16% from 5.26% to 6.09%

Au−Ag alloy popcornshaped NP coupled Au NPs

maximum photocurrent enhancement factor 4.7

increased PCE from 8.3% to 10.8%

TiO2@Au@TiO2 NP

Au NP

performance

4.6% increase compared to the hierarchical TiO2 sphere-based DSSC w/o Au NP


hierarchical TiO2 sphere/ Au NP

Table 1. continued effects

• • • •

improved optical absorption of the dye by plasmonic scattering to trap the light retarded the charge recombination with Ag acting as an electron sink rapid interfacial charge transfer by formation of Schottky barrier scattering effect by the core−shell sphere

strong far-field scattering in the electrolyte for small Ag NP efficient near-field effect in the photoanode for large Ag NP enhanced optical absorption of dye as a result of the intensified electromagnetic field long-term stability of plasmonic structures via cobalt redox mediator reduced transport resistance and recombination process by accelerated charge transport under the Au NP’s plasmon near-field • decreased charge-transfer resistance • enhancement of dye adsorption • reduced recombination and positive shift of conduction band edge of TiO2 by Ag-doping induced an impurity level • improved electron transfer properties and plasmon-enhanced light absorption

• • • • •

increased conductivity and reduced recombination of charges strong SPR effect of long Au NR at longer wavelength than short Au NR or Au NP broadband balanced light harvesting and panchromatic solar energy conversion plasmon-enhanced photoabsorption in the less harvested region of solar spectrum broadband optical absorption enhancement by simultaneous excitation at different LSPR wavelengths • photocurrent enhancement in the long-wavelength region by increased electric field intensity due to a plasmon coupling effect for larger NP • broadband enhancement of dye absorption by coupled plasmonic system

• • • • •

effective dye excitation by localized electric fields enhanced electron transport due to conducting pathway increased the light absorption by scattering discussion of the charge separation and photocurrent formation phenomenon and the relationship between SPR and photocurrent formation (plasmon-sensitized solar cell) • increased dye absorption by near-field effect • reduced photoanode thickness for better electron collection • increased dye absorption by local-field optical enhancement • Voc enhancement by modulated quasi-Fermi level and reduced charge recombination rate • co-contribution of the photocharging effect, plasmonic effect, and scattering effect of Au NP and their dependence on Au NP sizes • enhanced light-harvesting efficiency (LHE) of dye without changing the electron lifetime and diffusion coefficient • enhanced light harvesting due to scattering, plasmonic near-field, and SPP at the interface of Ag/ TiO2 • higher surface area, strong light scattering, and efficient electron transport

• • • •

• synergistic effect of SPR with constructed TiO2 nanostructures

ref

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2013124

2013144

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34% improvement in DSSC performance

15% increase in PCE by introducing 0.05% Au@SiO2 nanoprisms

24% enhancement with PCE 9.7%

optimal performance of 8.43% (40% higher than w/o Au@Ag NR)

increased PEC and Jsc by 20.8% and 29.9% highest efficiency of 8.4% for Au@SiO2@TNS double shell-based ss-DSSC

Au@SiO2 nanocubes

Au@SiO2 triangular nanoprisms

Au@Ag decorated TiO2 hollow NP

Au@Ag NR

Au−TiO2−Ag film Au@SiO2@TiO2 nanosheet 1D HeteroNT Ag NW@TiO2

photoelectric conversion efficiency of 23.42% under monochromatic light (486.7 nm, 10 mW/cm2),

increased PCE by ∼20% from 7.1% to 8.4% for N179 cells and by ∼30% from 3.9% to 5.0% for N749 cells

26% enhancement in device performance with optimized particle density (0.44 wt %) (7.51% PCE relative to 5.97% for reference DSSCs)

increased PCE from 7.39% to 9.12% for DSSCs based on organic liquid electrolyte PCE of 10.49% with 7.9% enhancement compared with pure TiO2 microspherebased DSSCs

Au@Ag@SiO2 NPs

Au nanostars

Au/Ag/SiO2 core/shell nanostructures

mixture of AuNSs@SiO2 and AuNRs@SiO2 Au-TiO2 microspheres

PCE improvement from 4.68% to 5.31% (in an optimum content of 3 wt% in photoanode)

13.3% increase in PCE and 75% decrease in scattering layer thickness

performance

Au@TiO2 NR


Table 1. continued effects

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• improved light trapping by far-field coupling of scattered light • plasmon resonance energy transfer (PRET) from Au/Ag core−shell nanostructures to TiO2 (demonstrated by transient absorption spectroscopy study) • broadband light-harvesting at both low- and long wavelengths • efficient dye excitation by localized electric fields • inhibited electron recombination, resulting in slightly increased Voc

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2014145

ref

• reduced TiO2/dye/electrolyte interfacial charge transfer impedance: facial electron transfer • enhanced light harvesting by SPR and light scattering effects of NW • Ag shell thickness effect: broaden and improved LSPR • increase of light absorption and decrease of recombination photo carrier • enhanced light absorption in the NIR region (500 nm ∼1000 nm) and thus the generation of more charge carriers • decreased charge recombination resistance • increased light harvesting enabled by the intense near-field interaction

• increasing the spectral overlap with the absorption spectrum of dye after morphological charges at high temperature • enhancement of the light harvesting by strong and broadened LSPR (interaction between Au core and Ag shell) • sufficient dye loading and effective optical scattering by larger surface area of TiO2 hollow structures • enhanced light-harvesting by broadened absorption in the red and near-infrared (NIR) region • promoted suppression of charge recombination • improved light harvesting by plasmonic cooperation effect of Au and Ag • promoted charge transfer, electron transport, and plasmon-assisted light harvesting by presence of electromagnetic field

increased dye excitation through near-field enhancement generation of additional photocurrent by plasmon-induced hot electron generation increase in the optical path by introduction of Au@TiO2 plasmonic scattering layer reduction of the back reaction probability of electrons due to thinner anode increased plasmonic molecular coupling by intense electromagnetic fields at the edges and corners of nanocubes • panchromatic enhancement of the light-harvesting efficiency

• • • • •



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photocurrent generation.100,138 Bath et al. suggested possible plasmon-induced charge separation mechanisms on the basis of hot charge carrier transfer from the metal into the TiO2 conduction band or hole transport material (Figure 15C).138 Su et al. discussed the relationship between the SPR intensity and photovoltaic parameters.100 An increase in the SPR intensity (i.e., a stronger electric field) resulted in an increase in the photocurrent, photovoltage, and solar conversion efficiency. Earlier studies on plasmonic DSSCs mostly used single metal nanoparticles with spherical shapes, and plasmonic nanostructures of various sizes, shapes, and compositions have been applied since then to further increase the efficiency of solar cells. The dependence of the PCE on the size of metal nanoparticles in the DSSCs has been investigated.139−142 Liu et al. suggested Au nanoparticles had multiple plasmonic effects depending on their size.141 For smaller Au nanoparticles (∼5 nm), the photocharging effect was the dominant factor that increased the photovoltage. The plasmon-enhanced light absorption of the dye has been mainly attributed to the strong interaction between Au nanoparticles (∼45 nm) and dye and thus to an increase in photocurrent. The improvement in Jsc for 110 nm Au-TiO2 DSSCs was a result of superior light scattering at a long wavelength. However, there are conflicting reports on the effect of the Au nanoparticle size. Park et al. reported that the field enhancement of 100 nm diameter Au nanoparticles dominantly influenced the device performance rather than the light scattering effect.139 Sindhu et al. argued that for a smaller particle size, the Jsc value was less than that of the original value (without Au) due to the destructive interference.140 In this regard, there is room for further investigation on the effect according to size. Plasmonic nanostructures of various shapes, including NRs, nanocubes, nanoprisms, and nanostars, have been designed and incorporated into DSSCs to efficiently harvest low-energy sunlight as well as to intensify the electromagnetic field.51,143−147 When Ag2S-encapsulated plasmonic Au NRs were introduced into the photoanode, the photocurrent was significantly enhanced (enhancement ratio ∼37%) in the wavelength region from 600 to 720 nm due to the longitudinal SPR-enhanced light trapping.143 Another study compared the performance of DSSCs with different types of Au nanostructures [Au nanosphere (NS) and Au NR].144 The DSSCs with long Au NRs showed the best performance, which could be attributed to the strong SPR in the longer wavelength region. Shape-controlled Au nanocubes with a thin SiO2 layer were integrated into the DSSC photoanode, and the ∼34% improvement in PCE was attributed to both intense electromagnetic fields at the edges and corners of the nanocubes (Figure 16).51 Although plasmonic nanostructures with sharper features have stronger plasmonic effects than their spherical analogues, morphological changes to round or aggregated forms are often observed during device fabrication. The Scott and Kelly group examined the thermal stability of Au@SiO2 triangular nanoprisms by controlling their annealing temperature.146 At a temperature above 250 °C, the change in morphology and blue shift in LSPR were observed, which resulted in an increase in the spectral overlap with the absorption band of the dye and PCE. In this respect, broadband-balanced light harvesting with a spectral overlap is one of the most important factors to improve the solar cell performance. The integration of plasmonic

Figure 13. Distance dependence of the plasmon-enhanced photocurrent: (A) Configuration of DSSC containing Ag nanoparticles isolated by a TiO2 spacer. (B) IPCEs of cells containing dye only, Ag only, and both dye and Ag nanoparticles separated by an amorphous TiO2 layer (2.0 nm). (C) The evolution of IPCEs of the cells with dye and Ag nanoparticles according to an increase in the TiO2 thickness (direction of arrow). The top and bottom four curves represent an amorphous and anatase TiO2 layer, respectively. Reprinted from ref 127. Copyright 2009 American Chemical Society.

Figure 14. Proper protection of the metal nanoparticles and plasmonenhancement mechanisms. (A) Improvement in the charge collection by decreasing the thickness of the photoanode, (B) reduced recombination and back reaction of the electrons on the Ag surface by introducing a thin TiO2 shell. Reprinted with permission from ref 134. Copyright 2011 American Chemical Society. (C) Increase in photocurrent and photovoltage due to the plasmonic effect of Au@ SiO2 and the electron charging effect of Au@TiO2, respectively. (D) Illustration of electron equilibration and apparent Fermi level shift according to the metal oxide capping layer. Reprinted from ref 135. Copyright 2012 American Chemical Society.

photovoltaic cell composed of vertically aligned ZnO NR arrays sensitized with Au nanoparticles.137 Plasmon-induced electrons were transferred from the plasmon excited state to the conduction band of ZnO while remaining holes in the Au nanoparticles were compensated by electrolyte donors (Figure 15A). In addition, the performance could be further improved through the simultaneous tunneling of electrons from dye to ZnO (Figure 15B). Recently, solar cells with a plasmonicsensitizer were demonstrated in order to study the plasmoninduced charge separation, which is an essential process for 14995


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Figure 15. Plasmon-induced charge generation and separation: band diagram of the (A) ZnO/Au and (B) ZnO/Au/dye cells. Reprinted from ref 137. Copyright 2009 American Chemical Society. (C) Schematic illustration of self-assembled Au or Ag nanoparticle sensitized solid-state solar cell and proposed plasmon-induced charge separation mechanisms. Reprinted with permission from ref 138. Copyright 2012 Wiley-VCH.

Figure 16. Plasmon-enhancement in DSSC with shape-controlled plasmonic nanostructures: (A) TEM image and (B) calculated/experimentally obtained normalized extinction spectra of Au@SiO2 nanocubes (inset of B: electromagnetic intensity profile, ⟨E2/E02⟩). (C) Schematic representation of DSSC containing Au@SiO2 nanocube-embedded photoanode. (D) Current density and power density curves of the device with and without Au@SiO2 nanocubes. Reprinted from ref 51. Copyright 2014 American Chemical Society.

increase in light harvesting in the DSSCs.93,148−151 Plasmonic cooperation (i.e., plasmon hybridization)152 between Au and Ag produced a strong, broad LSPR behavior, resulting in stronger electric field enhancement. Liu et al. utilized popcornshaped Au−Ag alloy core−shell nanoparticles to further develop the broadband light absorption in DSSCs.93 In this system, different LSPR modes could be simultaneously excited at different parts of irregular popcorn-shaped nanoparticles at different wavelength due to the different size, shape, and proportions of Au and Ag on the popcorn nanoparticles (Figure 17B). Jang et al. fabricated Au@Ag core−shell nanoparticledecorated TiO2 hollow structures and implemented them as potential photoanodes in DSSCs.148 Wu et al. also introduced

components in the DSSCs is a promising method to increase the light absorption of the dye. However, the narrow LSPR band of noble metal nanostructures may limit their broadband light harvesting, and several strategies to overcome this limitation have been suggested. Hammond and co-workers demonstrated panchromatic DSSCs using multiple-core−shell structured oxide-metal-oxide (TiO2−Au−TiO2, TAuT) plasmonic nanoparticles, and the highest efficiency achieved was 10.8%.124 The photoabsorption in the weakly absorbing region (λLo > 600 nm) could be improved by matching the spectra between the LSPR of TAuT structures and the less-harvested region (Figure 17A). Moreover, the effect of the plasmonic interaction between two noble metals can contribute to the 14996


Chemical Reviews

Review

Figure 17. Strategies to achieve broadband-balanced light harvesting: (A) Spectral overlap of N719-dye and three plasmonic core−shell structures (Ag−TiO2, AgT; Au−TiO2, AuT; and TiO2−Au−TiO2, TAT). LSPR-enhanced light harvesting by matching λLSPR of AgT and AuT with λHi of N719. Enhancing photoabsorption of weakly absorbing region of N719 by matching λLSPR of TAuT with λLo of N719. Reprinted from ref 124. Copyright 2013 American Chemical Society. (B) LSPR-field distribution of Au−Ag alloy “popcorn-shaped” nanoparticles. Better light trapping by simultaneously excited LSPR mode arising from various fine structures. Reprinted with permission from ref 93. Copyright 2013 Macmillan Publishers. (C) Illustration of intensified electrical field between assembled Au nanoparticles by interparticle plasmon coupling effect. Reprinted from ref 153. Copyright 2013 American Chemical Society.

Au@Ag NRs into DSSCs to utilize the low-energy range of the solar spectrum by broadening the absorption.149 The coupled plasmon effect by coupled Au nanoparticle assemblies or nanostructured plasmonic aggregates have also been demonstrated to achieve a broadband enhancement of quantum efficiency (Figure 17C).153,154 In the last part of this section, we introduce the plasmon nanoparticle-modified photoanode with various morphologies [nanowire (NW), nanotube (NT), sphere, hollow sphere, and core−shell types], which can offer advantages over conventional nanoparticle-based photoanodes. The NW- or NT-based plasmonic photoanodes offer efficient electron collection as well as plasmon-enhanced light absorption.141,155,156 The Wang and Tang group used hollow Au@TiO2 submicro spheres as the photoanode of DSSCs and achieved a remarkable increase of 30% in the conversion efficiency, which was mainly attributed to the plasmonic effect of the Au nanoparticles and the scattering effect of the hollow structures.157 To systematically demonstrate the effect of the LSPR of a pair of nanostructures with mutually antagonistic morphology, we constructed plasmonic DSSCs utilizing Au−TiO2 core− shell nanostructures as additives in the photoanode.158 Plasmonic Au−TiO2 core−shell nanostructures supported on SiO2 spheres with tailored configurations (SiO2@TiO2@Au and SiO2@Au@TiO2) were embedded into the photoanode in DSSCs to achieve an improved performance. We suggested that the increase in efficiency might be a result of the combined effects of LSPR and scattering (Figure 18). In spite of the vast amount of literature on the subject to date, there is still significant controversy as to the role of the plasmonic nanoparticles in DSSCs, and further in-depth investigations on the enhancement mechanism are still required.

Figure 18. (A) Current density−voltage curves of plasmonic DSSCs with different types of plasmonic core−shell structures. (B) Plasmonenhancement mechanisms: scattering, near-field enhancement, and LSPR-induced direct electron transfer. Reprinted with permission from ref 158. Copyright 2014 Royal Society of Chemistry.

electrons flow from organic semiconductors (electron donors) to fullerene derivatives (electron acceptors) (Figure 19B). (2) Charge transport layers for electrons and holes [electron transport layer (ETL) and hole transport layer (HTL), respectively] that selectively accept one charge carrier type (electrons or holes) from the active layer and facilitate charge extraction to the electrode. LiF, Ca, ZnO, and TiOx are typically employed for the ETL, and MoOx and poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) are representative materials for HTL. (3) Electrodes that finally collect charge carriers. Electrons are transferred to the cathode, and holes are accumulated at the anode. Indium tin oxide (ITO), Al, and Ag are conventionally used as transparent conducting electrodes (TCE) or metal electrodes. When the device is assembled in the order of TCE (bottom electrode)/HTL/active layer/ETL/metal electrode (top electrode), it is referred to as a conventional type. When the device is fabricated in the order of TCE/ETL/active layer/HTL/metal electrode, it is referred to as an inverted type. 3.3.2. Principal Consideration in Designing HighPerformance Plasmonic OPVs. One of the challenges is the trade-off between two goals: minimizing the cell thickness down to the level comparable to the exciton diffusion lengths and maximizing the thickness to provide sufficient photon absorption. Therefore, strategies that concentrate the light supply in a thin polymer layer are necessary to improve the device efficiency. This has been progressively achieved by

3.3. Plasmonic Effects in Organic Photovoltaics (OPVs)

3.3.1. General Concept of OPVs. OPV devices have many advantages over other types of solar cells based on costeffectiveness, flexibility, and facile molecular engineering, which is related to the design of the light absorption property of the organic sensitizers.165 As illustrated in Figure 19A, OPV devices consist of three major parts depending on their function.165,180,181 (1) Active layers are a blend of organic semiconductors and fullerene derivatives. Under illumination, organic semiconductors can generate excitons, and the electrically bound charge carriers are separated at the interface of the two components. Due to the energy level alignment, 14997


Chemical Reviews

Review

Figure 19. Schematic illustration of (A) an OPV device and (B) working mechanism of an OPV device.

plasmonic nanostructures are placed in the device (e.g., the plasmonic nanostructures can be integrated in either the active layer, buffer layer, or in both at the layer interfaces and as bottom or top electrodes) (see the second and fourth column of Table 2). (1) The introduction of plasmonic nanostructures in the active layer is the best approach for near-field enhancement. In that configuration, an intimate contact between the plasmonic nanostructures and sensitizers is intended, and the electric field induced by the plasmonic nanostructures is strong enough to affect the optical properties of the sensitizers. (2) The thickness of the charge transport layers determines a dominant plasmonic contribution. Assuming that the plasmonic nanostructures are deposited with charge transport layers onto bottom electrodes, plasmonic nanoparticles with a size smaller than ∼30 nm are completely covered with the charge transport layer and do not protrude out of the active layer. As a result, they rather affect the electron and hole transfer to the electrodes. Plasmonic nanoparticles of a larger size can be integrated into the charge transport layer, but such a configuration may exert a negative effect on the surface morphology of the active layers due to the increased roughness and contribution to back scattering, which interfere with the light absorption by the active materials. (3) The utilization of plasmonic nanostructures in OPV devices can often be problematic due to the miscibility or recombination issues that were discussed above. In that case, the placement of the plasmonic nanostructures at the interface of the two layers can be a solution. Most of all, this approach is advantageous in that multiple plasmonic effects are exhibited in both layers to be faced with metal nanostructures. (4) On the basis of the metallic characteristics of plasmonic nanostructures, they can be combined with TCE to extract charge carriers in a facile way. Plasmon enhancement in OPV devices are closely correlated with the aforementioned conditions, and it is highly necessary to understand the theoretical and experimental analysis techniques to determine a plausible reason for the plasmonic effect in the OPVs. In the following section, we summarize the main concepts and strategies reported in ∼70 articles to attain higher PCE by using plasmonic nanostructures in OPVs. 3.3.3. Progress and Research Trends in Plasmonic OPVs. Plasmonic OPVs have been actively developed, and some reviews have already discussed this progress in part.44,45,188 The concept of the plasmonic effect in OPVs has rapidly emerged since 2010, and a series of interesting studies have been introduced by leading research groups. In the early stage, the Chen group developed a simple model to observe the plasmonic effect in OPV based on LSPR-induced local field enhancement by incorporating Au prisms into HTL. JSC and PCE values improved to 10.22 mA/cm2 and 4.24% from 9.16 mA/cm2 and 3.57% after the introduction of the Au prisms.189

utilizing the SPR effect (i.e., by incorporating nanostructured metals into OPVs). As summarized in the first column of Table 2, various classes of plasmonic nanostructures (e.g., spheres, rods, wires, prisms, cubes, gratings, pyramids, disks, and stars of different sizes, lengths, and widths, ranging from a few nanometers to micrometers) have been employed in OPV devices because the structural properties are critical to determine the efficiency of the plasmonic OPVs. As discussed in section 2, the portion of light scattering becomes more dominant compared to that of the light absorption when the size of the plasmonic nanostructures increases. Light scattering is correlated with the optical path length in the device, so the size of the plasmonic nanostructures is intimately correlated to the improvement in the solar cell performance. Meanwhile, the shape and dimension of the plasmonic nanostructures affect the spectral range of absorption. It tunes the intrinsic optical properties of the sensitizers in the way that (1) the inclusion of plasmonic nanostructures with an LSPR band at the wavelength where sensitizers exhibit weak absorption enhances light utilization in a broader range, and (2) the introduction of plasmonic nanostructures with an LSPR band at the wavelength where sensitizers exhibit strong absorption intensifies the absorption cross section of the sensitizers. The compositional and surface properties of the plasmonic nanostructures are also important in OPV devices to improve the solar cell performance. As was already mentioned in section 2, the absorption cross section as well as the position of the LSPR bands can be controlled by organizing the types of metals. In general, Au or Ag have been chosen individually, but the use of both metals in one device can be realized by integrating them into different layers or by developing them into bimetallic or core−shell structures (see the first column of Table 2). In most cases, the layers in the OPV device are prepared via the solution process. To readily distribute the plasmonic nanostructures without agglomeration in the active layers, the outer surface of the plasmonic structures needs to be decorated with hydrophobic ligands.182−187 With regard to application into charge transport layers, the inclusion of plasmonic nanostructures into PEDOT:PSS HTL is preferable because the plasmonic nanostructures, which are generally synthesized in aqueous solution, are easily dispersed inside the HTL (see the second column of Table 2). Apart from the miscibility issue, a surface modification is required to reduce the charge carrier recombination facilitated by the plasmonic nanostructures. Decoration with insulating materials such as SiOx and TiOx has been suggested (see the first column of Table 2). Finally, the origin of the plasmonic enhancement in the OPVs can be altered depending on the position where the 14998


14999

JSC

HTL and ETL

Ag prisms

JSC

−

PCE

HTL and ETL

Between ITO and active layer

70 nm (HTL)/50 nm (ETL)

50 nm Au spheres

40−100 nm Ag@SiO2 prisms

−

D. S. Ginger and A. K−Y. Jen and H. Chen group Ag prisms between glass and active layer

PCE

22 nm

JSC

HTL and active layer

Au sphere/graphene composites

JSC PCE

HTL

PCE

18 nm (Au)

Au sphere/graphene composites

55 nm/49 nm, AR: 4.1

JSC

combination of Au spheres and Au rods

HTL

PCE

45 nm

JSC

HTL

Au prism

JSC

PCE (%)

PCE

HTL

location of plasmonic structures

JSC (mA/cm2)

30−40 nm

F.−C. Chen group Au spheres

plasmonic structures (composition/size/shape)

Table 2. Summary of Plasmonic OPVs

absorption enhancement; light trapping by dual scattering; increase in the total loading of plasmonic NPs

12.90 6.65 7.50 12.70 14.36

prolonged optical paths generated by multiple scattering between two plasmonic interfacial layers

elucidating the charge transfer from polymers to Ag prisms; blocking the charge recombination at Ag prisms by incorporating silica shell

higher photon absorption followed by an increase in polaron yields

different dielectric environments in HTL and active layer induced an extension of light absorption range; more photons were trapped in the device

increased photon absorption by the near-field effect

Au spheres: light scattering and LSPR effect; Au rods: LSPR effect

local field enhancement; plasmon-exciton coupling

enhanced light harvesting and electromagnetic field by the LSPR effect, leading to an increase in JSC and FF

effect

11.69

12.5 3.70 4.66

8.95 9.50 3.48 3.89 9.16 10.22 3.57 4.24 9.28 11.49 3.46 4.28 8.61 9.94 4.02 5.05 9.8

plasmonic cell

reference cell

plasmonic cell

reference cell

performances

2014195

2013194

2012193

2010192

2015187

2014191

2012190

2011189

2009228

ref



15000

JSC

combination of Ag spheres and Ag prisms

active layer

PCE

JSC

Au spheres

15 nm

PCE

50 nm/period: 750 nm ETL

JSC

active layer and anode

Au spheres/Ag grating

JSC

anode PCE

PCE

active layer

Au spheres 18 nm Ag grating

width: 700 nm, height: 40 nm

PCE

18 nm

JSC

HTL

Au spheres

JSC PCE

HTL

PCE

thickness: 2 nm

Au film

18 nm (HTL)/35 nm (active layer)

JSC

W. C. H. Choy group Au spheres HTL and active layer

PCE

20 nm

JSC

HTL

Au sphere/MoS2 composites

PCE (%)

PCE


JSC (mA/cm2)

40 nm (HTL)/20 nm (ETL)


Table 2. continued

18.07 8.02 8.74 8.99

18.39 7.59 8.79 17.23

9.74 3.16 3.85 3.8 5.2 0.59 1.24 8.5 8.94 3.10 3.51 1.64 2.17 14.05 15.50 7.20 7.73 17.09

8.35

15.44 6.18 7.25

7.66 9.02 13.36

plasmonic cell

reference cell

plasmonic cell

reference cell

performances

broadband absorption by incorporating Ag nanostructures with multiple shapes

charge transfer from metal to TiO2; plasmonically excited electrons fill the traps in TiO2, leading to a reduction in the effective extraction barrier

series resistance decrease; electron/hole mobility balancing; broadband absorption

absorption enhancement in a wider wavelength range based on the SPR effect on a metal grating structure

absorption enhancement by the LSPR effect; higher hole mobility leading to an increase in charge collection

low resistance of HTL; higher roughness caused by Au NSs increased hole collection efficiency

change in work function of anode depending on ultraviolet-ozone (UVO) treatment or duration of UVO treatment

in PEDOT:PSS layer: interfacial contact area between active layer and buffer layer facilitated hole collection at the anode; in active layer: absorption enhancement due to near-field and scattering from Au NSs

absorption increase by near-field enhancement; more intimate contact between Au NSs and active layer by replacing thick PEDOT:PSS layer with thin MoS2 sheets

effect

2013202

2013204

2012200

2012199

2012198

2011197

2011230

2011229

2014196

ref



15001

JSC

J. H. Park and A. J. Heeger group Ag spheres active layer

PCE

AR: ∼4

JSC

Au@SiO2 rods

active layer

PCE

70−80 nm

JSC

Y. Yang group Au spheres between two active layers

PCE

70 nm

JSC

Au stars

between HTL and active layer

PCE

thickness: 100 nm

JSC

Ag grating

anode

PCE

15 nm

JSC

Ag spheres

ETL

PCE

15 nm

JSC

ETL

Au spheres

PCE (%)

PCE


JSC (mA/cm2)

20 nm/60 nm, thickness =10 nm


Table 2. continued

Ag cluster formation in the active layer during film casting; light trapping and optical reflection caused by scattering from Ag clusters; downward shift of vacuum level, leading to a smaller electron transport barrier

plasmonic spectral tuning to the poor light absorption region of polymers

6.92 5.22 6.24 10.35 12.20 4.93 5.64 10.79

absorption and near-field enhancement

in active layer: near-field generation and light scattering effect resulting in the enhancement of absorption; the placement of Au nanostars across both layers induced a high-order plasmonic resonance leading to a broadband absorption enhancement; in HTL: relocation of exciton generation region close to anode and increase in hole extraction

absorption enhancement and spatial redistribution of light absorption in active layer; elimination of SCL effect by enhancing hole mobility

electron transfer from TiO2 to metal nanoparticles; electron accumulation effect on metal nanoparticles (under UV irradiation) and work function change in TiO2

effect

6.06

18.72 9.26 10.25

5.76 0.73 1.73 17.08

16.21 7.31 8.20 15.02 16.32 7.31 7.87 3.30

10.61 3.60 4.30 15.02

plasmonic cell

reference cell

plasmonic cell

reference cell

performances

2011208

2013207

2011206

2016205

2014201

2013203

ref



15002

JSC

active layer

E. Kymakis and E. Stratakis group Au spheres

JSC

PCE


PCE

diameter, 60 nm; length, several μm

Ag wires

24.7 nm

JSC

Ag spheres

active layer

PCE

side: 40 nm, height: 20 nm

JSC

between anode and HTL

Ag triangles

JSC

PCE


PCE

height: 7−9 nm, width: 100−150 nm

N. Mathews and T. C. Sum group Au wires

15−18 nm

JSC

Au spheres

active layer

PCE

70 nm

JSC

active layer

Au truncated octahedra

PCE (%)

PCE


JSC (mA/cm2)

40 nm


Table 2. continued

8.27 9.77

9.01 3.06 3.59 LSPR effect; Au NS migration to HTL, leading to higher hole collection at anode

increase in exciton and polaron densities

competition between light absorption/exciton generation and trap/recombination by plasmonic nanostructures

absorption enhancement by scattering and near-field coupling, leading to a higher population of hole polarons

9.02 2.44 2.72 8.55

9.57 4.24 4.52 8.98 7.88 3.10 2.60 8.36

absorption enhancement in the active layer by near-field generation and light scattering

dispersity of Au NSs in active layer could control the light scattering, field localization, surface roughness, shunt, and series resistances

multiple scattering and longer optical paths; efficient hole extraction reducing the possibility of electron/hole recombination

effect

7.78

15.30 6.58 7.02

11.18 3.54 4.36 15.31

11.61 6.3 7.1 10.65

plasmonic cell

reference cell

plasmonic cell

reference cell

performances

2013213

2014211

2013183

2012212

2012210

2014182

2011209

ref



15003

17 nm/11.3 nm

PCE

JSC

Au@Ag sphere clusters


PCE

3 nm (Ag)/6 nm (CD)

JSC

CD-supported Ag spheres (clusters)


PCE

50 nm/10 nm

JSC

B.−S. Kim and J. Y. Kim group Ag@SiO2 spheres between HTL and active layer

PCE

2−13 nm (Au)

JSC

Au sphere/WS2 composites

active layer

PCE

length: 30 nm, width: 10 nm

JSC

Au rods

ETL

PCE

8 nm

JSC

Au@P3HT spheres

active layer

PCE

15−18 nm

JSC


Au spheres/GO composite

PCE (%)

PCE


JSC (mA/cm2)

2−15 nm


Table 2. continued

16.28 7.77

16.0 7.53 8.31 14.53

16.65 7.51 8.92 14.4

14.64

10.54 2.66 3.65 10.87 12.03 5.96 6.75 10.6 12.3 5.6 6.3

10.2 2.90 3.37 8.30

3.26 3.71 9.59

plasmonic cell

reference cell

plasmonic cell

reference cell

performances

strong absorption by the Ag shell; scattering effect by Au and Ag coupling; light absorption in whole visible region by core−shell characteristics; block copolymer acted as a protective layer of plasmonic nanostructures by reducing exciton quenching

clustering effect of Ag NSs; absorption enhancement

light scattering and near-field generation; absorption enhancement; stronger coupling effect due to the close distance between the Ag NSs and active layer

absorption enhancement; preferable band alignment of WS2 for electron transfer to PCBM

light reuse via back scattering; near-field generation

light scattering and absorption enhancement; surface roughness increase resulted in higher charge carrier dissociation and hole mobility

absorption enhancement

effect

2015218

201323

2013217

2015215

2015216

2015184

2013214

ref



15004

4 nm (Ag)

PCE

JSC

cross patterned Ag NS/PFBT fibers


PCE

5 nm (Ag)

JSC

W.−C. Chen group cross patterned Ag NS/PVP wires between anode and HTL

PCE

15−20 nm (Au)/3 nm (PEG)

9.45 3.54

9.87 3.53 4.19 8.57

8.54

18.50 8.31 9.98

9.48 16.71

JSC

Au spheres and Au sphere@PEG/B-doped CNT


17.75 8.29

PCE

16.39

11.22 12.67 6.4 7.6 16.43 17.38 7.78 8.74 17.20 18.15 9.00 10.05

9.00

plasmonic cell

reference cell

plasmonic cell

reference cell

performances

21 nm (small Au)/97 nm (Au NPCs)/0.5−2 nm (interparticle spacing)

JSC

S. O. Kim group triangle-shaped Au clusters (NPCs) between HTL and active layer

PCE

50 nm

JSC

Ag spheres

ETL

PCE

45 nm/10 nm

JSC

HTL

Au@Ag cubes

JSC

PCE (%)

PCE

HTL


JSC (mA/cm2)

67 nm

J.−Y. Lee group Ag spheres


Table 2. continued

absorption enhancement by the LSPR effect

energy transfer from metal to polymers by near-field scattering; increase in hole and electron mobilities; higher conductivity

in active layer: absorption enhancement generating excess excitons; in PEDOT:PSS layer: light scattering

near-field coupling occurred at the gap between individual Au NSs in clusters and at the outer surface; increase in light absorption (exciton generation) and excition dissociation

nanofunneling effect; Ag NSs attracted charge carriers and directly extracted them to ITO

enhanced scattering effect by the Ag shell; broad-band absorption

light scattering and absorption enhancement

effect

2015232

2014231

2015223

2014222

2015221

2014220

2013219

ref



15005

side length: 110 nm, height: 30 nm

PCE

JSC

Au pyramids


PCE

diameter: 110 nm, length: 5−8 μm

JSC

Ag wires

active layer

PCE

width: 55 nm, height: 40 nm

JSC

etc. Ag wires anode

PCE

length: 100 nm

JSC

active layer

Au arrowhead rods

JSC

PCE

active layer

PCE

4.1 0.36 1.10

3.72 5.21 0.96 1.32 8.47 9.32 3.31 3.91 2.7

15.54 5.75 7.40

16.10 5.74 6.83 13.06

13.87

11.01 4.57 6.55

7.87 11.06 4.67 6.63 7.91

4.11

plasmonic cell

reference cell

plasmonic cell

reference cell

performances

diameter: 15 nm, length: 60 nm

W. Guo group Au rods

2 nm (Ag)/1 nm (Au)

JSC

HTL

Ag and Au islands

JSC

PCE (%)

PCE

HTL


JSC (mA/cm2)

8 nm

L. Shen group Au islands


Table 2. continued

near-field generation; absorption enhancement

near-field generation and light scattering

SPR and waveguide effect; absorption enhancement

absorption enhancement, leading to a faster charge dissociation into free carriers; metal tip effect inducing higher LSPR effect

higher absorption in the range of 350−700 nm; increase in charge carrier mobility; higher exciton generation and dissociation; improved interfacial electrical property

introduction of both Ag and Au to cover the whole absorption range of P3HT; P-doping effect of 2 nm Ag decreasing the Fermi level of HTL and increasing the hole transport to the anode

absorption enhancement by the LSPR effect

effect

2012224

2011237

2010236

2016185

2015235

2016234

2015233

ref



15006

JSC


Ag spheres

JSC

PCE

between cathode and ETL

PCE

JSC

35 nm

Au spheres

active layer (sensitizer)

PCE

JSC

Au corroles (organometallics)

active layer (electron donor)

PCE

15 nm

JSC

Au spheres

ETL

PCE

5.5 nm (Ag)

JSC

Ag sphere decorated TiO2 rods

ETL

PCE

40−50 nm

JSC

HTL

JSC

combination of Ag and Au spheres

PCE (%)

light scattering; absorption enhancement; multireflection effect; active layer with undulating surface by Ag NSs

15.6 2.6 3.6 9.12

10.43

LSPR effect of Au NSs enhancing light absorption and charge carrier dissociation

long-lived triplet exciton generation

decrease in series resistance and better electron transport

enhanced charge extraction in ETL

broadened absorption; absorption enhancement; increase in charge carrier density, hole mobility, lifetime, and charge transport

near-field generation; absorption enhancement

effect

14.18 5.3 6.0 13.1

10.34 0.5 4.0 13.22

17.7 7.25 8.67 14.99 16.46 5.81 6.92 8.04 7.88 2.25 2.35 1.19

5.54 1.10 1.39 15.0

4.39

plasmonic cell

reference cell

plasmonic cell

reference cell

performances

PCE



JSC (mA/cm2)

∼8.5 nm

Au spheres


Table 2. continued

2014244

2014243

2014242

2013241

2013240

2013239

2013238

ref



PCE

JSC

Au spheres: 10 nm/Au rods diameter: 34 nm, length: 89 nm/SiO2 thickness: 6 nm

Au cubes

15007

diameter: 45 nm, length: 40 μm

PCE

JSC

Ag wire mesh

anode

PCE

20−25 nm/1−2 nm

8.97 12.15 3.63 4.47

13.76 3.10 4.03 light scattering between Ag NWs and Al electrode, resulting in the light trapping in the active layer

absorption enhancement due to electric field from Au@TiO2; effective charge generation due to electric field from Au prisms; less charge recombination because of TiO2 shell; enhanced polaron pair generation; longer exciton lifetime

JSC

active layer

9.67

no observable correlation between size/concentration of plasmonic NSs and crystallization of active materials

−

active layer

Au or Ag spheres Au: 5, 50, 80 nm/Ag: 10, 40, 60 nm Ag prism@TiO2

17.24 7.41 9.15

PCE

energy transfer between Au NSs and PTB7 because the emission region of Au NSs overlaps with the absorption range of PTB7; increase in the number of Au NSs in active layer

hot hole generation, leading to lowering the work function of metal oxide layer and a higher electron extraction

light scattering; absorption enhancement

microlens focused light into the absorber layer; the light was diffracted by the corrugated Ag cathode and exhibited waveguide modes; SPs, propagating at the periodically textured metal cathode, enhanced the field intensity near the active layer and cathode interface broadband absorption by Au@SiO2; increased absorption and scattering by dual nanoparticle system, resulting in a higher hole mobility

effect

1.6 nm

JSC

Au spheres

between active layer and HTL

PCE

15−60 nm, thickness: 0.5−0.6 nm

JSC

Ag spheres

ETL

PCE

70 nm

8.54 15.1 16.7 7.5 8.2 9.5 9.9 5.3 5.5 15.87

15.56 6.52

JSC


HTL

12.17

−

cathode

5.07 5.87

plasmonic cell

reference cell

Ag corrugation: Ag and microlens pitch: 500 nm/Ag height: 90 nm/microlens radius: 175 nm/microlens height: 300 nm Au spheres/Au@SiO2 rods

PCE (%)

plasmonic cell

reference cell

performances

PCE


JSC (mA/cm2)

40 nm


Table 2. continued

2015249

2015248

2015247

2015227

2015226

2014246

2014245

2014225

ref



15008

PCE

JSC

Au@Ag@SiO2 cuboidal rods

diameter: 20 nm, length: 60 nm/20 nm/10 nm

PCE

long axis: 65 nm, short axis: 30 nm, period: 230 nm between HTL and active layer

JSC

Ag oblate spheroids

cathode

PCE

period: 650 nm

JSC

Ag grating

anode

PCE

diameter: 24 nm, length: 48 nm/8 nm

JSC


Au@SiO2 rods

JSC

PCE

ETL

PCE

41 nm

Au spheres

100 nm

JSC

Au disks

active layer

PCE

75 nm

JSC

active layer

JSC

Au rhombic dodecahedra

PCE (%)

19.39 9.13 10.59

14.70 6.87 7.64 10.90 13.32 5.12 6.28 8.41 10.77 3.36 4.02 17.32

15.81 6.67 7.86 13.06

10.24 5.30 6.42 9.23 10.39 3.46 4.14 17.05 19.10 8.08 9.29 14.49

8.36

plasmonic cell

reference cell

plasmonic cell

reference cell

performances

PCE



JSC (mA/cm2)

35 nm

Ag spheres


Table 2. continued

broadband light absorption by the core−shell configuration

improvement of absorption, scattering and near-field effect by increasing the eccentricity of Ag nanoparticles

enhanced light absorption and trapping due to grating architectures

clustering effect which increases near-field in the vicinity of individual Au@SiO2 rods

absorption enhancement; improvement of charge generation and dissociation; higher conductivity to facilitate the charge transfer

increase in light absorption, exciton generation, and dissociation probability

LSPR effect; light absorption and trapping enhancement in the active layer

LSPR effect; absorption enhancement; increase in the surface roughness of anode, leading to an improvement of hole collection

effect

2016255

2015251

2015254

2015253

2015252

2015251

2015186

2015250

ref



Chemical Reviews 2016256

9.63

PCE

19.2 7.75

large enhancement of light absorption in the active layer by multiple-patterned Au electrode; nanoposts: SPR effect exhibited by LSP and PSP modes; grating: higher optical path length by scattering; corrugation: coupling the light into SPP modes 17.3

thickness (electrode): 100 nm, period (grating): 300 nm, height (grating): ∼10 nm, penetration depth (nanoposts): < 10 nm, period (nanoposts): ∼ 50 nm

anode Au nanoposts and grating



Table 2. continued

A further increase in the PCE by up to 24% was observed when a blend of Au NSs and Au NRs was introduced into the HTL.190 The same group has more recently focused on the utilization of graphene oxide (GO) in OPVs. When GO was inserted as HTL with a certain amount of Au NSs instead of the conventionally used HTL, PEDOT:PSS, the Au NS/GO composite enhanced the JSC and PCE values from 8.61 to 9.94 mA/cm2 and from 4.02 to 5.05%, respectively.191 PEGylated GO rendered Au NSs dispersed in both the buffer layer and photoactive layer of the OPVs due to the amphiphilicity of the PEG ligands. The difference in the dielectric environments, resulted in the same PEGylated GO/ Au NS composites exhibiting LSPR bands at different wavelengths in the HTL and active layers, inducing a plasmonic effect over a wider spectral range. The dual plasmonic devices increased the PCE by up to 20%.187 The Ginger, Jen, and Chen groups conducted in-depth studies with photoinduced absorption spectroscopy to measure the lifetime of the photogenerated charge carriers in a 35 nm thin layer of poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) in the presence of Ag prisms. Higher polaron yields were correlated with an increase in the optical path length in the polymer thin film due to the excitement of the Au prisms.192 Photoinduced absorption spectroscopy also confirmed that the accumulation of negative charges on the Ag prisms (charging effect) was inhibited after introducing a 2.5 nm thick insulating layer (silica shell), leading to the suppression of recombination losses in OPVs without turning off the plasmonic enhancement.193 Another approach to induce a higher plasmonic effect incorporated Au NSs or Ag prisms of different sizes in both the HTL and ETL of OPVs. The PCE values increased from 6.65% to 7.50%194 (Figure 20) and from 7.66% up to 9.02%195 when PIDT-PhanQ or PIDTTDFBT were blended with PC71BM in the active layer, respectively, based on the multiple scattering between two interfacial layers. In addition, the plasmonic effect could be improved by replacing the PEDOT:PSS layer with thinner MoS2 sheets because a more intimate contact is achieved for the Au NSs with the active layer than in conventional OPV configurations.196 Most studies have reported that the device performance improved as a result of plasmonic nanostructures manipulating the optical properties of sensitizers (i.e., increase the light absorption and extend the range of the light absorption via near-field enhancement and scattering). Meanwhile, plasmonic nanostructures can also alter the electrical properties of the devices by promoting (1) the photocarrier mobility affecting the electron/hole transfer and collection and (2) the conductivity based on their metallic characteristics. The two effects (i.e., plasmonic optical and electrical effects) can be simultaneously or separately exhibited, depending on the geometry of the plasmonic structures and the location where the plasmonic structures are deposited in the device. The Choy group specifically investigated the origin of the plasmonic effect in OPVs. When Au NSs were placed in HTL, no increase in light absorption but IPCE enhancement was observed, pointing out that the optical effect can be a minor contributor to the device performance.197 The case with Au NSs introduced in the active layer exhibited an increase in light absorption followed by an improvement in hole mobility and collection.198 However, the improvement in electrical properties, such as charge mobility, exciton decay rate, and exciton dissociation probability, did not match the absorption trend at a

PCE (%)

JSC

plasmonic cell

reference cell

plasmonic cell

reference cell JSC (mA/cm2)

performances

effect

ref

Review

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Figure 20. (A) Schematic diagram of a dual plasmonic OPV device. (B) Calculated EQE enhancement of OPV devices with dual plasmonic layers (dual layer-doped) or mono plasmonic layer (PEDOT-doped or C70-bis-doped). Relative to mono layer-doped devices, the dual layer-doped device doubly enhanced the EQE value. Reprinted with permission from ref 194. Copyright 2013 Wiley-VCH.

Figure 21. (A) Schematic illustration of a plasmonic OPV device employing truncated octahedral Au nanoparticles in the active layer. (B) SEM image of truncated octahedral Au nanoparticles. (C) Energy band alignment of the components in the OPV device. The incorporation of Au nanoparticles decreased the hole injection barrier (Φh) and facilitated hole transport. Reprinted with permission from ref 209. Copyright 2011 Wiley-VCH.

transfer was reversed (i.e., metal to TiO2) and the plasmonically excited electrons at the Au NSs achieved trap filling in the TiO2 layer, leading to a reduction in the effective barrier for electron extraction to the cathode. As a result, the PCE increased from 8.02 to 8.74%.204 The Choy group has recently attempted to fabricate plasmonic OPV devices that simultaneously exhibit plasmonic optical and electrical effects by depositing star-shaped Au nanoparticles across the HTL and active layer. The part of the Au nanoparticles embedded into HTL induced the relocation of the region where charge carriers were generated close to the anode, leading to a higher hole extraction, while the remaining part penetrated into the active layer contributed to the excitation of sensitizers via near-field generation. The geometrical novelty also improved the absorption over the entire visible region, resulting in an improvement of PCE by up to 10.7%.205 The Yang group focused on the optical effect produced by plasmonic nanoparticles in the OPV devices. They integrated Au NSs into an interconnecting layer between two subcells, and this structure improved the PCE of the tandem solar cell from 5.22 to 6.24%. The efficiency of both the top and bottom cells could be enhanced by light concentration of the Au nanoparticles.206 Inside the device, Au nanoparticles were added into the active layer after enclosing the surface with a SiO2 shell to provide an electrically insulating environment. The Au NR@SiO2 increased the PCE by up to 14.4% when the active materials were PBDTT-DPP:PC60BM due to the effective spectral overlap between them.207 Along with the investigations into the scattering effect of Au or Ag nanoparticles in active layers, the Park and Heeger groups exploited ultraviolet photoelectron spectroscopy (UPS) to elucidate the electron transfer mechanism in OPVs. The

certain content Au NSs. This indicates that approaches to simultaneously maximize the optical and electrical plasmonic effect should be considered in OPVs in order to obtain a higher device performance.198 In another study, the Choy group integrated a plasmonic metal grating top electrode in OPV devices. The SPR effect on the Ag grating structure improved and broadened the light absorption, leading to an increase in the PCE and EQE values.199 This was more effective when the active layer contained Au NSs and was simultaneously covered with an Ag grating electrode by enabling broadband absorption and charge carrier balancing through LSPR and waveguide mode coupling. The PCE increased from 7.59 to 8.79% in that study.200 The metal grating anode was further confirmed to possibly break the space charge limit (SCL) since it can generate dense photocarriers at the interface with the active layer facilitating hole collection.201 With regard to the dual plasmonic concept, the group proposed a combination of Ag nanoparticles with different shapes in the active layer. A wide-band absorption derived from Ag nanoparticles of multiple shapes was matched with the whole absorption range of the active layer, and the values of JSC and PCE were improved by up to 17.91% and 19.44%, respectively.202 The same group also investigated the plasmonic effect on the ETL function. When the TiO2 layer containing Au or Ag NSs was irradiated with UV light, the electron transfer occurred from the TiO2 layer to the metal nanoparticles, resulting in a large accumulation of electrons in the metal nanoparticles followed by a decrease in the work function of the ETL. The increased carrier density in the ETL reduced the resistance of the layer, and it finally facilitated the electron extraction.203 Under illumination with visible light, the direction of the charge 15010


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UPS results confirmed that the downward shift in the vacuum level of the PCDTBT/PC70BM blend by the inclusion of Ag NSs resulted in a decrease in the electron transport barrier in the device, facilitating electron extraction to the cathode.208 In contrast, truncated octahedral Au nanoparticles acted as a hole conductor in the P3HT layer for efficient hole extraction, reducing the possibility of electron/hole recombination (Figure 21).209 In addition, the Park group carried out one simple study to demonstrate that balancing the optical and electrical advantages is important to obtain a higher cell efficiency. Apart from the actual size of an individual Au NS, the size of the Au NSs in the active layer was determined by the degree of nanoparticle aggregation. In the study, the number of ligands on the surface of the Au NSs controlled the aggregation behavior. Devices with well-dispersed Au NSs showed weak light scattering and field localization, but higher shunt and reduced series resistance than devices containing Au NS aggregates, leading to a decrease in leakage current and an increase in fill factor (FF).182 The Mathews and Sum group utilized NWs in plasmonic OPV devices. The thickness of the HTL was controlled to allow the evanescent field of the Au NWs to be extended to the photoactive layer.210 On the basis of a high AR of the NWs, it was further developed into a system to determine the polarization-dependent photocurrent and PCE. By incorporating Ag NWs between the HTL and active layer, the JSC value increased by 36% over the reference value. The excitation of the Ag NWs along the transverse axis resulted in this improvement.211 In another work, Ag triangles were hexagonally arranged, similar to a honeycomb structure, between the bottom electrode and the HTL. This unique pattern enhanced the JSC value by up to 12% based on the strong local field and scattering.212 The same group measured the transient absorption and light intensity dependent electrical properties in order to monitor the exciton generation in the OPV devices. They confirmed that the excitation of the charge carriers could be initially promoted by the Ag NSs, but trap-assisted recombination increased the loss because the competition between light absorption/exciton generation and charge trap/ recombination takes place simultaneously with the plasmonic nanoparticles.183 The Kymakis and Stratakis groups applied high spatial resolution synchrotron X-ray diffraction stratigraphy to plasmonic nanoparticle-based OPVs. This study provided information on the polymer crystallinity, plasmonic nanoparticle distribution, and structural properties of the active and interfacial layers in the device.213 The group turned to a better design configuration for the plasmonic devices. First, they focused on the application of two-dimensional semiconductors, such as GO and WS2 in OPV devices.214,215 Recently, they have suggested a new concept to use P3HT itself as a protecting layer for the plasmonic nanoparticles. Compared with other organic ligands or metal oxide layers, such as SiO2 and TiO2 on the surface of the plasmonic nanoparticles, the P3HT shell on the Au NSs was more effective in charge separation and collection via the decrease in the distance between the active materials and Au nanoparticles (Figure 22). As a result, the Au@P3HT attained an enhancement in the PCE by up to 37.2% when it was incorporated into the active layer.184 The same group also paid attention to backscattering plasmonic nanoparticles in contrast with most other cases that placed plasmonic Au nanoparticles for forward scattering in the OPV devices. The light transmitted from the active layer could be

Figure 22. (A) Schematic illustration of a bulk heterojunction (BHJ) layer including Au NS@P3HT. (B) J−V characteristics of OPV devices with or without Au structures. The Au NS@P3HT-based device (red) showed a remarkable increase relative to the reference device (black). This indicates that the outer surface property of the Au nanoparticles is critical to maximize the photovoltaic performance. Reprinted from ref 184. Copyright 2015 American Chemical Society.

effectively scattered back by incorporating Au NRs in the ETL of a conventional device, and this contributed to a PCE enhancement of ∼13%.216 The importance of the plasmonic nanoparticle location in the OPV devices was discussed by Kim et al. The device containing Ag NSs@SiO2 structures between the HTL and active layer showed a higher efficiency than the device with plasmonic structures between the bottom electrode and HTL because the coupling effect between the Ag nanoparticles and the active layer was stronger when they were in close contact. The best plasmonic device showed a PCE value of 8.92%, representing an improvement of ∼19% over the Ag nanoparticle-free device.217 The same group utilized carbon dots (CDs) as a template to cluster the Ag nanoparticles. By increasing the number of Ag NSs decorated on one CD, electronic coupling between the CD−Ag and Ag−Ag became stronger, leading to an electric field enhancement. As a result, the PCE and internal quantum efficiency (IQE) values increased from 7.53 to 8.31% and from 91 to 99%, respectively.23 The incorporation of Au NS@Ag core−shell structures between the HTL and active layer was also carried out to increase the light absorption over the whole visible region. When excess Ag ions were introduced in the presence of a diblock copolymer template [poly(ethylene oxide)-block-poly(acrylic acid), PEO-b-PAA], the Ag shell developed into a bridgelike structure, increasing the light scattering in the device, and the PCE increased by up to ∼16% with this approach.218 The Lee group investigated the size-dependent plasmonic scattering effect in OPV devices (Figure 23) by incorporating size-controlled Ag NSs (10−100 nm diameters) into the HTL. The scattering effect of the Ag nanoparticles was visualized using near-field optical microscopy (NSOM), and the best PCE was obtained when 67 nm of Ag nanoparticles were used.219 They also designed a binary plasmonic structure to compensate for the rather weak scattering of the Au nanoparticles. The 15011


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Figure 23. (A−D) TEM images of size-controlled Ag nanoparticles. The numbers in the TEM images denote the average size of the Ag nanoparticles. (E) Summary of photovoltaic parameters obtained from the OPV devices, including Ag nanoparticles shown in A−D. Adapted with permission from ref 219. Copyright 2013 Macmillan Publishers, Ltd.

Figure 24. (A) Schematic diagram of PEIE-based plasmonic OPV device. (B) J−V curves obtained from the PEIE-based plasmonic OPV devices (Hybrid) and PEIE-based OPV devices without Ag nanoparticles (PEIE). Two electron donors (PTB7 or PTB7-Ph) were used separately in the fabrication of OPV devices. (C) IQE plots of hybrid and PEIE devices. (D) Schematic illustration to show the electron collection through an Ag nanoparticle funnel. Reprinted with permission from ref 221. Copyright 2015 Wiley-VCH.

Figure 25. (A) TEM image of Au NS decorated B-doped CNTs. (B) J−V profiles obtained from OPV devices without Au NS/CNT structures (black), with Au NS/N-doped CNTs (orange) and Au NS/B-doped CNTs (green). (C) Schematic diagram to show an additional photocarrier generation by Au NSs and facile charge transfer along the CNTs. Reprinted with permission from ref 223. Copyright 2015 Wiley-VCH.

Au@Ag core−shell nanocubes exhibited broad-band absorption that produced a 2.2-fold enhancement in EQE at wavelengths of 450−700 nm. The PCE increased from 7.78 to 8.74% as a result.220 Recently, they also reported that photovoltaic parameters, such as the JSC, PCE, IPCE, and stability could be remarkably enhanced when the Ag NSs create a connection between the bottom electrode and active layer because this connection provides a direct pathway for electron transfer,

facilitating charge extraction to the cathode (Figure 24). The replacement of a conventional ETL with a 10 nm thick polyethylenimine ethoxylated (PEIE) layer was key to realizing the concept of an Ag funnel. The IQE of the plasmonic OPV device improved by up to 100%, and an absolute PCE value of 10.05% could be achieved.221 Kim et al. incorporated Au NS clusters to exploit plasmonic near-field coupling at the small gap between the individual Au 15012


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Figure 26. (A) Schematic illustration of an OPV device with Ag NS embedded metal oxide ETL. (B) FF profiles depending on the irradiation time. Holes generated by the Ag NSs are transferred into the TiOx layer, followed by a continuous improvement in FF. (C) J−V characteristics of OPV devices with (blue) or without Ag NSs. Adapted with permission from ref 226. Copyright 2015 Macmillan Publishers, Ltd.

Recently, organic−inorganic hybrid perovskites (OHPs) have been determined to be an important class of materials with excellent magnetic, electrical, and optical properties. The general chemical formula for hybrid perovskites is AMX3, where ‘X’ (I, Br, and Cl) is an anion bonding with ‘A’ [CH3NH3 and HC(NH2)2] and ‘M’ (Pb2+, Sn2+, and Cu2+) cations of different sizes. This new class of materials can be easily processed in solution and its qualities approaching that of industry staples such as GaAs and Si. Miyasaka and co-workers attempted to improve upon the durability of OHPs in perovskite-sensitized liquid-electrolyte solar cells in 2009,257 and since then, thousands of researchers turned their attention toward PSCs. The certified PCE of PSCs has risen from about 3% to over 20% in just four years. PSCs are generally constructed based on an n-i-p or p-i-n junction with the intrinsic OHPs sitting between a hole and an electron extraction layer. The PSCs can be built in a “regular” configuration with a typical device structure of FTO/TiO2/ OHPs/Spiro-OMeTAD/Au258 or in an “inverted” configuration with a typical device structure of ITO/PEDOT:PSS/ OHPs/PCBM/TiOx/Al.259 The basic operating principle of PSCs is that light is absorbed in the bulk of the film, free charges are generated at room temperature, and then they diffuse throughout the film and selectively transfer to the electrodes.260 Although, numerous approaches have been developed to improve the PCE of PSCs, optical management is required to achieve a higher performance of PSCs with the high current density, especially for the semitransparent solar cells. Ag@TiO2 nanoparticle-incorporated PSCs fabricated via low-temperature processing showed a 16.3% PCE. Highly polarizable Ag@TiO2 nanoparticles can increase the radiative decay of excitons, and the reabsorption of emitted radiation increases (Figure 27).261 High performance PSCs (η = 16.2%) were also achieved by incorporating a TiOx-Au-TiOx layer under the CH3NH3PbI3−xClx layer. Due to the plasmon-mediated hot carrier injection from Au nanoparticles to TiOx, the charge extraction properties of the TiOx layer improved significantly (Figure 28).262 The improvement in light absorption due to the plasmonic Au nanoparticles within the perovskite solar cell was also demonstrated in a numerical analysis carried out by CarreteroPalacios et al. The plasmonic near-field enhancement and scattering effects as a function of the nanoparticle size were analytically studied via FDTD simulations (Figure 29).263 Two hundred nanometer thick perovskite films with 60 nm-radii Au nanoparticles and 300 nm thick films with 90 nm-radii Au

NSs, and the PCE improved from 8.29 to 9.48% using this approach.222 In another work, they suggested the hybridization of Au NSs and doped carbon nanotubes (CNTs) to promote charge carrier generation at Au NSs, facilitating charge separation and transport along the CNTs. As a result, the PCE remarkably improved from 8.31 to 9.98% (Figure 25).223 In addition to the studies already mentioned, there are some more examples where an extraordinary improvement in the device performance could be obtained or where a new aspect of the plasmonic effect in OPV devices was investigated. The Ren group designed a unique pattern with Au nanopyramids (NPYs) on ITO. The high near-field intensity at the sharp corners of the Au NPYs led to an increase in PCE of up to 200%.224 The Biswas group calculated that the improvements in absorbance and photocurrent were of up to 49 and 58%, respectively, when two photonic crystal layers consisting of a microlens and Ag corrugations were implemented in the device. Since the microlens concentrated light into the active layer and the light was diffracted along the Ag pattern, the formation of waveguide modes and SP generation at the interface of the active layer and metal cathode could be induced.225 Riedl et al. developed a system employing Ag spheres covered with few-nanometer-thick ZnO or TiOx ETL. Hot holes generated on the plasmonic nanoparticles under irradiation with visible light were transferred into the metal oxide layer first. Ionosorbed oxygen on the surface of the metal oxide layer was oxidized and desorbed by the hot holes, lowering the work function of metal oxide layer and increasing the electron extraction to the cathode (Figure 26).226 Cho et al. observed that when the size of the Au NSs decreased to 1.6 nm, the emission from the Au NSs was critical in achieving an increase in the number of excitons in the active layer. This process was possible due to the spectral overlap between the emission band of the Au NSs and the absorption range of the organic semiconductor in the active layer, PTB7. The energy transfer from Au NSs to PTB7 improved the PCE by ∼20%.227 3.4. Plasmonic Effects in Other Types of Solar Cells

LSPR arising from the excitation of noble metal nanostructures by incident photons enables broadband absorption in the solar spectrum by means of far-field scattering and near-field enhancement of the electromagnetic field. Plasmonic effects have also been exploited to improve the overall PCE, photocurrent generation, and spectral response in other types of photovoltaic devices, including organo-metal halide (AMX3) perovskite solar cells (PSCs), quantum dot-sensitized solar cells (QDSCs), and CuInSe2 (CIS) solar cells, as summarized in Table 3. 15013


15014

2014 269

266

2013 271

273

2012 272

2008 274

ref

FTO/TiO2/Au+PbS/ZnS/Au/Pd

Au@SiO2 NPs

FTO/TiO2/Al2O3 and CH3NH3PbI3−xClx and Au@SiO2/spiro-OMeTad/Ag

5 nm

Au NPs

100 nm mixing with Al2O3 colloid solution and spin-coating

SiO2@Au nanoshell ∼150 nm spin-coating

Au- 5 nm Ag- 10 nm Immersing deposition

Au or Ag NPs

18 nm thick thermal evaporation

Ag NPs

110 nm thermal evaporation using AAO template

Ag NPs

FTO/TiO2/PbS CQD and SiO2@Au/MoO3/Au/Ag

p+-GaAs/p GaAs/i AlGaAs/i InAs QD/i InGaAs/i AlGaAs/i GaAs/ i GaAs/n GaAs/AlGaAs/ n+GaAs/Au or Ag NPs

p+-GaAs/p−-AlGaAs/p−-GaAs/iGaAs/InGaAs QDs/n−-GaAs/ n+-GaAs/n+-GaAs/TiO2/Ag Nps

Au/n-GaAs/n-AlGaAs/n-GaAs/pGaAs/p-AlGaAs/Ag NPs/pGaAs/Au

cell configurations

shapes/materials/ sizes/incorp methods of plasmonic structures

on TiO2 layer mixed Au NPs+ PbS

in the perovskite layer

in the QD film

On the GaAs top contact layer

On the GaAs substrate with varying thickness of TiO2 layer

On the GaAs layer

locations of plasmonic structures

Table 3. Summary of Other Types of Plasmonic Solar Cells

JSC (mA/ cm2)

PCE (%)

JSC (mA/ cm2)

PCE (%)

JSC (mA/ cm2)

PCE (%)

JSC (mA/ cm2)

PCE (%)

JSC (mA/ cm2)

PCE (%)

JSC (mA/ cm2)

increase in photoconv eff

29.45

20.71

11.4

16.91 10.7

24.5 6.2 6.9 14.76

21.6

67.2 8.0 9.5

− 56.0

18.70 −

17.76

5.9

11.9 4.7

11.0

plasmonic cell

ref cell

development of plasmonic nanocrystal solar cell by enhancing near-field absorption

enhancements in photocurrent generation that the exciton binding energy reduced with the incorporation of Au@SiO2 NPs

near-field scattering effects (absorption increase)

efficient light scatters and broadband absorption enhancement

plasmonic light trapping increasing IR photoresponse of QD solar cell

photocurrent enhancements by incorporating Ag NPs on to GaAs layer

mechanisms

IPCE/FL-decay

IPCE/PL

FDTD/EQE

EQE/Photoresponse

photoresponse

EQE

methods/tools/ equipmt to prove plasmonic effects

Au NPs can transfer the plasmon energy to the band gap transition of PbS semiconductor nanocrystal and near-field emission of Au toward the charge carrier generation

−

Au NPs based cell and Ag NPs based cell demonstration

−

−

etc.



15015

ITO/TiOx-Au NPs-TiOx/ CH3NH3PbI3‑xClx/HTM/Ag

glass/perovskite/Spiro-OMeTAD

FO/TiO2/ Meso-TiO2/Au−Ag alloy+CH3NH3PbI3/Spiro-OMeTAD/Ag

262

263

265

FTO/TiO2/Ag@TiO2/Al2O3 and CH3NH3PbI3−xClx /spiro-OMeTad/Ag

stainless steel/Mo/CIGS and Au NPs/CdS/i-ZnO/AZO/Ag

270

2015 261

ITO/ZnO/Au NRs/HgTe QDs/ MoO3/Au

cell configurations

275

2014

ref

Table 3. continued

100 nm

60 and 90 nm − Au−Ag Alloy NPs

Au NPs

40 nm spin-coating

Au NPs

40 nm mixing with Al2O3 colloid solution and spin-coating

Ag@TiO2 NPs

10 nm spary coating of Au NPs solution

Au NPs

Mixing with TiO2 paste with certain ratio

within the perovskite layer

sandwiched between two layers of low temp-processed TiOx film

on the TiO2 layer, within Al2O3 scaffold layer

pn-junction interface of the CIGS/ CdS layer

15.51

JSC (mA/ cm2)

16.46

−

19.9 13.2 16.2 −

16.3 18.3

22.0 14.5

20.2

10.36

37.81 8.31

− 34.84

−

1.27

4.0 4.2 0.37

plasmonic cell

ref cell

PCE (%)

JSC (mA/ cm2)

PCE (%)

JSC (mA/ cm2)

PCE (%)

JSC (mA/ cm2)

PCE (%)

JSC (mA/ cm2)

PCE (%)

JSC (mA/ cm2)

Au NR wide:40−50 nm long: 90−110 nm thermal release tape dry transfer process

PCE (%) on the ZnO layer



spin-coating


broadband light absorption can be improved by incorporating Au−Ag alloy popcorn-shaped NPs within mesoporous TiO2 layer

numerical analysis of absorption enhancement in organo-metal halid peorvksite films embedding Au NPs

trap state sites in TiOx matrix is filled by plasmon-induced charge injection across the Schottky barrier and improve conductivity of TiOx layer

enhancements in the radiative decay of excitons and increases the reabsorption of emitted radiation by Ag@TiO2 NPs

LSPR effect of Au NPs increase the electron−hole generation and thus enhance the photocurrent

strong near-field effects induced by Au NRs

mechanisms

IPCE/PL

FDTD

IPCE/PL/Kelvin-probe

IPCE/PL/temperature-dependent Abs and PL

FEM/EQE

FDTD/EQE


improvement of photo generated-charge transfer in CH3NH3PbI3 perovskite was observed by using plasmonic Au−Ag alloy NPs

−

charge extraction properties are improved by plasmonmediated hot carrier injection from Au NPs to TiOx.

high polarizability of the Ag@TiO2 NPs can increase the exciton radiation probability and change in excition radiation geometry which can induce the increase of radiation reabsorption

HgTe QD heterojunction photodiode photodetctor

etc.



264

2016 276

267

2015

ref

Au NPs

FO/TiO2 Au@nanofiber +CH3NH3PbI3/Spiro-OMeTAD/Au Au decorated TiO2 nanofiber as ETL

within the HTL (Spiro-OMeTAD)

PCE (%)

JSC (mA/ cm2)

PCE (%)

JSC (mA/ cm2)

JSC (mA/ cm2)

CdSe or CdSe@ZnS

Au NPs

PCE (%) on the ITO/PEIE layer



spin-coating


FO/TiO2/ Meso-TiO2/ CH3NH3PbI3/Spiro-OMeTAD + AuNPs/Au

ITO/PEIE/QD/PTB7:PCBM/ MoO3/Ag

cell configurations

Table 3. continued

21.63 9.23 14.92

20.04 12.66 12.74 19.14

19.63

8.9 10.3 15.1

plasmonic cell

ref cell

photon absorption induces a strong LSPR phenomenon at the Au-TiO2 interface, increase generation of electron−hole pairs

Au NPs induce near-field coupling in the short-wavelength range and an increase in the Jsc

enhancements in EQE by incorporating QD layer which corresponds to enhancements in photocurrent

mechanisms

EQE/IQE/Impedence

EQE/IQE/Impedence

PL/EQE/IQE


enhanced light absorption and reduced charge recombination

solar cell performance observations indicate that the LSPR and electrical effects of Au NPs enhance the photovoltaic performance

QD can participate directly in light harvesting and also strong near-field around the QDs from SPR effect

etc.


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Figure 27. (A) SEM cross-section of a meso-superstructured solar cell (MSSC). (B) TEM image of Ag@TiO2 nanoparticles with 40 nm Ag cores and 2 nm titania shells. Inset: magnified image of a single Ag@TiO2 nanoparticle, 20 nm scale bar. (C) Absorbance of Ag nanoparticles with and without titania shell in ethanol. (D) Absorbance of high concentration of Ag@TiO2 nanoparticles in an Al2O3 thin film deposited on FTO glass and heated to 150 and 500 °C, respectively. Reprinted with permission from ref 261. Copyright 2015 Wiley-VCH.

The in situ synthesis of Au-embedded TiO2 nanofibers via electrospinning was employed to enable plasmon-enhanced charge generation in PSCs.264 The SPR from metallic nanostructures allows for significant control over the optical field, and the prospective utilization of metallic nanostructures may improve the absorption of light in TiO2 nanofibers and perovskites (Figure 30). This effect also contributes to a reduction in charge recombination in Au@TiO2 nanofiber electrodes. The increase in current density could be attributed not only to the increase in excitation of the perovskites by the LSPR associated with near-field enhancement and scattering but also to the generation of an additional photocurrent as a result of the direct, LSPR-induced transfer of hot electrons from plasmonic structures to the conduction band of TiO2. As a result, the photovoltaic performance for PSCs based on Au@ TiO2 nanofibers was shown to achieve a photocurrent density of 21.63 mA cm−2 and an efficiency of 14.92%. Au−Ag alloy popcorn-shaped nanoparticles were also reported to lead to an increase in the performance of the PSCs. Lu et al. observed an improvement in the broadband light absorption and efficient photon-generated-charge transfer in the PSCs.265 The photocurrent and efficiency enhancement was also explored for meso-superstructured organo-metal halide PSCs incorporating Au@SiO2 nanoparticles. Unexpectedly, the materials exhibited reduced exciton binding energy with the incorporation of the metal nanoparticles rather than enhanced light absorption.266 The Au@SiO2-containing device exhibited a significant improvement in its short-circuit photocurrent and PCE of 11.4% relative to that of the control device. Other attempts have also been made to improve the performance of QDSCs. Plasmon-mediated CdSe or CdSe@ ZnS QD interlayers in polymer solar cells contributed to an increase in the photocurrent, and an optical enhancement was achieved via light harvesting and a strong near-field effect of QDs through the SPR effect.267 Kawawaki and co-workers used plasmonic Ag nanocubes in a QD/ZnO NW BHJ solar cell.

Figure 28. Schematic electrical diagram of the hot carrier injection process from Au nanoparticles to TiOx. Reprinted with permission from ref 262. Copyright 2015 Wiley-VCH.

Figure 29. Schematic of a random particle location within the perovskite film and contour plot of the Au nanoparticle. Reprinted from ref 263. Copyright 2015 American Chemical Society.

nanoparticles exhibited a maximum integrated solar absorption enhancement of ∼10% and ∼6%, respectively. The presence of the Au dimers was shown to enhance the absorption up to ∼12% in the thinnest films that were considered. 15017


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Figure 30. (A) FE-SEM cross-sectional image of fabricated PSCs. (B) Higher-magnification region for the perovskite material with Au@TiO2 nanofibers. The inset shows a TEM image of perovskite covered with Au@TiO2 nanofibers. (C) Schematic of the device architecture. (D) Schematic of the energy levels for TiO2, Au nanoparticles, CH3NH3PbI3, spiro-MeOTAD, and gold contacts. (E) Image of the final perovskite solid-state device. Reprinted with permission from ref 264. Copyright 2016 Royal Society of Chemistry.

The position and amount of Ag nanocubes were controlled to improve the photocurrent enhancement, particularly in the near-infrared region.268 Kholmicheva et al. developed Au nanoparticles with near-field emission at 5 nm, which can contribute to the generation of charge carriers due to the transfer of plasmon energy to the band gap transition of PbS semiconductor nanocrystals. The radiation from the SPs can improve the optical density within the solar cell and consequentially improve the device performance (Figure 31).269 The incorporation of Au nanoparticles can effectively improve the efficiency of flexible CIGS solar cells through a strong absorption and reduction of surface recombination that increases the electron−hole generation and improves the carrier transportation in the pn-junction. The light-to-electricity efficiency increased from 8.31 to 10.36% upon the incorporation of Au nanoparticles that harvested more incident light energy at the plasmonic resonance regions.270 The near-field effects of noble metal nanoparticles can also be utilized to improve the performance of HgTe QD heterojuction photodiode photodetectors. Recent research has demonstrated that strong near-field effects induced by Au nanorods resulted in a depth-dependent enhancement. More than 80 and 240% increments in the average photocurrent could be achieved in devices with Au nanorods embedded below different ZnO layers.195 Embedding plasmonic enhancers directly into the QD absorber film would be necessary to achieve the intended near-filed enhancement. Plasmonicexcitonic Au nanoshells applied directly in the QD absorbed film lead to a strong enhancement in near-field absorption, resulting in a 35% increase in photocurrent in the performancelimiting near-infrared spectral region. PCE enhancement was reported to be 11% overall relative to a nonplasmonic device (Figure 32).271 QDSCs have also shown great promise to further improve the performance of the most efficient multijunction solar cell devices due to their flexibility in bandgap engineering. An

Figure 31. (A) Per-volume extinction cross sections of common nanoscale sensitizers. (B) Schematic representation of the SP relaxation small-diameter Au nanoparticles. Reprinted from ref 269. Copyright 2014 American Chemical Society.

increase in the IR photoresponse of QDSCs was achieved by incorporating Ag nanoparticles on the rear side of the cells to trap long-wavelength light in the absorbing layer, leading to a corresponding increase in efficiency by 7.6%.272 Au and Ag nanoparticles were coupled through surface functionalization of InAs/GaAs QDSCs, leading to a PCE enhancement from 8.0 to 9.5% for Au nanoparticles and to 8.9% for Ag nanoparticles, respectively. Those metallic nanoparticles served as efficient 15018


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Figure 32. Electric field intensity profiles in the plasmonic film and cross-sectional TEM showing a single Au nanoshell embedded in a PbS QD film (left), schematic of the PbS QD depleted heterojunction device with embedded nanoshell, and measured current−voltage characteristics under AM1.5 simulated solar illumination for representative control and plasmonic devices (right). Reprinted from ref 271. Copyright 2013 American Chemical Society.

noble metals easily damages the cells through the following reaction:277

light scatters and contributed to an improvement in broadband absorption. 273 The effects of plasmonic scattering on absorption and photocurrent collection were explored in the GaAs solar cells decorated with size-controlled Ag nanoparticles. The shape and density of the nanoparticles were uniformly and systematically controlled using anodized aluminum oxide (AAO) templates. Densely formed nanoparticles significantly enhanced the photocurrent by 8%, which can be attributed to strong scattering by interacting SPs.274

2Au + I−3 + I− ↔ 2[AuI 2]−

Wu et al. simply evaluated the corrosion resistance of the plasmonic nanostructures with or without a protective layer (thin TiO2 layer) to the common iodide/triiodide redox couple.149 The color of a plasmonic colloidal solution without a protective layer rapidly turns transparent when treated with an electrolyte, while the color of the protected plasmonic colloidal solution is retained for a certain period of time, indicating that the protective layer provides chemical stability against corrosion by redox compounds. Likewise, noncorrosive electrolytes (such as cobalt redox mediator, imidazolium-dicyanamide based ionic liquid, and polymer electrolyte) have also been applied to overcome this problem.149,156,171 The crucial point to note is that the optimal thickness of the passivation layer is required to ensure the electrical and chemical stability while maintaining the optical distance for near-field enhancement. If the separation distance between metal nanoparticles and photoactive materials increases, the influence of the plasmon-enhanced near-field is diminished because the electromagnetic fields of the metal nanoparticles are dependent on the distance.127

4. CRITICAL ISSUES IN PLASMONIC PHOTOVOLTAICS As previously discussed, plasmonic effects in photovoltaics have attracted an increasing amount of interest due to their versatile characteristics. Plasmonic nanostructures can effectively concentrate incident light and can increase the overall efficiency through various mechanisms. However, there are several limitations to the use of plasmonic nanostructures, including their chemical and physical stability, charge recombination, and narrowband absorption properties. In this section, we comprehensively discuss these problems and suggest several points that need to be addressed. 4.1. Electrical and Chemical Stability

The stability and long-term performance of plasmonic nanostructures are highly critical for their use in solar cells. The chemical and electrical stability must be guaranteed to produce ideal plasmonic solar cells, and thus, silica, titania, and organic materials (polymers) have been widely applied as passivation layers on the surface of the plasmonic nanostructures to prevent or minimize the loss of plasmonic activity.127,131,133,134,142 The incorporation of these passivation layers has several purposes. (1) Plasmonic nanostructures may deteriorate charge separation and current collection by acting as recombination centers. This is a huge drawback that limits the active use of plasmonic nanostructures in solar cells. A spacer can be inserted between plasmonic nanostructures and chargetransporting materials to prevent direct contact between them and therefore reduce recombination of electron−hole pairs on the surface of the metal. (2) In any case, where thermal treatment is involved in the fabrication process, noble metal nanostructures are likely to lose their optical properties due to deformation, agglomeration, or Ostwald ripening. For example, high-temperature annealing (∼500 °C) is necessary to fabricate DSSCs, and plasmonic metal nanostructures coated with SiO2 or TiO2 layers kept their shape after the annealing process, indicating that protective layers can provide thermal and structural stability.51,176 (3) In conventional DSSCs where iodine-based liquid redox couple is used, rapid corrosion of

4.2. Balanced Light Absorption

The LHE has been improved by incorporating properly designed metal nanostructures into various types of solar cells, and it can be maximized in one of two ways. First, efficient resonant energy transfer from metal nanoparticles to a neighboring light absorber can increase the LHE. The spectral overlap of the LSPR band and the absorption spectrum of the photoactive material is essential for efficient resonance energy transfer. The second possible strategy is to extend the plasmonic response to a broader range of the solar spectrum to improve the photoabsorption in the weakly absorbing region, resulting in broadband light harvesting (Figure 33). However, metallic nanostructures typically resonate with light in a narrow spectral range, which restricts the plasmonic enhancement to a specific wavelength region. Therefore, a crucial factor in achieving practical plasmonic photovoltaic devices with an intensified LHE is to properly tune the position of the LSPR band by controlling the size and shape of the nanoparticles.124 Anisotropic metal nanoparticles, such as rods, cubes, and prisms, can be utilized since shape-controlled nanoparticles with sharp features show strong plasmonic fields around their corners or edges as well as multiple resonance bands throughout the visible to NIR ranges.51,144,145,164 Plasmon 15019


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electrons at lower energies (i.e., valence electrons) can shift to the Fermi surface or electrons near the Fermi-surface can jump to the next higher unoccupied energy level.280 These processes are a cause of additional high losses at optical frequencies. In particular, large losses at shorter wavelengths in the visible and UV ranges are caused by the interband electronic transitions of metals. Another important issue is the technical challenge to integrate these into viable devices. Metals can be readily degraded under harsh manufacturing environments, such as those with excessive heat, oxygen, or humidity, and this can be a critical issue to integrating metals into devices. The high cost and scarce supply are another drawback of using noble metals. Competitive pricing in addition to a high-tech manufacturing process are essential to realize commercial applications. Many alternative plasmonic materials have been proposed to overcome the shortcoming of using conventional plasmonic materials. Non-noble metal plasmonic materials, such as other metals,279,281 alloys,282 graphene,283,284 indium−tin oxide,285 and others,286−288 have attracted a considerable amount of interest due to their plasmonic properties. Aluminum (Al) has been emerged as an alternative metal due to its low cost and LSPR in the UV spectral region. Al is a nonresonant plasmonic metal, which means that the LSPR of Al cannot interact with the major region of incident light (i.e., visible ranges), and thus plasmon-induced losses by parasitic absorption would not occur. As a result, Al is considered to be a highly desirable plasmonic metal for use in solar energy applications where the optical absorption in the photoactive region is improved. In 2010, Akimov and Koh investigated the effects of resonant (Ag) and nonresonant (Al) plasmonic NPs in a thin-film hydrogenated amorphous silicon (a-Si:H) solar cell.289 The absorption enhancement in the photoactive region could be limited by the severe parasitic absorption caused by resonant coupling of the Ag NPs. On the other hand,

Figure 33. Absorbance spectra of various noble metal nanostructures and N719 dye.

resonance coupling (i.e., plasmon hybridization) is another promising technique that can be used to cover a wide range of the solar spectrum. For instance, combined Au core and Ag shell (Au@Ag core−shell) structures exhibited strong plasmon bands that were broader than individual nanoparticles due to the interaction between the high-energy and low-energy plasmon bands.148,149 In addition, plasmon coupling in adjacent nanoparticles can also amplify near-fields and optical absorption.153

5. PROSPECTIVE APPROACHES 5.1. Non-Noble Metal Based Plasmonics

By far, coinage metals such as Au, Ag, and Cu have contributed to the rapid growth of plasmonic photovoltaics, as discussed above. However, there are several problems with the use of conventional plasmonic materials. Interband transitions at optical frequencies are a significant source of optical loss, even though metals have a small Ohmic loss and high DC conductivity.278,279 When metal absorbs an incident photon,

Figure 34. Spectral absorption rates of the nanoparticle-enhanced a-Si:H layer and optimized nanoparticle arrays (red solid lines): (A) Ag, (B) Au, (C) Cu, (D) SiO2, (E) SiC, and (F) TiO2. The spectral response of the reference cell is plotted using dark dashed lines. Enhanced a-Si:H cells with ideal nanoparticles with constant permittivity Re[ε(ω0)] ≡ ε0 and zero dissipation indicated using blue dash-dotted lines. Reprinted with permission from ref 293. Copyright 2010 Applied Physics Letters. 15020


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have steered researchers to investigate “graphene plasmons”299 (or graphene surface plasmon polaritons, GSPP).300 Graphene consists of a hexagonal array of carbon atoms with three sigma bonding orbitals (sp2) and one π bonding orbital (pz) that is responsible for the conductivity of graphene. Graphene plasmons can be classified into three types: π plasmons, π+σ plasmons, and 2D plasmons. π Plasmons and π+σ plasmons require high energy for excitation (∼4.7 and ∼14.6 eV, respectively), and 2D plasmons exist only in doped graphene and can be excited with low energy (< ∼3 eV). Many theoretical studies have revealed the physical properties of plasmons in graphene, and this issue has been summarized in several reviews.301−303 Graphene plasmons can be coupled with other quasi-particles, such as photons, electrons, and phonons, subsequently resulting in novel properties.301−303 Specifically, the interaction of graphene plasmons with photons results in SPPs with strong plasmonic confinement (Figure 35B). Moreover, the plasmonic properties of graphene can be tuned by doping or modulating the geometric parameters, such as the number of layers, shape, size, etc. Nevertheless, graphene does not have an absolute absorption suitable for actual devices, despite its versatility, adjustability, stability, and extraordinary plasmonic properties of low loss, long lifetime, and high confinement.301 The SP frequency of graphene is noted to be in the THz and infrared range, and the visible light excitation of graphene plasmons has also been reported.304 In this regard, the coupling of graphene with noble metal plasmonic materials has been suggested as a solution to the limitations of graphene.301 By combining these two materials, the optical properties of graphene in the visible range can be markedly enhanced and the plasmonic response of metal nanostructures can be changed by the presence of graphene as well.305,306 This synergistic effect can advance the performance of the light-harvesting devices, such as photodetectors and photovoltaics.26,27 Numerous studies have already utilized graphene in photovoltaics (regardless of the combinaiton with plasmonic noble metal nanostructures), including such uses as transparent electrodes,307,308 as a component in HTL in OPV,309 as an interface modifier in an Si solar cell decreasing the series resistances,143 as additives in photoanodes of DSSCs with improved dye adsorption and electron lifetime,298 or as the active layer of an OPV to improve the electron transport.310 However, the plasmonic properties of graphene have not been investigated in these studies, and only a few studies have addressed the plasmonic effects of graphene, which will be discussed hereafter. The photoconversion efficiency of solar cells can be improved by coupling metal plasmons with graphene plasmons. Chen et al. have incorporated wrinkled graphene sheets and aluminum nanoparticles in Si solar cells as an antireflecting layer.290 Al nanoparticles were synthesized via thermal evaporation on NaCl powder and were then dispersed in aqueous solution. Since Al nanoparticles absorb UV light, which is only a small fraction of the solar spectrum, Al nanoparticles were selected rather than Ag or Au nanoparticles in order to maximize the light pathway in the cell by the scattering effect and to minimize the absorption of solar light by the nanoparticles. Reduced graphene oxide (rGO) prepared using a modified Hummer’s method311 and Al nanoparticles were dropped on top of the cell composed of an Ag finger (75 nm, screen printed) and SiN (180 μm, antireflective layer) on p-type Si wafers (c-Si) and Ag/Al back contact (Figure 36A). The cell with 100 nm Al nanoparticles with 10% surface

nonresonant Al NPs show comparable or even better photocurrent enhancement than the Ag NPs. Chen et al. also demonstrated Al NP-enhanced textured screen-printed solar cells.290 The incorporation of Al NPs with UV plasmon resonance effectively eliminated the “Fano effect” arising from resonant scattering and causing the low light absorption in the low wavelength region.291,292 As a result, a 7.2% increase in photocurrent could be achieved by maximizing the light trapping and optical absorption. Dielectric nanoparticles with a quite high permittivity and low dissipation level in the visible wavelength can also provide a comparable, or even better, improvement in thin-film solar cells. Numerical modeling was performed using a threedimensional model of an amorphous silicon solar cell (ITO/ NPs/a-Si:H/Al) to compare the performance of thin-film solar cells containing metallic or dielectric nanoparticle arrays.293FigFigure 34 shows the spectral absorption rate of the photoactive a-Si:H layer with the optimized NP arrays. The enhancement in the a-Si:H layer decreased dramatically at the resonant regions of the metallic NPs (Ag, Au, and Cu) through the excitation of surface plasmons. In cases with a small dispersion of dielectric NPs (SiO2, SiC, and TiO2), this enhancement nearly approached that of ideal dielectrics while a decreased enhancement was observed when compared to ideal particles due to finite dissipations (SiC and SiO2). The crucial point to note is that the dielectric materials should have a low dissipation, low dispersion, and high permittivity to achieve a higher enhancement. Numerical modeling of a-Si:H solar cells containing silicon carbide (SiC) NPs with a quite high permittivity (Re[ε(ω0)] = 6.96) revealed a 30% improvement that is achieved through a significant improvement in photoelectron generation, which is almost a perfect match with the ideal enhancement of 32%. 5.2. Graphene Plasmonics

The outstanding chemical, physical, and electrical properties of isolated monolayer graphite294 (i.e., graphene) have been extensively investigated ever since its experimental discovery,295 and its application has been suggested in various fields, including electronics,296 photonics,297 and theranostics.68 As such, the use of graphene-based light−matter interaction has been demonstrated to be a promising prospect298 due to the strong absorption of light (∼2.3% by monolayer graphene) in spite of its extremely thin thickness of a single atom level. Moreover, the electrons in graphene show linear dispersion (Figure 35A) and can propagate like photons. These findings

Figure 35. (A) Dispersion relation of doped graphene and (B) calculated near-field distribution on a doped graphene sheet (Fermi level = 0.5 eV) upon coupling with a photon (0.5 eV) from a dipole emitter at 10 nm above the graphene. The real part (red) and imaginary part (blue) of the electric field are indicated. Reproduced from ref 299. Copyright 2011 American Chemical Society. 15021


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Figure 36. (A) Schematic of Al nanoparticles and wrinkle-like graphene incorporated Si solar cell and (B) EQE spectra of bare solar cell and Al/ graphene-enhanced solar cell. Reproduced with permission from ref 290. Copyright 2013 Nature Publishing Group. (C) Ag nanoparticle and graphene-introduced Si solar cell schematics and (D) J−V curves. Reprinted with permission from ref 312. Copyright 2015 Springer.

coverage showed an increase in Jsc by 6.3% when compared to the reference cell (without Al/graphene) and an increase in energy conversion efficiency from 18.24 to 19.36%. Cells with Al nanoparticles/wrinklelike graphene showed an increase in Jsc of 7.2% when compared to the reference cell, achieving 19.54% conversion efficiency. The enhanced photocurrent in the graphene-deposited cell was explained as the antireflecting effect of the graphene layer. Even planar graphene without wrinkles could slightly improve Jsc due to the lower refractive index of graphene than the SiN layer below 600 nm. Furthermore, wrinkled graphene could induce an EQE enhancement at both below 600 nm and at longer wavelengths due to the light trapping effect from the wrinkle structure (Figure 36B). Tripathi et al. also developed Ag nanoparticles and grapheneincorporated Si solar cells.312 Three nanometers Ag nanoparticles and graphene were introduced on top of the FTO, which is located on top of the antireflective layer, as light trapping materials (Figure 36C). Three nanometers Ag nanoparticles were prepared via thermal evaporation and annealed, and then the graphene solution was deposited. A maximum improvement in the Jsc by 6.35% and PCE by ∼10.84% was observed compared to the cell with Ag nanoparticles only (without graphene). This enhancement was described to be a result of the electromagnetic coupling of the Ag nanoparticles and graphene. So far, several efforts have been made to achieve synergistic effects by incorporating an ensemble of graphene and metal nanostructures in photovoltaics.187,191,230,290,312−316 However, most of these have interpreted the improved performance in terms of the improved carrier mobility and chemical stability of graphene. The optical contribution of graphene and the coupling of graphene plasmons with the SPR of metal nanostructures has been described only in the aforementioned

works, in which graphenes were employed as a light-trapping layer on top of the Si solar cell. However, even in these studies, the coupling effect between the two materials was merely assumed. First of all, a more clear and comprehensive method to prove the synergistic behavior of the two plasmonic materials in photovoltaics should be developed, and the effect of the plasmonic coupling between graphene and the metal nanostructures in different parts of solar cells should also be investigated. Second, graphene can exhibit different LSPR properties by adjusting the size and dimensionality,299 so LSPR coupling between graphene plasmons of zero dimension and metal plasmons should also be examined. 5.3. Plasmon-Upconversion Coupling

Upconversion is the process to release photons with a higher energy than that at excitation by the sequential absorption of two or more photons (e.g., UV or visible light emission can be realized upon NIR excitation). Although NIR light comprises almost 40% of solar irradiation, the materials developed for photovoltaic devices mainly respond to UV−visible light, indicating that a large portion of solar light is just transmitted and does not contribute to the cell performance at all. In this regard, the inclusion of upconverters in solar cells was expected to improve the performance.317 Lanthanide ion doped oxides have been commonly used as upconverters in phovoltaics. However, their low absorption cross section produced an insignificant effect. Coupling with plasmonics has generally been known to dramatically improve the emission of upconverters through resonant or nonresonant processes in close proximity. When the SPR frequency of metal structures is resonant with the emission wavelength of upconverters, PRET takes place in a way to increase the PL of upconverters in the UV or visible region, and in a nonresonant system, the intensity of the excitation source 15022


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exposed to the upconverters can be strengthened through an electromagnetic field generated at the metal structures.318 Thus, it is reasonable to anticipate that the coupling between SPs and upconversion may lead to an extraordinary improvement in photoconversion. Plasmonic structures of different shapes and sizes have been incorporated inside or outside the solar cell devices in the presence of upconverters. A scattering layer consisting of βNaYF4:Yb,Er@SiO2@Au core−shell structures was introduced into DSSCs via deposition on top of the TiO2 photoanode. This induced (1) higher excitation of the dye molecules by the emission from upconverters, (2) large scattering due to the size of the upconverters, and (3) a plasmonic effect by Au boosting dye excitation and the upconverter emission. The emission of the upconverters increased by a factor of more than two under laser excitation at 980 nm after decoration with Au nanoparticles, and the PCE improved from 7.17 to 7.56 and 8.23% by the inclusion of upconverter@SiO2 and upconverter@ SiO2@Au structures, respectively, under one sun AM 1.5G solar illumination.319 At the rear side of the DSSC, 30 mol % Fe3+ doped βNaGdF4:Yb,Er upconverters and Ag NSs were sequentially deposited. The upconverters were excited by a 980 nm light source and released visible light to sensitize N719 dye molecules, while the Ag nanoparticles induced scattering of the unabsorbed light back to the device and increased the intensity of the upconverted emission. The green and red emission of the upconverters showed enhancement factors of 3.1 and 2.2 after the introduction of the Ag nanoparticle layer, and the device including only upconverters and upconverters with Ag nanoparticles as rear reflectors showed 7.14 and 21.4% enhancement in PCE, respectively, under one sun illumination.320 In spite of the noticeable enhancement in the PCE for both cases, it was not clear that the synergy between the upconverters and the plasmonic structures indeed influenced the device performance since the quantum yield of the upconverters would be very low under simulated solar light irradiation. In that sense, Song et al. demonstrated hybrid upconverter/plasmonic structures to estimate their efficacy in generating power for the DSSCs under NIR light irradiation (Figure 37). In this study, Yb2O3 released white broad band light instead of the coherent light that is normally emitted by conventional upconversion processes because the excitation wavelength range could be extended to 770−980 nm via PRET from the Au NRs. The hybrids were integrated at the front side of the DSSC, and a PCE of 0.01% was achieved under 790 nm light illumination by sensitizing N719 dye molecules in the device with upconverted light.321 To practically exploit the upconversion phenomena in solar cells under one sun illumination, Ag-coated periodic nanohole/ post structures were designed as a platform to deposit βNaYF4:Yb,Er upconverters. On the basis of the SPP resonance of Ag nanohole/post structure that efficiently confines the incident light in the vicinity of the metal surface, the emission intensity of the upconverters increased by approximately 130 and 70 times at 660 and 540 nm, respectively, compared to that obtained from plain Ag film at an excitation of 968 nm. When 8 μm thick Si solar modules were embedded into a plasmonically engineered upconversion substrate, a 13% improvement was achieved compared to that of a cell containing Ag nanostructures without upconverters under solar light irradiation. These results open up the practical application of

Figure 37. (A) TEM image and (B) EDX mapping image of Yb2O3/ Au NR hybrid. The green color in (B) indicates that Au moieties are evenly distributed throughout the Yb2O3 nanoparticles. (C) DSSC device including Yb2O3/Au NR hybrids at the front side. (D) I−V characteristics obtained from a DSSC, including Yb2O3/Au NR hybrids (red) under 980 nm light illumination. The PCE value was comparable to that obtained from NaYF4:Yb,Er-based DSSC (black), which has been demonstrated in a number of previous studies. (E) I− V profiles of DSSC devices with and without Yb2O3/Au NR hybrids under 790−830 nm illumination. The PCE value reached 0.01% when the device was exposed to a 790 nm light source due to the incorporation of an Au element in the hybrid structure. Reprinted from ref 321. Copyright 2014 American Chemical Society.

upconversion photovoltaics by integrating an optimal configuration of plasmonic structures in solar cell devices.322 5.4. Plasmon-Enhanced FRET Solar Cells

As discussed above, a number of studies have investigated the SP-assisted energy conversion by appropriately incorporating plasmonic nanostructures into conventional photovoltaic devices. However, great challenges remain to maximize the plasmonic effect by applying plasmonic structures into multicomponent systems. The Taylor group suggested a ternary blend of P3HT:PCBM:squaraine (SQ) dye as an example of highly efficient multicomponent OPV devices.323 The emission of P3HT, an inevitable event following the recombination processes in P3HT, can excite SQ molecules via FRET, which turned the significant loss in the device performance into a chance to generate extra excitons. The FRET efficiency from P3HT to SQ was calculated up to 96%, and it contributed to a 38% increase in PCE compared with P3HT:PCBM-based OPVs without SQ dye. Recently, we showed that the FRET efficiency can be improved by coupling with plasmonic materials.24 Au nanoparticles were chosen as a core to generate a near-field around a pair of FRET shell consisting of CdSe nanoparticle donor and 15023


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of a new class of plasmonic materials into photovoltaic devices. A question may also arise as to which types of solar cells may actually benefit from plasmon enhancement in realistic device geometries with optimized fabrication approaches. The degree of improvement in the efficiency of plasmonic cells would be comparable among different types of cells. Despite the rapid improvement in perovskite cells, the stability and overall performance has failed to surpass that of commercial Si solar cells. Thus, a reasonable strategy involves optimizing the design of plasmonic Si solar cells in a cost-effective way. We also find some common ground in most of the reported works in that the stability of the metallic nanostructures has not been thoroughly assessed through a careful study of the morphology during fabrication, including the annealing step, or under harsh operational environments. In summary, it is challenging but important to establish a consensus on the improvement in performance of plasmonic solar cells as well as a universal strategy to anchor metallic nanostructures into the necessary elements of the cell in a stable manner so that researchers can design metallic nanostructures with the required configuration in conjunction with target-oriented solar cell types. In parallel with further studies on the alternative strategies proposed in this article (e.g., graphene plasmonics coupling, plasmon-upconversion coupling, or plasmon-enhanced FRET effect), it is desirable for more creative and challenging protocols to be discussed by the related community to realize plasmonic solar cells.

S101 dye acceptor. By inserting a spacer of optimum thickness between the Au nanoparticle and a FRET pair, the near-field enhancement became dominant, resulting in an increase in the PL of CdSe as well as an energy transfer from CdSe to the S101 dye. The FRET efficiency improved to 86.57%, and the observed value was four times higher than that observed from a FRET pair without an Au nanoparticle core. Put together, the combination of a FRET pair and plasmonic structures in multicomponent OPVs is expected to greatly improve the cell performance for the following reasons (Figure 38). In a ternary blend consisting of P3HT:PCBM:SQ,

Figure 38. A plausible mechanism of plasmon-enhanced FRET OPVs.

plasmonic nanostructures can improve charge carrier generation and recombination processes in P3HT via near-field enhancement under light irradiation, leading to an increase in the PL properties of P3HT. As a result, a stronger energy is provided to excite the SQ molecules, generating excess charge carriers, than in the case of excitation of neat P3HT. When the LSPR band of the plasmonic nanostructures overlaps with the absorption band of P3HT, and the plasmonic nanostructures are placed nearby P3HT molecules, PRET to P3HT additionally occurs, contributing to more charge carrier generation in P3HT. In the case of the incorporation of plasmonic nanostructures with multiple LSPR bands, PRET to both P3HT and SQ can be simultaneously induced, increasing the number of available charge carriers in the OPV devices. The utilization of FRET in solar cell devices has been wellestablished by incorporating additional light sensitizers together with conventional dye molecules or quantum dots.324−326 Therefore, the concept of plasmon-enhanced FRET solar cells can be extended to other types of solar cells, including DSSCs, QDSCs, and perovskite solar cells.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: +82-2-3277-4517. Fax: +822-3277-4546. ORCID

Yu Jin Jang: 0000-0001-8116-3618 Dong Ha Kim: 0000-0003-0444-0479 Present Addresses ‡ Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), 5 Hwarangno 14-gil, Seoul 02792, Republic of Korea. § Department of Chemistry, University of North Carolina at Chapel Hill, 131 South Road, Chapel Hill, North Carolina 27599−3290, United States. ∥ Department of Electrical and Computer Engineering, University of Toronto, 10 King’s College Road, Toronto, Ontario M5S 3G4, Canada.

6. CONCLUDING REMARKS AND OUTLOOK This review constitutes an attempt to comprehensively survey the effect of SPR on the performance of photovoltaic devices. Plasmonic effect is a valuable means to improve the solar cell functionality through the fascinating nature of the coexistence of electrons and photons in confined, nanostructured metal surfaces. Nevertheless, several issues need to be addressed to ensure that plasmonics can be applied to photovoltaics. First, representative noble metals like Ag and Au are expensive materials and the need to tailor the patterning of these metals to attain optimal plasmonic functionality may limit their practical application. In this regard, there is growing interest to develop viable plasmonic properties using inexpensive, nonnoble metals327,328 or semiconductors287,322,329−338 as alternatives. Researchers may actively investigate the incorporation

Author Contributions ⊥

Y.H.J. and Y.J.J. contributed equally to this work.

Notes

The authors declare no competing financial interest. Biographies Yoon Hee Jang received her B.S. degree in 2008 in the Department of Bio and Nano Chemistry from Kookmin University, Korea. She then earned her M.S. and Ph.D. degrees in the Department of Chemistry and Nano Science from Ewha Womans University. Her doctoral thesis on “Structural and optoelectrical manipulations to enhance the performance of dye-sensitized solar cells” was carried out under the supervision of Prof. Dr. Dong Ha Kim. She is currently a postdoctoral research fellow in Photoelectronic Hybrid Research Center at Korea 15024


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Institute of Science and Technology, and her research focuses on the development and optimization of efficient photovoltaic devices.

(2014R1A2A1A09005656, 2015M1A2A2058365, and 20110030255).

Yu Jin Jang obtained her B.S. and M.S. degree from Ewha Womans University in 2009 and 2011, respectively. Currently, she is pursuing a Ph.D. degree in the Department of Chemistry and Nano Science at the same university, under the supervision of Prof. Dong Ha Kim. She is interested in the development of energy storage and conversion devices combined with plasmonics.

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Seokhyoung Kim received a B.S. in Chemistry from Pohang University of Science and Technology (POSTECH) in 2012 and a M.S. in Polymer Science and Engineering from the University of Massachusetts at Amherst in 2013. He is currently a Ph.D. candidate at the University of North Carolina at Chapel Hill under the guidance of Prof. James Cahoon, where he studies optical properties of morphology-controlled silicon nanowires. Li Na Quan received her B.S. degree in 2011 in Department of Chemical Engineering from Yanbian University in China. She earned her Ph.D. degree in the Department of Chemistry and Nano Science at Ewha Womans University under the supervision of Prof. Dong Ha Kim. She is currently a postdoctoral research fellow in the Department of Electrical and Computer Engineering at the University of Toronto under the guidance of Prof. Edward H. Sargent. Her research interests focus on developing bright organo-lead halide perovskite light-emitting diode devices and highly stable perovskite solar cells. Kyungwha Chung received her B.S. degree in 2011 and M.S. degree in 2013 in Chemistry and Nano Science from Ewha Womans University. She is currently a Ph.D. candidate student in the Department of Chemistry and Nano Science at Ewha Womans University under the supervision of Prof. Dong Ha Kim. Her current doctoral research interests include the development of hybrid plasmonic nanostructures for applications in optical sensing. Dong Ha Kim received his B.S. in the Department of Textile Engineering and M.S. and Ph.D. degrees in the Department of Fiber and Polymer Science at Seoul National University in Korea. He carried out his postdoctoral research activities in the Polymer Science and Engineering Department at the University of Massachusetts at Amherst (from 2000 to 2003) with Prof. Thomas P. Russell and in the Materials Science Department at the Max Planck Institute for Polymer Research (from 2003 to 2005) with Prof. Wolfgang Knoll. After postdoctoral work, he joined Samsung Electronics Co. in the Memory Division of the Semiconductor R&D Center as a senior scientist. Then he assumed a faculty position at Ewha Womans University in 2006. Currently he is a Full Professor and Ewha Fellow in the Department of Chemistry and Nano Science of the School of Natural Sciences. His research interests include the development of multifunctional hybrid nanostructures for applications in energy storage and conversion, environmental remediation, memory devices, display devices, and theragnosis. Surface plasmon resonance mediated photovoltaics, photocatalysis, light emission, and theragnosis are also his lasting interests. He has authored or coauthored 124 peer-reviewed SCI publications. He holds 41 issued Korean (25 registered) and 4 PCT patents (1 registered U.S. patent). As for the professional offices and services, he is serving as Associate Editor of Science of Advanced Materials (American Scientific Publishers), an editorial board member of Scientif ic Reports (Nature Publishing Groups), and an advisory board member of Nanoscale (Royal Society of Chemistry).

ACKNOWLEDGMENTS This study was supported by the National Research Foundation of Korea Grant, funded by the Korean Government 15025


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