Plasmonic nanostructures for light trapping in
organic photovoltaic devices
Chun-Hsien Chouaand Fang-Chung Chen*b
Over the past decade, we have witnessed rapid advances in the development of organic photovoltaic devices (OPVs). At present, the highest level of efficiency has surpassed 10%, suggesting that OPVs have great potential to become competitive with other thin-film solar technologies. Because plasmonic nanostructures are likely to further improve the efficiency of OPVs, this Article reviews recent progress in the development of metal nanostructures for triggering plasmonic effects in OPVs. First, we briefly describe the physical fundamentals of surface plasmons (SPs). Then, we discuss recent approaches toward increasing the light trapping efficiency of OPVs through the incorporation of plasmonic structures. Finally, we provide a brief outlook into the future use of SPs in highly efficient OPVs.
1
Introduction
The use of traditional fossil fuels is leading to increasing envi-ronmental concerns, including air pollution, global warming, and possibly climatic anomalies. To sustain economic growth, renewable energy sources, such as solar energy and wind power, are being considered as promising energy alternatives. The development of solar energy is a particularly suitable solution to the energy crisis because sunlight is clean, sustainable, and
naturally abundant. Nevertheless, wafer-based solar cells, which have the major share of the current photovoltaic market, are not capable of producing energy at a price comparable to that of electricity generated from fossil fuels in most areas of the world.1Therefore, the quest remains for inexpensive, scalable, next-generation solar technologies.
Among emerging solar technologies, organic photovoltaics (OPVs) are receiving a great deal of attention because of their attractive properties: low cost, light weight, mechanical exi-bility,2,3and extremely short energy payback time.4,5Recently, the power conversion efficiencies (PCEs) of OPVs prepared using the concept of bulk heterojunctions (BHJs) have approached 10%,6,7with the highest recorded efficiency having broken through 10% (ref. 8–12) for a tandem structure. These
Chun-Hsien Chou received his Bachelor's degree from the Department of Physics in Fu Jen Catholic University in 2006 and his Master's degree from the Institute of Physics in National Taiwan Normal University in 2008. Prior to joining National Chiao Tung University (NCTU) in 2011, he worked in the PV industry as a R&D engineer for two years. He is currently a Ph.D. student in the eld of organic solar cells andexible waveguiding photovoltaics in the group of Prof. Chen at NCTU, Taiwan.
Dr Fang-Chung Chen is a
professor in the Department of Photonics at National Chiao Tung University (NCTU). He received his Bachelor's and Master's degrees in Chemistry from National Taiwan Univer-sity in 1996 and 1998, respec-tively, and his Ph.D. degree in Materials Science and Engi-neering from the University of California, Los Angeles (UCLA), USA, in 2003. He was a post-doctoral research associate in the Department of Materials Science and Engineering, UCLA, in 2003. Prof. Chen is the recipient of the Award for Junior Research Investigators of Academia Sinica 2008. His research interests include exible solar cells, organic elec-tronics, and low-dimentsional nanomaterials.
a
Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan
bDepartment of Photonics and Display Institute, National Chiao Tung University,
Hsinchu 30010, Taiwan. E-mail: [email protected] Cite this:Nanoscale, 2014, 6, 8444
Received 23rd April 2014 Accepted 19th May 2014 DOI: 10.1039/c4nr02191f www.rsc.org/nanoscale
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efficiencies remain, however, lower than those of other thin-lm solar technologies, such as CdTe and CIGS photovoltaics. In other words, continuous improvements in efficiency will be necessary if OPVs are to become commercially viable. Moreover, the reliability of OPVs must be improved substantially to meet commercial requirements.13
The working principle of a typical OPV involves six steps:14(i) photon absorption, (ii) exciton generation, (iii) exciton diffu-sion, (iv) exciton dissociation, (v) charge transport, and (vi) charge collection. Electron/hole pairs (excitons) are generated upon the absorption of photons in organic semiconductors. Because the exciton binding energy is usually large (0.3–1 eV), they can be dissociated into free charges only at the interfaces between strong electron donors and electron acceptors. There-fore, a BHJ structure is oen adopted to ensure a large-area donor–acceptor interface, thereby producing a high density of free charge carriers. The free charges at the heterojunctions are then transported through the donor and acceptor materials, respectively, to the electrodes. The collection of the charges by the electrodes results in the generation of electrical power. The efficiencies of the above-mentioned processes are oen evalu-ated in terms of the external quantum efficiency (EQE), also known as the incident photon-to-electron conversion efficiency (IPCE), using the equation,15
EQE(l) ¼ hab(l) hgen(l) hcoll(m) (1) wherehab(l), hgen(l), and hcoll(m) are the efficiencies of absorp-tion, carrier generaabsorp-tion, and charge collecabsorp-tion, respectively. The efficiency of carrier generation can reach almost 100% in a BHJ solar cell because of its high donor–acceptor interfacial area. It is very difficult, however, to simultaneously improve the effi-ciencies of absorption and charge collection. While thicker photoactive layers can be used to increase the value ofhab(l), the possibility of charge recombination and the inevitable increase in the device series resistance will result in a lower value of hcoll(m).16Therefore, the tradeoff always exists between a high degree of light absorption and efficient charge collection.
One approach toward ensuring efficient light absorption when using a thin photoactive layer is to develop light trapping schemes. Many trapping strategies for OPVs have been proposed, including using optical spacers,17–20 photonic crystals,21,22 and folded device architectures.23 More recently, metal nano-structures, which trigger surface plasmons (SPs), have proved to be promising materials for effective light trapping in thin-lm solar cells.24–27 Surface plasmons are coherent oscillations of electromagnetic waves propagating along the surface of a conductor. They wererst discovered by Faraday during his study of the colors of colloidal metal nanoparticles (NPs).28 Several particle shapes (e.g., spheres, prisms, cubes, rice grains, stars)29–37 and periodic structures on the nanoscale (e.g., metal nano-hole,38,39 nanodiscs,29,40 nanopillars41,42 and nanogratings43,44 arrays) have been successfully developed to induce SPs, with the optical properties readily tailored through modications in their sizes, shapes, and even the surrounding dielectric materials.45 Accordingly, these nanostructures have great potential as effective optical tools for enhancing the absorption properties of OPVs.
In this Review Article, werst introduce the concept of SPs. We then focus on the results of recent research into the devel-opment of potential plasmonic-enhanced OPVs. Finally, we provide a short summary and perspective regarding the future use of plasmonic nanostructures in highly efficient OPVs.
2
Physical properties of surface
plasmons
2.1. Localized surface plasmons
Localized surface plasmons (LSPs) are associated with the collective oscillations of electrons conned locally by metal nanostructures. As displayed in Fig. 1(a), the most representa-tive examples of LSPs are metal NPs. The particle plasmons are excited when the frequency of the incident photons matches the resonance frequency of the NPs. Their resonance wavelength depends on the particle shape, size, and the dielectric param-eters of the surrounding environment.32,46,47 From the quasi-static approximation, the polarizability (P) of a spherical NP can be expressed as:
P ¼ 4pa33 3m 3 þ 23m
(2) where a is the diameter of the NP and3 and 3mare the dielectric constants of the surrounding dielectric medium and of the metal NP itself, respectively. From eqn (2), we conclude that the value of P reaches its maximum when 3m is equal to –23, resulting in a resonance condition. Because the excited plas-mons are localized and cannot propagate within the nano-structure, this process is known as localized surface plasmon resonance (LSPR).
Fig. 1 (a) LSPs confined in metal NPs. The SPs are excited by the electricfield (E0) of the incident light of wavelengthl in spherical NPs of diametera. (b) Schematic representation of the SPP mode excited at the metal–dielectric interface.49(c) Dispersion curve of a typical SPP
mode; a momentum mismatch exists between the light and the SPP.49
If we assume that the size of the NP is much smaller than the incident wavelength (a l), then the cross-sections of light scattering (ssca) and absorption (sab) are given by:26,48
ssca¼ 1 6p 2p l 4 jPj2¼8p 3 k 4a63 3m 3 þ 23m sab¼ 2p l Im½P ¼ 4pka3 3 3m 3 þ 23m (3)
where k is the wavenumber of the light. Further, the scattering efficiency (Qsc) can be dened as:26
Qsc¼ ssca/(ssca+sab) (4)
According to eqn (3), the particle size is entirely responsible for the extinction process;25,46larger particles scatter light more efficiently, whereas smaller ones absorb most of the photons. One unique feature of LSPs is the local enhancement of the electromagneticeld as a result of the strong resonance effects, leading to effective light concentration. The enhancement factor can reach as high as 100.50Such a plasmonic neareld in the vicinity of an NP decays exponentially with respect to the distance from the surface of the NP. The decay lengths of the electricaleld are usually of the order of the particle size, but also depend on the dielectric properties of the surroundings and the metallic material itself.24,32,46
2.2. Surface plasmon polaritons
The other types of excited SPs on metal surfaces are surface plasmon polaritons (SPPs), collective oscillations of electro-magnetic waves at a metal–dielectric interface [Fig. 1(b)]. The electromagneticeld of an SPP is conned at the metal surface, with the electric eld also enhanced perpendicular to the surface. From Maxwell's equations, the dispersion relationship for a typical SPP can be derived using the equation:48
Ksp¼ u c ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3d3m 3dþ 3m r (5) where Kspis the wave vector of the SPP;3dand3mare the relative permittivities of the dielectric and metal materials, respectively; andu and c are the angular frequency and the speed of light in a vacuum, respectively. Fig. 1(c) displays the dispersion curve of an SP mode; a momentum mismatch exists between the photon
and the SPP, implying that direct excitation of SPPs with photons is not allowed in the case of a planar interface. Three approaches have been developed to match their momenta:24,49 (i) to use a prism covered with a thin layer of metal to increase the momentum of the incident light; (ii) to apply scattering centers to trigger SPs; and (iii) to employ periodic corrugated structures, such as gratings, at the interface. The third is the most common approach used in OPVs, presumably because of compatibility with thin-lm devices. In the next section, we discuss several examples of periodic nanostructures that have successfully improved the performance of OPVs.
2.3. Device structures for plasmonic-enhanced OPVs
Three common device structures have been developed employ-ing plasmonic nanostructures to enhance the OPV performance (Fig. 2).24–26Therst design involves the incorporation of metal nanostructures, such as NPs, on the surface of the active organic layers; they behave as scattering centers that can trap the light in the device. The optical path will be increased through multiple scattering events, thereby increasing the photon absorption efficiency. In the second scenario, NPs are directly embedded into the active photoactive layer. Relatively small NPs (5–20 nm) will act as subwavelength antennas upon photoexcitation, thereby inducing the local-eld enhancement. Because the absorption efficiency of the organic layers is proportional to the intensity of the electromagneticeld, the overall device efficiency can be improved. Nevertheless, such an ideal structure as dis-played in Fig. 2(b) is very difficult to realize because the NPs will phase-separate from the organic semiconductors, thereby increasing the device resistance. Therefore, the nanostructures should be carefully designed to control the distribution of the NPs in the photoactive layers.25,51In thenal case, as depicted in Fig. 2(c), periodic corrugated nanostructures are included at the interface between the metal electrode and the absorption layer to sustain propagating SPPs. The evanescent electromagneticelds excited by the incident light effectively direct the vertical energy into the semiconductor layer. Such a design ensures that photons are absorbed along the lateral direction and that the optical path is increased by several orders of magnitude with respect to the thickness of the active layer. In other words, high absorption efficiency can theoretically be achieved from a thin lm.
Fig. 2 Structure designs of plasmonic-enhanced OPVs.24,25,48(a) Nanostructures behaving as scattering centers; incident photons are scattered
mostly into the material having a higher dielectric constant at the thinfilm surface. (b) Embedded NPs positioned in organic semiconductors to enhance the near-field in the cell; scattering events also possibly occur in such solar cells. (c) A periodical structure induces SPPs, which can turn the incident solarflux by 90.
3
Experimental results of plasmonic
OPVs
3.1 NPs positioned in the vicinity of the photoactive layer 3.1.1 Spherical NPs. The use of metal NPs is the most common means of obtaining plasmonic-enhanced OPVs because of the simplicity of their fabrication.26,46,52–54In 2008, for example, Morfa et al. deposited Ag NPs through thermal evaporation on indium tin oxide (ITO) substrates55 and then spin-coated a layer of poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) buffer onto the NPs. When using a blend of poly-(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) as the photoactive layer, the device PCE increased from 1.3 to 2.2% aer the incorporation of the Ag NPs. Other methods, such as pulse-current electrode deposition, have been also used for the synthesis of the noble metallic NPs.56,57 Kim et al. demonstrated that a relatively uniform distribution of NPs could be achieved through such electrochemical deposition; their device PCE increased from 3.05 to 3.69% aer introduction of the Ag NPs.58
Another noble metal, gold, can also produce LSPR phenomena. In 2009, we blended Au NPs into the anodic buffer layer (PEDOT:PSS) of OPVs to trigger LSPR.54,59 Because NP solutions are frequently prepared in water through colloidal chemistry,60,61buffer solutions containing Au NPs are readily prepared through blending of Au NP and PEDOT:PSS solutions [Fig. 3(a)]. This relatively simple plasmon-enhancing layer improved the device PCE from 3.57 to 4.24%. Another benet of such a simple fabrication approach is highly repeatable results, allowing systematic studies of the mechanism behind the enhancement in PCE. Fig. 3(b) presents the increase in quantum efficiencies aer incorporating Au NPs, compared with the extinction spectrum of the Au NPs. The difference in the incident photon–electron conversion efficiency (DIPCE) spectrum coincides with the extinction range of the Au NPs, indicating that the LSPR did indeed improve the photocurrent. To further investigate the near-eld effect, steady-state photoluminescent (PL) spectra of the P3HT:PCBMlms were obtained; the sample deposited on the PEDOT:PSS layer doped with Au NPs exhibited higher intensity, suggesting an increased level of photoabsorption.48We also performed exciton lifetime mapping of the photoabsorbinglm by using a time-resolved PL measurement system together with confocal laser scanning microscopy, which allowed vertical evolution of the exciton lifetime (sexciton) in the thinlms at various z-axial positions.62 Fig. 3(c–f) display histograms of the vertical evolutions of sexciton in thinlms prepared under various conditions. For the refer-ence sample, the average value ofsexcitonwas 0.48 ns when the focal plane was close to the PEDOT:PSS layer. The lifetime decreased to 0.35 ns when the focal plane was positioned away from the PEDOT:PSS buffer. The inhomogeneous distribution of the PC60BM molecules in the P3HT could account for the lifetime evolution.62Completely opposite behavior was observed for the samples containing Au NPs in the PEDOT:PSS layers. The value of sexciton decreased to 0.23 ns, presumably because of
strong coupling between the plasmons and excitons.
Furthermore, the values ofsexcitonof the upper positions, away from the surfaces of the metal NPs, were less affected, sug-gesting that near-eld plasmonic effects contributed signi-cantly to the device enhancement.
Although PEDOT:PSS is the material most widely used for the anode buffers, its acidity and hygroscopic properties usually result in poor device stability.63 Therefore, several alterative buffer materials have been tested to improve the reliability of plasmonic-enhanced OPVs. For example, solution-processed molybdenum oxides (MoO3)64have been blended with Au NPs; the mixed solutions were then spin-coated onto ITO substrates, with the resulting nanocomposite layer becoming an anode buffer for OPVs.64,65 The device exhibited a remarkable enhancement in PCE aer the incorporation of Au NPs. More-over, the stability of devices prepared using a MoO3buffer is superior to that of devices based on PEDOT:PSS buffers.64 Furthermore, because inverted device architectures eliminate the need for low-work function metals, which are air-sensitive,63 they usually exhibit prolonged device lifetimes.66,67The upper polymer layers also naturally behave as protective shields from oxygen and moisture in the air. Plasmonic-enhanced OPVs possessing an inverted device structure have also been demonstrated.68–70 A mixed solution of Au NP/Cs
2CO3 was
applied in inverted OPVs having the structure ITO/Au NP:Cs2CO3/P3HT:PCBM/MoO3/Ag. The photocurrents and ll Fig. 3 (a) Device structure of OPV containing Au NPs and the method of preparation of the buffer solution containing Au NPs.59 (b) The
change in IPCE after blending the Au NPs, compared with the extinction spectrum of the Au NPs. (c–f) Histograms of the exciton lifetimes for the (c and e) reference and (d and f) plasmonic devices at the positions (c and d) away from and (e and f) close to the PEDOT:PSS buffer layer. Average values of sexciton: (c) 0.35, (d) 0.27, (e) 0.48, and (f) 0.23 ns.54
factors both improved aer the addition of Au NPs to the Cs2CO3 buffer layer; overall, the PCE increased from 3.12 to 3.54%. A study of the morphologies of the cathode interfaces revealed that rough surfaces might increase the device resis-tance; this drawback is, however, overshadowed by the advan-tageous plasmonic effects.68
Aggregation of NPs in PEDOT:PSS is oen observed unless a capping agent is applied.71 To overcome this issue and to further control the uniformity of the NP distribution, Fan et al. used graphene oxide (GO) as the template to construct metal nanostructures to trigger SPs in OPVs.71 Au NPs were rst adhered to the GO surface and then blended into the PEDOT:PSS layer. The GO template helped to avoid aggregation of the Au NPs and to introduce plasmonic effects without dramatically sacricing the device's electrical properties.71 Moreover, GO itself and its reduced form (rGO) are effective interlayers when positioned between the electrodes and the photoactive layer in OPVs.72,73 We recently reported the synthesis and characterization of Au NP/GO nanocomposites for the triggering of LSPRs.74The morphology of the Au NPs on the GO surface was manipulated by controlling the amount of capping agent during the synthesis steps. When the nano-materials served as the anodic interlayer, they introduced LSPR effects in the OPVs, leading to a noticeable enhancement in efficiency. That study implies new avenues for constructing plasmon-enhancing layers on the nanoscale.
In addition to blending with P3HT:PCBM, the most well-known model polymer mixture, to construct BHJ solar cells, the metal NPs have been also applied in other material systems. In principle, the open-circuit voltage (Voc) is highly related to the energy difference between the highest occupied molecular orbital (HOMO) of the polymer semiconductor and the lowest unoccupied molecular orbital (LUMO) of the fullerene deriva-tive.75,76In other words, if PC
60BM is replaced by another elec-tron acceptor and the value of Vocis increased, the overall PCE will be improved. The representative acceptor was indene-C60 bisadduct (ICBA); its LUMO, higher than that of PC60BM, has been proved to increase the photovoltage.77,78Cheng et al. used P3HT:ICBA blends to fabricate inverted devices, introducing a nanostructured scattering rear electrode to enhance the light-trapping efficiency.78 The Ag NPs were fabricated through thermal deposition and sandwiched between MoO3 layers, resulting in an interesting device structure: ITO/ZnO/ P3HT:ICBA/MoO3/Ag NPs/MoO3/Al. The PCE increased to 7.21% from a value of 6.26% for the reference cell prepared without plasmonic NPs at the electrode.
A single organic material can only absorb within a limited range of the solar spectrum. To extend the absorption region, many low-band gap (LBG) polymers have been developed to harvest low-energy photons from solar irradiation. For example, polythieno[3,4-b]thiophene/benzodithiophene (PTB7) is one of the most promising LBG polymers; a PCE of over 7% has been achieved for a device based on a PTB7:PC70BM polymer blend.77 Metal NPs have also been employed to further improve the performance of OPVs incorporating these LBG polymers. Recently, Baek et al. investigated the effect of size-control by blending Ag NPs into the anodic PEDOT:PSS buffer; the
photoactive layer was a blend of PTB7 and PC70BM.79 They compared the performance of the devices incorporating various particle sizes, with the PCE of the optimal PTB7-based device increasing to 8.6%. Meanwhile, Choi et al. reported a PTB7:PC70BM device exhibiting an impressive efficiency of 8.92%.80 As displayed in Fig. 4(a), they prepared silica-coated silver (Ag@SiO2) NPs and incorporated them into devices to trigger LSPR effects. The Ag@SiO2 NPs were either blended directly into the PEDOT:PSS layer (type I) or positioned at the interface between the PEDOT:PSS and the photoactive layer (type II) [Fig. 4(b)]. In the type II architecture, the presence of the silica shells avoided possible exciton quenching by preventing direct contact between the Ag cores and the active layer. Choi et al. found that both types of structures led to improved device efficiencies. They concluded that the multipositional and solution-processable properties of core/shell NPs offer the possibility of providing plasmonic effects when introducing these nanomaterials into various spatial locations.80 More recently, another interesting type of plasmonic Ag NPs was demonstrated. Choi et al. prepared carbon dot-supported Ag NPs (CD–Ag NPs) and used them in the fabrication of solution-processable polymer optoelectronic devices.81 The electron-donating properties of UV-excited CDs enable rapid reduction of Ag salts to form Ag NPs on the surfaces of the CDs. The CD– Ag NPs had average diameters of approximately 6 nm for the CDs and approximately 3 nm for the Ag NPs. A broad light extinction occurred with a peak near 460 nm [Fig. 4(c)]. In
Fig. 4 Modified Ag NPs for use in plasmonic-enhanced OPVs. (a) Absorption spectrum and TEM image of Ag@SiO2NPs. (b) Structures of OPVs.80(c) Absorption spectra of as-prepared CD–Ag NPs in different
forms. (d and e) Simulated (d) extinction spectrum and (e) electricfield distribution for CD–Ag NPs, plotted with respect to the number of Ag NPs on the surface of the CDs (NAg). The magnitude of the enhancement in the electricfield intensity is indicated by the colour scale.81
contrast, the peak absorption occurred near 420 nm for free Ag NPs having a similar diameter; thus, the plasmonic peak shied by approximately 40 nm. This wavelength shi was simulated using the three-dimensional nite-difference time-domain (FDTD) method; Fig. 4(d) and (e) present the predicted extinc-tion spectra and the simulated electricaleld intensity distri-bution, respectively, plotted with respect to the number of Ag NPs (NAg) attached to the CD surfaces. Red-shiing of the spectra occurred upon increasing the value of NAg, with a strong eld enhancement at the gaps between the NPs, suggesting a signicant clustering effect of the NPs. The application of the CD–Ag NPs in OPVs, having a device structure of ITO/CD–Ag NP/ PEDOT:PSS/PTB7:PC70BM/Al, resulted in an improvement in PCE up to 8.31%, clearly demonstrating the potential of these templated Ag NPs.
The incorporation of two different NPs can lead to coopera-tive plasmonic effects. As displayed in Fig. 5(a), Lu et al. mixed Ag and Au NPs and then incorporated them into the anodic PEDOT:PSS buffer.82They obtained a high PCE of 8.67%, cor-responding to a 20% enhancement of efficiencies. The dual NPs provided a broader absorption-enhancing range relative to that of either single type of NPs. UV-vis absorption spectra conrmed that the absorption efficiency of the PTB7:PC70BM cell in the range from 420 to 600 nm was enhanced aer adding Ag NPs; further incorporation of Au NPs increased the absorp-tion efficiency in the wavelength range from 520 to 750 nm [Fig. 5(b)]. The two complementary resonance peaks of these two different types of NPs resulted in OPVs having a much broader light absorption enhancement region.
Alloy NPs have also employed for plasmonic-enhanced OPVs.84,85 When a polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) copolymer was used as a template for fabricating
arrays of metal NPs on ITO substrates, metallic (Au and Cu) salts were blended and converted to either pure or alloy NPs.84For P3HT:PCBM BHJ systems having the device structure ITO/metal NP/PEDOT:PSS/P3HT:PCBM/Al, the PCE of the reference cell was 2.9%, improving to 2.98 and 3.22% when the Au and Cu NPs, respectively, were incorporated into the devices. For the device containing the Au–Cu alloy NPs, the efficiency improved even further to 3.35%.
3.1.2 Nanoparticles with non-spherical or abnormal shapes. While the majority of the existing studies on the plas-monic OPVs are focused on spherical nanoparticles, more and more recent articles study the shape effects of these plasmonic materials.39,46,86–89 We recently introduced both Au NPs and nanorods (NRs) into the P3HT:PCBM devices and found improved light trapping efficiencies.83Tunable LSPR bands at different wavelength regions from 520 to 850 nm have been achieved through altering the sizes and shapes (Fig. 6). Because of the two different resonance modes along the short and long axes—classied as the transverse and longitudinal mode, respectively, a wide range of LSPR become possible. L. Lu et al. further proved that one single type Au NRs could also achieve a broad light absorption enhancement region due to their dual plasmonic bands along the two axes of the nanorods [Fig. 5(c)].82A PCE of 8.52% has been achieved for the device based on the PTB7:PC70BM blends; this value is close to the PCE obtained using Ag and Au NP blends [Fig. 5(d)].82
Oo et al. synthesized ultrane Au nanowires (NWs), using a solution-phase approach, for application as plasmonic antennae in P3HT:PCBM solar cells.90Fig. 7(a) illustrates their device structure; notably, a layer of PEDOT:PSS was incorpo-rated as a spacer between the active layer and the Au NW network. The absorption of the device was enhanced over the spectral regime ranging from 400 to 700 nm. The photocurrent increased by 23.2% and the PCE was improved by 11.4%. Oo et al. concluded that a strong localized plasmon eld and
Fig. 5 Plasmonic nanostructures that result in broader absorption enhancement regimes.82(a) TEM images of Ag and Au NPs and their
mixture. (b) UV-vis absorption spectra of PTB7:PC70BM devices prepared with and without NPs; inset: absorption spectra of the NPs in water.82 (c) TEM image of Au rods in water. (d) J–V curve of the
PTB7:PC70BM cell after Au NRs had been blended into the device.
Fig. 6 (a and b) Dipolar oscillations in (a) spherical Au NPs and (b) Au NRs. (c) Normalized UV-vis-NIR absorption spectra of Au NPs of various sizes and shapes in water.83
increased far-eld scattering collectively led to the enhance-ment in absorption. Furthermore, Kozanoglu et al. compared the SP effects of abnormally shaped particles with those of nanospheres.34NPs having sharp features should contribute to a greater extent to device enhancement because of their strong local electromagnetic elds.91 Fig. 7(b) and (c) display TEM images and a corresponding UV-vis absorption spectrum, respectively, of the nanostars;34 the tip-to-tip distance was approximately 150 nm. Because of the various resonance modes, a broad plasmon absorption band appeared, ranging from 500 to 900 nm. Aer NPs of various shapes—nanostars, nanorods, and nanospheres—had been doped into the PEDOT:PSS buffers of the devices based on P3HT:PCBM blends, the PCEs increased by approximately 29, 14, and 11%, respec-tively. Among the tested nanostructures, Au nanostars exhibited the highest eld enhancement around the NPs, presumably because of their branched structures and sharp features.34
3.2 Metal NPs embedded into photoactive layers
In principle, directly embedding nanostructures into the pho-toactive layer would be a straightforward means of exploiting their plasmonic effects. From a theoretical point of view, Sha et al. developed a rigorous electrodynamic approach for inves-tigating the optical absorption in OPVs; they observed remark-able differences between the behavior of metal NPs placed inside the interlayer between the active layer and the anode and those embedded directly into the photoactive layer.92–94 The enhancement factors (Table 1) were generally higher when the NPs were blended directly into the active layer.95Nevertheless, the direct embedding method has many concerns, including possible exciton quenching and rapid charge recombination at
the surface of the metal NPs. As we will see in many examples below, serious phase separation between the nanostructures and the organic semiconductors also dampen their positive effects on device performance.
As early as 2004, Ag and Au NPs were directly doped into the photoactive layer of OPVs. Although the device efficiency was indeed improved, the authors concluded that the cell enhancement was due to the improved electrical conductivity.54 In 2010, Xue et al. also embedded Ag NPs into the P3HT:PCBM layer, but did not observe signicant device enhancement.96 Although the mobility of the active layer increased, the number of total extracted carriers decreased. From an analysis of the surface morphology of the active layers, Xue et al. found that the Ag NPs tended to phase-segregate from the organic compo-nents. On the other hand, the formation of such an Ag NP sub-network might explain the increased device mobility.96 More recently, many successful examples regarding blending metal NPs into the photoactive layers have been reported.97–99Wang et al. added truncated octahedral Au NPs in the OPVs fabricated from P3HT/PC70BM, poly[N-900 -hepta-decanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)] (PCDTBT)/ PC70BM, and poly{[4,40-bis(2-ethylhexyl)dithieno(3,2-b:20,30-d) silole]-2,6-diyl-alt-[4,7-bis(2-thienyl-2,1,3-benzothiadiazole-5,50 -diyl)]} (Si-PCPDTBT)/PC70BM, and found that the PCE increased from 3.54% to 4.36% for the devices based on P3HT/PC70BM blends.97For other polymer systems, the efficiencies were also improved from 5.77% to 6.45% (PCDTBT/PC70BM), and from 3.92% to 4.54% (Si-PCPDTBT/PC70BM). Because the average size of the truncated octahedral Au NPs was approximately 700 nm, Wang et al. suggested that multiple scattering, leading to longer optical paths within the active polymer materials, was responsible for the enhancement in the PCE. Furthermore, the lower series resistance extracted from the dark J–V curves indicated that improved charge transport also contributed to the device enhancement, because the Au NPs also behaved as hole conductors. Meanwhile, Ag NPs synthesized through solution polyol chemistry were also blended with the PCDTBT/ PC70BM active layers; again, device enhancement was observed.98
Although surface coating of NPs can help to improve their dispersion in a polymer matrix, insulating layers might atten-uate the plasmonic eld, resulting in a less effective Fig. 7 Non-spherical NPs for plasmonic-enhanced OPVs. (a) The
structure of an Au NW device and the high-resolution TEM image of as-synthesized ultrafine NWs.90(b) The TEM image of the nanostars. (c)
The UV-vis absorption spectrum of the nanostar colloids in aqueous medium.34
Table 1 Enhancement factors for metal NPs embedded into the spacer and the active layer100
NP types
Vertical incidence
Oblique incidence
Spacer layer Separated small 0.992 1.078
Close-packed small 0.989 1.174
Separated large 0.927 0.935
Close-packed large 0.725 0.960
Active layer Separated small 1.366 1.374
Close-packed small 1.985 1.821
Separated large 1.118 1.216
Close-packed large 1.342 1.589
enhancement in absorption. Spyropoulos et al. embedded surfactant-free Au NPs, synthesized through ultrafast laser ablation in liquids, into a P3HT:PCBM layer; they found that the PCE improved by 40%,99suggesting that the uncoated surface of the NPs could eliminate recombination pathways. In addition to noble metals, other metals, including Al (ref. 101–103) and Cu,84 have also been tested as promising candidates for improving the performance of OPVs. Kochergin et al. found that Al NPs are especially suitable for use in organic materials because Al has a higher plasma frequency, which ensures better overlap between its resonance peaks and the absorption band of the organic semiconductors.103 Kakavelakis et al. further demonstrated enhancements in both device efficiencies and stability when using Al NPs generated through laser ablation.101 The Al NPs behaved as effective optical reectors, allowing the solar radiation to pass multiple times through the P3HT:PCBM
active layer. The value of Jscincreased by 31.66%—from 8.59 to 11.31 mA cm2—aer incorporation of the Al NPs, while the PCE improved from 3.14 to 4.00%. More interestingly, stability tests indicated that the degradation rate of the Al NP-doped device was much slower than those of standard devices. Kaka-velakis et al. attributed the better device stability to the efficient quenching of triplets, thereby inhibiting photooxidation processes within the devices.101Furthermore, when Au–Ag alloy NPs were synthesized in organic media using a one-pot reac-tion,85 the efficiency of the P3HT:PCBM device increased to 4.37%—a 31% enhancement—aer doping the alloy NPs into the active layer. Chen et al. attributed the improved device performance to enhanced light trapping and better charge transport in the active layer. The device performance of the plasmonic-enhanced OPVs fabricated with NPs is summarized in Table 2.
Table 2 Device performance of the plasmonic-enhanced OPVs fabricated with NPsa
Plasmon scheme/active layer Jsc(mA cm2) Voc(V) FF (%) PCE (%) Ref.
Ag nanoprism/P3HT:PC61BM 1.99 (1.43) 0.50 (0.55) 52.3 (57.3) 5.21 (4.58) 30 Au@SiO2NR/PCPDTBT:PC70BM 12.7 (11.4) 0.597 (0.597) 58.1 (51.4) 4.4 (3.5) 35 Inorganic NP/P3HT:PC61BM 11.8 (10.1) 0.59 (0.59) 61 (59) 4.3 (3.5) 51 Au NP/P3HT:PC61BM 10.22 (9.16) 0.59 (0.59) 70.32 (66.06) 4.24 (3.57) 54 Ag NP/P3HT:PC61BM 6.9 (4.6) 0.581 (0.590) — 2.2 (1.3) 55 Ag NP/P3HT:PC61BM — — 55 (53) 3.69 (3.05) 58 Au NP/P3HT:PC61BM 10.18 (8.95) 0.59 (0.59) 69.8 (65.9) 4.19 (3.48) 59 MoO3+ Au NP/P3HT:PC61BM 10.9 (10.2) 0.60 (0.60) 64 (60) 4.20 (3.68) 65 Cs2CO3+ Au NP/P3HT:PC61BM 10.11 (9.73) 0.55 (0.55) 64 (59) 3.54 (3.12) 68 Au–GO/P3HT:PC61BM 9.43 (8.46) 0.63 (0.60) 59.7 (51.5) 3.55 (2.79) 71 Au–GO/P3HT:PC61BM 10.44 (8.87) 0.57 (0.57) 66.8 (64.5) 3.98 (3.26) 74 Au–GO/P3HT:ICBA 9.94 (8.61) 0.81 (0.81) 62.7 (57.7) 5.05 (4.02)
MoO3–Ag NP/P3HT:ICBA 12.86 (11.64) 0.85 (0.84) 66 (64) 7.21 (6.26) 78
Ag NP/PCDTBT 12.67 (11.22) 0.89 (0.89) 67 (64) 7.6 (6.4) 79 Ag NP/PTB7:PC70BM 16.33 (14.93) 0.75 (0.75) 70 (70) 8.6 (7.9) Ag@SiO2/PTB7:PC70BM 16.65 (14.64) 0.74 (0.74) 68 (67) 8.49 (7.26) 80 Dual Au + Ag NP/PTB7:PC71BM 17.7 (15.0) 0.71 (0.72) 69.0 (67.1) 8.67 (7.25) 82 Dual Au NP + NR/P3HT:PC60BM 11.49 (9.28) 0.59 (0.59) 63 (64) 4.28 (3.46) 83 Dual Au + Cu NP/P3HT:PC61BM 9.37 (8.08) 0.58 (0.60) 61 (60) 3.35 (2.90) 84 AuAg alloy NP/P3HT:PC61BM 12.21 (10.12) 0.63 (0.61) 61.5 (58.4) 4.73 (3.61) 85 Dual Ag NP + nanoprisim/P3HT:PC61BM 10.61 (8.99) 0.64 (0.64) 63.33 (62.33) 4.3 (3.6) 86 Au NW/P3HT:PC61BM 9.02 (7.87) 0.65 (0.65) 46 (48) 2.72 (2.44) 90 Au NP/P3HT:PC70BM 11.18 (—) 0.63 (—) 61 (—) 4.36 (3.54) 97 Au NP/PCDTBT:PC70BM 11.16 (—) 0.89 (—) 65 (—) 6.45 (5.77) Au NP/Si–PCPDTBT:PC70BM 13.13 (—) 0.57 (—) 61 (—) 4.54 (3.92) Ag NP/PCDTBT:PC70BM 11.61 (10.79) 0.86 (—) 69 (68) 7.1 (6.3) 98 Au NP/P3HT:PC61BM 9.77 (8.27) 0.6 (0.6) 63.38 (53.22) 3.71 (2.64) 99 Al NP/P3HT:PC61BM 11.31 (8.59) 0.6 (0.6) 59 (61) 4.00 (3.14) 101 Au–Al NP/PCDTBT:PC61BM 12.71 (11.27) 0.86 (0.86) 56 (55) 6.12 (5.33) 102 Au NP/tandem, P3HT:ICBA–PSBTBT:PC70BM 6.92 (6.06) 1.457 (1.455) 61.91 (59.22) 6.24 (5.22) 104 Au NP/MEH–PPV:PC61BM *Isc(mA), 74.39 (66.79) 0.78 (0.76) 44.8 (39.2) 2.36 (1.99) 105 Au NP/PFSDCN:PC61BM — — — 2.17 (1.64) 106 Ag NP/PCDTBT:PC71BM 12.12 (11.63) 0.87 (0.90) 61 (57) 6.4 (5.9) 107 Ag nanoplate/PCDTBT:PC71BM 13.19 (11.63) 0.87 (0.90) 57 (57) 6.6 (5.9) Ag NP/P3HT:PC71BM 9.33 (7.89) 0.58 (0.59) 53 (52) 2.82 (2.41) 108 Pt NP/P3HT:PC61BM 8.54 (9.53) 0.57 (0.53) 52.8 (45.3) 2.57 (2.29) 109 Au NP/P3HT:PC61BM 9.74 (8.35) 0.61 (0.61) 65.00 (61.92) 3.85 (3.16) 110 Dual Ag nanodots + NP/P3HT:PC61BM 11.20 (9.21) 0.64 (0.63) 67.0 (69.3) 4.80 (4.02) 111 Au@SiO2/P3HT:PC61BM 10.6 (10.0) 0.62 (0.61) 57 (54) 3.80 (3.29) 112 MoO3–Ag NP/PCDTBT:PC70BM 10.43 (9.12) 0.88 (0.88) 64 (64) 5.87 (5.07) 113 Ag NP/PCDTBT:PC70BM 10.10 (9.18) 0.89 (0.89) 61 (62) 5.57 (5.08)
aThe dash mark (—): data not provided; numbers in parentheses: data from reference devices.
3.3 Periodic metallic nanostructures
Another promising strategy for enhancing the performance of OPVs is to use periodic metal nanostructures, which can usually fulll the momentum mismatch conditions, as described in the previous section. While the preparation of metal NPs seems to be a simpler approach, it is difficult to precisely control their sizes, shapes, and, most importantly, monodispersity; manip-ulating their distribution within devices is also very chal-lenging. By tuning the geometrical factors of periodic nanostructures, the resonance wavelength of the SPP modes
can be tuned readily within the visible to near-infrared (NIR) spectral range.29,38–41,114–120 Furthermore, these structures are also easier to model, facilitating theoretical studies. For instance, Tvingstedt et al. constructed periodic Al gratings to induce propagating plasmons; Fig. 8(a) illustrates the device structure.43They used the grating as the bottom cathode and deposited two different polymers (APFO3, APFO Green5) con-taining PCBM molecules as active layers; a layer of PEDOT:PSS served as the top anode. Fig. 8(b) and (c) display the corre-sponding absorption spectra measured in different polarization directions. The absorption spectra were different from those of the planar structure. In addition, different absorption patterns were observed when the devices were illuminated with differ-ently directed polarized light. The results suggested that the transverse-electric (TE) polarized electromagnetic wave could not excite the plasmons. The IPCE spectra of the devices also revealed apparent polarization effects depending on the direc-tion of the spectra, implying that the propagating plasmons could indeed increase the photocurrent. A successful example of a grating structure was reported by You et al. in 2012.121Their metal grating back electrode was implemented using an imprinting technique [Fig. 8(d)]. Aer patterning the devices, the PCE increased from 7.20 to 7.73% when using an LBG polymer (PTB7) as the active polymer.
One-dimensional (1D) grating structures oen have high polarization-dependence, usually limiting their application in photovoltaic devices because of the randomly polarized nature of solar radiation. Sefunc et al. proposed and simulated both 1D and two-dimensional (2D) metal gratings for OPVs based on P3HT:PCBM blends, nding that the absorption efficiency could be increased by approximately 21% in all polarizations.122 Through FDTD numerical calculations, Kim et al. identied multiple SP modes for Ag grids in ultrathin OPVs.100 They attributed the absorption enhancements in the OPVs incorpo-rating the metal grids to both the SPP mode excited at the glass– Ag interface and the LSP mode of the metal grids. They also found another SPP mode at the interface between the metal and the active layer aer incorporation of 2D grids (Fig. 9). Indeed, light trapping signicantly differs between the TE and trans-verse-magnetic (TM) modes for 1D gratings. Nevertheless, 2D or quasi-3D structures, possessing higher-order symmetries, offer Fig. 8 (a) Periodic grating structure, shown in the middle, for
sup-porting the propagating SPs. The polymer materials were sandwiched between the Al grating cathode and the PEDOT:PSS top anode. (b and c) Absorption spectra of (b) APFO3/PCBM and (c) APFO Green5/PCBM polymer blends on the grating structures, recorded with illumination from various polarization directions.43 (d) Processing flow of the
patterned polymer solar cells.121
Fig. 9 (a) Schematic representation of a device structure featuring a 2D metal grid embedded in a buffer layer. (b) Absorption spectra of cells prepared with and without 1D and 2D metal grids, recorded under randomly polarized light; width and period of the metal grids: 40 and 150 nm, respectively. (c) Normalized E-field intensity distributions at a wavelength of 695 nm under polarized light in the x direction; the contour plot is displayed on the cross-section along thex–y plane passing through the vertical interface between the metal grid and the buffer layer.100
the opportunity for further optimization of the optical absorp-tion. Broadband absorption enhancement over the entire solar spectrum can also be potentially realized when using 2D metal grids.
Several plasmonic architectures have been employed to
prepare nanostructures with higher-order
symme-tries.44,89,115,117,119,123Dunbar et al. fabricated Ag nanovoid arrays,
through nano-imprinting on both PC61BM and
PCPDTBT:PC61BM layers;115 they observed optical absorbing enhancements of up to approximately 80 and 40%, respectively, for these lms. For practical applications, Kirkeminde et al. prepared Au plasmonic nanopyramid (NPY) structures, through self-assembled lithography, onto the substrates of silole-thio-phene conjugated polymer (P3):PC61BM devices.89They fabri-cated polystyrene (PS) nanospheres of various sizes as templates on an ITO substrate [Fig. 10(c)]; these tightly coordinated PS spheres behaved as excellent templates for NPYs with sharp tips and edges, enabling enhanced plasmonic resonance. Au NPY arrays, uniform in size and distribution, were obtained aer evaporating a 30 nm Au lm and removing the PS spheres through a li-off process [Fig. 10(a)]. Fig. 10(e) presents the absorption spectra of the Au NPY arrays and the photoactive layer; the 110 nm side-length Au NPYs exhibited the most
suitable plasmon resonance for effective coupling with the P3:PC61BM photoabsorption. A 50% increase in photocurrent was measured, with the average current density improving from 2.7 0.2 to 4.1 0.3 mA cm2; the average PCE (1.10 0.05%) was approximately 200% greater than that of the device prepared without the NPY arrays (0.36 0.02%). Meanwhile, Wu et al. fabricated large-area periodic Ag nanotriangle (NT) arrays [Fig. 10(d)] through self-assembled nanosphere lithog-raphy in PCDTBT:PCBM devices.116 The hexagonal NT arrays increased the absorption efficiency and improved the device PCE from 4.24 to 4.52%, presumably because of greater exciton generation induced by the strong local electriceld and the SP-induced scattering.
3.4 Dual plasmonic nanostructures
Many conventional plasmonic nanostructures, such as spher-ical NPs, can cover only a narrow spectral bandwidth; therefore, a single structure alone cannot harvest the broad spectrum of solar radiation. Furthermore, different nanostructures present different functionalities and operate through different device-enhancing mechanisms when incorporated at different posi-tions, as we have seen above. Therefore, many researchers have tested the effects of incorporating pairs of different plasmonic structures simultaneously in a single device (Table 3). For example, Xie et al. synthesized poly(ethylene glycol)
Fig. 10 Nanostructures with higher-order symmetries. (a) AFM images of the Ag–NT arrays. The height and size of the triangles are ca. 20 and 40 nm, respectively. The inset shows a zoom-in unit of the NT array. (b) Percentage enhancement in the IPCE of the devices with NTs. The inset shows the corresponding IPCE spectra of the two devices.116(c)
The self-assembled PS sphere nanolithography to prepare Au NPY arrays. The structure was then covered with PEDOT:PSS for device fabrication. (d) SEM images of the PS spheres (up) and Au NPYs with 110 nm side-length (down). (e) The absorptions of Au NPYs, P3/ PC61BM, and Au NPY plasmonic enhanced OPV.89Reproduced from ref. 89.
Fig. 11 (a) Chemical structures of PBDTTT-C-T and PC71BM (left). Schematic representation of the device structure: NP device (top), grating device (bottom), and dual metallic structures (right) (b) The IPCE spectra of the control (flat) and the optimized devices. The enhancement factor is also plotted.124
(PEG)-capped Au NPs and positioned them into both the PEDOT:PSS buffer and the active layer.110They demonstrated the accumulated benets of incorporating these NPs into all polymer layers. The Au NPs in the PEDOT:PSS layer contributed mainly to improved hole collection, while those in the active layer enhanced the optical absorption and helped to balance charge transport. Li et al. combined two different approaches— blending metal NPs and using 1D grating backcontacts—to improve the device efficiencies in inverted OPVs incorporating a blend of poly{[4,8-bis(2-ethylhexylthien-5-yl)benzo[1,2-b:4,5-b0 ]-dithien-2,6-diyl]-alt-[2-(20 -ethylhexanoyl)thieno[3,4-b]thien-4,6-diyl ]}(PBDTTT-C-T) and PC71BM as the active later.124
Fig. 11(a) presents the device structure and the chemical structures of the component materials. Theat control device, fabricated without any nanostructures, exhibited a PCE of 7.59 0.08%. Then, Au NPs having diameters of 20 and 50 nm were doped into the PBDTTT-C-T:PC71BM active layer. Meanwhile, Ag grating electrodes having periodicities of 750 and 350 nm were prepared, through vacuum-assisted nanoimprinting, as the back electrodes. For the best device, in which the grating interval was 750 nm, the PCE reached 8.38 0.20%. Aer the Au NPs had also been added to the active layer, the PCE increased further to 8.79 0.15%. Li et al. found that the Au NPs enhanced the device absorption in the spectral region from 480 to 600 nm, whereas the Ag grating had a greater impact on the absorptions in the regions below 400 nm and above 600 nm [Fig. 11(b)].124More recently, Liu et al. demonstrated the dual effects of Au nanodots (NDs) and NPs within OPVs.111 They incorporated octahedral Au NPs within the hole transport layer (PEDOT:PSS) and thermally evaporated Au NDs onto the P3HT:PCBM active layer. The device PCE increased by 15% aer positioning the Au NDs at the cathode interface and then increased further, by 20%, aer including both nanostructures, indicating the great potential of using such an approach.
4
Conclusion and outlook
Aer tremendous progress made in the past decade, PCEs exceeding 10% have been achieved for single-junction OPVs. An earlier research study revealed that the internal quantum effi-ciencies of OPVs could approach almost 100%.125,126 Hence, sufficient absorption of sunlight remains one of the limitations
affecting the development of higher-efficiency devices. While many light trapping approaches have been proposed, the use of plasmonic nanostructures is one of the most promising routes toward harvesting more solar radiation. The optical properties of these nanostructures are simply tuned through structural design and material selection. Moreover, they are readily incorporated into OPVs. In this Article, werst discussed the fundamentals of SPs and described the three main categories of plasmonic nanostructures that have been used to enhance the performance of OPVs. We have reviewed the recent literature, highlighting the rapid progress in thiseld with a focus on studies of BHJ poly-mer-based solar cells. Using most of these approaches, the photocurrents and/or PCEs have frequently increased by 20– 30%. The most simple and assessable method is the introduc-tion of metal NPs in the vicinity of the photoactive layer—that is, in the buffer layer or at the organic–electrode interface. The plasmoniceld, however, usually decays exponentially, resulting in limited near-eld effects on the absorption enhancement of the photoactive layer. A less-simple, but rather straightforward method, is to directly embed the nanostructures in the semi-conductor layer. The resulting morphology, however, should be carefully controlled because phase separation between the nanostructures and the organic materials can degrade the device. Furthermore, it is rather difficult to decouple the elec-trical and plasmonic effects of these nanostructures, at least under some experimental conditions. The use of periodic nanostructures is also a promising means of achieving positive plasmonic effects. These ordered architectures also simplify the structural design and are more amenable to theoretical investi-gations. Although 1D grating structures exhibit polarization dependence, some 1D nanostructures can provide certain benets; 2D or even higher-order symmetries can be used to overcome such polarization issues. The high cost of fabrication of the periodic nanostructures, however, remains a challenge for future commercialization. In addition to improvements in effi-ciency, reliability is another important feature of a practical solar module. While some reports have demonstrated that the addi-tion of metal NPs can improve the photostability, further efforts should be made to improve device stability.
The loading capacity of metal nanostructures in organic matrices is a very critical parameter in determining the device performance and should be further emphasized. Earlier reports Table 3 Device performance of the plasmonic-enhanced OPVs fabricated with periodic nanostructuresa
Plasmon scheme/active layer Jsc(mA cm2) Voc(V) FF (%) PCE (%) Ref.
Au NPY/P3:PC61BM 4.1 (2.7) — — 1.10 (0.36) 89
Au:SH–PEG NPY/P3:PC61BM — — — 1.1 (0.84)
Ag NT array/PCDTBT:PC61BM 9.57 (8.55) 4.52 (4.24) 53 (55) 4.52 (4.24) 116 Al nanodisk array/PCPDTBT:PC60BM — — — 4.52 (—) 117 Ag nanograting/PTB7:PC71BM 15.50 (14.05) 0.720 (0.726) 69.08 (70.62) 7.73 (7.20) 121 Au NPs + Ag nanograting/PBDTTT-C-T:PC71BM 18.39 (17.09) 0.76 (0.76) 62.87 (58.43) 8.79 (7.59) 124
MoOx/Ag nanoporouslm/SubPc-C60 5.15 (5.43) 1.03 (1.04) 65.03 (62.19) 3.45 (3.51) 127
Corrugated Au–Al electrode/CuPc-C60 5.5 (4.1) 0.46 (0.46) 57 (57) 1.44 (1.07) 128
aThe dash mark (—): data not provided; numbers in parentheses: data from reference devices.
have indicated that a high concentration of metal nanoparticles tends to result in serious aggregation.79,97,119The clustering and/ or stronger dipolar coupling between the particles might affect the plasmonic properties of the nanostructures.79As the metal surface probably quenches excitons and facilitates charge recombination, a high level of nanostructure loading could degrade the device performance. These loss mechanisms, however, are still not well understood yet, and needed to be elucidated. Further, controlled methods for the nanoparticle dispersion should be further developed. Together with the better understandings of the quenching mechanisms, the increased loading capacity might further benet the device efficiencies.
Although many examples regarding the plasmonic-enhanced performance have been reported, in-depth understanding of the enhancement mechanism is still required. Theoretical studies and simulations are needed to support the existing experi-mental studies, especially for the rather complicated nano-structures, and to provide design rules for structure development in the further. The inuence of the nanostructures on the electrical properties of the plasmonic-assisted OPVs is not clear yet. For example, the possibility of charge trapping on the metal NPs should be evaluated. Besides, the effect on the metal/organic interfaces of the nanostructures is another important concern.
In addition to conventional single-junction OPVs, plasmonic nanostructures have also been applied in other device archi-tectures. Because the electrochemical potential of charge carrier extraction can be increased, tandem cell structures have been used widely to allow theoretical PCEs to surpass the Shockley– Queisser limit. Yang et al. incorporated Au NPs in the inter-mediate layer, which connects the two subcells, when building tandem OPVs.104 The PCE increased by approximately 20%, revealing the great potential of the exploiting the plasmonic effect. We anticipate that many more device structures will be designed for ready incorporation of nanostructures. It is also worth noting that core/shell metal NPs have been employed recently in emerging meso-superstructured solar cells based on perovskite materials.129Because thinlm-type perovskite solar cells have been demonstrated as efficient,130 we expect the knowledge gained from studies of plasmonic-assisted OPVs to also benet in enhancing the efficiencies of these emerging solar technologies.
In conclusion, plasmonic light trapping technologies have great potential for achieving high degrees of photon absorption in OPVs. Dennler et al. proposed that a PCE of approximately 15% might be possible through the development of multi-junction OPVs.76We believe that plasmonic nanostructures will play an important role in pushing the record PCE toward these theoretical predications. Finally, in addition to their attractive properties of light weight and highexibility, we foresee that inexpensive, highly efficient, large-area OPV modules will be delivered in the near future.
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