Electron accumulation on metal nanoparticles in plasmon-enhanced organic solar cells

Theory of Hot-Carrier Generation in Bimetallic Plasmonic Catalysts

Bimetallic nanoreactors in which a plasmonic metal is used to funnel solar energy toward a catalytic metal have recently been studied experimentally, but a detailed theoretical understanding of these systems is lacking. Here, we present theoretical results of hot-carrier generation rates of different Au-Pd nanoarchitectures. In particular, we study spherical core-shell nanoparticles with a Au core and a Pd shell as well as antenna-reactor systems consisting of a large Au nanoparticle that acts as an antenna and a smaller Pd satellite nanoparticle separated by a gap. In addition, we investigate an antenna-reactor system in which the satellite is a core-shell nanoparticle. Hot-carrier generation rates are obtained from an atomistic quantum-mechanical modeling technique which combines a solution of Maxwell's equation with a tight-binding description of the nanoparticle electronic structure. We find that antenna-reactor systems exhibit significantly higher hot-carrier generation rates in the catalytic material than the core-shell system as a result of strong electric field enhancements associated with the gap between the antenna and the satellite. For these systems, we also study the dependence of the hot-carrier generation rate on the size of the gap, the radius of the antenna nanoparticle, and the direction of light polarization. Overall, we find a strong correlation between the calculated hot-carrier generation rates and the experimentally measured chemical activity for the different Au-Pd photocatalysts. Our insights pave the way toward a microscopic understanding of hot-carrier generation in heterogeneous nanostructures for photocatalysis and other energy-conversion applications.

Synergistic Effects of Localized Surface Plasmon Resonance, Surface Plasmon Polariton, and Waveguide Plasmonic Resonance on the Same Material: A Promising Hypothesis to Enhance Organic Solar Cell Efficiency

This work explores the utilization of plasmonic resonance (PR) in silver nanowires to enhance the performance of organic solar cells. We investigate the simultaneous effect of localized surface plasmon resonance (LSPR), surface plasmon polariton (SPP), and waveguide plasmonic mode on silver nanowires, which have not been thoroughly explored before. By employing finite-difference time-domain (FDTD) simulations, we analyze the plasmonic resonance behavior of a ZnO/Silver nanowires/ZnO (ZAZ) electrode structure. Our investigations demonstrate the dominance of LSPR, leading to intense electric fields inside the nanowire and their propagation into the surrounding medium. Additionally, we observe the synergistic effects of SPP and waveguide plasmonic mode, contributing to enhanced light absorption within the active layer of the organic solar cell. This leads to an improvement in photovoltaic performance, as demonstrated by our previous work, showing an approximate 20% increase in photocurrent and overall power conversion efficiency of the organic solar cell. The incorporation of metallic nanostructures exhibiting these multiple plasmonic modes opens up new opportunities for improving light absorption and overall device efficiency. Our study highlights the potential of these combined plasmonic effects for the design and optimization of organic solar cells.

Active control of dielectric nanoparticle optical resonance through electrical charging

A novel method for active control of resonance position of dielectric nanoparticles by increasing the excess charges carried by the nanoparticles is proposed in this paper. We show that as the excess charges carried by the particle increase, the oscillation frequency of excess charges will gradually increase, when it is equal to the incident frequency, resonance occurs due to resonant excitation of the excess charges. What is more, the formula of charges carried by an individual particle required to excite the resonance at any wavelength position is proposed. The resonance position can be directly controlled by means of particle charging, and the enhancement of resonance intensity is more obvious. This work has opened new avenues for the active control of plasmon resonances, which shows great promise for realizing tunable optical properties of dielectric nanoparticles.

Plasmon-induced trap filling at grain boundaries in perovskite solar cells

The deep-level traps induced by charged defects at the grain boundaries (GBs) of polycrystalline organic-inorganic halide perovskite (OIHP) films serve as major recombination centres, which limit the device performance. Herein, we incorporate specially designed poly(3-aminothiophenol)-coated gold (Au@PAT) nanoparticles into the perovskite absorber, in order to examine the influence of plasmonic resonance on carrier dynamics in perovskite solar cells. Local changes in the photophysical properties of the OIHP films reveal that plasmon excitation could fill trap sites at the GB region through photo-brightening, whereas transient absorption spectroscopy and density functional theory calculations correlate this photo-brightening of trap states with plasmon-induced interfacial processes. As a result, the device achieved the best efficiency of 22.0% with robust operational stability. Our work provides unambiguous evidence for plasmon-induced trap occupation in OIHP and reveals that plasmonic nanostructures may be one type of efficient additives to overcome the recombination losses in perovskite solar cells and thin-film solar cells in general.

Impact of hybrid plasmonic nanoparticles on the charge carrier mobility of P3HT:PCBM polymer solar cells

The solution processable polymer solar cells have shown a great promise as a cost-effective photovoltaic technology. Here, the effect of carrier mobility changes has been comprehensively investigated on the performance of P3HT:PCBM polymer solar cells using electro-optical coupled simulation regimes, which may result from the embedding of SiO₂@Ag@SiO₂ plasmonic nanoparticles (NPs) in the active layer. Firstly, the active layer thickness, stemmed from the low mobility of the charge carriers, is optimized. The device with 80 nm thick active layer provided maximum power conversion efficiency (PCE) of 3.47%. Subsequently, the PCE has increased to 6.75% and 6.5%, respectively, along with the benefit of light scattering, near-fields and interparticle hotspots produced by embedded spherical and cubic nanoparticles. The PCE of the devices with incorporated plasmonic nanoparticles are remarkably enhanced up to 7.61% (for spherical NPs) and 7.35% (for cubic NPs) owing to the increase of the electron and hole mobilities to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\upmu }_{e}=8\times {10}^{-7} \,{\text{m}}^{2}/\text{V}/\text{s}$$\end{document}μe=8×10-7m2/V/s and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\upmu }_{h}=4\times {10}^{-7} \,{\text{m}}^{2}/\text{V}/\text{s}$$\end{document}μh=4×10-7m2/V/s, respectively (in the optimum case). Furthermore, SiO₂@Ag@SiO₂ NPs have been successfully synthesized by introducing and utilizing a simple and eco-friendly approach based on electroless pre-treatment deposition and Stober methods. Our findings represent a new facile approach in the fabrication of novel plasmonic NPs for efficient polymer solar cells.

Large plasmonic absorption enhancement effect of triangular silver nanowires in silicon

Two-dimensional finite difference time domain (FDTD) simulations were performed for evaluating optical absorption enhancement and loss effects of triangular silver (Ag) nanowires embedded in silicon (Si) thin-film photovoltaic device structures. Near-bandgap absorption enhancement in Si was much larger than the reported values of other nanostructures from similar simulations. A nanowire with equal sides of 20 nm length showed 368-fold absorption enhancement whereas only 5× and 15× values of solid spherical and two-dimensional core-shell type nanostructures, respectively. Undesirable absorption loss in the metal of the nanowire was 3.55× larger than the absorption in Si which was comparable to the value reported for the spherical nanoparticle. Interestingly, as the height of the nanowire was increased to form a sharper tip, absorption loss showed a significant drop. For a nanowire with 20 nm base and 20 nm height, absorption loss was merely 1.91× larger than the absorption in Si at the 840 nm plasmon resonance. This drop could be attributed to weaker plasmon resonance manifested by lower metallic absorption in the spatial absorption map of the nanowire. However, absorption enhancement in Si was still large due to strong plasmonic fields at the sharper and longer tip, which was effective in enhancing absorption over a larger area in Si. Our work shows that the shape of a nanostructure and its optimization can significantly affect plasmonic absorption enhancement and loss performance in photovoltaic applications.

Plasmonic Metal Nanoparticles with Core-Bishell Structure for High-Performance Organic and Perovskite Solar Cells

To maximize light coupling into the active layer, plasmonic nanostructures have been incorporated into both active layers of organic solar cells (OSCs) and perovskite solar cells (PSCs) with the aim of increasing light absorption, but reports have shown controversial results in electrical characteristics. In this work, we introduce a core-bishell concept to build plasmonic nanoparticles (NPs) with metal-inorganic semiconductor-organic semiconductor nanostructure. Specifically, Ag NPs were decorated with a titania/benzoic-acid-fullerene bishell (Ag@TiO₂@Pa), which enables the NPs to be compatible with fullerene acceptors or a perovskite absorber. Moreover, coating the Ag@TiO₂ NP with a fullerene shell can activate efficient plasmon-exciton coupling and eliminate the charge accumulation, thus facilitating exciton dissociation and reducing the monomolecular recombination. The improved light absorption and enhanced carrier extraction of devices with Ag@TiO₂@Pa nanoparticles are responsible for the improved short-circuit current and fill factor, respectively. On the basis of the synergistic effects (optical and electrical), a series of plasmonic OSCs exhibited enhancement of 12.3-20.7% with a maximum power conversion efficiency of 13.0%, while the performance of plasmonic PSCs also showed an enhancement by 10.2% from 18.4% to 20.2%. This core-bishell design concept of plasmonic nanostructures demonstrates a general approach to improving the photovoltaic performance with both optical and electrical contributions.

Influence of Ag Nanostructure Location on the Absorption Enhancement in Polymer Solar Cells

The optical absorption enhancement in Ag nanocube (NC)- and nanosphere (NS)-embedded poly[ N-9'-heptadecanyl-2,7-carbazole- alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)]:[6,6]-phenyl C₇₁-butyric acid methyl ester active layer was calculated using three-dimensional finite-difference time domain simulations. The simulations were carried out by incorporating Ag nanostructures as a two-dimensional array at various locations in the active layer matrix. High absorption enhancements of 53 and 61% were achieved with NSs and NCs, respectively, when they were incorporated at the top portion of the active layer. The influence of various passivation layers on the absorption enhancement was also investigated. The simulation results revealed that the absorption enhancement is mainly due to the near-field enhancement around the nanostructures and the backward reflection of incident light from the nanostructure array.

Internally Supported Metal-Oxide Nanocatalyst for Hydrogenation of Nitroaromatics

The uncalcined but highly dispersive oxide-supported metal catalyst for liquid phase reactions may suffer from the agglomeration of metal nanoparticles and the drop of metal catalyst in solution, which will decrease the activity and shorten their life in catalysis. Here, a one-pot successive polyol reaction was developed to prepare M-E _xO _y colloidal particles as heterogeneous nanocatalysts, which merge the controlled synthesis of metal catalysts and oxide supports, the in situ loading of catalyst, and even the mesopore amplification into a highly integrated process. Unlike the traditional surface-deposited catalysts, the noble metal nanoparticles even with a large amount of loading are internally dispersed in the mesoporous oxide particles, which show higher activity and stability in the hydrogenation of nitroaromatics compared to the isolated nanocatalysts or surface-deposited catalysts. The improved activity and stability comes from the physical confinement of metal nanoparticles and good mass transportation of substrate/product within the support particles. This work proposed a novel method to prepare highly dispersed metal catalysts, which could be potentially useful to heterogeneous catalytic reactions with high-throughput and long-life demands.

Passivation of Plasmonic Colors on Bulk Silver by Atomic Layer Deposition of Aluminum Oxide

We report the passivation of angle-independent plasmonic colors on bulk silver by atomic layer deposition (ALD) of thin films of aluminum oxide. The colors are rendered by silver nanoparticles produced by laser ablation and redeposition on silver. We then apply a two-step approach to aluminum oxide conformal film formation via ALD. In the first step, a low-density film is deposited at low temperature to preserve and pin the silver nanoparticles. In the second step, a second denser film is deposited at a higher temperature to provide tarnish protection. This approach successfully protects the silver and plasmonic colors against tarnishing, humidity, and temperature, as demonstrated by aggressive exposure trials. The processing time associated with deposition of the conformal passivation layers meets industry requirements, and the approach is compatible with mass manufacturing.

A Flexible High-Performance Photoimaging Device Based on Bioinspired Hierarchical Multiple-Patterned Plasmonic Nanostructures

In insect eyes, ommatidia with hierarchical structured cornea play a critical role in amplifying and transferring visual signals to the brain through optic nerves, enabling the perception of various visual signals. Here, inspired by the structure and functions of insect ommatidia, a flexible photoimaging device is reported that can simultaneously detect and record incoming photonic signals by vertically stacking an organic photodiode and resistive memory device. A single-layered, hierarchical multiple-patterned back reflector that can exhibit various plasmonic effects is incorporated into the organic photodiode. The multiple-patterned flexible organic photodiodes exhibit greatly enhanced photoresponsivity due to the increased light absorption in comparison with the flat systems. Moreover, the flexible photoimaging device shows a well-resolved spatiotemporal mapping of optical signals with excellent operational and mechanical stabilities at low driving voltages below half of the flat systems. Theoretical calculation and scanning near-field optical microscopy analyses clearly reveal that multiple-patterned electrodes have much stronger surface plasmon coupling than flat and single-patterned systems. The developed methodology provides a versatile and effective route for realizing high-performance optoelectronic and photonic systems.

Recent Advances of Plasmonic Organic Solar Cells: Photophysical Investigations

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

Plasmonic Effects of Metallic Nanoparticles on Enhancing Performance of Perovskite Solar Cells

We report systematic design and formation of plasmonic perovskite solar cells (PSCs) by integrating Au@TiO₂ core-shell nanoparticles (NPs) into porous TiO₂ and/or perovskite semiconductor capping layers. The plasmonic effects in the formed PSCs are examined. The most efficient configuration is obtained by incorporating Au@TiO₂ NPs into both the porous TiO₂ and the perovskite capping layers, which increases the power conversion efficiency (PCE) from 12.59% to 18.24%, demonstrating over 44% enhancement, compared with the reference device without the metal NPs. The PCE enhancement is mainly attributed to short-circuit current improvement. The plasmonic enhancement effects of Au@TiO₂ core-shell nanosphere photovoltaic composites are explored based on the combination of UV-vis absorption spectroscopy, external quantum efficiency (EQE), photocurrent properties, and photoluminescence (PL). The addition of Au@TiO₂ nanospheres increased the rate of exciton generation and the probability of exciton dissociation, enhancing charge separation/transfer, reducing the recombination rate, and facilitating carrier transport in the device. This study contributes to understanding of plasmonic effects in perovskite solar cells and also provides a promising approach for simultaneous photon energy and electron management.

Scalable Production of Mechanically Robust Antireflection Film for Omnidirectional Enhanced Flexible Thin Film Solar Cells

Antireflection (AR) at the interface between the air and incident window material is paramount to boost the performance of photovoltaic devices. 3D nanostructures have attracted tremendous interest to reduce reflection, while the structure is vulnerable to the harsh outdoor environment. Thus the AR film with improved mechanical property is desirable in an industrial application. Herein, a scalable production of flexible AR films is proposed with microsized structures by roll-to-roll imprinting process, which possesses hydrophobic property and much improved robustness. The AR films can be potentially used for a wide range of photovoltaic devices whether based on rigid or flexible substrates. As a demonstration, the AR films are integrated with commercial Si-based triple-junction thin film solar cells. The AR film works as an effective tool to control the light travel path and utilize the light inward more efficiently by exciting hybrid optical modes, which results in a broadband and omnidirectional enhanced performance.

Compound Copper Chalcogenide Nanocrystals

This review captures the synthesis, assembly, properties, and applications of copper chalcogenide NCs, which have achieved significant research interest in the last decade due to their compositional and structural versatility. The outstanding functional properties of these materials stems from the relationship between their band structure and defect concentration, including charge carrier concentration and electronic conductivity character, which consequently affects their optoelectronic, optical, and plasmonic properties. This, combined with several metastable crystal phases and stoichiometries and the low energy of formation of defects, makes the reproducible synthesis of these materials, with tunable parameters, remarkable. Further to this, the review captures the progress of the hierarchical assembly of these NCs, which bridges the link between their discrete and collective properties. Their ubiquitous application set has cross-cut energy conversion (photovoltaics, photocatalysis, thermoelectrics), energy storage (lithium-ion batteries, hydrogen generation), emissive materials (plasmonics, LEDs, biolabelling), sensors (electrochemical, biochemical), biomedical devices (magnetic resonance imaging, X-ray computer tomography), and medical therapies (photochemothermal therapies, immunotherapy, radiotherapy, and drug delivery). The confluence of advances in the synthesis, assembly, and application of these NCs in the past decade has the potential to significantly impact society, both economically and environmentally.

Plasmonic Solar Cells: From Rational Design to Mechanism Overview

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.

Enhancement in Organic Photovoltaics Controlled by the Interplay between Charge-Transfer Excitons and Surface Plasmons

In this work, we investigate plasmonic enhancement in poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester organic photovoltaics (OPVs) by integrating shape- and size-controlled bimetallic gold core-silver shell nanocrystals (Au-Ag NCs) into the poly(3,4-ethylenedioxythiophene):polystyrene sulfonate hole-transport layer. We observed that the best-performing Au-Ag NC-incorporated OPVs improved the power conversion efficiency by 9% via a broadband increase in photocurrent throughout the visible spectrum. Our experimental and computational results suggest that the observed photocurrent enhancement in plasmonic OPVs originates from both enhanced absorption and improved exciton dissociation and charge collection. This is particularly achieved by placing metal NCs near the interface of the active layer and hole-transport layer. The impedance spectroscopy results suggest that Au-Ag NCs reduce recombination and also increase the internal exciton to carrier efficiency by driving the dissociation of bound charge-transfer states to free carriers.

Plasmon-Induced Broadband Light-Harvesting for Dye-Sensitized Solar Cells Using a Mixture of Gold Nanocrystals

The efficiency of dye-sensitized solar cells (DSSCs) is generally limited by the mismatch between the absorption spectrum of the photosensitizer and the solar irradiation spectrum. This work describes the use of a mixture that containing proper proportions of SiO2 coated Au nanospheres (AuNSs@SiO2 ) and Au nanorods (AuNRs@SiO2 ) (the mixture was denoted as AuNCs@SiO2 ) to enhance the sunlight utility in DSSCs. The incorporation of AuNCs@SiO2 into the TiO2 photoanode induced broadband light-harvesting at both low- and long- wavelengths and thus enhanced the photocurrent compared to that of plasmonic solar cells based on either AuNSs@SiO2 or AuNRs@SiO2 . Upon the doping of AuNCs@SiO2 , the overall power conversion efficiency (PCE) increased from 7.39 to 9.12 % for DSSCs based on organic liquid electrolytes.

Optimal Shell Thickness of Metal@Insulator Nanoparticles for Net Enhancement of Photogenerated Polarons in P3HT Films

Embedding metal nanoparticles in the active layer of organic solar cells has been explored as a route for improving charge carrier generation, with localized field enhancement as a proposed mechanism. However, embedded metal nanoparticles can also act as charge recombination sites. To suppress such recombination, the metal nanoparticles are commonly coated with a thin insulating shell. At the same time, this insulating shell limits the extent that the localized enhanced electric field influences charge generation in the organic medium. It is presumed that there is an optimal thickness which maximizes field enhancement effects while suppressing recombination. Atomic Layer Deposition (ALD) was used to deposit Al2O3 layers of different thicknesses onto silver nanoparticles (Ag NPs), in a thin film of P3HT. Photoinduced absorption (PIA) spectroscopy was used to study the dependence of the photogenerated P3HT(+) polaron population on the Al2O3 thickness. The optimal thickness was found to be 3-5 nm. This knowledge can be further applied in the design of metal nanoparticle-enhanced solar cells.

Plasmon-Polaron Coupling in Conjugated Polymer on Infrared Nanoantennas

We propose and demonstrate a novel type of coupling between polarons in a conjugated polymer and localized surface plasmons in infrared (IR) nanoantennas. The near-field interaction between plasmons and polarons is revealed by polarized photoinduced absorption measurements, probing mid-IR polaron transitions, and infrared-active vibrational modes of the polymer, which directly gauge the density of photogenerated charge carriers. This work proves the possibility of tuning the polaronic properties of organic semiconductors with plasmonic nanostructures.

Plasmonic Ag@oxide nanoprisms for enhanced performance of organic solar cells

Localized surface plasmon resonance (LSPR), light scattering, and lowering the series resistance of noble metal nanoparticles (NPs) provide positive effect on the performance of photovoltaic device. However, the exciton recombination on the noble metal NPs accompanying above influences will deteriorate the performance of device. In this report, surface-modified Ag@oxide (TiO2 or SiO2 ) nanoprisms with 1-2 nm shell thickness are developed. The thin film composed of P3HT/Ag@oxides and P3HT:PCBM/Ag@oxides is investigated by absorption, photoluminescence (PL), and transient absorption spectroscopy. The results show a significant absorption, PL enhancement, and long-lived photogenerated polaron in the P3HT/Ag@TiO2 film, indicating the increase of photogenerated exciton population by LSPR of Ag nanoprisms. In the case of P3HT/Ag nanoprisms, partial PL quench and relatively short-lived photogenerated polaron are observed. That indicates that the oxides layer can effectively avoid the exciton recombination. When the Ag@oxide nanoprisms are introduced into the active layer of P3HT:PCBM photovoltaic devices, about 31% of power conversion efficiency enhancement is obtained relative to the reference cell. All these results indicate that Ag@oxides can enhance the performance of the cell, at the same time the ultrathin oxide shell prevents from the exciton recombination.

Plasmon-enhanced light harvesting: applications in enhanced photocatalysis, photodynamic therapy and photovoltaics

This review article summarizes the recent progress on surface plasmon-enhanced light harvesting and its applications toward enhanced photocatalysis, photodynamic therapy, chemical transformations and photovoltaics.

Plasmon-enhanced light harvesting: applications in enhanced photocatalysis, photodynamic therapy and photovoltaics

This review article summarizes the recent progress on surface plasmon-enhanced light harvesting and its applications toward enhanced photocatalysis, photodynamic therapy, chemical transformations and photovoltaics.

Plasmon-enhanced light harvesting: applications in enhanced photocatalysis, photodynamic therapy and photovoltaics

This review article summarizes the recent progress on surface plasmon-enhanced light harvesting and its applications toward enhanced photocatalysis, photodynamic therapy, chemical transformations and photovoltaics.

Plasmon-induced hot carriers in metallic nanoparticles

Plasmon-induced hot carrier formation is attracting an increasing research interest due to its potential for applications in photocatalysis, photodetection and solar energy harvesting. However, despite very significant experimental effort, a comprehensive theoretical description of the hot carrier generation process is still missing. In this work we develop a theoretical model for the plasmon-induced hot carrier process and apply it to spherical silver nanoparticles and nanoshells. In this model, the conduction electrons of the metal are described as free particles in a finite spherical potential well, and the plasmon-induced hot carrier production is calculated using Fermi’s golden rule. We show that the inclusion of many-body interactions has only a minor influence on the results. Using the model we calculate the rate of hot carrier generation, finding that it closely follows the spectral profile of the plasmon. Our analysis reveals that particle size and hot carrier lifetime play a central role in determining both the production rate and the energy distribution of the hot carriers. Specifically, larger nanoparticle sizes and shorter lifetimes result in higher carrier production rates but smaller energies, and vice versa. We characterize the efficiency of the hot carrier generation process by introducing a figure of merit that measures the number of high energy carriers generated per plasmon. Furthermore, we analyze the spatial distribution and directionality of these excitations. The results presented here contribute to the basic understanding of plasmon-induced hot carrier generation and provide insight for optimization of the process.

Plasmonic Cu(x)In(y)S2 quantum dots make better photovoltaics than their nonplasmonic counterparts

A synthetic approach has recently been developed which results in Cu(x)In(y)S2 quantum dots (QDs) possessing localized surface plasmon resonance (LSPR) modes in the near-infrared (NIR) frequencies.1 In this study, we investigate the potential benefits of near-field plasmonic effects centered upon light absorbing nanoparticles in a photovoltaic system by developing and verifying nonplasmonic counterparts as an experimental control. Simple QD-sensitized solar cells (QD-SSCs) were assembled which show an 11.5% relative increase in incident photon conversion efficiency (IPCE) achieved in the plasmon-enhanced devices. We attribute this increase in IPCE to augmented charge excitation stemming from near-field "antenna" effects in the plasmonic Cu(x)In(y)S2 QD-SSCs. This study represents the first of its kind; direct interrogation of the influence of plasmon-on-semiconductor architectures with respect to excitonic absorption in photovoltaic systems.

Plasmonic organic solar cells employing nanobump assembly via aerosol-derived nanoparticles

We report the effect of a nanobump assembly (NBA) constructed with molybdenum oxide (MoO3) covering Ag nanoparticles (NPs) under the active layer on the efficiency of plasmonic polymer solar cells. Here, the NPs with precisely controlled concentration and size have been generated by an atmospheric evaporation/condensation method and a differential mobility classification and then deposited on an indium tin oxide electrode via room temperature aerosol method. NBA structure is made by enclosing NPs with MoO3 layer via vacuum thermal evaporation to isolate the undulated active layer formed onto the underlying protruded NBA. Simulated scattering cross sections of the NBA structure reveal higher intensities with a strong forward scattering effect than those from the flat buffer cases. Experimental results of the device containing the NBA show 24% enhancement in short-circuit current density and 18% in power conversion efficiency compared to the device with the flat MoO3 without the NPs. The observed improvements are attributed to the enhanced light scattering and multireflection effects arising from the NBA structure combined with the undulated active layer in the visible and near-infrared regions. Moreover, we demonstrate that the NBA adopted devices show better performance with longer exciton lifetime and higher light absorption in comparison with the devices with Ag NPs incorporated flat poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Thus, the suggested approach provides a reliable and efficient light harvesting in a broad range of wavelength, which consequently enhances the performance of various organic solar cells.

Surface plasmon enhanced organic solar cells with a MoO3 buffer layer

High-efficiency surface plasmon enhanced 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane:C70 small molecular bulk heterojunction organic solar cells with a MoO3 anode buffer layer have been demonstrated. The optimized device based on thermal evaporated Ag nanoparticles (NPs) shows a power conversion efficiency of 5.42%, which is 17% higher than the reference device. The improvement is attributed to both the enhanced conductivity and increased absorption due to the near-field enhancement of the localized surface plasmon resonance of Ag NPs.

Phase transfer of triangular silver nanoprisms from aqueous to organic solvent by an amide coupling reaction

In this paper, we describe a procedure for the phase transfer of silver nanoprisms (AgNPrs) from aqueous solution to chloroform via an amide coupling reaction. AgNPrs are first modified with 16-mercaptohexadecanoic acid (MHA), and then primary or secondary amines are attached to the carboxylic acid end of the MHA ligand through a carbodiimide-mediated amide coupling step. Secondary amines, such as dicyclohexylamine and diphenylamine, are found to solubilize the nanoparticles in chloroform, whereas primary amines (e.g., butylamine and hexadecylamine) do not result in phase transfer. It is found that the AgNPrs functionalized with dicyclohexylamine show the highest stability and the least aggregation after undergoing phase transfer; in contrast, with a less nucleophilic amine, such as diphenylamine, the amide coupling reaction does not go to completion and the resultant AgNPrs are less stable and more prone to aggregation.

Jointly tuned plasmonic-excitonic photovoltaics using nanoshells

Recent advances in spectrally tuned, solution-processed plasmonic nanoparticles have provided unprecedented control over light's propagation and absorption via engineering at the nanoscale. Simultaneous parallel progress in colloidal quantum dot photovoltaics offers the potential for low-cost, large-area solar power; however, these devices suffer from poor quantum efficiency in the more weakly absorbed infrared portion of the sun's spectrum. Here, we report a plasmonic-excitonic solar cell that combines two classes of solution-processed infrared materials that we tune jointly. We show through experiment and theory that a plasmonic-excitonic design using gold nanoshells with optimized single particle scattering-to-absorption cross-section ratios leads to a strong enhancement in near-field absorption and a resultant 35% enhancement in photocurrent in the performance-limiting near-infrared spectral region.

Identification of parameters through which surface chemistry determines the lifetimes of hot electrons in small Au nanoparticles

This paper describes measurements of the dynamics of hot electron cooling in photoexcited gold nanoparticles (Au NPs) with diameters of ∼3.5 nm, and passivated with either a hexadecylamine or hexadecanethiolate adlayer, using ultrafast transient absorption spectroscopy. Fits of these dynamics with temperature-dependent Mie theory reveal that both the electronic heat capacity and the electron-phonon coupling constant are larger for the thiolated NPs than for the aminated NPs, by 40% and 30%, respectively. Density functional theory calculations on ligand-functionalized Au slabs show that the increase in these quantities is due to an increased electronic density of states near the Fermi level upon ligand exchange from amines to thiolates. The lifetime of hot electrons, which have thermalized from the initial plasmon excitation, increases with increasing electronic heat capacity, but decreases with increasing electron-phonon coupling, so the effects of changing surface chemistry on these two quantities partially cancel to yield a hot electron lifetime of thiolated NPs that is only 20% longer than that of aminated NPs. This analysis also reveals that incorporation of a temperature-dependent electron-phonon coupling constant is necessary to adequately fit the dynamics of electron cooling.