Improved Internal Quantum Efficiency and Light-Extraction Efficiency of Organic Light-Emitting Diodes via Synergistic Doping with Au and Ag Nanoparticles

Effects of photon recycling and scattering in high-performance perovskite solar cells

Efficient external radiation is essential for solar cells to achieve high power conversion efficiency (PCE). The classical limit of 1/2n² (n, refractive index) for electroluminescence quantum efficiency (ELQE) has recently been approached by perovskite solar cells (PSCs). Photon recycling (PR) and light scattering can provide an opportunity to surpass this limit. We investigate the role of PR and scattering in practical device operation using a radiative PSC with an ELQE (13.7% at 1 sun) that significantly surpasses the classical limit (7.4%). We experimentally analyze the contributions of PR and scattering to this strong radiation. A novel optical model reveals an increase of 39 mV in the voltage of our PSC. This analysis can provide design principles for future PSCs to approach the Shockley-Queisser efficiency limit.

Feasibility of using bimetallic Au-Ag nanoparticles for organic light-emitting devices

Matching the resonant wavelength of plasmonic nanoparticles (NPs) and the emission band of organic materials is critical for achieving optimal plasmon-enhanced luminescence in organic light-emitting devices (OLEDs). However, the spectral matching is often unsatisfactory because the interior architecture of OLEDs limits the dimensions of the NPs to support the desired wavelength adjustment. In this article, we proposed a design strategy via Au_xAg_1-x alloy NPs to enable resonance tuning while preserving the size of the NP to suit the OLED design requirements. The bimetallic NPs, especially for x < 0.6, not only add one more degree of freedom to vary the plasmon wavelength but also provide the benefits of higher scattering and more intense and outspread electric fields over a broader spectrum compared to Au monometallic NPs. These features allow smaller NPs, which are more compatible with OLED interiors, to scatter electric fields more efficiently and increase the density of molecules interacting with the NP plasmons. In the presence of a nearby dipole emitter, the bimetallic NPs can simultaneously increase radiative enhancement and suppress non-radiative losses, which are advantageous for increasing the quantum yield and luminescence efficiency of the emitter. These improvements are associated with lower intraband and interband activities resulting from the higher molar fraction of Ag in the alloy NPs. We provided composition mappings to achieve enhanced luminescence for specified wavelengths at fixed NP sizes. Finally, we theoretically demonstrated that the bimetallic NPs could improve the light-extraction efficiency of OLEDs better than Au monometallic NPs. This work provides essential guidance to enable versatile plasmon-enhanced applications with predefined nanostructural geometries and wavelengths to match the device requirements.

Computational Study of Dipole Radiation in Re-Absorbing Perovskite Semiconductors for Optoelectronics

Compared to organic emitters, perovskite materials generally have a small Stokes shift and correspondingly large re-absorption of dipole emission. Classical optical modelling methods ignoring re-absorption do not provide an adequate description of the observed light emission properties. Here, optical modelling methods and design rules for perovskite light-emitting diodes are presented. The transfer-matrix formalism is used to quantify the Poynting vectors generated by a dipole radiating inside a perovskite optoelectronic device. A strategy is presented to deal with non-radiative coupling to nearby emissive material that can otherwise lead to non-physical divergence in the calculation. Stability issues are also investigated regarding coherence of the light propagating in the substrate and the absence of a light absorber in the system. The benefit of the photon recycling effect is taken into account by recursive calculation of the dipole generation profile. The simulation results predict that a high external quantum efficiency of ≈40% is achievable in formamidinium lead triiodide-based perovskite light-emitting diodes, by optimization of microcavity, dipole orientation, and photon recycling effects. Contrary to conventional device structures currently reported, this work highlights the benefits of thick charge transport layers and thick perovskite with small Stokes shift.

The role of photon recycling in perovskite light-emitting diodes

Perovskite light-emitting diodes have recently broken the 20% barrier for external quantum efficiency. These values cannot be explained with classical models for optical outcoupling. Here, we analyse the role of photon recycling (PR) in assisting light extraction from perovskite light-emitting diodes. Spatially-resolved photoluminescence and electroluminescence measurements combined with optical modelling show that repetitive re-absorption and re-emission of photons trapped in substrate and waveguide modes significantly enhance light extraction when the radiation efficiency is sufficiently high. In this manner, PR can contribute more than 70% to the overall emission, in agreement with recently-reported high efficiencies. While an outcoupling efficiency of 100% is theoretically possible with PR, parasitic absorption losses due to absorption from the electrodes are shown to limit practical efficiencies in current device architectures. To overcome the present limits, we propose a future configuration with a reduced injection electrode area to drive the efficiency toward 100%.

Perovskite light-emitting diodes have shown unexpected high external quantum efficiency of 20%, breaking the ray-optics limit. Here Cho et al. reveal that photon recycling is responsible for the enhancement and propose photonic structures to further improve the device efficiency.

Multi-bandgap Solar Energy Conversion via Combination of Microalgal Photosynthesis and Spectrally Selective Photovoltaic Cell

Microalgal photosynthesis is a promising solar energy conversion process to produce high concentration biomass, which can be utilized in the various fields including bioenergy, food resources, and medicine. In this research, we study the optical design rule for microalgal cultivation systems, to efficiently utilize the solar energy and improve the photosynthesis efficiency. First, an organic luminescent dye of 3,6-Bis(4′-(diphenylamino)-1,1′-biphenyl-4-yl)-2,5-dihexyl-2,5-dihydropyrrolo3,4-c pyrrole -1,4-dione (D1) was coated on a photobioreactor (PBR) for microalgal cultivation. Unlike previous reports, there was no enhancement in the biomass productivities under artificial solar illuminations of 0.2 and 0.6 sun. We analyze the limitations and future design principles of the PBRs using photoluminescence under strong illumination. Second, as a multiple-bandgaps-scheme to maximize the conversion efficiency of solar energy, we propose a dual-energy generator that combines microalgal cultivation with spectrally selective photovoltaic cells (PVs). In the proposed system, the blue and green photons, of which high energy is not efficiently utilized in photosynthesis, are absorbed by a large-bandgap PV, generating electricity with a high open-circuit voltage (V_oc) in reward for narrowing the absorption spectrum. Then, the unabsorbed red photons are guided into PBR and utilized for photosynthesis with high efficiency. Under an illumination of 7.2 kWh m⁻² d⁻¹, we experimentally verified that our dual-energy generator with C₆₀-based PV can simultaneously produce 20.3 g m⁻² d⁻¹ of biomass and 220 Wh m⁻² d⁻¹ of electricity by utilizing multiple bandgaps in a single system.

Investigation of the localized surface plasmon resonance of Ag@SiO2 core-shell nanocubes and its application in high-performance blue organic light-emitting diodes

A blue organic light-emitting diode (OLED) with silica-coated silver nanocubes (Ag@SiO₂ NCs) inserted at the hole transporting layer/emission layer interface is reported. The localized surface plasmon resonance (LSPR) properties of the Ag@SiO₂ NCs were characterized by the measured absorption spectra, stable-stated and transient photoluminescence, and calculated dipole radiation power. The results suggest that the Ag NCs significantly improved the radiation intensity of the nearby excitons due to their sharp corners and edges, but had less impact on the radiation rate of the excitons. The exciton recombination zone in the blue OLED was confirmed by a group of devices with an ultra-thin yellow emission layer located at different places, which helped to figure out the distribution of the excitons around the Ag@SiO₂ NCs and deeply understand the coupling between the excitons and the Ag@SiO₂ NCs. In our blue OLED, an appropriate distance between the Ag NCs and the excitons was realized by the SiO₂ coating layer and the exciton distribution, which greatly improved the energy transfer between the excitons and the Ag NCs. In addition, the LSPR enhanced electric field around the Ag@SiO₂ NCs improved the carrier injection at the hole transporting layer/emission layer interface and increased the current density of the blue OLED. Finally, the blue OLED with a simple triple layer structure achieved a high current efficiency of 51.1 cd A^-1.

Multifunctional Silver Nanoparticle Interlayer-Modified ZnO as the Electron-Injection Layer for Efficient Inverted Organic Light-Emitting Diodes

The insufficient electron injection constitutes the major obstacle to achieving high-performance inverted organic light-emitting diodes (OLEDs). Here, a facile electron-injection architecture featuring a silver nanoparticle (AgNPs) interlayer-modified sol-gel-derived transparent zinc oxide (ZnO) ultrathin film is proposed and demonstrated. The optimized external quantum efficiencies of the developed inverted fluorescent and phosphorescent OLEDs capitalized on our proposed electron-injection structure reached 4.0 and 21.2% at a current density of 20 mA cm^-2 and increased by a factor of 1.90 and 2.86 relative to a reference device without the AgNP interlayer, while simultaneously reducing the operational voltage and substantially ameliorating the device efficiency. Detailed analyses reveal that the local surface plasmon resonance emanated from AgNPs plays three meaningful roles simultaneously: suppressing the surface plasmon polariton mode loss, aiding in energy-level alignments, and inducing and reinforcing the local exciton-plasmon coupling electric field. Among these interesting and multifunctional roles, the enhanced local exciton-plasmon coupling electric field dominates the electron injection enhancement and substantial increases the device efficiency. Additionally, the light-scattering effect also helps in recovering the trapped light energy flux and thus improves the device efficiency. The proposed approach and findings provide an alternative path to fabricate high-performance inverted OLEDs and other related organic electronic or optoelectronic devices.

Controlled Enhancement in Hole Injection at Gold-Nanoparticle-on-Organic Electrical Contacts Fabricated by Spark-Discharge Aerosol Technique

We demonstrate that hole injection from a top electrode composed of Au nanoparticles (AuNPs) capped with a thick Au layer into an underlying organic semiconductor, N, N'-diphenyl- N, N'-bis-[4-(phenyl- m-tolyl-amino)-phenyl]-biphenyl-4,4'-diamine (DNTPD), is significantly enhanced compared to that in a control device whose top electrode is composed entirely of a thick Au layer. The fabrication of this organic hole-only device with the AuNP electrode is made possible by dry, room-temperature distribution of AuNPs onto DNTPD using a spark-discharge aerosol technique capable of varying the average diameter ( D̅) of the AuNPs. The enhancement in hole injection is found to increase with decreasing D̅, with the current density of a device with D̅ = 1.1 nm being more than 3 orders of magnitude larger than that of the control device. Intensity-modulated photocurrent measurements show that the built-in potentials of the devices with the AuNP electrode are smaller than that of the control device by as much as 0.68 V, indicating that the enhanced hole injection originates from the increased work functions of these devices, which in turn decreases the hole injection barrier heights. X-ray photoelectron spectroscopy reveals that the increased work functions of the AuNP electrodes are due to surface oxidation of the AuNPs resulting in AuN and Au₃N. The degree of oxidation of the AuNPs increases with decreasing D̅, consistent with the D̅-dependencies of the hole injection enhancement and the built-in potential reduction.

Improved Configuration and LSPR Response of Platinum Nanoparticles via Enhanced Solid State Dewetting of In-Pt Bilayers

Noble metallic nanoparticles (NPs) can exhibit valuable properties such as localized surface plasmon resonance (LSPR) and large surface to volume ratio, which can find various optoelectronic and catalytic applications. In this work, the improved configuration and uniformity of platinum (Pt) NPs are demonstrated by using a sacrificial indium (In) layer via the enhanced solid state dewetting of In-Pt bilayers on sapphire (0001). In a sharp contrast to the conventional dewetting of intrinsic Pt film, the introduction of In component can significantly enhance the global dewetting process and thus can result in the fabrication of well-defined Pt NPs with the improved uniformity. This can be due to the fact that In possess high diffusivity, low surface energy and low sublimation temperature. Upon annealing, the intermixing of In and Pt atoms can occur at the interface due to the inter-diffusion, which forms In-Pt alloy system. As a result, the overall diffusivity and dewetting degree of system can be significantly improved and this can produce more isolated, uniform and semispherical Pt NPs at much lower temperatures as compared to the pure Pt film dewetting. Conveniently, the In atoms preferentially can be removed from the NP matrix by the sublimation even at relatively low temperatures. These Pt NPs exhibit dynamic LSPR band in the UV-visible wavelength based on the excitation of dipolar, quadrupolar and higher order resonance modes. Specifically, the LSPR wavelength can be tuned between ~480 and 580 nm by the fabrication of small to large size Pt NPs with the distinct configuration and interparticle spacing. Furthermore, at a constant Pt thickness, the size, spacing and density of Pt NPs can be readily tuned by the control of In layer thickness. The introduction of sacrificial In component can enable an additional flexibility for the control of surface morphologies of metallic NPs with the low diffusivity materials.

Nearly 100% Efficiency Enhancement of CH3NH3PbBr3 Perovskite Light-Emitting Diodes by Utilizing Plasmonic Au Nanoparticles

Organic-inorganic hybrid perovskites have drawn considerable attention due to their great potentials in lighting and displaying. Despite great progress being demonstrated in perovskites light-emitting diodes (PeLEDs), the commercialization of PeLEDs was still limited by their low efficiencies and poor device stabilities. Utilizing the metallic nanoparticles was a feasible way to further improve the efficiencies of PeLEDs. Herein, substantially enhanced electroluminescent performance of CH₃NH₃PbBr₃-based PeLEDs were first demonstrated by incorporating plasmonic gold nanoparticles (Au NPs) into the hole injection layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Compared to the reference device without Au NPs, 109% enhancement in maximum luminance and 97% enhancement in maximum EQE were achieved upon 9 vol % Au NPs doping. Such enhancements can be ascribed to the localized surface plasmon resonance between Au NPs and CH₃NH₃PbBr₃ excitons, as well as the enhanced electrical conductivity of modified PEDOT:PSS. Our studies indicated great potential of Au NPs in developing highly efficient PeLEDs.

Facile one-step hydrothermal synthesis of noble-metal-free hetero-structural ternary composites and their application in photocatalytic water purification

The development of heterostructured and metal decorated photocatalysts using a relatively simple, efficient and economical one-step strategy is crucial for commercial applications.