Converting an Organic Light-Emitting Diode from Blue to White with Bragg Modes

Identifying the origin of delayed electroluminescence in a polariton organic light-emitting diode

Modifying the energy landscape of existing molecular emitters is an attractive challenge with favourable outcomes in chemistry and organic optoelectronic research. It has recently been explored through strong light-matter coupling studies where the organic emitters were placed in an optical cavity. Nonetheless, a debate revolves around whether the observed change in the material properties represents novel coupled system dynamics or the unmasking of pre-existing material properties induced by light-matter interactions. Here, for the first time, we examined the effect of strong coupling in polariton organic light-emitting diodes via time-resolved electroluminescence studies. We accompanied our experimental analysis with theoretical fits using a model of coupled rate equations accounting for all major mechanisms that can result in delayed electroluminescence in organic emitters. We found that in our devices the delayed electroluminescence was dominated by emission from trapped charges and this mechanism remained unmodified in the presence of strong coupling.

Green synthesis of ethyl cinnamates under microwave irradiation: photophysical properties, cytotoxicity, and cell bioimaging

A simple and green method for the synthesis of six ethyl cinnamates was performed via Horner-Wadsworth-Emmons reaction under microwave irradiation. The photoluminescent properties of all compounds in ethyl acetate solutions were evaluated demonstrating that all compounds exhibit fluorescence. Five compounds exhibited blue emissions in the 369-442 nm range, and another compound exhibited blue-green emission at 504 nm. This last compound showed the largest Stokes shift (134 nm), and the highest quantum yield (17.8%). Two compounds showed extinction coefficient values (ε) higher than 30 000 M-1 cm-1, which are appropriate for cell bioimaging applications. In this sense, cytotoxicity assays were performed using Vero cells at different concentrations; the results showed that these compounds were not cytotoxic at the highest concentration tested (20 μg mL-1). Finally, the analysis by fluorescence microscopy for localization and cellular staining using Vero cells demonstrated that the compounds stained the cytoplasm and the nuclei in a selective way.

Developing Solution-Processed Distributed Bragg Reflectors for Microcavity Polariton Applications

Improving the performance of organic optoelectronics has been under vigorous research for decades. Recently, polaritonics has been introduced as a technology that has the potential to improve the optical, electrical, and chemical properties of materials and devices. However, polaritons have been mainly studied in optical microcavities that are made by vacuum deposition processes, which are costly, unavailable to many, and incompatible with printed optoelectronics methods. Efforts toward the fabrication of polariton microcavities with solution-processed techniques have been utterly absent. Herein, we demonstrate for the first time strong light-matter coupling and polariton photoluminescence in an organic microcavity consisting of an aluminum mirror and a distributed Bragg reflector (DBR) made by sequential dip coating of titanium hydroxide/poly(vinyl alcohol) (TiOH/PVA) and Nafion films. To fabricate and develop the solution-processed DBRs and microcavities, we automatized a dip-coating device that allowed us to produce sub-100 nm films consistently over many dip-coating cycles. Owning to the solution-based nature of our DBRs, our results pave the way to the realization of polariton optoelectronic devices beyond physical deposition methods.

Improved green thermal activated delayed fluorescence OLEDs based on thermally evaporated distributed Bragg reflector (DBR) of MgF2/ZnS

Unlike the traditional fabrication of distributed Bragg reflector (DBR) structure via atomic layer deposition or spin-coating, here the 1-6 pairs of magnesium fluoride (MgF₂)/zinc sulfide (ZnS) alternative dielectric layers were grown via thermal evaporation. The absorption, transmission, reflection, and photoluminescence (PL) spectra were evaluated. 5 pair MgF₂/ZnS denotes the largest reflectance (88.5% at 535 nm) together with a stopband at 450-650 nm among the 1- 6 pair dielectric layers, exhibiting the potential for using as DBR. Relative to the bare 4,4'-bis(carbazol-9-yl)biphenyl(CBP):(4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl) isophthalonitrile (4CzIPN) film, the PL intensity of CBP:4CzIPN/5 pair MgF₂/ZnS DBR is enhanced and splitted into two peaks. The 5 pair alternative dielectric film presents more uniform aggregation over 4 pair MgF₂/ZnS. The cross-sectional scanning electron microscopic image denotes explicit layering for the MgF₂and ZnS. The organic light-emitting diode (OLED) incorporating 5 pair MgF₂/ZnS DBR layers illustrates significantly improved electroluminescent (EL) performance due to the photons concentrated in the direction perpendicular to the DBR. The slightly narrowed EL spectrum is originated from the microcavity effect between the two Al electrodes. Here we develop a universal method for the DBR fabrication suitable to most of OLEDs.

Engineering Dielectric Materials for High-Performance Organic Light Emitting Transistors (OLETs)

Organic light emitting transistors (OLETs) represent a relatively new technology platform in the field of optoelectronics. An OLET is a device with a two-fold functionality since it behaves as a thin-film transistor and at the same time can generate light under appropriate bias conditions. This Review focuses mainly on one of the building blocks of such device, namely the gate dielectrics, and how it is possible to engineer it to improve device properties and performances. While many findings on gate dielectrics can be easily applied to organic light emitting transistors, we here concentrate on how this layer can be exploited and engineered as an active tool for light manipulation in this novel class of optoelectronic devices.

Polarization and Phase Textures in Lattice Plasmon Condensates

Polarization textures of light may reflect fundamental phenomena, such as topological defects, and can be utilized in engineering light beams. They have been observed, for instance, in photonic crystal lasers and semiconductor polariton condensates. Here we demonstrate domain wall polarization textures in a plasmonic lattice Bose-Einstein condensate. A key ingredient of the textures is found to be a condensate phase that varies spatially in a nontrivial manner. The phase of the Bose-Einstein condensate is reconstructed from the real- and Fourier-space images using a phase retrieval algorithm. We introduce a simple theoretical model that captures the results and can be used for design of the polarization patterns and demonstrate that the textures can be optically switched. The results open new prospects for fundamental studies of non-equilibrium condensation and sources of polarization-structured beams.

Strong Coupling beyond the Light-Line

Strong coupling of molecules placed in an optical microcavity may lead to the formation of hybrid states called polaritons; states that inherit characteristics of both the optical cavity modes and the molecular resonance. Developing a better understanding of the matter characteristics of these hybrid states has been the focus of much recent attention. Here, as we will show, a better understanding of the role of the optical modes supported by typical cavity structures is also required. Typical microcavities used in molecular strong coupling experiments support more than one mode at the frequency of the material resonance. While the effect of strong coupling to multiple photonic modes has been considered before, here we extend this topic by looking at strong coupling between one vibrational mode and multiple photonic modes. Many experiments involving strong coupling make use of metal-clad microcavities, ones with metallic mirrors. Metal-clad microcavities are well-known to support coupled plasmon modes in addition to the standard microcavity mode. However, the coupled plasmon modes associated with a metal-clad optical microcavity lie beyond the light-line and are thus not probed in typical experiments on strong coupling. Here we investigate, through experiment and numerical modeling, the interaction between molecules within a cavity and the modes both inside and outside the light-line. Making use of grating coupling and a metal-clad microcavity, we provide an experimental demonstration that such modes undergo strong coupling. We further show that a common variant of the metal-clad microcavity, one in which the metal mirrors are replaced by distributed Bragg reflector also show strong coupling to modes that exist in these structures beyond the light-line. Our results highlight the need to consider the effect of beyond the light-line modes on the strong coupling of molecular resonances in microcavities and may be of relevance in designing strong coupling resonators for chemistry and materials science investigations.