The formation of triplet excitons in semiconducting organic materials plays an important role for the operation of organic optoelectronic devices. Triplet excitons are difficult to investigate spectroscopically at room temperature due to their non-radiative character. Here we show the measurement of the triplet decay dynamics by a highly sensitive time-resolved measurement of the triplet state absorption using pump and probe experiments within a waveguide configuration. Pump and probe pulse have to be separated spatially and in time to ensure a segregation of singlet and triplet excitons. The non-radiative triplet excitons are detected at room temperature by their absorption. A variation of the time delay between pump and probe pulse allows for the investigation of the dynamics of the excitons. Former experiments made use of the photoluminescence of the material under investigation itself as source of probe light. But since there is a spectral shift between the photoluminescence band and the triplet absorption band the spectral bandwidth of probe light has to be broadened for the identification of the triplet absorption. Here we show how a widening of the probe light band up to red part of the spectrum is achieved by an application of extra emission layers. With the help of this technique the triplet exciton dynamics of the hole transport material 2,2',7,7'-tetrakis(diphenylamino)-9,9'-spirobifluorene (SpiroTAD) was measured and analyzed.

In this paper, we review our recent work on triplet harvesting (TH) and its application in white organic light-emitting diodes (OLEDs). This includes the configuration of highly efficient single unit two-color and tandem four-color white OLEDs reaching efficacies of more than 30 lm/W at 1000 cd/m², as well as the development of new blue fluorescent emitters. The new compounds are chemically designed to exhibit a high triplet energy to allow TH by a green phosphorescent emitter. In a first step towards white TH OLEDs, we fabricated blue-green TH OLEDs to prove the validity of our concept.

We report on the development of a plasmonic-photonic coupled platform based on a plasmonic periodic nanostructure and a host matrix for active media in the visible-near infrared range, constituted by a thin film of sol-gel glass. Here, we report on preliminary results about two main tasks of the research work. On one side, we have studied and optimized the surface that supports plasmonic resonances with tunable wavelengths. On the other side, we focused on improving the sol-gel techniques to form and deposit thin films appropriate for covering the previous surface as well as to protect it (i.e. for sensing applications), embed suitable fluorophores (for active device applications) while avoiding metal-induced radiative emission quenching. Besides structural and optical characterization of the considered structures and films, finite-difference time-domain numerical simulations have been performed, in order to give a feedback on the structure features and thereby interpret its optical response.

High-brightness green phosphorescent hybrid inorganic-organic light-emitting diodes (HyLEDs) and inverted HyLEDs (IHyLEDs) have been demonstrated. The devices comprised a transparent and conductive WO3 layer deposited by thermal evaporation, which improved both hole injection and transport, and led to more balanced charge injection and significant performance enhancement. At 20 mA/cm2, the HyLEDs had a low operation voltage of 6.1 V, 0.8 V lower than that of OLEDs with an organic hole transport layer. With an optimized layer structure, the HyLEDs reached 104 cd/m2 brightness at 7.3 V. At this brightness level, the current efficiency was 55.2 cd/A, 57% higher than that of the OLEDs. In the IHyLEDs, facile hole injection and transport through WO3 was balanced by electron injection from the indium-tin-oxide (ITO) cathode overcoated with nanometer-thick Ca, leading to a low turn-on voltage of ∼6 V. Brightness of 8133 cd/m2 was reached at 20 mA/cm2 and the corresponding current efficiency was 40 cd/A. The hybrid devices also exhibited markedly improved stability under constant-current stressing due to the robust WO3 hole transport layer.

Excimer fluorescence of two-component thin films made of pyrene (Py) and polystyrene (PS) can be quenched by the vapor of nitro-aromatic and nitro-ester explosives with a high selectivity and sensitivity. Normally, an electrospun film can be quenched in minutes by the vapor of the explosives. In order to understand the origin of the mechanism, we have investigated the fluorescence quenching rate of the binary thin films as functions of the molecular weights (MW) of the polystyrene (from 2,500 to 900,000 g/mol) and film thicknesses (110nm and 610 nm) in presence of the vapor of 2,4-dinitrotoluene (2,4-DNT, a type of nitro-explosives). The diffusion coefficients of 2, 4-DNT in the solid films are found nearly independent of MW but have strong dependence on the film thicknesses.

A general approach to enhancing the photoluminescent quantum yield for a series of organic chromophores is presented. By bridging a chromophore symmetrically about a sulfur atom it was found that the photoluminescence could be systematically increased by oxidizing the bridge. Furthermore, the enhanced quantum yields were achieved without diminishing the solubility of these chromophores in common organic solvents. The photophysical characterization, as well as potential applications of these molecules will be discussed.