[Abstract]

Organic LEDs (OLEDs) have progressed rapidly due to their great potential for energy saving lighting and TV applications. The OLEDs comprising plural light-emitting-units (LEUs) stacked in series have been produced to achieve high luminance and long operational lifetime simultaneously. However, it can be made only by vacuum evaporation process due to required multilayer structures, resulting in high cost of the product. We report the fabrication of tandem OLEDs comprising two LEUs with solution-based processes.[1-5]

In terms of materials development for OLEDs, "blue" is one of most important issue.[6-7] In these days, to achieve high efficiency, phosphorescence of iridium complexes and thermally activated delayed fluorescence (TADF) with small triplet–singlet energy gap materials are main two approaches in molecular design of emitting materials. However, for blue emission, typical fluorescent molecules such as anthracene derivatives are still very important for the commercialized devices because of their higher chemical stability than phosphorescence or TADF materials, although device efficiency of the typical fluorescent molecules is lower than that of above mentioned other two approaches. To improve the efficiency of such typical anthracene molecules-based OLEDs, triplet-triplet annihilation (TTA) has been considered useful to recover non-emissive triplet excitons to emissive singlet excitons. There are a few theoretical studies or speculations on different high efficiency mechanism of typical fluorescent anthracene molecules in OLEDs, based on reverse intersystem crossing (RISC) process at highly excited states which are different from phosphorescence, TADF, and TTA. In this study, we first experimentally demonstrated the RISC process at highly excited states in OLEDs by systematic comparison between a monoanthracene compound and a spatially orthogonal bianthracene compound. Transient EL measurement and magnetic field dependence of EL revealed large TTA contribution of the monoanthracene and no TTA contribution of the bianthracene. PL quenching by oxygen and negative temperature dependence of EL only for the bianthracene support the RISC at the highly excited states. These are also strongly supported by TD-DFT calculation. High EQE of 11% with the bianthracene was obtained in blue OLEDs and this value is one of highest value among OLEDs with fluorescent anthracene compounds, demonstrating the practical effectiveness of the molecular design in this study.

Singlet fission (SF), which is the conversion of one singlet exciton into two triplet excitons through an intermediate of two neighboring molecules, has recently attracted much attention, as it may provide a way to overcome the theoretical upper limit for the power conversion efficiency of organic PVs. We report singlet fission of thienoquinoid compounds in organic photovoltaics.[8] The escalation of thienoquinoid length of the compounds realizes suitable packing structure and energy levels for singlet fission. Magnetic field dependence of the photocurrent and external quantum efficiency of the devices reveals singlet fission of the compounds and dissociation of triplet excitons into charges.