Organic compounds that exhibit highly efficient, stable blue emission are required to realize inexpensive organic light-emitting diodes for future displays and lighting applications. Here, we define the design rules for increasing the electroluminescence efficiency of blue-emitting organic molecules that exhibit thermally activated delayed fluorescence. We show that a large delocalization of the highest occupied molecular orbital and lowest unoccupied molecular orbital in these charge-transfer compounds enhances the rate of radiative decay considerably by inducing a large oscillator strength even when there is a small overlap between the two wavefunctions. A compound based on our design principles exhibited a high rate of fluorescence decay and efficient up-conversion of triplet excitons into singlet excited states, leading to both photoluminescence and internal electroluminescence quantum yields of nearly 100%.

The highest carrier mobility of polydiacetylene (PDA) thin films in field-effect transistors has been limited to less than 0.8 cm2 V-1 s-1, although the main chain conduction should show higher carrier mobility potentially. We revealed that the cause of the low carrier mobility is due to the presence of local upheaval regions generated by the volume change through the polymerization process of diacetylene monomers. In order to suppress the occurrence of the upheaval regions, we found that electron beam (EB) irradiation is effective, resulted in the highest carrier mobility of µmax = 3.8 cm2 V-1 s-1.

We investigated the influence of vacuum chamber impurities on the lifetime of highly efficient TADF-based OLEDs. Batch-to-batch lifetime variations are clearly correlated with the results of contact angle measurements, which reflect the amount of impurities present in the chamber. Introduction of ozone gas can clean the impurities out of the vacuum chamber, reducing the contact angle to less than 10°. In the vacuum chamber of a new deposition system designed using resin-free vacuum components, various plasticizers and additive agents were initially detected by WTD-GC-MS analysis, but these impurities vanished after ozone gas cleaning. Devices fabricated in the new chamber exhibited lifetimes that are approximately twice those of OLEDs fabricated in a pre-existing chamber. These results suggest that impurities, particularly from plasticizers, in the vacuum chamber greatly influence the OLED lifetime.

Conventional vacuum deposition onto a substrate coated with a self-assembled monolayer (SAM) was used to produce arrays of hemispherical nanostructures composed of organic semiconducting materials with low molecular weight. The nanostructures had a minimum diameter of 40 nm, indicating that cluster formation occurred in the gas phase during vacuum deposition. The size and shape of the nanostructures were controlled by the underlying SAMs, organic semiconducting materials, film thickness and substrate temperature.

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