Jin-Wuk Kim

High-performance bipolar host materials for blue TADF devices with excellent external quantum efficiencies

New bipolar host molecules composed of carbazole, pyridoindole, and dibenzothiophene (DBT) were synthesized for blue thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes (OLEDs). 2,8-Di(9H-carbazol-9-yl)dibenzo[b,d]thiophene, 9-(8-(9H-carbazol-9-yl)dibenzo[b,d]thiophen-2-yl)-9H-pyrido[2,3-b]indole, and 2,8-bis(9H-pyrido[2,3-b]indol-9-yl)dibenzo[b,d]thiophene were prepared based on the selective reactivity at the 2,8-positions of DBT. The new symmetric and asymmetric host materials exhibited high triplet energies (2.89–2.95 eV). 4,5-Di(9H-carbazol-9-yl)phthalonitrile (2CzPN) was selected as an emitting dopant for achieving sky-blue emissions in TADF-OLEDs. 2CzPN-doped TADF-OLEDs, whose configuration is ITO (50 nm)/HATCN (7 nm)/TAPC (75 nm)/host:6% 2CzPN (20 nm)/TmPyPB (50 nm)/LiF (15 nm)/Al (100 nm), showed low driving voltages and high external quantum efficiencies (EQEs). These results are attributed to the well-controlled bipolar character of the host giving a better charge balance in the emitting layer. In particular, the device containing ZDN:6% 2CzPN showed an unprecedentedly high EQE of 25.7% (at 0.074 mA cm−2).

Highly luminescent blue-emitting CdZnS/ZnS nanorods having electric-field-induced fluorescence switching properties

We demonstrate for the first time highly luminescent blue-emitting CdZnS/ZnS wurtzite core/shell nanorods (NRs) that show electric-field-induced fluorescence switching properties. Uniform CdZnS NRs were rapidly synthesized by injecting sulfur powder dissolved in 1-octadecene into a flask containing phosphonic acid ligands, and subsequently ZnS shells were coated using reagents consisting of sulfur powder, zinc sulfate heptahydrate, and oleylamine. The growth of high-quality ZnS shells resulted in a dramatically increased photoluminescence (PL) quantum yield (QY) of ∼40% with a minimal red-shift of the blue PL peak, which indicates that the combination of reagents successfully controlled a large number of defects appearing on the surface of the NR cores. By pre-annealing CdZnS cores before growing ZnS shells, we could achieve an additional increase in the maximum PL QY to 50%, decreases in both the full width at half maximum (FWHM) and the red-shift of the PL peak, and improved electric-field-induced fluorescence switching performance. Density functional theory calculations reveal that the effective relaxation of strain accumulating on the NR core during shell growth is the key to our successful synthesis of blue-emitting NRs, and that the additional improvement in performance obtained through the pre-annealing process results from the elimination of sulfur vacancies appearing at the surface of the NR core.

Broadening the reflection bandwidth of polymer-stabilized cholesteric liquid crystal via a reactive surface coating layer

We report a method of broadening the reflection bandwidth of polymer-stabilized cholesteric liquid crystal (PSCLC). The top substrate was consecutively coated with a polyimide (PI) and a reactive mesogen (RM) layer, while the bottom substrate was coated with only PI. We exposed the top substrate with the RM coating to UV light. The reflection bandwidth of the PSCLC samples where the top substrate was over-coated with RM was significantly broader than the samples where both substrates were coated with PI. In addition to the effect of the UV intensity gradient, the RM-coated top substrate has a chemical affinity to bulk RM, promoting formation of the pitch gradient and broadening the reflection bandwidth in the sample.