True-to sky-blue emitters bearing the thiazolo[5,4-d]thiazole electron acceptor for single and tandem organic light-emitting diodes
Résumé
A series of emitters displaying efficient photoluminescence and electroluminescence (EL) into the true-to sky-blue region is herein described and employed as electroluminescent materials for the fabrication of efficient blue organic light-emitting diodes (OLEDs). The compounds possess linear, yet twisted, donor-π-acceptor-π-donor (D-π-A-π-D) architecture, where D and A are a (substituted) carbazole (Cz) and thiazolo[5,4-d]thiazole (TzTz) moiety as the electron-donor and accepting unit, respectively. D = 9H-carbazol-9-yl and 3,6-di-tert-butyl-9H-carbazol-9-yl for compound TzTz-PCz2 and TzTz-PtbCz2, respectively. In dilute CH2Cl2 solution, both compounds display intense (photoluminescence quantum yield = 60%-74%) and short-lived (τ = ca. 0.8-2.0 ns) luminescence arising from an intramolecular charge transfer (1 ICT) excited state and falling into the sky-blue to the greenish-blue region (λem = 448-502 nm). Compared to the previously reported TzTz-TPA parental emitter that features two triphenylamine (TPA) donor groups, the highly twisted nature and less extended-conjugation of the donor-acceptor TzTz-PCz2 and TzTz-PtbCz2 counterparts herein presented enable to widen the HOMO-LUMO energy gap and warrant blue shifting of the emission. Finally, OLED devices fabricated by employing the newly proposed emitters are presented as well with both single-and tandem-device architectures. The so-prepared OLEDs display true-blue to sky-blue EL peaking at λEL,max = 460 nm (TzTz-PCz2) and 467-470 nm (TzTz-PtbCz2) and achieve maximum external quantum efficiency (EQE) and luminance of 10.2% and 44851 cd m-2 (for TzTz-PtbCz2) in the case of a tandem device, respectively, with mitigated roll-off efficiency. These results demonstrate that the tandem device employing these TzTz donor-acceptor blue emitters raises the feasibility of fluorescent compounds to compete with other potential emitters in practical applications.
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