Simultaneously enhancement of quantum efficiency and color purity by molecular design in star-shaped solution-processed blue emitters

• Flurorene-free bipolar star-shaped molecules have been developed as high efficient blue emitters for OLEDs.
• The solid PL quantum efficiency of the star-shaped molecules significantly increased with longer spacer.
• Nondoped OLED device based on S4-Cz-OXD emitted stable pure blue light with current efficiency of 4.96 cd A−1 and EQE of 4.20%.

A series of fluorene-free bipolar star-shaped molecules, Sn-Cz-OXD (n = 1–5), with increasing conjugated length in branches were synthesized as high efficient blue emitters for OLEDs. With the extension of conjugated branches, the solid PL quantum efficiency and external quantum efficiency of Sn-Cz-OXD significantly increased with longer spacer, while the emission spectrum of these materials exhibited a blue-shift with enhanced color purity due to the unique molecular design. All materials maintained exceptionally high thermal stability after prolonged heat treatment at 150 °C in air. The photophysical, electrochemical, thermal properties of these emitters were studied in relation to the molecular structure. Nondoped device based on S4-Cz-OXD with structure ITO/PEDOT:PSS/EML/TPBI/LiF/Al emitted stable pure blue light with CIE coordinates of (0.157, 0.146). It exhibited high current efficiency and external quantum efficiency of 4.96 cd A−1 and 4.20%, respectively. These values are among the best results for solution-processed non-doped blue device based on fluorene-free materials, indicating its potential for commercial applications.

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