Dumbbell effects of solution-processed pyrene-based organic semiconductors on electronic structure, morphology and electroluminescence

The number of bulky groups in hindrance-capped π-systems is closely relative to thin film morphology, stability and luminescent features in printing electronics. Herein, we demonstrated advantages of dumbbell-shape over mono-substituted organic semiconductors by designing two highly bulky 3′,6′-bis(octyloxy) spiro[fluorene-9,9′-xanthene] (OSFX)-functionalized pyrene derivatives, OSFXPy and DOSFXPy. Their differences in terms of thermal, photophysical and electrochemical properties, and film morphologies were characterized via thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), UV–vis absorption and photoluminescence (PL), cyclic voltammetry and atomic force microscopy (AFM). DOSFXPy has higher thermal and electrochemical stability, more suitable energy levels and smaller bandgap than that of OSFXPy. Thin film of DOSFXPy is smooth and pin-hole free, while OSFXPy is easy to crystallize and form large crystal domains. Solution-processed nondoped electroluminescent devices based on DOSFXPy exhibit stable deep-blue emission with excellent CIEx,y color coordinates (x = 0.15, y = 0.13) at the luminance of 1000 cd/m2, and the maximum luminance of up to 3200 cd/m2 and maximum current efficiency of 2.07 cd/A, respectively. Moreover, solution-processed doped OLEDs with DPAVBi show greenish-blue emission with optimum current efficiency of 4.52 cd/A at doping concentration of 3 wt%.