Solid state solvation effect of a donor–acceptor type fluorescent molecule and its application to white organic light-emitting diodes

We investigated the solid state solvation effect (SSSE) of a donor–acceptor (D–A) type green fluorescent emitter, 3,7-bis(benzimidazol-2-yl)-10-methylphenothiazine (BBMP), and fabricated white organic light-emitting diodes (OLEDs) by utilizing SSSE. After having confirmed that BBMP showed a large solvatochromic photoluminescence spectral shift not only in liquid solutions but also in solid solutions, we induced a sky-blue electroluminescence (EL) by doping the green-emitting BBMP in non-polar host materials. The emission spectrum of BBMP blue-shifted as the polarity of host material decreased, and white OLEDs were obtained by integrating an additional orange light-emitting material, Rubrene, into this sky-blue-emitting OLED structure. When 9, 10-di(naphtha-2-yl)anthracene(ADN) was employed as a non-polar host of BBMP, the device exhibited a sky-blue (λpeak,EL = 475 nm) emission by SSSE with evidently improved EL properties: the luminous efficiency, the external quantum efficiency, and the maximum luminance were 10.4 cd/A, 3.5%, and 46,600 cd/m2, respectively. Finally, the white OLED with an optimized structure of [ITO/NPB (20 nm)/ADN:Rubrene (20 nm)/NPB (2 nm)/ADN:BBMP (20 nm)/TPBi (20 nm)/Alq3 (10 nm)/LiF (1 nm)/Al (100 nm)] demonstrated a luminous efficiency of 5.3 cd/A, an external quantum efficiency of 1.3%, and a maximum luminance of 15,200 cd/m2 with CIE coordinates of (0.32, 0.42) at 100 mA/cm2.