Insight into the mechanism and outcoupling enhancement of excimer-associated white light generation

Fundamental insight into excimer formation has been gained by using 9,10-bis[4-(9-carbazolyl)phenyl]anthracene] (Cz) as a probe. Cz exhibits a highly emissive blue fluorescence in solution and is found to emit a panchromatic white light spectrum (400-750 nm) in film, powder and single crystal, in which an additional excimer band appears at ∼550 nm. Detailed structural analyses, emission relaxation dynamics and a theoretical approach conclude the formation of an anthracene*/phenyl ring excimer through an overlap between π* (anthracene) and π (phenyl ring) orbitals in a face-to-edge stacking orientation. The rate of excimer formation is determined to be 2.2 × 10⁹ s^-1 at room temperature, which requires coupling with lattice motion with an activation energy of 0.44 kcal mol^-1. Exploiting Cz as a single emitter, a fluorescent white organic light emitting diode (WOLED) is fabricated with a maximum external quantum efficiency (η_ext) of 3.6% at 1000 cd m^-2 (4.2 V) and Commission Internationale de L'Eclairage (CIE) coordinates of (0.30, 0.33). The white-light Cz reveals a preferred orientation of the transition dipole moment in the emitting layer to enhance light outcoupling. This non-doped, single component (Cz) WOLED greatly reduces the complexity of the fabrication process, rendering a green and cost-effective alternative among the contemporary display/lighting technologies.