Modulation of hybrid emissions from multiple exciplexes and Frenkel excitons: dipole characteristics toward facile fabrication of cool white OLEDs

Blue and white organic light-emitting diodes (OLEDs) are widely used in lighting, augmented reality, and high-resolution flat-panel displays. In this study, cool white OLEDs are fabricated through hybrid emissions combining multiple exciplexes (XPs, 500-700 nm) and Frenkel excitons (XFs, 430 nm), generated in two co-deposition layers (CDLs) comprising donor (D; m-MTDATA and TCTA) and acceptor (A; T2T and TmPyPB) molecules. The emission characteristics of the XPs and XFs are modulated by adjusting relative D/A concentrations. In m-MTDATA : TCTA (1 : 5)/T2T : TmPyPB (1 : 10) OLEDs, increasing the applied voltage from 4 to 13 V enhances electroluminescence (EL). High TmPyPB concentrations in A-CDL promote randomly oriented XP dipole moments at the D/TmPyPB heterojunction, thereby increasing blue XF generation in m-MTDATA (XF_mMT). The EL peak at 430 nm, associated with parallel-stacked XF_mMT, remains stable under external electric fields. The dipole orientations of XFs and XPs correlate with molecular stacking, as confirmed by grazing-incidence wide-angle X-ray scattering, variable-angle spectroscopic ellipsometry, and angle-resolved EL. Without tandem architectures, the OLEDs emit cool white light with color coordinates of x = 0.32 and y = 0.37 in the CIE 1931 color space. The proposed approach enables voltage-dependent spectral modulation and angle-dependent emission control in OLEDs using co-deposited D and A emissive layers.