Ambipolar Phosphine Derivatives to Attain True Blue OLEDs with 6.5% EQE

A family of new branched phosphine derivatives {Ph2N-(C6H4)n-}3P → E (E = O 1-3, n = 1-3; E = S 4-6, n = 1-3; E = Se 7-9, n = 1-3; E = AuC6F5 4-6, n = 1-3), which are the donor-acceptor type molecules, exhibit efficient deep blue room temperature fluorescence (λem = 403-483 nm in CH2Cl2 solution, λem = 400-469 nm in the solid state). Fine tuning the emission characteristics can be achieved varying the length of aromatic oligophenylene bridge -(C6H4)n-. The pyramidal geometry of central R3P → E fragment on the one hand disrupts π-conjugation between the branches to preserve blue luminescence and high triplet energy, while on the other hand provides amorphous materials to prevent excimer formation and fluorescence self-quenching. Hence, compounds 2, 3, 5, and 12 were used as emitters to fabricate nondoped and doped electroluminescent devices. The luminophore 2 (E = O, n = 2) demonstrates excellently balanced bipolar charge transport and good nondoped device performance with a maximum external quantum efficiency (EQEmax) of 3.3% at 250 cd/m(2) and Commission International de L'Eclairage (CIE) coordinates of (0.15, 0.08). The doped device of 3 (E = O, n = 3) shows higher efficiency (EQEmax of 6.5, 6.0 at 100 cd/m(2)) and high color purity with CIE (0.15, 0.06) that matches the HDTV standard blue. The time-resolved electroluminescence measurement indicates that high efficiency of the device can be attributed to the triplet-triplet annihilation to enhance generation of singlet excitons.