(2,2′-Binaphthyl-6,6′-diyl)bis(diphenylphosphine oxide) as a potentially simple and efficient electron-transport layer for stable organic light-emitting diodes

Abstract

The synthesis of organic electron-transport materials (ETMs) for organic light-emitting diodes (OLEDs) has been intensely pursued. Herein we report an organic phosphinyl compound (2,2′-Binaphthyl-6,6′-diyl)bis(diphenylphosphine oxide) (BiNa-BiDPO) with concise synthesis and purification. BiNa-BiDPO is thermally stable up to ca. 415 °C and exhibits distinct glass transition with a Tg of 112 °C after being cooled from the melt. Upon further heating, no crystallization or melting is observed. Ultraviolet photoemission spectroscopy studies reveal that EHOMO ≈ −6.12 eV for the new compound. Consequently, the LUMO level was roughly estimated as −2.77 eV based on the onset of the film absorption spectrum. BiNa-BiDPO possesses a higher electron mobility of 1.6–8.4 × 10−5 cm2 V−1 s−1 at E = 2–5 × 105 V cm−1, relative to a common ETM 1,3,5-tris(N-phenylbenzimidazolyl)benzene (TPBi), thus providing better OLED efficiency with lower working voltage. Moreover the OLED devices involving BiNa-BiDPO as the electron-transport layer showed a half life-time of 172 h at an initial luminance of ca. 1000 cd m−2, driven at a constant current density, in contrast with the TPBi device with a shortened t1/2 of 93 h. Further device engineering as well as molecular design may provide enhanced device durability.