Carbazole derivatives as electron transport and n-type acceptor materials for efficient organic light-emitting devices

Electron transport materials (ETMs) are typically designed by linking conjugated neutral aromatic rings with strong electron withdrawing groups to ensure deep HOMO and LUMO energy levels. Herein, two new A-D-A′ type ETMs of 3CNCzOXD and 4CNCzOXD are developed. The presence of both electron-donating carbazole and strong electron-accepting 1,3,4-oxadiazole and cyano moieties results in more pronounced positive and negative charge distributions, which may ultimately facilitate electron transport. In addition, the two electron-deficient groups are located at the peripheral sites of the molecules, providing efficient electron transport channels. Therefore, A-D-A′ type 3CNCzOXD and 4CNCzOXD exhibit 2-3 orders higher current than D-A type CN-free CzOXD and 3-5 orders higher electron current than the commercial A₃-π structured ETM TPBI over a wide voltage range. When used as ETMs for green phosphorescent OLEDs, a maximum external quantum efficiency (EQE) of over 20% was achieved compared to that of 18.7% for TPBI. 3CNCzOXD and 4CNCzOXD could also serve as efficient n-type electron acceptors in exciplex hosts for phosphorescent and TADF OLEDs with the maximum EQE above 22%. In particular, green phosphorescent OLEDs based on TCTA:3CNCzOXD and TCTA:4CNCzOXD exciplex hosts showed a similar maximum EQE of 20%, which is significantly higher than 16.9% for the single TCTA host. Our work indicates that the electron donating moiety can also exist in efficient ETMs through careful molecular design.