Interfacial electronic structures of buffer-modified pentacene/C60-based charge generation layer.

Interfacial electronic structures of MoO 3 and LiF-modified pentacene (PEN)/fullerene (C 60 )-based charge generation layer (CGL) have been investigated with photoemission spectroscopy. Important characteristics controlling its functional effectiveness have been analyzed for charge transport properties in tandem organic lighting-emitting diodes (TOLEDs). It is found that a small energy offset at the PEN/C 60 heterojunction makes it easy to transfer electrons from PEN to C 60 even under a small applied bias, facilitating the occurrence of charge generation. The band bending observed in both PEN and C 60 is beneficial to exciton-dissociation and charge transport in opposite directions. At the MoO 3 /PEN interface, the high work function (WF) of MoO 3 brings the highest occupied molecular orbital (HOMO) onset up to the Fermi level ( E F ) not only for PEN but also for most hole transport layer (HTL) materials of the adjacent electroluminescent (EL) unit as this CGL is connected into TOLED. Therefore, holes can be efficiently injected from PEN into this EL unit. Similarly, at the C 60 /LiF interface, the low WF of the LiF buffer layer makes the lowest unoccupied molecular orbital (LUMO) to pin close to the E F not only for C 60 but also for most electron transport layer (ETL) materials of the other EL unit, which induces the electrons to inject easily from C 60 into that EL unit by tunneling through the thin LiF film. The favorable energy level alignment can effectively enhance charge generation, transport, and injection. The advantage of the MoO 3 /PEN/C 60 /LiF structure is that thus formed CGL can greatly reduce the voltage drop and thus enhance the power efficiency (PE) of the corresponding TOLED. [ABSTRACT FROM AUTHOR]