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

Abstract

Interfacial electronic structures of MoO3 and LiF-modified pentacene (PEN)/fullerene (C60)-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/C60 heterojunction makes it easy to transfer electrons from PEN to C60 even under a small applied bias, facilitating the occurrence of charge generation. The band bending observed in both PEN and C60 is beneficial to exciton-dissociation and charge transport in opposite directions. At the MoO3/PEN interface, the high work function (WF) of MoO3 brings the highest occupied molecular orbital (HOMO) onset up to the Fermi level (EF) 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 C60/LiF interface, the low WF of the LiF buffer layer makes the lowest unoccupied molecular orbital (LUMO) to pin close to the EF not only for C60 but also for most electron transport layer (ETL) materials of the other EL unit, which induces the electrons to inject easily from C60 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 MoO3/PEN/C60/LiF structure is that thus formed CGL can greatly reduce the voltage drop and thus enhance the power efficiency (PE) of the corresponding TOLED.