AVS 50th International Symposium
    Contacts to Organic Materials Topical Conference Monday Sessions
       Session OM-MoA

Paper OM-MoA3
The Role of Charge Neutrality Level in Molecular Level Alignment at All-organic Heterojunctions

Monday, November 3, 2003, 2:40 pm, Room 318/319

Session: Contacts to Molecules and Molecular Films (II)
Presenter: W. Gao, Princeton University
Authors: W. Gao, Princeton University
A. Kahn, Princeton University
Correspondent: Click to Email
Alignment of molecular levels at organic-organic (OO) heterojunctions controls charge injection and transport in multilayer organic devices. Because of weak intermolecular interaction and absence of free charges, vacuum level alignment is often observed at heterojunctions between intrinsic (undoped) molecular films. We present here the first experimental evidence that an alignment mechanism based on the charge neutrality level of the molecular semiconductor becomes important when one of the interface constituents is doped. Our direct and inverse photoemission (UPS, XPS, IPES) investigation focuses on interfaces between hole transport layers (HTL) and electron transport layers (ETL).@footnote 1@ We use the strong electron acceptor F@sub 4@-TCNQ to p-dope the HTLs.@footnote 2@ We show that, unlike at metal-organic (MO) interfaces, molecular levels and energy barriers across the interfaces are not strongly anchored and shift with respect to each other upon doping of the HTL. The shift is accompanied by the formation of an interface dipole, the size of which depends on the constituents of the heterojunction. The key result is that the Fermi level position in the gap of the undoped ETL remains fixed and independent of the HTL when the HTL is doped. This important observation suggests that the introduction of charges and of electronic states at the interface in the gap of the HTL and ETL via doping results in a pinning of the Fermi level at or near a specific energy level, tentatively related to the charge neutrality level of the organic material. @FootnoteText@@footnote 1@W. Gao and A. Kahn, Appl. Phys. Lett., (in press) @footnote 2@W. Gao and A. Kahn, Organic Electronics 3, 53 (2002).