The interfaces of poly(p-phenylene vinylene) and fullerene derivatives with Al, LiF, and Al/LiF studied by secondary ion mass spectroscopy and x-ray photoelectron spectroscopy: Formation of AlF3 disproved

Abstract

Two mutually exclusive mechanisms have been proposed to explain the improved electron injection by the insertion of a LiF layer between the metal cathode and the active organic layer of organic photoelectronic devices: the dipole and the doping mechanism. The possibility of the doping mechanism was studied by investigating the interface of poly͓2-methoxy-5-͑3Ј,7Ј-dimethyl-octyloxyl ͒ -1, 4-phenylenevinylene ͔ ͑MDMO-PPV ͒ or 1-͑ 3-͑ methoxycarbonyl ͒ propyl͒-1-phenyl͓6,6͔C 61 ͑PCBM͒ with Al, LiF, or Al/LiF. In this mechanism, Li dopes the organic layer, after liberation via the reaction Alϩ3LiF→AlF 3 ϩ3Li. If this reaction takes place, AlF 3 should be detectable at the surface. However, SIMS measurements showed that AlF 3 is not present at the Al/LiF/MDMO-PPV and Al/LiF/PCBM interfaces. This is evidence that the proposed reaction does not occur. Other evidence that the doping mechanism cannot be the general mechanism to explain the enhanced electron injection comes from the presence of LiF on both organic surfaces. XPS measurements indicate that there is a reaction of Al with the carboxylic oxygen of PCBM, and that a LiF layer between PCBM and Al prevents this reaction.

Energy-Level Alignment at Metal–Organic and Organic–Organic Interfaces in Bulk-Heterojunction Solar Cells

“…The work function of metallic aluminum is ~4.1 eV. Note that LiFmodified Al contacts are often used for a variety of reasons and the resulting work function of LiF-modified Al can take a range of values depending on deposition order and materials involved [31][32][33][34][35]. In all cases, however, the resulting work function will be smaller than that for clean Al, and as we will show, from an interface energetics standpoint, the work function of clean Al already is low enough to pin the PCBM/Al contact.…”

“…Note that reducing the effective work function of the Al contact by a LiF sandwich layer will not change this, only increase the interface dipole at the metal contact. The LiF layer can effect the contact in other ways, however, including preventing covalent bonding between Al and PCBM and possible defects resulting thereof [32][33][34].…”

The Roles of Poly(Ethylene Oxide) Electrode Buffers in Efficient Polymer Photovoltaics

“…[4][5][6] The metal atoms also tend to react with conjugated molecules near the contact interface to break the conjugated length and cause localized defect states. [7][8][9] These largely degrade the device performance and limit the application of organic electronic devices.…”

Photoelectron Spectroscopy of the Contact between the Cathode and the Active Layers in Plastic Solar Cells: The Role of LiF

“…Competing mechanisms to explain the device performance improvement have been suggested, either suggesting a dipole formation at the Al/LiF/blend interface 20) or protection by an LiF barrier layer of the PCBM from Al-bonding induced degradation. 27) The aim of this paper is to if possible reconcile these two models and present a more comprehensive and detailed picture of the mechanisms involved.…”