The interaction of poly (p‐phenylenevinylene) with air

Abstract

Following the discovery of light emitting diodes"] based on conjugated polymers, ['] research on the physics and chemistry of the conjugated polymer materials in their semiconducting state has evolved considerably.[31 In particular, the materials science underlying light-emission-device development has progressed at a rapid pace.L4] Even though complex multilayer devices exist, in the simplest configuration, polymer-based light emission devices (LEDs) consist of an ITO-covered glass substrate, a spincoated active conjugated polymer layer, and an electron-injecting, low-work-function metal e l e~t r o d e .~~] Among the most widely used of the conjugated polymers for polymerbased LEDs are poly(p-phenylenevinylene), or PPV, and its substituted derivatives. Most of the substituted PPVs are soluble in common organic solvents. However, it has been established that the performance characteristics of devices based on the PPVs are unfortunately sensitive to exposure to air.[5361 The dependence of the electronic properties of the PPVs upon air exposure may thus prove to be a central issue in the development of polymer-based LEDs.This work reports some of the results of a study of the effects of air exposure on the electronic structure of unsub-stituted PPV, as studied by both X-ray and ultaviolet photoelectron spectroscopy (XPS and UPS, respectively), optical absorption spectroscopy, and model quantum chemical calculations. Reversible effects associated with the absorption of water vapor, which lead to detrimental changes in the frontier electronic structure, have been observed. The nature of these effects is identified through the use of quantum chemical calculations and verified experimentally. Sample treatment and photoelectron spectroscopy were carried out in a unique UHV system of our own design and construction. The system consists of separate sample introduction, preparation, and analysis chambers, pumped by a special combination of turbo-, ion-, and cryogenic pumps, enabling studies under UHV (53 x lo-'' mbar) conditions even for polymer samples inserted from the atmo~phere.'~] Optical absorption spectroscopy was carried out either in a commercial PE Lambda-9 spectrometer, or in a special UHV system, also of our own design and construction.[*] Figure 1 shows the O(1s) XPS spectra of a PPV film in the converted (clean) state, following overnight exposure to air. Note that the residual oxygen signal in the freshly converted PPV corresponds to 51.5 at.-% oxygen. Upon exposure, the O(1s) signal increases, and the increased signal can be deconvoluted from the original (residual) O(1s) peak as shown. The O(1s) signal decreases at elevated temperatures. Upon heat treating overnight in UHV at 180 "C, the O(1s) signal associated with the air exposure is completely removed. The process is thus fully reversible. The low[*] Prof.

25th Anniversary Article: Charge Transport and Recombination in Polymer Light‐Emitting Diodes

“…In order to elucidate the origin of the electron traps, a series of quantum‐chemical calculations was carried out to examine how potential defects within the polymer thin films could act as electron trap sites . We have considered: i) the presence of defects along the chains ranging from photo‐oxidation products to halogen termination; ii) the incorporation between the chains of water molecules interacting with the chains; and iii) the presence of hydrated oxygen complexes.…”

“…We have considered: i) the presence of defects along the chains ranging from photo‐oxidation products to halogen termination; ii) the incorporation between the chains of water molecules interacting with the chains; and iii) the presence of hydrated oxygen complexes. Previous photoelectron spectroscopy and quantum‐chemical studies suggested that water molecules appearing within PPV can form complexes with the PPV backbones and modify the polymer ionization energies . Following the model complexes investigated in these earlier studies, we evaluated a series of PPV oligomers (containing up to 6 phenylene rings) interacting with a single water molecule; we note that the calculations explicitly incorporate the dispersion effects in the complexes and implicitly take account of the polarization effects induced by the environment via a dielectric continuum model (with ε r = 3.0).…”

Energy‐Level Alignment at Organic/Metal and Organic/Organic Interfaces

“…The synthesis of p-conjugated molecules and polymers has produced tremendous improvement, but for many materials there exist in literature a conflicting range of values of ionization potentials, work functions, etc., which complicates both comparisons with theory and device design. Impurities can also be introduced either from the atmosphere, [28] which can lead to modifications of the electronic structure through doping, [29] or chemical degradation [30] of the material. Finally, local order in the molecular and polymer films can affect both interface and bulk electronic properties.…”

Optical properties of single crystals and vacuum-deposited thin films of a substituted oligo(p-phenylene vinylene)

“…Recently, it has been shown that exposure of PPV films to air in the dark leads to the formation of water-PPV complexes. In the same study no changes were seen upon exposure to a controlled oxygen environment [30]. In order to find out the relative importance of oxygen and water on photoluminescence quenching, the luminescence lifetime of a vacuum-deposited film just after deposition was measured subsequently in air, vacuum and water vapour.…”