N. Johansson

Indium–tin oxide treatments for single- and double-layer polymeric light-emitting diodes: The relation between the anode physical, chemical, and morphological properties and the device performance

We report combined studies of the influence of chemical and physical treatments on the properties of indium–tin oxide (ITO) thin films. The ITO films were also used as transparent anodes of polymeric light-emitting diodes (LEDs) incorporating poly(p-phenylene vinylene) (PPV) as the emitter material, with, or without, doped poly(3,4-ethylene dioxythiophene) (PEDOT) as a hole-injection/transport layer. Structures based on a soluble green derivative of PPV, poly(4,4′-diphenylene diphenylvinylene) were also tested. We studied chemical (aquaregia, degreasing, RCA protocol) and physical (oxygen and argon plasmas, Teflon, and paper rubbing) treatments and, in contrast to recently published work, we find that for Balzer Baltracon ITO, oxygen plasma and not aquaregia yields the highest efficiencies and luminances and the lowest drive voltages. For oxygen-plasma-treated anodes, the device efficiency clearly correlates with the value of the ITO surface work function, which in turn depends on the time of treatment. I...

Kelvin probe and ultraviolet photoemission measurements of indium tin oxide work function: a comparison

Abstract We report a comparison of the work functions of thin films of indium tin oxide (ITO), carried out by means of ultraviolet photoelectron spectroscopy (UPS) and by measurements of the contact potential difference with respect to a gold reference electrode (Kelvin probe (KP) method). We investigated commercially available ITOs both “as-received”, and after certain surface treatments, such as oxygen plasma. First, we find measurable discrepancies between KP values measured with three different instruments, and between the KP and the UPS values. Secondly, and unexpectedly, we find that the KP, although more sensitive than UPS, does not detect certain differences between ITOs with different surface treatments. We discuss the results in view of the different environments in which the measurements are carried out (UHV for the UPS and air/Ar for the Kelvin method), of the effects which may be induced by the high-energy photon irradiation in the UPS measurement, and of the stability of the gold probe work function in gas ambient. We conclude that UPS is better-suited for absolute work function determination, although KP remains a convenient and inexpensive tool for fast screening of contact potential differences.

Electronic structure of tris(8-hydroxyquinoline) aluminum thin films in the pristine and reduced states

The electronic structure of tris(8-hydroxyquinoline) aluminum (Alq3) has been studied in the pristine molecular solid state as well as upon interaction (doping) with potassium and lithium. We discuss the results of a joint theoretical and experimental investigation, based on a combination of x-ray and ultraviolet photoelectron spectroscopies with quantum-chemical calculations at the density functional theory level. Upon doping, each electron transferred from an alkali metal atom is stored on one of the three ligands of the Alq3 molecule, resulting in a new spectral feature (peak) in the valence band that evolves uniformly when going from a doping level of one to three metal atoms per Alq3 molecule.

Calcium electrodes in polymer LEDs

Abstract The qualitative results of a study of the fabrication of Ca/CN-PPV/ITO polymer light-emitting diodes (LEDs) in ultrahigh vacuum (UHV) are reported, where CN-PPV is a cyano-substituted poly(p-phenylenevinylene). If the calcium electrode is deposited on a clean polymer surface under UHV conditions, the device yield is very low, and, for the devices that do function, the electroluminescence lifetimes are extremely short. If the calcium electrode is deposited in the presence of a high partial pressure of oxygen, the diodes are much better in terms of yield, lifetime and luminescence.

Electronic structure and optical properties of electroluminescent spiro-type molecules

The electronic structure and optical properties of the propeller-shaped spiro molecules, 2,2′-bis(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)-9,9′-spirobifluorene, and 2,2′,7,7′-tetrakis (biphenyl-4-yl)-9,9-spirobifluorene, have been studied by photoelectron spectroscopy, Raman spectroscopy, and spectroscopic ellipsometry. The experimental spectra are analyzed with the help of results from quantum-chemical calculations. The excellent agreement between experiment and theory allows for a detailed characterization of the frontier electronic structure and lowest-energy optical transitions in these novel compounds. It follows that most of the electronic and optical properties of the these spiro-molecules can be deduced by consideration of a single branch; in this case a sexi-phenyl and PBD branch. In particular, although the branches in the spiro molecules are perpendicular to each other, there is considerable interaction at the spiro-center between the π-electron systems of the two branches, indicating, among other things, that chemistry involving either branch is dependent on the other branch.The electronic structure and optical properties of the propeller-shaped spiro molecules, 2,2′-bis(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)-9,9′-spirobifluorene, and 2,2′,7,7′-tetrakis (biphenyl-4-yl)-9,9-spirobifluorene, have been studied by photoelectron spectroscopy, Raman spectroscopy, and spectroscopic ellipsometry. The experimental spectra are analyzed with the help of results from quantum-chemical calculations. The excellent agreement between experiment and theory allows for a detailed characterization of the frontier electronic structure and lowest-energy optical transitions in these novel compounds. It follows that most of the electronic and optical properties of the these spiro-molecules can be deduced by consideration of a single branch; in this case a sexi-phenyl and PBD branch. In particular, although the branches in the spiro molecules are perpendicular to each other, there is considerable interaction at the spiro-center between the π-electron systems of the two branches, indicating, amon...

Characterisation of the properties of surface-treated indium-tin oxide thin films

Abstract We studied the surface properties of indium-tin oxide (ITO) modified by wet (aquaregia, ultrasonication, RCA) and dry (oxygen and argon plasmas) treatments. The influence of surface treatment on ITO was investigated by surface energy, surface roughness, sheet resistance, and workfunction measurements. We report that oxygen plasma treatment induces: (1) the highest surface energy with the highest polarity, (2) the smoothest surface, (3) the lowest sheet resistance, and (4) the highest workfunction. We also observe that the PL efficiencies of PPV, PDPV and PEDOT/PPV on ITO are affected by the surface treatment of ITO.

A study of the ITO-on-PPV interface using photoelectron spectroscopy

Abstract Angle-dependent photoelectron spectroscopy has been used to study some details of the chemistry at the interface between indium-tinoxide, deposited by argon plasma sputtering, and poly( p -phenylene vinylene). We find chemical species corresponding to different oxidized states of poly( p -phenylene vinylene) and we discuss their assignment with reference to those produced in photo-oxidation experiments.

A theoretical study of the chemical structure of the non-conjugated polymer poly(3,3′-phthalidylidene-4,4′-biphenylene)

Abstract The equilibrium geometry of an isolated chain of poly(3,3′-phthalidylidene-4,4′-biphenylene), or PPB, has been studied using quantum chemical calculations based upon the Austin Model 1 (AM1) method. The band structure of an isolated PPB chain has been calculated, using the AM1 geometry as input, with the help of the valence-effective Hamiltonian (VEH) model. From the electronic structure results, the density of valence states has been derived for comparison with experimental data. Both the neutral and the n-doped polymer chains have been simulated. The results are compared with features of the polymer deduced through other measurements reported in the literature.

Resonant and nonresonant x-ray scattering spectra of some poly(phenylenevinylene)s

The electronic structure of some poly(phenylenevinylene)s have been investigated by resonant and nonresonant x-ray inelastic scattering spectroscopies. The nonresonant as well as all resonant spect ...

Electronic structure of poly(9,9-dioctylfluorene) in the pristine and reduced state

The electronic structure of the conjugated polymer poly(9,9-dioctylfluorene) and the charge storage mechanism upon doping with lithium atoms have been studied using a combined experimental–theoretical approach. The density of states in the valence band region was measured using ultraviolet photoelectron spectroscopy, and the spectra interpreted with the help of the results of quantum-chemical calculations carried out at the Austin Model 1/Valence Effective Hamiltonian level. The valence band spectra as well as the calculated band structure and derived density of valence states of poly(9,9-dioctylfluorene) are compared to those of poly(p-phenylene) and a ladder-type derivative of poly(p-phenylene). The deposition of lithium atoms, in situ, onto the surface of the polymer films resulted in uniform (bulk) diffusion of the lithium atoms, charge transfer to the polymer (resulting in lithium ions), and the appearance of new electronic states in the gap. At low doping levels (up to about 0.1 Li-atom per repeat u...