Nikolay N. Barashkov

Controlling charge injection in organic electronic devices using self-assembled monolayers

We demonstrate control and improvement of charge injection in organic electronic devices by utilizing self-assembled monolayers (SAMs) to manipulate the Schottky energy barrier between a metal electrode and the organic electronic material. Hole injection from Cu electrodes into the electroluminescent conjugated polymer poly[2-methoxy,5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] was varied by using two conjugated-thiol based SAMs. The chemically modified electrodes were incorporated in organic diode structures and changes in the metal/polymer Schottky energy barriers and current–voltage characteristics were measured. Decreasing (increasing) the Schottky energy barrier improves (degrades) charge injection into the polymer.

The Schottky energy barrier dependence of charge injection in organic light-emitting diodes

We present device model calculations of the current–voltage (I–V) characteristics of organic diodes and compare them with measurements of structures fabricated using MEH-PPV. The structures are designed so that all of the current is injected from one contact. The I–V characteristics are considered as a function of the Schottky energy barrier to charge injection from the contact. Experimentally, the Schottky barrier is varied from essentially zero to more than 1 eV by using different metal contacts. A consistent description of the device I–V characteristics is obtained as the Schottky barrier is varied from small values, less than about 0.4 eV, where the current flow is space-charge limited to larger values where it is contact limited.

Systematic investigation of the effects of organic film structure on light emitting diode performance

We present a systematic investigation of the effects of organic film structure on light emitting diode (LED) performance. Metal/organic film/metal LEDs were fabricated using a five ring, poly(phenylene vinylene) related oligomer as the active layer. The structure of the vacuum evaporated oligomer films was varied from amorphous to polycrystalline by changing the substrate temperature during deposition. The intrinsic properties of the oligomer films and the LED performance were measured. The measured intrinsic film properties include: optical absorption, photoluminescence (PL) spectra, PL lifetime, PL efficiency, and effective carrier mobility. The measured device characteristics include current–voltage, capacitance–voltage, electroluminescence (EL) efficiency, and the contact metal/organic film Schottky barrier heights. The optical absorption and PL properties of the films are weakly dependent on film structure but the effective carrier mobility decreases with increasing crystallinity. The EL quantum effi...

Chemical potential pinning due to equilibrium electron transfer at metal/C60-doped polymer interfaces

We report electroabsorption measurements of the built-in electrostatic potential in metal/C60-doped polymer/metal structures to investigate chemical potential pinning due to equilibrium electron transfer from a metal contact to the electron acceptor energy level of C60 molecules in the polymer film. The built-in potentials of a series of structures employing thin films of both undoped and C60-doped poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) were measured. For undoped MEH-PPV, which has an energy gap of about 2.4 eV, the maximum built-in potential is about 2.1 eV, whereas for C60-doped MEH-PPV the maximum built-in potential decreases to 1.5 eV. Electron transfer to the C60 molecules close to the metal interface pins the chemical potential of the metal contact near the electron acceptor energy level of C60 and decreases the built-in potential of the structure. From the systematic dependence of the built-in potential on the metal work function we find that the electron acceptor e...

Synthesis and optical properties of oligo- and poly(2,5-dialkoxy-1,4-para-phenylenevinylene)s

Abstract Model compounds based on 1,4-distyrylbenzene and its 2,5-dialkoxy derivatives were synthesized by the Wittig and Wittig-Horner methods. Corresponding poly(2,5-dialkoxy-1,4- para -phenylenevinylene)s (DAPPVs) were obtained from their 2,5-bis (chloromethyl)benzene monomers. The photophysical behavior of the model compounds and DAPPVs was investigated through absorbance, excitation and fluorescence spectroscopy. The absorbance and fluorescence spectra of DAPPVs are red shifted in comparison to the corresponding three- and five-ring model compounds by 4600 and 7000 cm −1 , respectively. We also report the fluorescence excitation and emission spectra of the model compounds deposited as solid films on quartz and as solid solutions in poly(methyl methacrylate) (PMMA). The photophysical properties of these materials are interpreted from their capacity to form excimer-like molecular dimers. The fluorescence excitation spectra of DAPPV solutions are concentration dependent. We show evidence for the existence of fluorescent aggregates in the solid state.

Femtosecond dynamics of excitons in π-conjugated oligomers: the role of intrachain two-exciton states in the formation of interchain species

Abstract We report femtosecond transient absorption results for solutions and thin films of a substituted oligomer of poly(para-phenylene vinylene) performed over wide spectral and pump-intensity ranges. Solutions and films exhibit a photoinduced absorption (PA) band with dynamics matching those of the stimulated emission, demonstrating unambiguously that these features originate from intrachain singlet excitons. Thin films exhibit an additional short-wavelength PA band with pump-independent dynamics, indicating the formation of non-emissive interchain excitons. Correlations in the dynamics of the two PA features, as well as the intensity-dependence, provide strong evidence that the formation of interchain excitons is mediated by intrachain two-exciton states.

Excitations and optical properties of phenylene-based conjugated polymers and oligomers

Abstract We present a combined experimental and theoretical study of the ground and photoexcited optical properties of a model oligomer of PPV, MEH-DSB. Our theoretical picture is based upon a band description of electronic states of PPV oligomers, while invoking corrections from Coulomb interactions. The necessary discrete energy levels at low and intermediate energies appear naturally, in addition to the lower energy delocalized states. On this basis we identify the most important features in direct optical absorption for both high (4–6 eV) and low (2–4 eV) photon energies as well as in photoinduced absorption (PA) and stimulated photoemissions (SE) in MEH-DSB solutions, which represent the limit of noninteracting oligomers. While in agreement with previous interpretations for three absorption peaks (2.74, 4.46 and 6.2 eV), we give a new assignment for the most disputed 3.62 eV one as well as for the two PA peaks.

Copolymers with fragments of meta-pyridylvinylene and para-phenylenevinylene: synthesis, quaternization reaction and photophysical properties

Abstract Conjugated copolymers with fragments of 2,6-pyridylvinylene and 1,4-phenylenevinylene have been prepared by the Wittig reaction between the diphosphonium salt of 2,6-bis(chloromethyl)pyridine and terephthaldehyde or 2,5-dialkoxyterephthaldehydes. The protonation reaction of poly(1,4-phenylenevinylene)-co-(2,6-pyridylvinylene) with hydrochloric acid and the quaternization reaction of poly(2,5-dioctyloxy-1,4-phenylenevinylene)-co-(2,6-pyridylvinylene) with methyl triflate have been investigated by spectrophotometric and fluorescent methods. The absorption, excitation and fluorescence spectra of these copolymers as well as their corresponding model compounds were studied and compared. The photophysical properties of the investigated polymers suggest that these materials could be good candidates for the fabrication of efficient blue light-emitting diodes.

Influence of polymeric media on the association of aromatic hydrocarbons

Synchronous scanning over a range of excitation wavelengths and fluorescence measurements were used to study polystyrene, solid solutions of luminophores in poly(methyl methacrylate (PMMA), and methyl methacrylate copolymers doped with pyrene, anthracene, and naphthalene. The complicated nature of the aggregation of aromatic hydrocarbons in solid polymers is revealed. The origin of their excimer-like fluorescence is also discussed.

Synthesis and spectral-luminescence properties of aromatic polyamides and polyesters with chromophores in the polymer main chain

Abstract We report copolycondensation reactions of several benzimidazole-, benzoxazole- and diphenylanthracenyl-containing chromophores with m -phenylenediamine (MPD), isophthaloyl chloride (IPC) and bisphenol A (BPA) to form polyamides and polyesters. The absorbance and fluorescence spectra of the polymers were measured and the actual molar ratios of luminophoric and nonluminophoric moieties in their structures were determined by comparison to model compounds. Most copolymers showed intense blue fluorescence, from solution and film, with a maximum in the 410–515 nm range. The present results demonstrate that non-conjugated luminescent polymers, which are potentially useful for light-emitting diode applications, may be obtained by a one-state copolycondensation technique.