E. L. Alexandrova

Hybrid active layers from a conjugated polymer and inorganic nanoparticles for organic light emitting devices with emission colour tuned by electric field

We report on the investigation of the electrical and optical properties of hybrid active layers for organic devices consisting of a conjugated polymer MEH-PPV mixed with ZnO and Si nanoparticles. The effect of an electric field on the photoluminescence (PL) from a MEH-PPV : ZnO composite film is studied. We have found that in the absence of an electric field PL emission from the MEH-PPV : ZnO composites have two main maxima in the blue–red regions. Three additional minor PL maxima attributed to the exciplex states were found at ~420–480 nm. Application of a voltage bias to planar electrodes significantly suppresses the blue emission. Generation of excited states in the MEH-PPV : ZnO structures implies the presence of several radiative recombination mechanisms with the formation of polymer–nanoparticle complexes including exciplex states and charge transfer between the polymer and nanoparticles that can be controlled by an electric field. This effect provides the possibility to tune by an electric field the emission colour of organic light emitting diodes by combining an efficient emission from both organic/inorganic materials involved.

Effect of electric field on the photoluminescence of polymer-inorganic nanoparticle composites

Abstract We report on the effect of electric field on the photoluminescence (PL) from a composite consisting of a BEHP-co-MEH-PPV conjugated polymer mixed with ZnO nanoparticles. We have found that in the absence of electric field PL emission from the BEHP-co-MEH-PPV-ZnO composite has two maxima in the blue and green-yellow regions. Application of a voltage bias to planar gold electrodes suppresses the green yellow emission and shifts the only PL emission maximum towards the blue region. Current–voltage characteristics of the BEHP-co-MEH-PPV-ZnO composite exhibit the nonlinear behaviour typical of nonhomogeneous polymer-inorganic structures. Generation of excited states in the BEHP-co-MEH-PPV-ZnO structure implies the presence of several radiative recombination mechanisms including formation of polymer-nanoparticle complexes including “exciplex” states and charge transfer between the polymer and nanoparticle that can be controlled by an electric field.

Photophysical properties of indolo[3,2-b]carbazoles as a promising class of optoelectronic materials

The photophysical properties of new synthesized indolocarbazoles, i.e., indolo[3,2-b]carbazole and its derivatives, have been comparatively analyzed. It is shown that their photosensitivity (total photosensitivity to (5–8) × 10−2 (lx s)−1, spectral photosensitivity ∼105 cm2 J−1, and free carrier photogeneration quantum yield of 0.1) and transport (effective mobility in 5,11-dioctyl indolo[3,2-b]carbazole is more than 10−5 cm2/(V s)) parameters significantly exceed those of pentacene, which, among molecular media (organic crystals), exhibits the highest carrier photogeneration quantum yields. The high photoluminescence intensity of synthesized indolo[3,2-b]carbazole derivatives shows promise for their applicability in electroluminescent devices.

Switching effects in composite films based on the conjugated polymer polyfluorene and ZnO nanoparticles

The S- and N-shaped current—voltage characteristics have been studied for composite films of the conjugated polymer polyfluorene and ZnO nanoparticles deposited onto Al and In2O3/SnO2 electrodes with and without an intermediate sublayer of the conducting polymer PEDT/PSS. The differences in the current— voltage characteristics of the systems (the N- and S-types, respectively) are interpreted using the electro-thermal switching model, which takes into account the structural and electric properties of PEDT/PSS. The switching provides both alignment of polymer molecules and tunneling of charge carriers, which leads to an increase in conductivity. The current flow in this structure causes an increase in temperature of conducting channels; when the temperature reaches certain levels, the conductivity of the channels decreases because the alignment of polymer molecules is upset, which creates an N-shaped form of the current—voltage characteristics.

Electrical and optical properties of composites based on carbazole derivatives and silicon particles

The electrical and optical properties of polymer-silicon composites with particles incorporated by different means have been studied. It is shown that both when silicon particles are embedded in a carbazole-containing polymer matrix and in the case of a pure polymer, the I–V characteristics are nonlinear and asymmetric (the I–V characteristics of the carbazole-containing polyorganosiloxane, which has silicon atoms in the monomer link, behave in a more symmetric pattern). In all cases, the I-V characteristics can be fitted with power laws, I(V) ∼ Vp, with three different slopes for different voltage intervals, which remainds one of the pattern typical of the mechanism of space-charge-limited currents. It is shown that, in its luminescent properties, the carbazole-containing polyorganosiloxane is similar to a carbazole-containing polymer matrix with embedded silicon particles. The results obtained argue for charge transfer between the polymer and silicon nanoparticles if they are embedded in the matrix and for an formation of an interchain charge-transfer complex in the case of chemically bound silicon.

Current instability with an S-shaped current-voltage characteristic in thin films of composites based on polymers and inorganic particles

The switching and memory effects, which manifest themselves in the transition from a high-resistance state to a low-resistance state, are observed in thin films of composites based on polymers (carbazole derivatives) and silicon inorganic particles. It is established that, at small thicknesses of composite films, the resistance (with an initial value of no less than 100 MΩ) upon transition from the high-resistance state to the low-resistance state is changed by three or four orders of magnitude. This transition in the composites under investigation is accompanied by the appearance of an S-shaped current-voltage characteristic and its hysteresis. It is demonstrated that the observed effects are associated with the specific features of the thermomechanical and electrical properties of the polymer in the composite material.

Photophysical properties of silicon-containing poly[salicylidene azomethine]s

Photophysical properties of new silicon-containing “nonclassical” polyconjugated poly[salicylidene azomethine]s were studied and a free carrier photogeneration mechanism characteristic of them was proposed. It was shown that polymer properties are controlled by the structure of the azomethine fragment and acceptor properties of the silicon atom. Due to multiple specific donor-acceptor interactions of unshared electron pairs of azomethine fragments with vacant d orbitals of silicon atoms, polymers feature photosensitive and photoconductive properties. The estimated photosensitivity S0.1 ≈ 105 cm2/J and quantum yield of free carrier Photogeneration η ≈ 0.15 correspond to the level of “classical” polyconjugated nitrogen-containing aromatic polymers used in optoelectronics.

“Dimensional” effect due to the matrix isolation of luminescent composites of polyphenylquinolines

The dimensional effect previously observed in various matrices for 2,6-polyphenylquinoline containing oxygen bridge and carbazolediyl groups is confirmed for phenylamine-containing 2,6-polyphenylquinolines with carbazole- and indolocarbazolediyl moieties in a polymethyl methacrylate matrix. The effect is observed from the dependence of the luminescence parameters (intensity and peak position) on the film thickness and poly-4-phenylquinoline concentration in the matrix. It is shown that the luminescence intensity decreases as the film thickness decreases to less than 50 micrometers and at distances 〈D〉 between molecules in the composite of less than 9 nm, with the peak position shifted to shorter wavelengths. The dependence of the luminescence intensity on 〈D〉 has a clearly pronounced maximum at 6–9 nm.

Photosensitive properties of metal-containing polydisalicylidene azomethines

Photosensitive properties of new metal-containing polydisalicylidene azomethines were studied. It was shown that polymer properties are controlled by the nature of the metal atom (its electron affinity energy Aa and ionic radius ri) included in “nonclassical” polyconjugation. The photosensitivity S0.1 of studied polymers is ∼105cm2/J, and the quantum yield of free-carrier photogeneration is η ≈ 0.10–0.15, which corresponds to the level of organometallic complexes that have found application in optoelectronics.