Valentas Gaidelis

High hole mobilities in carbazole-based glass-forming hydrazones

The properties of a series of carbazole-based dihydrazones are reported. The dependence of their thermal and glass-forming properties on their chemical structure is discussed. The hydrazones having phenyl substituents at the N atom of the hydrazine moiety form glasses and their amorphous films on the glass or polyester substrates can be prepared by casting from solutions. The ionization potentials of the synthesized hydrazones measured by the electron photoemission technique range from 5.24 to 5.50 eV. Hole drift mobilities of some newly synthesized carbazole-based dihydrazones appproach 10−2 cm2 V−1 s−1 at an electric field of 6.4×105 V cm−1, at 22 °C.

Air stable electron-transporting and ambipolar bay substituted perylene bisimides

Four new perylene bisimides containing carbazolyl and triphenylamino electron-donor groups in the bay region have been designed, synthesized and characterized. The materials possess high thermal stability and form uniform films. They display a wide absorption window extending to the near infrared region of the spectrum and demonstrate efficient photoinduced intramolecular electron transfer. Ionization potential values of these perylene bisimide derivatives measured by photoelectron spectroscopy range from 5.8 eV to 6 eV. Charge-transporting properties were investigated by the xerographic time of flight (XTOF) technique. Complementary ambipolar charge-transport was observed in differently linked carbazolyl substituted perylene bisimides while the triphenylamino substituted material exhibited competent electron drift mobility (>10−3 cm2 V−1 cm−1) under ambient conditions. Density functional theory (DFT) calculations were performed for carbazolyl bay substituted perylene bisimides in order to understand the complementary ambipolar charge transport as well as the difference in the optical properties.

Hole-transporting molecular glasses based on carbazole and diphenylamine moieties

Abstract Various molecular designs based on carbazole and diphenylamine moieties are found to constitute novel hole transporting amorphous molecular materials, as characterized by differential scanning calorimetry and time of flight method. Hole drift mobility of synthesized materials were in the range 10 −4 –10 −6 cm 2 V −1 s −1 at an applied field of 3.6×10 5 V cm −1 .

Hole-Transporting Glass-Forming 3,3′-Dicarbazyl-Based Hydrazones

ABSTRACT 3,3′-Di(9-(4-butylphenyl)carbazyl) and 3,3′-di(9-ethylcarbazyl)-based hydrazones were synthesized by the multi-step synthetic route including oxidative dimerization of 9-(4-butylphenyl)carbazole and 9-ethylcarbazole, formylation of the dimmers obtained and the reactions of the formyl derivatives with different hydrazines. The chemical structure of the compounds was confirmed by 1HNMR, IR and mass spectroscopy. The hydrazones synthesized form stable glasses with the glass transition temperatures exceeding 140°C. The values of ionisation potentials measured by electron photoemission technique are in the range of 5,21–5,4 eV. The hole drift mobilities in the films of the 50% solid solutions of the hydrazones in bisphenol Z polycarbonate established by the xerographic time-of-flight technique exceed 10−5 cm2/Vs at high electric fields.

Phenylethenyl-Substituted Triphenylamines: Efficient, Easily Obtainable, and Inexpensive Hole-Transporting Materials

Star-shaped charge-transporting materials with a triphenylamine (TPA) core and various phenylethenyl side arm(s) were obtained in a one-step synthetic procedure from commercially available and relatively inexpensive starting materials. Crystallinity, glass-transition temperature, size of the π-conjugated system, energy levels, and the way molecules pack in the solid state can be significantly influenced by variation of the structure of these side arm(s). An increase in the number of phenylethenyl side arms was found to hinder intramolecular motions of the TPA core, and thereby provide significant enhancement of the fluorescence quantum yield of the TPA derivatives in solution. On the other hand, a larger number of side arms facilitated exciton migration through the dense side-arm network formed in the solid state and, thus, considerably reduces fluorescence efficiency by migration-assisted nonradiative relaxation. This dense network enables charges to move more rapidly through the hole-transport material layer, which results in very good charge drift mobility (μ up to 0.017 cm(2) V (-1) s(-1)).

Hole-drift mobility in phenylenediamine derivatives possessing methyl substituents

Abstract Substituent effects on the hole-drift mobility of N , N , N ′, N ′-tetraarylphenylenediamine derivatives have been investigated. The hole mobility values are dependent on the number of methyl substituents and their position in the investigated transporting materials (TM). The highest mobility was observed in the TM containing methyl groups at the para positions of the side benzene rings and at the central ring. On the other hand, the priority of the m -phenylenediamine derivative against the p -phenylenediamine homologue was proved.

New Carbazolyl Containing Molecular Glasses for Optoelectronics

The multistep synthesis, characterization and properties of the well defined six carbazolyl and branched aliphatic chain containing organic photosemiconductor is reported. It was prepared by the nucleophilic opening of the oxirane cycle of 1,6-di(carbazol-9-yl)-5-(carbazol-9-methyl)-4-oxa-2-hexanol glycidyl ether with the linking agent 4,4′-thiobisbenzenethiol in the presence of catalyst triethylamine (TEA). The molecular structure of this branched molecular glass prevents its crystallization in the layer and allows stable films to be prepared without binder material. The hole drift mobility, measured by the time-of-flight technique, reaches 10−5 cm2/V · s at an electric field of 106 V · cm−1. The value of ionization potential (5.92 eV) measured by the electron photoemission method in air is close to those reported for the other organic photoconductors containing electronically isolated carbazolyl moieties.

Features of charge-carrier transport in phthalocyanine dispersed in binder polymer

Abstract In layers of metal-free x -H 2 Pc and y -TiOPc phthalocyanine dispersed in polyvinylbutyral binder, the transport features of holes were studied by equilibrium charge extraction method by linearly increasing voltage. The mobility values estimated by this method correspond to ones measured by time-of-flight method, moreover, proposed method is more convenient.

Stable all organic radicals with ambipolar charge transport

A series of neutral long-lived purely organic radicals based on the stable [4-(N-carbazolyl)-2,6-dichlorophenyl]bis(2,4,6-trichlorophenyl)methyl radical adduct (Cbz-TTM) is reported herein. All compounds exhibit ambipolar charge-transport properties under ambient conditions owing to their radical character. High electron and hole mobilities up to 10-2 and 10-3  cm2  V-1  s-1 , respectively, were achieved. Xerographic single-layered photoreceptors were fabricated from the radicals studied herein, exhibiting good xerographic photosensitivity across the visible spectrum.

Relationship between measurement conditions and energy levels in the organic dyes used in dye-sensitized solar cells

The energy levels of new metal-free organic dyes for dye-sensitized solar cells have been investigated by the photoemission in air, UV-Vis absorption and cyclic voltammetry methods in the solutions of the dye molecules, in films of the pure dyes and in the dyes adsorbed on nanoporous TiO2. Significant differences of the energy levels have been found depending on the dye environment. For the best level of tuning in a solar cell, the energy levels are to be determined for the dyes adsorbed on the TiO2 surface. The absorbed photon conversion to current efficiency (APCE) of the solar cells was evaluated and compared with the incident photon quantum efficiency (IPCE). The results obtained show that the IPCE is dependent on the light quanta energy and reaches a maximum value when the light quanta energy is about 0.3 eV higher than the light absorption threshold.