Oleg V. Borshchev

Material solubility and molecular compatibility effects in the design of fullerene/polymer composites for organic bulk heterojunction solar cells

We report a systematic study of more than 100 bicomponent systems composed of 19 different fullerene derivatives blended with 9 different conjugated polymers (including previously investigated poly(3-hexylthiophene)). It was shown that short circuit current density (JSC) and light power conversion efficiency (η) of the fullerene/polymer photovoltaic devices depend on the solubility of the fullerene components in the solvent used for the blend film deposition (chlorobenzene). The revealed dependences have unusual “double branch” character because many fullerene derivatives possessing similar solubilities showed different photovoltaic performances. This behavior was related to the peculiarities of the molecular structures of the fullerene derivatives. Substituents attached to the cyclopropane ring fused with the fullerene cage in methanofullerenes affected both the morphology of their composites with conjugated polymers and their photovoltaic performance. It was demonstrated that variation of the fullerene component blended with a conjugated polymer might easily change its photovoltaic performance by a factor of 3–4. The obtained results proved that design of appropriate fullerene derivatives and novel conjugated polymers are equally important tasks on the way towards highly efficient organic photovoltaics.

Oligothiophene-based monolayer field-effect transistors prepared by Langmuir-Blodgett technique

Quinquethiophene-based monolayer organic field-effect transistors (OFETs) prepared by Langmuir-Blodgett (LB) technique show hole mobilities up to 10−2 cm2/Vs and On/Off ratios up to 106. Functional logic LB monolayer devices operating in air have been demonstrated. The performance of LB OFETs is comparable to self-assembled monolayer field-effect transistors (SAMFETs) devices prepared by self-assembly from solution using the same organosilicon oligothiophene despite the LB OFET monolayer is weakly bounded to the dielectric surface. Taking into account that the LB technique is a fast and rather easy process, these findings highlight a high potential of LB technique for ultrathin organic electronics.

Nanostructured organosilicon luminophores and their application in highly efficient plastic scintillators

Organic luminophores are widely used in various optoelectronic devices, which serve for photonics, nuclear and particle physics, quantum electronics, medical diagnostics and many other fields of science and technology. Improving their spectral-luminescent characteristics for particular technical requirements of the devices is a challenging task. Here we show a new concept to universal solution of this problem by creation of nanostructured organosilicon luminophores (NOLs), which are a particular type of dendritic molecular antennas. They combine the best properties of organic luminophores and inorganic quantum dots: high absorption cross-section, excellent photoluminescence quantum yield, fast luminescence decay time and good processability. A NOL consists of two types of covalently bonded via silicon atoms organic luminophores with efficient Förster energy transfer between them. Using NOLs in plastic scintillators, widely utilized for radiation detection and in elementary particles discoveries, led to a breakthrough in their efficiency, which combines both high light output and fast decay time. Moreover, for the first time plastic scintillators, which emit light in the desired wavelength region ranging from 370 to 700 nm, have been created. We anticipate further applications of NOLs as working elements of pulsed dye lasers in photonics, optoelectronics and as fluorescent labels in biology and medical diagnostics.

Polymer Surface Engineering for Efficient Printing of Highly Conductive Metal Nanoparticle Inks

An approach to polymer surface modification using self-assembled layers (SALs) of functional alkoxysilanes has been developed in order to improve the printability of silver nanoparticle inks and enhance adhesion between the metal conducting layer and the flexible polymer substrate. The SALs have been fully characterized by AFM, XPS, and WCA, and the resulting printability, adhesion, and electrical conductivity of the screen-printed metal contacts have been estimated by cross-cut tape test and 4-point probe measurements. It was shown that (3-mercaptopropyl)trimethoxysilane SALs enable significant adhesion improvements for both aqueous- and organic-based silver inks, approaching nearly 100% for PEN and PDMS substrates while exhibiting relatively low sheet resistance up to 0.1 Ω/sq. It was demonstrated that SALs containing functional -SH or -NH2 end groups offer the opportunity to increase the affinity of the polymer substrates to silver inks and thus to achieve efficient patterning of highly conductive structures on flexible and stretchable substrates.

Highly Luminescent Solution-Grown Thiophene-Phenylene Co-Oligomer Single Crystals

Thiophene-phenylene co-oligomers (TPCOs) are among the most promising materials for organic light emitting devices. Here we report on record high among TPCO single crystals photoluminescence quantum yield reaching 60%. The solution-grown crystals are stronger luminescent than the vapor-grown ones, in contrast to a common believe that the vapor-processed organic electronic materials show the highest performance. We also demonstrate that the solution-grown TPCO single crystals perform in organic field effect transistors as good as the vapor-grown ones. Altogether, the solution-grown TPCO crystals are demonstrated to hold great potential for organic electronics.

Self-assembled organic semiconductors for monolayer field-effect transistors

Recent data on the structures and properties of self-organized molecules used for the production of the semiconducting layer in self-assembled organic monolayer field-effect transistors are reviewed. Methods for fabrication of these transistors are presented together with their advantages and shortcomings. Electric characteristics of the produced devices are compared. Major structural regularities for selection of the reactive group in self-organized semiconductor oligomer molecules are elucidated with respect to the type of substrate.

Luminescence spectral properties of dendritic oligothiophenesilane macromolecules

The luminescence spectral properties of oligothiophenesilane dendrite macromolecules were studied. It was found that the chromophores responsible for the formation of the absorption and luminescence spectra were dendrimer fragments separated by silicon atoms; all the chromophores are equally involved in the formation of the absorption spectra of dendritic macromolecules of the zero, first, second, and third generations. It was shown that for dendrites of the first, second, and third generations, the luminescence spectrum is mainly formed by the chromophore fragments lying in the internal layers of the dendritic macromolecule due to the induction resonance energy transfer of electronic excitation from peripheral to internal chromophore fragments. The conclusion was drawn that synthesis of dendritic macromolecules with internal chromophore fragments possessing a high quantum yield of fluorescence can give actively luminescing nanosized objects with the molar extinction coefficient proportional to the number of fragments ɛmax ≈ 106 l/(mol cm). These compounds can find wide application in transducers for converting various types of ionizing radiation into optical radiation in electroluminescent devices.

A novel highly efficient nanostructured organosilicon luminophore with unusually fast photoluminescence

Synthesis and theoretical and experimental investigations of a novel nanostructured organosilicon luminophore (NOL) containing six 2,2′-bithienyl donor units connected via silicon atoms to a 1,4-bis(5-phenylthienyl-2-yl)-benzene acceptor unit with efficient intramolecular Forster resonance energy transfer are reported. The NOL shows a unique combination of optical properties: a high photoluminescence (PL) quantum yield of up to 91%, a fast PL decay time of down to 800 ps, a large pseudo-Stokes shift of 101 nm and a huge molar extinction coefficient of 1.4 × 105 M−1 cm−1. These peculiarities caused by the specific arrangement of the donor and acceptor fragments at the nanoscale distance within the NOL were correlated with the molecular structure of the NOL using theoretical calculations, which for the first time allowed successful prediction of the oscillator strength, PL decay time and intramolecular energy transfer efficiency. A comparison of the photophysical properties of the NOL with the standard laser dye POPOP in THF and toluene solutions revealed its huge application potential in organic photonics and high energy physics.

Luminescent Organic Semiconducting Langmuir Monolayers

In recent years, monolayer organic field-effect devices such as transistors and sensors have demonstrated their high potential. In contrast, monolayer electroluminescent organic field-effect devices are still in their infancy. One of the key challenges here is to create an organic material that self-organizes in a monolayer and combines efficient charge transport with luminescence. Herein, we report a novel organosilicon derivative of oligothiophene-phenylene dimer D2-Und-PTTP-TMS (D2, tetramethyldisiloxane; Und, undecylenic spacer; P, 1,4-phenylene; T, 2,5-thiophene; TMS, trimethylsilyl) that meets these requirements. The self-assembled Langmuir monolayers of the dimer were investigated by steady-state and time-resolved photoluminescence spectroscopy, atomic force microscopy, X-ray reflectometry, and grazing-incidence X-ray diffraction, and their semiconducting properties were evaluated in organic field-effect transistors. We found that the best uniform, fully covered, highly ordered monolayers were semiconducting. Thus, the ordered two-dimensional (2D) packing of conjugated organic molecules in the semiconducting Langmuir monolayer is compatible with its high-yield luminescence, so that 2D molecular aggregation per se does not preclude highly luminescent properties. Our findings pave the way to the rational design of functional materials for monolayer organic light-emitting transistors and other optoelectronic devices.

Novel Cross-Linked Luminescent Silicone Composites Based on Reactive Nanostructured Organosilicon Luminophores

New functional silyl hydride and vinyl containing methylphenylsiloxane oligomers were synthesized by a polycondensation reaction in the active medium being the solvent, the catalyst and the reactant simultaneously. Hydrosilylation of them in the presence of 0.1 - 3 wt. % of novel reactive nanostructured organosilicon luminophores (NOLs) containing different central luminescent and 2,2’-bithienyl peripheral light harvesting fragments with terminal undecylenic groups led to crosslinked silicone composites with valuable luminescence in different regions of the visible spectrum (blue, yellow or red). Investigation of their optical properties revealed that their absorption and luminescence spectra correspond to those of the diluted solutions of the reactive NOLs used. These findings indicate the absence of any aggregation of NOLs in the crosslinked silicon composites confirming their excellent optical quality. High thermal stability of the reactive NOLs (up to 370 ∘C in argon) and the silicones themselves indicate their great potential for creation of highly efficient thermo and radiation resistant plastic scintillators.