Sergei A. Ponomarenko

Bottom-up organic integrated circuits

Self-assembly—the autonomous organization of components into patterns and structures—is a promising technology for the mass production of organic electronics. Making integrated circuits using a bottom-up approach involving self-assembling molecules was proposed in the 1970s. The basic building block of such an integrated circuit is the self-assembled-monolayer field-effect transistor (SAMFET), where the semiconductor is a monolayer spontaneously formed on the gate dielectric. In the SAMFETs fabricated so far, current modulation has only been observed in submicrometre channels, the lack of efficient charge transport in longer channels being due to defects and the limited intermolecular π–π coupling between the molecules in the self-assembled monolayers. Low field-effect carrier mobility, low yield and poor reproducibility have prohibited the realization of bottom-up integrated circuits. Here we demonstrate SAMFETs with long-range intermolecular π–π coupling in the monolayer. We achieve dense packing by using liquid-crystalline molecules consisting of a π-conjugated mesogenic core separated by a long aliphatic chain from a monofunctionalized anchor group. The resulting SAMFETs exhibit a bulk-like carrier mobility, large current modulation and high reproducibility. As a first step towards functional circuits, we combine the SAMFETs into logic gates as inverters; the small parameter spread then allows us to combine the inverters into ring oscillators. We demonstrate real logic functionality by constructing a 15-bit code generator in which hundreds of SAMFETs are addressed simultaneously. Bridging the gap between discrete monolayer transistors and functional self-assembled integrated circuits puts bottom-up electronics in a new perspective.

Liquid crystalline carbosilane dendrimers: First generation

Abstract An approach to the synthesis of a new class of liquid crystalline (LC) compounds, dendrimers of regular structure with terminal mesogenic groups, was elaborated. LC dendrimers based on the carbosilane dendritic matrix of first generation were synthesized. Cyanobiphenyl, methoxyphenyl benzoate and cholesteryl groups were used as mesogenic fragments. Individuality and structure of all compounds obtained was proved by GPC together with 1H- and 29Si NMR methods. The mesomorphic behaviour and structure of the LC dendrimers synthesized were investigated. It is argued that different mesophases of the smectic type are realized in all cases. It is shown that the mesophase type of these compounds essentially depends on the chemical nature of the mesogenic groups.

Monolayer coverage and channel length set the mobility in self-assembled monolayer field-effect transistors

The mobility of self-assembled monolayer field-effect transistors (SAMFETs) traditionally decreases dramatically with increasing channel length. Recently, however, SAMFETs using liquid-crystalline molecules have been shown to have bulk-like mobilities that are virtually independent of channel length. Here, we reconcile these scaling relations by showing that the mobility in liquid crystalline SAMFETs depends exponentially on the channel length only when the monolayer is incomplete. We explain this dependence both numerically and analytically, and show that charge transport is not affected by carrier injection, grain boundaries or conducting island size. At partial coverage, that is when the monolayer is incomplete, liquid-crystalline SAMFETs thus form a unique model system to study size-dependent conductance originating from charge percolation in two dimensions.

High-mobility organic thin-film transistors based on α,α′-didecyloligothiophenes

We have fabricated organic thin-film transistors and integrated circuits based on the small-molecule organic semiconductors α,α′-didecylquaterthiophene, α,α′-didecylquinquethiophene, and α,α′-didecylsexithiophene. The organic semiconductors were deposited by thermal evaporation, with solution-processed and cross linked poly-4-vinylphenol serving as the gate dielectric layer. We have found that bottom-contact devices based on these materials have better electrical performance than top-contact devices, presumably due to more efficient carrier injection from bottom contacts due to the presence of the relatively long alkyl chains substituted at the α- and ω-positions of the oligothiophene molecules. Bottom-contact transistors have carrier mobility as large as 0.5 cm2/V s and on/off current ratio as large as 105, and ring oscillators fabricated using bottom-contact transistors and α,α′-didecylsexithiophene as the organic active layer have signal propagation delay as low as 30 μs per stage.

Liquid crystalline dendrimer of the fifth generation: From lamellar to columnar structure in thermotropic mesophases

The structure of a liquid crystalline (LC) carbosilane dendrimer of the fifth generation bearing 128 terminal cyanobiphenyl mesogenic groups has been studied. This dendrimer was synthesized by a hydrosilylation reaction and then the cyanobiphenyl mesogenic groups were chemically linked to the dendritic matrix via a-OOC-(CH2)10-Si(CH3)2OSi(CH3)2-spacer. Structural studies carried out by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction methods revealed unusual phase behaviour. At room temperature the dendrimer forms a lamellar (smectic A) phase which develops in-plane ordering above 40C.This is due toa tendency to form columns ofmolecules which areprobably perpendicular to the layers. Above 121C the material transforms into another more disordered mesophase which is probably a disordered hexagonal columnar phase. The proposed structures and molecular packing in these different types of mesophase are discussed.

Synthesis and thermal behaviour of α,α′-didecyloligothiophenes

α,α′-Didecylquater-, -quinque- and -sexi-thiophenes were synthesized by Kumada cross-coupling and oxidative coupling reactions. For the former reaction Pd(dppf)Cl2 was found to be a more efficient catalyst than the usually applied Ni(dppp)Cl2. Thermal behaviour of all new oligothiophenes was investigated by differential scanning calorimetry and polarizing optical microscopy. It was shown that all these compounds possess not only crystal phases but also high temperature ordered smectic mesophases and that the clearing point increases linearly with the number of conjugated thiophene rings. A degree of order in the crystal phase was estimated on the basis of thermodynamic data. The highest degree of order was proposed for α,α′-didecylquaterthiophene, which explains why the mobility of end-α,α′-capped quaterthiophenes in FET (field effect transistor) devices is comparable or sometimes better than those of corresponding quinque- and sexi-thiophene derivatives.

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.

Conjugated Organosilicon Materials for Organic Electronics and Photonics

In this chapter different types of conjugated organosilicon materials possessing luminescent and/or semiconducting properties will be described. Such macromolecules have various topologies and molecular structures: linear, branched and hyperbranched oligomers, polymers, and dendrimers. Specific synthetic ap- proaches to access these structures will be discussed. Special attention is devoted to the role of silicon in these structures and its influence on their optical and electri- cal properties, leading to their potential application in the emerging areas of organic and hybrid electronics.

Ordered Semiconducting Self-Assembled Monolayers on Polymeric Surfaces Utilized in Organic Integrated Circuits

We report on a two-dimensional highly ordered self-assembled monolayer (SAM) directly grown on a bare polymer surface. Semiconducting SAMs are utilized in field-effect transistors and combined into integrated circuits as 4-bit code generators. The driving force to form highly ordered SAMs is packing of the liquid crystalline molecules caused by the interactions between the linear alkane moieties and the pi-pi stacking of the conjugated thiophene units. The fully functional circuits demonstrate long-range order over large areas, which can be regarded as the start of flexible monolayer electronics.

A combination of Al-doped ZnO and a conjugated polyelectrolyte interlayer for small molecule solution-processed solar cells with an inverted structure

We successfully demonstrate a smart strategy to use aluminum doped ZnO (AZO) and the thiophene-based conjugated polyelectrolyte P3TMAHT as an interfacial layer in small molecule solution-processed inverted solar cells. Modification of AZO with a thin P3TMAHT layer increases the photovoltaic properties of the inverted cell as a result of reduction in the work function of the cathode with well aligned frontier orbital energy levels for efficient charge transport and reduced surface recombination. The inverted device achieved ∼16% performance improvement dominantly by recapturing part of the Voc losses when going from conventional to the inverted architecture. In addition, the inverted device using the AZO/P3TMAHT interlayer shows improved device stability in air compared to conventional devices.