Ambipolar organic field-effect transistors based on solution-processed single crystal microwires of a quinoidal oligothiophene derivative

Small-molecule ambipolar transistors

Ambipolar transistor properties have been observed in various small-molecule materials. Since a small energy gap is necessary, many types of molecular designs including extended π-skeletons as well as the incorporation of donor and acceptor units have been attempted. In addition to the energy levels, an inert passivation layer is important to observe ambipolar transistor properties. Ambipolar transport has been observed in extraordinary π-electron systems such as antiaromatic compounds, biradicals, radicals, metal complexes, and hydrogen-bonded materials. Several donor/acceptor cocrystals show ambipolar transport as well.

Optical-field driven charge-transfer modulations near composite nanostructures

Optical activation of material properties illustrates the potentials held by tuning light-matter interactions with impacts ranging from basic science to technological applications. Here, we demonstrate for the first time that composite nanostructures providing nonlocal environments can be engineered to optically trigger photoinduced charge-transfer-dynamic modulations in the solid state. The nanostructures explored herein lead to out-of-phase behavior between charge separation and recombination dynamics, along with linear charge-transfer-dynamic variations with the optical-field intensity. Using transient absorption spectroscopy, up to 270% increase in charge separation rate is obtained in organic semiconductor thin films. We provide evidence that composite nanostructures allow for surface photovoltages to be created, which kinetics vary with the composite architecture and last beyond optical pulse temporal characteristics. Furthermore, by generalizing Marcus theory framework, we explain why charge-transfer-dynamic modulations can only be unveiled when optic-field effects are enhanced by nonlocal image-dipole interactions. Our demonstration, that composite nanostructures can be designed to take advantage of optical fields for tuneable charge-transfer-dynamic remote actuators, opens the path for their use in practical applications ranging from photochemistry to optoelectronics.

Controlling and modulating charge transfer dynamics in composite nanostructures, though promising for optoelectronic applications, remains a challenge. Here, the authors report optical control of charge separation and recombination processes in organic semiconductor-based composite nanostructures.

Theoretical study on the charge transport properties of three series of dicyanomethylene quinoidal thiophene derivatives

It is very important to analyse the most advantageous connection style for quinoidal thiophene derivatives, which are used in n-type organic semiconductor transport materials. In the present work, the charge transport properties of three series of quinoidal thiophene derivatives, oligothiophene (series A), thienothiophene (series B) and benzothiophene (series C), are systematically investigated by employing full quantum charge transfer theory combined with kinetic Monte-Carlo simulation. The single crystal structures of the molecules we had constructed were predicted using the USPEX program combined with density functional theory (DFT) and considering the dispersion corrected. Our theoretical results expounded how the different connection styles, including oligo-, thieno-, and benzo-thiophene in the quinoidal thiophenes derivatives, effectively tune their electronic structures, and revealed how their intermolecular interactions affect the molecular packing patterns and hence their charge transport properties by symmetry-adapted perturbation theory (SAPT). In the meantime we also elucidated the role of end-cyano groups in noncovalent interactions. Furthermore, it is clarified that quinoidal thiophene derivatives show excellent carrier transport properties due to their optimal molecular stacking motifs and larger electronic couplings besides their low energy gap. In addition, our theoretical results demonstrate that quinoidal oligothiophene derivatives (n = 3-5) with more thiophene rings will have ambipolar transport properties, so quinoidal thienothiophene and benzothiophene derivatives should be promising alternatives as n-type OSCs. When we focused only on the electronic transport properties in the three series of molecules, quinoidal benzothiophene derivatives were slightly better than quinoidal oligothiophene or thienothiophene derivatives.

Quasi-Phase Diagrams at Air/Oil Interfaces and Bulk Oil Phases for Crystallization of Small-Molecular Semiconductors by Adjusting Gibbs Adsorption

The temperature and concentration dependencies of the crystallization of two small-molecular semiconductors were clarified by constructing quasi-phase diagrams at air/oil interfaces and in bulk oil phases. A quinoidal quaterthiophene derivative with four alkyl chains (QQT(CN)4) in 1,1,2,2-tetrachroloethane (TCE) and a thienoacene derivative with two alkyl chains (C8-BTBT) in o-dichlorobenzene were used. The apparent crystal nucleation temperature (T_n) and dissolution temperature (T_d) of the molecules were determined based on optical microscopy examination in closed glass capillaries and open dishes during slow cooling and heating processes, respectively. T_n and T_d were considered estimates of the critical temperatures for nuclear formation and crystal growth, respectively. The T_n values of QQT(CN)4 and C8-BTBT at the air/oil interfaces were higher than those in the bulk oil phases, whereas the T_d values at the air/oil interfaces were almost the same as those in the bulk oil phases. These Gibbs adsorption phenomena were attributed to the solvophobic effect of the alkyl chain moieties. The temperature range between T_n and T_d corresponds to suitable supercooling conditions for ideal crystal growth based on the suppression of nucleation. The T_n values at the water/oil and oil/glass interfaces did not shift compared with those of the bulk phases, indicating that adsorption did not occur at the hydrophilic interfaces. Promotion and inhibition of nuclear formation for crystal growth of the semiconductors were achieved at the air/oil and hydrophilic interfaces, respectively.

Para-Quinodimethanes: A Unified Review of the Quinoidal-Versus-Aromatic Competition and its Implications

In this article, some quinoidal p-quinodimethanes compounds that convert partially or completely to diradicals or biradicaloids are analyzed. The aromatic/quinoidal balance is revisited with the objective of providing a common interpretation for most of them. For that purpose, important structural and energetic parameters such as the bond length alternation pattern and the singlet-triplet gaps are analyzed and interpreted in the framework of double spin polarization and π-conjugation. p-Quinodimethanes based in oligothiophenes, polycyclic aromatic hydrocarbons, oligophenylenes, thienothiophenes, charged dications and cyclic conjugated molecules are discussed. There are excellent reviews in the field of singlet diradicals; however, a revision similar to that proposed here can help the reader to have another perspective on these promising new functional materials. The focus has been put on molecules which are well known by the author and another of relevance in the field. In this regard, the article finishes with a discussion of some important applications of these diradicals in organic electronics. New chemical systems based on the p-quinodimethane building blocks are waiting us around the corner, bringing us new and challenging structures and fascinating novel properties, which describe a very rich field of research in chemistry and in physics with an excellent present and a bright future.

Benzothiadiazole-Based Small-Molecule Semiconductors for Organic Thin-Film Transistors and Complementary-like Inverters

New benzothiadiazole derivatives, 4,7-bis(5-phenylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (PT-BTD) and 4,7-bis[4-(thiophen-2-yl)phenyl]benzo[c][1,2,5]thiadiazole (TP-BTD), were synthesized and characterized as small-molecule organic semiconductors for organic thin-film transistors (OTFTs) and complementary inverters. The thermal, optical, and electrochemical properties of the new compounds were fully characterized. Vacuum-deposition and solution-shearing methods were used to fabricate thin films based on these compounds. Thin films based on PT-BTD exhibited p-channel characteristics with hole mobilities as high as 0.10 cm²  V^-1  s^-1 and current on/off ratios >10⁷ for top-contact/bottom-gate OTFT devices. With an optimized blending ratio of PT-BTD and the representative n-channel semiconductor N,N'-1H,1H-perfluorobutyl dicyanoperylenediimide, bulk heterojunction ambipolar transistors were fabricated with balanced hole and electron mobilities of 0.10 and 0.07 cm²  V^-1  s^-1 , respectively. Furthermore, a complementary-like inverter was fabricated using ambipolar thin-film transistors, which showed a high voltage gain of 84.

Electronic energy and electron transfer processes in photoexcited donor-acceptor dyad and triad molecular systems based on triphenylene and perylene diimide units

We investigate the photophysical properties of organic donor-acceptor dyad and triad molecular systems based on triphenylene and perylene diimide units linked by a non-conjugated flexible bridge in solution using complementary optical spectroscopy techniques. When these molecules are diluted in dichloromethane solution, energy transfer from the triphenylene to the perylene diimide excited moieties is evidenced by time-resolved fluorescence measurements resulting in a quenching of the emission from the triphenylene moieties. Simultaneously, another quenching process that affects the emission from both donor and acceptor units is observed. Solution ultrafast transient absorption measurements provide evidence of photo-induced charge transfer from either the donor or the acceptor depending upon the excitation. Overall, the analysis of the detailed time-resolved spectroscopic measurements carried out in the dyad and triad systems as well as in the triphenylene and perylene diimide units alone provides useful information both to better understand the relations between energy and charge transfer processes with molecular structures, and for the design of future functional dyad and triad architectures based on donor and acceptor moieties for organic optoelectronic applications.

Microcrystallization of a Solution-Processable Organic Semiconductor in Capillaries for High-Performance Ambipolar Field-Effect Transistors

We report on the use of microcrystallization in capillaries to fabricate patterned crystalline microstructures of the low-bandgap ambipolar quinoidal quaterthiophene derivative (QQT(CN)4) from a chloroform solution. Aligned needle-shaped QQT(CN)4 crystals were formed in thin film microstructures using either open- or closed- capillaries made of polydimethylsiloxane (PDMS). Their charge transport properties were evaluated in a bottom-gate top-contact transistor configuration. Hole and electron mobilities were found to be as high as 0.17 and 0.083 cm(2) V(-1) s(-1), respectively, approaching the values previously obtained in individual QQT(CN)4 single crystal microneedles. It was possible to control the size of the needle crystals and the microline arrays by adjusting the structure of the PDMS mold and the concentration of QQT(CN)4 solution. These results demonstrate that the microcrystallization in capillaries technique can be used to simultaneously pattern organic needle single crystals and control the microcrystallization processes. Such a simple and versatile method should be promising for the future development of high-performance organic electronic devices.

Donor–acceptor co-assembled supramolecular nanofibers with high and well-balanced ambipolar charge transport properties under ambient conditions

We report an example of single-bundle nanofiber based OFETs with high and well-balanced ambipolar charge transport properties under ambient conditions.

Structure–charge transfer property relationship in self-assembled discotic liquid-crystalline donor–acceptor dyad and triad thin films

Dynamics of the photo-induced charge transfer are correlated with the structural properties of self-assembled discotic donor–acceptor dyad and triad films.

A new aspect of cyclopentadithiophene based polymers: narrow band gap polymers upon protonation

A new aspect of CPDT based conjugated polymers that CPDT units in the polymers can be protonated with a significant change in optical and electronic properties was reported.