2.2. Materials 929 2.3. Factors Influencing Charge Mobility 931 2.3.1. Molecular Packing 931 2.3.2. Disorder 932 2.3.3. Temperature 933 2.3.4. Electric Field 934 2.3.5. Impurities 934 2.3.6. Pressure 934 2.3.7. Charge-Carrier Density 934 2.3.8. Size/molecular Weight 935 3. The Charge-Transport Parameters 935 3.1. Electronic Coupling 936 3.1.1. The Energy-Splitting-in-Dimer Method 936 3.1.2. The Orthogonality Issue 937 3.1.3. Impact of the Site Energy 937 3.1.4. Electronic Coupling in Oligoacene Derivatives 938

Charge transport in organic semiconductors.

Organic solar cell research has developed during the past 30 years, but especially in the last decade it has attracted scientific and economic interest triggered by a rapid increase in power conversion efficiencies. This was achieved by the introduction of new materials, improved materials engineering, and more sophisticated device structures. Today, solar power conversion efficiencies in excess of 3% have been accomplished with several device concepts. Though efficiencies of these thin-film organicdevices have not yet reached those of their inorganic counterparts (η ≈ 10–20%); the perspective of cheap production (employing, e.g., roll-to-roll processes) drives the development of organic photovoltaic devices further in a dynamic way. The two competitive production techniques used today are either wet solution processing or dry thermal evaporation of the organic constituents. The field of organic solar cells profited well from the development of light-emitting diodes based on similar technologies, which have entered the market recently. We review here the current status of the field of organic solar cells and discuss different production technologies as well as study the important parameters to improve their performance.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400094498

Organic solar cells: An overview

In the 21st century, soft materials will become more important as functional materials because of their dynamic nature. Although soft materials are not as highly durable as hard materials, such as metals, ceramics, and engineering plastics, they can respond well to stimuli and the environment. The introduction of order into soft materials induces new dynamic functions. Liquid crystals are ordered soft materials consisting of self-organized molecules and can potentially be used as new functional materials for electron, ion, or molecular transporting, sensory, catalytic, optical, and bio-active materials. For this functionalization, unconventional materials design is required. Herein, we describe new approaches to the functionalization of liquid crystals and show how the design of liquid crystals formed by supramolecular assembly and nano-segregation leads to the formation of a variety of new self-organized functional materials.

Functional liquid-crystalline assemblies: self-organized soft materials.

Discotic (disc-like) molecules typically comprising a rigid aromatic core and flexible peripheral chains have been attracting growing interest because of their fundamental importance as model systems for the study of charge and energy transport and due to the possibilities of their application in organic electronic devices. This critical review covers various aspects of recent research on discotic liquid crystals, in particular, molecular design concepts, supramolecular structure, processing into ordered thin films and fabrication of electronic devices. The chemical structure of the conjugated core of discotic molecules governs, to a large extent, their intramolecular electronic properties. Variation of the peripheral flexible chains and of the aromatic core is decisive for the tuning of self-assembly in solution and in bulk. Supramolecular organization of discotic molecules can be effectively controlled by the choice of the processing methods. In particular, approaches to obtain suitable macroscopic orientations of columnar superstructures on surfaces, that is, planar uniaxial or homeotropic alignment, are discussed together with appropriate processing techniques. Finally, an overview of charge transport in discotic materials and their application in optoelectronic devices is given (234 references).

Discotic liquid crystals: a new generation of organic semiconductors

3.2. Thienothiophenes 1216 3.2.1. Thieno[3,4-b]thiophene Analogues 1216 3.2.2. Thieno[3,2-b]thiophene Analogues 1217 3.2.3. Thieno[2,3-b]thiophene Analogues 1218 3.3. , ′-Bridged Bithiophenes 1219 3.3.1. Dithienothiophene (DTT) Analogues 1220 3.3.2. Dithieno[3,2-b:2′3′-d]thiophene-4,4-dioxides 1221 3.3.3. Dithienosilole (DTS) Analogues 1221 3.3.4. Cyclopentadithiophene (CPDT) Analogues 1221 3.3.5. Nitrogen and Phosphor Atom Bridged Bithiophenes 1222

Functional oligothiophenes: molecular design for multidimensional nanoarchitectures and their applications.

Polycrystalline thin films of small molecules hold promise for organic thin-film transistors because of their large charge mobility, but are currently limited by poor film homogeneity and thermal durability. Here, Iino et al. design an ordered liquid crystal phase to overcome these two problems.

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Liquid crystals for organic thin-film transistors

The carrier transport in different phases of a new photoconductive calamitic liquid crystal, 2-(4\ensuremath{'}-heptyloxyphenyl)-6-dodecylthiobenzothiazole was studied by the time-of-flight technique: In the smectic A phase, a fast hole transient photocurrent was obtained in a nondispersive manner, in which the mobility was as high as $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0ex}{0ex}}{\mathrm{cm}}^{2}/\mathrm{V}\mathrm{s}$ and independent of applied electric field; in the isotropic phase, however, slower carrier transport was observed, probably due to positive or negative ions, and their mobilities were as low as ${10}^{\ensuremath{-}5}\phantom{\rule{0ex}{0ex}}{\mathrm{cm}}^{2}/\mathrm{V}\mathrm{s}$. These experimental results demonstrate the importance of local molecular alignment in creating the fast electronic conduction in calamitic liquid crystals.

Fast Hole Transport in a New Calamitic Liquid Crystal of 2-(4′-Heptyloxyphenyl)-6-Dodecylthiobenzothiazole

High Carrier Mobility up to 0.1 cm2 V–1 s–1 at Ambient Temperatures in Thiophene‐Based Smectic Liquid Crystals

Dialkylterthiphene derivatives exhibiting self-organization of molecular alignment were synthesized and their electrical properties were studied. It was found that these materials exhibited very fast electron transport in smectic mesophases as well as hole transport, both of which were independent of electric field and temperature. The mobility for electrons and holes was 5×10−4 cm2/V s in smectic C (SmC) and was increased up to 2×10−3 cm2/V s and even more up to 1×10−2 cm2/V s in accompany with upgrade of molecular alignment from SmC to SmF and from SmF to SmG, respectively. This abrupt increase in the mobility to the extent of about one order of magnitude corresponds to that in 2-phenylnaphthalene derivatives at the phase transition from SmA to SmB and from SmB to SmE whose molecular alignments are in good correlation with those of SmC, SmF, and SmG, respectively.

High ambipolar carrier mobility in self-organizing terthiophene derivative

1,4,8,11,15,18,22,25-Octaoctylphthalocyanine shows the highest time-of-flight (“long-range”) hole mobility so far reported for the columnar phase of a discotic liquid crystal. Unlike most other high long-range mobility columnar discotics, there is no clear evidence from x-ray diffraction of high order.

Jun-ichi Hanna

Papers

Liquid crystals for organic thin-film transistors

Fast Hole Transport in a New Calamitic Liquid Crystal of 2-(4′-Heptyloxyphenyl)-6-Dodecylthiobenzothiazole

High Carrier Mobility up to 0.1 cm2 V–1 s–1 at Ambient Temperatures in Thiophene‐Based Smectic Liquid Crystals

High ambipolar carrier mobility in self-organizing terthiophene derivative

Very high time-of-flight mobility in the columnar phases of a discotic liquid crystal