Yoshiko Koizumi

Comprehensive Approach to Intrinsic Charge Carrier Mobility in Conjugated Organic Molecules, Macromolecules, and Supramolecular Architectures

Si-based inorganic electronics have long dominated the semiconductor industry. However, in recent years conjugated polymers have attracted increasing attention because such systems are flexible and offer the potential for low-cost, large-area production via roll-to-roll processing. The state-of-the-art organic conjugated molecular crystals can exhibit charge carrier mobilities (μ) that nearly match or even exceed that of amorphous silicon (1-10 cm(2) V(-1) s(-1)). The mean free path of the charge carriers estimated from these mobilities corresponds to the typical intersite (intermolecular) hopping distances in conjugated organic materials, which strongly suggests that the conduction model for the electronic band structure only applies to μ > 1 cm(2) V(-1) s(-1) for the translational motion of the charge carriers. However, to analyze the transport mechanism in organic electronics, researchers conventionally use a disorder formalism, where μ is usually less than 1 cm(2) V(-1) s(-1) and dominated by impurities, disorders, or defects that disturb the long-range translational motion. In this Account, we discuss the relationship between the alternating-current and direct-current mobilities of charge carriers, using time-resolved microwave conductivity (TRMC) and other techniques including field-effect transistor, time-of-flight, and space-charge limited current. TRMC measures the nanometer-scale mobility of charge carriers under an oscillating microwave electric field with no contact between the semiconductors and the metals. This separation allows us to evaluate the intrinsic charge carrier mobility with minimal trapping effects. We review a wide variety of organic electronics in terms of their charge carrier mobilities, and we describe recent studies of macromolecules, molecular crystals, and supramolecular architecture. For example, a rigid poly(phenylene-co-ethynylene) included in permethylated cyclodextrin shows a high intramolecular hole mobility of 0.5 cm(2) V(-1) s(-1), based on a combination of flash-photolysis TRMC and transient absorption spectroscopy (TAS) measurements. Single-crystal rubrene showed an ambipolarity with anisotropic charge carrier transport along each crystal axis on the nanometer scale. Finally, we describe the charge carrier mobility of a self-assembled nanotube consisting of a large π-plane of hexabenzocoronene (HBC) partially appended with an electron acceptor. The local (intratubular) charge carrier mobility reached 3 cm(2) V(-1) s(-1) for the nanotubes that possessed well-ordered π-stacking, but it dropped to 0.7 cm(2) V(-1) s(-1) in regions that contained greater amounts of the electron acceptor because those molecules reduced the structural integrity of π-stacked HBC arrays. Interestingly, the long-range (intertubular) charge carrier mobility was on the order of 10(-4) cm(2) V(-1) s(-1) and monotonically decreased when the acceptor content was increased. These results suggest the importance of investigating charge carrier mobilities by frequency-dependent charge carrier motion for the development of more efficient organic electronic devices.

Thienoisoindigo-based low-band gap polymers for organic electronic devices

We synthesized a series of new low-band gap donor–acceptor copolymers containing 4,4′-bis(alkyl)-[6,6′-bithieno[3,2-b]pyrrolylidene]-5,5′(4H,4′H)-dione. This acceptor unit, so-called dithienoketopyrrole (DTKP), is an analogue of isoindigo, the phenyl rings of which are replaced by thiophenes. Donor moieties such as benzodithiophene, cyclopentadithiophene, fluorene, and dithienothiophene are polymerized with DTKP in an alternating fashion by Stille or Suzuki–Miyaura coupling methods. Exceedingly low-band gaps (Eg = 1.0–1.6 eV) were achieved in these copolymers through internal charge transfer interactions between the donor and acceptor moieties. The structural, photophysical, and electrochemical properties of the resultant copolymers were characterized, and field-effect transistor (FET) mobilities were measured. The copolymers showed electronic absorption spectra extending to the near infrared region (600–1400 nm) with absorption maxima at 745–971 nm, along with a low-lying LUMO of −3.8 eV. Density functional theory (DFT) calculation indicated high planarity for the copolymer backbone when compared to that of its phenyl-isoindigo counterparts. FET hole mobilities on the order of 10−4 to 10−3 cm2 V−1 s−1 were obtained, demonstrating a feasibility to use them in organic photovoltaic cells.

A Versatile Approach to Organic Photovoltaics Evaluation Using White Light Pulse and Microwave Conductivity

State-of-the-art low band gap conjugated polymers have been investigated for application in organic photovoltaic cells (OPVs) to achieve efficient conversion of the wide spectrum of sunlight into electricity. A remarkable improvement in power conversion efficiency (PCE) has been achieved through the use of innovative materials and device structures. However, a reliable technique for the rapid screening of the materials and processes is a prerequisite toward faster development in this area. Here we report the realization of such a versatile evaluation technique for bulk heterojunction OPVs by the combination of time-resolved microwave conductivity (TRMC) and submicrosecond white light pulse from a Xe-flash lamp. Xe-flash TRMC allows examination of the OPV active layer without requiring fabrication of the actual device. The transient photoconductivity maxima, involving information on generation efficiency, mobility, and lifetime of charge carriers in four well-known low band gap polymers blended with phenyl-C(61)-butyric acid methyl ester (PCBM), were confirmed to universally correlate with the PCE divided by the open circuit voltage (PCE/V(oc)), offering a facile way to predict photovoltaic performance without device fabrication.

Amphiphilic Design of a Discotic Liquid‐Crystalline Molecule for Dipole Manipulation: Hierarchical Columnar Assemblies with a 2D Superlattice Structure

Columnar liquid-crystalline (LC) materials composed of diskshaped aromatic molecules arranged in one-dimensional (1D) columns have attracted increasing attention owing to their potential utility for solution-processable organic electronic and ionic devices. Because carrier transport relies on these 1D columns, a long-range intracolumnar molecular order is particularly important. Charge-transfer complexation has been reported to be effective in reinforcing the intracolumnar 1D order of discotic columnar LC assemblies. Williams and co-workers demonstrated that the incorporation of electronwithdrawing substituents into aromatic mesogens results in the enhancement of p stacking to stabilize the LC state. This molecular-design strategy based on so-called “p polarization” probably gives rise to a dipole in the aromatic mesogens. As a result, LC molecules tend to p stack in a headto-tail manner, and the dipole is canceled out within individual columns (Figure 1b). In this context, one may wonder how LC molecules composed of p-polarized mesogens assemble when they bear a particular functionality that would hamper head-to-tail stacking, and in turn, how the entire LC assembly would cope with the large intracolumnar dipole generated upon head-to-head stacking (Figure 1c). Herein we report the interesting finding that such a molecular design, uncomfortable for the molecule in terms of dipole interactions, leads to the formation of a 2D superlattice structure that is unprecedented in columnar LC assemblies composed of disk-shaped aromatic molecules. We noticed that this hierarchical structure not only has the advantage that the dipoles are canceled out intercolumnarly but also that a homeotropic alignment of the LC columns is adopted. Dibenzo[a,c]phenazine, which possesses a dipole moment along the longer molecular axis, is known to serve as a mesogenic core for columnar LC assemblies (see Figure S1a and Table S2 in the Supporting Information). We previously reported that the derivative with six decyloxy side chains, 1C10 (Figure 1a), exhibits a hexagonal columnar (Colh) mesophase over a wide temperature range. [7] In study described herein, we designed an amphiphilic derivative, 1TEG (Figure 1a), with two triethylene glycol (TEG) chains on the phenazine ring and four decyloxy chains on the fused benzene rings. We anticipated that this amphiphilic derivative could adopt a head-to-head arrangement upon p stacking if microphase separation between immiscible TEG and paraffinic side chains occurred in preference to cancellation of the dipole (Figure 1c). Compound 1TEG was synthesized by a procedure similar to that reported previously (see Scheme S1 in the Supporting Information). As reference molecules, we also prepared 1EG (Figure 1a), with shorter oxyethylene side chains, and 2TEG, which was obtained by a ring-closing reaction of the corresponding 2,3-bisphenylnaphthalene derivative (see Scheme S2 in the Supporting Information). All new compounds were characterized unambiguously by H Figure 1. a) Molecular structures of dibenzo[a,c]phenazine derivatives 1C10, 1TEG, and 1EG and the benzo[b]triphenylene derivative 2TEG. b,c) Schematic illustrations of 1D columnar assemblies with head-totail (b) and head-to-head arrangements (c) of the aromatic mesogen. Yellow arrows indicate the dipole moment of the dibenzo[a,c]phenazine core.

Syntheses and Properties of Graphyne Fragments: Trigonally Expanded Dehydrobenzo[12]annulenes

We present herein the synthesis and properties of the largest hitherto unknown graphyne fragment, namely trigonally expanded tetrakis(dehydrobenzo[12]annulene)s (tetrakis-DBAs). Intramolecular three-fold alkyne metathesis reactions of hexakis(arylethynyl)DBAs 9 a and 9 b using Fürstners Mo catalyst furnished tetrakis-DBAs 8 a and 8 b substituted with tert-butyl or branched alkyl ester groups in moderate and fair yields, respectively, demonstrating that the metathesis reaction of this protocol is a powerful tool for the construction of graphyne fragment backbones. For comparison, hexakis(arylethynyl)DBAs 9 c-g have also been prepared. The one-photon absorption spectrum of tetrakis-DBA 8 a bearing tert-butyl groups revealed a remarkable bathochromic shift of the absorption cut-off (λcutoff ) compared with those of previously reported graphyne fragments due to extended π-conjugation. Moreover, in the two-photon absorption spectrum, 8 a showed a large cross-section for a pure hydrocarbon because of the planar para-phenylene-ethynylene conjugation pathways. Hexakis(arylethynyl)-DBAs 9 c-e and 9 g and tetrakis-DBA 8 b bearing electron-withdrawing groups aggregated in chloroform solutions. Comparison between the free energies of 9 e and 8 b bearing the same substituents revealed the more favorable association of the latter due to stronger π-π interactions between the extended π-cores. Polarized optical microscopy observations, DSC, and XRD measurements showed that 8 b and 9 e with branched alkyl ester groups displayed columnar rectangular mesophases. By the time-resolved microwave conductivity method, the columnar rectangular phase of 8 b was shown to exhibit a moderate charge-carrier mobility of 0.12 cm(2)  V(-1)  s(-1) . These results indicate that large graphyne fragments can serve as good organic semiconductors.

Nanopatterning of polyfluorene derivative using electron-beam lithography

Direct nanopatterning of polyfluorene derivative is demonstrated using electron-beam lithography. Polyfluorene, which has attracted much attention because of its strong fluorescence and application in organic light emitters, is a negative (crosslinking) type material upon exposure to radiation, and requires a large amount of exposure dose ∼2300μC∕cm2 to be patterned in the present case. This extremely large dose would lead to radiation damage of the polymer. To address this issue, a polyfluorene derivative for acid-catalyzed chemical amplification was synthesized to realize nanopatterning of π-conjugated polymer without degrading its optical property. The synthesized polyfluorene derivative was investigated in terms of sensitivity, optical absorption, and spatial resolution. Sensitivity of 4μC∕cm2 was achieved, and lines of 70 nm width were fabricated after optimization of the process conditions.

Molecular engineering of benzothienoisoindigo copolymers allowing highly preferential face-on orientations

Orientation of conjugated polymers is increasingly important in organic photovoltaics (OPV) to achieve high power conversion efficiency (PCE). The optimized orientation of conjugated backbones for photo-generated charge carriers in OPV devices is in contrast to organic semiconductor devices, demanding new strategies to control and realize face-on orientation of conjugated systems onto substrates. Here we report new conjugated polymers composed of electron-accepting benzothienoisoindigo (BTIDG), an asymmetric unit of isoindigo and thienoisoindigo. BTIDG was coupled with weakly electron-donating thiazolothiazole or benzobisthiazole, concurrently leading to moderate optical band gaps (1.41–1.52 eV) and the highest occupied molecular orbital (−5.35 to −5.50 eV). The alkylthiophene spacer between BTIDG and the donor unit provided a marked control over the orientation of polymers, among which the degree of face-on orientation as high as 95% was revealed by grazing incidence X-ray diffraction. The maximum PCE was improved up to 4.2% using the system with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). We present a useful basis on the structure (orientation)–property (OPV output) relationship to lay down new guidelines for the design of efficient solar cell materials.

Optical and electrical properties of dithienothiophene based conjugated polymers: medium donor vs. weak, medium, and strong acceptors

Herein we report the systematic study of the structure–property relationship of a few dithienothiophene (DTT) based donor–acceptor conjugated polymers using various techniques such as UV-vis absorption and fluorescence spectroscopy, cyclic voltammetry (CV), flash-photolysis time-resolved microwave conductivity (FP-TRMC), density functional theory (DFT), X-ray diffraction (XRD), and field-effect transistor (FET). A medium donor, DTT, was coupled in an alternating fashion with thiazole-based weak, medium, and strong acceptors. Though the optical properties showed good correlation with the donor–acceptor strength, the FET properties indicated significant deviation. The XRD analysis and DFT calculations revealed that the deviation is caused by the difference in structural ordering of the polymers in the film state. Since the FP-TRMC analysis reflects the properties of semiconducting organic materials at the molecular level such as the donor–acceptor strength and structural ordering in the film state, it showed good correlation with FET properties. Thus the present work illustrates that the study of charge carrier generation and mobility dynamics by FP-TRMC is a valuable addition to the conventional structure–property analysis methods, and is reliable to find the suitability of conjugated polymers for electronic device applications.