Itaru Osaka

Advances in Molecular Design and Synthesis of Regioregular Polythiophenes

Regioregular poly(3-alkylthiophene)s (rrP3ATs) are an important class of pi-conjugated polymers that can be used in plastic electronic devices such as solar cells and field-effect transistors. rrP3ATs can be ordered in three dimensions: conformational ordering along the backbone, pi-stacking of flat polymer chains, and lamellar stacking between chains. All of these features lead to the excellent electrical properties of these materials. Creative molecular design and advanced synthesis are critical in controlling the properties of the materials as well as their device performance. This Account reports the advances in molecular design of new functional polythiophenes as well as the associated polymerization methods. Many functionalized regioregular polythiophenes have been designed and synthesized and show fascinating properties such as high conductivity, mobility, chemosensitivity, liquid crystallinity, or chirality. The methods for the synthesis of rrP3ATs are also applicable to other functional side chains. Di- and triblock copolymers consisting of rrP3AT and polyacrylate or polystyrene have also been successfully synthesized, which can facilitate the assembly of the polythiophene segments. The synthesis of rrP3ATs has evolved into a simple and economical system in which the synthesis can be carried out quickly at room temperature and is thus suitable for large-scale manufacturing. Intensive study has revealed that the regioregular polymerization of 3-alkylthiophenes proceeds by a chain-growth mechanism and can be made into a living system. This feature enables precise control of the molecular weight and facile end-group functionalization of the polymer chains, leading to tailor-made regioregular polythiophenes for specific applications. In addition, researchers have recently designed and synthesized regiosymmetric polythiophenesthese are regioregular but not coupled in a head-to-tail fashionby various methods. These reports indicate that these regiosymmetric polymers show very high mobilities when used in field-effect transistors due to their highly ordered structure. The remarkable performance of regioregular polythiophenes in recent years has allowed for the rapid development in printable electronics and seems destined to lead to further advances in this field.

Thienoacene-based organic semiconductors.

Thienoacenes consist of fused thiophene rings in a ladder-type molecular structure and have been intensively studied as potential organic semiconductors for organic field-effect transistors (OFETs) in the last decade. They are reviewed here. Despite their simple and similar molecular structures, the hitherto reported properties of thienoacene-based OFETs are rather diverse. This Review focuses on four classes of thienoacenes, which are classified in terms of their chemical structures, and elucidates the molecular electronic structure of each class. The packing structures of thienoacenes and the thus-estimated solid-state electronic structures are correlated to their carrier transport properties in OFET devices. With this perspective of the molecular structures of thienoacenes and their carrier transport properties in OFET devices, the structure-property relationships in thienoacene-based organic semiconductors are discussed. The discussion provides insight into new molecular design strategies for the development of superior organic semiconductors.

Synthesis, Characterization, and Transistor and Solar Cell Applications of a Naphthobisthiadiazole-Based Semiconducting Polymer

We report the synthesis and characterization of a novel donor-acceptor semiconducting polymer bearing naphthobisthiadiazole (NTz), a doubly benzothiadiazole (BTz)-fused ring, and its applications to organic field-effect transistors and bulk heterojunction solar cells. With NTzs highly π-extended structure and strong electron affinity, the NTz-based polymer (PNTz4T) affords a smaller bandgap and a deeper HOMO level than the BTz-based polymer (PBTz4T). PNTz4T exhibits not only high field-effect mobilities of ~0.56 cm(2)/(V s) but also high photovoltaic properties with power conversion efficiencies of ~6.3%, both of which are significantly high compared to those for PBTz4T. This is most likely due to the more suitable electronic properties and, importantly, the more highly ordered structure of PNTz4T in the thin film than that of PBTz4T, which might originate in the different symmetry between the cores. NTz, with centrosymmetry, can lead to a more linear backbone in the present polymer system than BTz with axisymmetry, which might be favorable for better molecular ordering. These results demonstrate great promise for using NTz as a bulding unit for high-performance semiconducting polymers for both transistors and solar cells.

High-Lamellar Ordering and Amorphous-Like π-Network in Short-Chain Thiazolothiazole−Thiophene Copolymers Lead to High Mobilities

Owing to their superior transport properties, poly(alkylthiophenes) and their derivatives emerged as one of the most widely studied semiconducting polymers with potential applications in organic electronics. It is now generally acknowledged that one of the particularly effective ways to increase the carrier mobility in these materials is by increasing the length of the conjugated backbones. Some recent reports suggest also that carrier mobilities can be further enhanced by highly crystalline arrangement (and interdigitation) of alkyl side chains possibly because it promotes the formation of extensive layered structures favorable for carrier transport. Results presented here demonstrate that, surprisingly, none of these factors are actually necessary for good electronic performance of polythiophene-like systems. Thiophene-based semiconducting polymers bearing thiazolothiazole unit (PTzQT) described here were shown to have very high carrier mobilities (approximately 0.3 cm(2)/Vs) despite their low molecular weight and uneven spacing of alkyl side chains, which suppressed high side chain crystallinity/interdigitation as revealed by thermal analysis and X-ray scattering. The highly disordered nature of these materials extended to the nanoscale level as evident from atomic force microscopy images, which have shown only the presence of small domains packed into isotropic amorphous-like superstructures with lateral correlation lengths increasing with the length of alkyl side chains. The observed concomitant increase of carrier mobilities points to the possible role of characteristic length of surface roughness as the key parameter controlling carrier transport in disordered, noninterdigitating systems.

Thiophene–Thiazolothiazole Copolymers: Significant Impact of Side Chain Composition on Backbone Orientation and Solar Cell Performances

The backbone orientation in the thiophene-thiazolothiazole (TzTz) copolymer system can be altered by tuning of the alky side chain composition. We highlight that the orientation significantly impact their solar cell efficiency in particular when using thicker active layers.

High-efficiency polymer solar cells with small photon energy loss

A crucial issue facing polymer-based solar cells is how to manage the energetics of the polymer/fullerene blends to maximize short-circuit current density and open-circuit voltage at the same time and thus the power conversion efficiency. Here we demonstrate that the use of a naphthobisoxadiazole-based polymer with a narrow bandgap of 1.52 eV leads to high open-circuit voltages of approximately 1 V and high-power conversion efficiencies of ∼9% in solar cells, resulting in photon energy loss as small as ∼0.5 eV, which is much smaller than that of typical polymer systems (0.7–1.0 eV). This is ascribed to the high external quantum efficiency for the systems with a very small energy offset for charge separation. These unconventional features of the present polymer system will inspire the field of polymer-based solar cells towards further improvement of power conversion efficiencies with both high short-circuit current density and open-circuit voltage.

Drastic Change of Molecular Orientation in a Thiazolothiazole Copolymer by Molecular-Weight Control and Blending with PC61BM Leads to High Efficiencies in Solar Cells

A thiazolothiazole-thiophene copolymer is examined as the active material in bulk heterojunction (BHJ) solar cells. By optimizing the molecular weight, the polymer-based cells exhibit power conversion efficiencies as high as 5.7%. The increase in molecular weight improves the orientational order, and blending with phenyl-C61-butyric acid methyl ester (PC61BM) changes the orientational motif from edge-on to face-on, which accounts for the trend in photovoltaic performances. These results might give new insight into the structure-property relationships in BHJ solar cells.

Benzobisthiazole-based semiconducting copolymers showing excellent environmental stability in high-humidity air.

Scheme 1 . Chemical structures of the thiazolothiazole-thiophene copolymers. Printed organic fi eld-effect transistors (OFETs) are of particular interest today since they enable large area fl exible displays or ubiquitous cheap electronics, and thus greatly differentiate from silicon technology. [ 1 , 2 ] Use of semiconducting polymers, which offer excellent solution-processability, fi lm uniformity and thermal stability, as an active layer for OFETs is a promising approach to develop and commercialize such devices. [ 1 , 3 , 4 ]

Naphthodithiophenes as building units for small molecules to polymers; a case study for in-depth understanding of structure–property relationships in organic semiconductors

In this Feature Article, we report a family of organic semiconductors based on isomeric naphthodithiophenes (NDTs). The small-molecule- and polymer-based organic semiconductors exhibit field-effect mobilities as high as 1.5 cm2 V−1 s−1 and 0.77 cm2 V−1 s−1, respectively, revealing that NDTs are useful building units for organic semiconductors. We also highlight their structure–property relationships in depth by focusing on the HOMO geometry as well as the packing structure, which provide a clear understanding of how the core structures influence the properties of the resulting materials and thus provide new insight into the design of high-performance organic semiconductors.

Novel dibenzo[a,e]pentalene-based conjugated polymers

Dibenzo[a,e]pentalene (DBP), a ladder-type fused-ring system with 16π anti-aromatic nature, is integrated into conjugated polymers with oligothiophene building blocks to examine the potential of DBP as a new building block for semiconducting polymers. Depending on the incorporation manner of the DBP unit in the polymer backbone, via the 5,10- or 2,7-positions, the polymers show distinct colours, reflecting the different electronic structures, though the HOMO and LUMO energy levels estimated from cyclic voltammograms are almost the same. Interestingly, the impact of the incorporation manner was observed in the characteristics of their field-effect transistors (FETs). For PDTDBP2Ts, in which the DBP units are integrated into the polymer backbone via the 5,10-positions, the DBP units behave like a “dibenzo-annulated 1,3-butadiene” moiety, and their FET characteristics are strongly affected by orientational ordering and crystallinity, similar to ordinary “donor-only” polymers such as P3HT. On the other hand, iPDTDBP2Ts, in which the whole 16π DBP unit is integrated into the polymer backbone via the 2,7-positions, behave like a certain kind of donor-acceptor polymers, and the FET characteristics are independent of orientational order: the field-effect mobilities are higher than 0.1 cm2 V−1 s−1 regardless of the polymer orientation in the thin film. From these results, we can recognize the 16π anti-aromatic DBP unit as a useful building block with transmutable nature for the development of new conjugated polymers.