Cyclopentadithiophene-Benzothiadiazole Donor-Acceptor Polymers as Prototypical Semiconductors for High-Performance Field-Effect Transistors.

Donor-acceptor (D-A) conjugated polymers are of great interest as organic semiconductors, because they offer a rational tailoring of the electronic properties by modification of the donor and acceptor units. Nowadays, D-A polymers exhibit field-effect mobilities on the order of 10-10 cm V s, while several examples showed a mobility over 10 cm V s. The development of cyclopentadithiophene-benzothiadiazole (CDT-BTZ) copolymers one decade ago represents an important step toward high-performance organic semiconductors for field-effect transistors. The significant rise in field-effect mobility of CDT-BTZ in comparison to the existing D-A polymers at that time opened the door to a new research field with a large number of novel D-A systems. From this point, the device performance of CDT-BTZ was gradually improved by a systematic optimization of the synthesis and polymer structure as well as by an efficient solution processing into long-range ordered thin films. The key aspect was a comprehensive understanding of the relation between polymer structure and solid-state organization. Due to their fundamental role for the field of D-A polymers in general, this Account will for the first time explicitly focus on prototypical CDT-BTZ polymers, while other reviews provide an excellent general overview on D-A polymers. The first part of this Account discusses strategies for improving the charge carrier transport, focusing on chemical aspects. Improved synthesis as an essential stage toward high purity, and high molecular weight is a prerequisite for molecular order. The modification of substituents is a further crucial feature to tune the CDT-BTZ packing and self-assembly. Linear alkyl side chains facilitate intermolecular π-stacking interactions, while branched ones increase solubility and alter the polymer packing. Additional control over the supramolecular organization of CDT-BTZ polymers is introduced by alkenyl substituents via their cis-trans isomerization. The last discussed chemical concept is based on heteroatom variation within the CDT unit. The relationships found experimentally for CDT-BTZ between polymer chemical structure, solid-state organization, and charge carrier transport are explained by means of theoretical simulations. Besides the effects of molecular design, the second part of this Account discusses the processing conditions from solution. The film microstructure, defined as a mesoscopic domain organization, is critically affected by solution processing. Suitable processing techniques allow the formation of a long-range order and a uniaxial orientation of the CDT-BTZ chains, thus lowering the trapping density of grain boundaries for charge carriers. For instance, alignment of the CDT-BTZ polymer by dip-coating yields films with a pronounced structural and electrical anisotropy and favors a fast migration of charge carriers along the conjugated backbones in the deposition direction. By using film compression with the assistance of an ionic liquid, one even obtains CDT-BTZ films with a band-like transport and a transistor hole mobility of 10 cm V s. This device performance is attributed to large domains in the compressed films being formed by CDT-BTZ with longer alkyl chains, which establish a fine balance between polymer interactions and growth kinetics during solvent evaporation. On the basis of the prototypical semiconductor CDT-BTZ, this Account provides general guidelines for achieving high-performance polymer transistors by taking into account the subtle balance of synthetic protocol, molecular design, and processing.