Highly Disordered Polymer Field Effect Transistors: N-Alkyl Dithieno[3,2-b:2′,3′-d]pyrrole-Based Copolymers with Surprisingly High Charge Carrier Mobilities

Abstract

A series of novel electroactive and photoactive conjugated copolymers based on N-alkyl dithieno[3,2-b:2',3'-d]pyrroles (DTP) and thiophene (TH) units (DTP-co-THs) were synthesized using a Stille coupling reaction and exhibited molecular weights of 1.6 x 10(4) to 5.0 x 10(4) g/mol. The incorporation of soluble substituted thiophenes and planar DTP units resulted in low band gap, highly conductive polymers. DTP-co-THs exhibited excellent solubility in common organic solvents and formed high-quality films. Optical characterization revealed that the band gaps of DTP-co-THs were between 1.74 and 2.00 eV, lower than regioregular poly(3-alkylthiophenes). Electrochemical characterization showed that the HOMO energy levels of DTP-co-THs are between -4.68 and -4.96 eV. When doped, DTP-co-THs exhibited high conductivities up to 230 S/cm with excellent stability. The different thiophene substituent patterns' effect on the polymers' optical and electronic properties was then examined by density functional theory computations. The microstructure and surface morphologies of poly(2,6-(4-dodecyl-4H-bisthieno[3,2-b:2',3'-d]pyrrole)-random-2,5-(3-dodecylthiophene)) (P4) and poly(2-(4,4'-didodecyl-2,2'-bithiophen-5-yl)-4-octyl-4H-bisthieno[3,2-b:2',3'-d]pyrrole) (P6) thin films were studied by X-ray diffraction and atomic force microscopy. As-cast P4 and P6 thin films exhibited poorly defined, randomly ordered lamellar structure that improved significantly after thermal annealing. Field effect transistor devices fabricated from P4 and P6 showed typical p-channel transistor behavior. Interestingly, the mobilities of as-cast, less ordered samples were much higher than those observed after annealing. The highest values of maximum and average mobilities were observed for the polymer P6 as-cast (0.21 and 0.13 cm(2) V(-1) s(-1), respectively). One of our goals was to test the idea that high mobility and excellent electrical and structural reproducibility could perhaps be achieved by the creation of amorphous pi-conjugated materials that could possess long arrange pi connectivity on the microscopic scale. The results of these studies strongly suggest that the presence of highly ordered microcrystalline structures in thin films of organic semiconductors is not necessary for excellent performance of organic transistors.

Structure and Morphology Control in Thin Films of Conjugated Polymers for an Improved Charge Transport

“…A excess torsion could lead to a less ordered morphology and hamper the efficient transport of carriers, as reported by Park et al for a polythiophene with a larger density of thiophene side-chain and a lower mobility of 5.1 × 10 ( Figure 9I) [115]. Besides, the steric effect arising from the head-head linkage of alkyl thiophenes in backbones results in a huge torsion (Figure 13c), which destroys the planarity and conjugation of chains and leads to a bad carrier mobility [50]. Therefore, a high regio-regularity (fewer head-head linkages) is wanted for polythiophenes and their derivatives.…”

“…As an intrinsic property rather than doping, the amount of crystalline regions in films also plays an important role in the transport of charge carriers via determining the amount of transport pathways in the scale of devices for (semi-) crystalline conjugated polymers which contribute a large proportion in the catalogue of conjugated polymers, although several amorphous conjugated polymers such as dithieno [3,2- [50,51]. More crystallites in films means more efficient pathways for carriers, and fewer disordered regions and/or grain boundaries limiting mobility in films.…”

“…Thus, increasing the crystallinity is an important strategy to improve the morphology and electronic properties of conjugated polymer films. For those amorphous conjugated polymers with high carrier mobilities, the origin was attributed to the presence of long arrange π-conjugated pathways in microscopic scale [50] or the low bandgap combined with highly connected and compact structures indicated by a low surface roughness [51], as mentioned in Section 2.2.…”

Design and synthesis of indole-substituted fullerene derivatives with different side groups for organic photovoltaic devices

“…The similar changing trend was found in the cases of OIBC60P and OIB2FC60P with Dh values as 3.7 and 2.6, respectively. The results indicated that OIBC60P and OIB2FC60P had good miscibility with P3HT in the non-annealed blend films, and the crystallinity of the films increased greatly after annealing treatment [45,46]. However, the situation was quite different in the case of OIHC60P, a stronger (100)-P3HT peak was observed before the blend film was annealed compared to that of PCBM in the same condition, which indicated that the crystallinity in the blend film of P3HT and OIHC60P was stronger [47].…”

Effects of Molecular Structure on Intramolecular Charge Carrier Transport in Dithieno [3,2-b: -d] Pyrrole-Based Conjugated Copolymers

“…Dithieno [3,2-b: 2 ,3 -d] pyrrole (DTP) has been expected as a new class of π-conjugated unit in organic electronics [20][21][22][23][24][25][26]. DTP unit possesses a large planarity of the rigid and fused ring structure, leading to a small perturbation of effective conjugation length, where the low reorganization energy of molecules can facilitate intermolecular hopping process of charge carriers.…”