Poly(2,5‐bis(2‐thienyl)‐3,6‐dialkylthieno [3,2‐b]thiophene)s—High‐Mobility Semiconductors for Thin‐Film Transistors

Abstract

[1] represent a promising class of polymer semiconductors for solution-processed thin-film transistors (TFTs), [2][3][4][5][6] an emerging technology that has received phenomenal interest as a lowcost alternative to silicon-based TFTs to enable ubiquitous, large-area, flexible electronics. In general, low-cost TFT fabrication requires a noninert processing environment, which necessitates the semiconductors to possess a certain level of air stability, particularly resistance against photoinduced oxidative doping. From this perspective, several regioregular polythiophenes with improved air stability have been developed, [6][7][8] potentially enabling low-cost TFT manufacturing.The sensitivity of regioregular polythiophenes to photo-oxidative doping is a consequence of photoexcitation, leading to charge transfer to atmospheric oxygen and, hence, charge-carrier generation. [3,9] This results in increased semiconductor conductivity, and thus degraded semiconductor properties. This difficulty has recently been overcome by a simple structural design, schematically represented by structure A (Fig. 1). The effective p-conjugation length of the polythiophene system is curtailed by a conjugating moiety, M, suppressing its sensitivity by lowering its highest occupied molecular orbital (HOMO) level.[6] Thus, we observe that with a proper choice of M such as unsubstituted bithienylene as in poly(didodecylquarterthiophene) (PQT) 1, [6c] high-mobility TFTs can be fabricated under ambient conditions. Very recently, other members of this class have also been developed as functionally capable semiconductors. [7,8] Specifically, poly(2,5-bis(3-dedecyl-thiophen-2-yl)thieno[3,2-b]thiophene) was reported to give a mobility of 0.2-0.6 cm 2 V -1 s -1 , albeit under inert atmosphere.[10] This latest result has prompted us to report our independent study of another novel system, schematically described by structure B, in which the alkyl side chains are bound to M to optimize its processability, molecular organization, and electronic properties in the solid state. We show herein, through poly(2,5-bis(2-thienyl)-3,6-dialkylthieno[3,2-b]-thiophene), 2, that this class of polythiophenes affords high mobility at low processing temperatures under ambient conditions. The synthesis of poly(2,5-bis(2-thienyl)-3,6-dipentadecylthieno[3,2-b]thiophene) 2 is described in Scheme 1. 2,5-Dibromo-3,6-dipentadecylthieno[3,2-b]thiophene 3 was prepared from 3,4-dibromothiophene in good yields. Stille coupling of 3 with 2-tributylstannylthiophene, followed by bromination, gave 2,5-bis(2-bromo-thienyl)-3,6-dipentadecylthieno[3,2-b]thiophene 5 in ca. 60 % yield.[11] Subsequent dehalogenative coupling polymerization was carried out in the presence of a slight excess of Ni(COD) 2 (COD: 1,5-cyclooctadiene) and a catalytic amount of 2,2′-dipyridil in refluxing toluene. Soxhlet extraction of the crude product to remove oligomeric materials yielded 2 as a dark red solid in 92 % yield. High-temperature gel permeation chromatography (GPC) analysis gave a number-average...

Semiconducting Thienothiophene Copolymers: Design, Synthesis, Morphology, and Performance in Thin‐Film Organic Transistors

“…Increase in the alkyl chain length within the same polymer backbone series, typically had the most effect on the first melting transition, but also slightly [67] (see also [56] for C 9 H 19 ) -n.a. 1 Â 10 À6 4 [68] 5.00 0.30 5 [68] (see also [69] for C 15 H 31 and [34] for C 9 H 19 ))…”

“…[56] Acylation with decanoyl chloride in the presence of aluminum chloride afforded 2-decanoyl-3,4-dibromothiophene in good yield. Subsequent reaction with ethyl 2-sulfanylacetate under basic conditions afforded the mono-alkylated thieno [ [69] We developed an alternative synthesis from 2-thieno [3,2-b]thiophene-carboxylic acid, which is readily prepared as outlined in Scheme 1. [67,68] Treatment of this carboxylic acid with excess bromine in acetic acid affords tetrabromo thieno [3,2-b]thiophene, through a combination of bromo-decarboxylation and bromination of the unsubstituted aromatic positions (Scheme 2).…”

“…Ong and co-workers reported an alternative synthesis of poly(3,6-dipentadecylthieno[3,2-b]thiophene-co-bithiophene) (Scheme 4). [69] In this case a symmetrical monomer was constructed from the Stille cross-coupling of 2,5-dibromo-3,6-dipentadecylthieno[3,2-b]thiophene with 2-tri(butyl)stannylthiophene followed by bromination in the free a positions with N-bromosuccinimide. Dehalogenative polymerization (Yamamoto polymerization [93,94] ) was performed by refluxing in toluene in the presence of excess bis(cyclooctadiene)nickel (0) and 2,2 0 -bipyridyl, to afford the product in 92% yield.…”

Solvent‐Assisted Re‐annealing of Polymer Films for Solution‐Processable Organic Field‐Effect Transistors

“…[8] It has been demonstrated by different groups that many excellent polymers including regioregular poly(3-hexylthiophene) (P3HT), poly(2,5-bis(3-alkylthiophen-2-yl)thieno [3,2-b]thiophene (PBTTT), poly- (3,3 0 -didodecylquaterthiophene) (PQT-12), and cyclopentadithiophenebenzothiadiazole copolymer (CDT-BTZ) possess high field-effect mobility. [4,[8][9][10][11]14,23] Despite these achievements, the mobility of polymer FETs is still lower than those of smallmolecule semiconductor-based OFETs. [24] In addition, the device stabilities remain to be improved to meet the requirement of actual application.…”

The Influence of Morphology on High‐Performance Polymer Field‐Effect Transistors

“…[5][6][7] In the case of organic semiconductors, a main challenge is to reduce the number of trapping or scattering sites, as they inhibit charge-carrier transport. Structural defects within the organic semiconductor layer due to poor molecular self-assembly are one of the most important origins of such traps.…”