Efficient Charge Transport Driven by Strong Intermolecular Interactions in Cyclopentadithiophene‐Based Donor–Acceptor Type Conjugated Copolymers

Abstract

as switching devices for innovative electronic applications such as flexible and stretchable electronics. [1][2][3][4] Mobilities in the early stages of OFET development were poor, with polyacetylene having a fieldeffect mobility of only ≈10 −5 cm 2 V −1 s −1 . However, since the development of thiophene-based semicrystalline polymer semiconductors (e.g., regioregular poly(3hexylthiophene-2,5-diyl) (rr-P3HT), poly(3,3'-dialkyl-quaterthiophene) (PQT), and poly(2,5-bis(3-alkylthiophen-2-yl) thieno[3,2-b]thiophene) (PBTTT)) and then donor-acceptor (D-A) type conjugated copolymers with alternating electron-donating and electron-accepting moieties along the polymer backbone, several research groups have reported high-performance semiconducting polymers with field-effect mobilities in excess of 10 cm 2 V −1 s −1 which is comparable to that of their soluble oxide-based inorganic semiconductors counterparts. [5,6] Indeed, the D-A type conjugated copolymers are intriguing materials because they possess superior mobility values to the thiophene-based semi-crystalline homopolymers, despite exhibiting somewhat amorphous-like structural characteristics. This means that their structure-property relationships may not follow the trend of higher mobilities requiring higher degrees of semicrystalline molecular order Understanding charge transport in π-conjugated polymers is critical for the design of high-mobility polymers. However, charge transport in donor-acceptor (D-A) type copolymers and the roles of D and A units are still unclear. Herein, the charge transport properties of three cyclopentadithiophene (CDT)-based D-A type copolymers with CDT as a common donor and benzothiadiazole, fluoro-2,1,3-benzothiadiazole, or pyridyl-2,1,3-thiadiazole (PTz) as an acceptor are investigated. The best charge transport is found for CDT-PTz showing 0.79 ± 0.01 cm 2 V −1 s −1 hole mobility in field-effect transistors, with the lowest energetic disorder by temperature-dependent mobility measurements. Using various structural probes and density functional theory simulations, it is found that CDT-PTz has the most rigid and planar backbone structure and the strongest intramolecular dipole moment thanks to the noncovalent N-S interactions and the strong electron-withdrawing PTz acceptor unit. These structural properties lead to strong intermolecular interactions in CDT-PTz, reducing its π-π stacking distance, producing strong aggregation even in hot dilute solutions. The enhanced mobility in CDT-PTz is attributed to this high quality π-π stacking, driven by increased backbone rigidity and strong molecular dipoles. This work suggests that rigidifying the copolymer backbone with enhanced push-pull strength of the D-A units is a key requirement for high-mobility D-A copolymers.

Ambipolar Charge Transport in p-Type Cyclopentadithiophene-Based Polymer Semiconductors Enabled by D–A–A–D Configuration

“…The highest hole and electron mobilities of P1 were 1.50 and 0.41 cm 2 V −1 s −1 , respectively, which were approximately twice greater than those of a previously reported D−A-type PCDTPT. 28,29 The P1 devices exhibited hysteresis-free output characteristics (Figure 3e,f), which outperformed previously reported ambipolar OFETs based on PCDTPT (Table S5). 28 The enhanced electron mobilities and hysteresis-free transistor characteristics demonstrated improved electron transport in the D−A−A−D-type P1 thin films compared with that in the D−A-type PCDTPT.…”

“…Compared with those of its D–A-type counterpart based on the CDT–PT configuration (hereafter, PCDTPT), P1 exhibited improved electron and hole mobilities (0.41 and 1.50 cm 2 V –1 s –1 , respectively) with hysteresis-free transistor characteristics. These values are more than twice to those of the conventional PCDTPT. , Owing to the improved ambipolar charge transport characteristics of P1 , the CMOS inverters fabricated with the P1 charge transporting layers exhibited a high gain greater than 165. Given the p -type-dominant transistor characteristics of PCDTPT reported previously, , the improved ambipolar charge-transport characteristics of P1 enabled by the simple reconfiguration of the existing D and A moieties was an interesting result.…”

Low dark current density in organic photodiodes achieved by reduced trap states of conjugated polymers bearing a fluorinated moiety

“…This eventually causes a reduced trap DOS distribution in the D–A type conjugated copolymer film employed as the photoactive layer in the OPDs. 42…”

Tuning Charge Transport of Oligopeptide Junctions via Interfacial Amino Acids†

“…[ 33 ] The effects of other interactions on intermolecular charge transport have been subsequently investigated. [ 34‐40 ] For example, efficient intermolecular charge transport in π‐stacked pyridinium dimers was observed. [ 39 ] Recently, the introduction of dipole‐bearing moieties enhanced the π–π stacking between azulene‐based single molecules, leading to higher electrical conductivity.…”