Structurally Ordered Polythiophene Nanoparticles for High‐Performance Organic Thin‐Film Transistors

Abstract

Solution-processable organic semiconductors are potentially low-cost alternatives to mainstream amorphous-silicon materials for electronics applications.[1±6] Organic thin-film transistors (OTFTs) fabricated using solution-deposition techniques (e.g., spin-coating, screen printing, inkjet printing) are particularly well-suited for large-area devices (e.g., active-matrix displays) where high switching speeds are not essential; they may also be attractive for low-end electronic devices (e.g., radio frequency identification tags) where the high cost of packaging silicon circuits becomes limiting. For meaningful practical applications, these organic semiconductors need to provide field-effect transistor (FET) mobilities close to that of amorphous silicon. This will necessitate establishment of proper molecular order in the semiconductors to achieve high mobilities, [2,3] since charge-carrier transport in organic semiconductors is dominated by hopping, [6] and disordered materials are not efficient charge-transporting media. However, molecular ordering in solution-processed semiconductors, which relies primarily on molecular self-organization from solution after deposition, is often not facile. [2,7] Several approaches such as controlled solvent evaporation, [8] use of the directive influence of substrate surface chemistry, [9] and post-deposition annealing [10] were found to improve molecular ordering in several solution-deposited organic semiconductor systems. Molecular self-organization in solution depends on molecular structural features as well as concentration. Molecular aggregation in solution occurs at the critical saturation concentration where the solvency of the medium is reduced. This can be achieved through solvent evaporation, lowering the temperature, addition of a non-solvent, or a combination of all three. Solvatochromism and thermochromism of conjugated polymers, such as regioregular polythiophenes, to form polymer aggregates in solution, are two illustrative examples of solubility and temperature effects, respectively.[11±13] It should therefore be possible to utilize these solution phenomena to pre-establish desirable molecular organization in the semiconductor materials before deposition to enhance their chargetransport capability. We present here our studies of molecular self-assembly behavior of poly(3,3--dialkylquarterthiophene)s 1 (PQTs) [14] in solution, and our ability to capitalize on the unique molecular property of PQT for the preparation of structurally ordered semiconductor nanoparticles for highperformance OTFTs.In hot, dilute dichlorobenzene solutions (> 50 C), PQT-12, 1a (1, R = n-C 12 H 25 ) showed a broad peak at k max~4 80 nm in its UV-vis absorption spectrum (Fig. 1, trace a), but the absorption was slightly red-shifted with concomitant appearance of a weak absorption at k max~6 10 nm (Fig. 1, trace b) when the solution was cooled down to room temperature. This was obviously due to the migration of PQT-12 from the twisted disordered conformation in hot solution to a more ordered ...

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

“…Firstly, extending the conjugation system and strengthening intra-chain and inter-chain interaction is a direct way to improve electronic properties. Based on the classic conjugated polymers such as P3ATs (P1), a straight strategy is inserting more conjugated moieties [3,82] such as carbon double bond (-HC=CH-, P2) [83][84][85], unsubstituted multiple or fused aromatic rings (i.e., thiophenes and thieno[3,2-b]thiophene, etc., P3-P7) [13,14,16,37,82,[86][87][88][89][90][91], other heterocyclic rings containing nitrogen(and sulfur) (P8 and P9) or selenium atoms (P10) [13,15,81,[92][93][94][95][96][97] and their combination (P11) [98]. Thus, the intra-chain conjugation is extended and strengthened by coupling with the more conjugated moieties, and the conjugation length is elongated due to the improved planarity.…”

Organic Semiconductor Nanoparticle Film: Preparation and Application

“…31,63 Poly(3,3'''-dialkylquarterthiophene)s (PQTs) nanoparticles have been explored for using in OFETs. 64 As expected, the presence of lamellar stacking order in the nanoparticles can be verified by XRD and UV spectrum data. OFETs based on PQT nanoparticles show a 50 % improvement in mobility on bare SiO 2 dielectric layer and an order of magnitude improvement in mobility on surface modified SiO 2 dielectric layer relative to those based on normal films.…”

Organic semiconductors for device applications: current trends and future prospects

“…To alleviate these problems, side chains were, very often, added to improve solubility by distorting the molecular conjugation as a result. For example, in PQT-12, the presence of solubilizing side chains on the first and fourth thiophene rings [121][122][123] leaving two thiophenes without side chains created a slight twist in the molecular chain that did not disturb the conjugation much but otherwise created a larger band-gap, making it very stable under ambient conditions while still retaining higher mobility. Such stable PQT-12 films exhibited mobility of 0.07 to 0.18 cm 2 /Vs.…”