Chih-Jung Lin

Biaxially extended quaterthiophene-thiophene and -selenophene conjugated polymers for optoelectronic device applications

New biaxially extended quaterthiophene (4T) conjugated polymers, including poly(5,5′′′-di-(2-ethylhexyl)[2,3′;5′,2′′4′′,2′′′]quaterthiophene) (P4T) and their copolymers with thiophene(P4TT), bithiophene (P4T2T), selenophene(P4TSe) and biselenophene (P4T2Se) were synthesized by Stille coupling reactions under microwave heating. The effects of the ring number of thiophene and selenophene moieties on the physical properties and polymer structures were systematically investigated experimentally and theoretically. With the increased ring number of the unsubstituted thiophene and selenophene moieties, the band gaps and the main-chain torsional angles were reduced. However, the side-chain torsional angles were increased with increasing the ring number, and thus significantly affected the carrier transporting characteristics. Among these studied conjugated polymers, the field-effect transistor (FET) based on P4TSe showed the highest hole mobility of up to 4.28 × 10−2 cm2 V−1 s−1 and an on/off ratio of 1.12 × 104. The photovoltaic device prepared from P4TSe/PC71BM exhibited the highest power conversion efficiency (PCE) of 2.6%, which resulted from more balanced hole/electron mobility and a smaller band gap. The above results revealed that the conformation, charge-transporting and optoelectronic device characteristics of biaxially extended 4T-based conjugated copolymers could be manipulated by incorporating the heteroaromatic ring spacer.

Highly ordered luminescent microporous films prepared from crystalline conjugated rod-coil diblock copolymers of PF-b-PSA and their superhydrophobic characteristics

We have successfully prepared three crystalline conjugated rod–coil diblock copolymers of poly[2,7-(9,9-dihexylfluorene)]-block-poly(stearyl acrylate) (PF-b-PSA), PF7-b-PSA64, PF7-b-PSA93, and PF7-b-PSA166, via the combination of a Suzuki coupling reaction and atom transfer radical polymerization. The crystalline comb-like PSA coil segment effectively formed the highly ordered microporous films through a “breath figure” (BF) process. On the other hand, the blue-emission band of the PF block was blue-shifting and became narrower as the PSA block was increased. The effects of block ratio, humidity, and solution concentrations on the porous structures were studied. The longer PSA block length, larger humidity, and higher copolymer concentration formed a more regular PF-b-PSA microporous structure. The images of SEM and scanning laser confocal microscope of the PF7-b-PSA166 microporous film showed that all bubbles were independent and perfectly monodisperse with a pore diameter ca. 1.85 μm. Furthermore, a rod-co-valley-like structure exhibited superhydrophobicity (contact angle up to 163 ± 0.3°), which was obtained from the peeled skin layer of the ordered PF7-b-PSA166 microporous surface. The experimental results revealed that multifunctional ordered microporous films could be successfully prepared from crystalline conjugated rod–coil block copolymers.

Synthesis and characterization of novel polythiophenes with graphene-like structures via intramolecular oxidative coupling

Novel polythiophenes with graphene structures (GPTs) could be synthesized based on intramolecular oxidative coupling of the polythiophene precursors having phenyl groups at the 3-position, poly(3-(4′-(3′′,7′′-dimethyloctoxy)phenyl)thiophene) (P3PhT) and poly(3-hexylthiophene)-b-P3PhT (P3HT-b-P3PhT), using FeCl3 as an oxidative agent. The GPTs were characterized by 1H NMR spectroscopy, UV-vis spectroscopy, FT-IR spectroscopy, cyclic voltammetry and X-ray diffraction. The thin film transistor (TFT) characteristics were also performed for P3HT-b-GPT. The progression of oxidative coupling of P3PhTs was confirmed by 1H NMR and FT-IR spectroscopies. Moreover, GPTs showed significant red-shifts in the UV-vis spectra due to their high coplanarity, extended π-conjugation length, and strong π-stacking formation. P3HT-b-GPT also showed a red-shift in the UV-vis spectra and a reduced band gap as compared to the pristine P3HT. P3HT-b-GPT exhibited hole mobility (∼10−5 cm2 V−1 s−1) which is comparable to that of pristine P3HT having a similar molecular weight.

Nonvolatile organic field effect transistor memory devices using one-dimensional aligned electrospun nanofiber channels of semiconducting polymers

We report the nonvolatile memory characteristics of organic field effect transistors (OFETs) using one-dimensional aligned electrospun (ES) nanofibers of semiconducting poly(9,9-dioctyl-fluorene-co-bithiophene) (F8T2). The effects of the nanofiber diameter associated with the polymer chain orientation on the charge transport and storage ability were explored. The OFET devices using the F8T2 ES nanofibers exhibited a large average memory window of ∼30 V, an on–off ratio of 102–103 and a hole mobility of 10−4 to 10−2 cm2 V−1 s−1. The write, erase and read processes of the memory device were performed by voltages across the nanofiber channels for at least 100 cycles and could be stabilized for at least 1000 s. The reversible hysteresis characteristics were attributed to the shallow trapping in the ordered/disordered domains of the F8T2 nanofibers from the in situ grazing incidence wide angle X-ray scattering (GIWAXS) analysis. Our results suggest that the semiconducting ES nanofibers could have potential applications for high performance OFET-based nonvolatile organic memory devices.

Poly(3-hexylthiophene)–graphene composite-based aligned nanofibers for high-performance field effect transistors

We report the morphology and field effect transistor (FET) characteristics of aligned electrospun nanofibers prepared from poly(3-hexylthiophene) (P3HT)–graphene composites. The graphene flakes were more uniformly distributed in the nanofibers compared with spin-coated films, leading to different FET characteristics. The geometrical confinement from electrospun nanofibers resulted in enhanced π–π molecular packing with highly ordered orientation and reduced the grain boundaries under strong stretching forces, thereby increasing carrier mobility. The graphene behaved as an electrically conducting bridge between the P3HT domains in the composites, and thus the FET mobility generally increased as the graphene composition increased. Remarkably, the ES-PG4 FET had the highest hole mobility of 1.82 cm2 V−1 s−1 and a moderately high ION/IOFF of 5.88 × 104, which also exhibited good environmental stability for its transfer characteristics. The experimental results indicated that semiconducting composites based one-dimensional nanofiber devices offer advantages over conventional spin-coated thin films and provide a simple strategy for producing high-performance FET devices.

Molecular stacking structure and field-effect transistor characteristics of crystalline poly(3-hexylthiophene)-block-syndiotactic polypropylene through solvent selectivity

We investigate the molecular packing structures, morphologies and field-effect characteristics of the crystalline–crystalline poly(3-hexylthiophene)-block-syndiotactic polypropylene block copolymers (P3HT-b-sPP) using different solvent mixtures of chloroform–cyclohexane (CF–CH). For the P3HT-b-sPP with a shorter sPP segment length, the increase of CH solvent content led to the P3HT domain with highly crystalline nanofibrillar networks and thus improved the charge transporting characteristics. For the P3HT-b-sPPs with a longer sPP segment length, well-defined microstructure and device characteristics were only observed at the 70 vol% CF content. Furthermore, the self-encapsulation of the insulating sPP blocks effectively improved the air stability of the P3HT-b-sPP field transistor devices. This work highlights the significance of solvent selectivity and rod/coil block ratios on the molecular packing and the organic field-effect transistor performances.