Abstract
In this work, thermally crosslinkable conjugated copolymers have been developed to improve lifetime of organic opto-electronic devices. Poly[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene-2,6-diyl-alt-2,1,3-benzothiadiazole-4,7-diyl] (PCPDTBT), poly[N-9′-heptadecanyl-2,7-carbazole- alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and poly({4,8- bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7) are selected as the backbone structures because these polymers show high performances when they are used as active materials in organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). In Pd-catalyzed Suzuki or Stille coupling polymerizations, 5 mol% of dibromo-cyclopentadithiophene comprising crosslinkable 1-hexenyl groups was added as a comonomer followed by subsequent addition of 4-bromoanisole to cap the allyl groups on the comonomer by Heck reaction. The purpose of the Heck reaction is to reduce a thermal crosslinking temperature. After purification by Soxhlet extraction, crosslinkable polymers Cr-PCPDTBT, Cr-PCDTBT and Cr-PTB7 were obtained in 56-89% yield. To compare the optical and electrochemical properties and device performances, standard polymers (PCPDTBT, PCDTBT and PTB7) without crosslinkable moiety have also been synthesized. To undergo crosslinking reaction of Cr-PCDTBT and Cr-PTB7 in solid state, polymer films in vial tubes were annealed at 160 ºC for 8-16 hours. After the annealing and addition of chloroform into the vial tubes, insoluble particles were observed in the chloroform solutions. This poor solubility of the polymers results in the thermally crosslinking reaction. UV-vis spectra and cyclic voltammograms (CVs) of the polymers were measured to estimate optical bandgap and HOMO and LUMO levels of the polymers. The polymers show relatively low optical bandgap in a range of 1.40~1.85 eV. The crosslinkable polymers show very similar UV-vis spectra to the standard polymers, indicating that the addition of crosslinkable moiety to the standard polymers does not interrupt their original optical properties. Similarly, CVs of the crosslinkable polymers are similar to the standard polymers. The OFETs using thermally crosslinkable polymers were fabricated in ambient condition. The best performing device fabricated from Cr-PCPDTBT showed hole mobility of 4.63 × 10-4 cm2 V-1 s-1 after annealing at 160 ºC for 1 hour, which is higher than that from standard PCPDTBT (2.74 × 10-4 cm2 V-1 s-1). In addition, the lifetime of OFET using Cr-PCPDTBT was enhanced by crosslinking. Cr-PCDTBT exhibited the best hole mobility at 1.28 × 10-3 cm2 V-1 s-1 after annealing at 180 ºC for 1 hour. On the other hand, PCDTBT showed slightly higher hole mobility (1.83 × 10-3 cm2 V-1 s-1) than Cr-PCDTBT (1.28 × 10-3 cm2 V-1 s-1) under the same annealing condition (180 ºC for 1 hour). This is probably attributed to more ordered packing structure after annealing. Furthermore, the stability of OFET using Cr-PCDTBT was similar or slightly worse than standard PCDTBT. This is presumably because the effect of thermal annealing dominates over that of structural crosslinking for Cr-PCDTBT. The best performing device fabricated from Cr-PTB7 was measured at 1.21 × 10-4 cm2 V-1 s-1. The decreased hole mobility was observed in Cr-PTB7 after annealing, resulting from the less dense molecular packing. However, the lifetime of OFET using non-annealed Cr-PTB7 was significantly enhanced. The morphological analysis was conducted by Atomic Force Microscopy and X-ray diffraction. The chemical structural changes with ageing were analyzed by X-ray photoelectron spectroscopy.