Abstract
A series of poly-3-alkylthiophenes (P3ATs) with butyl (P3BT), hexyl (P3HT), and octyl (P3OT) side-chains and well-defined molecular weights (MWs) were synthesized using Grignard metathesis polymerization. The MWs of P3HTs and P3OTs obtained via gel permeation chromatography agreed well with the calculated MWs ranging from approximately 10 to 70 kDa. Differential scanning calorimetry results showed that the crystalline melting temperature increased with increasing MWs and decreasing alkyl side-chain length, whereas the crystallinity of the P3ATs increased with the growth of MWs. An MW-dependent red shift was observed in the UV–Vis and photoluminiscence spectra of the P3ATs in solution, which might be a strong evidence for the extended effective conjugation occurring in polymers with longer chain lengths. The photoluminescence quantum yields of pristine films in all polymers were lower than those of the diluted solutions, whereas they were higher than those of the phenyl-C61-butyric acid methyl ester-blended films. The UV–Vis spectra of the films showed fine structures with pronounced red shifts, and the interchain interaction-induced features were weakly dependent on the MW but significantly dependent on the alkyl side-chain length. The photovoltaic device performances of the P3BT and P3HT samples significantly improved upon blending with a fullerene derivative and subsequent annealing, whereas those of P3OTs mostly degraded, particularly after annealing. The optimal power conversion efficiencies of P3BT, P3HT, and P3OT were 2.4%, 3.6%, and 1.5%, respectively, after annealing with MWs of ~11, ~39, and ~38 kDa, respectively.
Highlights
The solubilities of the synthesized P3ATs were estimated in various solvents, and it was found that the P3OTs have relatively high solubilities in conventional organic solvents
gel permeation chromatography (GPC) results in conjunction with the solubility information reveal that the typical overestimation of such experimental molecular weights (MWs) compared to theoretically predicted MWs can be effectively reduced in the case of P3OTs because of their relatively high solubility in THF
The UV–Vis absorption spectra of the solutions showed a clear dependence on MW, which indicates the extended effective conjugation length (ECL) of the conjugated polymers
Summary
Poly-3-alkylthiophenes (P3ATs) have been intensively investigated and developed for use in various organic electronic devices, such as organic light-emitting diodes [1], organic photovoltaic cells (OPVs) [2,3,4,5,6], and organic field-effect transistors [7,8], because of their easy deposition from solution onto various types of substrates and low preparation cost [9].In OPVs, the polymer is known to be an effective semiconducting material with a relatively high hole mobility [10,11,12] and self-assembly capacity [13,14].P3ATs with simple chemical structures can be controllably synthesized using numerous methods, including McCullough [15,16,17], Rieke [18,19,20], and Grignard metathesis polymerization (GRIM) [21,22,23] syntheses. The GRIM method developed by Yokoyawa et al [24,25] with a high regioregularity (RR) of 98% head-to-tail coupling is currently preferred. This method is based on nickel-catalyzed cross-coupling polymerization and proceeds via living chain growth according to a catalyst transfer mechanism; it is attractive because of the ready availability of required reagents and relatively mild reaction conditions, as shown in Scheme S1 [25]. The dimers act as virtual initiators for the polymerization of the Grignard monomer via transmetalation. When the initiation rate is faster than or similar to the propagation rate, the molecular weight (MW) of the resulting polymer can be controlled through the feed ratio [monomer]0 /[Ni]0 [26,27,28]
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