Influence Of Alkyl Side Chain Length Research Articles

Vibronic States and Edge-On Oriented π-Stacking in Poly(3-alkylthiophene) Thin Films

The influence of alkyl side-chain length [CnH2n+1 (A), where n = 6 (hexyl), 8 (octyl), 12 (dodecyl)] and thermal annealing (TA) and the definite roles of these in the quantity and quality of π-stacked crystallinity and edge-on orientated (EO) ordering of spin-coated poly(3-alkylthiophene) [P3AT] thin films, which are of massive significance in their optoelectronic properties, were investigated using complementary optical absorption spectroscopy and X-ray reflectivity (XR) techniques. The energy-band diagram, corresponding to the vibronic levels, obtained from the optical absorption spectrum provides information about the percentage, planarity, local order, and average conjugation length of the crystalline aggregates, while the electron density profile obtained from the XR provides unique information about the EO ordering and its variation along the depth. A prominent EO ordering near the substrate with a gradually decreased ordering toward the top surface is observed in each film. An as-cast P3HT film of shorter side-chain (having a natural nucleation tendency and rigidity) shows a slightly higher crystallinity and weaker excitonic coupling, while as-cast P3DDT film (having longer side-chain flexibility) shows a slightly better EO ordering. After the TA, the planarity of the unfolded backbone improves in each film; however, the effect is maximum in the P3DDT film. The EO ordering also improves throughout the film (by a thermal energy-induced reorientation of crystallites) but more toward the top surface (by increasing the decay length), preserving the exponential decay nature. The P3HT film (with the better initial crystallinity and planarity) shows an appreciable improvement, while the P3DDT film (where the crystallinity increases and crystallites have a better reorientational ability) shows the maximum improvement in the EO ordering. The improvement of the crystallinity (in quantity and quality) and the EO ordering of P3AT in thin films (as active layers), especially near the film–substrate interfaces, are of immense importance for their better in-plane charge carrier mobility in a thin-film transistor.

Influence of Alkyl Sidechain Length on Lithium-Ion Conductivity for Polyether Electrolytes

Lithium-ion batteries (LIBs) have been used in various energy devices, from small electronic devices to electric vehicles. However, LIBs have safety concerns, mainly due to the flammable organic solvents used for electrolytes. The development of electrolytes based on flame-retardant materials, including polymer, is one of the potential solutions. Although various polymer electrolytes (PEs) were successfully synthesized, the low lithium-ion (Li+) conductivity hinders its practical application.It is considered that the Li+ conductivity in PEs is determined by two factors; (i) segmental mobility of polymer chains and (ii) Li+ concentration, which is connected to the Li+ mobility and the number of ion carriers, respectively. Most of the PEs failed to meet those requirements, which hinders the improvement of Li+ conductivity. The problem is that the polar groups in the polymer chains tend to actively coordinate with Li+ under high Li+ concentration, which inhibits the mobility of the polymer chains. Also, the Li+-polar group interaction anchors Li+ on the polymer chain, which may also suppress the Li+ conductivity.Therefore, to establish a polymer design strategy, it is necessary to elucidate the factors which affect the interaction between Li+-polar groups and Li+ transportation. Previous studies have focused mainly on the polar properties (e.g., donor number and dielectric constant) of the polymers to understand the interaction between Li+ and polar groups. In contrast, the effect of non-polar groups was often overlooked. Non-polar groups can affect the mobility and polarity of polymers as well as the Li+-polar group interaction, due to its plasticization effect, polarity, and steric hindrance.Here, in this study, we selected polyethers having alkyl sidechains with different lengths (–(CH2)m–H, m = 1, 2, 4, 6, 8, and 12), to clarify the contribution of the non-polar groups on the Li+ conductivity in PEs. Electrochemical impedance spectroscopy revealed that the ionic conductivity (σ) at low temperatures was dominated by glass transition temperature, which correlates with the segmental mobility of polymer chains. Furthermore, σ increase with the increase in the length of the alkyl group due to the internal plasticization effect of the long alkyl groups. On the contrary, the σ at high temperature was mostly determined by salt dissociation capacities of polymers, which tended to increase as the alkyl group became shorter. The Li+ transportation number (t Li+) gradually improved with the extension of the alkyl groups; 0.16 (m = 1) < 0.26 (m = 2) < 0.28 (m = 4) < 0.32 (m = 6) < 0.36 (m = 8) < 0.43 (m = 12). We believe that the gradual changes of t Li+ are due to the low polarity of long alkyl sidechain, giving low dielectric constant and/or low viscous properties. The activation energy of Li+ transportation at the electrode-electrolyte interface (E int) showed a large gap between m =1 and others; E int = 68–82 kJ mol–1 (m = 2, 4, 6, 8, and 12) < E int = 116 kJ mol–1 (m = 1). The coordination number of ether groups (n ether) estimated from ν(COC) intensity of infrared spectra showed a similar trend; n ether = 3.6–4.4 (m ≥ 2) < n ether = 6.2 (m = 1). The interaction between the ether group and Li+ can be affected by the length of alkyl sidechain, and it effectively suppressed for alkyl sidechain of m > 1, probably because of the steric hindrance. Considering the similar trend in both E int and n ether, the amount of ether-Li+ interaction can be a descriptor to determine Li+ transportation at the electrode-electrolyte interface.In summary, the Li+ conductivity, including ionic conductivity (σ) and Li+ transportation number (t Li+) strongly influenced by the properties of the alkyl group, such as the internal plasticization effect and low polarity. The Li+-ether group interaction, which was governed by the steric hindrance of alkyl sidechain, can be a dominant factor for the activation energy of Li+ transportation at the electrode-electrolyte interface (E int). Designing alkyl sidechains is thus a valid strategy to optimize segmental mobility and polarity of the bulk as well as the local interaction at the interface, which leads to the improved Li+ conductivity at bulk and electrode-electrolyte interfaces.

Influence of alkyl side-chain length on the carrier mobility in organic semiconductors: herringbone vs. pi–pi stacking

With the increase of alkyl chain length, the change trend of charge mobility is reversed between π–π and herringbone systems.

Influence of Alkyl Side-Chain Length on the Performance of Poly(3-alkylthiophene)/Polyfluorene All-Polymer Solar Cells

We have investigated the influence of the alkyl side-chain length of poly(3-alkylthiophene) (P3AT) on the performance of all-polymer blend photovoltaic devices with the polyfluorene copolymer F8TBT (poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2′′-diyl)) used as the electron acceptor. Butyl, hexyl, octyl, and decyl side-chains have been studied, with clear trends in the device performance observed. The properties of the unblended polythiophene are first reported, namely their thermal, photophysical, electrochemical, and morphological characteristics. The observed trends in photovoltaic device performance are interpreted in terms of the complex interplay between energy level offsets, blend morphology and charge carrier mobility. Device performance is especially sensitive to the annealing temperature employed that influences both the recrystallization of P3AT and the coarsening of phase separation. Poly(3-hexylthiophene) (P3HT) is found to be the optimum P3A...