ABSTRACT
The low volatility of ionic liquids (ILs) is one of their most interesting physico-chemical properties; however, the general understanding of their evaporation dynamics under vacuum is still lagging. Here, we studied the thermodynamics of IL evaporation by employing thermogravimetry (TG) measurements under vacuum. The thermodynamic parameters of ILs, such as the evaporation onset temperatures, enthalpies, entropies, saturation vapor pressures, and boiling points were quantified by analyzing the TG data. The obtained evaporation enthalpies (110-140 kJ mol-1) were higher than those of typical molecular liquids, and the entropies (>88 J mol-1 K-1) suggested that they are exceptions of the Trouton's rule. The obtained Clausius-Clapeyron equations demonstrated that the saturation vapor pressures of ILs only depend on temperature. Further, we derived the empirical equation for estimating the upper limit temperature of the liquid phase of IL under given external pressures. Using the evaporation behaviors of referential normal alkanes and charge-transfer complex and the evaporation entropies of the ILs, the vaporized IL structure was thermodynamically modelled. The ILs were found to evaporate as ion pairs, instead of as individual ions or higher-ordered cluster structures. By comparing a series of ILs with various cations and a fixed anion, it was found that the IL evaporation dynamics under vacuum is strongly and systematically affected by their chemical structures, charge balances between the cations and the anions, molecular weights, and the higher-ordered structures including polar and non-polar regions. Our concept, measurement method, and equation can be extended to other ILs and low-volatile liquids under vacuum, and help with the design of ILs with higher thermal stabilities.
ABSTRACT
The current-voltage characteristics of benzoporphine-fullerene solar cells were measured subsequent to the deposition of Al as a cathode material. Even in vacuum, a shift in the open circuit voltage was observed at 20 min after Al deposition. Moreover, the displacement of inert gases (N(2)or Ar) in the evaporation chamber enhanced the photovoltaic parameters. The power conversion efficiency was increased by 24% over the initial characteristics (from 1.04% to 1.29%), which indicates that the structure of the organic-metal interface changed rapidly after Al deposition, even if the process was performed in an air-free glovebox.
ABSTRACT
Multilayered oriented polyfluorene (PF) films were obtained by applying thermal treatment procedure to a multilayered PF film constructed with fluorene derivatives layer formed on top of a highly oriented friction-transferred crystalline poly(9,9-dioctylfluorene) (PF8) film. The orientations in the multilayered PF films were investigated by polarized photoluminescence (PL) spectroscopy and grazing incident X-ray diffraction (GIXD) analysis. The results of the multilayered PF film constructed with spin-coated PF8 on friction-transferred PF8 indicate that the rearrangement of the upper PF8 layer is induced from the orientation of lower PF8 layer by thermal treatment at the nematic phase temperature. Polarized green emission from the multilayered oriented PF film was demonstrated using the blend of PF8 and poly(9,9-dioctylfluorene-co-benzothiadiazol) (F8BT) as green light emitter for upper layer. By this method, the polarized emission color can be tuned using polymer blends for upper layer similar to the liquid-crystalline polymer arrangement without using different materials as an underlying layer such as the rubbed polyimide.
Subject(s)
Fluorenes/chemistry , Membranes, Artificial , Polymers/chemistryABSTRACT
Poly(2,5-dioctyloxy-1,4-phenylenevinylene) (DOPPV) was found to form a highly oriented film by a friction-transfer technique. Structural investigation of friction-transferred DOPPV was studied by means of polarized ultraviolet-visible (UV-vis) absorption spectroscopy, polarized photoluminescence (PL) spectroscopy, and synchrotron-sourced grazing incident X-ray diffraction (GIXD) analysis. The polarized UV-vis absorption and PL spectra indicate clear axial alignment. DOPPV backbones in friction-transferred film are highly aligned along the drawing direction of the friction-transfer. Further information of the molecular arrangement in friction-transferred DOPPV film was investigated by both the out-of-plane and the in-plane GIXD analyses with synchrotron source. The DOPPV molecules in friction-transferred films were perfectly arranged three-dimensionally: the backbones aligned along the drawing direction of friction-transfer, the alkyl side chains lay in the film plane, and the planar backbones were arranged parallel to the film surface. Additionally, two neighboring DOPPV molecules along the direction of inter-backbones separation by alkyl side chains were found to be shifted with respect to one another by the mean distance of half of a monomeric repeat.
