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Cui, Wei,Lansac, Yves,Lee, Hochun,Hong, Seung-Tae,Jang, Yun Hee The Royal Society of Chemistry 2016 Physical chemistry chemical physics Vol.18 No.34
<P>Complex formation between lithium (Li+) ions and electrolyte molecules would affect the ionic conductivity through the electrolyte in lithium-ion batteries (LIBs). We hence revisit the solvation number of Li+ in the most commonly used ethylene carbonate (EC) electrolyte. The solvation number n of Li+(EC)(n) in the first solvation shell of Li+ is estimated on the basis of the free energy calculated by the density functional theory combined with a hybrid solvation model where the explicit solvation shell of Li+ is immersed in a free volume of an implicit bulk solvent. This new hybrid solvation (implicit and explicit) model predicts the most probable solvation number (n = 4) and solvation free energy (-91.3 kcal mol(-1)) of Li+ in a good agreement with those predicted by calculations employing simpler solvation models (either implicit or explicit). The desolvation (n = 2) of Li-0(EC)(n) upon reduction near anodes is also well described with this new hybrid model.</P>
Ryu, Taekhee,Lansac, Yves,Jang, Yun Hee American Chemical Society 2017 NANO LETTERS Vol.17 No.7
<P>A fullerene derivative with five hydroxyphenyl groups attached around a pentagon, (4-HOC6H4)(5)HC60 (1), has shown an asymmetric current voltage (I-V) curve in a conducting atomic force microscopy experiment on gold. Such molecular rectification has been ascribed to the asymmetric distribution of frontier molecular Orbitals over its shuttlecock-shaped structure. Our nonequilibrium Green's function (NEGF) calculations based on density functional theory (DFT) indeed exhibit an asymmetric I-V curve for 1 standing up between two Au(111) electrodes, but the resulting rectification ratio (RR similar to 3) is. insufficient to explain the wide range of RR. observed in experiments performed under a high bias, voltage. Therefore, we formulate a hypothesis that high RR (>10) may come from molecular orientation switching induced by a strong electric field applied between two electrodes. Indeed, molecular dynamics simulations of a self-assembled monolayer of 1 on Au(111) show that the Orientation of 1 can be switched between standing-up and lying-on-the-side configurations in a manner to align its molecular dipole moment with the direction of the applied electric field. The DFT NEGF calculations taking into account such field-induced reorientation between up and side configurations indeed yield RR of similar to 13, which agrees well with the experimental value obtained under a high bias voltage.</P>
Ku, Jamin,Lansac, Yves,Jang, Yun Hee American Chemical Society 2011 The Journal of Physical Chemistry Part C Vol.115 No.43
<P>Organic solar cells need low-band gap polymers for efficient harvesting of sunlight. Alternating electron-rich and electron-deficient units in a copolymer can lower the band gap, and high efficiency would be reached with the optimum combination of those units. As the first step toward this goal, we constructed four systematically altered copolymers <B>1</B>–<B>4</B>, where a benzothiadiazole electron-deficient unit is connected to each of four fused-ring electron-rich units (fluorene, carbazole, cyclopentadithiophene, and dithienopyrrole). The density functional theory (DFT) and the time-dependent DFT calculations at the B3LYP/6-311G(d,p) level on the dimer models of <B>1</B>–<B>4</B> reproduced very well the geometries, the HOMO/LUMO (highest occupied and lowest unoccupied molecular orbital) energy levels, and the band gaps – important parameters determining the solar-cell efficiency – of these copolymers. The calculation shows that the dithieno-type copolymers, <B>3</B> and <B>4</B>, exhibit lower band gaps and stronger absorption of visible light than the dibenzo-type copolymers, <B>1</B> and <B>2</B>. The calculation also indicates that <B>3</B>, which has deeper HOMO/LUMO levels than <B>4</B>, exhibits the highest solar cell efficiency (∼3%) among the four systems. On the basis of these results, we propose several derivatives of <B>3</B> with electron-withdrawing groups (<B>6</B>–<B>8</B>), which have even deeper HOMO/LUMO levels than <B>3</B>, as promising donor copolymers for high-efficiency (∼6%) organic bulk heterojunction solar cells.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2011/jpccck.2011.115.issue-43/jp2062207/production/images/medium/jp-2011-062207_0002.gif'></P>
Jang, Yun Hee,Lansac, Yves,Kim, Jae-Ki,Yoo, Hee-Soo,Chae, Chang-Geun,Choi, Cheol Ho,Samal, Shashadhar,Lee, Jae-Suk The Royal Society of Chemistry 2014 Physical chemistry chemical physics Vol.16 No.45
<P>Sodium benzanilide (Na<SUP>+</SUP>BA<SUP>−</SUP>) initiators have opened a new route to living anionic polymerization of <I>n</I>-hexylisocyanate (HIC) with 100% yield and controlled molecular weight. The NaBA initiators not only provide initiation points for polymerization by attacking HIC monomers but also successfully prevent <I>back-biting</I> side reactions without any help from additives. Our hypothesis on this dual function of the NaBA initiators is that they self-assemble to form protection shields around the chain ends. Indeed, our density functional theory calculations performed under experimental conditions on the free energy of formation of (NaBA)<SUB><I>n</I></SUB> clusters of various sizes and conformations searched by Monte Carlo simulations show that the BA<SUP>−</SUP> moiety forms a stable complex with Na<SUP>+</SUP> in a fan-like circular-sector shape owing to its double binding sites (N<SUP>−</SUP>–C&z.dbd;O ↔ N&z.dbd;C–O<SUP>−</SUP>) and that the tightly-bound NaBA units spontaneously self-assemble to form small (NaBA)<SUB><I>n</I></SUB> clusters (<I>n</I> = 2 and 4). The growing end of the polymer chain [(BA)(HIC)<SUB><I>n</I></SUB><SUP>−</SUP>], which resembles BA<SUP>−</SUP>, would also assemble with <I>n</I>− 1 NaBA units to form an <I>n</I>-mer cluster. We expect that the chain end in this cluster would be more available to attack small HIC monomers coming into the cluster (leading to chain growth) rather than folding back to attack the middle of the chain (leading to cyclotrimerization to isocyanurates and depolymerization).</P> <P>Graphic Abstract</P><P>The origin of the living nature of isocyanate polymerization by sodium benzanilide (Na<SUP>+</SUP>BA<SUP>−</SUP>) initiator is understood from the relative stabilities of (NaBA)<SUB><I>n</I></SUB> clusters in THF solution. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c4cp03596h'> </P>
Ku, Jamin,Gim, Yeongrok,Lansac, Yves,Jang, Yun Hee The Royal Society of Chemistry 2016 Physical chemistry chemical physics Vol.18 No.2
<P>Low-band-gap push-pull copolymers are promising donor materials for bulk heterojunction organic solar cells. One of the best push-pull copolymers are composed of bridged dithiophene pushing units and benzothiadiazole (BT) pulling units, but BT has no proper position to accommodate alkyl side chains introduced to enhance the solubility of the resulting copolymers in organic solvents. On the other hand, N-alkylthienopyrroledione (TPD), which has an alkyl side chain attached to its pyrrole moiety, has been combined with various bridged dithiophene pushing units to give high-solubility donor polymers whose power conversion efficiencies are higher than those of the BT-based polymers especially after a morphology control. However, our well-validated time-dependent density functional theory calculation on the intrinsic (single-chain) electronic structure, which has been proved powerful to estimate the efficiency, gives a contradictory prediction that both polymers would show essentially the same efficiency. Intrigued by this, we subsequently perform density functional theory calculations on their p-stacked-pair models in a number of stacking configurations and conclude that the enhanced performance of the TPD-based polymers is ascribed to their enhanced inter-chain interaction resulting from their enhanced dipole moments in the push-pull direction. Enhanced morphological ordering (p-stacking and p-conjugation) in their solid films, which is not considered in electronic-structure calculations, would reduce the band gap (as proved by the low-energy shoulders in UV/vis absorption spectra), improve the charge transfer (as shown by the calculated transfer integral, transfer rate, and hole mobility), and enhance the power conversion efficiencies (as observed after a morphology control).</P>
구자민,김대균,류택희,정은환,Yves Lansac,장윤희 대한화학회 2012 Bulletin of the Korean Chemical Society Vol.33 No.3
Push-pull-type copolymers − low-band-gap copolymers of electron-rich fused-ring units (such as cyclopentadithiophene; CPDT) and electron-deficient units (such as benzothiadiazole; BT) − are promising donor materials for organic solar cells. Following a design principles proposed in our previous study, we investigate the electronic structure of a series of new CPDTBT derivatives with various electron-withdrawing groups using the time-dependent density functional theory and predict their power conversion efficiency from a newlydeveloped protocol using the Scharber diagram. Significantly improved efficiencies are expected for derivatives with carbonyl [C=O], carbonothioyl [C=S], dicyano [C(CN)2] and dicyanomethylene [C=C(CN)2] groups, but these polymers with no long alkyl side chain attached to them are likely to be insoluble in most organic solvents and inapplicable to low-cost solution processes. We thus devise several approaches to attach alkyl side chains to these polymers while keeping their high efficiencies.