Incoherent Charge-hopping Research Articles

Improvement of photovoltaic performances by optimizing π-conjugated bridge for the C217-based dyes: A theoretical perspective

In this work, a series of novel metal-free dyes were designed based on the C217 dye by changing the π-conjugated bridge, and then their electronic and optical properties were theoretically studied with density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. Moreover, the photovoltaic performances of designed dyes were quantitatively estimated by means of the (TiO2)38 cluster and bidentate bridging adsorption mode coupled with the incoherent charge hopping model. Results showed that bithiophene (BT) and biselenophene (BS), which redshift the strongest absorption peak about 19.7 nm and 42.7 nm, are superior to π-conjugated bridges than thieno[3,2-b]thiophene (TT). Furthermore, the photoelectric conversion efficiency (PCE) of C217 dye was predicted to be as high as 9.40%, which is in excellent agreement with the measured value (9.60% ∼9.80%). As expected, the cell sensitized by the designed dye 4 and 7 exhibited higher PCE of 11.50% and 12.69% than that sensitized by the C217 dye, an increase of 22.5% and 35.1%, denoting the dye 4 and 7 are more promising dye sensitizer candidates, and are worth further experimental studies. In addition, our results also revealed that it is essential to accurately estimate the electron injection rate and efficiency for designing and developing high- performable dye sensitizers.

Charge transport properties of 5,6,7-trithiapentacene-13-one (TTPO) and its fluorinated derivatives

Three fluorinated 5,6,7-trithiapentacene-13-one (TTPO) derivatives were designed by replacing some hydrogen atoms of TTPO with fluorine atoms. Based on the X-ray structure of TTPO, the crystal structures of the fluorinated TTPO were optimized by the dispersion-corrected density functional theory (DFT-D). The charge transport properties of TTPO and its derivatives as potential n-channel organic semiconductors (OSCs) have been investigated at molecular and crystal level by means of density functional theory (DFT) calculations coupled with the incoherent charge hopping model. TTPO molecule has a low-lying LUMO level while its fluorinated derivatives have a further low LUMO level that enable their stability when exposed to air. Electron affinities (EA) also show this increasing stability after fluorination. The reorganization energies for both electron and hole transports increase after fluorination. The electron transport mobility at room temperature for TTPO and its fluorinated derivatives are in the range from 0.09 to 1.01cm2V−1s−1. This indicates the four compounds are candidates as stable n-channel OSC materials under favorable device conditions. In addition, the angular-dependent analysis for electron mobility shows that the electron transport is remarkably anisotropic and the maximum μe appears along face-to-face stacking direction with the largest electronic couplings.

Molecular stacking character and charge transport properties of tetrabenzoheptacenes derivatives: the effects of nitrogen doping and phenyl substitution.

The nitrogen doping and phenyl substitution effects on the geometries, molecular stacking character, electronic, and charge transport properties of tetrabenzoheptacene (TTBH) have been investigated by means of density functional theory (DFT) calculation and incoherent charge hopping model. Our results indicate that the nitrogen doping (TTH) at the 6,8,15,17 positions improves its stability in air and the ability of electron injection and in the meantime slightly changes the molecular stacking due to the C-H···N interaction. For both TTBH and TTH, large hole transport mobility (μh) and electron transport mobility (μe), which are on the same order of magnitude, are given rise by their dense displaced π-stacking in crystal. Comparatively, the phenyl substitution (Ph-TTBH) at the 6,8,15,17 positions adopts a non-planar conformation, adverse to close packing and therefore leads to smaller electron/hole transport mobility (μ) than those of TTBH and TTH. The calculations suggest TTBH and TTH are promising candidates for excellent ambipolar OFET materials.

Electronic and Charge Transport Properties of peri-Xanthenoxanthene: The Effects of Heteroatoms and Phenyl Substitutions

The electronic and charge transport properties of peri-xanthenoxanthene (PXX) and its phenyl-substitued derivative (Ph-PXX) are explored via quantum chemical calculations. To gain a better understanding of the physical properties of PXX, a comparative study is performed for its analogue, that is, anthanthrene. By employing Marcus electron transfer theory coupled with an incoherent charge hopping and diffusion model, we estimate the charge mobilties of PXX and Ph-PXX. Our calculated results indicate that the introduction of a heteroatom (oxygen) at the reactive sites of anthanthrene can stabilize the extended π-system and improve the effiecient charge injection in electronic devices. The phenyl substitution of PXX makes a remarkable change of charge transport characteristics from a p-type semiconductor to an n-type semiconductor, which shed light on molecular design for an n-type semiconductor through simple chemical structural modification.

Transition from Tunneling to Hopping in Single Molecular Junctions by Measuring Length and Temperature Dependence

The charge transport characteristics of a family of long conjugated molecular wires have been studied using the scanning tunneling microscope break junction technique. The family consists of four wires ranging from 3.1 to 9.4 nm in length. The two shortest wires show highly length dependent and temperature invariant conductance behavior, whereas the longer two wires show weakly length dependent and temperature variant behavior. This trend is consistent with a model whereby conduction occurs by two different mechanisms in the family of wires: by a coherent tunneling mechanism in the shorter two and by an incoherent charge hopping process in the longer wires. The temperature dependence of the two conduction mechanisms gives rise to a phenomenon whereby at elevated temperatures longer molecules that conduct via charge hopping can yield a higher conductance than shorter wires that conduct via tunneling. The evolution of molecular junctions as the tip retracts has been studied and explained in context of the characteristics of individual transient current decay curves.

Incoherent charge hopping and conduction in DNA and long molecular chains

We establish some relations between the kinetics of incoherent, hopping charge transport in bridged large scale chemical systems or in a single-component duplex DNA, and the electrical properties (electric current ( j) and conductance ( g)) of these systems connected by two electrodes. We considered two distinct voltage distributions across the equienergetic chain (with N bridge elements, and an intersite hopping rate k), which involve the voltage being biased only across the edge bridge elements (case (i)), and the voltage being equally distributed across the bridge (case (ii)). For sufficiently long chains in the low voltage ( Φ) domain, we find that j = ( ek/ N) G( κ 1, κ −1)( eΦ/ k B T), where G() is a function of charge injection rates κ 1( κ −1) to (from) the electrode. The low field (constant) conductance is g = 1.6 × 10 −19( k/N) G Ω −1. At high voltages we established the existence of a maximal, constant, Φ independent current ( j max), where g → 0. For case (i) j max = ek/ N, being determined by the intersite hopping rate and by N −1, as appropriate for diffusional charge transport. For case (ii) j max = eκ 1, being independent of the chain length, and determined by the rate of charge injection from the electrode. Finally, we applied our kinetic model for the description of incoherent charge transport in and the electronic properties of a donor–acceptor pair connected by two electrodes.

Making a molecular wire: charge and spin transport through para-phenylene oligomers.

Functional molecular wires are essential for the development of molecular electronics. Charge transport through molecules occurs primarily by means of two mechanisms, coherent superexchange and incoherent charge hopping. Rates of charge transport through molecules in which superexchange dominates decrease approximately exponentially with distance, which precludes using these molecules as effective molecular wires. In contrast, charge transport rates through molecules in which incoherent charge hopping prevails should display nearly distance independent, wirelike behavior. We are now able to determine how each mechanism contributes to the overall charge transport characteristics of a donor-bridge-acceptor (D-B-A) system, where D = phenothiazine (PTZ), B = p-oligophenylene, and A = perylene-3,4:9,10-bis(dicarboximide) (PDI), by measuring the interaction between two unpaired spins within the system's charge separated state via magnetic field effects on the yield of radical pair and triplet recombination product.