Non-Condon effect on charge transport in dithiophene-tetrathiafulvalene crystal

The effects of side chain engineering on the morphology and charge transport of the A-DA1D-A type of non-fullerene acceptor: a multiscale study

The appearance of the A-DA1D-A type of non-fullerene acceptor Y6 and its derivatives significantly improves the power conversion efficiency of organic solar cells. However, the effects of the modulation of the side chains of Y6 on its morphology and charge transport in organic thin films are still not well understood. In this work, we have systematically studied the effects of symmetric modifications of the length of alkyl side chains and the types, such as branched or straight alkyl chains, and the introduction of heteroatoms to side chains on these properties. A multiscale study, including density functional theory and classical molecular dynamics simulations, has been used to answer this open question. We find that face-on configurations are generally dominant for the AA, A1A1, and DD stacking of molecular pairs. With respect to prototype Y6, the introduction of oxygen atoms to outer alkyl side chains could enhance AA stacking but worsen the electrical network and enlarge the reorganization energy during electron transfer, and changing outer side straight alkyl chains to branched chains ruins π-π stacking of all units significantly. Finally, we discover that shortening outer alkyl side chains appropriately or changing inner branched chains to straight chains with the same number of carbon atoms is a good strategy to improve the molecular π-π stacking and electron mobility of Y6 while changing outer straight side chains to branched chains or introducing oxygen atoms to outer straight chains is the opposite. This study provides a new insight into the relationship between morphology and electron mobility and will be helpful for the design of future high-performance non-fullerene acceptors.

Generalization on Entropy-Ruled Charge and Energy Transport for Organic Solids and Biomolecular Aggregates

Herein, a generalized version of the entropy-ruled charge and energy transport mechanism for organic solids and biomolecular aggregates is presented. The effects of thermal disorder and electric field on electronic transport in molecular solids have been quantified by entropy, which eventually varies with respect to the typical disorder (static or dynamic). Based on our previous differential entropy (h _s )-driven charge transport method, we explore the nonsteady carrier energy flux principle for soft matter systems from small organic solids to macrobiomolecular aggregates. Through this principle, the synergic nature of charge and energy transport in different organic systems is addressed. In this work, entropy is the key parameter to classify whether the carrier dynamics is in a nonsteady or steady state. Besides that, we also propose the formulation for unifying the hopping and band transport, which provides the relaxation time-hopping rate relation and the relaxation time-effective mass ratio. The calculated disorder drift time (or entropy-weighted carrier drift time) for hole transport in an alkyl-substituted triphenylamine (TPA) molecular device is 9.3 × 10^-7 s, which illustrates nuclear dynamics-coupled charge transfer kinetics. The existence of nonequilibrium transport is anticipated while the carrier dynamics is in the nonsteady state, which is further examined from the rate of traversing potential in octupolar molecules. Our entropy-ruled Einstein model connects the adiabatic band and nonadiabatic hopping transport mechanisms. The logarithmic current density at different electric field-assisted site energy differences provides information about the typical transport (whether trap-free diffusion or trap-assisted recombination) in molecular devices, which reflects in the Navamani-Shockley diode equation.

From Y6 to BTPT-4F: a theoretical insight into the influence of the individual change of fused-ring skeleton length or side alkyl chains on molecular arrangements and electron mobility

Organic solar cells (OSCs) based on non-fullerene acceptor (NFA) Y6 have drawn tremendous attention due to the great progress in their power conversion efficiencies (PCEs).

Theoretical insights into the 1D-charge transport properties in a series of hexaazatrinaphthylene-based discotic molecules

Discotic liquid crystal (DLC) materials have attracted considerable attention mainly due to their high charge carrier mobilities in quasi-one-dimensional columns. In this article, five hexaazatrinaphthylene-based DLC molecules were investigated theoretically, and their frontier molecular orbital energy levels, crystal structures, and electron/hole drift mobilities were calculated by combination of density functional theory (DFT) and semiclassical Marcus charge transfer theory. The systems studied in this work include three experimentally reported molecules (1, 2, and 3) and two theoretically designed molecules (4 and 5). Compared with the 1-3 compounds, 4 and 5 have three more extended benzene rings in the π-conjugated core. The present results show that the orders of the frontier molecular orbital energy levels and electron drift mobilities agree very well with the experiment. For 4 and 5, the electron/hole reorganization energies are lower than those of compounds 1-3. Furthermore, the calculated electron/hole transfer integral of 5 is the largest among all the five systems, leading to the highest electron and hole mobilities. In addition, the hydrophobicity and solubility were also evaluated by DFT, indicating that compound 5 has good hydrophobicity and good solubility in trichloromethane. As a result, it is expected that compound 5 can be a potential charge transport material in electronic and optoelectronic devices. © 2017 Wiley Periodicals, Inc.

Forth-back oscillated charge carrier motion in dynamically disordered hexathienocoronene molecules: a theoretical study

Electronic structure calculations were performed to investigate the charge transport properties of hexathienocoronene (HTC) based molecules. The effective displacement of the charge carrier along the π-orbital of nearby molecules is calculated by monitoring the forth and back oscillations of the charge carrier through kinetic Monte Carlo simulation. The charge transport parameters such as charge transfer rate, mobility, hopping conductivity, localized charge density, time average effective mass and degeneracy pressure are calculated and used to study the charge transport mechanism in the studied molecules. The existence of degeneracy levels facilitates the charge transfer and is analyzed through degeneracy pressure. Theoretical results show that the site energy difference in the dynamically disordered system controls the forth-back oscillation of charge carrier and facilitates the unidirectional charge transport mechanism along the sequential localized sites. The ethyl substituted HTC has good hole and electron hopping conductivity of 415 and 894 S cm(-1), respectively, whereas unsubstituted HTC has the small hole mobility of 0.06 cm(2) V(-1) s(-1) which is due to large average effective mass of 1.42 × 10(-28) kg.

Electron mobilities of n-type organic semiconductors from time-dependent wavepacket diffusion method: pentacenequinone derivatives

The electron mobilities of two n-type pentacenequinone derivative organic semiconductors, 5,7,12,14-tetraaza-6,13-pentacenequinone (TAPQ5) and 1,4,8,11-tetraaza-6,13-pentacenequinone (TAPQ7), are investigated with use of the methods of electronic structure and quantum dynamics. The electronic structure calculations reveal that the two key parameters for the control of electron transfer, reorganization energy and electronic coupling, are similar for these two isomerization systems, and the charge carriers essentially display one-dimensional transport properties. The mobilities are then calculated by using the time-dependent wavepacket diffusion approach in which the dynamic fluctuations of the electronic couplings are incorporated via their correlation functions obtained from molecular dynamics simulations. The predicted mobility of TAPQ7 crystal is about six times larger than that of TAPQ5 crystal. Most interestingly, Fermi's golden rule predicts the mobilities very close to those from the time-dependent wavepacket diffusion method, even though the electronic couplings are explicitly large enough to make the perturbation theory invalid. The possible reason is analyzed from the dynamic fluctuations.