Dan Mendels

Thermoelectricity in Disordered Organic Semiconductors under the Premise of the Gaussian Disorder Model and Its Variants.

Using Monte Carlo simulations, we investigate the thermoelectric properties of disordered organic semiconductors under the premise of the Gaussian disorder model and its variants. In doing so, we provide much needed additional dimensions for comparison between these theoretical frameworks and real systems beyond those based on extensively studied charge-transport properties and aim to provide a frame-of-reference for rising interest in these systems for thermoelectric-based applications. To illustrate the potential existing in the implementation of combined transport and thermoelectric investigation, we discuss strategies to experimentally deduce a systems DOS shape and the temperature dependence of its transport energy (which can discern hopping transport from multiple trapping transport), infer whether a systems activation energy originates from inherent energetic disorder or a polaron activation energy (while deducing the given polaron activation energy), and discerning whether a systems energetic disorder is spatially correlated or accompanied by off-diagonal disorder.

A Comprehensive study of the Effects of Chain Morphology on the Transport Properties of Amorphous Polymer Films.

Organic semiconductors constitute one of the main components underlying present-day paradigm shifting optoelectronic applications. Among them, polymer based semiconductors are deemed particularly favorable due to their natural compatibility with low-cost device fabrication techniques. In light of recent advances in the syntheses of these classes of materials, yielding systems exhibiting charge mobilities comparable with those found in organic crystals, a comprehensive study of their charge transport properties is presented. Among a plethora of effects arising from these systems morphological and non morphological attributes, it is shown that a favorable presence of several of these attributes, including that of rapid on-chain carrier propagation and the presence of elongated conjugation segments, can lead to an enhancement of the system’s mobility by more than 5 orders of magnitude with respect to ‘standard’ amorphous organic semiconductors. New insight for the formulation of new engineering strategies for next generation polymer based semiconductors is thus gathered.

Field dependent thermoelectric properties of organic semiconductors—A tool to determine the nature of charge transport in materials exhibiting thermally activated transport

By implementing Monte Carlo simulations and employing the concept of effective temperature, we explore the effects of an applied field bias on the charge carrier statistics and Peltier coefficient in hopping systems subject to the parameter range applicable to disordered organic semiconductors. Distinct differences are found between the observed field dependences as obtained from systems in which energetic disorder is spatially correlated and those in which it is not. Considerable differences are also found between the charge carrier statistics and the Peltier coefficients field dependence in systems in which charge is transported by bare charge carriers and systems in which it is propagated by polarons. Peltier coefficient field dependence investigations are, hence, proposed as a new tool for studying charge transport and thermoelectricity in disordered organic semiconductors and systems which exhibit thermally activated transport in general.

Collective Variables from Local Fluctuations

We introduce a method to obtain one-dimensional collective variables for studying rarely occurring transitions between two metastable states separated by a high free energy barrier. No previous information, not even approximated, on the path followed during the transition is needed. The only requirement is to know the fluctuations of the system while in the two metastable states. With this information in hand, we build the collective variable using a modified version of Fishers linear discriminant analysis. The usefulness of this approach is tested on the metadynamics simulation of two representative systems. The first is the freezing of silver iodide into the superionic α-phase, and the second is the study of a classical Diels-Alder reaction. The collective variable works very well in these two diverse cases.

Metadynamics with Discriminants: A Tool for Understanding Chemistry

We introduce an extension of a recently published method [ Mendels et al. J. Phys. Chem. Lett. 2018 , 9 , 2776 - 2781 ] to obtain low-dimensional collective variables for studying multiple states free energy processes in chemical reactions. The only information needed is a collection of simple statistics of the equilibrium properties of the reactants and product states. No information on the reaction mechanism has to be given. The method allows studying a large variety of chemical reactivity problems including multiple reaction pathways, isomerization, and stereo- and regiospecificity. We applied the method to two fundamental organic chemical reactions. First we study the SN2 nucleophilic substitution reaction of a Cl in CH2Cl2 leading to an understanding of the kinetic origin of the chirality inversion in such processes. Subsequently, we tackle the problem of regioselectivity in the hydrobromination of propene revealing that the nature of empirical observations such as Markovnikovs rule lies in the chemical kinetics rather than the thermodynamic stability of the products.