arXiv subjects

Marco Bardini

Publications and source records attributed to Marco Bardini.

3 recordsLinked to original sources

arXiv2026

A hidden bulk polymorph governs charge transport dimensionality in an organic semiconductor

Organic semiconductors (OSCs) are widely explored for flexible optoelectronic technologies, with performance governed not only by molecular design, but also by solid-state packing, which can give rise to polymorphism. Dinaphthothienothiophene (DNTT) is a benchmark OSC that has long been considered monomorphic. Here, we discover, isolate, and resolve the crystal structure of a previously unrecognised bulk polymorph of DNTT, termed blue DNTT owing to its characteristic blue emission. Coexisting with the well-known (green) DNTT in commercial powders, yet previously overlooked, blue DNTT represents the thermodynamically stable form. By combining X-ray diffraction, Raman, and THz spectroscopy with simulations, we demonstrate that polymorphism in DNTT reshapes the low-frequency phonon landscape and transfer-integral network, impacting charge transport. While green DNTT exhibits two-dimensional charge transport with holes more mobile than electrons, blue DNTT shows charge transport along all crystallographic directions enabled by a distinct herringbone packing. Electron mobility along the crystallographic a and b-axes in blue DNTT exceeds twice the hole mobility in the green phase. To our knowledge, this is the first reported acene-based semiconductor exhibiting three-dimensional charge transport. Polymorphism emerges as a key lever to tune charge transport dimensionality and carrier efficiency in organic semiconductors.

cond-mat.mtrl-sci↗

arXiv2025

Accurate and Efficient Phonon Calculations in Molecular Crystals via Minimal Molecular Displacements

Vibrational dynamics governs the fundamental properties of molecular crystals, shaping their thermodynamics, mechanics, spectroscopy, and transport phenomena. However desirable, the first-principles calculation of solid-state vibrations, i.e.\ phonons, stands as a major computational challenge in molecular crystals characterized by many atoms in the unit cell and by weak intermolecular interactions. Here we propose a formulation of the harmonic lattice dynamics based on a natural basis of molecular coordinates consisting of rigid-body displacements and intramolecular vibrations. This enables a sensible \emph{minimal molecular displacement} approximation for the calculation of the dynamical matrix, combining isolated molecule calculations with only a small number of expensive crystal supercell calculations, ultimately reducing the computational cost by up to a factor 10. The comparison with reference calculations demonstrates the quantitative accuracy of our method, especially for the challenging and dispersive low-frequency region it is designed for. Our method provides an excellent description of the thermodynamic properties and offers a privileged molecular-level insight into the complex phonons band structure of molecular materials.

cond-mat.mtrl-sci↗

arXiv2023

Transiently delocalized states enhance hole mobility in organic molecular semiconductors

There is compelling evidence that charge carriers in organic semiconductors (OSs) self-localize in nano-scale space because of dynamic disorder. Yet, some OSs, in particular recently emerged high-mobility organic molecular crystals, feature reduced mobility at increasing temperature, a hallmark for delocalized band transport. Here we present the temperature-dependent mobility in two record-mobility OSs: DNTT (dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]-thiophene), and its alkylated derivative, C8-DNTT-C8. By combining terahertz photoconductivity measurements with fully atomistic non-adiabatic molecular dynamics simulations, we show that while both crystals display a power-law decrease of the mobility (μ) with temperature (T, following: μ\propto T^(-n)), the exponent n differs substantially. Modelling provides n values in good agreement with experiments and reveals that the differences in the falloff parameter between the two chemically closely related semiconductors can be traced to the delocalization of the different states thermally accessible by charge carriers, which in turn depends on the specific electronic band structure of the two systems. The emerging picture is that of holes surfing on a dynamic manifold of vibrationally-dressed extended states with a temperature-dependent mobility that provides a sensitive fingerprint for the underlying density of states.

cond-mat.soft↗