Electron-phonon coupling in crystalline organic semiconductors: microscopic evidence for nonpolaronic charge carriers

Influence of the electron-vibration coupling on the interfacial charge transfer in organic solar cells: a simple quantum model

In this paper, we analyze the influence of the electron-vibration interaction on the charge transfer process at the donor-acceptor interface in an organic solar cell. We present an essentially exact numerical analysis for a minimal model with only one vibration mode which is coupled to the charge transfer state. We show that the charge transfer state can be hot or cold depending on the parameters and in particular on the value of the energy of the lowest unoccupied molecular orbital on the donor side. We analyze also different regimes where electron-hole attraction or hybridization effects at the interface can modify the quantum yield of the transfer. We discuss also briefly the possible effects of the other vibration modes that are coupled to the charge either on the donor side or on the acceptor side.

Emissive spin-0 triplet-pairs are a direct product of triplet-triplet annihilation in pentacene single crystals and anthradithiophene films

Singlet fission and triplet-triplet annihilation represent two highly promising ways of increasing the efficiency of photovoltaic devices. Both processes are believed to be mediated by a biexcitonic triplet-pair state, ¹(TT). Recently however, there has been debate over the role of ¹(TT) in triplet-triplet annihilation. Here we use intensity-dependent, low-temperature photoluminescence measurements, combined with kinetic modelling, to show that distinct ¹(TT) emission arises directly from triplet-triplet annihilation in high-quality pentacene single crystals and anthradithiophene (diF-TES-ADT) thin films. This work demonstrates that a real, emissive triplet-pair state acts as an intermediate in both singlet fission and triplet-triplet annihilation and that this is true for both endo- and exothermic singlet fission materials.

Chasing the "Killer" Phonon Mode for the Rational Design of Low-Disorder, High-Mobility Molecular Semiconductors

Molecular vibrations play a critical role in the charge transport properties of weakly van der Waals bonded organic semiconductors. To understand which specific phonon modes contribute most strongly to the electron-phonon coupling and ensuing thermal energetic disorder in some of the most widely studied high-mobility molecular semiconductors, state-of-the-art quantum mechanical simulations of the vibrational modes and the ensuing electron-phonon coupling constants are combined with experimental measurements of the low-frequency vibrations using inelastic neutron scattering and terahertz time-domain spectroscopy. In this way, the long-axis sliding motion is identified as a "killer" phonon mode, which in some molecules contributes more than 80% to the total thermal disorder. Based on this insight, a way to rationalize mobility trends between different materials and derive important molecular design guidelines for new high-mobility molecular semiconductors is suggested.

Dynamic Formation of Bipolaron-Exciton Complexes in Conducting Polymers

The recombination dynamics of two oppositely charged bipolarons within a single polymer chain is numerically studied in the scope of a one-dimensional tight-binding model that considers electron-electron and electron-phonon (e-ph) interactions. By scanning among values of e-ph coupling and electric field, novel channels for the bipolaron recombination were yielded based on the interplay between these two parameters. The findings point to the formation of a compound species formed from the coupling between a bipolaron and an exciton. Depending on the electric field and e-ph coupling strengths, the recombination mechanism may yield two distinct products: a trapped (and almost neutral) or a moving (and partially charged) bipolaron-exciton. These results might enlighten the understanding of the electroluminescence processes in organic light-emitting devices.

Fröhlich Electron-Phonon Vertex from First Principles

We develop a method for calculating the electron-phonon vertex in polar semiconductors and insulators from first principles. The present formalism generalizes the Fröhlich vertex to the case of anisotropic materials and multiple phonon branches, and can be used either as a postprocessing correction to standard electron-phonon calculations, or in conjunction with ab initio interpolation based on maximally localized Wannier functions. We demonstrate this formalism by investigating the electron-phonon interactions in anatase TiO(2), and show that the polar vertex significantly reduces the electron lifetimes and enhances the anisotropy of the coupling. The present work enables ab initio calculations of carrier mobilities, lifetimes, mass enhancement, and pairing in polar materials.