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Towards a Multi-Scale Theory for Charge and Spin Transport in Organic Single Crystal-based Devices

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Constructing a materials-speci c theory of charge and spin transport in organic single crystal-based devices is a complex problem, where the computation of accurate structural and vibrational properties needs to be coupled to ways of determining the charge mobility and the spin-di usion characteristics. Here I will present a few steps towards a multi-scale theory of charge and spin transport in organic devices, in particular focussing on architectures where the organic channel is formed by a high-mobility organic single crystal.

Firstly I will discuss the interface between an organic crystal and a standard inorganic metallic electrode. In this case once the interface geometry is optimised with state of the art density functional theory, including dispersive forces, the electronic structure and the correct level alignment is extracted rigorously and accu- rately from constrained density functional theory (1). This allows one to identify the necessary quasi-particle corrections to apply to the spectrum. The obtained electronic structure can be then compared with available UPS data (2). Intriguingly, often the dynamics of the injection of charge and spin at the interface is not simply dominated by the density of states of the constituents. In particular I will discuss a case where dynamical interaction between spin-carrier and molecular vibration can a ect dramatically the spin lifetime at the interface (3).

Then I will move to discuss a scheme to calculate the charge carrier mobility of pure organic crystals at nite temperature, which is material-speci c, it accounts for van der Waals interactions and it includes vibrational contributions from the entire phonon spectrum of the crystal. Such an approach is also based on density functional theory, which now is combined with the construction of a tight-binding e ective model via Wannier transformation. The nal Hamiltonian includes coupling of the charge carriers to the crystals phonons, which are also calculated from density functional theory. Furthermore, in the case of spin transport both spin-orbit and hyper ne interaction complete the picture. I will apply this methodology to a range of molecular crystals, and here I will present in detail the case of durene (4), a small -conjugated molecule, which forms a high-mobility herringbone-stacked crystal. I will show that accounting correctly for dispersive forces is fundamental for obtaining a high-quality phonon spectrum, in agreement with experiments. Then the mobility as a function of temperature is calculated along di erent crystallographic directions and the phonons most responsible for the scattering are identi ed.

REFERENCES

1 A.M. Souza, I. Rungger, C.D. Pemmaraju, U. Schwingenschloegl and S. Sanvito, Constrained-DFT method for accurate energy-level alignment of metal/molecule interfaces Phys. Rev. B 88 , 165112 (2013). 2 A. Droghetti, S.Steil, N. Grossmann, N. Haag, H. Zhang, M. Willis, W.P. Gillin, A.J. Drew, M. Aeschli- mann, S. Sanvito and M. Cinchetti, Electronic and magnetic properties of the interface between metal- quinoline molecules and cobalt , Phys. Rev. B 89 , 094412 (2014). 3 A. Droghetti, I. Rungger, M. Cinchetti, and S. Sanvito, Vibron-assisted spin relaxation at a metal/organic interface, Phys. Rev. B 91 , 224427 (2015). 4 C. Motta and S. Sanvito, Charge Transport Properties of Durene Crystals from First-Principles, J. Chem. Theory Comput. 10, 4624 (2014).

This talk is part of the Optoelectronics Group series.

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