Nanoscale physics

Electrofluorescence Molecular Diode: The Asymmetry of 
the Voltage Dependence
Shevchenko Ye.V., Leonov V.O., and Petrov E.G.
Bogoliubov Institute of Theoretical Physics, NAS of Ukraine. 
Metrologichna str. 14-B, Kyiv-03680, Ukraine.
E-mail: shevchenko@bitp.kiev.ua

We have carried out the detailed analysis of the kinetics of electrofluorescence formation in the systems with symmetric and asymmetric couplings between the molecule and the electrodes. For this purpose, the model of transmission of an electron in the molecular compound “electrode 1–molecule–electrode 2” (1M2 system) is studied in the framework of the method of a nonequilibrium density matrix. The active center of the fluorophore molecule is considered to be coupled to the electrodes through inactive structural groups of the molecule (spacers). Within the model where the energy states of the active center (fluorophore group) are determined by the HOMO and LUMO, we have obtained analytical expressions for the steady-state powers of electrofluorescence, which are valid in a wide range of interelectrode bias voltage.
It is shown that the optically active molecular state is formed as a result of electron hops between the molecule and each of the electrodes, as well as due to inelastic interelectrode tunneling of the electron. The two physical mechanisms of the dependence of electrofluorescence’s power on the sign of the voltage bias at the identical absolute values has been revealed (the effect of polarity of electrofluorescence). The first mechanism is due to the difference between the Stark shifts of the HOMO and LUMO levels in an electric field, which is caused by the asymmetry in geometry of the 1M2 system. In the second case, the polarity of electrofluorescence appears because of the difference in the kinetics of formation of populations of molecular states when the coupling of the fluorophore group of the molecule to one of the electrodes is much stronger than its coupling ot the other electrode. The performed estimates of the radiation power indicate that the electrofluorescence is much more efficient in 1M2 systems exhibiting nonpolarity irrespective of the Stark shift of levels [1,2].

1.	Petrov E.G., Leonov V.O., and Shevchenko Ye.V. Bipolar and Unipolar Electrofluorescence in a Molecular Diode // JETP Letters-2017.-105.-P. 89-97.
2.	Petrov E.G., Leonov V.A., and Shevchenko E.V. Electrofluorescence Polarity in a Molecular Diode // JETP-2017.-125.-P. 856–874.