SCL seminar by Marko Mladenovic

SCL's Marko Mladenovic presented a seminar:

"Electronic States at Low-Angle Grain Boundaries in Polycrystalline Naphthalene"


Organic semiconductors are materials of great promise for electronic devices, such as organic
field-effect transistors (FETs), organic light-emitting diodes (LEDs), and organic solar cells (OSCs).
Their advantage over inorganic counterparts is that they are flexible and have a low processing
cost. However, devices made of organic semiconductors still have relatively low charge mobility
and low efficiency. Thin films of crystalline organic semiconductors have a polycrystalline form,
which is composed of many different crystalline grains. Previous works indicate that grain
boundaries are the most limiting intrinsic factor for efficient charge transport in small molecule
based polycrystalline organic semiconductors. We have investigated the influence of grain boundaries on electronic properties of polycrystalline
organic semiconductor naphthalene. Atomic structure of grain boundaries was found using a
Monte Carlo method, while electronic structure calculations were performed using the density
functional theory based charge patching method. We found that grain boundaries introduce trap
states within the band gap of the material. Wave functions of these states are localized on closely
spaced pairs of molecules from opposite sides of the boundary. The energies of trap states are
strongly correlated with the distances between the molecules in the pair. These findings were used
to calculate the electronic density of trap states, which was found to exhibit a qualitatively different
behavior for grain boundaries perpendicular to the a and b direction of the naphthalene unit cell [1]. [1] M. Mladenovic, N. Vukmirovic and I. E. Stankovic, J. Phys. Chem. C 117, 15741 (2013).