Vibrational Anharmonicity in Organic Semiconductors

  Maor Asher  ,  Omer Yaffe  
Weizmann Institute of Science

Organic semiconductors have drawn a lot of interest in the last few decades due to their applications in the (opto)electronic industry. 
These applications include solar cells, light emitting diodes and field-effect transistors. 
Theoretical models used for organic semiconductors treat the motion of the molecules as harmonic. 
These models fail to predict important electronic properties such as the charge-carrier mobility. 
Since organic solids have weak intermolecular interactions, the vibrational motion of the molecules is expected to be highly anharmonic. 
These vibrations are crucial for determining their charge transport properties. 
Understanding their anharmonic nature is expected to improve the predictive power of the theoretical models at finite temperatures.

I will show our experimental investigation of the vibrational anharmonicity of single crystal organic semiconductors, through an advanced Raman spectroscopy technique known as Raman crystallography. 
This technique is used for measuring the symmetry of the different vibrations in the crystal. 
Using our state of the art custom-built Raman system we measured the evolution of the vibrational symmetry with temperature. 
The results show strong anharmonic characteristics. 
We found a gradual transition to a "liquid-like" behavior with increasing temperatures and a change in vibrational symmetry which may indicate a subtle phase transition.