Investigation of the structure-transport correlation in metal phthalocyanine thin films

The correlation of thin films structure and orientation of small organic molecules with the physical properties like charge carrier transport is not only important for basic understanding but also crucial for various application as functional devices. Small organic semiconductor molecules assemble in different configuration in thin films resulting in exhibiting a wide variety of optoelectronic properties. Here, we have investigated the structure of thin films of three metal phthalocyanines (MPcs) having a metal ion as Cu, Co, and Ni using a combination of x-ray scattering techniques and microscopic images. The x-ray results reveal that the molecules assemble in sticklike nanograins of well-ordered structure on Si surface. These nanograins packed differently for three different metal ions and CuPc growth are more of a columnar type whereas NiPc form much smoother films. The $I\text{−}V$measurements reveal higher conductivity for the thinner films, both under optical illumination and dark condition, in contrary to the normal convention. This observation was explained in terms of the transport of carriers through the region of trapped charge states in the ordered phase of the nanograins and the porous region between the grains resulting in different threshold voltages for differently ordered films.