Crystallographic and Electronic Structures of Solution-Processed Tetrabenzoporphyrins

Patrick Shea and Jerzy Kanicki, University of Michigan, Ann Arbor; Lisa Pattison and Pierre Petroff, University of California – Santa Barbara; Hiroko Yamada and Noboru Ono,Ehime University – Japan

The electrical performance of organic semiconductors is directly related to their crystallographic structure and the means by which the semiconductor is processed.  This project utilizes the unique resources of the member universities to examine the crystallographic properties of metallo-tetrabenzoporphyrin (MTBP) thin-films, and employs advanced quantum mechanical software packages to both analyze and drive material development. 

The novelty of TBP-based organic semiconductors is two-fold due to the customizability of the synthetic process by which the molecules are created.  First, TBP can be synthesized in a bulky, soluble precursor form allowing for easy solution processing.  Thin-films are deposited via solution in an amorphous, insulating form, then converted to a polycrystalline semiconductor by a thermal annealing step.  Second, the synthetic process can be modified to substitute particular atoms in the porphyrin core, or attach side-molecules.  Therefore, significant control can be exercised from the very start to enhance field-effect mobility and alter thin-film morphology.

We have extensively studied the thin-film morphology of metal-substituted TBPs, including Cu, Ni, and Zn substitutes.  X-ray diffraction (XRD) studies indicate a similar unit cell structure to metal-free TBP (monoclinic, P21/n).  Metal-substitution does exhibit an effect on morphology, however, as evidenced by variations in morphology observed via polarized optical microscopy and atomic force microscopy.  Metal-free and metallo-TBPs form nanorod aggregates upon thermal annealing, with rod groupings in turn forming larger domains.  Nanorod density has been observed to have film-thickness dependence.   The current focus of this research initiative is exploration of nanorod anisotropic electrical behavior, and altering thin-film morphology from the present “herringbone” structure to face-to-face packing that will likely enhance charge transport. 

With the computing grid at the Center for Advanced Computing at the University of Michigan, quantum mechanical simulators are used to calculate the energy band structure, density of states, and optical properties of MTBP films.  These calculations shed light on the thin-film electrical behavior, and explain the high performance of the Ni- and Cu-TBP thin-films.  Furthermore, development of new molecules is assisted by predicting their properties and focusing on molecules expected to have properties advantageous to OFET performance, such as oxygen insensitivity and high charge carrier mobility.

[1]            P. B. Shea, et al., Phys. Rev. B., submitted.
[2]            P. B. Shea, et al., Appl. Phys. Lett., submitted.

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