Deiveces using organic semiconductors, such as organic light-emitting diodes (OLED), organic field effect transistors (OFET), and organic solar cells, have been developping rapidly in recent years. Most of organic semiconductor devices use the "thin films" rather than the single crystals. Here electrons moves in position and energies in organic semiconductors to functionilize the diveces. Therefore electroic structure of the organic thin films are important to understand the device performances. We have been investigating the electronic structure of the organic thin films using photoemission spectroscopy (UPS, XPS) and inverse photoemission spectroscopy (IPES).
It has been known for a long time that some of organic molecules show different crystal structures called "polymorphs". Perylene and phthalocyanines are well-known cases. These different crystal structure, i.e. "intermolecular" structure (we often devide various properties of organic solids into "inter-" and "intra-" molecules for convinience), give a crucial effect on the device performances because the charge carriers (electrons and holes) in the organic semiconductors moves through the overlap of the molecular orbitals between the adjascent molecules.
The electronic structure of organic solis are first determined by that of the constituent molecules because intra-molecular interactions are much stronger than inter-molecular ones. Therefore photoemission spectra of oraganic solids resemble with those of gas phase molecules (isolated states) except for the spectal width and ionization energies . The intermolecular interaction usually play a secondary role.
Fig. 1 shows photoemission spectra of the energy region of the HOMO-bands (highest occupied molecular orbital)of pentacene thin films taken from Ref. . The two spectra measured at the film thickness of 9 nm and 230 nm look smilar, but apparently different in the spectral line shapes. The spectum on your left is broader in the width. The origin of the difference is "polymorphs" as I will show you later. In order to analyze these sepctral differences, we have to know the film structure. That is the reason why I started analyzing the film structure.
In many cases thin films of organic semiconductor are polycrystalline or amorphous. The polycrystalline film consist of small crystallites with the size from a few nanometer to mm. Here I am going to discuss crystallographic structure and orientation of the crystallites.
The stucture of single crystals can be determined using the established X-ray diffraction analysis. In constrast the structure of the thin film cannnot be easily examined. The reasons are,
We are going to examine the film strcture (crystallographic structues and orientations) using various experimental techniques including X-ray diffraction and infrared spectroscopy. K. Seki, Y. Harada, K. Ohno, and H. Inokuchi, Bull. Chem. Soc. Jpn. 47, 1608 (1974).