Pentacene (PEN) is currently the most potential conjugated organic molecule as active material in novel electronic devices, such as organic thin-film field effect transistors (OFFT). Therefore, extensive research efforts are being undertaken to investigate its electronic and electrical properties from both of fundamental and applied aspects. A newly synthesized molecule of perfluoropentacene (PFP) was reported to act as n-channel OFET and fabricated ambipolar transistors with pentacene [1].
The intramolecular charge reorganization energy (λ), which is related to the electron-phonon coupling, is important parameter for efficient transport of charges in organic materials and across related interfaces. However, no detailed discussion on λ for the films has been performed so far due to lack of high-resolution ultraviolet photoelectron spectra (UPS) of the films. Hence, λ has been discussed based on highly-resolved spectra of gas-phase molecules [2]. Recently, we have succeeded to assess λ directly from the fine features in high-resolution UPS of organic solid systems by fabricating a well-ordered monolayer deposited on graphite [3].
In this paper, we compare the electronic structures of the well-ordered monolayer both for PEN/graphite and PFP/graphite. The hole injection barrier is ca. 0.4 eV larger for the PFP/graphite. The UPS band derived from the highest occupied state (HOMO) for both monolayer systems shows fine structures clearly, indicating the molecular vibrational modes strongly couple to photoelectron (HOMO hole). The observed HOMO band shape measured at 50 K is much different between the PEN and PFP systems due to change in the vibration energy and the coupled modes by fluorination. The λ for PFP is about two times larger than PEN, leading a conclusion of a higher hopping mobility for PEN. The detail discussion under the Franck-Condon analysis would be presented.
[1] Y. Inoue et al, Jpn, J. Appl. Phys. 44, 3663 (2005) and Y. Sakamoto et al, J. Am. Chem. Soc. 126, 8138 (2004).[2] J-L. Bredas et al, Chem. Rev. 104, 4971 (2004). [3] S. Kera et al, Chem. Phys. Lett. 364, 93 (2002) and H. Yamane et al, Phys. Rev. B 72, 153412 (2005).
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