A Meaningful Analogue of Pentacene: Charge Transport, Polymorphs, and Electronic Structures of Dihydrodiazapentacene
journal contributionposted on 14.04.2009, 00:00 by Qin Tang, Dieqing Zhang, Shenglong Wang, Ning Ke, Jianbin Xu, Jimmy C. Yu, Qian Miao
6,13-Dihydro-6,13-diazapentacene (DHDAP) has two nitrogen atoms replacing two carbon atoms of pentacene, the leading organic semiconductor for organic thin film transistors (OTFTs). This report details a comprehensive investigation of DHDAP highlighting the relationship between charge transport, polymorphs, and electronic structures. Three crystalline polymorphs are found from the thin films of DHDAP according to their (001) spacing (layer periodicity). Our surprising finding is that the field effect mobility of DHDAP is extremely sensitive to the polymorphs with the “12.9 Å phase” yielding a mobility of 0.45 cm2 V−1 s−1, which is over 5000 times higher than those of the other two phases. This unusually large effect of the crystalline polymorph on charge transport can be understood in terms of molecular packing using the models developed by de Wijs and Brédas. The comparable field effect mobilities and highly relevant structures of DHDAP and pentacene imply that the common structural features shared by the two molecules may be key factors that benefit the electrical performance while the particular structural features of pentacene should be unimportant to the electrical properties. In this point of view, DHDAP is a meaningful analogue of pentacene allowing better understanding on the structure−property relationship of pentacene. The electronic structure of DHDAP is studied in comparison with that of pentacene using both computational and experimental methods. It is found that DHDAP has a delocalized HOMO with the energy level essentially the same as that of pentacene, although the HOMO−LUMO gap of DHDAP is significantly larger. The environmental stability of DHDAP suggests that a relatively high HOMO energy level does not necessarily lead to environmentally unstable organic semiconductors. These results may lead to better understanding on the structure−property relationship of organic semiconductors.