Pentacene and its substituted derivatives are among the most employed semiconductors in organic electronic devices. Over the past few years, a lot of effort has been devoted to elucidate the relationship between i) molecular packing and morphology, ii) reactivity and iii) charge mobility and efficiency of pentacene-based Organic Field Effect Transistors (OFETs). The high reactivity of pentacene and its low degree of order at grain boundaries of polycrystalline thin films represent some of the major issues preventing further development and commercialization of its applications. However, only a few experimental studies focused on the impact of the electric field produced by the device operation on pentacene reactivity and molecular organization in the thin film.(1,2) A single investigation performed by Raman spectroscopy showed a electric field-induced structural modification in pentacene OFETs.(3) In the present study, DFT and molecular mechanics simulations were used to investigate the effect of OFET operation on the thin film structure and reactivity, and results were compared to XRD measurements performed in real time on functioning devices. Firstly, the thermal dimerization pathway(4) of pentacene under the influence of an external electric field was reproduced. Results indicate that pentacene cycloaddition may be enhanced by the applied source-drain bias in working OFET devices, in correspondence to disordered domains of polycrystalline thin films. Dimerization would then result in an irreversible loss of charge carriers and contribute to cause the mobility decrease observed after long operation time.(5) Secondly, the effect of gate bias was simulated on a pentacene monolayer, showing that the electric field slightly perturbs the molecular arrangement of crystallites by inducing a small molecular tilt towards the direction of the applied field. This has been confirmed by XRD results, which also highlight how molecules seem to re-organize at grain boundaries making themself responsible for the observed threshold voltage shift. The interplay between simulations and experimental results allows to understand the structural evolution of operating transistors, to identify possible causes of fatigue and bias stress phenomena and has the potential to lead to future improvements in organic-based devices.
References:
(1) Mottaghi, M.; Horowitz, G. Org. Electron. physics, Mater. Appl. 2006, 7, 528–536. (2) Mandal, T.; Garg, A. J. Appl. Phys. 2013, 114, 154517. (3) Cheng, H. L. L.; Chou, W. Y. Y.; Kuo, C. W. W.; Wang, Y. W. W.; Mai, Y.
S. S.; Tang, F. C. C.; Chu, S. W. W. Adv. Func. Mater. 2008, 18, 285–293. (4) Zade, S. S.; Zamoshchik, N.; Reddy, A. R.; Fridman-Marueli, G.; Sheberla, D.; Bendikov, M. J. Am. Chem. Soc. 2011, 133, 10803–10816. (5) Di, C.; Yu, G.; Liu, Y.; Guo, Y.; Sun, X.; Zheng, J.; Wen, Y.; Wang, Y.; Wu, W.; Zhu, D. Phys. Chem. Chem. Phys. 2009, 11, 7268–7273.