3D reconstruction of pentacene structural organization in top-contact OTFTs via resonant soft X-ray reflectivity

Year: 2018

Authors: Capelli R, Nardi M.V., Tuccoli T., Verucchi R., Dinelli F., Gelsomini C., Kpshmak K., Giglia A., Nannarone S., Pasquali L.

Autors Affiliation: Istituto Officina Dei Materiali, IOM-CNR, s.s. 14, Km. 163.5 in AREA Science Park, Basovizza, Trieste, 34149, Italy; IMEM-CNR, Institute of Materials for Electronics and Magnetism, Trento Unit, Via alla Cascata 56/C Povo, Trento, 38123, Italy; Department of Industrial Engineering, University of Trento, Via Sommarive 9, Trento, 38123, Italy; Istituto Nazionale di Ottica, INO-CNR, Area della Ricerca di Pisa di S. Cataldo, via Moruzzi 1, Pisa, I-56124, Italy; Dipartimento di Ingegneria E. Ferrari, Università di Modena e Reggio Emilia, Via Vivarelli 10, Modena, 41125, Italy; Department of Physics, University of Johannesburg, PO Box 524, Auckland-Park, 2006, South Africa

Abstract: Herein, we describe the use of soft X-ray reflectivity at the carbon K-edge to study the molecular organization (orientation, structure, and morphology) of pentacene active films in a top-contact transistor geometry. This technique is not affected by sample charging, and it can be applied in the case of insulating substrates. In addition, the sampling depth is not limited to the near-surface region, giving access to buried device interfaces (metal/organic and dielectric/organic). Spectral lineshape simulations, based on ab-initio calculations using a realistic 3D layer-by-layer model, allow us to unravel the details of the molecular organization in all the specific and crucial areas of the active film, overcoming the limitations of conventional approaches. The tilt angle of the long molecular axis in the whole film is found to progressively decrease with respect to the substrate normal from 25° to 0° with the increasing film thickness. A full vertical alignment, optimal for in-plane charge hopping, is reached only after the complete formation of the first five monolayers. Remarkably, starting from the first one in contact with the dielectric substrate, all the monolayers in the stack show a change in orientation with the increasing thickness. On the other hand, at the buried interface with a gold top-contact, the molecules assume a flat orientation that only propagates for two or three monolayers into the organic film. Top-contact devices with the highest performances can thus be obtained using films of at least ten monolayers. This explains the observed thickness dependence of charge mobility in pentacene transistors.


Volume: 112 (3)      Pages from: 031602-1  to: 031602-5

More Information: – Financial support from INFN-CNR national project (PREMIALE 2012) EOS “Organic Electronics for Innovative research instrumentation” is gratefully acknowledged. The authors thank Dr. L. Aversa, Dr. R. Tatti, and Dr. A. Borgono from IMEM-CNR Trento and Dr. C. Collini from FBK Institute of Trento for the support. Dr. N. Mahne is acknowledged for useful discussions and advice in the use of OPAL code.
KeyWords: thin film molecular packing, resonant X-ray reflectivity, transistor performance, OTFT, Pentacene, Organic electronics, Thin films, RSXRR
DOI: 10.1063/1.5008941

Citations: 3
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