8:30 AM - EL01.11.03
Enhancing Long-Term Device Stability Using Thin-Film Blends of Small Molecule Semiconductors and Insulating Polymers to Trap Surface-induced Polymorphs
Tommaso Salzillo1,2,Antonio Campos1,Adara Babuji1,Raul Santiago3,Stefan Bromley3,4,Carmen Ocal1,Esther Barrena1,Remi Jouclas5,Christian Ruzie5,Guillaume Schweicher5,Yves Geerts5,6,Marta Mas-Torrent1
Institut de Ciència de Materials de Barcelona (ICMAB-CSIC)1,Weizmann Institute of Science2,Universitat de Barcelona3,Institució Catalana de Recercai Estudis Avanzats (ICREA)4,Faculté des Sciences Université libre de Bruxelles5,Université Libre de Bruxelles6
The 2,7-dioctyloxybenzothieno[3,2-b]benzothiophene (C8O-BTBT-OC8) molecule, one of the most promising organic semiconductors, presents two different crystalline phases: a co-facial structure (bulk phase) and a herringbone structure also defined as surface-induced phase (SIP phase).1 From solution deposition on Si/SiO2 always the SIP phase has been detected so far and previous works have reported that with aging at ambient conditions or by solvent vapor annealing it transforms to the thermodynamically stable bulk structure.2 The Bulk C8O-BTBT-OC8 phase crystallizes in a triclinic P-1 system with two molecules per unit cell presenting a displaced cofacial stacking, while the SIP phase has a monoclinic P21/c structure with a herringbone packing.1,3
In this work we report the stabilization of the SIP phase by blending the C8O-BTBT-OC8 with an insulating polymer and preparing the thin film transistors (TFTs) by the bar-assisted meniscus shearing (BAMS) technique. All the TFTs based on C8O-BTBT-OC8 with polystyrene (PS) of low molecular weight (i.e., 3K) and high molecular weight (i.e., 100K) present the SIP structure and display good electrical properties with a field-effect mobility close to 1 cm2/V s, a threshold voltage around 0 V and an on/off current ratio of 107-108. However, after 3 months in ambient conditions the C8O-BTBT-OC8:PS100K film retain the same morphology, while the film based on the lower molecular weight PS presents the formation of different crystal domains on top of the pristine layer. The structural inhomogeneity of the C8O-BTBT-OC8:PS3K was investigated by micrometric spatially resolved Lattice phonon Raman spectroscopy.4
In order to compare the hole transport tendencies in the Bulk and SIP polymorphs of C8O-BTBT-OC8, we analyzed all independent pairwise HOMO-HOMO (Highest occupied molecular orbital) intermolecular electronic couplings (JHOMO) in the two crystal structures using density functional theory (DFT) based calculations. Bulk phase show a highly anisotropic one dimensional (1D) electronic structure with only a single dominant JHOMO value of 42 meV in a direction parallel to the a-axis of the crystal with all other JHOMO values being ≤ 6 meV. In contrast, the SIP polymorphic phase has two main independent intermolecular couplings with moderate JHOMO values of 12-13 meV, which span over three different directions in the ab plane of the material, indicative of a more 2D electronic isotropy. Thus, although a larger electronic coupling is found in the Bulk phase, the electronic dimensionality in the SIP phase is enhanced.
Using the polymer blend we demonstrate the possibility to stabilize, during the time period studied of one year and a half, of the metastable SIP phase which present a 2D electronic isotropy compared with the 1D π-stacking arrangement of the bulk structure and thus to be preferred for thin film devices fabrication.5
 N. Bedoya-Martínez, B. Schrode, A. O. F. Jones, T. Salzillo, C. Ruzié, N. Demitri, Y. H. Geerts, E. Venuti, R. G. Della Valle, E. Zojer and R. Resel, J. Phys. Chem. Lett., 2017, 8, 3690–3695.
 B. Schrode, A. O. F. Jones, R. Resel, N. Bedoya, R. Schennach, Y. H. Geerts, C. Ruzié, M. Sferrazza, A. Brillante, T. Salzillo and E. Venuti, ChemPhysChem, 2018, 19, 993–1000.
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 T. Salzillo, R. G. Della Valle, E. Venuti, A. Brillante, T. Siegrist, M. Masino, F. Mezzadri and A. Girlando, J. Phys. Chem. C, 2016, 120, 1831–1840.
 T. Salzillo, A. Campos, A. Babuji, R. Santiago, S. T. Bromley, C. Ocal, E. Barrena, R. Jouclas, C. Ruzie, G. Schweicher, Y. H. Geerts and M. Mas-Torrent, Adv. Funct. Mater., 2020, 2006115, 1–9.