In order to understand how additives influence the structure and electrical properties of active layers in thin‐film devices, a compositionally identical but structurally different guest–host system based on the
Organic field‐effect transistor (OFET) performance is dictated by composition and geometry, as well as the quality of the organic semiconductor (OSC) film, which strongly depends on purity and microstructure. When present, impurities and defects give rise to trap states in the band gap of the OSC, lowering device performance. Here, 2,8‐difluoro‐5,11‐bis(triethylsilylethynyl)‐anthradithiophene is used as a model system to study the mechanism responsible for performance degradation in OFETs due to isomer coexistence. The density of trapping states is evaluated through temperature‐dependent current–voltage measurements, and it is discovered that OFETs containing a mixture of syn and anti‐isomers exhibit a discrete trapping state detected as a peak located at ≈0.4 eV above the valence‐band edge, which is absent in the samples fabricated on single‐isomer films. Ultraviolet photoelectron spectroscopy measurements and density functional theory calculations do not point to a significant difference in electronic band structure between individual isomers. Instead, it is proposed that the dipole moment of the syn‐isomer present in the host crystal of the anti‐isomer locally polarizes the neighboring molecules, inducing energetic disorder. The isomers can be separated by applying gentle mechanical vibrations during film crystallization, as confirmed by the suppression of the peak and improvement in device performance.
more » « less- PAR ID:
- 10234459
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Electronic Materials
- Volume:
- 3
- Issue:
- 1
- ISSN:
- 2199-160X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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syn andanti isomers of triethylsilylethynyl anthradithiophene (TES ADT) is systematically explored. The mobility of organic thin‐film transistors (OTFTs) comprisinganti TES ADT drops with the addition of only 0.01% of thesyn isomer and is pinned at the mobility of OTFTs having puresyn isomer after the addition of only 10% of the isomer. As thesyn isomer fraction increases, intermolecular repulsion increases, resulting in a decrease in the unit‐cell density and concomitant disordering of the charge‐transport pathway. This molecular disorder leads to an increase in charge trapping, causing the mobility of OTFTs to drop with increasingsyn‐ isomer concentration. Since charge transport is sensitive to even minute fractions of molecular disorder, this work emphasizes the importance of prioritizing structural compatibility when choosing material pairs for guest–host systems. -
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Abstract The
syn andanti isomers of [FeIV(O)(TMC)]2+(TMC=tetramethylcyclam) represent the first isolated pair of synthetic non‐heme oxoiron(IV) complexes with identical ligand topology, differing only in the position of the oxo unit bound to the iron center. Both isomers have previously been characterized. Reported here is that thesyn isomer [FeIV(Osyn )(TMC)(NCMe)]2+(2 ) converts into itsanti form [FeIV(Oanti )(TMC)(NCMe)]2+(1 ) in MeCN, an isomerization facilitated by water and monitored most readily by1H NMR and Raman spectroscopy. Indeed, when H218O is introduced to2 , the nascent1 becomes18O‐labeled. These results provide compelling evidence for a mechanism involving direct binding of a water moleculetrans to the oxo atom in2 with subsequent oxo–hydroxo tautomerism for its incorporation as the oxo atom of1 . The nonplanar nature of the TMC supporting ligand makes this isomerization an irreversible transformation, unlike for their planar heme counterparts.
