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We report on the structural and electronic interface formation between ITO (indium-tin-oxide) and prototypical organic small molecular semiconductors, i.e., CuPc (copper phthalocyanine) and alpha-NPD (N,N'-di(naphtalen-1-yl)- N,N'-diphenyl-benzidine). In particular, the effects of in situ oxygen plasma pretreatment of the ITO surface on interface properties are examined in detail: Organic layer-thickness dependent Kelvin probe measurements revealed a good alignment of the ITO work function and the highest occupied electronic level of the organic material in all samples. In contrast, the electrical properties of hole-only and bipolar organic diodes depend strongly on the treatment of ITO prior to organic deposition. This dependence is more pronounced for diodes made of polycrystalline CuPc than for those of amorphous alpha-NPD layers. X-ray diffraction and atomic force microscopic (AFM) investigations of CuPc nucleation and growth evidenced a more pronounced texture of the polycrystalline film structure on the ITO substrate that was oxygen plasma treated prior to organic layer deposition. These findings suggest that the anisotropic electrical properties of CuPc crystallites, and their orientation with respect to the substrate, strongly affect the charge carrier injection and transport properties at the anode interface.
Moderate doping leads to high performance of semiconductor/insulator polymer blend transistors
(2013)
Polymer transistors are being intensively developed for next-generation flexible electronics. Blends comprising a small amount of semiconducting polymer mixed into an insulating polymer matrix have simultaneously shown superior performance and environmental stability in organic field-effect transistors compared with the neat semiconductor. Here we show that such blends actually perform very poorly in the undoped state, and that mobility and on/off ratio are improved dramatically upon moderate doping. Structural investigations show that these blend layers feature nanometre-scale semiconductor domains and a vertical composition gradient. This particular morphology enables a quasi three-dimensional spatial distribution of semiconductor pathways within the insulating matrix, in which charge accumulation and depletion via a gate bias is substantially different from neat semiconductor, and where high on-current and low off-current are simultaneously realized in the stable doped state. Adding only 5 wt% of a semiconducting polymer to a polystyrene matrix, we realized an environmentally stable inverter with gain up to 60.
Current models for molecular electrical doping of organic semiconductors are found to be at odds with other well-established concepts in that field, like polaron formation. Addressing these inconsistencies for prototypical systems, we present experimental and theoretical evidence for intermolecular hybridization of organic semiconductor and dopant frontier molecular orbitals. Common doping-related observations are attributed to this phenomenon, and controlling the degree of hybridization emerges as a strategy for overcoming the present limitations in the yield of doping-induced charge carriers.
Variation in dative resumption among and within Alemannic varieties of German strongly favors an Evaluator component that makes use of optimality-theoretic evaluation rather than filters as in the Minimalist Program (MP). At the same time, the variation is restricted to realizational requirements. This supports a model of syntax like the Derivations and Evaluations framework (Broekhuis 2008) that combines a restrictive MP-style Generator with an Evaluator that includes ranked violable (interface) constraints.
In recent experimental work, arguments for or against Condition C reconstruction in A'-movement have been based on low/high availability of coreference in sentences with and without A'-movement. We argue that this reasoning is problematic: It involves arbitrary thresholds, and the results are potentially confounded by the different surface orders of the compared structures and non-syntactic factors. We present three experiments with designs that do not require defining thresholds of 'low' or 'high' coreference values. Instead, we focus on grammatical contrasts (wh-movement vs. relativization, subject vs. object wh-movement) and aim to identify and reduce confounds. The results show that reconstruction for A'-movement of DPs is not very robust in German, contra previous findings. Our results are compatible with the view that the surface order and non-syntactic factors (e.g. plausibility, referential accessibility of an R-expression) heavily influence coreference possibilities. Thus, the data argue against a theory that includes both reconstruction and a hard Condition C constraint. There is a residual contrast between sentences with subject/object movement, which is compatible with an account without reconstruction (and an additional non-syntactic factor) or an account with reconstruction (and a soft Condition C constraint).