Recently, it has been shown by several groups that the electrical characteristics of organic thin‐film transistors (OTFTs) can be significantly influenced by depositing self‐assembled monolayers (SAMs) at the organic semiconductor/dielectric interface. In this work, the effect of such SAMs on the transfer characteristics and especially on the threshold voltage of OTFTs is investigated by means of two‐dimensional drift‐diffusion simulations. The impact of the SAM is modeled either by a permanent space charge layer that can result from chemical reactions with the active material, or by a dipole layer representing an array of ordered dipolar molecules. It is demonstrated that, in both model cases, the presence of the SAM significantly changes the transfer characteristics. In particular, it gives rise to a modified, effective gate voltage Veff that results in a rigid shift of the threshold voltage, ΔVth, relative to a SAM‐free OTFT. The achievable amount of threshold voltage shift, however, strongly depends on the actual role of the SAM. While for the investigated device dimensions, an organic SAM acting as a dipole layer can realistically shift the threshold voltage only by a few volts, the changes in the threshold voltage can be more than an order of magnitude larger when the SAM leads to charges at the interface. Based on the analysis of the different cases, a route to experimentally discriminate between SAM‐induced space charges and interface dipoles is proposed. The developed model allows for qualitative description of the behavior of organic transistors containing reactive interfacial layers; when incorporating rechargeable carrier trap states and a carrier density‐dependent mobility, even a quantitative agreement between theory and recent experiments can be achieved.

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The gyrokinetic paradigm in the long wavelength regime is reviewed from the perspective of variational averaging (VA). The VA-method represents a third pillar for averaging kinetic equations with highly-oscillatory characteristics, besides classical averaging or Chapman-Enskog expansions. VA operates on the level of the Lagrangian function and preserves the Hamiltonian structure of the characteristics at all orders. We discuss the methodology of VA in detail by means of chargedparticle motion in a strong magnetic field. The application of VA to a broader class of highly-oscillatory problems can be envisioned. For the charged particle, we prove the existence of a coordinate map in phase space that leads to a gyrokinetic Lagrangian at any order of the expansion, for general external fields. We compute this map up to third order, independent of the electromagnetic gauge. Moreover, an error bound for the solution of the derived gyrokinetic equation with respect to the solution of the Vlasov equation is provided, allowing to estimate the quality of the VA-approximation in this particular case.• strong error estimate for the gyrokinetic solution with respect to f , solution of (1).We are able to prove existence with a new ansatz for the GY-transformation as a finite power series in ε, algebraic in the generating functions, in contrast to the usual Lie-transform approach, which relies on operator exponentials of Poisson brackets. Prerequisites for understanding existing formal VA-theories [9,22,23,39,44] include a firm knowledge about exterior calculus, differential forms and Lie transforms, with rare exceptions [34,41]. Our theory does not rely on these concepts and is thus more accessible for non-specualists. The long wavelength regime is considered, hence the inclusion of finite-Larmor radius effects postponed to a future work. We stress the non-uniqueness of transformations leading to GY-Lagrangians, which is overlooked in the existing VA-theories. A new GY-transformation is presented which leads to simpler equations of motion; this is possible due to the freedom of "unloading" complicated terms into the transformation (the generating functions), rather than keeping them in the Lagrangian.The methodology of VA is carefully developed in this work. The concept of the "tangent map" between two coordinate representations of a manifolds's tangent bundle is introduced in detail. We then shift the focus to a particular class of Lagrangian functions of the form (13), linear in the tangent vectors. The VA-theory developed here could in principle be applied to a large class of highly-oscillatory problems, formulated in terms of this generic Lagrangian. The charged particle is a prototypical example and treated in detail.Historically, the first approach towards reduced GY-models stems from averaging Newton's equation of motion for the charged particle [28,37]. Assuming a uniform static magnetic field, these can be solved exactly to yield the spiraling motion around the straight field lines. In this case the GY is well-defined ...

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