Robby Janneck

Predictive Model for the Meniscus-Guided Coating of High-Quality Organic Single-Crystalline Thin Films

A model that describes solvent evaporation dynamics in meniscus-guided coating techniques is developed. In combination with a single fitting parameter, it is shown that this formula can accurately predict a processing window for various coating conditions. Organic thin-film transistors (OTFTs), fabricated by a zone-casting setup, indeed show the best performance at the predicted coating speeds with mobilities reaching 7 cm2 V-1 s-1 .

Highly Crystalline C8-BTBT Thin-Film Transistors by Lateral Homo-Epitaxial Growth on Printed Templates

Highly crystalline thin films of organic semiconductors offer great potential for fundamental material studies as well as for realizing high-performance, low-cost flexible electronics. The fabrication of these films directly on inert substrates is typically done by meniscus-guided coating techniques. The resulting layers show morphological defects that hinder charge transport and induce large device-to-device variability. Here, a double-step method for organic semiconductor layers combining a solution-processed templating layer and a lateral homo-epitaxial growth by a thermal evaporation step is reported. The epitaxial regrowth repairs most of the morphological defects inherent to meniscus-guided coatings. The resulting film is highly crystalline and features a mobility increased by a factor of three and a relative spread in device characteristics improved by almost half an order of magnitude. This method is easily adaptable to other coating techniques and offers a route toward the fabrication of high-performance, large-area electronics based on highly crystalline thin films of organic semiconductors.

Contact resistance characterization in organic thin film transistors (Conference Presentation)

Proper thin film transistor (TFT) operation requires that its contact resistance Rc remains only a fraction of its channel resistance Rch. The integration of thin films based on latest generation organic semiconductors into downscaled TFTs with short channel length and high capacitance dielectric results in devices with very low Rch. Matching this with a low enough Rc is very challenging, due to the notoriously poor charge injection into organic semiconductors. The viability of integrated circuit technologies based on organic TFTs hinges on solving this critical contact resistance issue. To properly address this, it is important to use a common metric based on simple, comparable contact resistance measurements. Rc is commonly measured using the Transfer Length Method (TLM) that involves the characterization of TFTs of different channel lengths in the linear regime. We find, however, that the precision and the absolute value of the extracted Rc is greatly influenced by the conditions used to characterize each TFT. This seriously complicates the comparison to other literature values. In this talk, we present an in-depth study of the TLM technique aimed at solving these particular problems. Our TLM structures are based on high mobility organic TFTs, fabricated with different technologies and topologies. We conduct a systematic comparison of voltage- and current-controlled measurements with constant lateral electric field and charge density. As a result, we delineate the conditions to conduct TLM characterization and data treatment for clean Rc extraction. We also identify the measurement parameters that count in establishing a good Rc benchmark.

Negative field‐dependent charge mobility in crystalline organic semiconductors with delocalized transport

Charge-carrier mobility has been investigated by time-of-flight (TOF) transient photocurrent in a lateral transport configuration in highly crystalline thin films of 2,7-dioctyl[1]benzothieno [3,2-b][1] benzothiophene (C8-BTBT) grown by a zone-casting alignment technique. High TOF mobility has been revealed that it is consistent with the delocalized nature of the charge transport in this material, yet it featured a positive temperature dependence at

Determination of crystal orientation in organic thin films using optical microscopy

T \ge 295\,{\text{K}}

Influence of the Surface Treatment on the Solution Coating of Single‐Crystalline Organic Thin‐Films

T≥295K. Moreover, the mobility was surprisingly found to decrease with electric field in the high-temperature region. These observations are not compatible with the conventional band-transport mechanism. We have elaborated an analytic model based on effective-medium approximation to rationalize the puzzling findings. The model considers the delocalized charge transport within the energy landscape formed by long-range transport band-edge variations in imperfect organic crystalline materials and accounts for the field-dependent effective dimensionality of charge transport percolative paths. The results of the model calculations are found to be in good agreement with experimental data.

Single-crystalline organic thin films: A predictive model that enables fast process optimization

Organic thin film transistors (OTFTs) based on single crystalline thin films of organic semiconductors have seen considerable development in the recent years. The most successful method for the fabrication of single crystalline films are solution-based meniscus guided coating techniques such as dip-coating, solution shearing or zone casting. These upscalable methods enable rapid and efficient film formation without additional processing steps. The single-crystalline film quality is strongly dependent on solvent choice, substrate temperature and coating speed. So far, however, process optimization has been conducted by trial and error methods, involving, for example, the variation of coating speeds over several orders of magnitude. Through a systematic study of solvent phase change dynamics in the meniscus region, we develop a theoretical framework that links the optimal coating speed to the solvent choice and the substrate temperature. In this way, we can accurately predict an optimal processing window, enabling fast process optimization. Our approach is verified through systematic OTFT fabrication based on films grown with different semiconductors, solvents and substrate temperatures. The use of best predicted coating speeds delivers state of the art devices. In the case of C8BTBT, OTFTs show well-behaved characteristics with mobilities up to 7 cm2/Vs and onset voltages close to 0 V. Our approach also explains well optimal recipes published in the literature. This route considerably accelerates parameter screening for all meniscus guided coating techniques and unveils the physics of single crystalline film formation.