Quantitative modeling of charge transport in organic semiconductor devices [electronic resource]
Keywords: Electrical Engineering.

Organic semiconductors have attracted significant interest in recent years for applications in low-cost and large area electronics; for example, flexible displays and solid state lighting, photovoltaics, biosensors, disposable electronics, and low cost RFID tags. Their unique properties make them compatible with high throughput roll-to-roll printing and low temperature deposition, thus allowing the utilization of inexpensive and flexible substrates. Although some commercial applications, such as organic light emitting diode displays, already exist; organic semiconductors still need further development. The success of organic semiconductors in commercial applications requires a deeper understanding of the factors limiting or degrading their performance. In particular those creating defects that lead to reduction of mobility or creation of electronic traps. Identifying those traps and linking them to their physical origin is therefore an important step forward in the evolution of organic semiconductors. Modeling electrical characteristics is an interesting technique that can be used to understand how processing parameters or other environmental factors affect material and device performance. However, attention must be paid to assess that the model fully describes measured devices in order to obtain reliable parameters estimations. In this thesis a series of models are described that allow to estimate semiconductor properties, such as mobility and trap density, from electrical measurements of thin film transistors and unipolar diodes. First, the analysis of transfer curves from polymeric transistors is used to understand the effect that regioregularity defects, degree of crystallinity, and angular distribution of crystallites have on the electrical properties of the material. Results indicate that none of them play a significant role on the total concentration of trap states. The model is then extended to study the electrical properties in unipolar diodes, in which current is space-charge limited. This particular geometry requires the model to account for diffusion current, asymmetries in the contacts, and non-homogeneities in the semiconductor; three factors that are typically ignored in the literature. A thorough error analysis allows us to estimate the energy range where the trap distribution can be estimated reliably. Finally, defects are induced in a rubrene single-crystals by means of ultra-violet ozone exposure and X-ray irradiation. The models developed in this work are used to determine how different the energetic and spatial signatures of the induced traps are. Oxygen-related states centered around 0.35 eV and spatially located near the surface of the crystal, are generated after ultra-violet ozone exposure. In addition the mobility in the same region is severely affected. X-ray irradiation, in contrast, generates a much broader distribution of traps, with no preferred energy. Surprisingly, the spatial distribution indicates that, even though X-ray are supposed to be absorbed uniformly through the crystal, the induced defects have a higher concentration near the top and bottom surfaces of the crystal.