Numerical methods for organic optoelectronic devices: simulations and experiments

Abstract

In recent years, the field of organic electronics has been experiencing a great expansion, due to several characteristics which candidate it as a main player in the definition of new markets comprising low-cost, flexible and biocompatible electronics. Although many experimental works on the optimization of organic devices have been performed, a real improvement in performance is subordinate to a good understanding of the underlying physical phenomena. At this purpose, computer-based simulations are of great importance for the determination of suitable high-level models and the identification of limiting factors. This thesis is focused on the application of state-of-the-art Technology Computer Aided Design (TCAD) tools to organic electronics, aiming to show how models peculiar to this field can be integrated into a commercial, massproduction oriented software and exploited for the analysis and design of novel devices. In this respect, particular importance is given to Organic Phototransistors (OPTs) and Organic Photodiodes (OPDs), which rely on Bulk Heterojunction (BHJ) organic semiconductors in order to enhance the photogeneration quantum yield. To study the transport properties of a BHJ, testbed Organic Field-Effect Transistors (OFETs) are fabricated on Silicon substrates with conventional techniques, such as spin-coating deposition. The current-voltage characteristics and impedance curves of the OFETs are described using TCAD simulations. This analysis shows how the transport of charge is limited by the presence of electronic traps in the material, which negatively affect the subthreshold swing and cut-off frequency of the OFET. These considerations can be directly applied to vertical OPTs. A comparative modeling study is performed in comparison to a planar OPT with means of TCAD simulations. Results show that vertical devices can outperform the planar ones in both electrical and optical characteristics, which confirms vertical OPT a promising technology due to the advantages of reduced channel length and large sensitive area. The TCAD methodology also applies to the design rather than analysis only. This concept is demonstrated on a novel OPD architecture, in which a wire-grid polarizer is directly integrated into the device in order to make the photocurrent sensitive to light polarization. The OPD is studied and optimized using numerical simulations, stressing the effect of important physical and geometrical parameters. Consequently, a proof-of-concept of the OPD is demonstrated and the model is refined. A Monte Carlo approach is also proposed in order to enhance the semiconductor models used for the simulation of BHJ materials. In conclusion, this work describes a complete framework in which organic electronics models are integrated with state-of-the-art TCAD tools. It is our opinion this approach will set the basis for a better understanding and design of organic electronic devices in the near future. [edited by author]