Numerical methods for organic optoelectronic devices: simulations and experiments

2016 - 2017In 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]XVI n.s

Simulation and fabrication of polarized organic photodiodes

Several important applications rely today on the detection of polarized light, as demonstrated by the wide range of sensing devices exploited over the years. Nevertheless, the miniaturization of such systems has been little explored. In this work, a possible solution towards the direct integration of the sensing optics within an electronic device has been established, utilizing a wire-grid polarizer in conjunction with a photodetector realized with organic semiconductors. The optical and electronic properties of the device have been studied and optimized using physically based numerical simulations. Consequently, a proof of concept of the photodetector has been demonstrated, having a polarization extinction ratio of 50 at a wavelength of 550 nm

Conductive Adhesive Based on Mussel-Inspired Graphene Decoration with Silver Nanoparticles

Decoration with silver nanoparticles was obtained by coating graphene with a polydopamine layer, able to induce spontaneous metallic nanoparticles formation without any specific chemical interfacial modifier, neither using complex instrumentation. The choice of dopamine was inspired by the composition of adhesive proteins in mussels, related to their robust attach to solid surfaces. The synthesis procedure started from graphite and involved eco-friendly compounds, such as Vitamin C and glucose as reducing agent and water as reaction medium. Silver decorated graphene was inserted as secondary nanofiller in the formulation of a reference conductive adhesive based on epoxy resin and silver flakes. A wide characterization of the intermediate materials obtained along the step procedure for the adhesive preparation was carried out by several techniques. We have found that the presence of nanofiller yields, in addition to an improvement of the thermal conductivity (up to 7.6 W/m\ub7 K), a dramatic enhancement of the electrical conductivity of the adhesive. In particular, starting from 3 \ub7 10 2 S/cm of the reference adhesive, we obtained a value of 4 \ub7 10 4 S/cm at a nanofiller concentration of 11.5 wt%. The combined double filler conductivity was evaluated by Zallen’s model. The effect of the temperature on the resistivity of the adhesive has been also studied