The Stretchable OLED Display

Documenting the evolution of flexible and stretchable electronics, this thesis highlights the shift from traditional circuits to innovative designs that leverage materials like organic semiconductors and advanced substrates for enhanced mechanical, optical, and electrical properties.

Recent advancements in flexible electronics, primarily due to effective management of bending strain, have unlocked diverse applications including wearable technology, flexible solar cells, bio-integrated devices, and notably, stretchable organic displays, with the latter being a focal point of this project. While bendable designs have successfully entered commercial production, the development of stretchable applications continues to pose significant challenges.

The focus here is on optimizing each element of organic light-emitting diodes, from elastomeric substrates and electrodes to active organic layers, balancing performance with cost-effectiveness for potential mass production. In particular, this project leverages a novel surface design to enhance the stretchability of thin film devices. Through the utilization of microscopic surface waves molded on substrates, thin films applied are subjected to reduced stress, when the devices are stretched, as the designed waves convert tensile into bending strain.

The effect is documented numerically and experimentally on substrate and thin film level. The deposition process for indium tin oxide was optimized to ensure homogeneous, defect-free thin films that seamlessly conform to the contours of the surface design. Experimentally, these high performance inorganic electrodes endure a three-fold increased stretch, when stretching devices with the incorporated surface design, relative to planar counterparts. The report focuses intensively on outcomes, emphasizing the experimental methodologies and essential processes uncovered in the pursuit of creating thin film devices that adapt seamlessly to the designed surface contour.

The substrates are produced using doctor-blade coating, while the thin films are deposited through processes like magnetron sputtering and evaporation techniques. Each process is investigated and optimized utilizing an extensive collection of characterization tools. This includes sophisticated, conventional, and project-specifically developed equipment, encompassing techniques like atomic force microscopy, scanning electron microscopy, gallium focused ion beam helium ion microscopy, and grazing incidence x-ray diffraction, alongside optical, electrical, and custom-built in situ electro-mechanical characterization systems. Additionally, the project utilizes finite element models for the numerical quantification of mechanical properties and in assessing the efficiency of organic semiconductor devices.

This project has successfully achieved the creation and characterization of functional light-emitting diodes on the stretchable substrates utilizing the intended surface wave design. These have been compared to their counterparts on rigid substrates, displaying comparable, though slightly lower, current efficiencies. However, the tensile characterization of these devices in operando has not been accomplished in this phase of the project.

In conclusion, this thesis underscores advancements and challenges in the ongoing journey towards the realization of fully functional stretchable organic light-emitting diode displays. Examining each aspect of the devices, from substrates to active layers and interconnects, is a methodical approach for in-depth research going forward, and crucial for addressing specific challenges and fostering innovation, paving the way for promising breakthroughs in stretchable electronics.