Flexible GaAs Optoelectronics for Photovoltaics and Biomedical Applications

Abstract

In recent years, flexible electronics have gained considerable attention from both academia and industry because of their potential applications in flexible photovoltaics, flexible displays, healthcare sensors, disease diagnostics, therapies, etc. Compared to traditional electronic applications, these unusual applications require higher standards to mechanical flexibility, weight, and electrical performances. Among various approaches to enable these unusual applications, one promising approach is transfer-printing. Transfer-printing is the technology that releases thin active layers of the semiconductor devices from an original wafer and integrates them onto a flexible substrate using an elastomeric stamp with a sticky surface. After transfer-printed onto a flexible substrate, the thin-filmed semiconductor devices exhibit not only high electronical performances, but also mechanical flexibility. However, transfer-printing has several challenges: anchors complicate conditions of the process; and interlayer adhesives increases electrical and thermal resistances. This thesis introduces the novel releasing and printing technology which requires no adhesives nor anchors, and then demonstrates its application for photovoltaic, wearable devices and bio-medical electronics. First, initial research has been focused on the development of the new transfer-printing technique which doesn’t require interlayer adhesives by using cold-welding process with overlayer adhesive. By using the proposed printing technology, ultra-thin vertical GaAs photovoltaics can be integrated onto metal surfaces. Both experimental and theoretical analysis proved that the ultra-thin GaAs photovoltaics can tolerate a bending radius of 1.4 mm without degradations in electrical performances. Moreover, the electrical performances of the ultra-thin GaAs photovoltaics have been improved due to photon recycling. Then, this thesis successfully demonstrated an attachable and flexible pulse sensor to monitor essential cardiovascular signals, and a skin-attachable micro LED light source patch to wirelessly power implantable devices. Operating in a reflection mode, the attachable pulse sensor could measure heart pulse waveforms from various locations on the human body such as fingers, fingertips, nails, and forearms. The attachable micro LED light source patch, which was integrated onto a wide and thick metal heat sink through the new release process using etch-holed film stamp without anchors, could wirelessly charge implantable devices in a body without electrical wires penetrating through skins. Computational and experimental analysis demonstrated that the attachable micro LED light source patch can be safe and comfortable on the skin, due to its wide and thick metal heat sink and its mechanical bridge design on the metal interconnects. In experiments on live animals, the skin-attachable micro LED light source patch successfully recharged battery and operated an implantable stimulator without any skin burns on the subject, proving feasibility of our proposed concept.