Materials and Devices for Energy Autonomous Distributed Electronics

The growing energy demand of humankind and the concern about climate change require rethinking how energy is provided, both on a macroscopic scale and a microscopic one, e.g. for distributed electronics. For the latter, a promising approach is to use energy harvesting from ambient sources such as light. Energy harvesting needs intermediate energy storage, such as a supercapacitor or a battery. In contrast to batteries, which store electrical energy chemically, energy is stored in the electric field at the interface between electrode and electrolyte in supercapacitors. Supercapacitors are promising energy storage due to potentially non-toxic materials and longer cycle life compared to batteries. By using printing processes, we can fabricate flexible supercapacitors suitable for Internet of Things purposes.

The goal of the research in this thesis is to develop an environmentally friendly, printable, and flexible energy source for distributed electronics. One focus on the path to this goal is the fabrication and study of flexible printed supercapacitors using novel methods, materials, and architectures with a target of improved performance and manufacturability. Monolithically fabricated supercapacitors were improved by a novel composite made from chitosan and micro-fibrillated cellulose, used as a printable separator in aqueous supercapacitors. The electrical performance of the devices was improved to a level comparable with laminated supercapacitors while maintaining the advantages of monolithic supercapacitors for manufacturing and system integration. We also studied current collectors made from graphite foil and aluminum coated with graphite inks for flexible supercapacitors when low equivalent series resistance (ESR) is required. The ESR decreased by more than 80 % compared to devices using graphite ink alone. The use of a dense graphite protective layer on top of aluminum prevented corrosion, and the supercapacitors' performance was stable for at least 950 days.

The second focus of the research was integrating monolithic printed supercapacitor modules and flexible photovoltaic modules onto a single substrate to provide a monolithic energy module for energy-autonomous, low power Internet of Things devices. The energy module comprised a flexible organic series-connected photovoltaic module and a series-connected supercapacitor module. Indoor light was sufficient for the Photovoltaic (PV) module to charge the supercapacitors to a level that can drive low-power wireless sensor nodes when there is no external energy input, e.g. overnight.

The last part of the thesis aims to improve long-term indoor light harvesting by investigating new materials that can be used in PV modules. This work is related to dye-sensitized solar cells and looks at novel inorganic-organic hybrid systems. To understand and use novel materials in dye solar cells, it is necessary to understand the photo-induced energy and charge transfer processes in the materials. To achieve such an understanding, precise optical spectroscopy tools are needed, such as steady-state and time-resolved spectroscopic methods. The last part of the thesis reports a detailed study of photo-induced processes between an organic material, a novel phthalocyanine, and an inorganic semiconductor quantum dot. The study confirmed hole transfer from the quantum dot valence band to the phthalocyanine highest occupied molecular orbital after photo-excitation of quantum dots. The studied materials can be used potentially for making solar cells.