The state-of-the-art organic solar cells (OSC) use bulk heterojunction (BHJ) blend architecture in the photo-active layer. The BHJ is formed by finely mixing polymer donor and small molecule acceptor, which was predominantly fullerene derivatives until the last five years. However, the emergence of non-fullerene acceptor (NFA) materials has been the viable alternative to overcome high synthetic costs, limited optical absorption, and poor bandgap tunability of fullerene-based acceptors. These unique properties of NFA has resulted in a rapid improvement of OSC efficiency and opened doors for wide variety of applications including building integrated photovoltaics, green houses and agrivoltaics. Despite these advantages, the shorter device lifetime under light and heat is a major concern for their commercialization. This dissertation is focused on improving poor photo- and thermal stability of high efficiency OSC based on the widely used NFA, ITIC and Y-series derivatives. The light-induced changes in the acceptor molecular structure and the active layer nanostructure results in the photo-induced traps in photo-aged devices. The selective addition of third component to the active layer impedes the changes in the active layer nanostructure and suppress trap formation. Under constant thermal stress, the growth of acceptor crystals results increases the trap-assisted recombination in thermally aged devices. Similar to photo-stability the selective addition of third or more component/s arrests the crystal growth by minimizing the Gibbs free energy. The results suggest that the fabricated hexanary and ternary OSC display a superior thermal stability than the respective binary devices. In addition, the hexanary devices displayed thickness independent thermal stability, which is essential for the active layer thermal stability printed via high throughput techniques.

Non-imaging Fresnel lenses have been playing an important role in improving the efficiency of the solar energy systems. Many researchers and scientists have devoted their research to optimize the design of the Fresnel lenses. Before it can contribute to energy efficiency increase, a Fresnel lens with optimized design will first need to be fabricated with the most cost-effective method as well as the best quality fabrication as possible. If targeted in a commercial market, feasibility of mass production with a minimum fabrication time would also be a consideration. To bring the design optimization of a Fresnel lens from a conceptual theory to a real-life increase in energy efficiency, the lens needs to be fabricated, tested, compared, and analyzed. This research thesis is intended to explore the performance of the lenses with optimized design through experimental investigations. The design optimization was achieved by a previous PhD student at UNT. A total of six lenses fabricated with four different methods along with two purchased lenses were tested with two different approaches. Multiple testing routes were conducted within a 10-month period to observe the effects of material decomposition and degradation on the lens performance. The resulting experimental data has provided a solid base for analyzing the performance of the lenses, in particular, the energy conversion efficiency increase of the solar cell by using each lens. The potential cause of the performance variation can be extracted from the comparison and evaluation.