Processing and Optical Engineering of Polymer-Fullerene Solar Cells Towards High Efficiency and Environmentally Friendly Fabrication for Real-Life Applications

Thesis (Ph.D.)--University of Washington, 2014Facing the tremendous challenges of energy shortage and global warming, embracing renewable energies is an obvious choice and necessary action. Solar energy is arguably the most important source of renewable energy. Organic photovoltaics (OPV) has the potential to make the solar energy much more affordable. However, OPV technology is still relatively new and immature and requires substantial improvements on device performance, stability, fabrication, and many other aspects to make OPV actually useful. The studies described in this dissertation are aimed to address some of the critical challenges and provide ideas for future developments. In chapters 1 and 2 the motivations and current developments of OPV research are briefly overviewed. In chapter 3, two high performance polymer:fullerene bulk-heterojunction solar cells used in the works of this dissertation are described. Chapter 4 describes our developments of a device processing approach that involves completely halogen-free solvents, which is desirable to achieve sustainable large-scale fabrication of OPV cells. The solvent system, consisting of small amount of a novel solvent additive 1-methylnaphthalene (Me-naph) in common halogen-free solvent matrix, can drastically improve phase-separated morphology of OPV devices to achieve efficient charge separation and yield high-performance. In Chapter 5, OPV devices in a novel optical resonant cavity structure are systematically investigated. Such structure has exhibited several advantages: first, light trapping of thin film OPV devices can be boosted by carefully tuning the optical field in the resonant cavity. Second, replacing the typical transparent ITO electrode with semi-transparent TeO2/Ag electrode simultaneously solves or ameliorates multiple problems associated with the ITO electrode such as high cost, brittleness, and limited conductance, making the cavity structure viable for making flexible and large-area device. In Chapter 6, semi-transparent organic photovoltaic (OPV) cells with high device performance and tunable transparency are demonstrated. The devices not only possess high performance to transparency ratio but also close to perfect color rendering index (nearly 100), making it a strong candidate for power-generating window applications