Hybrid Inorganic/Organic Nanostructured Tandem Solar Cells: Simulation and Fabrication Methods
Organic solar cell technologies continue to be an extremely active area of scientific research. With their promise of providing low-cost, easily-processable, multiapplication photovoltaics, these devices could very possibly be the most viable and practical form of renewable energy among many being explored. However, significant technological obstacles remain that must be overcome if this technology is to successfully realize the goal of providing abundant energy while simultaneously reducing
... neously reducing dependence on fossil fuel-based sources. Compared with inorganic solar photovoltaics, power conversion efficiencies in organics are still too low to compete economically. Much research has been accomplished over the past three decades in an attempt to optimize the performance characteristics of planar organic solar cell devices. Unfortunately, the limitations of device physics and the optical and electrical characteristics inherent in semiconducting polymers restrict the achievable efficiency for this type of structure. A "next generation" approach to surmounting this shortcoming is to stack multiple planar devices in such a way that enhanced performance is achieved. This tactic requires hybridization through the inclusion of inorganic metal oxide components that serve a number of functions such as electron and/or hole transporters/blockers and optical spacers. In this work, these "tandem" devices are modeled and their functionality simulated using computer-based algorithms in an effort to ascertain the ideal structural, optical, and electrical properties that must be designed into actual hybrid inorganic/organic photovoltaic devices. Additionally, several solar cells are fabricated and the methods described to show the many factors to be accounted for and controlled to achieve high-efficiency devices. Results produced in this study show that hybrid inorganic/organic solar cells can significantly improve power conversion efficiency over standard planar devicessimulated results show efficiencies over 9% are possible. Such factors as electron and hole mobilities, structural layer thicknesses, and choice of polymer and fullerene materials were found to be critical to the optimization of these structures. A key finding is that charge carrier mobilities in the subcells must be balanced so that space charge current limitations are avoided, thereby ensuring the maximum achievable current through the tandem structures. Hybrid inorganic/organic solar cells have tremendous promise as an alternative means of renewable energy production. Modeling and simulation are valuable tools that allow for the assessment of a multitude of various interdependent factors that impact the performance of these devices. By conducting this type of analysis prior to fabricating actual solar cells, considerable time and materials resources can be conserved while, at the same time, rapid prototyping can be accomplished and improvements in performance characteristics realized more quickly. V To my wife, Julie, and my two sons, Andrew and Robert, for their love, support and patience as we persevered through our shared "odyssey" toward whatever comes next. VI ACKNOWLEDGMENTS I thank my teacher, advisor, and mentor, Dr. Gon Namkoong, for his wisdom, guidance, and patience during my journey toward achieving this Ph.D. He's been a guiding force, an inspiration, and a true blessing to me and my family.