Renewable energy sources are becoming increasingly important as the world's energy demands continue to grow and climate change continues to occur. Photovoltaic power generation has great potential, but most commercial photovoltaic cells have typically been made from single-crystal or polycrystalline silicon which are expensive and energy intensive to produce. Organic semiconductors are one class of solar cell materials that have the potential for low cost due to solution processability, roll-to-roll fabrication, material tunability, and the very small amounts of material required to absorb light compared to crystalline silicon. Organic photovoltaic (OPV) efficiencies have increased rapidly in the past few years and are currently at about 11% power conversion efficiency.
To find new pathways to improve OPV performance, a deeper understanding of the relationships between morphology, electronic properties, and OPV performance is required. This work will explore these relationships, focusing on the relationship between morphology and charge transfer states. We focus on a small molecule material system with only pure donor and acceptor domains, unlike the typical bulk heterojunction composed of donor, acceptor, and mixed domains. The advantage of studying this system is a more well-defined interfacial area and simpler morphology that allows us to better characterize charge transfer states at the donor-acceptor interface.
We first review the basic principles of OPV operation, fabrication and characterization methods, and charge transfer states. We then use grazing incidence wide angle X-ray scattering to characterize the crystal structures and textures of the polymorphs of the donor and acceptor in our small molecule system, which are important for understanding their blend morphology. Next, we demonstrate a new method to control morphology in this system with the use of a thermally degradable binder polymer. Afterwards, we vary interfacial area and charge transfer state density in this system through processing and characterize it using sub-bandgap external quantum efficiency measurements, allowing us to gain valuable insights about the photophysics of this system.
Finally, many of the insights and techniques used to study this system are also useful in studying other systems of organic or solution-procesed semiconductors. We optimize the morphology and performance of OPVs made with novel low-bandgap donor-acceptor copolymers. We then find that ionic photoconductivity plays an important role in the behavior of photodetectors made with solution processed amorphous ZnO and in their interaction with organic semiconductors.