Overcoming Energy Losses for Efficient Organic Solar Cells

University of Minnesota Ph.D. dissertation. September 2019. Major: Material Science and Engineering. Advisor: Russell Holmes. 1 computer file (PDF); xiv, 219 pages.Organic photovoltaic cells (OPVs) are promising for low-cost solar energy harvesting as they are light-weight, mechanically flexible and compatible with large area fabrication methods. The power conversion efficiency of OPVs, however, still lags behind inorganic counterparts, which limits their widespread commercial application. This dissertation is focused on understanding the various energy loss mechanisms in OPVs and devising general design rules for minimizing these losses. Among all parasitic energy loss pathways in OPVs, charge recombination is a major source of inefficiency in state-of-the-art systems. It can take place at the donor-acceptor interface when charges are bound by electrostatic forces as charge-transfer (CT) states, or in bulk active layers when free charge carriers are in transit to electrodes. To develop a detailed understanding of recombination loss mechanisms, a novel technique based on transient photovoltage has been developed, which allows quantitative elucidation of the dominant recombination mechanisms (CT state vs. free charge carrier losses) in OPVs. Using the information obtained from photovoltage measurement, strategies have been developed to suppress charge recombination losses in various OPV systems, including optimizing thin film morphology that facilitates charge transport for dipolar donor materials system and devising advanced device architectures that can stabilize CT states and maximize charge collection for metal phthalocyanine-fullerene materials system. Efforts have also been devoted to understanding and engineering the transport of CT states, a potential strategy that reduces recombination losses in OPVs. Based on the detailed understanding of charge recombination losses, a device-based methodology has been developed to probe exciton losses in OPVs. It is the first method capable of probing the intrinsic active material exciton diffusion length, equally applicable to both luminescent and dark materials. With this novel technique, exciton transport has been investigated in various excitonic semiconductor systems, including dark small molecules, polymers, inorganic semiconductor quantum dots. Moving forward, topics like exploring long-range CT state migration and understanding singlet fission mechanisms are pathways towards enhanced device efficiency