Charge Dynamics and Device Physics in Bulk-Heterojunction Organic Photovoltaics

Organic photovoltaic (OPV) technology is still advancing towards commercialization with its potential for low-cost and light-weight flexible electricity generating applications. However, the lack of thorough understanding on the charge dynamics including charge generation, recombination, and transport processes still hinders the further boost of OPV device performance. The aim of my research is applying advanced characterization techniques to gain comprehensive understandings on charge dynamics in various OPV systems. Particularly the impedance spectroscopy was extensively applied as a powerful and non-destructive tool to study charge carrier collection and recombination. Meanwhile, time-resolved charge extraction measurement was performed as a complementary method for recombination study. External quantum efficiency spectroscopy was also carefully adapted to probe the device energetics. On the other hand, to better understand the experimental results, numerical drift-diffusion simulation and transfer matrix method modeling were carried out to provide theoretical guidance. With the combination of these techniques morphology effects, including the donor-acceptor interface and structural order, on recombination in OPV devices were thoroughly investigated. The photocurrent loss mechanisms in a novel fullerene-based OPV system were systematically and quantitatively analyzed. Moreover, the defect characterization using capacitance-frequency analysis was discovered to have severe artifacts when applied to OPV devices. In general, this research not only carefully addressed the charge dynamics in several OPV systems, but the multifaceted approaches it demonstrated also provide insights for future photovoltaic research.