The objective of this research proposal focuses on understanding the fundamental optical and electronic properties of advanced semiconductor systems with an emphasis on examining how intentional or radiation-induced defects influence their carrier transport and recombination, and the role of defect passivation in enhancing transport. Chapter 2 reviews select semiconductors and optoelectronics with an emphasis on the infrared (IR) and visible spectra absorption. Chapter 3 presents the preliminary research used to extract fundamental material properties. The proposed research is detailed in Chapter 4, further investigating the trap states in TiO2-x Schottky diodes, the indirect-to-direct band transition in Ge1-xSnx with increasing Sn concentration, and the performance degradation of on-orbit Si, CZTS, and perovskite solar cells due to radiation displacement damage.