Layered black phosphorus (BP) has been recognized as a promising elemental two-dimensional (2D) material for its contrast in properties with graphene and other layered transition-metal dichalcogenides semiconductors. However, its tendency to oxidize under ambient conditions hampers its adaptability for applications in electronics and optoelectronics. While defective BP is considered undesirable for real world applications, the presence of defects in other materials has been utilized to create new functionalities. In this chapter, an overview is provided on the new functionalities and their applications in optoelectronic devices ranging from health management to neuromorphic computation, all generated by defect states in layered BP.