, at which the insulator-to-metal transition occurs. We discuss the electrical properties of Se point defects and Se complexes, shedding light on the formation and the nature of the impurity band in the bandgap and how the presence of different types of complexes may increase the broadening of the impurity band and affects the insulator-to-metal transition. We identify the best doping range in which the properties of the impurity band can be engineered according to the needs of the electronic industry. Simulations of the structural properties of the complexes complete the work. Our findings are relevant for intermediate impurity band applications.