Interface layers play important roles in thermoelectric (TE) devices. The present study employs a phenomenological model to study the effects of internal and/or external interface layers on TE devices performance. Sets of general performance formulae are derived for both TE coolers and generators. Some simplifications are presented, and familiar performance formulae are found by introducing effective or equivalent properties such as electrical resistivity, thermal conductivity and Seebeck coefficient. Moreover, temperature-entropy diagrams are built to understand the effects of interface layers on TE thermodynamic cycles. Together with the power-efficiency curves, TE devices performance can be fully comprehended. In addition, a popular example is analysed, and new results are presented.