The past years has observed a significantly boost of the thermoelectric materials in the scale of thermoelectric figure-of-merit, i.e. ZT, because of its promising application to harvest the widely distributed waste heat. However, the simplified thermoelectric materials' performance scale also shifted the focus of thermoelectric energy conversion technique from devices-related efforts to materials-level works. As a result, the thermoelectric devices-related works didn't get enough attention. The device-level challenges behind were kept unknown until recent years. However, besides the thermoelectric materials properties, the practical energy conversion efficiency and service life of thermoelectric device is highly determined by assembling process and the contact interface. In this perspective, we are trying to shine some light on the device-level challenge, and give a special focus on the thermoelectric interface materials (TEiM) between the thermoelectric elements and electrode, which is also known as the metallization layer or solder barrier layer. We will go through the technique concerns that determine the scope of the TEiM, including bonding strength, interfacial resistance and stability. Some general working principles are summarized before the discussion of some typical examples of searching proper TEiM for a given thermoelectric conversion material.