Improved Stability of Thin-Film Multijunction Thermal Converters

We have demonstrated the long-term stability and environmental durability of a high-performance thin-film multijunction thermal converter (TMJTC) with an improved thin-film thermopile design. We fabricated a new TMJTC with an improved deposition pattern for a Bi-Sb-Cu-based thin-film thermopile in order to avoid interface delamination between the Bi and the Cu layers. Our TMJTC exhibited good frequency properties, with an ac-dc difference of less than 5 V/V in the range from 10 Hz to 100 kHz, at a root-mean-square (rms) voltage of 10 V. The long-term stability of the TMJTC has been significantly increased owing to the performance of the developed thin-film thermopile, as compared with that of previously fabricated TMJTCs, exhibiting an ac-dc difference of less than 2 V/V after 18 months at room temperature. We also evaluated the temperature, relative humidity, and pressure characteristics of the TMJTC in accordance with the International Electrotechnical Commission (IEC) standards for electronic devices. The measurement results showed that the dependence of the ac-dc transfer difference on the temperature, humidity, and pressure was less than 1 V/V, which is comparable to the results of thermal converters, constituting the primary standard.