Making better contacts faster: Combining electronic defect calculations with microprobe measurements for accelerated Mg2X-based TE device development

Kurzfassung

"Thermoelectric devices require, on the one hand, high performance functional materials and, on the other hand, an optimized interface design to minimize losses on the device level. During the development of Mg2X (X = Si,Sn)-based thermoelectric generators we have repeatedly ob-served a (local) change of the Seebeck coefficient of the TE material arising from the interface with the contacting electrode. This phenomenon is observed for several electrode materials at-tached to Mg2X but also for other thermoelectric materials, and is related to the diffusion of doping species from the electrode into the TE material forming charged point defects. As TE materials require optimized carrier concentration, a change of carrier concentration results in a performance degradation. Hence, electrode-induced point defect formation is a major obstacle for the design of thermoelectric devices. To address this challenge, we have developed an electrode pre-selection method based on first-principles electronic structure calculations of charged defect formation energies. Comparing the formation energies of intended point defects (to adjust optimal doping concentration) and potential electrode induced defects we can predict changes in the carrier concentration of the TE material. The calculation predictions for various candidate electrode materials are compared with microprobe measurements of the Seebeck coefficient. Excellent agreement was obtained, qualifying our approach as a method to exclude or shortlist elements as interface materials. A further challenge in understanding the electrode-TE material interaction (and the develop-ment of Mg2X-based TEG) is that degradation can also arise due to Mg loss from the Mg2X TE material, which changes the density of Mg-related intrinsic point defects and causes a detri-mental change in carrier concentration. To distinguish between both degradation mechanisms, we have performed comparative stability investigations on contacted Mg2X samples with and without ALD coating, where for the latter samples Mg sublimation is surpressed. We find that using Cu as an electrode causes a significant carrier compensation, with Mg loss by sublimation being a small additive effect (for unprotected samples) while for Al contacts, Mg sublimation is the more relevant degradation mechanism, in agreement with the predictions from the DFT re-sults. The principle approach is transferable to other material systems."