25. Probing the Origin of 2D Covalent Organic Frameworks with High Thermoelectric Performance Down to the Ångström Level

Abstract

Covalent organic frameworks promise inexpensive, low-toxic, and lightweight thermoelectrics for energy harvesting and temperature control. Nevertheless, systematic materials development has been hitherto impeded by a poor understanding on the relationships among performance, microscopic transport processes, and chemical structures. Here, by using a multi-scale first-principles computational scheme hybridizing the model-driven and data-driven learning approaches, an integrated framework describing their lattice and charge-carrier dynamics is established, and thus advance the existing knowledge of synthesis and performance to the atomistic-level understanding. It is unveiled that continuous tunable thermoelectric figure of merits of covalent organic frameworks can be achieved through the atomic-scale chemical modifications, in which not only the high mobility but also the low lattice thermal conductivity lies at the heart of their decent thermoelectric efficiency. It is corroborated that the enhanced interlayer electronic couplings by substituting with relatively heavier elements minimizes the negative impacts caused by the inherent stacking disorder. Such a strategy concomitantly gives rise to the more low-frequency optical phonons and the softened acoustic modes, markedly strengthening the vibrational anharmonicity and suppressing the thermal transport. It is anticipated that our new insights lay the groundwork for designing new thermoelectric covalent organic frameworks with higher performance.