2D Metal-Organic Frameworks as a New Class of Thermoelectric Material*

Organic materials are desired in many fields of thermoelectric applications due to material abundance, light weight, and flexibility. Electrically conductive 2D metal-organic frameworks (MOFs) are a new class of organic material, and the two dimensionality and nanopores have a potential to realize a comparable thermoelectric figure of merit in 2D MOFs. As such, we measured thermal conductivity, electrical conductivity, and Seebeck coefficient of Copper Benzenehexathiol (Cu-BHT), an electrically conductive 2D MOF, in a mesoscopic device scale. We found that phonon mean free path is of the order of the length scale associated with nanopores and interlayer distance and it is responsible for the observed low thermal conductivity of 0.25 W/mK at room temperature. This result ensures low thermal conductivity in any 2D MOFs without artificially imposing nanostructures. Electronic transport and Seebeck coefficient measurements revealed that the charge carrier is n-type and the electronic transport is governed by the degree of polycrystallinity. However, unlike conductive polymers, each domain retains a definite crystal structure, resulting in an electronic band in each domain. This leads to a possibility of enhancing Seebeck coefficient by band engineering.