Aspects of semiconductivity in soft, porous metal-organic framework crystals

Metal-organic frameworks (MOFs) are known for their vast design space of possible structures, covering a wide range of often porous crystal structures and physical properties. Electrical conductivity, though, was-until very recently-not a feature usually associated with MOFs. On the other hand, well defined porous media such as MOFs, showing some measure of conductivity, could find uses in a huge number of fields ranging from electrochemistry to electronics and sensing. In this work, we therefore investigate the different aspects contributing to the bad conductivity in MOFs. Using Bardeen-Shockley deformation potential theory, we devise an approach that allows us to gauge all factors influencing the conductivity, including the availability of free charge carriers and their mobility. The latter itself is determined by the effective masses of the charge carriers, the material's elastic constants, and the deformation potential constants, which measure an effective electron-phonon coupling. Based on these parameters, we study charge carrier mobility in metal (1,2,3)-triazolate MOF crystals, M(ta)₂, where the metal is either iron, zinc, or ruthenium. Thereby, Zn(ta)₂ was experimentally shown to have little to no conductivity, while Fe(ta)₂ is one of the best currently known MOF semiconductors. Disregarding the fact that all three investigated MOFs show near-zero carrier densities due to their large bandgaps, our calculations reproduce the trends between Zn(ta)₂ and Fe(ta)₂. In contrast to that we find the Ru(ta)₂ MOF, which to date has not been synthesized experimentally, to yield even better performance than iron triazolate. In summary, assuming, fox example, light doping to counter the large bandgap, our analysis of the factors influencing conductivity in MOFs allows us not only to confirm experimental trends but also to predict new, as yet unknown semiconducting MOF crystals.