Formation of an intermediate band in isoreticular metal–organic framework-993 (IRMOF-993) and metal-substituted analogues M-IRMOF-993

Intermediate band (IB) materials are attractive for multiple photon harvesting in solar cells thus increasing their efficiency beyond the Shockley–Quassier limit. However, it has so far been demonstrated that this can only be achieved in a few inorganic solids by appropriate doping. Here we demonstrate that it may be possible to achieve intermediate band materials with the isoreticular metal–organic framework IRMOF-993 and metal-substituted analogues. The equilibrium crystal structures, electronic structures, formation enthalpies, chemical bonding, and optical properties of M-IRMOF-993 (M = Zn, Cd, Be, Mg, Ca, Sr, Ba) were systematically investigated using density functional theory methods. The unit cell volume and atomic positions were optimized with the Perdew–Burke–Ernzerhof (PBE) functional; there was good agreement between the current theoretical equilibrium structural parameters and previously reported structural data for Zn-IRMOF-993. The calculated bulk moduli indicate that Zn-IRMOF-993 and its analogues are soft materials. The estimated fundamental bandgap values from the electronic structure studies for the whole series are ca. 3.5–3.6 eV, indicating a semiconducting character. The bandgap values estimated from the bottom of the IB to the top of VB are ca. 1.5–1.6 eV, and those from the top of IB to the bottom of CB are ca. 2.0 eV, suggesting that these materials may be suitable for enhancing the efficiency of solar cells. As MOFs are considered as potential materials for photocatalysts, active components in hybrid solar cells, electroluminescence cells, organic semiconducting devices such as field-effect transistors, and organic light-emitting devices, the optical properties and chemical bonding of M-IRMOF-993 were also systematically investigated.