**5. Conclusions**

*Optoelectronics*

One way to achieve this is designing lithium-based metal organic frameworks (Li MOF) where excess lithium is transferred through the defects in the MOF structure. However, research regarding Li MOFs as solid electrolytes is currently lacking. The majority of MOF/electrolyte studies are only focused on employing MOFs as a host of ionically conductive materials rather than utilizing MOFs as solid-state electrolytes. Therefore, we identified this area of research in the proposed roadmap to stimulate investigating the potential of Li-based MOFs as solid electrolytes. There are different types of Li-MOFs already developed [110–112], but many of them are designed for applications other than battery electrolytes. We believe that Li- MOF structures can be tuned for lithium transport. Overall, Li MOFs show potential for the use as solid ionic conductors and much research should be performed to explore

Exploring thermoelectric properties of MOFs emerges five years ago along with exploring the electronic properties of MOFs by systematic structural modifications and introducing guest molecules onto MOFs. The first thermoelectric property measurements on MOFs has introduced by Erikson in 2015 [113]. then, up to date, there have been less than ten publications in thermoelectric MOFs, thus this field of research is relatively new. Highly nanoporous MOFs are promising since porosity can reduce the lattice thermal conductivity. The effect the conjugation length of the organic linker that tailors the pore dimension for lattice thermal conductivity must be investigated. The thermoelectric figure of merit that measures the efficiency of a thermoelectric device can be improved by decreasing the lattice thermal conductivity. It is believed that changing the conjugation length or the complexity of the organic linker changes phonon scattering, thereby changing the lattice thermal conductivity [77, 114]. The ligand modifications can be successfully achieved by isoreticular synthesis approaches. Also, increasing the porosity of MOFs increases phonon scattering that also reduces thermal conductivity [114]. Therefore, in order to utilize MOFs as active materials in thermoelectric devices, understanding the contribution of phonon vibrations to lattice thermal conductivity is essential and must be investigated. Directing future research on thermoelectric MOFs towards experimentally investigating thermoelectric properties of MOF based thin films to find ways of decreasing thermal conductivity by structural modifications to the

In order to use MOFs as photoactive layer for energy harvesting and conversion, MOFs should possess decent light harvesting capability in the region from visible light to near-infrared (NIR). As the material's light-harvesting window is primarily determined by its band gap, synthesizing a MOF with a semiconducting band gap that can absorb light in the solar spectrum should be a requirement for it to serve as the photoactive material. Given that the electronic configuration of MOFs is contributed by both the constituent metal ion and the organic linker, the resultant bandgap and semiconducting properties of MOFs can thus be tailored by their structural design and engineering. Since most MOFs possess large band gap due to lack of overlap between metal ion and the organic linker and low degree of conjugation, they cannot effectively absorb light in the solar spectrum. The ligand center of MOFs plays a dominant role in its resulting light harvesting behavior [77, 114]. Tailoring the structure and its composition, MOFs charge transfer processes can be improved to enable the photocurrent of MOFs and fulfilling the photoactive

To effectively reduce the band gap of MOFs and enrich their semiconducting properties for photovoltaic applications, three strategies can be implemented and have been identified [115]. These strategies are: (1) selecting electron rich metal nodes and conjugated-based organic molecules, (2) enhancing the conjugation of the organic linker, and (3) functionalizing the organic linker with

their possibility for solid state electrolytes and battery components.

**158**

functions.

organic ligand is beneficial.

Owing to synthetic processability using reticular chemistry, MOFs offer unusual properties paving the path for many opportunities and their use in optoelectronic devices. Their use in devices so far is limited to sensors and gas storage. However, MOFs field is moving towards exploring their optical and electrical properties to use in electronic devices. There are many MOFs with tunable bandgap, both ultralow-k and high-k dielectric constants, varied magnetic properties, luminescence, and a few with semiconducting behavior, suggesting MOFs as emerging material with unique properties exceeding any other class of materials. Combining the solvothermal synthesis method with self-assembly processes, we can achieve highly ordered nanoporous structures with precise dimensionality that creates the potential for electronics and self-assembly with atomic-scale resolution and precision. In order to become MOFs for electronic devices, many challenges must be solved, and electronic structures of MOFs should be revealed. The MOFs-device roadmap should be one meaningful way to reach MOFs milestones for optoelectronic devices and will enable MOFs to be performed in their best, as well as allowing the necessary integration with other materials to fabricate fully functional devices.
