Facile interfacial charge transfer across hole doped cobalt-based MOFs/TiO₂ nano-hybrids making MOFs light harvesting active layers in solar cells

Efficient separation of charges and their mobility are key challenges in metal-organic-framework (MOF) based devices. In the present study, thin films of cobalt-based metal organic frameworks (MOFs) are synthesized using a layer-by-layer technique, and their electrical/optoelectronic properties are studied. The as-prepared MOF films show electrically insulating behavior, which after hole doping demonstrate p-type conduction behaviour. The measured HOMO-LUMO energy states of the MOF films are found to be well matched for sensitizing TiO₂, and the photoluminescence quenching experiment demonstrates a facile photoelectron transfer path from the doped frameworks to TiO₂. Consequently, the doped MOFs are employed successfully as light harvesting and charge transporting active layers in a fully devised TiO₂-based solar cell. Two different organic ligands viz., benzene dicarboxylic acid and naphthalenedicarboxylic acid are used to synthesize two kinds of Co-MOFs having different geometrical dimensions of unit cells and pores, and their influence on hole doping and charge transportation is studied. Under optimized conditions, the Co-MOF based device demonstrates a solar-to-electric energy conversion efficiency of 1.12% with a short circuit current of 2.56 mA cm^-2, showing promising future prospects of the application of Co-MOFs in photovoltaic devices. Further, the photovoltaic performance of the Co-MOF based device is comparatively studied with that of the previously reported Cu-MOF and Ru-MOF based similar devices, and the influence of different metal centers of MOFs on their light harvesting performance is discussed.