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

Abstract

a Efficient charge separations and their mobility is a key challenge in metal-organic-frameworks (MOFs) based devices. In the present study, thin films of cobalt-based metal organic frameworks (MOFs) are synthesized using 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 conducting material. 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 a light harvesting and charge transporting active layer in a fully devised TiO2based 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 are studied. Under an optimized condition, the Co-MOF based device demonstrates the solar-to-electric energy conversion efficiency of 1.12 % with short circuit current of 2.56 mA.cm -2 , showing a promissing future prosprect on application of Co-MOFs in photovoltaics. Further, the photovoltaic performance of the Co-MOF based divice is comaparatively studied with the previously reported Cu-MOF and Ru-MOF based similar devices, and the influence of different metal centers to MOFs on their light harvesting performce is discussed.

Electrically Conductive Metal–Organic Frameworks

“…Hall effect measurements on films of CoBDC and CoNDC grown on TiO 2 and doped with I 2 showed similar mobility values of 21.2 and 87.6 cm 2 V –1 s –1 , respectively. 253 …”

Electrical conductivity and magnetic bistability in metal–organic frameworks and coordination polymers: charge transport and spin crossover at the nanoscale

“…67 Afterwards, the I2-doping strategy has been successfully reproduced with a considerable number of insulating MOFs. The appearance of electrical conductivity was due to either donor-acceptor interactions between I2 and ligand π electrons, [68][69][70][71][72] the oxidation of the network [73][74][75] or conduction through polyiodide ions. [76][77][78] Although in some of these conduction mechanisms were not clearly elucidated.…”

Efficient MOF-Sensitized Solar Cells Featuring Solvothermally Grown [100]-Oriented Pillared Porphyrin Framework-11 Films on ZnO/FTO Surfaces

“…Marking the advances of the MOF field in recent years, at first, optically and redox-silent MOF thin films were employed in DSSCs as simple dye-loading materials to enhance the dye-loading capacity and prevent dye aggregation. , Although this strategy helped improve V OC by impeding charge recombination, the insulating MOFs also hindered long-range charge movement and injection processes, diminishing the photocurrent density and overall efficiency of devices compared to those without MOFs . The thin films of several traditional MOFs based on colorless aromatic ligands also displayed photovoltaic response upon iodine infiltration (PCE up to 1.2%), − but their inabilities to absorb visible–NIR light and generate photocurrents without iodine doping left the origin of photovoltaic behavior unclear and highlighted the need for intrinsically light-harvesting frameworks based on more powerful chromophores, such as porphyrin, perylene, and Ru­(bpy) 3 2+ dyes. − Although the energy-transfer capability and catalytic activities of such light-harvesting MOFs have been widely explored, − their photovoltaic properties remained largely overlooked until lately − possibly because the latter also required properly oriented MOF films attached to electrode surfaces to support requisite charge separation, movement, and injection processes. Recently, Wöll, , Allendorf, and others have demonstrated that porphyrin-based MOFs could produce modest photocurrents under visible light (PCE: 0.0026–0.45%), whereas Morris et al demonstrated that Ru­(bpy) 3 2+ -based MOFs experienced slightly greater PCE than corresponding molecular Ru­(bpy) 3 2+ complexes (∼0.12 vs 0.08%).…”