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

Owing to their abilities to assemble and organize a large number of redox and photoactive components in highly ordered periodic fashion, crystalline porous metal-organic frameworks (MOFs) have the potential to execute myriad complex functions, including charge transport and light to electrical energy conversion when the required conditions are fulfilled. Herein, we demonstrate an unprecedented spontaneous solvothermal growth of precisely [100]-oriented pillared porphyrin framework-11 (PPF-11) films featuring vertically aligned Zn-tetrakis(4-carboxyphenyl)porphyrin (ZnTCPP) walls and horizontally aligned 2,2'-dimethyl-4,4'-bipyridine beams attached to annealed ZnO-fluorine-doped tin oxide (FTO) surfaces and their remarkable photovoltaic performance in liquid-junction solar cells. The [100]-oriented PPF-11/ZnO-FTO photoanodes displayed excellent photovoltaic response (short-circuit current ( J_SC): 4.65 mA/cm², open-circuit voltage ( V_OC): 470 mV, power conversion efficiency: 0.86%) that easily outperformed all control devices as well as previously reported porphyrin and Ru(bpy)₃²⁺-based visible light-harvesting MOFs with 10-1000 times greater photocurrent density and 2-375 times higher efficiency. The superior photovoltaic behavior of [100]-oriented PPF-11/ZnO films compared to epitaxially grown MOF thin films on insulating self-assembled monolayers and drop-cast PPF films with different orientations can be attributed to several factors, including better charge separation, transport, and injection capabilities of the former. The noncatenated PPF-11 was able to host electron-deficient C₆₀ guests, filling in nearly half of its cavities and engage them in ZnTCPP/C₆₀ charge-transfer interaction. However, the C₆₀-doped PPF-11/ZnO films displayed much weaker photovoltaic response than undoped [100]-oriented PPF-11/ZnO films presumably due to exclusion of I^-/I₃^- electrolyte from the C₆₀-occupied cavities and the inability of isolated C₆₀ guests to support long-range charge movement.