Enhancing Photovoltages at p-Type Semiconductors Through a Redox-active Metal-Organic Framework Surface Coating
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Metal-organic frameworks (MOFs) interfaced with visible-light-absorbing semiconductors offer a novel approach to improve photoelectrochemical performances. When tested under 1-sun illumination, a naphthalene diimide (NDI)-based monolayer immobilized at p-type Si(111) undergoes two sequential one-electron reductions close to their thermodynamic potentials. No photovoltage is observed until the NDI monolayer is expanded in three dimensions in a PIZOF-type Zr(NDI) MOF (PIZOF = porous interpenetrated zirconium organic framework). The surface-grown MOF thin film promotes photo-induced charge separation and electron transfer across the interface and through the film, resulting in reduction of the molecular linkers at formal potentials >300 mV positive of their thermodynamic potentials. The apparent diffusion coefficient is similar to that measured at a conductive electrode (10-10 cm2 s-1), indicating that the observed photocurrent is governed by charge diffusion through the Zr(NDI) MOF film. The charges accumulated in the NDI-based MOF can be extracted by an external electron acceptor, demonstrating sufficient conductivity throughout the MOF film to power reductive transformations. When grown on GaP(100), the potentials of the NDI reductions in the MOF film are shifted anodically by >700 mV compared to those of the same MOF on conductive substrates. This photovoltage, among the highest reported for GaP in photoelectrochemical applications, illustrates the power of MOF thin films to improve photocathodic performance.