Switching between Limiting Charge Extraction Regimes in an Illuminated Semiconductor-Metal-Organic Framework Junction

Surface modification is an effective method to realize high performance photoelectrodes. While current investigations mostly aim to leverage surface layers for improved charge carrier kinetics during charge separation, interfacial charge transfer, and decreased recombination, carrier transport within the surface layer is largely unattended. Herein, we explore this charge transport process on a model photocathode consisting of p-Si and GaP semiconductors (SCs) that are coated with a redox-active Zn-NDI (NDI = naphthalene diimide bis-pyrazolate) metal-organic framework (MOF) surface layer. The MOF layer is able to accept photogenerated electrons and support a large photovoltage of the underlying SC. In addition to well-established carrier generation and interfacial transfer processes that are frequently considered to control photocurrents, experimental photoelectrochemical data of the MOF@SC electrodes expose limitations that arise from electron transport in the surface layer coating. The transport-limited regime becomes relevant when the illumination intensity is gradually increased and is sensitive to the nature of the underlying semiconductor as well as the electrolyte. The phenomenon reported in this work is likely present in other surface-modified photoelectrodes with thick cocatalysts or redox-active polymer coatings but can easily be overlooked. In the MOF@SC construct, the transition between different limiting regimes can be visualized owing to the well-behaved cation-coupled photoelectron hopping transport in the MOF layer. These findings support the design and realization of efficient photoelectrodes.