Layer-by-layer growth and photocurrent generation in metal-organic coordination films

A series of metal-organic coordination films with tetra(4-carboxyphenyl) porphyrin (TCPP) and trimesic acid (TMA) ligands and Cu²⁺ ion linkers were grown layer-by-layer on TiO₂-modified indium tin oxide electrodes and the cathodic short-circuit photocurrent that they generate was studied. The layer-by-layer growth was verified by using absorption spectroscopy in the visible and X-ray photoelectron spectroscopy, highlighting the consistent coverage of TCPP in each layer. The photocurrent increases nonlinearly with the number of TCPP layers and with the number of TMA layers introduced as a buffer between TCPP and substrate. These observations are rationalized by considering substrate screening of the TCPP photoexcited state and charge transport across the metal-organic multilayers.