Quantifying Charge Carrier Dynamics in Fullerene-Sensitized ZnO Nanorod Array Photoanodes for Enhanced Photoelectrochemical Water Splitting

Abstract

Hybrid photoanode interfaces that integrate organic semiconductors with inorganic electron transport layers (ETL) are crucial for achieving high performance and stability in photoelectrochemical (PEC) oxidation reactions. In this study, we employed phenyl-C61-butyric acid methyl ester (PCBM)-coated ZnO nanorod arrays (ZnO/PCBM) as photoanodes to enhance water splitting in the PEC process. ZnO nanorods were grown on ZnO seed layers of varying thicknesses by adjusting the precursor concentration. Morphological analysis using field emission scanning electron microscopy (FE-SEM) confirms that nanorod density is highly dependent on the seed layer concentration, which in turn dictates the final PCBM coverage. The introduction of PCBM led to an improvement in PEC performance, achieving a maximum efficiency of 5.49 % and a peak photocurrent density of 0.603 mA/cm2 under UV light illumination. To fundamentally investigate this enhancement, we analysed the interface using X-ray Photoelectron Spectroscopy (XPS) and Electrochemical Impedance Spectroscopy (EIS). XPS revealed a downward shift in the binding energies of Zn and O elements following PCBM introduction, indicating a favourable electronic coupling and increased surface activity that contributes to the enhanced PEC process. Crucially, EIS analysis, modelled using a general equivalent circuit model (ECM), provided a quantified correlation: the first time constant (τ1), associated with charge separation, ranged from nanoseconds to tens of microseconds, and this time constant decreased across all PCBM-coated samples, directly demonstrating PCBM's role in accelerating interfacial charge transfer. Conversely, the second time constant (τ2), related to surface reactions and/or ion mobility (tenths of a second), remained largely unaffected. Despite these mechanistic advantages, no improvement in photocurrent stability was observed. Our findings offer vital, quantified insights into designing efficient ZnO/PCBM hybrid photoanodes.