Current and Voltage Limiting Processes in Thin Film Hematite Electrodes

Atomic layer deposition was used to grow conformal thin films of hematite on transparent conductive oxide substrates and used as electrodes in regenerative photoelectrochemical cells (PECs). Systematic variations of the contacting electrolyte and redox shuttles were performed to determine the rate limiting processes of hematite PECs. Steady state current density vs applied potential measurements were employed to show that when the fast, one electron redox shuttles are used, hole collection is not the rate limiting step; however, hole collection is the rate limiting step when hematite is employed for water oxidation. Electron transfer was found to be the dominant recombination mechanism which limits the open circuit photovoltage. We further demonstrate that with variations of the pH and redox potentials of the contacting electrolyte, increases in both the photocurrent density and photovoltage can be achieved, which is attributed to increasing the built-in voltage.