PHOTORESPONSE OF UNDOPED AND IODINE-DOPED IRON-OXIDE THIN-FILM ELECTRODES

Undoped and iodine doped iron oxide thin films were prepared spray pyrolytically and their photoresponse towards the water splitting reaction in alkaline medium (pH 13) was studied. The optimum spray time and the corresponding film thickness for maximum photoresponse were determined. An undoped iron oxide film approximately 50 nm thick gave rise to a maximum photocurrent density and its wavelength-dependent absorption coefficients of light were also measured. Raman spectroscopic analysis indicates the formation of alpha-Fe2O3 film by the spray pyrolytic method. The band gap of this undoped film was found to be 1.97 eV. A theoretical expression of photocurrent density at the thin film electrode-solution interface was developed. A good correlation was found between theoretical and experimental photocurrent densities. Importantly, such an iodine-doped iron oxide film approximately 100 nm thick gave rise to a fivefold increase in maximum photocurrent density compared with an undoped optimum thick film of approximately 50 nm. These observations were attributed to increased mobility of carriers and an increase in the absorption of light in the iodine-doped film. Model calculations show that five undoped iron oxide thin film electrodes of optimum thickness in stacks generate a photocurrent density of 5 mA cm-2 whereas an optimized stack of iodine-doped film produced a photocurrent density of 15 mA cm-2 at 0.82 V vs. SHE.