Charge Transfer Characterization of ALD-Grown TiO₂ Protective Layers in Silicon Photocathodes

A critical parameter for the implementation of standard high-efficiency photovoltaic absorber materials for photoelectrochemical water splitting is its proper protection from chemical corrosion while remaining transparent and highly conductive. Atomic layer deposited (ALD) TiO₂ layers fulfill material requirements while conformally protecting the underlying photoabsorber. Nanoscale conductivity of ALD TiO₂ protective layers on silicon-based photocathodes has been analyzed, proving that the conduction path is through the columnar crystalline structure of TiO₂. Deposition temperature has been explored from 100 to 300 °C, and a temperature threshold is found to be mandatory for an efficient charge transfer, as a consequence of layer crystallization between 100 and 200 °C. Completely crystallized TiO₂ is demonstrated to be mandatory for long-term stability, as seen in the 300 h continuous operation test.