Operando electrochemical study of charge carrier processes in water splitting photoanodes protected by atomic layer deposited TiO2

Abstract

Semiconductor-based solar energy conversion devices are often multilayer structures with each layer serving a distinct purpose towards generating an efficient and stable device. In water splitting, the use of atomic layer deposited TiO₂ (ALD-TiO₂) layers enables the stable operation of materials that would normally photocorrode in the aqueous electrolyte. Interestingly, thick ALD-TiO₂ (>50 nm) has been successfully used to protect high performance photoanodes, despite an apparent band mismatch that should preclude charge transfer. The understanding of the charge transfer through the relatively thick TiO₂ layer remains controversial and warrants further study. Here, we introduce an operando methodology to study charge carrier processes in the ALD-TiO₂ protected photoanode by utilizing photoelectrochemical impedance spectroscopy (PEIS) combined with the dual-working-electrode (DWE) technique to resolve if the charge transport through the TiO₂ is a conduction band process or involves a hopping through defect states. Two silicon-based systems were evaluated, one featuring a buried homojunction (np⁺Si/TiO₂/Ni) and the other a purely n-type Si directly interfaced with TiO₂ (nSi/TiO₂/Ni). The additional series resistance imparted by the TiO₂ layer (R_TiO2) was extracted from the PEIS measurements. Both the potential and thickness dependence of R_TiO2 were analyzed, and the DWE technique enabled the sensing of the potential of the TiO₂ layer under operation, indicating a strong band bending with the conduction band even more positive than the oxygen evolution potential. Together, these data suggest a conduction band-based transport mechanism, in spite of the presence of defect states in the bandgap of ALD-TiO₂, and a detailed picture of the charge transfer through the multilayer structured photoanodes was obtained.