Defect Engineering of Ta₃N₅ Photoanodes: Enhancing Charge Transport and Photoconversion Efficiencies via Ti Doping

Ta₃N₅ shows great potential as a semiconductor photoanode for solar fuel applications. However, its performance is hindered by poor charge carrier transport and trapping due to a high density of defects that introduce electronic states deep within its bandgap. Here, we demonstrate that controlled Ti-doping of Ta₃N₅ can dramatically reduce the concentration of deep-level defects and enhance its photoelectrochemical performance, yielding a seven-fold increase in photocurrent density and a 300 mV cathodic shift in onset potential compared to undoped material.[1] Comprehensive characterization, including structural, compositional, optical, electrical, and photoelectrochemical methods, reveals that Ti⁴⁺ ions substitute Ta⁵⁺ lattice sites, thereby introducing compensating acceptor states, reducing concentrations of nitrogen vacancies, and reduced Ta³⁺ states, and suppressing trapping and recombination. Importantly, Ti doping offers distinct advantages compared to Zr, an intensively investigated dopant in the same group. Specifically, Ti⁴⁺ and Ta⁵⁺ have more similar atomic radii, allowing for substitution without introducing lattice strain, and Ti exhibits a lower affinity for oxygen than Zr, enabling its incorporation without increasing the oxygen donor content. Consequently, we demonstrate that Ti doping decreases the conductivity immensely by lowering the charge carrier density but simultaneously increases the mobility of free charge carriers due to reduced recombination at nitrogen vacancies. Thus, these findings provide a powerful basis for precisely engineering the optoelectronic characteristics of Ta₃N₅ and to substantially improve its functional characteristics as an advanced photoelectrode for solar fuel applications.<br/><br/>[1] L.I. Wagner, Adv. Funct. Mater. 2023, 2306539