Enhancing Charge Transport and Extraction in Ta₃N₅ Photoanodes Via Ti Doping and TiN Contact Layers

${\mathrm{Ta}}_3{N}_5$ is a promising semiconductor for solar-driven water splitting, but its performance is limited by poor charge transport and inefficient carrier extraction. We present a systematic approach to overcome these limitations by combining Ti compensation doping with engineered TiN back contact interlayers. Time-resolved terahertz and microwave photoconductivity reveal that Ti incorporation suppresses trapping at mid-gap defect states and reduces grain boundary barriers, yielding enhanced mobilities, longer lifetimes, and reduced carrier localization. These results elucidate the mechanisms of improved transport in Ti-doped ${\mathrm{Ta}}_3{N}_5$ (Ti: ${\mathrm{Ta}}_3{N}_5$ ) and the role of compensation doping in suppressing bulk recombination losses. Despite these improved bulk transport characteristics, efficient photoelectrochemical (PEC) function also requires optimized back contacts. To address interfacial losses, we introduce ultrathin ( $\sim$ 8 nm) TiN interlayers that remain metallic during high-temperature ammonolysis, act as effective diffusion barriers that protect substrates, and enable efficient majority carrier extraction on both fused silica and Si. Tandem integration of Ti: ${\mathrm{Ta}}_3{N}_5$ with TiN interlayers on n-type Si yields significantly improved PEC performance. Overall, this work establishes a fundamental basis for advancing nitride-based photoelectrodes through coordinated defect and interface engineering, while enabling cost-effective fabrication of semi-transparent Ti: ${\mathrm{Ta}}_3{N}_5$ photoanodes for in situ optical studies and nitride-based tandem solar cells.