Transition metal nitride semiconductors are rapidly emerging as a promising class of materials that provide a combination of properties well suited for advanced optoelectronic and energy conversion applications. Compared to oxides, nitride semiconductors offer narrower bandgaps, stronger bond covalency, and improved carrier transport properties that make them well suited for harvesting sunlight in photovoltaic and photoelectrochemical systems. Despite this considerable promise, dramatically fewer nitrides than oxides have been experimentally investigated due to their synthetic complexity. Consequently, a broad range of new compounds remain to be explored as functional semiconductors. Furthermore, synthesis challenges have led to poorly controlled defect and impurity properties within this class of materials. In this work, we overcome these limitations using reactive co-sputtering as a non-equilibrium deposition approach to synthesize thin film nitride semiconductors with precisely controlled compositions, exploring both dopants and new compounds in the Ti-Ta-N and Zr-Ta-N composition spaces. Starting with orthorhombic Ta₃N₅, which stands as the best performing photoanode material within this class, we investigate the critical roles of nitrogen vacancy (v_N), substitutional oxygen (O_N), and reduced Ta centers (Ta³⁺) on thin film photoconversion efficiencies [1]. Using in situ photoelectrochemical transient absorption spectroscopy, we identify spectral signatures of photogenerated holes and assess the competition between recombination and chemical reaction at the interface, which is directly impacted by point defect concentrations. Armed with these insights, we show that substitutional Ti doping on Ta sites (TiTa) can be used to improving photoconversion efficiencies [2]. While the structural properties and optical bandgap are only minimally affected by the incorporation of Ti impurities, we observe substantial changes in surface photovoltage, band bending, and recombination dynamics with Ti content. At low Ti content, Ti doping substantially improves the photoelectrochemical performance, manifesting in increased photocurrent densities and favorably reduced onset potentials. While high Ti contents lead to the precipitation of a secondary TiN phase, different behavior is observed for the case of Zr incorporation. In particular, introduction of a 1:1 Zr:Ta ratio leads to formation of a new ternary nitride compound, bixbyite-type ZrTaN₃. Comprehensive investigation of the properties of this material reveal that it is a strong visible light absorber and is an active photoanode material. Complementary DFT calculations indicate a direct bandgap that is tunable based on cation site occupancy. Thus, this material offers exciting prospects not only for solar energy conversion but also for optoelectronics applications. Overall, these results highlight the promise of both established and new transition metal nitride semiconductors for solar energy harvesting, as well as the importance of precise composition engineering to tune optoelectronic and charge transport characteristics.