This research explores the promising field of metal oxynitride semiconductors for solar energy conversion, aiming to advance the fundamental understanding and practical application of these materials in photoelectrochemical (PEC) systems. Multinary oxynitrides, with their unique combination of oxygen and nitrogen anions, offer tunable band gaps, enhanced light absorption, and improved charge carrier dynamics, making them ideal candidates for PEC energy conversion. The study delves into the fundamental aspects of oxynitride chemistry, focusing on these materials' formation mechanisms, structural stability, and physicochemical properties. Advanced synthesis techniques, including plasma growth processes and flash photonic heating, are employed to control the composition and crystallinity of the oxynitrides, which are critical for optimizing their electronic and optical properties. The research further investigates the influence of these synthesis methods on the efficiency and stability of oxynitrides in PEC applications, aiming to overcome current limitations in their use for solar-driven water splitting and other energy conversion processes. By bridging the gap between material chemistry and device performance, this study seeks to unlock the full potential of multinary metal oxynitride semiconductors, paving the way for next-generation solar energy technologies that are both efficient and durable. The findings could significantly contribute to developing sustainable energy solutions, addressing the global need for clean and renewable energy sources.