Science News: 2021 Adv. Mater., 61.8% FE! How do carbon nitride catalyst and photocathodes decoupling enhance light-harvesting?

　　Advanced Materials (IF 30.849) published a study this year. The research team used a porous carbon nitride catalyst to decouple photocatalysis and electrocatalysis with a Si photocathode structure, achieving a Faradaic efficiency (FE) of 61.8%.

　　Photoelectrochemical (PEC) approach for nitrogen reduction reaction (NRR) to synthesize ammonia (NH₃) is an attractive and quite promising method. However, the collaborative management of optical, electrical, and catalytic performance is full of challenges, and it also limits the efficiency of the photoelectrochemical nitrogen reduction reaction. In this research, the research team used a cascaded n⁺np⁺-Si photocathode structure to decouple the light-harvesting and electrocatalysis in the device, and then enhanced light trapping, carrier separation/transport and catalytic reactions. This decoupling design concept, in addition to eliminating parasitic light blocking, could also improve the optical and electrical characteristics of the n⁺np⁺-Si photocathode without affecting the efficiency.

　　However, although the unique decoupling method improved the light-harvesting ability of the photocathode, the electrical design was another major issue. In order to achieve better electrical design, the research team used appropriate doping and surface passivation layers to build a cascade of n⁺np⁺-Si (Figure d below). This electrical design improved the separation and transfer of photogenerated carriers, thereby increasing the availability of carriers for catalytic reactions. Under the irradiation of the AM 1.5G solar simulator, the light-harvesting ability of the Si cell was evaluated. As shown in the figure (e) below, the open circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF) of the n⁺np⁺-Si battery were 0.58 V, 33.43 mA cm^-2 and 59.09%, respectively. In addition, the reflectivity and external quantum efficiency (EQE) of the n⁺np⁺-Si battery were measured through the quantum efficiency measurement system. As shown in the figure (f) below, the n⁺np⁺-Si battery has low reflection and high EQE (>90%) in the 450-900 nm wavelength range, proving its excellent light absorption and charge carrier separation capabilities.