Figures and. Furthermore, our DFT calculations predict the formation of self-trapped excitons, see Figure and Table , being -together with emission from halide vacanciesresponsible for the broad emission features in the pure Cs2AgBiCl6 phase, Figure , in line with previous reports. To rationalize the trend in HER, we study the band alignment of the g-C3N4/Cs2AgBiCl6 heterostructure for the Cs2AgBiCl6 with and without the g-C3N4 monolayer adsorbed, representative for the low and high-wt% of g-C3N4 regions, respectively; see Figure and from the interface to the Cs2AgBiCl6 bulk. Increasing the amount of g-C3N4 may consequently increase the lifetime of charge carriers due to efficient charge separation, but also suppresses the formation of STEs as seen experimentally, Figure , resulting in the increase in HER, Figure .