a: The developed process consisting of the 1st step (spin-coating and thermal annealing) and 2nd step (sulfurization and crystallization). b: Photograph of a bismuth sulfide thin film. c: Atomic force microscope image of a bismuth sulfide film prepared using the reported process. The dark/bright colors represent the height of the film surface. d: Atomic force microscope image of a bismuth sulfide film prepared using a conventional process (spin-coating of nanoparticles).