Excellent CuBr films exhibiting ∼104 times higher room-temperature free-exciton PL efficiency than conventional CuBr films

High-quality CuBr films are fabricated by film-substrate chemical reaction of CuCl on KBr. The films exhibit more than ∼104 times higher free-exciton photoluminescence (PL) efficiency at room temperature than conventional CuBr films. The room-temperature PL efficiency exceeds that of conventional CuCl films by a factor of ∼103 and even the cryogenic-temperature free-exciton PL efficiency of the same films by a factor of ∼102, despite the general recognition that the free-exciton PL efficiency of CuBr is much weaker than that of CuCl. The extreme PL efficiency of the CuBr is concluded to be mediated by the high-quality KBr− crystal surroundings, into which and out of which Cu+ and K+ ions migrate, respectively, during the film-substrate chemical reaction, thus generating high-quality CuBr crystallites in the surroundings.