Dramatic improvement of excitonic photoluminescence in metal halide films

So far, excitonic photoluminescence (PL) in metal halides (MHs) has exhaustively been studied by many workers, not only with the interest in the fundamental properties of excitons, but also with the expectation that excitonic luminescence in MHs is useful for application in optoelectronics because of its highly-light-emitting mechanism. In the actual MHs, however, excitonic PL is so weak that their excitonic luminescence is far beyond the scope of application-this is due to defect-rich problems native to MHs. Here we show dramatic improvements of excitonic PL in MHs recently achieved by two different methods, both of which employ simple techniques, i.e., solid-state chemical interaction/reaction (method 1) and amorphous-to-crystal transformation (method 2)-they are both applicable in several MHs. In the present paper, a systematic overview is given on the improvements achieved by method 1, together with a brief description of the improvements achieved by method 2. Method 1 is applicable to copper halides, model compounds for studying excitonic PL in MHs. For example, CuBr films generated by film-substrate chemical reaction of CuCl films on KBr-crystal exhibit more than 104 times stronger excitonic PL than conventional CuBr films. The improvements are based on migrations of the Cu+ and K+ ions across the film-crystal interface governed by their chemical affinities for negative ions-the vacancies-rich problem native to CuBr is solved by exploiting the advantages of the vacancy-free KBr-crystal. Method 2 is typically applicable to cesium-lead halides: for example, CsPbCl3 films prepared by crystallization via the amorphous phase exhibit more than ∼103 times stronger excitonic PL than conventional CsPbCl3 films-defects are dislodged from the crystallites in the films during the crystallization and subsequent crystal growth. These improvements disclose hitherto not-observed, inherent excitonic PL in MHs, demonstrating that inherent excitonic PL in MHs is indeed dominated by the highly-light-emitting mechanism as expected, and thus suggesting that MHs are useful for application in UV/VUV-lights-emitting devices (LED, LD).