Features of the absorption spectra of thin films of M2Ag1 − xCuxI3 solid solutions (M = Rb, Cs)

The absorption spectra of thin films of (MI)1 − y(Ag1 − xCuxI)y solid solutions (M = Rb, Cs) with the initial molar concentration y = 0.33 have been investigated. It is established that, at low concentrations x, a local exciton band due to Cu+ ions is split off from the main long-wavelength exciton bands. In Rb2Ag1 − xCuxI3 solutions, the concentration shift of exciton bands indicates the formation of a persistent-type exciton spectrum. However, in Rb2Ag1 − xCuxI3 with x ≥ 0.5 and in Cs2Ag1 − xCuxI3 with x > 0.2, exciton spectra of amalgamation type are observed, which are related to the formation of more stable M3Ag2 − 2xCu2xI5 solid solutions. The formation of these solutions leads to broadening of the exciton bands and to the concentration transition from persistent-to amalgamation-type exciton spectra.