Temperature-dependent luminescence originating from the recombination of excitons in organic light-emitting materials

Based on experimental measurement and theoretical simulation, the temperature dependence of the photoluminescence (PL) of amorphous thin films of tris-(8-hydroxyquinoline) aluminum (Alq) and 4-(dicyanomethylene)-2-t-butyl -6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) has been studied. It is proposed that three types of excitons with different charge separations are involved in the steady state PL process, which, with increasing energy, are accordingly called intra-molecule localized, intra-molecule delocalized, and inter-molecule delocalized, while the corresponding radiative decay rates are relatively low, high, and negligible, respectively. The blue shift of the DCJTB PL spectra with increasing temperature has been observed and explained. It is suggested that the temperature-dependent PL could be possibly used to evaluate the radiative and conductive characteristics of organic emitters and to distinguish whether a given material would be of molecular or semiconductor properties.