Temperature-dependent photoluminescence of organic light-emitting materials: Types and characteristics of excitons involved in the emitting process

Based on experimental measurement and theoretical analysis, 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. It is found that all three types of excitons participate in the PL process of Alq, whereas, only the first two types of radiative excitons dominate the PL process of DCJTB in the temperature range covered by the present work. The blue shift of the DCJTB PL spectra with increasing temperature has been observed and analyzed. It is suggested that the temperature-dependence of PL could be used to evaluate the radiative and conductive characteristics of organic emitters and to distinguish whether a given material would tend to be of molecular or semiconductor properties.