Dicyanoperylene-diimide thin-film growth: a combined optical and morphological study

N,N′-bis (n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI8-CN₂) molecules represent an important example of novel n-type organic materials for realization of air-stable n-channel organic thin film transistors. In this work, the growth by evaporation of PDI8-CN₂ thin films deposited on silicon/silicon dioxide substrates has been investigated as a function of the film thickness through the combination of optical and morphological analyses. A continuous transition from two-dimensional to three-dimensional growth is observed at increasing thickness, accompanied by a modification of the photoluminescence spectrum of the films. Correlations between morphology and optical emission (photoluminescence) of the films are evidenced: namely, an emission band, red shifted with respect to the excitonic transition, emerges at increasing thickness. Time-resolved photoluminescence analysis demonstrates that the decay kinetics of such a band is different from the one associated with exciton recombination. Such a feature may thus represent a useful signature of defect-related trap states.