Ultrafast photoexcited carrier dynamics in violanthrone-79 thin films

Violanthrone-79 (VO-79) is a promising organic semiconductor, molecules of which consist of large aromatic cores and aliphatic side chains. Strong π - π interactions promote stacking of individual molecules and forming nanoaggregates in solutions when dissolved in polar solvents like toluene and chloroform. Introduction of small amount of poor solvents such as hexane enhances aggregation and results in highly crystalline morphology. We have studied optical and electronic properties of VO-79 thin films prepared from VO-79/chloroform solution at a concentration above the aggregation threshold, as well as those prepared from VO-79 solutions in chloroform/hexane mix. Using optical pump – THz probe spectroscopy, we have demonstrated that delocalization of π electrons in the film results in band-like carriers that remain free within individual aggregates for tens of picoseconds after photoexcitation. We correlate carrier dynamics in films containing VO-79 nanoaggregates with their morphology and optical properties as revealed by the photoluminescence spectroscopy. Observation of long-lived free, mobile photocarriers suggests possible applications of VO-79 in organic optoelectronic and solar energy conversion devices.