Ultrafast exciton dynamics in polyacene organic semiconductors

The strong potential for new technological applications provided by organic semiconductors has spurred extensive research efforts in these materials. Applications include thin film transistors, light-emitting diodes (OLED), photodiodes and solar cells. To successfully utilize organic semiconductors in these technologies it is vital to understand the nature of their photo-generated states and the relaxation dynamics of these states. It is also important to understand the role of defects which may change relaxation dynamics in ways that can be either beneficial or detrimental to device performance. Here, we present a comparative study of ultrafast photo-generated state dynamics in pentacene (Pc) and tetracene (Tc) single crystals and Pc films using optical pump-probe spectroscopy. We show that in Pc and Tc single crystals exciton dynamics is dominated by singlet-triplet fission. In Pc, triplet production is temperature independent, while in Tc it displays activated behavior. By comparing undoped and C60-doped Pc films, triplet production is shown to be quenched by electron traps that result in competing charge transfer dynamics. Electron traps can thus be used to enhance free carrier formation at the expense of triplet exciton generation which can be benecial to solar cell and photodiode performance.