Thickness-Dependent Exciton Dynamics In Thermally Evaporated Rubrene Thin Films

In this study, exciton dynamics and singlet fission (SF) in amorphous rubrene thin films with various thicknesses from 5 to 100 nm were investigated. No clear X-ray diffraction peaks could be observed from these thin films, and their absorption and photoluminescence (PL) spectra were similar, although the PL slightly red-shifted with increasing rubrene thickness. However, temperature-dependent and time-resolved PL measurements showed dramatic differences between 5 and 100 nm thick films. There were two different relaxation channels in a 100 nm rubrene film, one of which is fission capable (channel a) and the other fission inactive (channel b). The SF process of species a was endothermic with activation energy 58 meV, as determined by time-resolved PL measurements carried out over a temperature range 300-77 K. On the other hand, for a 5 nm rubrene thin film, both this endothermic SF route and a weaker, exothermic SF channel below 160 K were observed with a SF rate of 0.22 ns(-1). This was attributed to a new fission channel (channel c) that is probably due to molecular packing in the beginning of film growth. Channel c indicated a lower coupling molecular strength together with higher singlet energy that compensated the required thermal energy barrier for SF. A phase transition from amorphous to polycrystalline rubrene was observed when a thermal annealing treatment was applied to the 100 nm rubrene film. The PL spectral profile was dominated by microcrystals oriented with the crystal c-axis parallel with the substrate, and these high density of SF "hotspots" increased the SF rate with only a weak temperature dependence from 120 to 300 K.