Tunable self-trapped excitons in 2D layered rubrene

As emerging efficient emitters, two-dimensional (2D) organic semiconductors offer an intriguing potential to the low-cost and high-performance light emitting devices. However, organic semiconductors usually suffer from serious luminescence quenching owing to the self-trapped exciton formation which prevalently occur in materials with soft lattice and strong exciton-phonon coupling. Therefore, revealing the underlying mechanism that leads to self-trapped excitons is a prerequisite for increasing the photoluminescence (PL) efficiency in organic materials. Here, we grew high-quality layered rubrene films on hexagonal boron nitride through a physical vapor transport method. Combined with time-resolved photoluminescence (TRPL) spectra and laser power dependent TRPL spectra, we confirmed the free exciton (FE) and self-trapped exciton (STE) emission. We found that STEs evolved from FEs from the temperature dependent PL characterization. In addition, we observed tunable STEs in a 2D layered rubrene: reducing the layer number could strongly suppress the exciton transferring efficiency and the rate from a free exciton state to a self-trapped exciton state which result from the decreased self-trapping depth and increased barrier height. Consequently, the average PL intensity is strongly enhanced about seven times, whereas STE emission is quenched. The results provide a method for suppressing the STE formation process and contribute to improving the photoluminescence efficiency in optoelectronic applications.