Extremely efficient exciton fission and fusion and its dominant contribution to the photoluminescence yield in rubrene single crystals

Measurements of photoluminescence yield over a wide range of excitation power in rubrene single crystals reveal a transition between a low-yield region and a region with a yield more than an order of magnitude larger. This transition occurs at an excitation density of 3 x 10(20) cm(-3) absorbed photons per second. This power dependence is predicted in case of an extremely efficient conversion between singlet and triplet excitons through fission and fusion. Triplet fusion starts contributing to rubrene's photoluminescence already at intensities of a few mW/cm(2) at room temperature, corresponding to triplet densities of similar to 10(16) cm(-3). (C) 2013 AIP Publishing LLC.