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We visualize exciton diffusion in rubrene single crystals using localized photoexcitation and spatially resolved detection of excitonic luminescence. We show that the exciton mobility in this material is strongly anisotropic with long-range diffusion by several micrometers associated only with the direction of molecular stacking in the crystal, along the b axis. We determine a triplet exciton diffusion length of 4.0 ± 0.4 μm from the spatial exponential decay of the photoluminescence that originates from singlet excitons formed by triplet-triplet fusion.

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 × 1020 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/cm2 at room temperature, corresponding to triplet densities of ∼1016 cm−3.

Fundamental understanding of photocarrier generation, transport and recombination under a steady-state photoexcitation has been an important goal of organic electronics and photonics, since these processes govern such electronic properties of organic semiconductors as, for instance, photoconductivity. Here, we discovered that photoconductivity of a highly ordered organic semiconductor rubrene exhibits several distinct regimes, in which photocurrent as a function of cw (continuous wave) excitation intensity is described by a power law with exponents sequentially taking values 1, 1/3 and ¼. We show that in pristine crystals this photocurrent is generated at the very surface of the crystals, while the bulk photocurrent is drastically smaller and follows a different sequence of exponents, 1 and ½. We describe a simple experimental procedure, based on an application of “gauge effect” in high vacuum, that allows to disentangle the surface and bulk contributions to photoconductivity. A model based on singlet exciton fission, triplet fusion and triplet-charge quenching that can describe these non-trivial effects in photoconductivity of highly ordered organic semiconductors is proposed. Observation of these effects in photoconductivity and modeling of the underlying microscopic mechanisms described in this work represent a significant step forward in our understanding of electronic properties of organic semiconductors.

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