Steady-state photoconductivity and multi-particle interactions in rubrene single crystals

We demonstrate that photoconductivity of pristine rubrene crystals 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 1/4. We show that this photocurrent is generated almost exclusively at the surface of pristine rubrene crystals, while the bulk photocurrent is dramatically smaller and follows a different set of exponents, 1 and 1/2. A model based on exciton fission, fusion and triplet-charge quenching is developed to describe these non-trivial effects in photoconductivity of highly ordered organic semiconductors.