  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.
