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The rubrene molecular crystal has the unique property of showing a strong photoconductivity for light wavelengths that are close to the absorption edge. We studied the microsecond dynamics of the photoconductivity induced by short light pulses to characterize the way in which photoinduced excitons efficiently ionize to produce free charge carriers. We found that the photoconductivity is produced by carriers released after an average time of $100\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$ from a ``reservoir state'' that originates from the photoexcited molecular excitons. The conversion of photoexcited excitons into this reservoir state happens only at the surface of the crystal within a depth of the order of a few micrometers, but in this region close to the surface, a photoexcited molecular exciton has a probability of the order of unity to ultimately lead to a mobile charge carrier. This high carrier photoexcitation efficiency leads to a pronounced shortening of the photocurrent rise time for decreasing wavelength or increasing energy of the excitation pulses because of the effect of quadratic recombination of the photocarriers.

By simultaneously measuring the excitation spectra of transient luminescence and transient photoconductivity after picosecond pulsed excitation in rubrene single crystals we show that free excitons are photoexcited starting at photon energies above 2.0 eV. We observe a competition between photoexcitation of free excitons and photoexcitation into vibronic states that subsequently decays into free carriers, while molecular excitons are instead formed predominantly through the free exciton. At photon energies below 2.25 eV, free charge carriers are only created through a long-lived intermediate state with a lifetime of up to 0.1 ms and no free carriers appear during the exciton lifetime., Comment: 4 pages, 3 figures