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Inset: typical photocurrent transients observed after illumination with 20 ps pulses at 600 nm wavelength, showing a delayed photocurrent buildup and a fast buildup that occurs in less than 10 ns, and an increase in current for growing pulse energies in the high-exposure region above an absorbed photon density of . Main figure: dependence of the maximum photocurrent (, filled symbols) and of its fast component (, open symbols) on the absorbed photon density at the surface of the crystal. Green squares and red circles are for illumination wavelengths of 580 nm and 600 nm, respectively. The crosses represent the photoluminescence intensity, and the triangles are the buildup rate of the photocurrent. The solid lines represent a linear dependence and the dashed lines represent a square-root dependence.
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(a) Changes in the photocurrent dynamics induced by 600 nm pulsed illumination before and after a short exposure of the crystal surface to free radicals created by a high vacuum gauge (see Ref. 12). The curve with the largest amplitude was obtained before exposure to the gauge effect. The other curves are labeled with the time in hours after the sample was removed from the vacuum environment. (b) Changes in photocurrent dynamics with temperature, measured in another sample. Each curve is labeled with the temperature in degree celsius.
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Excitons in rubrene single crystals dissociate into free charge carriers via two mechanisms whose relative importance depends on the illumination wavelength through the optical penetration depth into the crystal. The first mechanism is defect-induced dissociation in less than 10 ns after photoexcitation. For low photoexcitation densities, about 10% of the excitons that survive radiative recombination dissociate through this channel. The second mechanism, affecting the remaining 90% of the excitons, involves a previously reported state localized close to the surface of the crystal that leads to a delayed release of photocarriers a fraction of a millisecond after photoexcitation.
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