Optical pump-probe spectroscopy of photocarriers in rubrene single crystals

The photocarrier dynamics of rubrene, an organic molecular semiconductor, are investigated by transient absorption spectroscopy over a wide energy range (0.1\ensuremath{-}2.2 eV). By photoirradiation, we observed three kinds of absorption bands in the infrared region, which are attributable to induced absorptions of excitons, free carriers, and polarons. The results show that for photoirradiation with energies higher than \ensuremath{\sim}2.7 eV (\ensuremath{\sim}0.4 eV higher than the lowest exciton energy), free carriers are photogenerated just after the photoirradiation. Some of those free carriers are stabilized as polarons and the others are relaxed to excitons. For the resonant excitation to the lowest-exciton, polarons can also be generated from the excitons. The temperature dependence of the decay dynamics of polarons reveals that polaron mobility increases with decreasing temperature, indicating bandlike transport of large polarons. Below 80 K, photocarriers are trapped by defects or impurities and their decay dynamics are dominated by the escape time from traps.