Extrinsic optical recombination in pentacene single crystals: Evidence of gap states

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Abstract
Two luminescence bands observed in pentacene single crystals with different degrees of purity are identified as due to extrinsic optical emissions. A band at 1.49 eV remains in the crystal with the highest purity. Its redshift of about 0.3 eV from the free exciton optical recombination suggests that the extrinsic transitions could involve gap states recently discovered in pentacene transistors. Absence of resonance Raman scattering when photon energies overlap the extrinsic recombination suggests that the gap states are likely due to impurities. The temperature dependence of luminescence intensities is interpreted by activated decay of excitons to radiative and nonradiative states.