Advanced Functional Materials

Spatially resolved STM spectroscopy of charge injection at the ladder-type poly(para -phenylene)/Au(111) interface

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The effect of the morphology on charge-carrier injection into methyl-substituted ladder-type poly(para-phenylene) (Me-LPPP) thin films deposited on a Au(111) substrate has been studied by scanning-tunneling-microscope-based spectroscopy. We find that the charge-carrier injection barrier as well as the single-particle bandgap, Egsp, of the polymer show significant variations at different locations of the sample surface. Normally, we find that the values of Egsp are larger than the optical absorption edge, the energy difference being attributed to the exciton binding energy. In some regions of the sample, however, Egsp appears to be close to or below the optical absorption edge, pointing to the effect of aggregates within the polymer film which act as hole-trapping centers with a depth of a few 100 meV. Density functional calculations are used to elucidate the dependence of the electronic states on the polymer packing density. Our results show that in this polymer morphological inhomogeneities strongly influence the charge carrier injection and transport properties. This points to a common behavior of materials exhibiting a tendency to form aggregates. In addition, the exciton binding energy of Me-LPPP is determined to be approx. 0.85 eV. Moreover, the comparison between the charge-injection energy gap and the photocurrent action spectrum indicates that the photoionization threshold is not directly related to the exciton binding energy.