The localization of injected charges in polymers and molecular solids

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Abstract
Polymers and molecular solids are characterized by two fundamental features: the retention of molecular identity in the solid state and the occurrence of relatively weak interactions between the individual molecular entities. The weak interactions lead to a high degree of disorder caused both by defects and by thermally induced motions of the molecules. A model of charges injected into polymers and molecular solids is developed. Photoemission and transport measurements are utilized to evaluate the parameters in this model. Results for metal free phthalocyanine are presented as an illustrative example. For molecular glasses and aromatic pendant-group polymers this analysis predicts that disorder causes injected charges to form a Fermi glass of self-trapped polarons, i.e., they occupy localized states characterized by a high degree of local atomic and electronic polarization. Successful interpretations of photoemission spectra, contact charge exchange, and injected carrier drift mobilities verify this prediction. The boundary of the Fermi-glass region of localized-state molecular-ion behavior is identified by consideration of molecular crystals in which injected charges may occupy extended energy-band states at low temperatures.

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