Optical absorption from polarons in a diatomic polymer.

Abstract

Several recent experiments' on polythiophene have focused renewed attention on the optical-absorption properties of polaronlike excitations in conducting polymers. In particular, strong intragap absorption features are observed in both photoinduced photoabsorption (PA) and in optical absorption from doped samples. In PA and in moderately doped ( )4 mol % ) ' samples, two peaks are clearly observed, whereas for lightly doped ( & 2.5 mo1%)' samples, a third peak occurs at a frequency intermediate between the other two. Importantly, both this pattern of peaks and its change with doping are exactly what one would expect on the basis of prior theoretical calculations of polaron absorption within the continuum limit of the Su-SchriefferHeeger —(SSH)-like model' appropriate to a conducting polymer (like polythiophene) with a nondegenerate ground state. For low doping, where single polarons are expected to be dominant excitations, the two localized electronic levels (at +coo, coo & 6,o) associated with a polaron lead to three distinct intragap (co & 2b.o) contributions to a(co), the optical absorption: (1) a'(co), a 5-function transition between the two localized levels occurring at co = 2coo, (2) a' '(co), an absorption which begins at