Opto-electrochemical spectroscopy of trans-(CH)x

Detailed quantitative studies of the visible near-ir absorption spectra of trans-(CH)x have been carried out in the semiconducting, transitional, and metallic regimes. With the use of the opto-electrochemical technique to take advantage of the control and precision of electrochemical doping, the absorption data were obtained in situ, during the doping process. Thin (CH)x films were polymerized on conducting glass and used as one electrode in an electrochemical cell (Li+ClO4- in propylene carbonate) with a strip of Li metal as the counter electrode. As the trans-(CH)x was doped from y<0.003 to y0.08, the interband absorption peak decreased and the midgap absorption increased in intensity. The quantitative agreement with the calculations of the soliton model and the universal nature of the data provide direct evidence for the existence of charged solitons in doped trans-(CH)x. The results show that in the concentration range 2×10-3<y<5×10-2, where independent experiments have established that the high conductivity is due to spinless carriers, the semiconductor gap persists with little change in magnitude. Moreover, the absorption data identify these carriers as charged solitons. Kinetic studies following a step change in applied cell voltage indicate a time constant for the approach to equilibrium (after an initial faster transient) in the range from 6 to 10 h. This long time constant results from diffusion of the dopant ions within the (CH)x fibrils. Analysis of the approach to equilibrium and direct analysis of the spectra at intermediate doping levels demonstrate that phase separation with the formation of metallic "particles" does not occur. Hysteresis in the intensity of the midgap transition as a function of applied voltage has been interpreted as a direct indication of single-charge injection via polarons which subsequently combine to form lower-energy soliton pairs.