ESR study of electrochemical doping in the conducting polymer polypyrrole

Steady-state and transient measurements of spin concentration have been carried out as a function of electrochemical charge injection in polypyrrole. A maximum of one spin per 10 pyrrole units has been observed for about 50% of the maximum charge. The data are interpreted in terms of quasiequilibrium polaron and bipolaron populations resulting from a two-step redox process. Surprisingly, it appears that at low doping levels, bipolaron formation is not favorable compared to formation of two polarons.