  The conducting polymer polyaniline (PANI) has a wide range of optoelectronic
applications due to its unique electronic and optical characteristics. Although
extensive works have been performed to understand the equilibrium properties,
the nature of the charge type that governs its non-equilibrium optical response
has been barely understood; a number of studies have debated the nature of
photo-generated charge type in PANI, specifically whether it is polaron or
exciton based. Here, we report experimental evidence that the charge relaxation
dynamics of PANI are dominated by excitons. Utilizing ultrafast spin-resolved
pump-probe spectroscopy, we observed that PANI charge dynamics are strongly
spin-polarized, exhibiting a spin Pauli-blocking effect. Investigations
including both spin-independent and spindependent dynamics reveal that there is
no spin-flip process involved in charge relaxation. This provides compelling
evidence of an exciton-dominated photo-response in PANI.
