Direct observation of a frozen junction in polymer light-emitting electrochemical cells

Doping propagation in polymer light-emitting electrochemical cells (LECs) has been studied based on the direct imaging of extremely large planar LECs at various temperatures and driving voltages. In situ electrochemical p doping, manifested as strong photoluminescence quenching in the polymer film, propagates at a nearly constant rate from anode toward cathode until it encounters an n-doped region to form a p-n junction. The propagation rate is found to be proportional to the applied bias at a fixed temperature. At a fixed bias, the propagation rate has a superexponential dependence on temperature. Below the glass transition temperature (T_g), the propagation rate becomes negligible, and a frozen junction has been confirmed by direct imaging of the doping profiles.