Chemical stabilization of doping in conjugated polymers

Conjugated polymers can be electrochemically doped to high-conductivity states under applied voltage, but such in situ formed doping structures are dynamic and dissipate when the formation voltage is removed. For some applications it is highly desirable to permanently stabilize the doping after its formation. Here, we report and compare results on four different approaches to chemical stabilization of an emissive and rectifying p-n junction doping structure in a light-emitting electrochemical cell (LEC) application: (i) polymerization of the dopant counter-ions, (ii) polymerization of the counter-ions utilizing a radical-initiator compound, (iii) polymerization of the ion-transport material, and (iv) polymerization of both the counter-ions and the ion-transport material utilizing a radical-initiator compound. We found that approach (i) resulted in LEC devices with poor stability, current rectification and light-emission, and that approach (ii) solely yielded a notable improvement in the light-emission. Approach (iii) resulted in good current rectification and stability, but the overall best results were clearly attained with approach (iv) as such stabilized LEC devices exhibited a respectable current rectification ratio of ∼2000, as well as a decent light-emission efficiency and long-term stability under idle conditions.