Doping Effectiveness and Stability in Semiconducting Polymers

Molecular doping of semiconducting polymers has emerged as a prominent research topic in the field of organic electronics with new dopant molecules introduced regularly. FeCl₃ has gained attention as a p-type dopant due to its low-cost, availability, ability to dope high ionization energy co-polymers, and its use as a dopant that can be used with anion exchange. Here, we use a combination of UV-Vis-NIR<br/>spectroscopy, four-probe sheet resistance measurements, and X-ray absorption near-edge structure (XANES) spectroscopy to perform lifetime measurements to assess the stability of the doped polymers over time, which is crucial for evaluating the long-term performance and reliability of the doped films. FeCl₃ can cause radical side reactions that damage the conjugated polymer backbone, leading to degradation of the electronic properties. The rate of this degradation is orders of magnitude higher when the film is exposed to air. Anion exchange doping can reduce the [FeCl₄]⁻ concentration, but does not necessarily improve the doping lifetime because anion exchange electrolytes can serve<br/>as co-reactants for the degradation reaction. By comparison, doping with (2,3,5,6-Tetrafluoro-2,5-cyclohexadiene-1,4-diylidene)dimalononitrile (F4TCNQ) as the reactive dopant results in lower initial conductivity, but the lifetime of the doped polymer is almost tripled as compared to FeCl₃ doped polymer films. These findings highlight that the use of FeCl₃ as a molecular dopant requires a cost/benefit analysis between higher initial doping levels and lower film stability.