UC Davis

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 copolymers, and its use as a dopant that can be used with anion exchange. Here, we use a combination of UV-vis-NIR 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. We used gas chromatography-mass spectrometry (GCMS) to prove that FeCl3 can cause radical side reactions that damage the conjugated polymer backbone, leading to the 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 as coreactants 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 that of 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.