Anion Exchange Doping: Tuning Equilibrium to Increase Doping Efficiency in Semiconducting Polymers.

Area of Science:

• Materials Science
• Polymer Chemistry
• Organic Electronics

Background:

• Doping of low ionization energy (IE) polymers with high electron affinity (EA) molecules establishes equilibrium based on energetic driving force (IE-EA), reorganization energy, and dopant concentration.
• Anion exchange doping (AED) involves replacing a dopant anion with an ion from an electrolyte, offering a method to tune polymer properties.

Purpose of the Study:

• To demonstrate and predict the outcomes of anion exchange doping (AED) in organic semiconductors.
• To investigate the spontaneity and resulting doping levels of AED using an isotherm equilibrium model.

Main Methods:

• Utilized an isotherm equilibrium model to predict anion exchange doping (AED) levels.
• Investigated the exchange of FeCl3- dopant anions for TFSI- anions in poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly[3-(2,2-bithien-5-yl)-2,5-bis(2-hexyldecyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione-6,5-diyl] (PDPP-2T).

Main Results:

• The AED process was shown to be either nonspontaneous or spontaneous depending on the polymer system.
• In P3HT, FeCl3 doping yielded high levels but unstable counterions; exchange for TFSI- resulted in air stability with marginal doping increase.
• In PDPP-2T, FeCl3 was a weak dopant, but spontaneous AED with TFSI- led to a >10-fold increase in doping level.

Conclusions:

• The isotherm equilibrium model effectively predicts AED outcomes.
• AED offers a viable strategy for significantly enhancing doping levels in specific polymer systems like PDPP-2T, leading to improved material properties.