Morphology of Organic Semiconductors Electrically Doped from Solution Using Phosphomolybdic Acid

The field of organic electronics aspires to enable the fabrication of low-cost, solution-processed optoelectronic devices with unique mechanical, electrical, optical, and chemical properties. Critical to the success of these aspirations is the ability to fabricate controlled doping profiles vertically or laterally (i.e., to a limited depth or area extension). However, the fabrication of stable doping profiles in polymer films has proven particularly challenging, as neither solution processing nor evaporation of dopants, such as 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ), leads to vertical doping profiles due to fast diffusion on the length scale of the typical film thickness (∼100 nm). This challenge was surmounted in 2017 with the first demonstration of a successful solution-based technique to fabricate doping profiles in semiconducting polymer films through immersion into a phosphomolybdic acid (PMA) solution (Kolesov et al., 2017). Still, to date, no clear picture that explains the doping phenomena has emerged. In an attempt to identify some of the key variables that govern the PMA doping process and shed light onto why this technique produces vertical doping profiles in organic films, we here report on a study of the morphology of PMA doped semiconducting polymer films, complemented theoretically with ab initio quantum chemistry calculations. We believe these results may foster the extension of the technique to other organic optoelectronic systems.