Trions as Fundamental Species in Chemically Doped Polymer Semiconductors

Abstract

Doping is a cornerstone strategy for enhancing charge transport in semiconducting polymers, important for their application in, for example, semi-transparent electrode materials, thermoelectric devices, and antistatic coatings. Both chemical and electrochemical doping have, for this purpose, been the focus of extensive research resulting in considerable progress. However, the interactions between neutral excitons and doping-induced charges to form multi-particle states are largely unexplored in soft organic semiconductors, and their signatures remain poorly understood. Here, we demonstrate that coupling between excitons and polarons in doped polymers can lead to bound states such as trions (i.e., quasiparticles of an electron and two holes delocalized across three chromophores for p-doping), or bound exciton-hole pairs. Combining spectroscopic evidence with theoretical insights, we hypothesize that polymer architecture, dopant chemistry, and charge delocalization govern the formation and stability of these multi-particle states. More broadly, our findings reveal that trions and bound exciton-hole pairs-that is, three-body entities-are a key species in organic semiconductors that could open new pathways toward optoelectronic functionalities beyond conventional doping, including enhanced charge transport and quantum-coherent excitations.