Critical dopant concentrations govern integer and fractional charge-transfer phases in doped P3HT

The conjugated polymer poly(3-hexylthiophene) (P3HT) p-doped with the strong acceptor tetrafluorotetracyanoquinodimethane (F4TCNQ) is known to undergo ion-pair (IPA) formation, i.e. integer-charge transfer, and, as only recently reported, can form ground state charge-transfer complexes (CPXs) as a competing process, yielding fractional charge transfer. As these fundamental charge-transfer phenomena differently affect doping efficiency and, thus, organic-semiconductor device performance, possible factors governing their occurrence have been under investigation ever since. Here, we focus on the role of a critical dopant concentration deciding over IPA- or CPX-dominated regimes. Employing a broad, multi-technique approach, we compare the doping of P3HT by F4TCNQ and its weaker derivatives F2TCNQ, FTCNQ, and TCNQ, combining experiments with semi-classical modeling. IPA, CPX, and neutral-dopant ratios (estimated from vibrational absorption spectroscopy) together with electron affinity and ionization energy values (deduced from cyclic voltammetry) allow calculating the width of a Gaussian density of states (DOS) relating to the highest occupied molecular orbital in P3HT. While a broader DOS indicates energetic disorder, we use grazing-incidence x-ray diffraction to assess spatial order. Our findings consider the proposal of nucleation driving IPA formation and we hypothesize a certain host-dopant stoichiometry to be key for the formation of a crystalline CPX phase.