Molecular electrical doping of organic semiconductors, that is, conjugated polymers (CPs) and conjugated organic molecules (COMs) has emerged as a valuable strategy to improve the performance of organic (opto-)electronic devices and to extend the range of achievable functionality. Molecular p-doping is done by admixing strong molecular acceptors as dopants to CP/COM films with an electron affinity in the range of the ionization energy of the CP/COM. In line with common perceptions of molecular doping, the prototypical CP poly(3-hexylthiophene) (P3HT) doped with tetrafluoro-tetracyanoquinodimethane (F4TCNQ) shows ground-state integer charge transfer (ICT) with essentially every dopant being ionized [1]. Here, we aim at comparing the molecular doping of conjugated oligo- and polymers by juxtaposing the p-doping of P3HT, where ICT was reported to occur localized to one quaterthiophene (4T) unit of the polymer chain [2], to that of the 4T oligomer itself. Overall, we observe a vastly different phenomenology for the two materials classes: Comparably to P3HT, doping 4T films significantly increases their conductivity. However, in marked contrast to P3HT [2], surprisingly small shifts of the characteristic cyano-vibrational bands of F4TCNQ in infrared spectroscopy, which allow quantifying the degree of charge transfer reveal that, instead of ICT, only partial charge transfer (≤ 0.25) occurs for the 4T oligomer. Optical absorption spectroscopy evidences the formation of ground-state charge transfer complexes (CPXs) [3, 4] of 4T and F4TCNQ instead of ICT, which leads to a substantial energy-level splitting between a doubly occupied bonding and an empty antibonding supramolecular hybrid orbital. The frontier electronic states of the CPXs were directly observed by combining ultraviolet and inverse photoelectron spectroscopy (supported by a theoretical treatment on the DFT level). Via grazing-incidence X-ray diffraction, already at a low dopant loading of 1:75 we observe phase separation between pure 4T and 1:1 co-crystallites of 4T and F4TCNQ that form such CPXs. Therefore, instead of individual dopants being ionized through ICT, as clearly the case in P3HT, it is these 1:1 co-crystallites with their peculiar electronic structure of CPXs that take over the role of the dopant. Their unoccupied supramolecular hybrid states lie by ca. 0.5 eV higher in energy than the highest occupied molecular orbitals of the circumjacent 4T film. Through Fermi-Dirac occupation of all available states, only a fraction are occupied at room temperature, which then leads to negatively charged 1:1 co-crystallites and mobile holes within the 4T matrix. Our study highlights intrinsic differences between the fundamental doping mechanisms at work in COMs and CPs of identical chemical composition, thereby constituting an important thrust area of future research, which must aim at deriving a unified picture of molecular electrical doping valid for both classes of organic semiconductors.
[1] P. Pingel et al., Phys. Rev. B 87, 115209 (2013). [2] P. Pingel et al., J. Phys. Chem. Lett. 1, 2037 (2010). [3] I. Salzmann et al., Phys. Rev. Lett. 108, 035502 (2012). [4] H. Méndez et al., Angew. Chem. Int. Ed. 52, 7751 (2013).