Factors Promoting or Inhibiting Charge Transfer in Doped Organic Semiconductors

To control the conductivity of organic semiconductors (OSCs) and their energy level alignment in heterostructures and at interfaces, OSCs are being electrically doped by introducing guest species into the semiconductor host. Common doping agents added to initiate electron transfer include strong molecular donors/acceptors and Lewis acids. Complementary to direct integer charge transfer between OSC and dopant to form ion pairs (the ultimate goal of doping OSCs), fractional charge transfer can occur forming ground-state charge transfer complexes instead via intermolecular orbital rehybridization of OSC and dopant, which is detrimental to the doping efficiency [1]. For OSCs including oligothiophenes of different chain length p-doped with strong electron acceptors [2, 3], I will discuss the role that (i) thin film structure plays for the respective doping scenario, (ii) that of the dopant strength, and (iii) of OSC conjugation length. These findings suggest exploiting steric hindrance to suppress CPX formation which we recently demonstrated for a non-planar p-dopant of 3D shape [4]. Finally, I will contrast above doping scenarios to those that have been proposed to be at work for the doping of OSCs using Lewis acids as alternative doping agents.<br/> <br/>[1] I. Salzmann et al., Acc. Chem. Res. 49, 370 (2016)<br/>[2] H. Hase et al., J. Phys. Mater. 6, 014004 (2023)<br/>[3] J.T. Liu et al., Angew. Chem. Int. Ed. 59, 7146 (2020)<br/>[4] S. Charoughchi et al., Angew. Chem. Int. Ed., DOI: 10.1002/anie.202304964 (2023, in press)