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.

Molecular p‐dopants designed to undergo electron transfer with organic semiconductors are typically planar molecules with high electron affinity. However, their planarity can promote the formation of ground‐state charge transfer complexes with the semiconductor host and results in fractional instead of integer charge transfer, which is highly detrimental to doping efficiency. Here, we show this process can be readily overcome by targeted dopant design exploiting steric hindrance. To this end, we synthesize and characterize the remarkably stable p‐dopant 2,2′,2′′‐(cyclopropane‐1,2,3‐triylidene)tris(2‐(perfluorophenyl)acetonitrile) comprising pendant functional groups that sterically shield its central core while retaining high electron affinity. Finally, we demonstrate it outperforms a planar dopant of identical electron affinity and increases the thin film conductivity by up to an order of magnitude. We believe exploiting steric hindrance represents a promising design strategy towards molecular dopants of enhanced doping efficiency.

The energy‐level alignment (ELA) at interfaces ubiquitous in organic optoelectronic applications is decisive for the device performance. Due to the notoriously low density of free charge carriers in organic thin films, the ELA at organic––inorganic interfaces is determined by gap states and/or tailing states of the frontier molecular orbitals, that is, the highest occupied molecular orbital and the lowest unoccupied molecular orbital (LUMO). Informed by modeling defect energies on the density‐functional theory level, it is deduced from ultraviolet and X‐ray photoelectron spectroscopy data that chemical‐defect induced gap states lead to the substrate‐independent pinning of the Fermi level (EF) to the LUMO for 1,4,5,8,9,12‐hexaazatriphenylene‐2,3,6,7,10,11‐hexacarbonitrile thin films. For 5,6,11,12,17,18‐hexaazatrinaphthylene thin films, the ELA is instead governed by tailing states due to energetic disorder, which put the EF closer to midgap position. It is highlighted in the study that the susceptibility of conjugated organic material to forming chemical and structural defects is key for the ELA at interfaces and, therefore, must be considered in the synthesis of novel materials and their processing into functional structures.

Molecular p‐dopants designed to undergo electron transfer with organic semiconductors are typically planar molecules with high electron affinity. However, their planarity can promote the formation of ground‐state charge transfer complexes with the semiconductor host and results in fractional instead of integer charge transfer, which is highly detrimental to doping efficiency. Here, we show this process can be readily overcome by targeted dopant design exploiting steric hindrance. To this end, we synthesize and characterize the remarkably stable p‐dopant 2,2′,2′′‐(cyclopropane‐1,2,3‐triylidene)tris(2‐(perfluorophenyl)acetonitrile) comprising pendant functional groups that sterically shield its central core while retaining high electron affinity. Finally, we demonstrate it outperforms a planar dopant of identical electron affinity and increases the thin film conductivity by up to an order of magnitude. We believe exploiting steric hindrance represents a promising design strategy towards molecular dopants of enhanced doping efficiency.