Chemical doping of semiconducting polymers predominantly takes place via integer charge transfer (ICT), where an electron is entirely removed from the host conjugated polymer and transferred to reside on the dopant guest species. In contrast, chemical doping of small conjugated molecules and oligomers often leads to the formation of charge transfer complexes (CTCs), which have significant orbital overlap and shared electron density between the host and guest species. To date, the observation of fractional charge transfer in doped conjugated polymers is relatively rare, occurring only under extreme processing conditions that can be difficult to achieve, which is fortunate given that CTC formation generally yields fewer mobile carriers per dopant. In this work, we use the classic conjugated polymer/dopant pair of P3HT and F4TCNQ to demonstrate how simply adjusting the casting solvent for the dopant in sequential processing can fundamentally alter the nature of doping in this well-studied system, leading to tunable production of CTCs. Using solvent blends of dichloromethane and chloroform, selected for their low and high solubility toward P3HT, respectively, we show that the relative amount of polymer-dopant CTCs can be readily controlled over an order of magnitude. Increasing the amount of chloroform in the dopant solvent blend favors the creation of CTCs, while increasing the dichloromethane content results in doping by the more standard ICT; the results allow us to explain why CTC formation is common in charge-transfer salts but generally less so in doped conjugated polymers. We also explore the role of the doping method and the crystallinity of P3HT films in controlling the relative amounts of ICT and CTC formation. We find that the use of evaporation doping and higher-crystallinity material discourages CTC formation, but that even in the most favorable case of evaporation doping with high polymer crystallinity, fractional charge transfer always occurs to some extent. Finally, we show that brief thermal annealing can convert CTCs to integer charge transfer species, indicating that ICT is the thermodynamically preferred doping mechanism in conjugated polymers, and that fractional charge transfer is the result of kinetic trapping. With this understanding, we offer guidelines for limiting the occurrence of charge transfer complexes during sequential doping of conjugated polymers, thus avoiding the deleterious effects of CTCs on charge transport.

For thermoelectric and other device applications there has been great interest in the chemical doping of conjugated polymer films. Solution doping followed by film deposition generally produces poor-quality films, but this issue can be alleviated by sequential doping: a pure polymer film is deposited first, and the dopant is then added as a second processing step, preserving the quality and structure of the original polymer film. In this paper, we compare two methods for sequential doping of conjugated polymer films: evaporation doping, where a controlled thickness of dopant is added via thermal sublimation to a temperature-controlled polymer film, and sequential solution doping, where the dopant is spin cast from a solvent chosen to swell but not dissolve the underlying polymer film. To compare these two different types of sequential doping, we examine the optical, electrical, and structural properties of poly(3-hexylthiophene-2,5diyl) (P3HT) films doped by each method with the small-molecule dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ) as a function of the polymer film thickness. Although each method intercalates dopant in fundamentally unique ways, we find that both vapor and solution doping methods produce films that share many of the same properties. Interestingly, both methods can produce doped P3HT films with conductivities of ∼5 S/cm and comparable thermoelectric properties, even for films as thick as 400 nm. For the evaporation method, an "overhead" dopant film thickness of ∼6 nm is required, either to promote reorganization of existing crystallites or to fill preexisting trap states in the polymer film. After the overhead amount has been deposited, the thickness of the dopant layer that must be evaporated to reach the optimal electrical conductivity is ∼1/3 that of the underlying polymer film. For a given P3HT film thickness, the amount of evaporated dopant needed to produce the highest conductivity corresponds to a thiophene monomer to ionized dopant ratio of ∼8.5:1. For solution processing, with the appropriate choice of solvent and dopant concentration, we show that P3HT films as thick as 2 μm can be doped to achieve conductivities of ∼5 S/cm and thermoelectric power factors approaching 2 μW/mK 2 . For either method, if excess dopant is applied, it remains in neutral form either in the amorphous regions or on top of the film, reducing the conductivity by increasing the film thickness. For both methods, UV−vis absorption can be used as a quick proxy to easily monitor whether saturation doping levels have been reached or exceeded. Fourier transform infrared spectroscopy (FTIR) and grazing-incidence wide-angle X-ray scattering (GIWAXS) both show that vapor-doped films and thicker solution-doped films have improved morphologies that result in more mobile carriers. Overall, we demonstrate that it is a straightforward process to select a sequential doping method for a desired application: evaporation doping is more amenable to large-area films, while solution doping is lower cost ...

The nitrile vibrational modes of the 2,3,5,6-tetrafluoro-7,7,8,8tetracyanoquinodimethane (F 4 TCNQ) anion occur across a range of different frequencies in doped conjugated polymers. We show that these shifts can be understood as resulting from the vibrational Stark effect. F 4 TCNQ − serves as a sensitive vibrational Stark probe, and its nitrile stretches provide a direct readout of the locally experienced electric fields from the nearby polaron on the doped polymer backbone. The shift of the F 4 TCNQ − B 1u mode and broadening of the B 2u mode in doped poly(3-hexylthiophene-2,5-diyl) (P3HT) confirm that the dopant anion's long axis is oriented perpendicular to the P3HT backbone. The magnitude of the vibrational shifts prove that the anion−polaron distance in F 4 TCNQ − -doped P3HT films is ∼6 Å. We also show that the intrachain polaron coherence can be varied over a range spanning 6 to about 7.5 P3HT monomer units by controlling the local polymer order and crystallinity. At the highest degree of local order, polaron delocalization is limited by the minimum intrinsic strength of the anion−polaron Coulombic interaction. This work provides the first direct experimental measurement of polaron delocalization in doped conjugated polymer films and verifies theoretical models relating polaron coherence to the shape of the doped polymer's mid-IR electronic absorption spectrum.

Organic molecular crystals (OMCs) are of significant interest due to their potential use in transistors, photovoltaic devices, light emitting diodes, and other applications. However, conventional vacuum-based methods of growing crystalline OMC films are costly and provide limited control over crystal growth. In this study, we present a new method for preparing high performance single-crystal tetracene field-effect transistors under near-ambient conditions using organic vapor-liquid-solid (OVLS) deposition. We find that the mobility of OVLS-grown tetracene is comparable to high quality crystalline films prepared by physical vapor deposition. These results establish OVLS deposition as a relatively low cost, low substrate temperature, and ambient pressure method for growing high quality OMC films for device applications.

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