Comparison of solution-mixed and sequentially processed P3HT:F4TCNQ films: effect of doping-induced aggregation on film morphology

Abstract

Doping polymeric semiconductors often drastically reduces the solubility of the polymer, leading to difficulties in processing doped films. Here, we compare optical, electrical, and morphological properties of P3HT films doped with F4TCNQ, both from mixed solutions and using sequential solution processing with orthogonal solvents. We demonstrate that sequential doping occurs rapidly (o1 s), and that the film doping level can be precisely controlled by varying the concentration of the doping solution. Furthermore, the choice of sequential doping solvent controls whether dopant anions are included or excluded from polymer crystallites. Atomic force microscopy (AFM) reveals that sequential doping produces significantly more uniform films on the nanoscale than the mixed-solution method. In addition, we show that mixedsolution doping induces the formation of aggregates even at low doping levels, resulting in drastic changes to film morphology. Sequentially coated films show 3-15 times higher conductivities at a given doping level than solution-doped films, with sequentially doped films processed to exclude dopant anions from polymer crystallites showing the highest conductivities. We propose a mechanism for doping induced aggregation in which the shift of the polymer HOMO level upon aggregation couples ionization and solvation energies.To show that the methodology is widely applicable, we demonstrate that several different polymer:dopant systems can be prepared by sequential doping.

Quantitative Dedoping of Conductive Polymers

“…8,44 As a result, fluorescence intensity is highly sensitive to the density of mobile charges (p free ) and therefore doping level, allowing for simple detection of residual dopants. In these measurements we use sequential doping from an orthogonal solvent 18,19,45 to separate the effects of additional dopants from impurity levels in the initial polymer film coating. This also allows us to measure the fluorescence intensity of each film before doping as well as after dedoping, controlling for sample-to-sample variation in fluorescence intensity (e.g., from thickness variations).…”

“…18 In the following experiments, all films are sequentially doped from an F4TCNQ/AN solution to achieve a doping level of approximately 3.7 mol %. 19 After doping, each film is rinsed with CB to allow F4TCNQ to intercalate into the P3HT crystallites. 19 Finally, the films are dedoped by by immersion in a 10% amine solution in a polar carrier solvent which does not dissolve the polymer.…”

“…19 After doping, each film is rinsed with CB to allow F4TCNQ to intercalate into the P3HT crystallites. 19 Finally, the films are dedoped by by immersion in a 10% amine solution in a polar carrier solvent which does not dissolve the polymer. See Supporting Information Section 5 for additional information on optimization of dedoping conditions.…”

“…35 However, even if the reaction goes through a neutral intermediate or if the donor is not strong enough to reduce the polymer, thermal fluctuations could transiently transfer charge back from the ionized dopant to the ionized donor. In either case, since the driving force for solvation of F4TCNQ in P3HT is ionization and charge screening, 19 after reaction the product should prefer to dissolve in the solvent rather than remain in the film. A schematic of the process is shown in Figure 1d,f.…”

“…18 We call this process doping-induced solubility control (DISC). 18,19,25,26 However, in many device applications intrinsic (undoped) material properties are required; therefore, the ability to quantitatively dedope films after patterning or layering is of paramount importance. In previous work, 18 we found that when undoped P3HT films were doped with F4TCNQ and then dedoped with a 9:1 mixture of acetone:ethylenediamine at 60°C, the dedoped film's optical, electrical, and chemical properties returned almost exactly to their as-cast values.…”