Evaporation vs Solution Sequential Doping of Conjugated Polymers: F₄TCNQ Doping of Micrometer-Thick P3HT Films for Thermoelectrics

Abstract

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 ...

Microstructure and heteroatom dictate the doping mechanism and thermoelectric properties of poly(alkyl-chalcogenophenes)

“…1 dopant per 5 rings), which is consistent with previous P3HT-F4TCNQ reports. 30,31 Inspection of the N-1s spectra shown in Figure 2e provides additional insight as Watts et al…”

“…Vapor doped films were chosen because it has been demonstrated that this maintains higher degrees of structural ordering compared to solution doped films, and we observe comparable spectroscopic properties with both types of doping. 26,30 A brief discussion of the pristine GIWAXS scattering shown in Figure 3a-c can be found in Note S5. Upon doping with F4TCNQ in the vapor phase, a clear change in the scattering pattern occurs for all the polymers (Figure 3d-f).…”

Tunable Dopants with Intrinsic Counterion Separation Reveal the Effects of Electron Affinity on Dopant Intercalation and Free Carrier Production in Sequentially Doped Conjugated Polymer Films

“…Interestingly, for all the DDB dopants, we see coexistence of doped and undoped phases over a broad range of dopant concentrations. This is not observed with F 4 TCNQ, where minimal amounts of dopant appear to induce a phase transition in nearly all regions of the film at once, [24] and then F 4 TCNQ continues to add to and "fill up" those transformed crystallites. [25] By contrast, with the DDB clusters, the structural change is so extreme that "partially filled" but expanded crystallites are more energetically favorable.…”

“…[6,16,[30][31][32] A dominant idea in the literature has been that highly crystalline polymer films are necessary to achieve good charge transport properties, with many molecular doping papers focusing on improving polymer solid state order. [5,16,[20][21][22][23][24]31] In a previous work, we showed that we could use solution SqP to control the crystallinity of doped conjugated polymer films. [5] For the widely used poly(3-hexylthiophene-2,5-diyl) (P3HT) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ) materials combination, we indeed found higher carrier mobilities in more crystalline films.…”

Electron transport in a sequentially doped naphthalene diimide polymer

“…3,4 Sequential doping avoids possible complications that arise if the doped material is poorly soluble in the casting solvent and in some cases may allow for the preservation of some of the structural order present in the pristine semiconductor film. Although widely applied to the p-doping of P3HT [5][6][7][8] and PBTTT, 9,10 there are only few reports of the sequential n-doping of solution-processed polymers. 11 The air-stability of oxidants, hole-transport materials, and their doped combinations often allow for easy handling in air.…”