Molecular layer deposition and chemical vapor deposition are emerging and promising techniques for the incorporation of high-performance conductive polymers into high surface area devices, such as sintered tantalum anodes for electrolytic capacitors. Until recently, vapor-phase synthesis of poly(3,4-ethylenedioxythiophene) (PEDOT) has relied on solid reactants which require relatively high temperatures and complex dosing schemes for sequential layer-by-layer processes. This work introduces a facile and high-performing layer-by-layer oxidative molecular layer deposition (oMLD) scheme using the volatile liquid oxidant antimony(V) chloride (SbCl5) to deposit PEDOT thin films. Effects of reactor parameters on PEDOT film characteristics are described, and the necessary foundation for future studies aiming to understand the nucleation and growth of layer-by-layer oMLD PEDOT is detailed.

Since its discovery, poly(3,4-ethylenedioxythiophene) (PEDOT) has emerged as one of the most popular intrinsically conductive polymers. Oxidized PEDOT's transparency, electrical conductivity, thermal and electrochemical stability, biocompatibility, and thermoelectric properties make it suitable for a wide range of electrical, chemical, and biomedical applications.1–3 PEDOT belongs to a class of π-conjugated conducting polymers with nondegenerate ground states. The most widely accepted oxidative polymerization mechanism of PEDOT is a step-growth polymerization which can be divided into three main steps: (i) oxidation of 3,4-ethylenedioxythiophene (EDOT) monomers to radical cations, (ii) dimerization of free radical cations, and (iii) deprotonation. P-doped PEDOT is formed from the subsequent oxidation of neutral PEDOT chains, which results in polaron (radical cation) and bipolaron (dication) charge defects that are stabilized by counter anions, usually by-products of the redox reaction.4 In crystalline domains with short interchain distances, π-orbital overlap of adjacent chains promotes the formation of bipolaron and polaron networks, corresponding to semimetallic and metallic character, respectively.5 

Although the rigid conjugated bond structure of oxidized PEDOT provides favorable electric properties, the thermal stability and insolubility of the material make it difficult to process into thin films. To overcome these issues, the hydrophilic stabilizing anion poly(styrenesulfonic acid) (PSS) has been widely incorporated into PEDOT to make it solution processable. However, PSS stabilizing anions tend to inhibit crystallization into the form desired for high conductivity.6 Moreover, for PEDOT:PSS cathodes widely used in electrolytic capacitors, PSS molecules have been correlated to anomalous currents which may impede long-term performance.7,8 Another challenge in tantalum-PEDOT electrolytic capacitors is that in situ polymerized PEDOT leads to spatial accumulation of charge carriers at the oxide-semiconductor interface leading to an unfavorable decrease in break down voltages (BDV).9 

Vapor-phase deposition is a promising approach to integrate high-performance PEDOT films into a wide range of electronic devices. Vapor-phase synthesis methods, including vapor-phase polymerization (VPP), oxidative chemical vapor deposition (oCVD), and oxidative molecular layer deposition (oMLD), have been used to deposit PEDOT thin films and shown improved conductivity compared to solution phase synthesis.10–12 In VPP of PEDOT, a substrate is first coated with an oxidant by solution spinning, then growth proceeds by continuously exposing the surface to the EDOT monomer vapor. In the oCVD process, growth proceeds when the substrate is simultaneously exposed to EDOT and oxidant vapors. The oMLD process is a variant of oCVD where the monomer and oxidant vapors are introduced sequentially onto the depositing polymer surface using an inert carrier gas, separated by inert reaction purge steps.12 Analogous to atomic layer deposition (ALD), film growth can proceed by repeating self-limiting half-reactions on the substrate surface. Inherent advantages of oMLD include the ability to independently control substrate exposure to monomer and oxidant vapors and control film thickness. The sequential reactant dosing also allows effective purging of by-products to mitigate their accumulation within the film.

In the VPP, oCVD, and oMLD methods, several solid oxidants have been evaluated including FeCl3, MoCl5, and Fe(Tos)3.12–14 Compared to solid oxidants, volatile liquid oxidants are favorable for oMLD applications because of their facile dosing at lower operating temperatures and shorter processing times. The volatile liquid SbCl5 has been applied as a nonoxidative Lewis catalysts and dopant in PEDOT synthesis and was reported recently as an oxidant during PEDOT oCVD.3 

This report introduces an oMLD deposition strategy utilizing sequential doses of SbCl5 and EDOT to create conductive PEDOT thin films. The effects of film thickness, deposition temperature, and postdeposition rinses on conductivity are described, thereby laying the foundation for linking deposition conditions to film performance. Film growth and properties are also compared to previous reports of PEDOT formation by oMLD and oCVD,11,12,15 and we find that the high volatility of SbCl5 has benefits for deposition. Similar to previous oMLD studies, we find the half-reactions show partial saturation, indicating some oCVD component within the layer-by-layer oMLD sequence.

Monomeric 3,4-ethyelenedioxythiophene (EDOT, 99%) and anhydrous antimony(V) chloride (SbCl5, 99%) were obtained from Fisher-Scientific and used without further purification and were handled in a nitrogen atmosphere. Ultrahigh purity N2 (Arc3 Gases) was further purified using an inert gas filter (Gatekeeper) and used as the carrier and purge gas for reactants. Mass-Vac Sodasorb and activated charcoal filters were attached to the inlet of a Fomblin oil rotary vane vacuum pump to improve pump lifetime.

Films were deposited on double-sided polished p-type silicon (100) substrates (30–60 Ω cm) for IR analysis. For deposition, silicon wafers were cleaved into approximately 2 × 2 cm2 coupon substrates. For conductivity measurements, films were coated onto thermally oxidized silicon (100) wafers with an oxide thickness of 100 nm. For some experiments, silicon wafers were used that had native silicon oxide present. Prior to deposition, all silicon substrates were cleaned in a piranha solution with a 1:1 volume ratio of H2O2:H2SO4 to increase the uniformity of the oxide layer and remove organic residue. Substrates with the thick (∼100 nm) oxide are referred to as “SiO2 substrates.” Silicon with only the piranha chemical oxide is referred to as “hydroxylated Si-OH substrates.”

PEDOT layer-by-layer oMLD was performed in a custom-built hot-wall viscous flow reactor described previously.12 Reactor temperature ranged between 100 and 150 °C with 150 °C set as the upper limit to prevent the dissociation of SbCl5. The steady-state operating pressure was ∼1.4 Torr under a continuous total flow of 185 standard cubic centimeters per minute (sccm) of purified N2 delivered equally through two flow lines connected, respectively, to the EDOT and SbCl5 delivery vessels. To achieve sufficient precursor vapor pressure for consistent dosing, EDOT and SbCl5 were heated to 80 and 45 °C, respectively. To aid the monomer precursor delivery, the precursor vessels for both EDOT and SbCl5 were constructed in a flow-through geometry. Before each reactant dose, the N2 flow was directed into the EDOT vessel for 0.25 s before opening the vessel output valve to start the dose. SbCl5 vapor was delivered via directing N2 to flow through the reactant vessel. Each precursor vessel was equipped with computer-controlled pneumatic diaphragm valves as well as high-temperature manual ball valves.

For a typical reaction cycle, the monomer was dosed for 4 s and the SbCl5 oxidant was dosed for 0.15 s, each separated by a 60 s N2 purge step. This process sequence followed: EDOT/N2/SbCl5/N2 = 4/60/0.15/60 s, respectively. To evaluate half-cycle saturation, EDOT and SbCl5 dose times were varied independently using the following process sequences: EDOT/N2/SbCl5/N2 = x/60/0.15/60 s, where x ranged from 0 to 10 s, and EDOT/N2/SbCl5/N2 = 4/60/y/60 s, where y ranged from 0 to 4 s, respectively. In our reactor system, the volume of the precursor delivery vessels was ∼70 ml, initially filled with ∼30 ml of reactant. For this setup, we found that for deposition at 150 °C, increasing the dose times/cycle (>6 s or 2 s for EDOT or SbCl5, respectively) lead to a decrease in a net film growth rate, which is ascribed to a combination of limited precursor delivery and extended precursor desorption. Specifically, the reactants in the precursor source bottle evaporate relatively slowly so that during the long exposure step, the reactant partial pressure delivered into the reactor decreases during the dose. The reduced partial pressure and longer time favors reactant desorption so that an apparent longer dose time leads to less growth on the surface.

For quartz crystal microbalance (QCM) measurements, data were collected at 6 MHz oscillation frequency on unpolished gold-coated quartz crystals (Inficon). For reliable analysis, the QCM was modified to allow N2 purging to protect the backside of the crystal. The N2 purge flow was 25 sccm which corresponded to a 0.20 Torr increase in operating pressure. Prior to PEDOT deposition, QCM crystals and housing were stabilized at the reaction temperature under N2 flow for 360 min, ensuring that the temperature of the QCM crystal was the same as the deposition substrate in the isothermal hot-wall reactor. Before PEDOT deposition, the reactor walls and gold-coated QCM crystal were coated with 50 cycles of ALD Al2O3 to ensure a consistent starting chamber condition. The QCM analysis was performed within the steady-growth regime, i.e., after many cycles of PEDOT growth, thereby ensuring that the data were characteristic of PEDOT growth on PEDOT rather than nucleation on Al2O3.

For some sample analysis measurements, deposited films were treated with various solutions. Rinsing was performed with 0.5 M solutions of d-tartaric acid (99%, Sigma-Aldrich), sulfuric acid (99.999%, Sigma-Aldrich), nitric acid (ACS reagent, ≥90.0%), hydrochloric acid (37%, Sigma-Aldrich), and methanol (anhydrous 99.8%, Sigma-Aldrich) which were prepared using de-ionized water. In order to remove excess antimony oxide contaminants, films were rinsed in acids for 15 min with sonication, followed by 15 min in methanol with sonication at room temperature before drying under N2. Unless specified otherwise, data were collected on samples in the as-deposited state without acid or other solution treatment.

Film thickness values were determined by ellipsometry (SE, J.A. Woollam alpha-SE). For some samples, thickness measurements were also performed by cross-sectional SEM imaging (FEI Verios 460L) and by step height measurements (Dektatk 150 profilometer). For SEM cross-sectional images, PEDOT-coated silicon wafers were scored and then immersed in liquid nitrogen for 6 min before cracking. Thickness values obtained by the different techniques on the same samples gave consistent results. In-plane conductivity of the layer-by-layer oMLD PEDOT films was measured using a Jandel RM 3-AR four-point probe with 1 mm spacing. Conductivity measurements were performed on PEDOT thin films deposited on 100 nm thermal oxide silicon wafers. Because the thermal oxide layer electrically insulated PEDOT thin films from Si, probe contact with the underlying SiO2 will not affect the conductivity values. Measurements were also performed using a custom-built four-point probe with 2 mm spacing and Keithley 2400 source meter. The reported conductivity is determined directly from the measured data without applying geometric correction factors. Because the sample size (i.e., ∼2 × 2 cm2) is relatively small, the sample geometry can cause the measured resistivity values to be larger than the actual material value (i.e., the conductivity will be less than the actual material value). This means that the conductivity reported here may be smaller (possibly by a factor of 10%–30% depending on sample size and probe spacing used) than the bulk values. However, since the measured conductivity depends strongly on deposition conditions (i.e., values change by more than a factor of 10), any variations due to sample measurement geometry will be relatively small and will not substantively influence the observed trends.

PEDOT film composition was verified using Fourier transform infrared spectroscopy (FTIR), time-of-flight secondary ion mass spectroscopy (Tof-SIMS), and x-ray photoelectron spectroscopy (XPS). The FTIR analysis was performed in transmission mode using a ThermoNicolet 6700 IR bench with a deuterated triglycine sulfate (DTGS) detector. ToF-SIMS analyses were conducted using a TOF-SIMS V (ION TOF, Inc. Chestnut Ridge, NY) instrument equipped with a Bi-n m+ (n = 1–5, m = 1, 2) liquid metal ion gun. For depth profiling, 1 keV low energy Cs+ with 20 nA current was used to create a 150 × 150 μm2 area and the middle 30 × 30 μm2 area was analyzed using a 0.3 pA Bi3+ primary ion beam. The negative secondary ion mass spectra obtained were calibrated using C, O, Si, and S and normalized to total ion counts. XPS was performed using a SPECS system with a PHOIBOIS 150 analyzer and 10 kV Al source. Some deposited PEDOT samples were measured by x-ray reflectivity (XRR). Measurements were performed with a Rigaku SmartLab x-ray diffractometer in general reflectivity mode (Cu Kα source, 1.54 Å, 44 mA, 40 kV) in a parallel beam geometry at an angle of incidence of 0°–4°. Fits to the data were obtained using the genx software package.16 X-ray diffraction (XRD) and grazing incidence x-ray refraction (GIXRD) were used to analyze the crystal structure of the deposited PEDOT. These measurements were performed on the same instrument as for XRR but in Bragg–Bretano and parallel beam configuration for XRD and GIXRD, respectively. Samples were also analyzed by Raman spectroscopy using a Horiba XploRA PLUS confocal Raman microscope (Horiba Scientific CCD detector) with 532 and 785 nm excitation sources.

In the layer-by-layer oMLD process, the oxidant and reactant are sequentially introduced to the deposition substrate using a flow of inert N2 carrier gas. In an ideal ALD or MLD reaction, stepwise reactant exposure produces a binary sequence of self-limiting complementary half-reactions, allowing the deposited film to grow with atomic or molecular scale precision. For the PEDOT layer-by-layer oMLD process developed here, the extent of reaction saturation as a function of reactant exposure time was investigated at deposition temperatures of 100 and 150 °C, and results are shown in Fig. 1. In Fig. 1(a), the SbCl5 dose time was set at 0.15 s/cycle, and the EDOT dose time was varied from 0 to 10 s/cycle. The N2 dose time after SbCl5 and EDOT was fixed at 60 s/cycle. For each deposition experiment, PEDOT growth occurred simultaneously on three different ∼2 × 2 cm2 oxidized silicon pieces that were equally spaced along the direction of gas flow on a 12-in.-long sample holder located in the 2.5 in. diameter tubular hot-wall reaction chamber. The growth rates were calculated from the film thickness measured by ex situ ellipsometry after 100 oMLD cycles. The points in Figs. 1(a)1(c) correspond to the thickness/cycle obtained from the average of three measurements on each of the three samples, and the error bars correspond to the range of thickness values obtained from the three sample pieces. For deposition at both 100 and 150 °C, increasing EDOT dose time from 0 to ∼2 s/cycle lead to the increased PEDOT growth rate.

FIG. 1.

PEDOT growth rate of as-deposited samples as a function of (a) EDOT and (b) SbCl5 dose time using 100 and 150 °C, and (c) PEDOT growth per cycle at 100 °C vs EDOT dose time using 0.05 and 0.15 SbCl5 dose times.

FIG. 1.

PEDOT growth rate of as-deposited samples as a function of (a) EDOT and (b) SbCl5 dose time using 100 and 150 °C, and (c) PEDOT growth per cycle at 100 °C vs EDOT dose time using 0.05 and 0.15 SbCl5 dose times.

Close modal

Figure 1(b) shows the average growth per cycle using a fixed EDOT dose time of 4 s/cycle with SbCl5 dose times between 0 and 4 s/cycle. At both 100 and 150 °C, the resulting film thickness increased approximately linearly with SbCl5 dose time, indicating that the half-reaction during the SbCl5 dose does not readily saturate under these growth conditions. As a further saturation test at 100 °C, the SbCl5 dose time was fixed at 0.05 s/cycle and the EDOT dose was varied from 0 to 10 s/cycle. Results are shown in Fig. 1(c) along with data repeated from Fig. 1(a) using an SbCl5 dose time of 0.15 s/cycle. The longer SbCl5 dose time leads to a net larger growth rate. Figure 2 shows photographs of sample pieces from the middle of the sample holder prepared during the experiments shown in Fig. 1. For deposition at 100 and 150 °C, PEDOT samples appear uniform in color, indicating uniformity in the film thickness, which was confirmed with ellipsometry measurements at several locations across the sample. Good uniformity was also observed across the 12 in. growth zone in the flow-tube reactor.

FIG. 2.

Photographic images of as-deposited PEDOT films corresponding to the samples plotted in Fig. 1: (a) varying EDOT dose time and (b) varying SbCl5 dose time at 100 and 150 °C. Uniform color indicates thickness uniformity across the wafers.

FIG. 2.

Photographic images of as-deposited PEDOT films corresponding to the samples plotted in Fig. 1: (a) varying EDOT dose time and (b) varying SbCl5 dose time at 100 and 150 °C. Uniform color indicates thickness uniformity across the wafers.

Close modal

Using fixed EDOT and SbCl5 dose times of 4 and 0.15 s, respectively, the film thickness deposited on SiO2 increased linearly with deposition cycle at 100 and 150 °C, as shown in Fig. 3(a). Some nucleation inhibition was observed during the first ∼30 cycles on the clean SiO2 substrate. For these samples, the deposition sequence followed EDOT/N2/SbCl5/N2 = 4/60/0.15/60 s, respectively, and the thickness was measured by ellipsometry and confirmed using cross-sectional SEM (vide infra). The net growth rate was ∼0.14 nm/cycle at 100 and 150 °C, consistent with the data shown in Fig. 1.

FIG. 3.

(a) As-deposited PEDOT film thickness as a function of deposition cycles at 100 and 150 °C on SiO2. Steady-state growth proceeds after 100 cycles. (b) XRR data from two PEDOT samples deposited at 100 °C, including calculated data fits. (c) In situ QCM of PEDOT deposited at 100 and 150 °C. (d) A magnified view of in situ QCM results from panel (c), collected at 150 and 100 °C. (e) PEDOT film thickness per cycle as a function of purge time for films deposited using 0.15 s SbCl5 and 8 or 2 s EDOT doses.

FIG. 3.

(a) As-deposited PEDOT film thickness as a function of deposition cycles at 100 and 150 °C on SiO2. Steady-state growth proceeds after 100 cycles. (b) XRR data from two PEDOT samples deposited at 100 °C, including calculated data fits. (c) In situ QCM of PEDOT deposited at 100 and 150 °C. (d) A magnified view of in situ QCM results from panel (c), collected at 150 and 100 °C. (e) PEDOT film thickness per cycle as a function of purge time for films deposited using 0.15 s SbCl5 and 8 or 2 s EDOT doses.

Close modal

Film thickness and density were also estimated for representative samples using XRR, and the resulting data are shown in Fig. 3(b). The plots also include fits to the data obtained using the genx software package.16 The substrates for these tests were silicon pieces with a 100 nm thick thermally grown SiO2 layer. Deposition was performed using 2 s dose of EDOT and 0.05 s dose of SbCl5 at 100 °C for 200 and 400 cycles, respectively. From the data fits, the density of the PEDOT is ∼1.7 g/cm3, which is larger than the value of 1.47 g/cm3 estimated for PEDOT using density functional theory,17 and larger than 1.49 g/cm3 measured by the floating method for PEDOT produced with iron tosylate oxidant.18 The larger density value obtained here is ascribed to the presence of relatively heavy Sb remaining in the film, as observed by XPS (shown below).

Figure 3(c) shows in situ QCM results collected during PEDOT deposition in the steady-growth regime using EDOT/N2/SbCl5/N2 = 2/60/0.1/60 s at 100 and 150 °C. When the mass change is averaged over 20 cycles, the net mass change is +61 and +65 ng/cm2/cycle at 100 and 150 °C, respectively, consistent with data in Fig. 1 showing the growth rate is nearly independent of temperature in this range. Using the density of 1.7 g/cm3 from the XRR results, the mass change of +61 ng/cm2/cycle observed by QCM corresponds to a thickness increase of 0.36 nm/cycle, which is larger than the values determined by ellipsometry. The excess mass uptake is ascribed to surface roughness on the QCM crystal so that the growth area is larger than the planar area of the circular crystal. Quantifying results from QCM is also difficult because the measured frequency change is sensitive to both mass and temperature. The adsorption of relatively cool reactants would increase the frequency change, leading to an apparent excess mass gain. Therefore, the in situ QCM data were used to compare relative mass changes during each half-cycle under steady-growth conditions, showing reliable results under repeated growth cycles.

Figure 3(d) is a magnified view of a portion of the data from Fig. 3(c), showing mass uptake over ∼3 cycles at 100 and 150 °C. At both 100 and 150 °C, the SbCl5 dose produced a mass gain followed by slow mass loss during the purge step. The data collected at 150 °C show a relatively larger mass gain during the SbCl5 dose as well as a faster rate of mass loss during the subsequent purge step. The larger rate of mass change at higher temperature is ascribed to thermally driven oxidant diffusion, where at higher temperature, the oxidant more readily diffuses into the growing film during the oxidant dose, and it more readily diffuses out during the following purge.

The net layer-by-layer oMLD film growth rate was also influenced by the purge time used between the reactant doses. Figure 3(e) shows the average growth thickness per cycle measured after 100 cycles on SiO2 at 100 °C. Fixing the SbCl5 dose time at 0.15 s and the EDOT dose at either 2 or 8 s, the purge time after SbCl5 and EDOT doses was set at either 60, 120, or 240 s/cycle. As shown in Fig. 3(e), increasing purge time leads to a net decrease in film growth per cycle. This decrease is ascribed to desorption of unreacted physisorbed oxidant and/or monomer during the purge period. The overall growth mechanisms, including effects of subsurface reactant diffusion and reactant desorption, are presented below in Sec. III D.

Figure 4 shows cross-sectional SEM images of PEDOT films deposited for 400 cycles using EDOT/N2/SbCl5/N2 = 4/60/0.15/60 s at 100 °C on SiO2 and at 125 and 150 °C on hydroxylated silicon (Si-OH). Films show uniform thickness and smooth surface texture. The measured thickness corresponds to thickness per cycle of 0.25, 0.17, and 0.14 nm/cycle at 100, 125, and 150 °C, respectively, which are reasonably close to the values shown in Fig. 3(a) obtained by ellipsometry under the same conditions.

FIG. 4.

Cross-sectional SEM of as-deposited PEDOT thin films grown using EDOT/N2/SbCl5/N2 = 4/60/0.15/60 s for 400 cycles at different temperatures on (a) 100 nm thermal oxide SiO2 and (b) and (c) Si-OH substrates.

FIG. 4.

Cross-sectional SEM of as-deposited PEDOT thin films grown using EDOT/N2/SbCl5/N2 = 4/60/0.15/60 s for 400 cycles at different temperatures on (a) 100 nm thermal oxide SiO2 and (b) and (c) Si-OH substrates.

Close modal

1. Vibrational spectra

FTIR spectra from layer-by-layer oMLD PEDOT films deposited at 100, 125, and 150 °C using EDOT/N2/SbCl5/N2 = 4/60/0.15/60 s are shown in Fig. 5. The intensity of the spectra is normalized to the peak at 1083 cm−1. The spectra show peaks at 1518 and 1473 cm−1 corresponding to asymmetric and symmetric C=C stretching, associated with PEDOT in quinoid and benzoid structures, respectively. Stretching vibrations of the ethylenedioxy moiety (C—O—C) appear at 1190, 1083, and 1047 cm−1. Stretching vibrations of the thiophene ring (C—S—C) are seen at 975, 835, and 685 cm−1. Interring C—C vibrations and ethylenedioxy C—H deformations correspond to peaks at 1307 and 995 cm−1. The absence of thiophene α—H stretching and bending (3112 and 779 cm−1) indicates that acid catalyzed polymerization from HCl by-products is not prevalent.19 The frequency of C=Cνas and C=Cνs bands was the same for all samples, indicating similar proportions of quinoid to benzoid structures.20 Previous studies suggest that the peak intensity ratio at 830 and 670 cm−1, C—H out of plane stretching for thiophenes and 2,5− disubstituted thiophenes, respectively, is inversely proportional to the degree of PEDOT polymerization.21,22 The spectra in Fig. 5 show decreasing intensity at 830 and 670 cm−1 with increasing temperature with some possible decrease in intensity ratio indicating increased polymerization with increasing deposition temperature.

FIG. 5.

Transmission IR spectra of as-deposited PEDOT films deposited at 100, 125, and 150 °C using EDOT/N2/SbCl5/N2 = 4/60/0.15/60 s, normalized to the peak at 1083 cm−1. The position of the C=C asymmetrical stretching modes is independent of deposition temperature, indicating no change in the relative amounts of quinoid and benzoid confirmations between films.

FIG. 5.

Transmission IR spectra of as-deposited PEDOT films deposited at 100, 125, and 150 °C using EDOT/N2/SbCl5/N2 = 4/60/0.15/60 s, normalized to the peak at 1083 cm−1. The position of the C=C asymmetrical stretching modes is independent of deposition temperature, indicating no change in the relative amounts of quinoid and benzoid confirmations between films.

Close modal

Raman spectroscopy with two different excitation sources (λ = 532, 785 nm) was also used to investigate the effect of deposition temperature and film thickness on PEDOT conjugation length. Representative spectra are shown in Fig. 6, and the peak positions obtained from the spectra are plotted vs the film thickness in Fig. 7. In PEDOT films, an increase in the quinoid conformation is expected to correlate with a red shift in the Cα=Cβ symmetric stretching mode (1386–1481 cm−1).23 For all samples measured, the positions of the Cα=Cβ symmetric stretching modes are approximately constant, indicating the PEDOT retains a quinoid conformation for all values of the film thickness and deposition temperature studied.

FIG. 6.

Raman spectra of as-deposited PEDOT films using (a) 532 and (b) 785 nm sources. The Cα=Cβ symmetric modes showed consistent peak positions for each deposition temperature and film thickness for films deposited using EDOT/N2/SbCl5/N2 = 4/60/0.15/60 s.

FIG. 6.

Raman spectra of as-deposited PEDOT films using (a) 532 and (b) 785 nm sources. The Cα=Cβ symmetric modes showed consistent peak positions for each deposition temperature and film thickness for films deposited using EDOT/N2/SbCl5/N2 = 4/60/0.15/60 s.

Close modal
FIG. 7.

Raman shift values for the Cα=Cβ symmetric and asymmetric stretching modes for as-deposited PEDOT films using (a) 532 and (b) 785 nm sources, deposited at 100, 125, and 150 °C plotted vs film thickness. The peak positions are independent of thickness and deposition temperature, indicating the relative extent of quinoid and benzoid confirmation is constant for the deposition temperatures and film thicknesses studied.

FIG. 7.

Raman shift values for the Cα=Cβ symmetric and asymmetric stretching modes for as-deposited PEDOT films using (a) 532 and (b) 785 nm sources, deposited at 100, 125, and 150 °C plotted vs film thickness. The peak positions are independent of thickness and deposition temperature, indicating the relative extent of quinoid and benzoid confirmation is constant for the deposition temperatures and film thicknesses studied.

Close modal

2. X-ray diffraction

PEDOT film conductivity has been reported to be affected by crystallite orientation, which in turn depends on deposition temperature and the film thickness.11 Specifically, face-on stacking of crystallites, formed in thinner films at higher deposition temperatures, leads to the most conductive films.11,12 For films produced here, x-ray diffraction was used in Bragg–Brentano geometry to determine the effect of the film thickness and deposition temperature on film crystallinity and crystallite orientation, and results are shown in Figs. 8(a)8(c). XRD peaks at 2θ ≈ 26° correspond to 0k0 reflections of PEDOT chains with d = 0.34 nm. Integrated normalized peak intensity increased with the film thickness. For a constant thickness of ∼100 nm, increasing deposition temperature led to a decrease in the peak width. For example, a 117 nm film deposited at 100 °C had a full width half maximum (FWHM) = 6.2, whereas a 112 nm thick film deposited at 150 °C shows FWHM = 2.6 [Fig. 8(d)]. This may indicate an increase in film crystallinity for similar thicknesses with increased deposition temperature. Some samples were also analyzed using grazing incidence x-ray diffraction (GIXRD), and results are shown in Fig. 8(e). The peak position at 2θ = 26° confirms π—π stacking orientation for PEDOT deposited at 100, 125, and 150 °C. However, no preferred crystallite orientation is apparent for all films measured.

FIG. 8.

XRD of as-deposited PEDOT thin films deposited at (a) 100, (b) 125, and (c) 150 °C using EDOT/N2/SbCl5/N2 = 4/60/0.15/60 s. Peaks at 2θ = 26° correspond to 0k0 reflection of π-conjugated PEDOT chains. (d) Normalized integrated intensity and FWHM values of peaks at 2θ = 26°. (e) GIXRD diffractograms of PEDOT thin films. Peaks at 2θ = 26.2° indicate π—π stacking of PEDOT chains.

FIG. 8.

XRD of as-deposited PEDOT thin films deposited at (a) 100, (b) 125, and (c) 150 °C using EDOT/N2/SbCl5/N2 = 4/60/0.15/60 s. Peaks at 2θ = 26° correspond to 0k0 reflection of π-conjugated PEDOT chains. (d) Normalized integrated intensity and FWHM values of peaks at 2θ = 26°. (e) GIXRD diffractograms of PEDOT thin films. Peaks at 2θ = 26.2° indicate π—π stacking of PEDOT chains.

Close modal

3. Composition uniformity in growth direction

PEDOT composition uniformity in the direction of film growth was analyzed using ToF-SIMS for three samples deposited at 100, 125, and 150 °C, respectively, and results are presented in Fig. 9. Because of matrix effects in SIMS, where the ion yield of an element can depend on its surrounding chemical state, SIMS data are not used directly for quantitative analysis. However, the ToF-SIMs results are useful to observe trends in compositional depth uniformity and to analyze qualitative differences between samples measured under similar conditions. At each deposition temperature, the data show signals due to oxygen, chlorine, carbon, sulfur, and antimony, and the relative signal intensity in the film bulk is consistent for each sample measured. The remaining Sb is ascribed to a relatively small amount of SbCl3 (i.e., reduced SbCl5) and other antimony chlorides, oxides, and oxychlorides. For the film deposited at 150 °C, the Sb signal increases somewhat with increasing sputtering depth, indicating that more Sb may have desorbed from the top surface during film deposition.

FIG. 9.

ToF-SIMs depth profiling of as-deposited PEDOT films deposited at (a) 100, (b) 125, and (c) 150 °C using EDOT/N2/SbCl5/N2 = 4/60/0.15/60 s. The film deposited at 150 °C shows a decrease in residual antimony near the growth surface.

FIG. 9.

ToF-SIMs depth profiling of as-deposited PEDOT films deposited at (a) 100, (b) 125, and (c) 150 °C using EDOT/N2/SbCl5/N2 = 4/60/0.15/60 s. The film deposited at 150 °C shows a decrease in residual antimony near the growth surface.

Close modal

4. XPS analysis and postdeposition rinsing

X-ray photoelectron spectroscopy (XPS) was used to verify PEDOT composition in the as-deposited state, and after rinsing in several different solutions, high resolution XPS spectra from O 1s, C 1s, Cl 2p, O 1s/Sb 3d, and S 2p 1s regions are given in Fig. 10. The SbxOy peak in the Sb 3d spectrum peak is ascribed to residual by-products of the SbCl5 that become oxidized upon air exposure. XPS results in Fig. 10 also show the effect of postdeposition rinsing on the film composition. All rinses resulted in the removal of Cl, as indicated by a lack of Cl signal in the survey scans. Rinsing films with sulfuric acid resulted in the appearance of a feature in the S 2p spectrum at 169.6 eV consistent with sulfate. This indicates that the rinse promotes ion exchange, where the Cl is replaced by HSO4. Given the absence of Cl 2p peaks in sulfuric acid rinsed films (Tables I and II), it can be inferred that HSO4 has become the stabilizing anion.

FIG. 10.

XPS results for 120 nm thick PEDOT films deposited at 100 °C using EDOT/N2/SbCl5/N2 = 4/60/0.15/60 s, before and after various solution treatments as indicated: (a) Cl 2p survey scans; (b) Cl 2p high resolution spectrum for the as-deposited film; (c) O 1s high resolution spectra; (d) S 2p high resolution spectra; and (e) C 1s high resolution spectra.

FIG. 10.

XPS results for 120 nm thick PEDOT films deposited at 100 °C using EDOT/N2/SbCl5/N2 = 4/60/0.15/60 s, before and after various solution treatments as indicated: (a) Cl 2p survey scans; (b) Cl 2p high resolution spectrum for the as-deposited film; (c) O 1s high resolution spectra; (d) S 2p high resolution spectra; and (e) C 1s high resolution spectra.

Close modal
TABLE I.

XPS atomic percent of elements in ∼120 nm PEDOT thin films deposited at 100 °C after postdeposition acid treatment. The uncertainty in the values is ∼ ± 0.5 at. %.

At. %
Wash  C 1s  O 1s  S 2p  Sb 3d 3/2  Cl 2p 
No wash  63.0  21.3  8.0  5.6  2.0 
Tartaric acid  58.7  33.0  6.3  2.1  n.d. 
Nitric acid  59.5  31.5  8.1  1.0  n.d. 
Sulfuric acid  56.0  35.2  7.6  1.3  n.d. 
Hydrochloric acid  57.8  33.9  6.9  1.4  n.d. 
At. %
Wash  C 1s  O 1s  S 2p  Sb 3d 3/2  Cl 2p 
No wash  63.0  21.3  8.0  5.6  2.0 
Tartaric acid  58.7  33.0  6.3  2.1  n.d. 
Nitric acid  59.5  31.5  8.1  1.0  n.d. 
Sulfuric acid  56.0  35.2  7.6  1.3  n.d. 
Hydrochloric acid  57.8  33.9  6.9  1.4  n.d. 
TABLE II.

Conductivity values of PEDOT films in the as-deposited state and after rinsing with 0.5 M H2SO4 or methanol. The uncertainties reflect the uncertainty in film thickness for each sample, which is ∼ ± 5%.

Deposition temperature
(°C)
Thickness
(nm)
Conductivity
(S cm−1)
No wash Sulfuric acid Methanol
100  83  4165 ± 200  4152 ± 200  <3.9 ± 1 
125  79  4329 ± 200  4321 ± 200  <8.0 ± 2 
150  51  6668 ± 300  6668 ± 300  <7.2 ± 2 
Deposition temperature
(°C)
Thickness
(nm)
Conductivity
(S cm−1)
No wash Sulfuric acid Methanol
100  83  4165 ± 200  4152 ± 200  <3.9 ± 1 
125  79  4329 ± 200  4321 ± 200  <8.0 ± 2 
150  51  6668 ± 300  6668 ± 300  <7.2 ± 2 

From the XPS results in Fig. 10, we also find that rinsing the PEDOT films in HCl, C4H6O6, or HNO3, led to a shift in the C 1 s and S 2p peaks to lower binding energies. This is also consistent with removal of Cl anions, which reduces the effective charge transfer around the thiophene sulfur and α-carbon. Likewise, treatment with H2SO4 lead to a shift in the C 1s and S 2p peaks to higher binding energies, consistent with the different electronegativity upon the Cl/HSO4 ion exchange.24 In the Sb 3d spectrum, PEDOT treated with HNO3 had the largest decrease in the Sb 3d3/2 and Sb 3d5/2 peak intensities. These results indicate that nitric acid was the most effective rinse for the removal of excess antimony. Table I shows the atomic percent of C, O, S, Sb, and Cl in the PEDOT thin films after acid treatment, as determined by XPS. For all films, a relatively large oxygen content is ascribed to oxidation upon air exposure. In addition, XPS data in Fig. 10 show that washing with MeOH removes chlorine atoms with relatively less change in antimony content, indicating a small amount of antimony (∼1%–2%) remains in the film as a mixed oxychloride.

In-plane conductivity of layer-by-layer oMLD PEDOT films deposited at 100, 125, and 150 °C were measured by four-point probe in the as-deposited state, and results are plotted as a function of thickness in Fig. 11. Results after postdeposition rinse are also shown in Table II. For each deposition temperature, the highest conductivity films were between 50 and 100 nm thick. The highest conductivity measured was nearly 6700 S cm−1, larger than the previously reported value of 5400 S cm−1 for oMLD PEDOT.12 In addition, high conductivity films (>1000 S cm−1) were achieved at deposition temperatures as low as 100 °C, whereas oMLD of PEDOT at 100 °C using MoCl5 as the oxidant showed conductivity <100 S cm−1.12 Similar to the previous oMLD PEDOT report,12 there was a wide variation in conductivity with film thickness with smaller conductivity at larger film thicknesses. This variation may be due to difficulty in securing consistent surface contacts.25 To determine possible process effects on conductivity, various process parameters were tested, including purity of the inert gas purge, and gas pumping speed (i.e., reactor gas residence time), with none showing clear correlation with measured conductivity. In addition, we tested possible effects of precursor quality by replacing the EDOT and SbCl5 precursors with newly purchased materials and found no substantive effect on the conductivity results.

FIG. 11.

Measured conductivity of as-deposited PEDOT films as a function of thickness for deposition temperatures of 100, 125, and 150 °C using EDOT/N2/SbCl5/N2 = 4/60/0.15/60 s. The lines are drawn as a guide for the eye to indicate the general trend that conductivity tends to first increase and then decrease with increasing film thickness.

FIG. 11.

Measured conductivity of as-deposited PEDOT films as a function of thickness for deposition temperatures of 100, 125, and 150 °C using EDOT/N2/SbCl5/N2 = 4/60/0.15/60 s. The lines are drawn as a guide for the eye to indicate the general trend that conductivity tends to first increase and then decrease with increasing film thickness.

Close modal

The data presented above allow reaction schemes for layer-by-layer oMLD PEDOT polymerization and growth using EDOT/SbCl5 to be identified, as shown in Figs. 12 and 13. The polymerization scheme in Fig. 12 follows the widely accepted neutral dimer mechanism for PEDOT polymerization.12 In this scheme, oxidative polymerization proceeds via the formation of EDOT monomer cations, followed by reaction of EDOT cations to form neutral dimers. Monomer and dimer oxidation then enables step-growth polymerization. Further oxidation leads to the oxidative doping of neutral PEDOT chains. The oxidative doping steps can involve the loss of 1 e (formation of polaron) or 2 e (formation of bipolaron), which are associated with a change from benzoid to quinoid structures. As discussed in relation to Fig. 5, the location of C=C asymmetric stretching modes in FTIR analysis indicates the PEDOT formed here is primarily in the quinoid conformation. Because of the carbon double bond on the PEDOT backbone, PEDOT chains in the quinoid structure are rigid and planar which aids orbital overlap and polaron/bipolaron mobility. Within the mechanism presented, SbCl5 is ultimately reduced to SbCl3. Although excluded for clarity in Fig. 12, SbCl5 species can dimerize to Cl4Sb—Cl2—SbCl4, which can chlorinate EDOT/PEDOT species and lead to SbCl3.26 Given the oxidation potential of EDOT monomers, SbCl5 could also react to form SbCl3 as well as SbCl6anions, which could stabilize EDOT cation radicals.26 Although further work is needed to understand the reaction pathways taken by SbCl5 during the oxidative polymerization of PEDOT, our spectroscopic and chemical analysis showing PEDOT in quinoid confirmation as well as SbClx by-products is consistent with the general polymerization and doping steps presented in Fig. 12.

FIG. 12.

Neutral dimer mechanism for the oxidative polymerization of PEDOT using SbCl5 as an oxidant.

FIG. 12.

Neutral dimer mechanism for the oxidative polymerization of PEDOT using SbCl5 as an oxidant.

Close modal
FIG. 13.

Schematic of the proposed surface reaction sequence for PEDOT growth during the EDOT and SbCl5 oMLD exposure half-cycles.

FIG. 13.

Schematic of the proposed surface reaction sequence for PEDOT growth during the EDOT and SbCl5 oMLD exposure half-cycles.

Close modal

For the oxidative MLD sequence used here, the polymerization steps in Fig. 12 proceed sequentially on the surface as shown schematically in Fig. 13. While polymerization requires the EDOT monomer to react with the SbCl5 oxidant, polymerization proceeds on the surface by dosing each reactant separately. During the SbCl5 dose, the SbCl5 adsorbs on the surface and may also diffuse into the subsurface region of the growing film. Subsurface oxidant diffusion is consistent with relatively weak reaction saturation and the observed decrease in growth per cycle with increasing purge time. Exposing the growing PEDOT to SbCl5 vapor also aids in oxidative doping. The purge step removes reduced SbClx by-products with some SbClx and SbCl5 remaining behind. The amount of SbCl5 remaining and, therefore, the amount of PEDOT growth during the subsequent EDOT dose will depend on the net SbCl5 exposure as well as the purge time after SbCl5 dosing. As shown in Figs. 1(b), 1(c), and 3(d), longer SbCl5 dose times tend to increase the thickness per cycle, whereas longer purge times lead to decreased growth. Upon EDOT exposure, EDOT reacts with surface SbCl5 species leading to monomer oxidation and polymerization, as well as the loss of HCl and SbClx by-products. For crystal orientation along the growth surface, the monomers connect and extend in a predominant lateral direction. The net vertical growth then results from tilted crystal orientation and from adsorption of oxidant and monomers on top of the lateral planes. This reaction sequence is consistent with QCM results in Fig. 3 showing a net mass gain during the SbCl5 dose (SbCl5 adsorption) followed by a net mass loss during the EDOT dose/purge cycle where the mass loss from SbClx and HCl desorption offsets the mass gain due to EDOT. This trend was also observed by QCM during oMLD using MoCl5/EDOT.12 The SbCl5 oxidant used in this work is more volatile than MoCl5, so growth is enabled using very short doses (<0.15 s) of SbCl5 compared to ∼5 s dose time used previously for MoCl5.12 

As the deposition temperature increases from 100 to 150 °C, QCM results show a larger net mass loss after the EDOT dose step, consistent with a larger extent of polymerization reaction and faster desorption of the HCl and SbClx reaction by-products. This is supported by the ToF-SIMs depth profile data in Fig. 9 which showed a smaller amount of antimony relative to Cl at the surface of the film deposited at higher temperatures. For the as-deposited films, composition analysis from XPS (Table III) showed Sb:Cl atomic ratios ranging from <0.1 to >4, indicating a small concentration of various SbClx compounds present in the films. We note that the trend in the Sb/Cl ratio in the films as a function of temperature determined by XPS in Table III is somewhat different from that determined from SIMS in Fig. 9 and is likely due to difficulty in quantifying the chlorine content.

TABLE III.

XPS data for as-deposited PEDOT thin films deposited at 100, 125, and 150 °C using EDOT/N2/SbCl5/N2 = 4/60/0.15/60 s. For each condition, films with two different thicknesses were analyzed. The uncertainty in the composition values is ∼ ± 0.5 at. %.

Deposition temperature
(°C)
Thickness
(nm)
At. %
C 1s O 1s S 2p Sb 3d3/2 Cl 2p Sb:Cl
100  100  63.0  21.3  8.0  5.6  2.0  2.80 
210  52.4  20.6  7.4  3.1  16.5  0.19 
125  80  63.1  21.4  9.8  4.6  1.1  4.14 
194  54.7  27.4  6.6  3.3  6.2  0.53 
150  60  53.9  18.6  7.4  5.1  2.5  2.04 
112  62.1  26.9  8.8  0.1  1.8  0.05 
Deposition temperature
(°C)
Thickness
(nm)
At. %
C 1s O 1s S 2p Sb 3d3/2 Cl 2p Sb:Cl
100  100  63.0  21.3  8.0  5.6  2.0  2.80 
210  52.4  20.6  7.4  3.1  16.5  0.19 
125  80  63.1  21.4  9.8  4.6  1.1  4.14 
194  54.7  27.4  6.6  3.3  6.2  0.53 
150  60  53.9  18.6  7.4  5.1  2.5  2.04 
112  62.1  26.9  8.8  0.1  1.8  0.05 

The conductivity results in Fig. 11 demonstrate that the oMLD sequence using SbCl5 and EDOT can produce conductive PEDOT films. Common spin coated PEDOT:PSS films typically have conductivity near 100 S/cm,27 while films formed from solution sheared PEDOT:PSS have reached conductivities of up to 4600 S/cm.28 Most notable among previous studies, oxidative CVD of PEDOT using FeCl3 as the oxidant have shown conductivity up to 6259 S/cm for films deposited at 300 °C.11 This high conductivity was attributed to favorable film morphology with low thickness and increased deposition temperatures. PEDOT conductivity is also expected to depend on crystallite orientation, which in turn will depend on the substrate, deposition temperature, and thickness. Examining the XRD spectra in Fig. 8, for the thicker films (>100 nm), the 0k0 peaks tend to become sharper with increasing the growth temperature between 100 to 125 and 150 °C, suggesting a thermally driven transition in crystallite formation and orientation. Moreover, we observe 0k0 reflections for all films but are unable to observe h00 peaks. This suggests that confinement effects, which can result in preferred crystallite formation and orientation in thin films, do not contribute substantially to film crystallinity.

Compared to other oxidants, the crystallization observed here using SbCl5 may correlate with more rapid by-product desorption which could improve oligomer surface mobility to promote π—π stacking chain orientation. The reduction of SbCl5 to SbCl3 may also result in higher levels of polymerization and oxidation per deposition cycle since two cationic species may be formed per oxidant molecule. This may result in higher coverage of PEDOT in conductive, rigid, quinoid conformation on the surface and enhanced interchain stacking.

This work reports a facile layer-by-layer oMLD process for the deposition of PEDOT using SbCl5 as an oxidant. Compared to previous oMLD processes using MoCl5, the SbCl5 oxidant allowed for well-controlled film growth with shorter dose times and lower temperatures. The effect of precursor dose time, purge time, and process temperatures on the film growth rate were identified as well as the effects of process temperature on conductivity and residual antimony content. Under the conditions used here, extended SbCl5 dose times led to increased growth/cycle, whereas growth/cycle decreased with increasing purge time, consistent with growth rate dependent on physisorbed oxidant, likely on and within the near-surface region.

Using SbCl5 as an oxidant, semicrystalline films with high conductivity (>1000 S/cm) were obtained at moderate deposition temperatures (100 °C), although further work is needed to resolve conductivity variations across deposition condition replicates. Additional film properties, such as spatial homogeneity, i.e., no ionic accumulation at the substrate–film interface, may make these films advantageous over in situ polymerized solution phase PEDOT for electronic applications which require uniform PEDOT thin films. Overall, this work, using the facile SbCl5 oxidant, helps expand the understanding of oxidative layer-by-layer growth of PEDOT and helps identify opportunities for continued research on PEDOT nucleation and growth to achieve well-controlled film properties.

This work was supported by the National Science Foundation (NSF), as part of the Center for Dielectrics and Piezoelectrics under Grant Nos. IIP-1841453 and IIP-1841466. Support is also acknowledged from NSF under Award No. 1704151 by the Semiconductor Research Corporation, Task No. 2873.001.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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