An out of the box vision over oxidative chemical vapor deposition of PEDOT involving sublimed iron trichloride

Highlights

• PEDOT film formation is monitored in real time using a QCM microbalance.

• Discontinuous FeCl₃ evaporation is observed in a conventional sublimation hardware.

• Uniform PEDOT thicknesses and conductivities are observed over large-scale.

• It is confirmed that formation of PEDOT by oCVD involves no gas phase mechanisms.

• Detailed links between deposition conditions and film characteristics are analyzed.

Abstract

Oxidative chemical vapor deposition (oCVD) is an efficient technique to produce highly conductive films of Poly (3,4-ethylenedioxythiophene) (PEDOT). Despite numerous studies on the oCVD of PEDOT films, there is limited information on the stability of the sublimation of solid oxidants and on their impact on the polymerization reactions. In this work, we use an in situ Quartz Crystal Microbalance to monitor film formation over time. Through a series of deposition experiments between 20 °C and 100 °C and for FeCl₃/EDOT molar gas ratios between 17.3 and 75.3, we analyze in detail the correlations between process parameters and film morphology, composition, surface topography and electrical conductivity on 10 cm silicon wafers. By using multiple substrates at different positions into the reactor, we demonstrate that the formation of PEDOT occurs uniformly through purely surface reactions, following step growth polymerization principles. These results pave the way towards highly conductive oCVD PEDOT films processed from convenient solid oxidants.

Introduction

Poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the most significant conductive polymers used in the growing field of organic devices. It is widely used in organic solar cells [1], organic light-emitting diodes (OLEDs) [2], and supercapacitors [3], thanks to its mechanical stability, optical transparency and high electrical conductivity in comparison to other conductive polymers [4]. Its high electrical conductivity (up to 8797 S/cm for single-crystal nanowires [5]) comes from a conjugated bond structure that permits π-orbital overlap along with the alternating double- and single-bonds in the polymer backbone [6].

Since 1990, the main route of PEDOT production has been synthesizing in liquid solution through a series of oxidation and deprotonation steps [[7], [8], [9]]. Vapor phase processes, mainly vapor phase polymerization (VPP) [10] and oxidative chemical vapor deposition (oCVD) [9] have been developed more recently to provide conformal coatings on complex porous substrates and to avoid the substrate-solvent incompatibility in liquid deposition processes. While the VPP process still involves a liquid step, the oCVD is a one-step purely gas phase process where the oxidant serves both to promote monomer polymerization and to subsequently oxidize the neutral polymer to form the conductive, doped polymer. Another advantage of the oCVD is better control over film thickness by varying the deposition time. In the VPP, the polymerization stops when there is no oxidant even if EDOT is still present [11]. By sending oxidant in a controlled manner, oCVD can control the film properties with more precision. Since the introduction of oCVD PEDOT by Gleason and coworkers [9], several solid oxidants have been used to produce films of PEDOT from EDOT monomer. Among them, iron trichloride (FeCl₃) is the most studied one [9,[12], [13], [14]], because of its availability and relatively low toxicity [15,16]. Solid oxidants are commonly placed on a crucible in the reactor and are sublimated at a temperature between 170 °C and 350 °C. Despite the wealth of the literature on oCVD PEDOT via FeCl₃, the sublimation rate and stability over time have never been reported [9,[12], [13], [14],[17], [18], [19], [20], [21], [22], [23], [24]]. These two parameters are important since the sublimation rate of the oxidant allows controlling the gas phase composition and, subsequently, the film's characteristics. The unreacted excess oxidant is often found in as processed PEDOT films, requiring an additional step of post deposition rinsing to improve the electrical conductivity [25]. On the other hand, the control of the stability of the sublimation rate over time is a prerequisite for process implementation. In order to circumvent such sublimation drawbacks [26], solid oxidants have been replaced by more conveniently volatilized liquid ones, such as bromine (Br₂) [19] and later on sulfuric acid (H₂SO₄) [27], molybdenum(V) chloride (MoCl₅) [28], antimony pentachloride (SbCl₅) [29] and vanadium oxytrichloride (VOCl₃) [[29], [30], [31]].

Despite the advantages of liquid oxidants, there are other issues related to their use, such as safety, availability, oxidation efficiency, and subsequent equipement corrosion [32]. Consequently, the oCVD of PEDOT from EDOT and solid oxidants, especially FeCl₃, remains a promising process, provided sublimation issues are settled [15,16]. This condition comes together with the need to get a better insight into the gas-phase or surface reactions involved between the oxidant and the monomer. Indeed, deposition parameters such as the FeCl₃/EDOT ratio in the gas phase are not always detailed in the literature, and except for the substrate temperature (T_sub), the correlations between the deposition conditions and the film characteristics and properties are not clearly established [9,16,22,23,29,32,33]. As a result, the potential of scaling up the oCVD of PEDOT remains to be more deeply analyzed in terms of film uniformity over large areas. The first effort toward large-scale PEDOT film synthesis has been made by Kovacik et al. who developed a roll-to-roll process [23]. They obtained deposition of PEDOT on 21 × 33 cm pre-cleaned glass slides with a thickness deviation of 5.5 % and a conductivity variation of 10 % after a post deposition rinsing in methanol. However, they did not correlate process parameters and film uniformity. Except for one study about PEDOT synthesis by oxidative Molecular Layer Deposition [28], no in situ measurements of the deposited mass were published, although they could bring new insights about stability and the gas phase and surface mechanisms involved.

The objective of the present work is to extend the knowledge on the oCVD process yielding PEDOT films from EDOT and FeCl₃ by getting insight into the involved mechanisms. We in-situ monitor the deposition of PEDOT using a Quartz Crystal Microbalance (QCM). We analyze in detail the film uniformity over a large scale and establish correlations between key process parameters like precursors ratio and substrate temperature and film characteristics, namely thickness, roughness, morphology, chemical composition, and electrical conductivity, both in as-processed and after standard rinsing with methanol.