Surface smoothness and conductivity control of vapor-phase polymerized poly(3,4-ethylenedioxythiophene) thin coating for flexible optoelectronic applications Thuy Le Truong a , Dong-Ouk Kim a , Youngkwan Lee b , Tae-Woo Lee c , Jong Jin Park c , Lyongsun Pu c , Jae-Do Nam a, ⁎ a Department of Polymer Science and Engineering, SKKU Advanced Institute of Nanotechnology (SAINT), South Korea b Department of Chemical Engineering, Sungkyunkwan University, 300 Chunchun-dong, Jangan-gu, Suwon, 440-746, South Korea c Samsung Advanced Institute of Technology, Mt. 14-1, Nongseo-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-712, South Korea Received 24 October 2006; received in revised form 14 March 2007; accepted 25 October 2007 Available online 11 December 2007 Abstract The surface morphology of poly(3,4-ethylenedioxythiophene) (PEDOT) was investigated in the vapor-phase polymerization of the thiophene monomer on a flexible polyethyleneterphthalate (PET) substrate film. The PET surface was modified with ethylene diamine maintaining the surface roughness within 2 nm to create amine and amide groups for the enhanced hydrophilic interaction with Fe(III)-tosylate (Fe(OTs) 3 ) and for the desirable hydrogen bonding with thiophene monomer as well as PEDOT. Polymerization rate was reduced by incorporating pyridine as a reaction retardant to control the surface roughness and conductivity of PEDOT thin films. The optimal conditions of pyridine and glycerol were found at a pyridine/Fe(OTs) 3 molar ratio of 0.5 and a glycerol concentration of 4–5 wt.%, respectively, providing the conductivity up to 500 S/cm and the surface roughness b 2 nm. © 2007 Elsevier B.V. All rights reserved. Keywords: Poly(3,4-ethylenedioxythiophene); Vapor-phase polymerization; Fe(III)-tosylate polyethyleneterphthalate 1. Introduction There have been many studies on poly(3,4-ethylenediox- ythiophene) (PEDOT) over recent years on account of its many advantageous properties such as high conductivity, transpar- ency and stability [1–3]. This makes PEDOT very attractive for applications including electrochromic windows [4], organic electrodes for photovoltaics [5,6] and hole transport layers of organic/polymer light emitting device [7–11]. In most of those optoelectronic applications as buffer or electrode layers, the interface with the PEDOT coating layer plays an important role in determining the operating characteristics, quantum efficiency and stability [12,13]. In general, the surface roughness of the PEDOT thin films is required not to exceed several nanometers (b 10 nm), and a uniform composition is usually required. Therefore, the main issues in most electronic device applica- tions are not only the electrical conductivity but also the film surface morphology such as film thickness, surface roughness, uniformity, etc. Oxidized PEDOT can be produced in several forms using different polymerization techniques. Solution processing is most commonly be used in synthesizing PEDOT in the form of spin- coating, solvent-casting, or ink-jet printing. However, the PEDOT system is relatively insoluble in most solvents, which makes it necessary to derivatize it with soluble side chains or dope the polymer with stabilizing polyelectrolytes [14]. One of the most widely used systems is an aqueous dispersion of poly(3,4- ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT–PSS), Baytron P, which is a stable polymer system with a high transparency up to 80% [15,16]. However, the PEDOT–PSS film exhibits a relatively low electrical conductivity, ∼ 10 S/cm [15,16], which does not meet the high conductivity requirements in most applications. In addition, according to scanning-tunneling Thin Solid Films 516 (2008) 6020 – 6027 www.elsevier.com/locate/tsf ⁎ Corresponding author. E-mail address: jdnam@skku.edu (J.-D. Nam). Available online at www.sciencedirect.com 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.10.114