Delivered by Ingenta to: Nanyang Technological University IP: 5.62.157.79 On: Tue, 14 Jun 2016 05:17:57 Copyright: American Scientific Publishers RESEARCH ARTICLE Copyright © 2013 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 13, 4045–4051, 2013 A Simple Chemical Vapor Deposition Method for the Production of Highly Conductive Polymer Poly(3,4-ethylenedioxythiophene) Dalmau Reig-i-Plessis ∗ , Eugen Panaitescu, Akshai Baskaran, and Latika Menon Department of Physics, Northeastern University, Boston, MA 02199, USA Conductive thin films of Poly(3,4-ethylenedioxythiophene) (PEDOT) have been successfully deposited on a variety of substrates by a simple chemical vapor deposition (CVD) method starting from the liquid monomer (EDOT) and using FeCl 3 as a catalyst. Resistivity measurements indicated very good conductivity of the material, comparable with other previously reported values for PEDOT deposited by CVD. Further cyclic voltammetry measurements indicated a value of around -4.7 eV for the HOMO level of PEDOT in the deposited films, in agreement with other reported values. This value is within the bandgap of most semiconductors, and together with the relative low internal resistance makes our material an ideal candidate as a solid-state hole transport material for dye sensitized solar cells. Keywords: PEDOT, Hole Transport Material, HOMO, Vapor-Phase Polymerized, DSSC. 1. INTRODUCTION Conjugated polymers have sparked much interest since their discovery in 1970 because they are able to com- bine the low cost and physical properties of polymers with the electrical properties of metals and semiconductors. 1 Poly(3,4-ethylenedioxythiophene) (PEDOT) is a conju- gated polymer which has the desirable properties of being optically transparent and highly stable, 2 making it an ideal material for low cost LEDs, 3 organic solar cells, 4–6 and dye sensitized solar cells 7–10 and LCDs 11 where PEDOT is used as a transparent electrode or a hole acceptor. About two decades ago applications for PEDOT boomed with the synthesis of a compound of PEDOT and poly(styrenesulfonate) (PSS) which forms a stable water dispersion. 12 The dispersion allows for simple spin casting in order to form uniform films, a much simpler method than the previously used electropolymerization of EDOT monomer which requires a conducting substrate. Unfortu- nately with PEDOT:PSS films the conductivity is limited to 1 S/cm in pristine films, up to 80 S/cm for glyc- erol modified films 13 and past 1000 S/cm with a PSS removing post treatment. 5 This is because spin casted films have regions of conducting PEDOT within a matrix of non-conducting PSS. 13 14 Electropolymerization on the other hand 15 can achieve conductivities up to 400 S/cm. ∗ Author to whom correspondence should be addressed. Alternatively, PEDOT films have recently been manu- factured by way of Vapor Phase Polymerization (VPP- PEDOT) 16 using FeCl 3 . This can be achieved either by a wet FeCl 3 pretreatment followed by the exposure to EDOT vapors 17 (the resultant films had conductivities of up to 1000 S/cm) or by the simultaneous CVD of EDOT and iron chloride 16 with careful control of substrate tem- perature and gas flow (the resulting film conductivities ranging from 40 S/cm, 18 100 S/cm, 16 up to 300 S/cm 19 ). This was a very important advancement since the CVD method allows for excellent uniformity even on very rough substrates 19 as well as much higher conductivity than the PEDOT:PSS methods. A key application of such a material was demonstrated in solid-state Dye Sensitized Solar Cells (DSSC) 7–10 where the two main concerns are pore filling of nanostructures as well as improving charge transport and hole injection of the hole conducting layer. 20 The energy level of the hole conduction band also plays an important role in hole injection since a higher energy level will allow faster hole injection. Previous studies 4 21 have shown a higher energy for VPP-PEDOT (-4.1 eV and -4.3 eV) for the wet FeCl 3 pretreatment method than for PEDOT:PSS (-4.7 eV and -5.1 eV) using cyclic voltammetry 21 and ultra-violet photoemission spectroscopy 4 respectively. In this study we demonstrate a new, simpler method for the CVD fabrication of PEDOT that requires no flow control for any of the reactants, but only for the carrier gas (argon). The reaction may be performed in a regular J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 6 1533-4880/2013/13/4045/007 doi:10.1166/jnn.2013.7432 4045