Emission ellipsometry as a tool for optimizing the electrosynthesis of conjugated polymers thin films Eralci M. Therézio a, ⁎, Flavio Franchello c , Ivan F.L. Dias c , Edson Laureto c , Maurício Foschini a , Otávio L. Bottecchia b , Henrique de Santana d , José L. Duarte c , Alexandre Marletta a a Instituto de Física, Universidade Federal de Uberlândia, CP 593, 38400-902 Uberlândia, MG, Brazil b Instituto de Química, Universidade Federal de Uberlândia, CP 593, 38400-902 Uberlândia, MG, Brazil c Departamento de Física, Universidade Estadual de Londrina, 86051-990 Londrina, PR, Brazil d Departamento de Química, Universidade Estadual de Londrina, 86051-990 Londrina, PR, Brazil abstract article info Article history: Received 24 January 2012 Received in revised form 24 November 2012 Accepted 26 November 2012 Available online 7 December 2012 Keywords: poly[3-octylthiophene] Electrochemical synthesis Emission ellipsometry Stokes theory Emission ellipsometry technique was used as a tool to optimize the electrochemical synthesis of luminescent poly 24 [3-alkylthiophenes] conjugated polymers. This methodology was applied to poly[3-octylthiophene] (P3OT) films. P3OT films were synthesized by the chronocoulometry technique, using tetraethylammonium tetrafluoroborate and lithium perchlorate, as supporting electrolytes to control empirically the polymer/polymer interaction. After excitation with polarized light, a partially polarized emission from these films was observed. The polarization degree of the light emitted by the polymer was calculated by Stokes theory. Emission of polar- ized light is attributed to the limitation of energy transfer and/or diffusion processes of the excited carriers. These energy transfer and/or diffusion processes are dependent on the concentration and on the chemical nature of the electrolyte in the polymer film. The best condition for synthesis of P3OT films is correlated with the maximum of the polarization degree. The proposed methodology can be applied to optimize the electrochemical synthesis of luminescent conjugated polymers. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Poly[3-alkylthiophenes] (P3ATs) have been studied as a promising active layer in organic electronic devices, mainly in solar cells [1]. Solvatochromism [2], electrochromism [3,4], thermochromism [5], elec- troluminescence [6], and photoluminescence [2,4–8] properties were also observed for these conjugate polymers. In solid state films, the emis- sion efficiency, among other factors such as energy transfer processes or structural defects [8,9], depends on the chemical synthesis route, film processing, molecular packing, supra-molecular ordering and the poly- mer/polymer interaction. The solvent used [10], the regioregularity [11–14] or chemical doping [15] affects directly the P3ATs electronic structure (effective conjugation degree). A small bandgap and an en- hancement in emission efficiency have been found in polymeric films processed with regioregular P3ATs [13,16,17]. Despite the progress in the chemical synthesis of regioregular P3ATs [16], the regioregularity does not ensure the supra-macromolecular order of polymeric films processed by using common techniques, such as spin-coating, casting or electrochemistry. Moreover, polymeric blends have been utilized to produce opto-electronic devices, such as organic field effect transistors or light emission diodes [18,19]. Thus, the doping process and the poly- mer/polymer interaction are the most important physical parameters to enhance the electric or optical characteristics of the devices. In general, P3ATs are obtained by chemical synthesis using 3-alkylthiophene monomer catalyzed by FeCl 3 [2,4,5,7]. High regioregularity polymeric chains (~98% HT coupling) can be obtained using Ni(dppp)Cl 2 as a catalyst [12,14]. The advantage of the chemical synthesis is its ability to remove the dopant to obtain a pristine P3AT poly- mer. In the electrochemical synthesis process, however, the electrolyte used in P3ATs polymerization and film growth process remains in the film as a dopant. The spectroscopic features of polythiophenes depend strongly on film thickness, mainly because the doping level increases dur- ing the film growth [20]. It has been established that the ideal supporting electrolyte (SE) concentration for electrochemical techniques is about 0.1 mol.L −1 [21–24]. This paper deals with a quantitative method to de- termine the best electrochemical synthesis conditions for luminescent conjugated polymers. poly[3-octylthiophene] (P3OT) films were synthe- sized using the chronocoulometry technique to verify the proposed methodology. Tetraethylammonium tetrafluoroborate ((C 2 H 5 ) 4 NBF 4 ) and lithium perchlorate (LiClO 4 ) were used as supporting electrolytes. Emission ellipsometry measurements were carried out to quantify the po- larization degree (P) of the emitted light for P3OT films [25]. Considering the energy transfer processes and/or diffusion of photo-excited carriers and the polymer/polymer interaction, we correlated the parameter P with the electrochemical processing conditions. Thin Solid Films 527 (2013) 255–260 ⁎ Corresponding author at: Instituto de Física, Universidade Federal de Uberlândia, Av. João Naves de Ávila 2121, CP 593, 38400-902 Uberlândia, MG, Brazil. Tel.: + 55 34 3239 4190; fax: +55 34 3239 4106. E-mail addresses: therezio@doutorado.ufu.br, therezio@gmail.com (E.M. Therézio). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.11.093 Contents lists available at SciVerse ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf