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Received Nov 23, 2017; Accepted Feb 8, 2018
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1. Introduction
Electrochromic devices have gained widespread attention over the past decade due to their robust design, ease of manufacturing, and energy saving low power operation mechanisms [1]. Likewise, traditional materials like transition metal oxides and phthalocyanines of rare earth metals like actinides and lanthanides [2] have been largely exploited for exhibiting electrochromic properties in devices. At present, indium tin oxide (ITO) is a widely used transparent conductive electrode in electronics and organic photovoltaics (OPVs) [3–5]. The cost of manufacturing of indium as a rare earth metal rises invariably with depleting resources. Moreover, deposition of ITO on flexible substrates is challenging as it increases its defect densities due to ambient temperature deposition methods and greatly reduces its carrier concentration [6]. While the concept of electrochromic switching has been reported as early as the 1970s [7], nowadays research is more focused on developing organic polymers [8–10] which exhibit similar functionality and properties when compared to their metallic counterparts while being more viable, sustainable, and environmentally friendly [11]. PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) is an organic polymer from the family of thiophenes that has shown very promising results owing to its enhanced conductivity, higher environmental and electrochemical stability, and its conformity as a water-processable polymer [12] leading to extensive research in the field of flexible electronics and as an alternative to conventional metal oxide based electrochromic systems [13, 14]. Electrochromic contrast is an important parameter in the performance evaluation of such systems. PEDOT:PSS provides high optical contrasts between its redox states [12] enabling this monochromatic transition between colored and bleached states useful in developing absorption and transmission type devices like smart windows and visors.
Moreover, graphene has superior mechanical [15], optical [16], and electrical properties enabling its use in transparent electronic [17, 18] and photonic devices [19, 20]. These properties of graphene have made it a great candidate as a transparent conductive electrode. Graphene synthesized via chemical vapor deposition (CVD) on transition metals results in formation of large scale films with minimized structural defects, enabling its applications on an industrial...