# On the Use of PEDOT as a Catalytic Counter Electrode Material in Dye-Sensitized Solar Cells
## 1 Introduction  
Dye-sensitized solar cells (DSSCs), which were developed by [PERSON] and [PERSON] in the early 1990s [1], represent a regenerative photoelectrochemical device type characterized by unique properties, such as transparency, low production cost, capability to harvest the diffuse light, wide choice of color palette, building integration capabilities in both indoor and outdoor conditions [1; 2; 3; 4; 5]. DSSCs are considered among the new-generation photovoltaic technologies which aim at achieving power conversion efficiency (PCE) comparable with that of existing technologies, but with low cost per meter square, tunability in color and shape, transparency, flexibility, and roll-to-roll production. In particular, organic and hybrid solar cells are considered as an \"emerging photovoltaic technologies\" class. Dye-sensitized solar cells (DSSCs) have gathered much attention due to their potential, such as low cost, feasibility, building integration, and scalability [6]. Other significant emerging technologies in this field are organic/polymer solar cells (OSC) and metal organic perovskite solar cells (PSC) [7; 8]. Although PSCs are rapidly overcoming the efficiency record for hybrid photovoltaic technologies (up to 25%), their long-term stability under real working conditions remains an issue [9]. In particular, PSCs suffer from severe instability upon exposure to atmospheric O2 and humidity, light, high temperature, and an intrinsic instability upon prolonged working conditions as a result of ion migration and interface degradation [10; 11; 12]. The optimization of the whole DSSC has been achieved over the last 30 years through dedicated research oriented towards each part of the device, i.e., dye sensitizers, redox mediators, and catalytic counter electrodes (CEs) [2]. In particular, CEs play the important role of regenerating the reduced form of the redox electrolyte, maintaining a steady-state concentration of reduced mediator, which is essential to intercept the photo-oxidized dye atthe photoanode, preventing charge recombination. Since the early developments of DSSC in the 1990s, the iodine/iodide (i.e., \(\text{I}^{-}/\text{I}_{3}^{-}\)) couple was the most universally used mediator to conduct charge across the liquid electrolyte. Platinum coated counter electrodes ensured excellent electrocatalysis toward the iodine/iodide couple, but at the beginning of the 2000s, researchers moved to the search for alternative, lower cost, and more abundant electrode materials [3; 4]. One of the causes of limited DSSC stability was indeed identified in the reaction of the electrolyte with the platinum counter electrode (CE), which can lead to the dissolution of Pt from the substrate [13]. From this perspective, low-cost conductive polymers, such as poly(3,4-ethylenedioxy-thiophene) (PEDOT), which also play a prominent role as hole transporting layers in OSC and PSC architectures, have emerged to replace Pt-coated CEs in DSSCs, thanks to its high room-temperature conductivity and remarkable stability [5; 14; 15; 16]. It was later found that PEDOT electrodes displayed excellent electrocatalytic behavior not only with respect to the iodide/iodine couple but also toward newer high-efficiency redox mediators as well [17; 18; 19]. So far, among other conductive polymers, including polyaniline (PANI) and polypropylene (PPy), PEDOT is the most commonly used CE material. It was reported that PEDOT outperformed both PANI and PPy when used as a counter electrode material [20]. In addition, PANI is not an ideal CE material because it suffers from instability, self-oxidation, and carcinogenic properties [21]. Still, the deposition of PANI tends to give non-uniform films on TCO with insufficient conductivity and low surface area [22]. On the other hand, the main drawback with the PPy counter electrode is its higher charge transfer resistance Rct and the lower conductivity of the films [23].