Dye-sensitized solar cells (DSSCs) offer cost-effective and versatile energy generation. Due to their exceptional power conversion efficiency (ηPCE) under artificial light and indirect daylight, they are promising candidates for indoor photovoltaic applications. The devices' counter electrode (CE) determines the overall ηPCE. It should possess several critical qualities, including electrical conductivity and electrocatalytic activity, affordability, high electrochemically active surface area, corrosion resistance, and multi-process deposition [1]. Platinum (Pt) is the most used material for CEs in liquid-junction DSSCs due to its exceptional electrical conductivity and catalytic activity; ηPCE above 12% has been reported for the devices with Pt CE. Nevertheless, Pt presents several challenges, including its high cost, limited stability over extended periods of device operation, detachment of the nanoparticles, and migration to the photoanode, provoking recombination on the photoanode and photocurrent degradation [2].

Carbon-based materials are emerging as strong candidates for CE materials in DSSCs due to their low cost, abundance, extensive surface area, electrical conductivity, corrosion resistance, and reactivity for redox mediator reduction [3]. The most common way to implement carbon CE in the DSSC is to deposit it onto a conductive substrate, such as fluorine-doped tin oxide (FTO) glass, and encapsulate it with an electrolyte gap between the working electrode (WE) or by placing the carbon on the TiO2 using an insulating layer to prevent physical contact with the WE. However, they often suffer from poor adhesion, leading to electrode degradation. Alternatively, pre-made materials in sheet form carbon papers can easily be processed as CE with minimal impact on the DSSCs structure. The carbon paper porous structure enhances the surface area and enables electrolyte loading; excellent electrical conductivity facilitates swift electron transfer within the electrode [4]. The dispute with pre-made carbon sheets is incorporating them into the DSSC structure with guaranteed electrical contact with the FTO layer or on top of the sensitized TiO2 layer without causing a short-circuit.

In the present work, we studied the I−/I3− mediated liquid-junction DSSC assembly by pressing a carbon paper composite against the TiO2 WE and evaluated three assembly combinations: a single macro-porous carbon fiber (Carb); a bilayer of the Carb structure pressed against TiO2, and, on the CE-FTO side, a micro-porous carbon-based with PTFE layer (Carb/PTFE); and a compact ultrathin TiO2 blocking layer (BL) was added to the macro-porous structure of the Carb/PTFE bilayer by spray pyrolysis (TiO2BL/Carb/PTFE). Current-voltage performance from carbon-based DSSCs and conventional devices with Pt CE (Ref) were measured and revealed that the current density and fill factor (FF) were significantly reduced in DSSCs with Carb (12 mA·cm-2 and 0.48). This is likely due to recombination losses caused by poor electrical contact between the carbon sheet and the FTO and an insufficient rate of I3- reduction, resulting in electron and hole recombination [5]. This effect was lower in Carb/PTFE devices, where current density and FF increased significantly (15 mA·cm-2 and 0.58). The difference is explained by the porosity of each layer and their respective orientation; the micro-porous structure provides good electrical contact to the FTO, while the macro-porous is better for redox mediator infiltration when pressed against TiO2. However, the FF remained very low compared to Ref (0.73), most likely caused by a short-circuit from the charge transfer competition between WE and CE. This issue was mitigated using the TiO2BL/Carb/PTFE, rendering a superior FF (0.62) and a competitive ηPCE of 6.9 %, corresponding to 97% obtained with Pt-CE. With ongoing research, carbon papers could significantly advance the commercial prospects of DSSCs for flexible devices and module manufacturing. Moreover, its integration aligns with sustainability goals and reduces the carbon footprint in clean energy applications.