ABSTRACT Sustainable fabrication of photovoltaic devices remains challenging due to the widespread use of non‐renewable substrates and organic solvents. In this work, we investigate biodegradable cellulose‐based substrates for flexible counter electrodes in aqueous dye‐sensitized solar cells (DSSCs). Ethyl cellulose films prepared from different solvents were characterized in terms of rheology, mechanical flexibility, and biodegradability. The optimized formulation was coated with Ti/Au/Pt catalytic three‐layer and integrated into aqueous DSSC devices. The resulting solar cells achieved a maximum power conversion efficiency of 1.06%, comparable to reference devices fabricated on glass substrates using the same architecture. These results demonstrate that biodegradable cellulose substrates can support functional photovoltaic devices, providing a promising pathway toward more sustainable optoelectronic technologies.