Operational stability of organic solar cells (OSCs) is a key issue for the commercialization. In order to enhance the stability of OSCs, various strategies are being used. In this study, pyrroles and isoindigo monomers with tert-butoxycarbonyl side chains were synthesized and used to prepare a D–A-type polymer, P(TBPyTBIID), via Stille coupling. The obtained polymer dissolved readily in organic solvents, such as chloroform. However, the formation of interchain hydrogen bonds after side-chain cleavage via thermal treatment rendered the resulting polymer insoluble in organic solvents. In addition, strong π· · ·π stacking due to face-on orientation enabled dense packing of the structure, which led to an increase in resistances to organic solvents, acid, and ultraviolet (UV) light. P(TBPyTBIID) was introduced as a UV and external protective layer, along with silver nanoparticles, at the back surface of the glass substrate. The lightsoaking stability was enhanced without significantly reducing the efficiencies of the organic solar cells. This work indicates that introducing an UV blocking layer can be an effective strategy to increase the stability of organic solar cells.