The　past　decade　has　witnessed　the　prosperity　of　organic　photovoltaic　cells　(OPVs).　In　addition　to　efficiency　and　cost,　stability　is　another　challenge　that　OPVs　face　in　their　practical　application.　Continuous　thermal　stress　often　accelerates　the　crystallization　and　phase　separation　of　active　layers,　leading　to　a　decreased　photovoltaic　performance.　Developing　simple　and　effective　strategies　to　prolong　the　lifetime　of　OPVs　under　heating　has　attracted　increasing　attention.　In　this　work,　an　unexplored　random　copolymer　named　PY　was　incorporated　as　a　high-temperature-resistant　aid　to　the　emerging　nonfullerene　OPVs　to　control　and　stabilize　the　film　morphology.　Using　the　well-known　low-cost　P3HT:O-IDTBR　as　the　model　photoactive　layer,　OPVs　with　small　weight　amounts　(less　than　30%)　of　PY　achieved　comparable　photovoltaic　efficiencies.　Notably,　the　10%-ternary　blend　maintained　over　70%　of　its　initial　efficiency　after　annealing　at　150　°C　for　8　days,　while　the　efficiency　of　the　binary　system　dropped　rapidly.　Notably,　this　approach　was　also　effective　for　the　high-efficiency　PM6:BTP-eC9　blend.　Combined　with　the　morphology　and　crystallization　characterization　techniques,　the　relationship　between　microstructure　and　thermal　stability　was　constructed.　This　work　thus　demonstrated　an　effective　and　broadly　applicable　strategy　to　improve　the　thermal　stability　of　OPVs　without　sacrificing　the　initial　efficiency.　©　2022　American　Chemical　Society.　All　rights　reserved.