Polymer solar cells (PSCs) featuring nonfullerene acceptors have attracted a great deal of attention in the research community.Exposure of these active layers to sunlight during operation demands excellent thermal stability.Optimizing PSCs often requires a step of thermal annealing,while long-term annealing often places a constraint on their stable operation at elevated temperature and results in a progressive loss of photovoltaic performance.Hence,it is crucial to understand and control the fundamental affecting factors on the thermal stability of nonfullerene PSCs.This review highlights the relationships between microstructure and thermal stability of PSCs and summarizes promising strategies to push this technology toward the level of practical application.Finally,we present the current challenges and prospective of this research area,intending to advance the practical applications of stable PSCs based on nonfullerene acceptors.