Thermal Stability of Non-Fullerene Polymer Solar Cells: The Role of Metal Oxide Diffusion and Materials Design
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This study investigates the thermal stability of non-fullerene acceptor-based polymer solar cells (NFA-PSCs). Despite high morphological stabilities of the active layers, our results show that NFA-PSCs may not necessarily have high thermal stabilities. Using two NFA-PSCs made of well-known polymer donors and non-fullerene acceptors such as PTB7-Th, PBDB-T-2Cl, and ITIC-4F, we found that while the morphologies of the PTB7-Th:ITIC-4F and PBDB-T-2Cl:ITIC-4F blends were highly stable at 110 ℃, the corresponding NFA-PSCs degraded severely under the same temperature. The efficiency of all NFA-PSCs dropped by more than 30% within an hour, primarily due to a significant decrease in short-circuit current density and fill factor. Further analysis using time-of-flight secondary ion mass spectrometry revealed that degradation was caused by the penetration of metal oxide molecules into the photoactive layer, which acted as charge recombination centers. We also provide suggestions for preventing this undesirable penetration through materials design.