Unexpected MoO₃/Al Interfacial Reaction Lowering the Performance of Organic Solar Cells upon Thermal Annealing and Methods for Suppression

Understanding the degradation mechanism and improving the thermal stability of organic solar cells are essential for this new photovoltaic technology. In this work, we found that the high-performance polymer solar cells suffer from significant performance decay upon thermal annealing at 150 °C owing to the fast decay of V_OC and FF. We demonstrated that the thermal annealing process leads to a severe chemical reaction of MoO₃ with Al, forming an Al₂O₃ barrier layer at the MoO₃/Al interface, which lowers the built-in potential (V_bi) of the cells and consequently reduces charge collection efficiency. Inserting a thin C₆₀ interlayer between MoO₃/Al slows the chemical reaction of MoO₃ with Al, which ensures a high V_bi and charge collection efficiency for the annealed MoO₃/C₆₀/Al cells. Such a protection effect of the C₆₀ layer in improving device performance against thermal annealing was also confirmed for cells with different polymer photoactive layers and metal electrodes, demonstrating the generality of the interfacial degradation of the cells and the protection effect of the C₆₀ layer. Finally, we demonstrated that the inverted polymer solar cells with the C₆₀-modified anode showed almost no performance decay upon high-temperature hot-press encapsulation, demonstrating excellent heat tolerance of this new device structure.