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 MoO 3 with Al, forming an Al 2 O 3 barrier layer at the MoO 3 /Al interface, which lowers the built-in potential ( V bi ) of the cells and consequently reduces charge collection efficiency. Inserting a thin C 60 interlayer between MoO 3 /Al slows the chemical reaction of MoO 3 with Al, which ensures a high V bi and charge collection efficiency for the annealed MoO 3 /C 60 /Al cells. Such a protection effect of the C 60 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 C 60 layer. Finally, we demonstrated that the inverted polymer solar cells with the C 60 -modified anode showed almost no performance decay upon high-temperature hot-press encapsulation, demonstrating excellent heat tolerance of this new device structure.