The device operational lifetime and the understanding of the degradation pathways are critical steps for inverted organic solar cells (iOSCs) to enter the energy market. In this work, a combined study of impedance spectroscopy (IS) and photovoltage/photocurrent transient (TPV/TPC) techniques was employed to analyze the degradation of PTB7-Th:PC70BM-based iOSCs in accordance with the ISOS-D1 protocol under ambient conditions without encapsulation. Both techniques provide real information on how devices with different electron transport layers (PFN, TiOx, and ZnO) were degraded, affecting the stability of the active layer. We found that the main cause of degradation is the formation of interface traps in the dark and upon air exposure. The increase of trap density in the active layer upon degradation was revealed by TPV. The change in interface trap density due to the interfacial defect over the shelf-lifetime was further confirmed by the data from IS measurements. Last but not least, modelling dark current–voltage (J–V) using the IS data as well as the morphological changes of each ETL verified our hypothesis.