One of the limitations in understanding the performance of organic solar cells has been the unclear picture of morphology and interfacial layers developed at the active layer/cathode interface. Here, by utilizing the shadow-Focused Ion Beam technique to enable energy-filtered transmission electron microscopy imaging in conjunction with X-ray photoelectron spectroscopy (XPS) experiments, we examine the cross-section of polythiophene/fullerene solar cells to characterize interfacial layers near the semiconductor-cathode interface. Elemental mapping reveals that localization of fullerene to the anode interface leads to low fill factors and S-shaped current - voltage characteristics. Furthermore, the combination of elemental mapping and XPS depth profiles of devices demonstrate oxidation of the aluminum cathode at the active layer interface for devices without S-shaped characteristics and fill factors of 0.6. The presence of a thin dielectric at the semiconductor-cathode interface could minimize electronic barriers for charge extraction by preventing interfacial charge reorganization and bandbending. (Chemical Equation Presented).