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The enhancement of device performance achieved by inserting a LiF (lithium fluoride) interfacial layer between the active layer and aluminum (Al) cathode was investigated. Energy band diagrams and interfacial chemical reactions were examined using high-resolution synchrotron-radiation photoemission spectroscopy. If only a thin layer of Al is deposited on the P3HT:PCBM/LiF, the highest occupied molecular orbital level of the active layer does not change. This indicates that shifts in the energy band are only strongly influenced by the insertion of LiF, not the subsequent deposition of an Al layer. We found that the molecular energy band shifts occurred at the interface because of the presence of LiF increases the open-circuit voltage. Furthermore, the thin layer of LiF reduces the work function and improves contact with the cathode, thereby reducing the series resistance and improving the short circuit current density. Thus, without the influence of the Al layer, inserting LiF improves the device performance. The level of performance differs substantially from that observed at the organic layer/LiF/Al interfaces in organic light-emitting devices.