In this study, we investigate the mechanisms leading to the power conversion efficiency improvement in poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) based organic solar cells using calcium (Ca) in cathode structures. Ultraviolet and x-ray photoemission spectroscopy (UPS and XPS) indicate that chemical reactions occur at the P3HT/Ca interface. Upon Ca deposition, UPS results illustrate a 0.8 eV-downward shift in energy levels of P3HT, but not in those of PCBM. In addition to forming an ohmic contact at the cathode the presence of Ca widens the energy difference between the HOMO of P3HT and the LUMO of PCBM at the cathode interfaces, which results in the increase of open circuit voltage and the enhancement of device efficiency. (C) 2011 Elsevier B.V. All rights reserved.