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In this paper, the open-circuit voltage Voc of a poly(3-hexylthiophene-2,5-diyl) (P3HT) and a [6,6]-phenyl C61 -butyric acid methyl ester (PCBM) bulk heterojunction organic solar cell device was measured at temperatures ranging from 15°C to 145°C. The temperature dependence of the vacuum shift and of the highest occupied molecular orbital (HOMO) energy level of P3HT and PCBM were measured by ultraviolet photoelectron spectroscopy in the same temperature range. The temperature dependence of the absorption edge was also studied in the same temperature range to obtain the temperature variation of the optical band gap energy Eg. The measured Voc of the devices shows a clear decreasing trend with increasing operating temperature, and the total decrease is found to be about 0.1 V. However, by utilizing the measured values of the HOMO for the P3HT (donor) and of the determined lowest unoccupied molecular orbital (LUMO) for the PCBM (acceptor), we have found that the calculated values of Voc and its temperature dependence do not agree with the measured Voc values. This is a clear indication that the expression (HOMO-LUMO)-exciton energy does not hold and that other factors are impacting the value of Voc.