The challenges of experimentally probing the physical and electronic structures of the highly intermixed organic semiconductor blends that comprise active layers in high-performance organic photovoltaic (OPV) cells ultimately limit the fundamental understanding of the device performance. We use Fourier-transform IR (FTIR)-absorption spectroscopy to quantitatively determine the interfacial electric field in blended poly(3-hexylthiophene) (P3HT):phenyl- C61-butyric acid methyl ester (PCBM) thin films. The interfacial electric field is ≈0.2 V nm−1 in the as-spun film and blends annealing at temperatures as high as 150 °C, which is the optimal annealing temperature in terms of OPV performance. The field decreases to a negligible value upon further annealing to 170 °C, at which temperature PCBM changes from amorphous to crystalline and the open-circuit voltage of the solar cell decreases from 0.62 to 0.4 V. In addition, our measurements also allow determination of the absolute degree of crystallinity within the acceptor material. The roles of interfacial field and local crystallinity in OPV device performance are discussed.