A highly efficient planar heterojunction OSC based on zinc phthalocyanine (ZnPc)/fullerene (C60) by controlling the orientation of the ZnPc by using copper iodide (CuI) as the interfacial layer is reported. The proportion of face-on ZnPc molecules was increased significantly on the CuI layer compared to the layer without the CuI layer, which was analyzed with wide-angle X-ray scattering (WAXS) and optical absorption. The power conversion efficiency (PCE) of the orientation controlled planar heterojunction OSC was remarkably enhanced to 3.2 ± 0.1% compared with 1.2 ± 0.1% of the conventional OSCs without the control of the molecular orientation. By inserting the 3-nm-thick CuI layer, JSC, VOC and FF have increased from 4.6 ± 0.2 to 8.9 ± 0.2 mA cm−2, from 0.48 ± 0.01 to 0.59 ± 0.02 V, and from 0.56 ± 0.01 to 0.61 ± 0.02, respectively. VOC enhancement is discussed with the result of the ultraviolet photoemission spectra (UPS) measurements.