For high performance of organic thin-film photovoltaics (OPVs), high absorption and efficient charge transport of active materials are critical to compensate the short exciton diffusion length and low carrier mobility in organic semiconductors. It is desirable to control over molecular orientation in active layers to enhance both optical and electrical properties. In this letter, we show that amorphous donors having a planar molecular shape can be horizontally oriented, enabling active layers in OPVs to be thin while keeping high absorption in the devices. Based on anisotropic properties in optical design, a power conversion efficiency of more than 4% was achieved from a simple planar heterojunction. This result shows that using horizontally oriented amorphous materials is an effective approach to enhance OPV efficiency. (C) 2011 Elsevier B.V. All rights reserved.