Organic photovoltaics (OPVs) have recently received increasing attention as alternatives to inorganic solar cells because of their light weight, compatibility with flexible electronics, and low production cost. In this work, an ideal device structure for ordered bulk heterojunction (OBHJ) organic photovoltaics is proposed. Also suggested is a parameter related to the power conversion efficiency (PCE) of the devices. Such a parameter could serve as a rule of thumb for researchers. For cases in which it is difficult to reduce the pillar size and spacing, the proposed parameter dictates that an increase in the active layer thickness could be one way to increase the PCE of an OBHJ. To generate maximum PCE, (1) the pillar size and spacing must be two to three times less than the exciton diffusion length and (2) the thickness of the active layer has to be greater than half of the photon mean free path, which is the inverse of the absorption coefficient. (C) 2011 Elsevier B.V. All rights reserved.