We present a study of performances of organic photovoltaic cells (OPVs) using a wide variety of small molecules. These OPVs use two or three molecules to form binary or ternary cells, i.e. consisting of two or three flat stacked layers. Different ternary OPVs designs for efficient light harvesting and energy band structure combinations are studied. Molecules have been chosen to study all possible energy band structure alignments in ternary configuration. We discuss various parameters limiting their efficiency, such as carrier mobility, organic layer morphology, energy band structure alignment. Special attention has been paid to understand the device behavior. It is shown that, if light absorption domains, carriers mobility, energy band structure alignment are decisive for achieving performing OPVs, the geometry of the molecules is also decisive. In ternary OPVs, the third organic layer, intercalated between the electron donor and the electron acceptor, must serve as a bridge in the energy levels of the outer layers, which permits an energetic cascade effect. For this effect to be positive it is necessary that this intermediate layer facilitates creation and transport of charge carriers, but also that it allows transport of energy, all leading to an improvement of the performance of the OPVs.