Molecular alignment at the interface between the electron donor and acceptor (D/A) materials in thin films may have a large effect on the charge generation process in organic solar cells (OSCs). Edge-on (main chain parallel and π-plane perpendicular to D/A interface) and face-on (both main chain and π-plane parallel to D/A interface) orientations were previously obtained for the same π-conjugated semiconducting material at D/A interface by proper solvent selection and several studies have been performed to clarify the effects of these orientations’ difference on OPV performance.^[1-2] Recently, we reported the formation of end-on orientation (main chain vertical to the film surface). ^[3-4] This is based on the formation of surface segregated monolayer (SSM) of P3BT-F₁₇ on P3HT film (Fig. 1a). For the end-on orientation, strong electronic and optical anisotropy and unique photophysical behavior were observed. ^[3-4]

 In this study, P3DDFT and P3BT-F₁₇ (Fig. 1a) were utilized to form SSMs on top of P3HT with the edge-on and the end-on orientations on the film surface, respectively. Ultraviolet photoelectron spectroscopy (UPS) and low energy inverse photoelectron spectroscopy (LEIPS) were performed on P3HT, P3DDFT/P3HT and P3BT-F₁₇/P3HT films to evaluate the energy levels. As the result, rigid energetic shift of ~0.5 eV was observed between films with edge-on and end-on orientations indicating the change of electronic structure of film surface influenced by orientation-dependent quadrupole moments (Fig. 1b). By contact film transfer (CFT) method^[5], bilayer OSCs with the architecture of ITO/ZnO/PCBM//P3DDFT and P3BT-F₁₇/P3HT/MoO₃/Ag were fabricated with edge-on and end-on orientations at D/A interface, respectively (Fig. 1c). We found that the end-on oriented polymer chains at the D/A interface showed a smaller photovoltage loss expected from the energy structure and a more efficient charge separation than the device with the edge-on oriented polymer chains. These improvements were attributed to charge delocalization along the end-on oriented polymer chains normal to the D/A interface, through which the effective binding energy of the charge pairs in the interfacial charge-transfer state was reduced (Fig. 1c). These results provide important information about molecular arrangements at the D/A interface that may enable further improvements in OSC performance. ^[6]