The Impact of Molecular Orientation on the Photovoltaic Properties of a Phthalocyanine/Fullerene Heterojunction

The anisotropy inherent to many planar organic molecules leads to a high sensitivity of various fundamental processes to the orientation of molecules within films and at heterojunctions. Such processes include absorption, charge and exciton transport, energy levels, and charge transfer, all of which are critical to organic solar cell operation. Here,an in-depth study of bilayer cells consisting of a donor/acceptor interface between zinc phthalocyanine (ZnPc) and fullerene (C60) is conducted and devices with the typically deposited standing up (edge-on) orientation are compared to those with ZnPc lying flat (face-on). The face-on ZnPc-based device allows for an increase in all solar cell parameters, substantially increasing power conversion efficiency from 1.5% to 2.8%. Spectrally resolved photocurrent measurements reveal a >50% increase in ZnPc signal, from which only 12% is accounted for by the increase in absorption associated with the face-on orientation. The increase in internal quantum efficiency is accounted for via an improved charge transfer. The results of this study indicate that proper consideration of the orientation between donor and acceptor needs to be taken in order to fully optimize the numerous processes required for photovoltaic energy conversion.