Highlighting the possibility of parallel mechanism in planar ternary photovoltaic cells

Abstract

Ternary and binary planar heterojunctions (PHJs) have been realized and characterized. The outer layers of the active organic layers are pentathiophene (5T) and fullerene (C60), while the intercalated layer is AlPcCl. The binary, 5T/C60, 5T/AlPcCl and ternary 5T/AlPcCl/C60 PHJs were characterized by J-V and EQE measurements. The morphology of the organic layers was studied by scanning electron microscopy and atomic force microscopy, while the band structure of 5T was estimated by cyclic voltammetry. The study shows that it is possible to overcome the difficulties linked to the need for the good band matching of the three successive organic layers by using as a first electron donor layer, a layer whose morphology allows the ternary structure to behave as it was two diodes in parallel. Actually, due to this specific morphology the intercalated layer is discontinuous, which allows achieving parallel mechanism in planar ternary photovoltaic cells. This parallel mechanism in the 5T/AlPcCl/C60 organic photovoltaic cells (OPVCs) allows achieving efficiency of 1.25%, which represents a 65 % increase by comparison with the best binary 5T/C60 OPVC. It means that the morphology of the 5T layer, with its many protrusions and holes, allows ternary OPVCs to behave like parallel-linkage diodes. This behaviour allows the ternary OPVCs to achieve efficiencies higher than those obtained with the binary 5T/C60 OPVCs.