Asymmetrical I-V curves from a symmetrical devices structure of Organic Photovoltaics

Abstract

<p>The energy diagram for organic photovoltaics (OPV), involving the bulk heterojunction (BHJ), on which the device analysis is usually based, has long been a subject of debate. The widely used Metal-insulator-Metal model and P-type Schottky Junction model, both of which are based on inappropriate assumptions, could be incorrect to explain the working principle of BHJ OPV.</p> <p>To further explore the controversy, we start the investigation from the opposite direction, to the usually asymmetrical OPV, involving electron and hole passages, by introducing a pair of symmetric electrodes to a BHJ, to form a completely symmetrical device structure, which, in theory, would produce zero output.</p> <p>Surprisingly, it is found that such a symmetrical device exhibits asymmetrical I-V curves. In particular, it produces a non-zero open-circuit voltage, and a finite short-circuit current. The cause of the output was the asymmetrical charge carrier distribution due to the asymmetrical illumination. To explain the operational mechanism of the symmetrical device, the equivalent circuit including a pair of inverse-parallel diodes and a new model for the BHJ energy diagram are introduced. Those findings would certainly improve the understanding of the device physics of OPV, especially the working principle for BHJ.</p>