Charge accumulation induced S-shape J-V curves in bilayer heterojunction organic solar cells

We study the origin of the S-shape effect in the current-voltage curve of bilayer heterojunction organic solar cell induced by the exciton blocking layer (EBL). By gradually increasing the BCP thickness, the J-V curves change from exponential shape to S-shape, leading to a twofold degraded in the fill factor (FF). The variations of the J-V curves are attributed to the charge accumulation effect induced by the BCP layer. The charge accumulation phenomenon is further verified by the C-V measurements and a numerical model based on continuity and transport equations is developed to qualitatively reproduce the S-shape effect. Our finding provides insight into the origin of S-shape effect in bilayer heterojunction photovoltaic devices induced by the exciton blocking layer and reveals the importance of controlling charge accumulation for high performance OPVs especially the tandem devices. (C) 2011 Elsevier B.V. All rights reserved.