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.