Novel cathode buffer layer of Ag-doped bathocuproine for small molecule organic solar cell with inverted structure

2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (C26H20N2), known as bathocuproine (BCP), is a commonly used cathode buffer layer in conventional structure organic solar cells (OSCs). We demonstrated that BCP layer can also be used as a buffer layer in inverted structure OSCs. Unfortunately, the device exhibited an anomalous kink in the current density-voltage (J-V) characteristics, namely, an S-shaped J-V curve, leading to a low fill factor and low power conversion efficiency (PCE). To improve device performance, Ag-doped BCP layer (Ag:BCP) was used to replace the BCP layer. The results showed that the Ag: BCP layer can eliminate the S-kink in the J-V curve, resulting in a large improvement of fill factor and PCE. The origin of the S-shaped J-V curve was demonstrated to originate from the charge accumulation at the fullerene (C-60)/BCP interface. On the contrary, the C-60/Ag: BCP interface has favorable electronic properties with beneficial gap states for the transport of free carriers. Together with the good conductivity of Ag: BCP layer and the smooth morphology properties, the device performance was greatly improved by Ag: BCP buffer layer. (C) 2014 Elsevier B. V. All rights reserved.