Fabrication of Inverted Bulk-Heterojunction Organic Solar Cell with Ultrathin Titanium Oxide Nanosheet as an Electron-Extracting Buffer Layer

The contributions and deposition conditions of ultrathin titania nanosheet (TN) crystallites were studied in an inverted bulk-heterojunction (BHJ) cell in indium tin oxide (ITO)/titania nanosheet/poly(3-hexylthiophene) (P3HT): phenyl-C-61-butyric acid methylester (PCBM) active layer/MoOx/Ag multilayered photovoltaic devices. Only one or two layers of poly(diallyldimethylammonium chloride) (PDDA) and TN multilayered film deposited by the layer-by-layer deposition technique effectively decreased the leakage current and increased both open circuit voltage (V-OC) and fill factor (FF), and power conversion efficiency (eta) was increased nearly twofold by the insertion of two TN layers. The deposition of additional TN layers caused the reduction in FF, and the abnormal S-shaped curves above V-OC for the devices with three and four TN layers were ascribed to the interfacial potential barrier at the ITO/TN interface and the series resistance across the multilayers of TN and PDDA. The performance of the BHJ cell with TN was markedly improved, and the S-shaped curves were eliminated following the the insertion of anatase-phase titanium dioxide between the ITO and TN layers owing to the decrease in the interfacial potential barrier. (C) 2012 The Japan Society of Applied Physics