Photovoltaic Properties of Bulk-Heterojunction Organic Solar Cell with Ultrathin Titanium Oxide Nanosheet as Electron Selective Layer

The contributions of ultrathin titania nanosheet (TN) crystallites were studied in both an inverted bulk-heterojunction (BHJ) cell in indium tin oxide (ITO)/titania nanosheet (TN)/poly(3-hexylthiophene) (P3HT): phenyl-C-61-butyric acid methylester (PCBM) active layer/MoO3/Ag and a conventional BHJ cell in ITO/MoO3/P3HT: PCBM active layer/TN/Al multilayered photovoltaic devices. The insertion of only one or two layers of poly(diallyldimethylammonium chloride) (PDDA) and TN multilayered film prepared by the layer-by-layer deposition technique effectively decreased the leakage current and increased the open circuit voltage (V-OC), fill factor (FF), and power conversion efficiency (eta). The abnormal S-shaped curves observed in the inverted BHJ cell above V-OC disappeared with the reduction in the work function of the bottom electrode in the inverted cells. The power conversion efficiency was increased nearly twofold, and better photovoltaic performance was observed in the conventional BHJ cells. Although the active layer has a hydrophobic surface, the active layer was fully covered by the insertion of only two or three layers of PDDA/TN multilayered film. The TN layer effectively decreased the leakage current by a factor of 100, and increased the open circuit voltage by 0.25 V probably owing to the complete hole blocking at the P3HT/TN/Al interface. (C) 2013 The Japan Society of Applied Physics