Roll-to-Roll Sputter Coating of Aluminum Cathodes for Large-Scale Fabrication of Organic Photovoltaic Devices

Abstract

We report the demonstration of sputter-coated aluminum contacts directly onto P3HT:PCBM organic photovoltaic de vices using a R2R process without detrimentally influencing the performance of the devices. The final sputtered devices do not require any protective buffer layers to produce effi cient performance. Depth profiling analysis of sputtered films using X-ray photoelectron spectroscopy (XPS) indicat ed the presence of a 5–6 nm insulating oxide layer generated at the cathode interface for all sputtering target power densi ties greater than 1.4 W cm ¢2 . The aluminum penetration into the P3HT:PCBM film was found to be consistent with the depth of this oxide layer, suggesting that aluminum penetra tion into the organic film is not the primary reason for per formance limitations in sputtered devices. Introduction of thermally evaporated aluminum buffer layers prior to deposi tion of sputtered aluminum cathodes demonstrated that the performance of devices after annealing matched those of ref erence devices prepared with no sputtering for a buffer layer thickness of only 20 nm. Further analysis of the device J--V curves revealed an S-shaped kink prior to annealing, indicat ing that the major reason for the poor performance in sput tered devices was the introduction of a charge extraction bar rier at the cathode, which was subsequently removed upon annealing. Rigorous removal of oxygen from the sputtering chamber prior to aluminum deposition onto the P3HT:PCBM active layer was subsequently observed to pro duce a device with an efficiency close to that of the thermally evaporated reference device without the requirement for evaporated buffer layers. The results presented here highlight a pathway towards an alternative R2R cathode fabrication technique that allows the highly efficient aluminum cathodes employed in small-scale devices to be transferred onto large scale, flexible, and low-cost R2R printed organic electronic devic