Efficient Light-soaking-free Inverted Organic Solar Cells with Aqueous Solution Processed Low-Temperature ZnO Electron Extraction Layers

Low-temperature processes are unremittingly pursued in the fabrication of organic solar cells. The paper reports that the highly efficient and light-soaking-free inverted organic solar cell can be achieved by using ZnO thin films processed from the aqueous solution method at a low temperature. The inverted organic solar with an aqueous-processed ZnO thin film annealed at 150 C shows an efficiency of 3.79%. Even when annealed at a temperature as low as 80 C, the device still shows an efficiency of 3.71%. With the proper annealing temperature of 80 C, the flexible device, which shows an efficiency of 3.56%, is fabricated on PET. This flexible device still keeps the efficiency above 3.40% after bent for 1000 times with a curvature radius of 50 mm. In contrast, a low annealing temperature leads to an inferior device performance when the ZnO thin film is processed from the widely used sol gel method. The device with sol gel processed ZnO annealed at 150 degrees C only shows a PCE of 1.3%. Furthermore, the device shows a strong light-soaking effect, which is not observed in the device containing an aqueous-processed ZnO thin film. Our results suggest that the adopted aqueous solution method is a more efficient low temperature technique, compared with the sol gel method.