Performance improvement of inverted organic solar cells by adding ultrathin Al_2O_3 as an electron selective layer and a plasma enhanced chemical vapor deposition of SiO_x encapsulating layer

摘要

In this paper, we report the performance improvement of inverted organic solar cells by adding an ultrathin electron selective layer of Al_2O_3 prepared between the indium tin oxide (ITO) electrode and the active transport layer through atomic layer deposition (ALD). We evaluated the cell shelf-life after encapsulating with SiO_x-coated polyethylene terephthalate, where the SiO_x layer was made by plasma enhanced chemical vapor deposition (PECVD). It was found that the devices with ALD Al_2O_3 have a higher open circuit voltage than those without the ALD Al_2O_3 layer. Al_2O_3 deposited on an ITO electrode decreased the work function of ITO. Furthermore, based on the current density-voltage curves of the initial devices showing a pronounced S-shape, we soaked the cells with the ultraviolet (UV) light process. Then we obtained a higher efficiency in these ALD Al_2O_3 treated devices. With a careful analysis by atomic force microscopic and X-ray photoelectron spectroscopy, we believe that the UV light soaking process affected both ITO and Al_2O_3. Further, after the encapsulation by PECVD SiO_x, our devices achieved a shelf-life of over 500 h for 50% retained cell efficiency.