Performance improvement of inverted organic solar cells by adding ultrathin Al2O3 as an electron selective layer and a plasma enhanced chemical vapor deposition of SiOx encapsulating layer

Abstract

In this paper, we report the performance improvement of inverted organic solar cells by adding an ultrathin electron selective layer of Al2O3 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 SiOx-coated polyethylene terephthalate, where the SiOx layer was made by plasma enhanced chemical vapor deposition (PECVD). It was found that the devices with ALD Al2O3 have a higher open circuit voltage than those without the ALD Al2O3 layer. Al2O3 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 Al2O3 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 Al2O3. Further, after the encapsulation by PECVD SiOx, our devices achieved a shelf-life of over 500h for 50% retained cell efficiency.