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

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 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 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 SiO_x, our devices achieved a shelf-life of over 500 h for 50% retained cell efficiency. - Highlights: • Atomic layer deposition of Al₂O₃ improved the property of inverted organic solar cells. • Ultraviolet light soaking affected electrode work function and Al₂O₃ conductivity. • SiO_x coating enveloped cells can achieve a 500 h shelf-life