Air-stable efficient polymer solar cells incorporating solution-processed titanium oxide layer

Abstract

By introducing a titanium oxide (TiO_x) layer between the active layer and the aluminum cathode in polymer based electronic devices, we have demonstrated devices with excellent air stability and with enhanced performance. The TiO_x layer acts as a shielding and scavenging layer which prevents the intrusion of oxygen and humidity into the electronically active polymers, thereby improving the lifetime of unpackaged devices exposed to air by nearly two orders of magnitude. We have also fabricated polymer tandem solar cells with a power conversion efficiency of 6.5%, with each layer processed from solution. A transparent TiO_x layer is used to separate and connect the front cell and the back cell. The TiO_x layer serves as an electron transport and collecting layer for the first cell and as a stable foundation that enables the fabrication of the second cell to complete the tandem cell architecture. We use an inverted structure with the low band-gap polymer/fullerene composite as the charge separating layer in the front cell and the high band-gap polymer composite as the charge separating layer in the back cell.