Titanium–Aluminum Bilayer Cathode for Small-Molecular Organic Solar Cells with Prolonged Life upon Exposure to Air

The in-air stability of unencapsulated small-molecular organic solar cells (OSCs) was improved by 3 orders of magnitude by using a Ti (inner)–Al (exposed to air) bilayer cathode, in comparison with that of Al-monolayer-cathode OSCs. The stability of the unencapsulated bilayer-cathode OSCs (AlTi-OSCs) was comparable to that of encapsulated ones. These devices were degradation-free in air for more than 3 months. Under weathering tests, the unencapsulated AlTi-OSCs recovered their performance in the idle periods. This provides us with an effective and economical approach to prevent degradation of the extensively studied aluminum-cathode OSCs.