Impact of Contact Evolution on the Shelf Life of Organic Solar Cells

For organic photovoltaics (OPVs) to become a truly viable technology, it is critical to correctly identify their degradation mechanism(s) and pave the way for strategies to improve device lifetime. Changes in the active organic component are often cited as the leading degradation pathway in OPVs. In this shelf life study, we monitor device performance after storage in the dark at ambient conditions to show that decay behavior is dominated by changes at the top metal electrode and/or the metal-organic interface. Cross-sectional TEM reveals extensive void formation to be the primary degradation mechanism for Ca/Al contacts. Kelvin probe measurements show the work function of Ag contacts increases over time, consistent with silver oxide formation detected by TOF-SIMS depth profiles. The evolution of the work function is found to be advantageous for Ag as a hole-extracting contact, but unsuitable for electron-extraction. The combination of Ag positive contacts (to collect holes) and ITO/ZnO negative contacts (to collect electrons) yields devices that are stable up to one year when stored in ambient conditions.