Influence of the hole-collecting interlayer on the stability and lifetime of inverted organic solar cells

In organic photovoltaics (OPVs), interlayers between the photoactive layer and the electrodes are often used to modify the work-function of the electrode, provide charge-blocking selectivity, and improve the lifetime of the solar cell. To date, PEDOT:PSS has been the most commonly-used interlayer; however, due to its acidic and hygroscopic nature, it can facilitate degradation. To improve the stability of the device, molybdenum oxide (MoO$_3$) has emerged as an attractive alternative to PEDOT:PSS, and solar cells utilizing MoO$_3$ have shown significantly enhanced lifetimes. Furthermore, degradation of low work-function cathode metals such as calcium can be eliminated when the typical cell design is inverted. In inverted solar cells, interlayers remain a critical component but we find their role in the degradation of the OPV changes. Contrary to what is observed in a conventional-architecture OPV, degradation studies of inverted solar cells under constant illumination lasting $>$1000 hours reveal solar cells utilizing a MoO$_3$ interlayer degrade faster than those with PEDOT:PSS. Understanding the influence of the charge-collecting interfaces in OPVs provides a pathway to increased reproducibility and longer lifetimes.