Stability of Inverted Organic Solar Cells with ZnO Contact Layers Deposited from Precursor Solutions

We report on investigations of the stability of inverted organic solar cells with ZnO electron collecting interlayer that are solution-processed from zinc acetate (ZnAc) or diethylzinc (deZn) precursors. Characterization of the respective solar cells suggests that the two materials initially function similarly in devices, however, we find that devices with ZnO from the deZn precursor are more stable under long-term illumination and load than devices with ZnO from the ZnAc precursor. A dipolar phosphonic acid that reduces the ZnO work function also improved device performance and stability when compared with unmodified ZnAc-based ZnO, but was problematic for deZn-based ZnO. The long-term device degradation analyses shows that the improved devices had increased and significantly more stable open-circuit voltage and fill factor characteristics. Chemical analyses suggests that defects in the ZnO films, most likely interstitial zinc, may be responsible for the observed disparities in stability within organic solar cells.