MoO₃-Au composite interfacial layer for high efficiency and air-stable organic solar cells

Efficient and stable polymer bulk-heterojunction solar cells based on regioregular poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PC₆₁BM) blend active layer have been fabricated with a MoO₃-Au co-evaporation composite film as the anode interfacial layer (AIL). The optical and electrical properties of the composite MoO ₃-Au film can be tuned by altering the concentration of Au. A composite film with 30% (weight ratio) Au was used as the AIL and showed a better performance than both pure MoO₃ and PEDOT:PSS as AIL. The surface morphology of the MoO₃-Au composite film was investigated by atomic force microscopy (AFM) and showed that the originally rough ITO substrate became smooth after depositing the composite film, with the root mean square roughness (RMS) decreased from 4.08 nm to 1.81 nm. The smooth surface reduced the bias-dependent carrier recombination, resulting in a large shunt resistance and thus improving the fill factor and efficiency of the devices. Additionally, the air stability of devices with different AILs (MoO₃-Au composite, MoO₃ and PEDOT:PSS) were studied and it was found that the MoO ₃-Au composite layer remarkably improved the stability of the solar cells with shelf life-time enhanced by more than 3 and 40 times compared with pure MoO₃ layer and PEDOT:PSS layer, respectively. We argue that the stability improvement might be related with the defect states in MoO₃ component.