Solution-processed MoO₃:PEDOT:PSS hybrid hole transporting layer for inverted polymer solar cells

Solution-processed organic-inorganic hybrids composing of MoO₃ nanoparticles and PEDOT:PSS were developed for use in inverted organic solar cells as hole transporting layer (HTL). The hybrid MoO₃:PEDOT:PSS inks were prepared by simply mixing PEDOT:PSS aqueous and MoO₃ ethanol suspension together. A core-shell structure was proposed in the MoO₃:PEDOT:PSS hybrid ink, where PEDOT chains act as the core and MoO₃ nanoparticles connected with PSS chains act as the composite shell. The mixing with PEDOT:PSS suppressed the aggregation of MoO₃ nanoparticles, which led to a smoother surface. In addition, since the hydrophilic PSS chains were passivated through preferentially connection with MoO₃, the stronger adhesion between MoO₃ nanoparticles and the photoactive layer improved the film forming ability of the MoO₃:PEDOT:PSS hybrid ink. The MoO₃:PEDOT:PSS hybrid HTL can therefore be feasibly deposited onto the hydrophobic photoactive polymer layer without any surface treatment. The use of the MoO₃:PEDOT:PSS hybrid HTL resulted in the optimized P3HT:PC₆₁BM-and PTB7:PC₆₁BM-based inverted organic solar cells reaching highest power conversion efficiencies of 3.29% and 5.92%, respectively, which were comparable with that of the control devices using thermally evaporated MoO₃ HTL (3.05% and 6.01%, respectively). Furthermore, less HTL thickness dependence of device performance was found for the hybrid HTL-based devices, which makes it more compatible with roll-to-roll printing process. In the end, influence of the blend ratio of MoO₃ to PEDOT:PSS on photovoltaic performance and device stability was studied carefully, results indicated that the device performance would decrease with the increase of MoO₃ blended ratio, whereas the long-term stability was improved.