Annealing-free highly crystalline solution-processed molecular metal oxides for efficient single-junction and tandem polymer solar cells

Polyoxometalate (POM) layers have been used to realize efficient and long-term stable single-junction polymer photovoltaic devices with diverse configuration and donor : acceptor combination in the photoactive blend and polymer tandem cells through functioning as effective hole extraction layers and, also, as recombination layers in the interconnecting unit of the tandem cell. Their unique properties, such as their extremely high work function (WF) of 6.0-6.2 eV, their high degree of crystallinity without any post annealing requirements and, especially, the position of their lowest unoccupied molecular orbital (LUMO), were used to control the characteristics of optoelectronic devices. It was found that POMs having a deep LUMO level lying below the highest occupied molecular orbital (HOMO) of the donor polymer are highly beneficial for device operation due to the interfacial p-doping of the latter. We demonstrated conventional and reverse structure single-junction cells reaching an efficiency of 7.9% in the latter case and a tandem cell with an efficiency of 9.9% using an all solution processed inverted structure, where a POM layer simultaneously offers enhanced hole extraction in the sub-cells and minimal losses in the recombination unit. The specific properties of four POM materials and their role as functional layers in those different types of polymer photovoltaic devices are discussed.