  Two-terminal tandem cell architectures are believed to be an effective way to
further improve the power conversion efficiency in solution processed
photovoltaics. To design an efficient tandem solar cell, two key issues need to
be considered. Firstly, subcells with well-matched currents and complementary
absorption characteristics are a prerequisite for high efficiency. Secondly
identifying the appropriate intermediate layer (IML) to connect the subcells is
necessary to minimize the optical and electronic losses. PbS colloidal quantum
dots (CQDs) are a notable choice for the subcells due to their low cost,
solution processibility and remarkable wide range band gap tunability. Single
layer Graphene (Gr) has been proposed to be a promising IML due to its high
transparency and conductivity. Here, as a proof of concept, we demonstrate a
solution processed two terminal PbS CQDs tandem solar cell employing chemical
vapor deposited Gr as the IML. In doing so, we report a PbS CQD cell comprising
subcells with bandgaps of 1.4 and 0.95 eV that delivers power conversion
efficiency in excess of 7%, substantially higher than previously reported CQD
tandem cells.
