Colloidal Quantum Dot Tandem Solar Cells Using Chemical Vapor Deposited Graphene as an Atomically Thin Intermediate Recombination Layer

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. First, subcells with well-matched currents and complementary absorption characteristics are a prerequisite for high efficiency. Second, 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 processability, 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 that of previously reported CQD tandem cells.