Stable and Highly Efficient PbS Quantum Dot Tandem Solar Cells Employing a Rationally Designed Recombination Layer

This study reports the fabrication of stable, high-performance, simple structured tandem solar cells based on PbS colloidal quantum dots (CQDs) under ambient air. This study also reveals detailed device engineering to deposit each functional layer in the subcells at low temperature to avoid damage to the PbS CQDs and meanwhile makes the fabrication process compatible to flexible plastic substrate. Two efficient recombination layers (RLs) are rationally designed to connect the two subcells in series. The use of solution-processed RL with an organic PEDOT:PSS (poly(3,4-ethylenedioxythiophene): polystyrene sulfonate) interlayer leads to the fabrication of the tandem devices in solution process. The use of robust inorganic RL containing an ultrathin Au interlayer results in more efficient device performance and remarkably improved device lifetime. The optimal PbS CQDs tandem cells based on inorganic RL demonstrate a high power conversion efficiency approaching 9%. This efficiency is more than two times higher than the previous record of 4.2%, which has been kept for more than five years. The remarkable stability, high performance, and low-temperature processing of these tandem devices may provide insight into the commercialization of flexible and large-area CQDs tandem solar cells in the near future.