Lead sulfide (PbS) colloidal quantum dots (CQDs) have emerged as attractive candidates for thin film photovoltaic applications for their tunable near-infrared absorption, multiple exciton generation effect and solution processability. In PbS CQD solar cells, however, insulating ligands such as 1,2-ethanedithiol (EDT) and 3-mercaptopropionic acid (MPA) are often used and do not contribute to optical absorbance, thereby limiting short-circuit current density (Jsc). To solve this issue, we chose CH3NH3PbI3 perovskite as a promising capping ligand of PbS CQDs by taking advantage of its long carrier diffusion length and complementary optical spectrum with PbS CQDs. In our work, a solid-state ligand exchange method instead of complicated solution-phase ligand exchange method is employed to obtain CH3NH3PbI3 capped PbS CQDs, which exhibit significantly improved optical and electrical properties than neat PbS CQDs. Saturated solution of CH3NH3PbI3 in acetonitrile enables to attain thick PbS films with complete ligand replacement in the layer-by-layer deposition process. In inverted solar cells of CH3NH3PbI3 capped PbS CQDs, CH3NH3PbI3 ligands can not only create a p/n heterojunction with PbS to facilitate charge separation, but also act as an energy relay between PbS and the TiO2 layer to form the cascade energy alignment and reduce the energy loss. Thus the optimal solar cell reaches an impressive Jsc of ~25 mA/cm2 and a power conversion efficiency (PCE) of 4.25%. Furthermore, upon the addition of EDT-capped PbS CQDs, the bilayer solar cells yields a remarkably enhanced PCE up to 5.28%, due to more balanced and efficient charge transport in the device.
J. Peng, Y. Chen, X. Zhang, A. Dong, Z. Liang, Solid-State Ligand-Exchange Fabrication of CH3NH3PbI3 Capped PbS Quantum Dot Solar Cells. Adv. Sci. 3, 1500432 (2016).