lodide-Passivated Colloidal PbS Nanocrystals Leading to Highly Efficient Polymer:Nanocrystal Hybrid Solar Cells

Current state-of-the-art hybrid polymer:lead chalcogenide nanocrystal solar cells require postdeposition, thin film chemical treatments to remove insulating organic ligands from the nanocrystal surface, which is a kinetically hindered process. This is compounded by the fact that it can be especially difficult to obtain colloidally stable suspensions of PbS nanocrystals ligand exchanged with small ligands, and many atomic ligands require dispersion in solvents that are incompatible with polymer solubility. Herein, we report a novel one-step colloidal ligand exchange process for PbS nanocrystals employing lead iodide (PbI2) or ammonium iodide (NH4I) as surface ligands along with n-butylamine that allow the ligand-exchanged nanocrystals to be suspended in solvents compatible with polymer dissolution. While ligand exchange is shown to be near quantitative for both iodide sources, when compared to NH4I-exchanged PbS nanocrystals, the PbI2-exchanged PbS nanocrystals not only exhibit better colloidal stability but also display superior photovoltaic performance. When the PbI2-passivated PbS nanocrystals are combined with the donor polymer poly[2,6-(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2'3'-d]silole)-alt-4,7-(2,1,3-benzothiadiazole) (Si-PCPDTBT), the optimized hybrid solar cells give a broad spectral response into the NIR, leading to a power conversion efficiency (PCE) of 4.8% under AM 1.5G illumination. Time-resolved photoluminescence and transient absorption spectroscopies suggest that excitonic processes between the PbS nanocrystals and Si-PCPDTBT are more favorable when PbS nanocrystals are ligand exchanged with PbI2, leading to superior device performance.