Sensitization of SnO2 Single Crystals with Multidentate‐Ligand‐Capped PbS Colloid Quantum Dots to Enhance the Photocurrent Stability

A PbS quantum dot (QD) sensitized SnO2 solar cell has the advantage of producing photocurrent from near‐infrared to the ultraviolet that covers much of the higher energy solar spectrum. However, the long‐chain ligands, that are frequently used to cap as‐synthesized QDs, inhibit the charge transfer ability to the sensitized substrate, decreasing the photovoltaic efficiency in the device. Herein, we present a fast and efficient ligand‐exchange strategy for the replacement of synthetically convenient hydrophobic oleic acid (OA) ligands with short‐chain multidentate hydrophilic ligands such as meso‐2,3‐dimercaptosuccinic acid (DMSA) that increase the electronic coupling to the SnO2 substrate. Results from ultraviolet‐visible‐near‐infrared (UV‐Vis‐NIR) spectroscopy, X‐ray diffraction (XRD) measurements, and high‐resolution transmission electron microscopy (HR‐TEM) micrographs verify that this ligand‐exchange method produces no significant QD size or crystal‐structure changes. X‐ray photon spectra (XPS) results suggest that thiol groups from DMSA are likely bound to the PbS QDs compared to other bifunctional ligands. The PbS QDs sensitized SnO2 single crystals were characterized with atomic force microscopy (AFM), and with incident photon current efficiency (IPCE) spectra. Furthermore, it was shown that the IPCE of DMSA‐capped PbS QDs sensitized SnO2 single crystals retains >80% of its initial value after >7 hours of illumination within an electrolyte containing a high concentration of NaI whereas other ligand capped PbS QDs degraded much more rapidly. Meso‐2,3‐dimercaptosuccinic acid (DMSA)‐capped PbS QDs sensitized SnO2 single crystals by fast and efficient ligand exchange to enhance the photocurrent stability. The incident photon current efficiency of DMSA‐capped PbS QDs sensitized SnO2 single crystals was more stable than other ligand‐capped PbS QDs.